Introduction To Steels

 

Introduction to Steels

 

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Introduction to Steels Processing, Properties, and Applications

P. C. Angelo B. Ravisankar

 

CRC Press Taylor & Francis Group 52 Vanderbilt Avenue, New York, NY 10017 © 2019 by Taylor & Francis Group,&LLC CRC Press is an imprint of Taylor Francis Group, an Informa business No claim to original U.S. Government works Printed Print ed on acid-free paper International Standard Book Number-13: 978-1-138-38999-1 (Hardback) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if  permission to publish in this form has not been obtained. If any copyright material has not  been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, micro 󿬁lming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com  (http://www.copyright.com/  ( http://www.copyright.com/ ) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-forpro󿬁t organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of  payment has been arranged. Trademark Notice:  Product or corporate names may be trademarks or registered trademarks, and are used only for identi󿬁cation and explanation without intent to infringe. Library Librar y of Congre Congress ss Catalo Cataloging-in ging-in-Publi -Publication cation Data

Names: Angelo, P.C., author. | Ravisankar, B., author. Title: Introduction to steels : processing, properties, and applications / P. C. Angelo and B. Ravisankar. Description: New York, NY : CRC Press/Taylor & Francis Group, 2019. | Includes  bibliographical references and index. Identi󿬁ers: LCCN 2018052858| ISBN 9781138389991 (hardback : acid-free paper) | ISBN 9780429423598 (ebook) Subjects: LCSH: Steel–Metallurgy. Classi󿬁cation: LCC TN730 .A58 2019 | DDC 669/.142–dc23 LC record available at  at   https://lccn.loc.gov/201805 https://lccn.loc.gov/2018052858 2858

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Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix Author Aut hor Bios Bios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

1 Iron Iron-C -Car arbo bon n Di Diag agra ram m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1 1. 1.11 Iron Iron-C -Carb arbon on Diag Diagra ram m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 1.2 Slow Slow Cool Cooling ing of Eute Eutecto ctoid id Comp Composi ositio tion. n. . . . . . . . . . . . . . . . . . . . 7 1.3 Slow Slow Cooli Cooling ng of of Hypoe Hypoeute utecto ctoid id Com Compos positi ition on . . . . . . . . . . . . . . . 8 1.4 Slow Slow Cool Cooling ing of Hype Hypereu reutec tectoi toid d Comp Composi ositio tion n . . . . . . . . . . . . . .9 2 He Heat at T Tre reat atme ment nt of S Ste teel elss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   11 2. 2.11 Anne Anneal alin ing g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 . 12

2. 2.22 2. 2.33 2. 2.44 2. 2.55 2. 2.66 2. 2.77 2. 2.88 2. 2.99 2. 2.10 10

Norm Normal aliz izin ing g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Hard Harden enin ing g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 . 14 Hard Harden enab abil ilit ity y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Temp Temper erin ing. g. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20 . 20 Mart Martem empe peri ring. ng. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21 . 21 Auste Austemp mper erin ing. g. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21 . 21 Ausfo Ausform rmin ing. g. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Isof Isofor ormi ming. ng. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23 . 23 Prec Precip ipit itat atio ion n Hea Heatt Trea Treatm tmen entt . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3 Sur Surfa face ce H Har arden denin ing g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   27 3. 3.11 Case Case Har Harde deni ning ng . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27 . 27

3. 3.22 Chem Chemic ical al Sur Surfa face ce Hard Harden enin ing g . . . . . . . . . . . . . . . . . . . . . . . . . . . .28 . 28 4 Pla Plain in C Car arbo bon n St Stee eels. ls. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   31 4. 4.11 Low Low Car Carbo bon n (or) (or) Mil Mild d Stee Steels ls . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 4. 4.22 Medi Medium um Carb Carbon on St Stee eels ls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32 . 32 4. 4.33 High High Car Carbo bon n Ste Steel elss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32 . 32 4.4 Appli Applicat cation ionss as Engine Engineeri ering ng Mate Materia riall . . . . . . . . . . . . . . . . . . . . . 33 5 Eff Effect ect o off All Alloyi oying ng Ele Elemen ments ts in St Steel eel . . . . . . . . . . . . . . . . . . . . . . . . . . .   35 5.1 Effect Effect of Alloyi Alloying ng Elemen Elements ts on on Iron Iron Carbon Carbon Diagra Diagram m . . . . . . . 35 5. 5.22 Ferri Ferrite te Sta Stabi bili lize zers rs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36 . 36 5. 5.33 Auste Austeni nite te Stab Stabil iliz izer erss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 5. 5.44 Neut Neutra rall Sta Stabi bili lize zerr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39 . 39 5.5 Effect Effect of Variou Variouss Elem Element entss as as Disso Dissolve lved d in in Fer Ferrit ritee Matrix Mat rix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

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vi

5.6 5. 5.77 5. 5.88 5. 5.99 5. 5.10 10 5.111 5.1 5.122 5.1 5.133 5.1 5.144 5.1

Contents

Effect Effect of Variou Variouss Elem Element entss as as Carb Carbid ides es in Ferrit Ferrite/ e/ Austen Au stenite ite Matri Matrixx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Summ Summar ary y of of Carb Carbid ides es . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Disp Disper erse sed d Meta Metall llic ic Par Parti ticl cles es . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Nonm Nonmet etal alli licc Inc Inclu lusi sion onss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Inte Interm rmet etal alli licc Com Compo poun unds ds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Alloy Alloy Dist Distrib ributi ution on in in Austen Austenite ite . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Effect Effect of Allo Alloyin ying g Elem Element entss on Harden Hardening. ing. . . . . . . . . . . . . . . . . 46 Effect Effect of Allo Alloyin ying g Eleme Elements nts on Hard Hardena enabil bility ity . . . . . . . . . . . . . .48 . 48 Effect Effect of Allo Alloyin ying g Eleme Elements nts on Temp Temperi ering. ng. . . . . . . . . . . . . . . . .49 . 49

6 Lo Low w Al Allo loy y St Stee eels ls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   51 6.1 Plain Plain Carb Carbon on Stee Steels ls (1XX (1XXX X Serie Series) s) . . . . . . . . . . . . . . . . . . . . . . . . 52 6. 6.22 Nick Nickel el Ste Steel elss (2XX (2XXX) X) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 6.3 Nickel Nickel-Ch -Chrom romium ium Steels Steels (3XXX) (3XXX) . . . . . . . . . . . . . . . . . . . . . . . . .54 . 54 6. 6.44 Moly Molybd bden enum um St Stee eels ls (4X (4XXX XX)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

6. 6.5 6. 6.656 6. 6.77 6. 6.88 6. 6.99 6. 6.10 10 6. 6.11 11 6.122 6.1

Chro Ch romi mium umSt St Stee eels ls (6X (6XXX 5XXX XX) Vana Vanadi dium um Stee eels ls ((5X XX) )). .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 55 55 Tung Tungst sten en Ste Steel elss (7XX (7XXX) X) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55 . 55 Trip Triple le All Alloy oy Ste Steel elss (8XX (8XXX X and and 9XXX 9XXX)) . . . . . . . . . . . . . . . . . . . . . 55 Sili Silico con n Stee Steels ls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 Boro Boron n Stee Steels ls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 Inte Interst rstit itia iall Fre Freee Stee Steels ls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57 . 57 Appli Applicat cation ionss as Engine Engineeri ering ng Mate Materia riall . . . . . . . . . . . . . . . . . . . . . 62

7 Hi High gh St Stre reng ngth th St Stee eels. ls. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   65 7.1 De󿬁nitio nition n and Need of High High Strength Strength Steels Steels . . . . . . . . . . . . . . . 65 7.2 Types Types and Proble Problems ms in Develo Developin ping g Hig High h Str Streng ength th

7.3 7.4

Steels ...y . . of . . High . . . .h. Stren . .rength . . .gth . . . Steels . . .els . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .67 Metall Metallurg urgy Hig St Ste . 78 Appli Applicat cation ionss Parti Particul cularl arly y for for Autom Automoti otives. ves. . . . . . . . . . . . . . . . . 80

8 Hi High gh A All lloy oy S Ste teel elss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   83 8. 8.11 Mara Maragi ging ng St Stee eels ls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 8. 8.22 Stai Stainl nles esss Stee Steels ls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 8. 8.33 Tool Tool St Stee eels ls.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 116 9 Sel Selec ecti tion on o off Ma Mate teri rial als. s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   125 9.1 Tools Tools Used Used for Select Selection ion of Mate Materia rials ls . . . . . . . . . . . . . . . . . . . . 125 9.2 System Systemize ized d Selec Selectio tion n of Materi Materials als . . . . . . . . . . . . . . . . . . . . . . . 126 9. 9.33 Case Case Stu Studi dies es . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 127

 

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Subje Sub ject ctiv ive e Qu Quest estio ions ns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Objective questions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 Bibliography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 Inde In dex x . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

 

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Preface Steel is adapting itself to emerging structural standards and conditions. From 1778, when the   󿬁rst iron bridge was built, to the present, steel is the most commonly used material for various applications. Steel is appropriate to most critical requirements by the addition of alloying elements and by suitable heat treatment. Steels are also most suitable for manufacturing by formin for ming, g, weldin welding, g, castin casting, g, and machin machining ing.. There There are severa severall sta standa ndard rd  books that deal with the metallurgy of steel but they are meant for meta me tall llur urgi gica call engi engine neer erss and and ar aree not not suit suitab able le fo forr engi engine neer erss from from ot othe herr disciplin disci plines es includin including g mechanic mechanical, al, production production,, electrical electrical,, elect electronic ronics, s, and aero ae rona naut utic ical al engi engine neer erin ing g stude student nts. s. It is to ful ful󿬁ll th this is requ requir irem emen entt th that at Introductio Introd uction n to Steel: Steel: Processing Processing,, Properties, Properties, and Applications Applications has been written. It will also be useful for practicing engineers. We also expect the book to satisfy the needs of students preparing for competitive exams. This book consists of nine chapters covering most of the important types of steels and their physical metallurgy, microstructure, and engineering applications. Chapter 1 introduces the iron-carbon diagram and discusses the importance and interpretation of it as well as the resultant microstructuree under tur under equili equilibri brium um condit condition ionss with with respect respect to carbon carbon conten content. t. The effect of nonequilibrium cooling (heat treatment) on properties and methodology of heat treatment are discussed in Chapter 2. The most important surface hardening methods and methodology are described in Chapter 3. Chapter 4 deals with the various plain carbon steels and their heat treatment as well as important applications as engineering material. The effect of alloying elements on phase stability, microstructure, and properties are discussed Chapter elements 5. The properties and applications with low amounts ofinalloying are described in Chapter of 6. steel High-strength steel ste elss ar aree di disc scus usse sed d in Chap Chapte terr 7, in part partic icul ular ar th thei eirr ap appl plic icat atio ions ns fo forr spec sp ecia iali lize zed d uses uses in auto automo mobi bile less and and aero aerosp spac ace. e. Chap Chapte terr 8 co cove vers rs high high alloy all oy steels steels   –   in part partic icul ular ar mara maragi ging ng st stee eels ls,, stain stainle less ss st stee eels ls,, and and to tool ol steels   –   descri describin bing g their their specia speciall uses uses in variou variouss applic applicati ations ons.. The basic basic techniques of the selection of material illustrated through case studies are included in Chapter 9. In addition, the book contains both subjective and objective questions as an aid to students preparing for semester-end and competitive exams. This book is based on our many years of teaching undergraduate and postgr pos tgradu aduate ate studen students ts of variou variouss engine engineeri ering ng disci discipl pline ines. s. It is our fond fond hope that the present book will serve as a textbook for courses in engineering and also as a useful reference book for practicing engineers. We look forward to receiving your valuable response and suggestions.

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 Author Bios P.C. Ange Angelo lo, PhD, PhD, is intern internati ationa onally lly renown renowned ed in the   󿬁elds of Powder Metallurgy, Materials Characterisation for over 40 years. He has taught, resear res earche ched, d, and publi publishe shed d extens extensive ively ly in the areas areas of optic optical al emissi emission on spectr spe ctrosc oscopy opy,, optic optical al micros microscop copy, y, X-ray X-ray diffra diffracti ction, on, X-ray X-ray   󿬂uorescence, electron probe microanalysis, scanning electron microscopy, and scanning Auger microprobe. Dr. Angelo began his career in 1962 as a scientist at the Defenc Def encee Metall Metallurg urgica icall Resear Research ch Labora Laborator tory y (DMRL (DMRL), ), Hydera Hyderabad bad,, India, India, one of the laboratories of Defence Research and Development Organization (DRDO), Government of India, New Delhi. After 35 years of service he retired from DRDO and joined PSG College of Technology in the Department of Metallurgical Engineering and worked as Director, Metal Testing and Research Centre and as Professor and Dean for 20 years and retired in

Powder Metallu Metallurgy rgy:: Scienc Science, e, Techno Technolog logyy and Applic Applicatio ations, ns, 2017 20 17.. His His book bookss   Powder  Materials Characterisation, and   Non Ferrous Alloys   have been well received as textbooks by numerous institutions. Presently, he is Visiting Professor at PSG College of Technology, Coimbatore, Tamil Nadu, India.

Dr. B. Ravisankar,   Professor, Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli, has more than tha n 25 years years of teach teaching ing-cu -cumm-rese researc arch h experi experienc ence. e. He has contri contribut buted ed a chapter on Equal Channel Angular Pressing (ECAP) to the   Handbook of   Mechanical Nanostructuring. He al also so publi ublish sheed a book book on   Non Ferrou Ferrouss  Alloys.

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1  Iron-Carbon Diagram

1.0 Int Introd roduct uction ion Steel is frequently the   “gold-standard”   even among the emerging structural materials. Steels have numerous uses. Steel is used more than any  󿬁

other forter producing alloys. 1778 the1818 rst iron was built. Ins 1788 17 88,, metal ir iron on wate wa r pi pipe pe li line nes s were wereInlaid la id.. In 1818 th thee   󿬁bridge rs rstt st stee eel l sh ship ip was wa launched. In 1889 Gustav Eiffel, a French engineer, built the Eiffel Tower with wi th steel steel.. Ei Eiff ffel el’s cont contem empo porar rarie iess th thou ough ghtt th that at his his 300300-me mete terr latti lattice ced d structure would prove too fragile to last. But Eiffel argued that his creation would stand at least for a quarter of a century. Even today the Eiffel Tower   –  the true landmark of Paris   –  is intact and attracting visitors from all over the world. Steel is a moving standard, since regular and exciting discoveries are  being made. made. This makes steel to remain the most succ successful essful and costcost-effec effective tive of all materia materials ls ever. ever. Major Major reason for the the overwhel overwhelming ming dominanc dominancee of steel is the variety of microstructures and properties that can be generated by solidstate transformation and processing. Therefore, in studying advanced steels, it is useful to discuss, 󿬁rst the nature and behavior of pure iron, then the ironcarbon alloys, and   󿬁nally the complexities that arise when further solutes are added. At leas leastt four four allo allotro trope pess of ir iron on occu occurr natu natura rall lly y in bulk bulk fo form rm:: body body-centred cubic (bcc,   α   and   δ, ferrite), face-centred cubic (fcc,   γ, austenite) and hexagonal close packed (hcp,   ε). As molten iron cools past its freezing point of 1538°C, it crystallizes into a body-centered cubic (bcc)   δ  allotrope. As it cools further to 1394°C, it changes into a face-centered cubic (fcc)   γiron allotrope known as austenite. At 912°C and below, the crystal structuree again tur again becom becomes es bcc bcc   α-ir -iron on allotr allotrope ope,, or ferrite ferrite.. At pressu pressures res above above approximately 10 GPa and temperatures of a few hundred Kelvin or less, α-iron changes into a hexagonal close-packed (hcp) structure that is also known as   ε-iron; the higher-temperature   γ-phase also changes into   ε-iron,  but does so at still higher pressure. The phase diagram for pure iron is illustrated in Figure in  Figure 1.1. 1.1.

1  

2

 

Introduction to Steels

FIGURE 1.1 The phase diagram for pure iron

The phase   β  in the alphabetical sequence   α,   β,   γ,   δ   …  is missing because the magnetic transition in ferrite was at one time incorrectly thought to be the   β   allo allotr trop opee of ir iron on.. In fact fact,, th ther eree are are ma magn gnet etic ic tran transi siti tion onss in all all allotropes of iron. Some controversial experimental evidence exists for a   󿬁fth natural allotrope of iron in the core of the earth, where the pressure reaches some thre threee mill millio ion time mess th that at at th the e surf su rfac ace e the and andearth wher where th thee temp temper erat atur ure e is estimated tonbeti about 6000°C. The core of ise predominantly iron, and an d cons consis ists ts of a soli solid d in inne nerr core core surr surrou ound nded ed by a liqu liquid id ou oute terr core core.. Knowledge of the core is uncertain, but it has been suggested that the crystal structure of the solid core may be an orthorhombic or double hcp, also denoted as   β  (not to be confused with  β  named for magnetic transition for ferrite). Calculations that assume pure iron, indicate that the   ε   iron remains the most stable under inner-core conditions. These high-pressure phases of iron are of no practical importance, but are important as the end member models for the solid parts of planetary cores. Pure iron (99.99%) is not an easy material to produce and also, iron of  high hi gh puri purity ty is extr extrem emel ely y weak weak;; th thee re reso solv lved ed shea shearr st stre ress ss of a si sing ngle le crys cr ysta tall of iron iron at room room te tem mpera peratu ture re is as lo low w as 10 MPa, Pa, whil whilee th thee yield stress of a polycrystalline sample of iron at the same temperature is well below 50 MPa and has a hardness of 20 –30 Brinell. But pure iron has nevertheless been made with a total impurity content of less than

 

 Iron-Carbon Diagram

 

3

60 parts per million (ppm), of which 10 ppm is accounted for by nonmetallic impurities such as carbon, oxygen, sulfur and phosphorus, with the the rema remain inde derr re repr pres esen enti ting ng meta metall llic ic impu impuri riti ties es.. With With th thee cont contro roll lled ed amoun am ountt of nonme nonmetal tallic lic impur impuriti ities es espec especial ially ly car carbon bon (betw (between een 0.002% 0.002% and an d 2. 2.1% 1%)) prod produc uces es st stee eell that that may may be up to 1000 1000 tim times hard harder er th than an pure iron. Maximum hardness of 65 HRc is achieved with 0.6% carbon content. The possibility of achieving a variety of microstructure due to the allotropic behavior of iron, good workability (deformability) due to the softness of iron and the strength achieved due to carbon, make the steel suitable for a wide variety of applications. Steel is an enigma as it rusts easily, yet it is the most important of all metals. Steel is 90% of all metals being re󿬁ned today as steel is useful in terms of its mechanical, physical and chemical properties. It is well known that steel is an interstitial solid solution (alloy) of iron and carbon and it is more often referred to as a metal and while with subsequent addition of other solutes it is called as alloy steel, though it is a misnomer. Alloy steels are by far the most common industrial metals as they have a great range of desirable properties. Steel, with smaller carbon content than pig iron (about 4 wt.% C) but more than wrought iron (almost a pure iron), was   󿬁rst produced in antiquity by using a bloomery. By 1000 BCE, blacksmiths in Luristan in western Persia were making good steel. The improved versions such as Wootz steel by India and Damascus steel were developed around 300 BCE. These methods were specialized, and so steel did not become a major commodity until 1850s. New methods of  producing it by carburizing bars of iron in the cementation process were devised in the 17th century. In the Industrial Revolution, new methods of  producing bar iron without charcoal were devised and these were later applied to produce steel. In the late 1850s, Henry Bessemer invented a new steelmaking process, involving blowing air through molten pig iron, to prod pr oduc uce miwrou ld ough st stee eel. l. This Th made ma denost stee eel much mu more mo re ecinonom omic ical , anti th ther ereb eby y. lead le adin ing ge tomild wr ght t ir iron onis bein be ing g long lolnger er ch pr prod oduc uced edecon larg la rge e al, qu quan titi ties es. Carbon steels are least expensive of all metals while stainless steels are costly.

1.1 Iro Iron-C n-Carb arbon on Dia Diagra gram m Of all the binary alloy systems, the one that is possibly the most important is iron and carbon. Both steels and cast irons, primary structural materials in every every te tech chno nolo logi gica call lly y adva advanc nced ed cult cultur ure, e, are es esse sent ntia iall lly y iron iron–carbon alloys. This section deals with the study of the phase diagram and the solidi󿬁cation and development of several of the possible microstructures in iron-carbon alloys.

 

 

4

Introduction to Steels

Iron–carbon phase diagram is presented in  Figure 1.2. 1.2. Pure iron, upon heating, experiences two changes in crystal structure before it melts. At room temperature the stable form, called alpha ferrite, or iron, has a BCC crystal structure. Ferrite experiences a polymorphic transformation to FCC austenite, or iron, at 912°C. This austenite persists up to 1394°C; at that temperature tempe rature the FCC austenite austenite reverts back to a BCC phase known as delta ferrite that   󿬁nally melts at 1538°C. All these changes are shown along the left vertical axis of the phase diagram. Thee comp Th compos osit itio ion n axis axis in   Figure Figure 1.2   extends only to 6.70 % C as at this concentration the intermediate compound iron carbide, or cementite (Fe3C) C),, is form formed ed th that at is re repr pres esen ente ted d by a vert vertic ical al line line on th thee phas phasee diagram. Thus, the iron-carbon system may be divided into two parts: an iron-rich portion, as in Figure in  Figure 1.2; 1.2; and the other (not shown) for compositions between 6.70 and 100% C (pure graphite). In practice, all steels and cast irons have carbon contents less than 6.70% C and therefore, only the iron–iron carbide system is considered. Figure considered.  Figure 1.2 should 1.2  should be more appropriately labeled as Fe–Fe3C phase diagram, as Fe3C is now considered to  be a component. Convention and convenience dictate that composition be

1600

1538°C 1493°C γ

δ 

1400

γ

γ

 L

γ

γ

1394°C

γ+L

γ γ

γ γ

γ

γ

γ

γ

γ γ

γ

Fe3C

γ

1200

Fe3C

1147°C 2.14

γ 1 Austenite

  a 1000   r   e   p   m 912°C   e    T

α

A2

γ

γ

A4     )    C    °     (   e   r   u    t

Pearlite Fe3C

α + γ 

800

ACM

4.30 γ  +  + Fe3C

Austenite ledeburite and cementite

A3

Cementite and ledeburite

A1

727°C 0.76

γ

0.022 600

α + Fe3C

α1 Ferrite

Pearlite

Pearlite and Ferrite

Fe3C

Pearlite and Cementite

Cementite, pearlite and transformed ledeburite

Cementite (Fe3C)

Proeutecto-d α Eutecto-d α

400

0

Hypo (Fe) eutectoid

1

Hyper eutectoid

2

3

4

STEEL

CAST IRON γ γ Fe C 3

FIGURE 1.2 Iron-carbon (iron-iron carbide) phase diagram

5

Composition (wt% C)

γ γ

6

6.7

 

 Iron-Carbon Diagram

 

5

expressed in   “wt% C”  rather than   “wt% Fe3C”; 6.70 wt% C corresponds to 100 wt% Fe3C. In th this is book book,, all all perc percen enta tage gess refe referr on only ly to wt% wt% unle unless ss otherwise mentioned. Carbon is an interstitial impurity in iron and forms a solid solution with ferrites, and austenite, 1.2th . eInmaxi the BCC ferri fe rrite te,, only on ly also smal smalllwith conc concen entr trat atio ions nsasofindicated carb carbon on are arine   Figure solu solubl ble; e;1.2. the ma ximu mum m solubility is 0.022% at 727°C. The limited solubility is explained by the shape and size of the BCC interstitial positions that make it dif 󿬁cult to accom acc ommod modate ate the carbon carbon atoms. atoms. Even Even though though presen presentt in relati relativel vely y low concentrations, carbon signi󿬁cantly in󿬂uences the mechanical properties of  ferrite. This particular iron–carbon phase is relatively soft, may be made magnetic at temperatures below 768°C. Figure 768°C.  Figure 1.3 (a) 1.3  (a) is a photomicrograph of ferrite. Thee auste Th austeni nite te,, or gamm gammaa phas phasee of iron, iron, whic which h is nonm nonmag agne neti tic, c, wh when en alloyed with just carbon, is not stable below 727°C, as indicated in  Figure 1.2.. The maximum solubility of 2.14% carbon in austenite, occurs at 1147°C. 1.2 This solubility is approximately 100 times greater than the maximum for BCC ferrite, as the FCC interstitial positions are larger, and therefore, the strains imposed on the surrounding iron atoms are much lower. Also the phase transformations involving austenite are very important in the heat treating treatin g of steels. steels. Figure  Figure 1.3 (b) 1.3 (b) shows a photomicrograph of austenite phase. The delta ferrite is virtually the same as alpha ferrite, except for the range of temperatures over which each exists. Because the delta ferrite is stable only at relatively high temperatures, it is of no technological importance and hence is not discussed further. Cementite (Fe3C) forms when the solubility limit of carbon in ferrite is exceeded below 727°C (for compositions within the Fe 3C phase region). As

FIGURE 1.3 Photomicrograph of (a) ferrite and (b) austenite

 

6

 

Introduction to Steels

indicated in Figure in  Figure 1.2, 1.2, Fe3C will also coexist with the phase between 727 and 1147°C. Cementite is very hard and brittle; the strength of some steels is greatly enhanced by its presence. Strictly speaking, cementite is only meta me tasta stabl ble; e; that that is, is, it will will re rema main in as a comp compou ound nd inde inde󿬁nite nitely ly at room room temperature. if heated to between 650iron andand 700°C for several it will graduallyBut change or transform into carbon in the years, form of  graphite that will remain upon subsequent cooling to room temperature. Thus Th us,, the the phas phasee di diag agra ram m in   Figure Figure 1.2   is not not a true true equi equili libr briu ium m one one  because cementite is not an equilibrium compound. compound. However, in as much as the decomposition rate of cementite is extremely sluggish, virtually all the carbon in steel will be as Fe 3C instead of graphite, and the iron–iron carbide phase diagram is, for all practical purposes, valid. According to the iron-carbon diagram, alloys with less than 2% carbon are known as steels and alloys with more than 2% carbon are cast irons. There are three invariant (phase) reactions occurring in the iron-carbon system: (i) A peritectic reaction at 1493°C at 0.16% carbon delta ferrite + Liquid austenite (0.08%C) (0.5%C) (0.18%C)   ↔

This reac This reacti tion on is of mino minorr impo import rtan ance ce in st stee eels ls alth althou ough gh it has has some some impor im portan tance ce in weldi welding ng of austen austeniti iticc stainl stainless ess steels, steels, which which will will be disdiscussed later in this book. (ii) The second one is the eutectic reaction, at 4.30% C and 1147°C; Liquid austenite + cementite (4.3%C) (2%C) (6.67%C) ↔

Subsequent cooling to room temperature will promote additional phase chang ch anges. es. This This reacti reaction on has signi signi󿬁cant impact in cast irons. The eutectic mixture is also called as ledeburite. (iii) The third is eutectoid invariant reaction at a composition of 0.76%C at a temperature of 727°C. Austenite ferrite + cementite (0.76%C) (0.025%C) (6.67%C) ↔

The eutectoid phase changes are very important, being fundamental to the heat he at trea treatm tmen entt ofisst stee eels ls,, pearlite. as expl explai aine ned d in subs subseq eque uent nt disc discus ussi sion ons. s. The The eutectoid mixture called Although a steel alloy may contain as much as 2.0%C, in practice, carbon concentrations rarely exceed 1.0%.

 

 Iron-Carbon Diagram

 

7

Various microstructures that develop depend on both the carbon content and heat treatment (cooling rate and pattern). Initially, the discussion is con󿬁ne ned d to very very slow slow cool coolin ing g of st stee eell allo alloys ys,, in wh whic ich h equi equili libr briu ium m is continuously maintained. A more detailed exploration of the in 󿬂uence of  heat ondiscussed microstructure, and ultimately on the mechanical propertiestreatment of steels, is subsequently.

1.2 Slo Slow w Coolin Cooling g of Eutect Eutectoid oid Composit Composition ion Phase changes that occur upon passing from the austenite region into the ferrite + cementite phase   󿬁eld are relatively complex. An alloy of eutectoid composition cooled from a temperature within the austenite phase region, above eutectoid temperature (Figure (Figure 1.4) 1.4) and moving down the vertical li line ne R is in init itia iall lly y comp compos osed ed enti entire rely ly of th thee aust austen enit itee ph phas asee havi having ng a composition of 0.76%C and  Figure 1.4  shows the corresponding microstructure. As the alloy is cooled, no changes will occur until the eutectoid temperature (727°C) is reached. Upon crossing this temperature to point E, the austenite transforms into ferrite and cementite (eutectoid reaction). The microstructure for eutectoid steel that is slowly cooled through the eutectoid temperature consists of alternating layers or lamellae of the two

FIGURE 1.4 Microstructure developed during slow cooling of various steels

 

 

8

Introduction to Steels

phases (ferrite and Fe3C) that form simultaneously during the transformation. This microstructure, represented schematically in Figure in  Figure 1.4, 1.4, point E, is called pearlite because it has the appearance of mother of pearl when viewed under the microscope at low magni󿬁cations. The pearlite exists as  “

”

grains gra ins,, often oft en termed termedin the colonies   and and with within in each each colo colony nyfrom th thee one laye layers rs are are oriented essentially same direction, while varying colony to another. Pearlite has properties intermediate between the soft, ductile ferrite and the hard, brittle cementite.

1.3 Slo Slow w Coolin Cooling g of Hypoeutect Hypoeutectoid oid Composi Composition tion Left of the eutectoid, between 0.022 and 0.76%C is termed as hypoeutectoid (less than eutectoid) alloy. Let us consider slow cooling an alloy of  composition of 0.4%C represented by moving down the vertical line S in Figure Fig ure 1.4. 1.4. At abou aboutt 875° 875°C, C, th thee micr micros ostr truc uctu ture re will will co cons nsis istt enti entire rely ly of  grains of the austenite phase, as shown schematically in Figure in  Figure 1.4 1.4.. Cooling to about 775°C, which is within the ferrite + austenite phase region,  both of these phases will coexist as in the schematic microstructure. Most of the small particles will form along the original austenite grain boundaries. While cooling an alloy through the ferrite + austenite phase region, the composition of the ferrite phase changes with temperature along the ferrite   –   (ferrite + austenite) phase boundary (line UL) becoming slightly richer in carbon. On the other hand, the change in composition of the austen aus tenite ite is more more dramat dramatic, ic, procee proceedin ding g along along the (ferri (ferrite te + austen austenite ite))   – ferrite boundary (line UE) as the temperature is reduced. Cooling from just above the eutectoid but still in the ferrite + austenite region, will produce an increased fraction of the phase and a microstructure similar to that shown: the particles would have 0.022%C, grown larger. point, the composition of the ferrite phase will contain whileAt thethis austenite phase will be of the eutectoid composition, 0.76%C. As the temperature is lowered just below the eutectoid, all the austenite phase present will transform to pearl pearlite, ite, accord according ing to the the eutecto eutectoid id react reaction ion.. There There will will be virt virtual ually ly no change in the phase after crossing the eutectoid temperature and the microstructure will appear as schematically shown in Figure in  Figure 1.4. 1.4. Thus the ferrite phase will be present both in the pearlite (eutectoid mixture) and also as the phase that formed while cooling through the ferrite + austenite phase region. Thee ferrit Th ferritee that that is pres presen entt in the the pear pearli lite te is call called ed eu eute tect ctoi oid d ferrit ferrite, e, wh wher erea eass the other, othe r, that forme formed d above above eutect eutectoid oid tempe temperat rature, ure, is termed termed proeut proeutecto ectoid id (meaning pre- or before eutectoid) ferrite. The amount of proeutectoid ferrite and pearlite can be determined by drawing a tie line between the compositions of 0.008% carbon (almost left axis ax is of the the iron iron carb carbon on di diag agra ram) m) and and 0.76 0.76% % carb carbon on.. Alte Altern rnat ativ ivel ely y th thee amount of phases can also be found by simple thumb rule namely, the

 

 Iron-Carbon Diagram

 

9

0.8% 0.8% carb carbon on steel steel cont contai ains ns 100% 100% pear pearli lite te and and so 0.4% 0.4% carb carbon on st stee eell wi will ll contain about 50% pearlite and rest 50% proeutectoid ferrite. To   󿬁nd the total ferrite content, a tie line should be drawn between the solubility limit of carb carbon on in ferr ferrit itee and and to th thee ri righ ghtt vert vertic ical al axis axis of th thee iron iron-- carb carbon on diagram, that is, 6.67% carbon or 100% cementite.

1.4 Slow Cool Cooling ing of Hypereute Hypereutectoid ctoid Com Compositio position n Hypereutectoid alloys are those containing between 0.76 and 2.14% carbon, which are cooled from temperatures within the austenite phase   󿬁eld. Consider an alloy of composition of 1.2% carbon in   Figure 1.4, 1.4, which, upon cooling, moves down the line Q. Above line EV only the austenite phase will  be present with a com compositi position on of 1.2% carbon; carbon; the microstruc microstructure ture will appear as shown, having only austenite grains. Upon cooling into the austenite + cementite phase   󿬁eld, the cementite phase will begin to form along the initial austenite grain boundaries, similar to the phase in Figure in  Figure 1.4. 1.4. This cementite is called proeutectoid cementite   – that forms before the eutectoid reaction. Of  course, the cementite composition remains constant (6.70%C) as the temperature changes. However, the composition of the austenite phase will move along line EV toward the eutectoid. As the temperature is lowered through the eutectoid point, all remaining austenite of eutectoid composition is converted into pearlite; thus, the resulting microstructure consists of pearlite and proeutectoid cementite as microconstituents (Figure (Figure 1.4). 1.4). In the photomicrograph of a 1.2% carbon steel, note that the proeutectoid cementite appears light. Because it has much the same appearance as proe pr oeut utec ecto toid id ferri ferrite te,, th ther eree is some some di dif  f 󿬁cu culty lty in distin distingui guishi shing ng betwee between n hypoeutectoid and hypereutectoid steels on the basis of microstructure. theenonequilibrium cooling thentaine iron-carbon alloys, conditions of During metast metastabl able equili equilibri brium um have hav e been beenofmainta mai ined d con contin tinuou uously sly; ; that that is, suf 󿬁cient time has been allowed at each new temperature for necessary adjustment in phase composition and relative amounts of microconstituents predicted from the iron-carbon diagram. The microstructure developed due to the nonequilibrium cooling plays major role in determining the mechanical properties of the steels. The two nonequilibrium effects of  practical importance are: (i) the occurrence of phase changes or transformations at temperatures other than those predicted by phase boundary lines on the phase diagram (ii) not the existence room temperature appear onatthe phase diagram. of nonequilibrium phases that do Both are discussed in Chapter in  Chapter 22..

 

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2  Heat Treatment of Steels

2.0 Int Introd roduct uction ion The transformation of carbon steel from one microstructure or crystalline structure to another makes the material heat treatable. In other words, it allows for changes in the properties of the material just by going through various heating and cooling cycles, without a change in the overall chemicall comp ca compos osit itio ion n of th thee mate materi rial al.. This This chara charact cter eris isti ticc ca can n also also resu result lt in property changes during fabrication processes such as hot bending/forming, welding, and brazing. Heat treatment of steel is de󿬁ned as the process of heating and cooling steell to impro stee improve ve proper propertie tiess such such as toughn toughness ess,, machin machinabi abilit lity, y, hardne hardness, ss, duct du ctil ilit ity, y, as well well as to re remo move ve re resi sidu dual al st stre resse sses, s, wear wear resi resista stanc nce, e, grai grain n re󿬁nement, and so on. Basically, there are four types of heat treatments: (i) annealing, (ii) normalizing, (iii) hardening, and (iv) tempering. There is one more heat treatment process called precipitation hardening. Although precipitation hardening has minor role in plain carbon steels and low alloy steel ste els, s, it steels, pl play ayss amaraging majo majorr role rosteels, le in st stre reng ngth then enin g high hithe gh allo al loy y of st stee eels ls such such as stainless and so on.ing For sake completion, prec pr ecip ipit itat atio ion n hard harden enin ing g is also also brie brie󿬂y desc descri ribe bed. d. Heat Heat treatm treatmen entt also also covers cov ers surfac surfacee harden hardening ing method methodss such such as   󿬂ame hardening hardening,, induction induction hardenin hard ening, g, carburizin carburizing, g, nitriding nitriding,, cyanidin cyaniding, g, carbo nitriding nitriding,, and so on, which are dealt in next chapter. Thee mate Th materi rial al spec specii󿬁ca cati tion onss prov provid idee th thee spec specii󿬁c he heat at trea treatm tmen ents ts to achieve the properties for the speci 󿬁c material or application. Heat treatment is highly dependent on the manufacturing methods used for that product and the requirements can range from no heat treatment to subcritical criti cal heat treatments treatments (such as precipitat precipitation ion heat treatment, treatment, tempering, tempering, or stress relief) to high-temperature (austenitizing) heat treatments (such as quench hardening, annealing, or normalizing) that might be followed by a tempering heat treatment. Descriptions of common heat treatments are given in the following sections.

11  

 

12

Introduction to Steels

2.1 Ann Anneali ealing ng Annealing is a very broad term used to describe a variety of heat treatments and itFor is athe process customarily applied toused remove or work hardening. purpose of heat treatment on stresses carbon steels in the material speci󿬁cations, the more speci󿬁c term   “full annealing”   better describes the process. Full annealing is de 󿬁ned as   “annealing a steel object  by austenitizing it and then cooling it slowly through the transformation range.”   This results in maximum transformation to ferrite and to coarse pearlite giving the lowest hardness and strength. Full annealing of carbon steels would require the material to be heated to austenitizing temperature which must be determined from the iron-carbon diagram. Generally, the aust au sten enit itiz izin ing g te temp mper erat atur uree for for all all prac practi tica call pu purp rpos oses es is 50°C 50°C abov abovee th thee austen aus teniti itizin zing g temper temperatu ature re determ determin ined ed from from the iron iron carbon carbon diagra diagram m to have ha ve a homo homoge geno nous us struc structu ture re and and to null nullif ify y th thee effe effect ct of heat heatin ing g rate. rate. Duee to anne Du anneal alin ing, g, th ther eree is no chan change ge in micr micros ostr truc uctu ture re or amou amount nt of  microconstituents. They differ only in morphology and distribution of the phases. The   󿬁nal microstructure will be the same as that of slow cooled cond co ndit itio ions ns.. Anne Anneal alin ing g is appl applie ied d to forg forgin ings gs,, cold cold-w -wor orke ked d sh shee eets ts and and wires, welded parts as well as castings. The three main annealing processes used for steels are described here. 1.  Subcritical Annealing  Subcritical annealing is carried out below A1 temperature and there is no auste au steni nite te tran transf sfor orma matio tion n invo involv lved ed.. The The main main aim aim is to relie relieve ve resid residua uall stres stresse sess caused by previous processing such as cold working, welding, or casting (stress relief anneal) or to recrystallize cold-worked material. The sub critical annealing is also referred to as stress relieving annealing. Stress relieving is de󿬁ned as   “heating a steel object to a suitable temperature, holding it long enough to reduce residual stresses, and then cooling it slowly enough to minimize the development of new residual stresses.”   Locked in (residual) stresses in a component cannot exist at a greater level than the yield strength of the material. An increase in the temperature of steel lowers the yield strength and thus relieves some of the stresses. Further reduction in the residual stress can occur due to a creep mechanism at high stress relief  temperatures. Stress relieving has a time-temperature relationship. Although some stress relief occurs occurs very quickly quickly as a result of the lower yield strength strength at temperature, additional stress relief occurs by the primary creep mechanism. Stress relief temperatures are typically 595–675ºC for carbon steels. Stress relieving annealing temperature is lower than annealing temperatures. The other subclass of subcritical annealing is process annealing. It is commonly employed for wires and sheets and carried out between 500° to 690°C for several hours. Process anneal is carried out at higher temperatures

 

 Heat Treatment of Steels

 

13

FIGURE 2.1 Spherodized microstructure

(usually 11° to 22°C below A1) than stress relief anneal (usually between 500° to 650°C). 2.  Spherodise Annealing  Spherodise annealing is carried out between A1 and A3 for hypoeutectoid steels or between A1 and Acm for hypereutectoid steels. Steel is heated to within 55°C above A1 and then transformed at a temperature less than 55°C  below A1 to produce produce a structure structure consisting consisting of spherodize spherodized d carbi carbide de parti particles cles in a ferr ferrit itee matr matriix (Figure Figure 2.1). 2.1). Alte Altern rnat ativ iveely, ly, th thee st stee eell ca can n be held held at a temperature just below A1 for suf 󿬁cient time for the cementite lamellae of  the pearlite to spherodize. This happens because it leads to a reduction in surface energy of the cementite-ferrite interfaces. The spherodized structure (spheroidite) the minimum hardness, the maximum ductility, as well as the maximumhas machinability especially in higher carbon pearlitic steels. 3.  Supercritical or Full Annealing  Supercritical or Full annealing is carried out above A3 for hypoeutectoid steels and between A1 and Acm for hypereutectoid steels. In full annealing, there is transformation to austenite on heating and back to ferrite and pearlite upon cooling.

2.2 Nor Normal malizin izing g Normal Norm aliz izin ing g is a spec specii󿬁c term de󿬁ned as   “he heat atin ing g a st stee eell ob obje ject ct to a suitable temperature above the transformation range and then cooling it

 

14

 

Introduction to Steels

FIGURE 2.2 Pearlite in (a) annealed and (b) normalized conditions.

in ai airr to a te temp mper erat atur uree subs substa tant ntia iall lly y belo below w th thee tran transf sfor orma mati tion on rang range. e.” Typical normalizing temperatures are 55°C above A3 for hypoeutectoid steels and 27°C above Acm for hypereutectoid steels. The air cooling may  be natural or slightly forced air convection based on the dimensions of the

FIGURE 2.3 Heating and cooling cycles for stress relief, process, full and normalizing of hypo- and

hypereutectoid steels

 

 Heat Treatment of Steels

 

15

steel billet. The high cooling rate (cooling in air) results in ferrite and pearlite but the phase fraction changes. There is no suf 󿬁cient time for the prec pr ecip ipit itat atio ion n of proe proeut utec ecto toid id fe ferri rrite te and and henc hencee am amou ount nt of 󿬁ne pear pearllite ite is mo more re.. Forr exam Fo exampl ple, e, 0.4% 0.4% carb carbon on steel steel with with 50% 50% ferr ferrit itee an and d 50% 50% pear pearli lite te when when normalized will have about 65%   󿬁ne pearlite and 35% ferrite. And so the stre stren ngth gth of the the norm normal aliz ized ed st stee eell is high higher er th than an anne anneal aled ed st stee eells. In addi additi tio on, th thee morphology of pearlite in the annealed and normalized conditions differs. The lamell lamellar ar structu structure re of ferrite ferrite and and ceme cementi ntite te is is 󿬁ne in norm normal aliz ized ed p pea earl rlit itee as compared to annealed pearlite as shown in Figure in  Figure 2.2. 2.2. It is often used after hot rolling, where a high   󿬁nishing temperature can ca n le lead ad to a coar coarse se micr micros ostr truc uctu ture re.. Bene Bene󿬁ts of norm normali alizin zing g inclu include de improved machinability, grain size re 󿬁nement, homogenization of composit po sitio ion n and and modi modi󿬁ca catio tion n of resid residua uall stress stresses. es.   Figure Figure 2.3 2.3   represents various types of annealing and normalizing for hypo and hypereutectoid steels.

2.3 Har Harden dening ing “Harde Hardening ning

a steel object is by austenitizing austenitizing it and then cooling it rapidly rapidly enough enoug h so that some or all of the austenite austenite transforms to martensite. martensite.”  Th  Thee rapid cooling suppresses the formation of austenite to   α  + Fe3C, instead it produces hard martensite for applications that demand a hard material (e.g., (e. g., knives knives,, razor razor blades blades,, surger surgery y tools, tools, cuttin cutting g tools, tools, etc.). etc.). Only Only the austenite transforms to martensite. If austenite + ferrite is quenched, all of the austen austenite ite transfo transforms rms compl complete etely ly to marten martensit site, e, while while the ferrite ferrite remains unchanged. Similarly, when austenite + cementite is quenched, agai ag ain n al alll of the the auste austeni nite te comp comple lete tely ly tr tran ansf sfor orms ms to marte martens nsite ite whil whilee th thee cementite remains unchanged. Hence, complete austenitizing is the important step in hardening process. The hardening is a nonequilibrium cooling process and so the phase fo form rmed ed due due to hard harden enin ing g (mar (marte tens nsit ite) e) will will no nott be fo foun und d in th thee iron iron-carbon diagram since they are plotted in equilibrium cooling conditions. Hence, a scheme for understanding the transformations with respect to time is called transformation diagrams. Transformation diagrams were 󿬁rs rstt publ publis ishe hed d by Bain Bain and and Dave Davenp npor ortt in th thee Unit United ed St Stat ates es in 1930 1930 paving the way to the detailed understanding of nonequilibrium cooling, isothermal cooling and the effects of alloying elements on the heat treatment response in steels. 2.3.1 Trans Transforma formation tion Diagram Diagramss There are several several different different variations variations of transformat transformation ion diagrams while the mostt common mos commonly ly used used and refere reference nced d are the isothe isotherma rmall tra transf nsform ormati ation on

 

16

 

Introduction to Steels

(IT) diagram (commonly called as time-temperature-transformation(TTT) diagram) and the continuous cooling transformation (CCT) diagram. All the transformation diagrams plot temperature versus log time, with the display showing the expected crystalline structures and microstructures. The IT diagram shows the expected result when the steel is held for varyin var ying g length lengthss of time, time, keepin keeping g the temper temperatu ature re ess essent ential ially ly constan constantt (after (af ter an initia initiall austen austeniza izatio tion). n). The CCT diagra diagram m shows shows the expect expected ed result when the steel is cooled continuously at varying rates from the austen aus teniti iticc phase. phase. These These transf transform ormati ation on diagra diagrams ms appear appear to be simila similar, r,  but the CCT transformatio transf ormation n curves are typically depressed depress ed and moved to the right during continuous cooling from those in the IT diagram. Thee IT diagr Th diagram amss are are more more comm common only ly used used in pred predic icti ting ng struc structu ture ress duri du ring ng heat heat tr trea eatm tmen entt whil whilee CCT CCT di diag agra rams ms are are used used du duri ring ng weld weldin ing g and casting of steels. Thee isot Th isothe herm rmal al di diag agra rams ms (IT) (IT) ar aree pl plot otte ted d as fo foll llow ows: s: A steel steel is   󿬁rst heated to a temperature in the austenitic range, typically 20°C above the A1 or Acm line, and then cooled rapidly rapid in a bath to aprogress lower temperatur temp erature, e, allowing isothermal transformation tolyproceed. The of transformation mat ion can be follow followed ed by dilato dilatomet metry, ry, the degree degree of transf transform ormati ation on depending upon the holding time at the temperature.   Figure 2.4   illustrate tra tess sche schema mati tica call lly y th thee start start and and   󿬁nish nish of tran transf sfor orma mati tion on to ferri ferrite te,, pearlite, bainite, and martensite on a diagram as a function of temperature and time. Figure 2.4 is 2.4  is a schematic IT diagram for carbon steel. The transformation curve cur ve shows shows transfo transforma rmatio tion n with with respec respectt to time time (ra (rate te of coolin cooling). g). The cool co olin ing g curv curves es (l (lin inee 1 to 7) ar aree supe superi rimp mpos osed ed fo forr se self lf-e -exp xpla lana nati tion on.. When the cooling rate is fast (line 6 and 7), it ends up with martensite struc str uctu ture re once once it is coole cooled d below below Mf (mart (marten ensit sitee   󿬁nish) temperature. temperature. The caution is that the cooling rate line should not touch the beginning of the softer products such as ferrite and pearlite. The critical cooling rate (CCR) is important to achieve full martensitic transformation. The cooling curve (line 7) represents the critical cooling rate. When cooling ra rate te is sl slig ight htly ly lower ower than than cr crit itic ical al cool coolin ing g rate rate (lin (linee 5), 5), th thee soft softer er products start precipitating but the cooling is fast enough so that the transformation is not completed. It results in partial softer products and marte mar tensi nsite te upon upon coolin cooling g below below marte martensi nsite te   󿬁nish nish temper temperat ature ures. s. On further decreasing the cooling rate (lines 1 to 4), the transformation of  aust au sten enit itee to soft softer er prod produc uctt is comp comple lete ted d and and neve neverr en ends ds up with ith marten mar tensit site, e, whic which h is annea annealin ling g or norma normaliz lizin ing. g. The The IT diagr diagrams ams are drawn for a   󿬁xed composition. The addition of carbon or other alloying elem elemen ents ts al alte terr th thee di diag agra rams ms.. The The ef effe fect ct is disc discus usse sed d furt furthe herr in late laterr sections. During hardening, the cooling rate is too rapid to allow nucleation and growth mechanisms (critical cooling rate) and the result is that the trapped

carb ca rbon on is forc forced ed into into th thee cr crys ysta tall llin inee latti lattice ce.. Inst Instea ead d of fo form rmin ing g ferr ferrit itee

 

 Heat Treatment of Steels

 

17

FIGURE 2.4 IT (TTT) diagram for a carbo carbon n steel superimposed superimposed with cooling cooling curves (Schematic) (Schematic)

structu struc ture res, s, the the au auste steni nite te latti lattice ce shea shears rs and and resu result ltss in a body body-c -cen ente tere red d tetrago tetr agonal nal struct structure ure called called marten martensit site. e. This This marten martensit sitic ic transf transform ormati ation on occurs without diffusion of the carbon and therefore occurs very rapidly. In addition, once the austenitic structure is undercooled to the point at which the carbon cannot diffuse and additional ferrite cannot form, the only remaining transformation that can occur upon further cooling is to ma marte rtens nsit ite. e.is The Th te temp mper erat atur ure e at (Ms) whic which h marte martens nsit itee Because begi begins ns to fo form rmcannot from from austenite thee martensite start temperature. ferrite form, martensite will continue to form as the temperature decreases from

any an y exis existi ting ng auste austeni nite te unti untill all all of th thee aust austen enit itee is tran transf sfor orme med, d, whic which h

 

18

 

Introduction to Steels

occurs at the martensite   󿬁nish (Mf) temperature. This carbon steel martensitic structure is known to be both hard and strong but lacks ductility and toughness in the untempered state. There are two types of martensite; Lath and Plate. The formation of the type of martensite depends on the carbon content. Lath type martensite is formed when carbon content is low while plate type martensite is formed when carbon content in steels is higher. The resulting maximum hardness is closely related to the carbon content of  the steel and the percentage of martensite formed.

2.4 Har Harden denabi ability lity Hardenability is de󿬁ned as the depth to which the hard martensite is able to form upon quenching. Since for an actual component it depends upon the thickness, the cooling rate varies across the cross section (Figure ( Figure 2.5). 2.5). The surface cools fast upon quenching and further moving toward the center the cooling rate decreases in different microphases across the cross section. The hardenability of steels is measured by various methods; the Jominy end quench test is the most popular.   Figure 2.6   schematically represents the Jominy end quench test as per ASTM standard A 255. The specimen is heated to the austenitizing temperature and   󿬁xed in the holder. A water jet is forced from the bottom end. The sample experiences different cooling rate along its length. After cooling, the sample is taken from the holder and its hardness is measured along its length from quenched end to the other end.

FIGURE 2.5 Variation in cooling rate between surface and core superimposed on IT (TTT) diagram

(Schematic)

 

 Heat Treatment of Steels

 

19

FIGURE 2.6 Schematic representation of the Jominy end quench test

The Jominy end-quench test is related to the CCT diagram because the speci spe cimen men is raised raised to an austen austeniti itizin zing g temper temperatu ature re and then then quench quenched ed from one end with water. The result is a varying cooling rate along the specimen length that can then be plotted on the CCT to determine the expe ex pect cted ed micr micros ostr truc uctu ture re.. Dete Determ rmin inat atio ion n of th thee micr micros ostru truct ctur uree is a bit bit tedious process and as the microstructure obtained is directly related to the hardness which is easy to measure. Hence a graph plotted between the distance fromthe thehardenability quenched end the other end against hardness is used to determine ofto the steel. The critical cooling rate decides the depth to which the component can  be hardened which in turn is decided by the IT (TTT) diagram of the steel. If the beginning of transformation of softer products (commonly referred as nose of the C curve) is moved toward right (away from the Y-axis-Figure Y-axis- Figure 2.4), 2.4 ), the critical cooling rate to achieve martensite is low and hence the portion which experiences lower cooling rate can also form martensite. This results resu lts in a steel steel with with higher higher harden hardenab abili ility. ty. Genera Generally lly all alloyi oying ng elemen elements ts added to steel increase the hardenability, as shown in Figure in  Figure 2.7. 2.7. By contrast, if the nose of the C curve is moved toward the left (towards Y axis-Figure axis-Figure 2.4), 2.4), the critical cooling rate is high to achieve martensite. Such Su ch a high high cool coolin ing g rate rate is almo almost st impo imposs ssib ible le to achi achiev evee in prac practi tica call conditions. A typical example is the plain carbon steels with carbon less than 0.25%. In practical conditions, minimum of 0.3% carbon is required to

achieve martensite on the surface.

 

20

 

Introduction to Steels

FIGURE 2.7 Effect of alloying elements on hardenability (Schematic)

Apart from water, a range of quenchants are used. The order of severity is from 5% caustic soda, 5–10% brine, cold water, warm water, mineral oil, anim an imal al oi oil, l, and and vege vegeta tabl blee oi oil. l. When When th thee orde orderr of seve severi rity ty is high high (5% (5% caus ca usti ticc soda soda), ), th thee cool coolin ing g ra rate te is fast faster er.. As ment mentio ione ned d in th thee prev previo ious us paragraph, steels with less than 0.25%C cannot be hardened by quenching even in 5% caustic soda. The quenching medium is selected based on the C curve (Figure (Figure 2.4) 2.4) or the critical cooling rate. Care should be taken in the selection of the quenchants since if the cooling rate is lower than critical cool co olin ing g rate rate,, mart marten ensi site te may not not fo form rm fo forr adeq adequa uate te dept depth h and and if th thee cooling rate is more, possibility of quench cracks will be more due to high thermal stresses.

2.5 Tem Temper pering ing Quench Quenc h harden hardening ing is norma normally lly the   󿬁rst step in a heat treatment which would then include a subsequent tempering heat treatment. The martensitic steel is excessively hard and strong with characteristic low toughness and hence the tempering treatment is used to recover some of the more desirable properties. Tempering is de󿬁ned as   “rehea reheating ting hardened hardened steel subject to a temperature below Ac1 and then cooling it at any desired rate to increase softness and ductility by reducing the brittleness of the martensite.”  Tempering allows some of the carbon atoms in the strained martensitic structure to diffuse and form iron carbides or cementite. This reduces the hardness, tensile strength, yield strength, and stress level but increases

the the duct ductil ilit ity y and and to toug ughn hnes ess. s. Temp Temper erin ing g temp temper erat atur ures es and and time timess are are

 

 Heat Treatment of Steels

 

21

interdependent, but tempering is normally done at temperatures between 175°C and 705°C and for times from 30 minutes to 4 hours, depending on the the comp compos osit itio ion n of th thee steel steel.. Allo Alloyi ying ng elem elemen ents ts gene genera rall lly y slow slow down down tempering, which will be discussed in later chapters. Apar Ap artt from from the the re regu gula larr heat heat tr trea eatm tmen ents ts,, ther theree are are is isot othe herm rmal al he heat at trea treatm tmen ents ts,, which produce variety of microstructure and properties. In these treatments, the sample is held above the martensite start temperature and subsequently cooled before it touches the starting curve of softer product transformation or allowed to transform to softer products, which could be done by quenching the the stee steell in a hig high te tem mper erat atur uree bath bath of mol olte ten n sa sallt or meta metal. l. Th Thee soft softer er prod roducts ucts thus formed have different morphology and characteristics. The IT diagram is useful in helping to predict the resulting microstructures in isothermal heat treatm tre atmen ents. ts. Ther Theree are th thre reee ty type pess of is isoth other erma mall he heat at treat treatme ment ntss know known n as martempering, ausforming, and austempering.

2.6 Mar Martem temper pering ing Martempering is not a tempering operation but a treatment that leads to low levels of residual stresses and minimizes distortion and cracking. Martempering is also known as marquenching. The main aim of martempering is primarily to minimize distortion and cracking due to thermal shock loads of heat-treated steels during hardening processes. The steps include the following: (i) austenitize the steel at the appropriate temperature based on ir iron on carb carbon on diag diagram ram;; (ii) (ii) quen quench ch to a temp temper erat atur uree just just abov abovee th thee Ms (usually, into an oil or molten salt bath) based on the IT (TTT) diagram and critical cooling rate; (iii) hold in the quenchant to obtain uniform temperature throughout the cross section; and (iv) cool at a moderate rate thro throug ugh h the the mart marten ensi site te trans transfo form rmat atio ion n regio region n in such such a way way th that at th thee transformation of softer products will not start. As the sample is cooled (quenched) from just above Ms temperature and is held for certain period for temper temperatu ature re distrib distributi ution on to become become unifo uniform rm throug throughou houtt the cross cross section the distortion and cracking due to thermal shock will be minimum. After martempering, the steel is tempered. Figure tempered.  Figure 2.8 schematically 2.8  schematically shows the martempering process.

2.7 Aus Austem temper pering ing Austempe Austem perin ring g is desig designe ned d to prod produc ucee baini bainite te in carbo carbon n steel steels. s. Bain Bainit itee is similar to pearlite having ferrite and cementite but with different morphology. There are two types of bainite; lower and upper. The hardness and

strength of bainite are comparable to hardened and tempered martensite with

 

 

22

Introduction to Steels

Surface Tempered Martensite

   E    R    U    T    A    R    E    P    M    E    T

(a) Center

Bainite (b)

Ms

Mf  Transformation TIME

FIGURE 2.8 Schematic representation of (a) martempering process (b) austempering process

 better ductility  better ductility,, toughness, toughness, and uniform uniform mechan mechanica icall prop properti erties es than tempered tempered martensite. In addition, austempered products do not require tempering. Austempering consists of the following steps: (i) austenitize the steel at the appropriate temperature; (ii) quench to a temperature just above the Ms in a slat bath; (iii) hold isothermally in the salt bath until austenite transforms to bainite; and (iv) quench or air cool to room temperature. Figure 2.8 schematically 2.8  schematically shows the austempering process. The essential requirement is that the cooling rate must be fast enough to avoid the formation of pearlite and the isothermal treatment must be long enough to complete the transformation to bainite.

2.8 Aus Ausfor formin ming g Ausforming, also known as low-temperature thermomechanical treatment (LTMT), involves the deformation of austenite in the metastable bay between the ferrite and bainite curves of the IT (Figure ( Figure 2.9) 2.9) diagram. After deformation, the steel is cooled rapidly to form martensite. The combined effect of  austenite deformation and high dislocation density results in a very   󿬁ne martensite microstructure with high yield strength and high toughness. The ausforming process cannot be carried out for all types of steel. It can

 be done only for steels which show distinct distinct ferrite and bainit bainitee transformation transformation

 

 Heat Treatment of Steels

A3

 

23

Stable austenite

A1

  e   r Metastable austenite   u    t   a   r   e   p   m Ausforming   e    T

Isoforming

Ms Time

FIGURE 2.9 Schematic representation of ausforming process and isoforming

curves. Alloying elements produce such distinct bays in steels, which will be discussed later in this book.

2.9 Iso Isofor formin ming g This is similar to ausforming, with the deformation of austenite continued at the temperature where austenite transforms to ferrite and iron carbide (Figure 2.9). 2.9). Martensite is not formed. The microstructure of   󿬁ne ferrite and spheroidal carbides gives a large toughness improvement.

2.10 Preci Precipitation pitation Heat Treatme Treatment nt Precipitation heat treatment is less common in carbon steels because the desired precipitates generally are carbides of alloying elements rather than intermetallics. However, some of the low and high alloyed carbon steels containing chromium, molybdenum, niobium, vanadium, and so on respond to precipitation hardening. Figure hardening. Figure 2.10 2.10 schematically  schematically represents the precipitation hardening treatment. All the alloys will not respond to precipitation hardening also called age hardening. The alloy should satisfy few conditions (i) the system should have nega have negati tive ve soli solid d solu solubi bili lity ty,, th that at is is,, th thee solv solvus us curv curvee sh shou ould ld ha have ve negative slope (Figure (Figure 2.10a); 2.10a); (ii) the precipitate formed should be hard

inte interm rmet etal alli licc or carb carbid ide; e; and and (iii (iii)) th thee amou amount nt of prec precip ipit itate ate shou should ld be suf 󿬁cient to be distributed in the parent matrix.

 

 

24

Introduction to Steels

FIGURE 2.10 Schematic representation of (a) phase diagram showing negative solvus line and (b) solutionizing and aging

The alloy with composition C (Figure (Figure 2.10a) 2.10a) when cooled under equili brium conditions has alpha (α) and coarse theta (θ) phase as shown in Figure 2.10. 2.10. On heating above the solvus line, to a temperature of Ts as shown in Figure in  Figure 2.10, 2.10, the second phase theta dissolves in alpha since the solubility limit at the temperature Ts is high. Upon sudden quenching, the second phase theta has no time for precipitation and results in supersaturated alpha (Figure (Figure 2.11). 2.11). Again, on heating to temperature Tp and slow slow cool coolin ing g re resu sult ltss in   󿬁ne nely ly di dist stri ribu bute ted d seco second nd phas phasee th thet etaa in alph alphaa matrix, as shown in Figure in  Figure 2.11. 2.11.

α

θ

Start: α + coarse θ

θ

After quench: supersaturated α

Fine precipitate θ appears in α to strengthen

FIGURE 2.11

Microstructural changes during precipitation hardening (schematic)

 

 Heat Treatment of Steels

 

25

Ageing temperature T2   s   s   e   n     d   r   a    H

T1 T3

T1 < T2 < T3

Ageing time

FIGURE 2.12 Schematic representation of aging behavior

The   󿬁ne hard hard second second phase phase obstru obstructs cts the motion motion of disloc dislocati ations ons and increases the strength of the alloy. The mechanism of hardening is beyond the the scop scopee of the the book book.. The The agin aging g is like like temp temper erin ing g of hard harden ened ed ste steel elss depending on temperature and time. Figure time.  Figure 2.12 schematically 2.12  schematically represents the aging behavior.

 

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3 Surface Hardening 

3.0 Int Introd roduct uction ion Surface hardening, a heat treatment process that includes a wide variety of  techniques, is used to improve the wear resistance of parts without affecting the softer, tough interior of the part. This combination of hard surface and resistance to breakage upon impact is useful for cam or ring gear,  bearings or shafts, parts for turbine applications, and automotive components that must have a very hard surface to resist wear, along with a tough interior to resist the impact that occurs during operation. Most surface treatments result in compressive residual stresses at the surface that reduce the probability of crack initiation and help arrest crack propagation at the case-c cas e-core ore interf interface ace.. Furthe Further, r, the surfac surfacee harden hardening ing of steel steel can have have an advantage over through hardening because less expensive low-carbon and medium carbon steels can be surface hardened with minimal problems of  distortion and cracking associated with through hardening of thick sections. There are two type of surface hardening: one involves heating the surface alone to the austenitizing temperature and quenching, called   “case hardening”   and the second involves heating the surface to austenitizing temperature with hardening species such as carbon, nitrogen, boron, and so on, on, and and subs subseq eque uent ntly ly quen quench chin ing g base based d on need needs, s, call called ed   “chemical surface hardening.”

3.1 Cas Case e Hard Hardeni ening ng In the the case case of hard harden enin ing g proc proces esse ses, s, th thee co comp mpos osit itio ion n of th thee mate materi rial al is unchanged while only the surface is hardened by austenitizing and subsequent quenching. The inner part will not reach the austenitizing temperature and hence even upon quenching, the core will retain the soft and

ductile ferrite pearlite phases as shown in Figure in  Figure 3.1. 3.1. The depth (thickness) to which it forms martensite is called   “case depth.”

 27  

 

28

Introduction to Steels

Martensite Ferrite and pearlite   e   r   u    t   a   r   e   p   m   e    T

 A3  A1

Case depth

FIGURE 3.1 Schematic representation of case hardened microstructure across its cross section

The selective heating of surface is achieved by induction coils in the case of induction hardening, oxy-acetylene torch in the process of   󿬂ame hardening as shown in Figure in  Figure 3.2 3.2.. Apart from induction coils and oxy acetylene 󿬂ame, heating by high-frequency resistance, electron beam, and laser beam are also used for heating the surface leaving the core unaffected. Generally case hardening process is applied to 0.30 –0.50% C containing steels to get hardness values in the range 50 –60 HRc. Steels with less than 0. 0.3% 3% carb carbon on cann cannot ot be case case hard harden ened ed si sinc ncee th thee crit critic ical al cool coolin ing g rate rate required requi red to achieve achieve martensite is not practically practically possi possible. ble. So, for hardening hardening such low carbon steels, chemical surface hardening methods are used.

Power Supply 

Current Flow 

Heated Zone

Water Cooled Copper Tubing

Quench Water

(a)

FIGURE 3.2 Schematic representation of (a) induction hardening and (b)   󿬂ame hardening.

 

Surface Hardening 

 

29

FIGURE 3.2 (Cont.)

3.2 Chemi Chemical cal Su Surface rface Harde Hardening ning In chemical hardening methods, the chemical composition of the surface is modi󿬁ed by diffus diffusing ing harden hardening ing inters interstit titial ialss such such as carbon carbon,, nitrog nitrogen, en,  boron, and so on. The most widely known chemical surface hardening process is case carburizing practiced since ancient times. Case carburizing involves the diffusion of carbon into the surface layers of a low carbon steel by heating it in contact with a carbonaceous material. Carburizing is carried at temperatures the rangethe of transport 825 –925°Cofincarbon solid, from liquidthe or gaseousout media but, in each in treatment, carb ca rbur uriz izin ing g medi medium um ta take kess pl plac acee via via th thee gase gaseou ouss state state,, usua usuall lly y as CO. CO. Carburizing results in high carbon at the surface and low carbon at the core. Following the carburizing operation, the components are subjected to hardening heat treatments resulting in hard martensite at the surface due to high carbon and soft ferrite and pearlite in the core due to low carbon content. Another most important case hardening process is nitriding. As its name suggests, nitriding involves the introduction of atomic nitrogen onto the surface of a steel but, unlike in carburizing, it is carried out in the ferritic state at temperatures of the order of 500–575°C. Like carburizing, it can be performed in solid, liquid or gaseous media but the most common method involves ammonia gas (gas nitriding), which dissociates to form nitrogen

and hydrogen. Nascent, atomic nitrogen diffuses into the steel, forming

 

30

 

Introduction to Steels

nitrides in the surface region. Nitriding is carried out on steels containing strong nitride-forming elements such as aluminium, chromium and vanadium which form their respective nitrides in the nitride layer. Given the low temperature involved in the process, nitriding can be carried out on through-hardened steels after the conventional hardening and tempering treatments have been applied. Another common process is carbonitriding (nitro-carburizing). Carbonitriding can be regarded as a variant of gas carburizing in which both carbon and nitrogen are introduced onto the steel surface. This is achieved  by the introduction of ammonia gas into the carburizing atmosphere, which liberates nascent nitrogen. Carbonitriding is carried out at temperature tu ress of the the orde orderr of 850° 850°C. C. The The in intr trod oduc ucti tion on of nitr nitrog ogen en prod produc uces es a signi󿬁cant increase in the hardenability of the case region such that high surface hardness levels can be produced even in steels of relatively low alloy content. In the same way, if the other interstitial element boron is diffused to the surface of steel, it is called boriding or boronizing. The boron diffused surface contains metal borides, such as iron borides, nickel borides, and cobalt borides. As pure materials, these borides provide extremely high hardness and wear resistance. These favorable properties are manifested even when they are a small fraction of the bulk solid. Boronized steel parts  being extremely wear resistant will often last two to   󿬁ve times longer than components treated with conventional heat treatments such as hardening, carbu car buriz rizing ing,, ni nitrid triding ing,, nitroc nitrocarb arburi urizin zing g or induct induction ion harden hardening ing.. Most Most  borided steel surfaces will have an iron boride layer with hardness ranging from 1200–1600 HV. Apar Ap artt from from th thes esee proc proces esse ses, s, el elem emen ents ts such such as chro chromi mium um,, (cal (calle led d as chromizing), aluminium, (called as aluminizing), silicon (called as silico󿬁

nizing), are diffused ontoand thehigh steeltemperature surface for achieving such as wear, corrosion, resistance.speci c properties

 

4  Plain Carbon Steels

4.0 Int Introd roduct uction ion The term steel is usually taken to mean an iron-based alloy containing carbon less than about carbon te term rmed ed incarb caamounts rbon on st stee eels ls,, ordi or dina nary ry steel ste2%. els, s,Plain or st stra raig ight ht steels carb carbon on(sometimes steel steels) s) can canalso be de󿬁ned as steels that contain other than carbon, only residual amounts of  elements such as silicon and aluminum added for deoxidation and others such suc h as mangan manganese ese and cerium cerium added added to counte counterac ractt certai certain n delete deleterio rious us effects of residual sulfur. However, silicon and manganese can be added in amounts greater than those required strictly to meet these criteria so that arbitrary upper limits for these elements are set. Usually, 0.60% silicon and 1.65% manganese are accepted as limits for plain carbon steel. Carbon is cheap but an effective hardening element for iron and hence a large tonnage of commercial steels contain very little alloying elements. Figure 4.1 shows 4.1  shows the effect of carbon on the strength and ductility.

FIGURE 4.1 Effect of carbon on mechanical properties of steel (Schematic)

 31  

32

 

Introduction to Steels

Plain carbon steels are classi󿬁ed based on their carbon content as low, medium and high: low-carbon steel or mild steel, < 0.3%C, medium-carbon steel, 0.3% ~ 0.6%C, and high-carbon steel, > 0.60% C. Most of the plain carbon steel is used in the hot   󿬁nished condition, that is, straight from hot roll ro llin ing g with withou outt subs subseq eque uent nt cold cold roll rollin ing g or heat heat treatm treatmen ent. t. This This is th thee chea ch eape pest st form form of st stee eel. l. Howe Howeve ver, r, hot hot   󿬁nish nished ed cond condit itio ion n is usua usuall lly y restricted to low carbon and medium carbon grades, because of the loss of ductility and weldability at high carbon contents.

4.1 Low Ca Carbo rbon n (or) Mild Mild Steels Steels These generally contain less than about 0.25% C and are unresponsive to heat treat tre atme ment ntss inte intend nded ed to form form mart marten ensi site te and and stren strengt gthe heni ning ng is acco accomp mpli lish shed ed by cold work. Microstructures consist of ferrite and pearlite constituents. As aductility consequ cons equenc ence, e, these these alloys alloys rela tively ely soft sof t and aare nd weak, wea k, but have hav e outstan outstandin ding g and toughness andare in relativ addition, they machinable, weldable, and, of all steels, are the least expensive to produce.

4.2 Med Medium ium C Carb arbon on Ste Steels els Medium carbon steels (0.25–0.55% carbon) are capable of being quenched to form martensite and subsequently tempered to develop toughness with good strength and hence the medium carbon steels are often used in heat treated trea ted (quenc (quenched hed and temper tempered) ed) condit condition ions. s. Temper Tempering ing in higher higher temtemperature regions (i.e., 350–550°C) produces a spherodized carbide which toughens the steel suf 󿬁ciently for speci󿬁c applications. In addition, ausforming of medium carbon steels produce even higher strengths without signi󿬁cantly reducing the ductility. Thee pl Th plai ain n medi medium um carb carbon on st stee eels ls have have lo low w hard harden enab abil ilit ity y an and d can can be succ su cces essfu sfull lly y heat heat treate treated d only only in very very thin thin secti section onss and and with with very very rapi rapid d quenching rates. Additions of chromium, nickel, and molybdenum improve the capacity capacity of these alloys to be heat treated to a variety variety of strength strength–ductility combinations. These heat-treated medium carbon steels are stronger than the low carbon steels, with a sacri󿬁ce of ductility and toughness.

4.3 High Carbon Carbon S Stee teels ls

The high carbon steels, normally having carbon contents between 0.60 and 1.4%, are the hardest, strongest, and least ductile of carbon steels. They are almost always used in a hardened and tempered condition and, as such,

 

 Plain Carbon Steels

 

33

are especially wear resistant and also capable of holding a sharp cutting edge. The high carbon steels are usually tempered at 250°C to develop considerable strength with suf 󿬁cient ductility. Their limitations are poor hardenability and rapid softening properties at moderate tempering temperatures and so, the tool and die steels are of high carbon steels, usually containing chromium, vanadium, tungsten, and molybdenum. These alloying elements combine with carbon to form very hard and wear resistant carbide compounds, which are dealt with in Chapter in  Chapter 88..

4.4 Applic Applications ations a ass Engineerin Engineering g Materia Materiall Typical applications of plain carbon steels, include automobile body components, structural shapes (I-beams, channel and angle iron), and sheets that are used in pipelines, buildings, bridges, and tin cans. They typically have a yield strength of of 25%. 275 MPa, and 550 MPa and a ductility Low tensile carbonstrengths steels arebetween the most415 important group of hot   󿬁nished plain carbon steels. Hot rolled low carbon steel sheet is an important product and used extensively for fabrication where surface 󿬁nish is not of prime importance. Cold rolling is used as   󿬁nal pass where  better   󿬁nish and the additional strength from cold working are needed. However, for high quality sheet to be used in intricate pressing operations, it is nece necess ssar ary y to anne anneal al th thee cold cold work worked ed st stee eell to caus causee th thee ferr ferrit itee to recrystallize. This is done below the A1 temperature (subcritical annealing). The other important   󿬁eld of application of plain carbon steels is as cast ca stin ings gs.. Low Low carb carbon on cast cast st stee eels ls cont contai aini ning ng up to 0. 0.25 25% % carbo arbon n are are widely used as castings for miscellaneous jobbing as reasonable strength and ductility levels are readily obtained. Yield strengths of 240 MPa and elongations of 30% are fairly typical for this type of steel. Of all the different steels, those produced in the greatest quantities fall within the low carbon classi󿬁cation. Medium carbon steels are used in railway wheels and tracks, shafts, gears, crankshafts, dies for closed die or drop forgings and other machine parts and also for high high strength strength structu structural ral compo components nents calling calling for a combinat combination ion of high high strength, wear resistance, and toughness. High carbon steels are utilized as cutting tools and dies for forming and shaping materials, as well as in knives, razors, hacksaw blades, springs, and high strength wire. Though the plain carbon steels are widely used because of their properties, they have certain lacuna. They are not corrosion resistant, lose their strength upon heating, limited hardenability, which restricts section thickness ne ss,, and and so on. on. With With a vi view ew to in incr crea ease se spec specii󿬁c pro proper pertie ties, s, alloyi alloying ng

elements other than carbon are added which are called as alloy steels. Although it is a misnomer, it is well accepted in the society. Effect of  alloying elements on steels are discussed in the next chapter.

 

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5 Effect of Alloying Elements in Steel

5.0 Int Introd roduct uction ion The purpose of adding alloying elements in steel is: 1. To increase strength 2. To increase toughness and plasticity 3. To increase hardenability 4. To decrease quench hardening capacity 5. To increase rate of strain hardening 6. To increase machinability 7. To increase abrasion (wear) resistance 8. To decrease warping and cracking 9. To increase magnetic permeability 10. To decrease hysteresis loss 11. To increase corrosion resistance 12. To increase oxidation resistance. Although all of the properties are listed in general, each alloying element pla lays ys its own own role role in inc incre reaasi sing ng the the spe speci󿬁c prop proper erti ties es fo forr spec specii󿬁c requirements.

5.1 Eff Effect ect of Alloyin Alloying g Elemen Elements ts on Iron Iron Carbo Carbon n Diag Diagram ram It woul would d be impo imposs ssib ible le to in incl clud udee a deta detail iled ed surv survey ey of th thee effe effect ctss of 

alloying elements on the iron carbon equilibrium diagram in this chapter. Even in the simplest simplest version, version, this would would require require analysis of a large numb number er of ternary alloy diagrams over a wide temperature range. However, iron  binary equilibrium systems fall into two main categories: open and closed  35  

 

36

  e   r   u    t   a   r   e   p   m   e    t     d    i   o    t   c   e    t   u    E

Introduction to Steels

Si

Ti

  n   o    i    t     d    i    i   s   o   o    t   c   p   e    t   u   m    E   o   c Ti

W 

 Mo Cr 

727° C  Mn  Ni

0.76% C  Ni Cr 

Si  Mo

W 

 Mn

Alloy Content

FIGURE 5.1 Effect of alloying elements on eutectoid temperature and composition (schematic)

γ-󿬁eld systems, and expanded and contracted  γ -󿬁eld systems. This approach

indicates that alloying elements can in󿬂uence the equilibrium diagram in two tw o ways ways:: (a) (a) som some el elem emen ents ts by con contr trac acti ting ng th thee   γ-󿬁eld eld en enco cour urag agee th thee formation of ferrite over wider compositional limits and these elements are called   α-stabilizers. (b) some other elements by expanding the   γ-󿬁eld encourage the formation of austenite over wider compositional limits and these element elem entss are called called   γ-stabilizers.   Figure Figure 5.1   schema schematical tically ly repre represents sents the effect of alloying elements on the steel portion of the iron carbon diagram especially on eutectoid temperature and composition. It is clear from Figure from  Figure 5.1, 5.1, that all alloying elements decrease the eutectoid compos comp osit itio ion n (0.8 (0.8%C %C)) of the st stee eels ls.. In th thee same same way, way, exce except pt nick nickel el an and d manganese, all the other alloying elements increase the eutectoid temperature (727°C). Based on this fact, the alloying elements are classi 󿬁ed broadly as ferrite stabilizers and austenite stabilizers. The ferrite stabilizers increase the eutectoid temperature while austenite stabilizers decrease it.

5.2 Fer Ferrit rite e Stabiliz Stabilizers ers Ferrite stabilizers can be divided into two groups based on their tendency to form carbides. Aluminium and silicon will dissolve in ferrite matrix but do

not form form any carbid carbides es at steel steel melting melting temper temperatur ature, e, whereas whereas chrom chromium ium,, molybdenum, tungsten, tantalum, vanadium, niobium, zirconium and titanium will dissolve in ferrite matrix and also tend to form carbides. All ferrite stabilizers shrink the austenitic   󿬁eld and make steel non heat treatable (or)

 

Effect of Alloying Elements in Steel

 

37

make a narrow range of temperature for austenitizing. Chromium around 20%, molybdenum around 7%, tungsten around 12%, silicon around 8.5%, ti tita tani nium um aroun around d 1%, 1%, ni niob obiu ium m ar arou ound nd 1%, 1%, and and va vana nadi dium um aroun around d 4.5% 4.5%  󿬁

eliminate austenitic eld completely and make the steel non-heat-treatable.  Alumin  Alu minium ium : Traditionally, aluminium is added as deoxidizer during the melting of steel. Normally it is present as aluminium oxide (alumina) or as alumin alu minium ium nitrid nitridee (AlN). (AlN). These precip precipita itates tes inhibi inhibitt austen austeniti iticc gra grain in boundarie boundariess at high temperatures, and by pinning these boundaries they prevent excessive grain growth. On transformation to ferrite and pearlite, grain sizes around 12 ASTM (5–6   μm diameter) can be achieved with as little as 0.03% AlN in steel. Thee amou Th amount nt of alum alumin iniu ium m ra rare rely ly exce exceed edss 0.01 0.01 to 0.07 0.07%, %, exce except pt in specialized steels for nitriding or forging applications. At present there is a great commercial interest in aluminium additions, of the order of 0.5–2%, to low low carb carbon on high high st stre reng ngth th st stri rip p steel steelss to prod produc ucee a high highly ly de desi sira rabl blee mult mu ltip ipha hase se micr micros ostr truc uctu ture re.. Thou Though gh th ther eree has has been been some some work work on th thee effect of aluminium in steel microstructures, there has been no reports on addi ad diti tion on of al alum umin iniu ium m as allo alloyi ying ng el elem emen entt in st stee eels ls.. The The solu solubi bili lity ty of  aluminium in ferrite is more as compared to austenite and it is a ferrite stabilizer. It will not form any carbides in steels. Silicon: Like aluminium, silicon is added as deoxidizer. The residue silicon is present as its oxide. Silicon is a ferrite stabilizer and it is one of the ferrite stabilizers that does not form carbides. Silicon is normally added in magnetic steels as it improves magnetic properties and in valve steels as it improves the high temperature properties of steels. Precaution has to be taken while adding add ing silicon silicon becaus becausee silico silicon n is a powerf powerful ul grapha graphatiz tizer er that accele accelerate ratess metastable cementite to reach its stable state as iron and carbon (graphite). Chromium, molybdenum, tungsten, tantalum, vanadium, niobium, zirconium and titanium: All of these elements will dissolve in ferrite in more amou am ount ntss than than in auste austeni nite te.. All All th thes esee el elem emen ents ts have have grea greatt in󿬂ue uenc ncee on incr increa easi sing ng the the eute eutect ctoi oid d te temp mpera eratu ture re (Figu Figure re 5.1 5.1). In the the pres resence of  carbon car bon these these ferrit ferritee stabil stabilize izess form form their their respec respectiv tivee carbid carbides, es, which which are more stable than cementite. The carbide forming tendency of these elements are as follows: Mn < Fe < Cr < Mo < W < Ta < V < Nb < Zr < Ti. These carbides improve strength, hardness, wear resistance, and high temperature strengt stre ngth h of the steel. steel. Apart Apart from increa increasin sing g mec mechan hanica icall prop properti erties, es, these these stable carbides lock austenite grain boundaries, and thus allow much   󿬁ner ferrite grain sizes to be achieved when the austenite transforms.

5.3 Austen Austenite ite Stabilizers Stabilizers Manganese, nickel, and nitrogen are the most common austenite stabilizers. They reduce the eutectoid temperature and widen the gamma   󿬁eld. They make the decomposition of austenite sluggish and the formation of 

 

38

 

Introduction to Steels

ferrite and pearlite is avoided during cooling. Solubility of these elements in gamma phase is much higher and less in ferrite  Manganese:   Manga Mangane nese se is gene genera rall lly y adde added d as deox deoxid idiz izer er duri during ng th thee melting of steels. Itabout is also to counteract theindeleterious effect of  sulfur. Manganese 1%added will always be present plain carbon steels and up to this limit it is not considered as an alloying element. Manganese is an austenite stabilizer though it dissolves in ferrite to a limited extent. Mang Ma ngan anes esee caus causes es th thee eute eutect ctoi oid d comp compos osit itio ion n to occu occurr at lowe lowerr carb carbon on contents, and so increases the proportion of pearlite in the microstructure. Manganese is also an effective solid solution strengthener, and also has a grain re󿬁ning in󿬂uence.  Nickel:  Nickel is an austenite stabilizer and about 8% nickel stabilizes austenite at room temperature. Nickel is an important alloying element in austenitic stainless steels and in maraging steels.  Nitrogen:   All All st stee eels ls cont contai ain n nitr nitrog ogen en in trac traces es,, and and it impr improv oves es th thee mechanical and corrosion properties of steel. Nitrogen is being used as an alloying element in steel since 1940s as a substitute for nickel to produce stainless steels. Nitrogen is a very strong austenite former and also signi󿬁cantly increases the mechanical strength. It also increases resistance to localized corrosion, especially in combination with molybdenum. In ferritic stainless steels, nitrogen strongly reduces toughness and corrosion resistance.. In martensiti tance martensiticc grades, grades, nitrogen nitrogen increases both hardness hardness and strength  but reduces toughness. Nitrogen has ef 󿬁cient solubility in steel as a solid solution or as a chemical compound. While steel is in molten form, nitrogen is present in solution. However, solidi󿬁ca cati tion on of st stee eell can can re resu sult lt in th thre reee nitr nitrog ogen en-r -rel elate ated d phen phenom omen ena, a, name na mely ly,, form format atio ion n of bl blow owho hole les, s, prec precip ipit itat atio ion n of one one or more more nitr nitrid idee compounds and/or the solidi󿬁cation of nitrogen in interstitial solid solution. The maximum solubility of nitrogen in liquid iron is approximately 450 ppm and less than 10 ppm at ambient temperature. The other elements in liquid iron affect the solubility of nitrogen in the molten steel. More importantly, the presence of dissolved sulfur and oxygen limit the absorption tion of nitro nitroge gen n beca becaus usee th they ey ar aree surf surfac acee-ac acti tive ve elem elemen ents. ts. This This fact fact is exploited during steelmaking to avoid excessive nitrogen pickup, particularly during tapping. The addition of nitrogen to plain carbon steels results in three different microstructures: (i) for low nitrogen additions, the precipitate formed is Fe3C and the nitrogen enters into the ferrite matrix as interstitial, (ii) for lo low w carb carbon on addi additi tion ons, s, th thee prec precip ipit itate ate fo form rmed ed is Fe4N and the carbon goes into the ferrite matrix as interstitial, and (iii) for elevated concen-

trations of is both carbonand andferrite nitrogen above approximately the matrix austenite and(both no precipitates are formed.0.4%) With increa inc reasin sing g nitrog nitrogen en and/or and/or carbon carbon additi additions ons,, alloys alloys with with each each of the three different microstructures, increased hardness, and wear resistance are formed.

 

Effect of Alloying Elements in Steel

 

39

However, in carbon or low alloy steels, dissolved nitrogen being undesirable, is kept to a minimum because high nitrogen content may result in inconsistent mechanical properties in hot rolled products, embrittlement of  the heat affected zone (HAZ) of welded and poor cold formability. In particular, nitrogen can result in strainsteels, ageing and reduced ductility of  cold rolled and annealed steels although it improves yield, grain size, and mechanical properties of steels. In general limited amounts of nitrogen if present as   󿬁ne and coherent nitri nitride dess or carb carbon onit itri ride dess of ir iron on or allo alloyi ying ng elem elemen ents ts have have a bene bene󿬁cial effect eff ect on the mechan mechanica icall proper propertie ties. s. Nitrid Nitrides es of alumi aluminum num,, vanadi vanadium, um, niobium and titanium result in the formation of   󿬁ne grained ferrite which improv imp roves es mechan mechanica icall proper propertie ties, s, lowers lowers the ducti ductile le to brittl brittlee tra transi nsitio tion n temperature and improves toughness. But if nitrogen is present in ferritic matrix as interstitial element, the ductile to brittle transition temperature increases and toughness decreases. Therefore, it is necessary to carefully control, not only the nitrogen content, but also the form in which it exists, in order to optimize impact properties. When Wh en nitr nitrog ogen en is adde added d to auste austeni niti ticc st stee eels ls,, it can can simu simult ltan aneo eous usly ly improv imp rovee fatigu fatiguee life, life, strengt strength, h, work work harden hardening ing rate rate,, wear wear and locali localized zed corro co rrosi sion on re resi sist stan ance ce.. High High nitr nitrog ogen en mart marten ensi siti ticc st stai ainl nles esss st stee eell sh show owss improved resistance to localized corrosion, like pitting, crevice and intergranular corrosions, over their carbon containing counterparts. Because of  this, the high nitrogen steels are considered as a new promising class of  engineering materials. The effect of nitrogen in steel can be either detrimental or bene 󿬁cial, depending on the presence of other alloying elements in it, the form and quantity of nitrogen present, and the required behavior of the particular steel product. When properly employed, nitrogen is a friendly addition to steels. Apart from these elements, inert metals ruthenium, rhodium, palladium, osmi os mium um,, irid iridiu ium m and and pl plat atin inum um stabi stabili lize ze th thee aust austen enit itee bu butt th they ey are are to too o expensive and hence not used in steels as alloying elements.

5.4 Neu Neutra trall Sta Stabil bilize izerr Cobalt can dissolve both in ferrite (about 80%) and in austenite (100%). It stabilizes austenite when the amount exceeds 90%. It has no role to play in increasing the eutectoid composition or temperature. Like aluminium and silicon, it also does not form any carbides. Hence it can be classi󿬁ed as

a neutral stabilizer. Cobalt is the only alloying element that increases the critical cooling rate of steel steel and accel accelera erates tes pearli pearlitic tic transf transform ormati ation on thus thus reduci reducing ng harden harden-ability. Also, it has a tendency to graphitization and is a very expensive

 

40

 

Introduction to Steels

component, hence not used as an alloying addition in normal steels. It is neve ne verr used used in th thee stand standar ard d heat heat treat treatab able le st stee eels ls.. Henc Hencee coba cobalt lt is not not a popular element that is commonly added to steels. Whatever way that  󿬁

it nds use in tool steels, steels,its 18% nickeleffect maraging steels, and several other ultrahigh strength adverse on hardenability overcome by the remaining alloying constituents. The presence of cobalt in steel improves its durability and hardness at higher temperatures, reduces the fall in hardness of both austenite and ferrite due to increase in temperature, and thus it increases hot hardness. Therefore, cobalt is used as a supplement to some grades of high speed steels and tool steels, so that tools made from cobalt bearing steel can operate at high temperatures, maintaining their cutting edge. Cobalt is also a component of creep resistant steels. Magnetic steels containing from 9% to 40% cobalt have been used for compass needles, hysteresis motors and electrical instrumentation. Cobalt is used in stainless steel of grade 348, which contains 0.20% of cobalt. The dissolved cobalt in ferritic or austenitic matrix has a high-work hardening sensitivity, combines with the carbide fraction and allows to achieve excellent wear resistance along with a high degree of corrosion resistance.

5.5 Eff Effect ect of Variou Variouss Elements Elements as Dissolve Dissolved d in Ferri Ferrite te Matri Matrix x In the absence of carbon, all the alloying elements irrespective of whether ferrite or austenite stabilizer increase the strength of the matrix by solid solution strengthening as shown in Figure in  Figure 5.2. 5.2. The increase in strength/hardness is only due to solid solution hardening. Phosphorous the most effective solid solution hardener above 0.25% it forms Fe3P,iswhich is extremely brittle and reduces thebut ductility considerably. Therefore, it will be normally restricted to below 0.25%. Solid soluti sol ution on strengt strengthen hening ing effect effect increa increases ses tremend tremendous ously ly if lit little tle carbon carbon is present.

5.6 Effec Effectt of Various Eleme Elements nts as Carbides Carbides in Ferrite/Auste Ferrite/Austenite nite Matrix All carbides are brittle in nature and alloy carbides have less tendency of  coalescence and also stronger than cementite (Fe 3C). The dissolution of 

thes thesee carb carbid ides es in auste austeni nite te is slug sluggi gish sh and and need needss high high temp temper erat atur uree fo forr austenitizing, which is the main reason for increase in eutectoid temperature. The type of carbides are MC, M7C3, M6C, M3C, and M23C6, where M refers to metal. The type of carbides formed is based on the

 

Effect of Alloying Elements in Steel

 

41

FIGURE 5.2

Effect of alloying elements on solid solution strengthening of the steel (schematic)

amount of the element and the carbon content. The non-carbide-forming elements such as nickel, silicon, and aluminium accelerate graphatization of cementite. Chromium Chrom ium carbides: carbides:   Chromium forms (FeCr)3C, which is orthorhombic having 15% Cr, (FeCr)7C3, which is trigonal having 36% Cr, and (FeCr) 23C6, whic wh ich h is cubi cubicc havi having ng 70% Cr. Cr. The The chro chromi mium um carb carbid ides es tend tend to coal coales esce ce easily and form coarse carbides, which depends on cooling rate and so the stren str engt gthe heni ning ng due due to chro chromi mium um carb carbid ides es is cont control rolle led d by the the mode mode of  cool co olin ing. g. Air Air cool cooled ed chro chromi mium um st stee eels ls gi give ve high higher er hard hardne ness ss than than fu furn rnac acee  󿬁

cooled air ed cooled steels carbides higher of  pearlit pear liteebecause as compar compared to furnac fur nacee have cooled coolednewhich wh ich have haveand coarse coar se carbi caramount bides des.. The chromium forms an intermetallic phase (σ) with iron and chromium content varying between 20 and 80% in the  σ  phase. Usually, this intermetallic phase is observed in stainless steels.  Molybdenum carbides:   Molybdenum forms carbides such as (FeMo)6C, or Fe7Mo5C2. Molybdenum forms Fe3Mo2   intermetallic, which is able to dissolve 0.1% carbon and this intermetallic is expected to precipitate in low carbon age hardenable molybdenum containing steels. Tungsten carbides: Like molybdenum, tungsten forms (FeW)6C or Fe7W5C2 and an d is prim primar aril ily y foun found d in High High Spee Speed d St Stee eels ls (HS (HSS) S).. In th thee abse absenc ncee of  chromium and vanadium or even if present and are dissolved in carbide replacing iron, when tungsten is about 18% it forms (FeW)3C.

Tantalum carbides: Tantalum carbides:   Tant Tantal alum um form formss TaC TaC in st stee eels ls,, whic which h play playss an important role in lowering DBTT through its effect on prior austenitic grain size re󿬁nement and also improves the strength and toughness. However, higher tantalum content can result in the formation of extremely coarse TaC

 

42

 

Introduction to Steels

during the melting process, which is deleterious to the mechanical properties. With the increase of tantalum content, the content of chromium rich M23C6 carbides decreases. Because of its high cost tantalum is not used in low alloy steels. However of late, steels tantalum has been increasingly used as an alloying   –  super strong, corrosion resistant, and refracelement only in special tory. The addition of tantalum increases the creep resistance of steels. Vanadium carbides:  The iron, vanadium, and carbon diagram shows the absence of intermetallic compounds of iron and vanadium unlike in chromium, tungsten, and molybdenum systems. Vanadium addition introduces Vanadi dium um is by far far th thee most most used used elem elemen entt to impr improv ovee wear wear (VFe)4C3. Vana resistance in tool steels. At present there are several vanadium-alloyed tool steels with up to 18% vanadium in the market.  Niobium carbides:   The effect of niobium on steel is similar to that of  tantalum. Niobium is a strong ferrite and carbide former like tantalum and even a small addition of niobium to steels is effective for austenitic grain size re󿬁ning due to the precipitation of   δ-niob -niobiumium-carbi carbide de and half the amount (in wt%) of niobium is suf 󿬁cient compared to tantalum, to obtain the same   󿬁ne grain size. Niobium, about 0.03 to 0.04%, is added to re 󿬁ne remarkably the cast structure and austenite structure of steel. Addition of niobium (up to 1%) to chromium steel and in stainless steels improves its ductility and corrosion resistance by preventing precipitation of chromium carbides along grain boundaries. An addition of 0.7% nio bium decreases the ductile to brittle transformation temperature of steel to 80°C below zero by re󿬁ning the grain size of the matrix. matrix. During welding welding of  austen aus teniti iticc stainl stainless ess steels steels,, niobiu niobium m stabil stabiliza izatio tion n is better better than than titani titanium um stabilization as titanium can be easily oxidized during welding. Niobium addition plays major role on microstructures and formability of  TRIP and HSLA steels. However, higher contents of more than about 2.5% of niobium cause considerable dif 󿬁culties during melting. Niobium car bides directly precipitate at higher temperatures before the ferritic crystallization of the melt starts and the melt becomes more and more viscous and pulpy. Due to the viscosity, melts with more than about 3% niobium cannot be poured in moulds. For this reason the niobium content in tool steels is limited to about 2%. There are many attempts to improve the poor solubility of niobium in the melt and until now no experiment has been successful and satisfactory. Zirconium Zircon ium carbides: carbides:   Zirco Zirconi nium um is bein being g used used as an allo alloyi ying ng elem elemen entt in steels since the early 1920s, but has never been universally employed, unlike chromium, niobium, titanium, and vanadium. Zirconium is highly reactive and has a strong af 󿬁nity, in decreasing order, for oxygen, nitrogen, sulfur, and

carbon. Zirconium is a strong carbide and forms zirconium carbide (ZrC), containing carbon in the range of former 6 to 12%. Zirconium is used mainly in micro mic ro alloye alloyed d steels. steels. Zirconi Zirconium um also forms forms zircon zirconium ium nitrid nitridee (ZrN (ZrN)) when when nitrogen content is above 9%. There is extensive solid solubility between ZrC and ZrN to form Zr (CN) precipitates. Zirconium carbide, nitride and carbo

 

Effect of Alloying Elements in Steel

 

43

nitride inhibit grain growth and prevent strain aging. However, their use for either of these reasons is limited. Because of its relatively high price and also due to the availability of cheaper replacements, general acceptance of zirconium as an However, recent days there the re for has hasuse been been a alloying re rene newe wed delement in inte teres resttininsteels th thee is addi adlimited. diti tion on of zirc zircon oniu ium m to steel ste elss especially micro alloyed steels because of the following facts: Carbid Car bides, es, nitrid nitrides, es, and carbon carbonitr itride idess of zircon zirconium ium effect effective ively ly pin the austenite grains and avoid grain coarsening during heat treatment or hot working process. Addition of zirconium to plain carbon steels effectively re󿬁nes the grain as compared to other alloying elements and also reduces the interstitial carbon and nitrogen from ferrite. These two effects reduce the ductil ductilee to brittl brittlee transfo transforma rmatio tion n temper temperatu atures res to a greate greaterr ext extent ent in steels containing zirconium. Titanium Carbides:  Titanium is the strongest ferrite and carbide/nitride former. It forms more stable TiC, TiN, and Ti (CN). There are no complex carbides of iron and titanium. Titanium bearing steels contain less cementitee (less tit (less pearli pearlite) te) becaus becausee titani titanium um combin combining ing with with carbon carbon or nitrog nitrogen en forms the highly stable carbide (TiC) or nitride (TiN). Carbon scavenged in this this mann manner er is abou aboutt 0.25 0.25% % of th thee avai availa labl blee tita titani nium um.. This This mean meanss th that at pearlite is completely absent in steels containing 0.25% carbon if titanium amount is equal to more than four times (1%) the carbon content. Titanium is also used for the purpose of grain re 󿬁ning in many steels especially in micro alloyed steels. Titanium increases the grain coarsening te temp mper erat atur ure. e. In th this is re resp spec ectt it is much much more more effe effect ctiv ivee th than an alum alumin iniu ium m when concentrations of either element exceeds 0.035%. Titanium as an alloying element is mainly used in stainless steel for carbide stabilization. In austenitic stainless steels, TiC, which is quite stable and hard to dissolve in steel, tends to minimize the occurrence of intergranula gran ularr corros corrosion ion when when approx approxima imatel tely y 0.25% 0.25%–0.60 0.60% % Ti is adde added. d. The The carbon combines with the titanium in preference to chromium, preventing a tie up of corrosion resisting chromium as intergranular carbides and the acco ac comp mpan anyi ying ng loss loss of corr corros osio ion n re resis sistan tance ce at th thee grai grain n bo boun unda dari ries es.. In addition, it also increases the mechanical properties at high temperatures. In ferritic grades, titanium is added to improve toughness, formability, and corrosion resistance. In martensitic steels, titanium lowers the martensite hardness by combining with carbon and increases tempering resistance. In precipitation hardening steels, titanium is used to form the intermetallic compounds to increase the strength.

5.7 Sum Summar mary y of Carbi Carbides des As a summary, it can be stated that the alloying elements in steels form MC, M6C, M7C3, and M23C6   type of carbides where M represents the

 

44

 

Introduction to Steels

metall meta llic ic elem elemen ent(s t(s). ). El Elem emen ents ts li like ke ti tita tani nium um,, niob niobiu ium, m, ta tant ntal alum um,, and and zirconium form MC type of carbides called primary carbides. Less reactive metals met als like like molybd molybdenu enum, m, tungste tungsten, n, and vanadi vanadium um can also also substi substitute tute in thes thesee prim primary carb caM rbid ides es but but weak weaken en the bind bindin ing g fo force rcess and and degr degrad adati ation on reaction toary form 23C6   and M6C type of carbides. Primary carbides with titanium, niobium, tantalum, and zirconium counteract the degradation of  MC carbides even at temperatures of the order 1200–1260°C. M23C6  carbides tend to form with moderate to high chromium content. They hey form form duri rin ng low ower er te tem mper erat atur uree heat eat treat reatm ment ent and and serv serviice (760–98 980° 0°C) C) lead leadin ing g to dege degene nera rati tion on of MC carb carbiide dess and and fo form rm solu solubl blee carbon residual in the alloy matrix. Preferred sites for formation of these carbides are at grain boundaries and occasionally along twin lines, stacking faults and at twin ends. M23C6  has a complex cubic structure and if carbon atomss are remove atom removed, d, the struct structure ure closely closely resemb resembles les topogr topograph aphica ically lly clo close se packed (TCP) sigma phase which is deleterious phase in stainless steels. In fact, coherency between M23C6  and sigma is very high and sigma phase very often nucleates on M23C6 particles. Molybdenum and tungsten can substitute to certain extent and is found to have Cr21(Mo,W)2C6 composition. It has also  been shown that consi considerab derable le quantity quantity of nickel nickel and small portions portions of cobalt cobalt or iron could also substitute for chromium. Formation of M23C6 type carbides at grain grain bounda boundarie riess reduces reduces grain grain bound boundary ary sliding sliding and hence hence improve improvess rupture strength but cellular structure (network) of M 23C6 initiates premature rupt ru ptur uree fa fail ilur ures. es. The The grai grain n boun bounda dary ry M23C6   type type carb carbid ides es are pron pronee to intergranular corrosion. M6C carbides have a complex cubic structure and form at comparatively higher temperatures (815–980°C) when molybdenum and/or tungsten contents are high (above 6–8%). M6C carbides also form when molybdenum or tungsten replace chromium in other carbides. Because M6C carbides are stable at higher temperatures than M23C6, it is more bene󿬁cial to control the grain size during the processing of wrought alloys. M 6C also tends to form M23C6  in some alloys. Thee type Th type of carb carbid idee form formed ed also also depe depend ndss on th thee amou amount nt of carb carbon on available. The different type of carbides formed with respect to carbon content and temperature. The properties of the steel entirely depends on the type of carbides and nitrides formed. It should be noted that the formed carbides must be   󿬁ne and an d homo homoge gene neou ousl sly y di distr strib ibut uted ed in th thee matr matrix ix.. An Any y coar coarse se carb carbid ides es/ / nitrides distributed homogenously will lower the properties.

5.8 Disper Dispersed sed Metallic Particles Particles Apartt from Apar from the the el elem emen ents ts di disc scus usse sed d in th this is chap chapte ter, r, a few few elem elemen ents ts are are added to steels to improve speci󿬁c properties. These elements will not

 

Effect of Alloying Elements in Steel

 

45

form any compounds or dissolve in the matrix. They will be present in the steel as metallic particles. Copper:  Copper is soluble in austenite up to 2% and has no solubility in ferrite. At 600°C, the solubility is less than 0.3% and hence rejected as elemental copper. It is neither a ferrite nor austenite stabilizer. It does not fo form rm carb carbid ides es or comp compou ound nds. s. It is used used as a prec precip ipit itat atio ion n ha hard rden ener er (1.5–1.75%) in precipitation hardenable stainless steels. Lead:   Like copper, lead also neither dissolves in ferrite nor forms any compound. About 0.25% lead is added to improve machinability because lead has low cohesive strength and thus easy to remove as chips. However, the addition of lead affects hot working characteristics being a low melting point metal.

5.9 Nonmet Nonmetallic allic Inclusions Inclusions During steel melting, some elements that are not added intentionally as alloyi all oying ng elemen elements ts enter enter the ingots ingots as nonmet nonmetall allic ic inclus inclusion ions. s. Howeve However, r, a few elements such as manganese, tantalum, zirconium, and titanium are intentionally added to form compounds to improve speci 󿬁c properties.  Manganese:  As discussed previously, manganese is added to counteract the the effe effect ctss of sulf sulfur ur in st stee eels ls.. It form formss mang mangan anes esee sulp sulphi hide de (MnS (MnS)) as elongated inclusions, which act as a miniature notch to aid breaking of  chips easily by shear improving machinability. But MnS has deleterious in󿬂uence on the impact toughness of wrought and welded steels. Tantalum:  Tantalum addition improves pitting resistance in duplex stainle less ss steel steelss by cont contro roll llin ing g incl inclus usio ions ns such such as mang mangan anes esee sulp sulphi hide de (MnS (MnS), ), MnCr2O4, and (Ti,Ca)-oxides known to act as pitting initiators in environments containing chloride ions. The addition of tantalum leads to the formation of (Ta,Mn)-oxysulphides and these inclusions are electrochemically stable. Zirconium:  The main use of zirconium additions to steel is to preferentially combine with sulfur, to avoid the formation of manganese sulphide (MnS) known to have a deleterious in󿬂uence on the impact toughness of  wrought and welded steel. Also, it controls the other nonmetallic oxysulphide inclusions and for the   󿬁xation of nitrogen mainly in boron steels. The function of zirconium is not to remain in solution in steel but to scav scaven enge ge impu impuri riti ties es (oxy (oxyge gen, n, sulf sulfur ur,, nitro nitroge gen) n) or modi modify fy incl inclus usio ions ns through the formation of complex sulphides and oxysulphides. The ef 󿬁cacy of zirconium additions will therefore be measured not by the amount

of residual zirconium, but by the extent to which inclusions are bene󿬁cially modi󿬁ed. Titanium:  Like zirconium, titanium is also used for   󿬁xing sulfur in order to redu reduce ce its its harm harmfu full effe effect cts. s. Tita Titani nium um fo form rmss stabl stablee comp compou ound ndss with with oxygen and sulfur at the steelmaking temperatures. With respect to   󿬁xing

 

46

 

Introduction to Steels

oxygen,, titani oxygen titanium um functi functions ons simila similarr to alumi aluminiu nium. m. Howeve However, r, tit titani anium um is more expensive and hence rarely used as a deoxidizer.  Aluminium:  Aluminium oxide formed due to excess aluminium available after deoxidation pins the austenite grains restricting their growth. Graphite:  Graphite is an inclusion in steels but not in cast irons. Cementite in the pearlite forms as graphite when excess amount of graphatizer such as silicon or nickel are available and if exposed to high temperature for long periods.

5.10 Inter Intermetalli metallicc Compou Compounds nds Normally, no intermetall Normally, intermetallic ic compo compound und is is found found in plain plain carbo carbon n steels. steels. Nitrides Nitrides of aluminium, zirconium, vanadium, and titanium form as intermetallics in plain carbon steels mainly with a view to obtain   󿬁ne-grained structures for somee speci som special al applic applicati ations. ons. A few few high high allo alloy y steels steels such such as maragi maraging ng st steel eelss and precip pre cipita itatio tion n harden hardening ing steels steels have have interm intermeta etalli llics cs of nickel nickel,, tit titani anium um,, and aluminium that help in age hardening of those alloys; this will be discussed in Chapter in  Chapter 8. 8. Some intermetallics like sigma phase, which usually form in stainless steels, are undesirable.

5.11 5.1 1 Allo Alloy y Distrib Distributi ution on in Austenite Austenite The el The elem emen ents ts such such as nick nickel el,, mang mangan anes esee and and nitr nitrog ogen en th that at diss dissol olve ve in austenite matrix minimize eutectoid temperature and try to stabilize austenite room temperature. Intesteels, about 8% nickel retains room ro om at te temp mper erat atur ure. e. The The ferri ferrite st stab abil iliz izer erss fo form rmin ing g ca carb rbid ides es,, austenite if th they ey are arate undissolved in the austenite, pin grain growth of austenite resulting in 󿬁ne ferrite and pearlite grains upon cooling. The undissolved nonmetallic inclusions (such as alumina, titanium dioxide, and vanadium oxide) have the same effect as carbides if they are   󿬁ne and uniformly distributed. Table 5.1 gives 5.1  gives the abstract of the effect of alloying elements on steels.

5.12 5.1 2 Eff Effect ect of Alloyin Alloying g Elemen Elements ts on Harden Hardening ing Most alloying elements forming solid solution in austenite lower the Ms

temperature, with the exception of cobalt and aluminium. However, the interstitial solutes carbon and nitrogen have a much larger effect than the metallic solutes. About 1% of carbon lowers the Mf by over 300°C. The relative effect of other alloying elements is indicated in the following empirical relationship due to Andrews (concentrations in wt%):

 

Effect of Alloying Elements in Steel

 

47

TABLE 5.1 Effect of alloying elements on steel Maximum (in wt%) solubility in pure Element

austenite

Aluminium   0.6 Chromium   12–20

in󿬁nite Copper 2 Lead   N il Ni Manganese   in󿬁nite Cobalt

   

Molybdenum   3 Nickel

     

Niobium Nitrogen

in󿬁nite ~ 0.9 ~ 200 ppm

Phosphorous   0.5 Silicon Titanium Tungsten Tantalum

       

Vanadium  

2 0.76 6 ~ 0.9

1–2 Zirconium   ~ 4

ferrite

Carbides formed

30 No in󿬁ni nite te Cr23C6, Cr7C3 and Cr3C 80 No Nil No Nil No 10 No

Nonmetallic inclusion

Special Intermetallic Compounds

In elemental state

Al2O3 No

AlN Nil

–

No No No MnS, MnFeO,

 

Nil Nil No Nil

   

–

 

–

Cu Pb  

32 25–30 Nil less than 10 ppm 2.5 18.5 6 32 Nil

Mo6C No NbC No

MnO.SiO2 No No No No

No No TiC W6C, W3C TaC

No SiO2   TiO2   No (Ta,Mn)-

30 Nil

V4C3 ZrC

oxysulphides VO2   VN   ZrS ZrN, Zr (CN)

 

–

Fe3Mo2   Nil   Nil   nitrides with Al, Ti, V, Zr

–

Fe3P   Nil   TiN, Ti(CN) Nil   Nil  

–

– – –

–   – – –

–   –

Ms(°C) = 539–423 (%C) − 30 30.4 .4 (%Mn (%Mn))   − 17.  17.77 (%Ni (%Ni))   − 12.1(%Cr)   − 7.5 (%Mo). The equation applies to a limited class of steels. A better approach is to express Ms in terms of the driving force for transformation. Abov Ab ovee 0. 0.7% 7% C, th thee Mf te temp mper erat atur uree is belo below w room room temp temper erat atur uree an and d cons co nseq eque uent ntly ly high higher er carb carbon on steel steelss quen quench ched ed into into wate waterr will will no norm rmal ally ly contain substantial amounts of retained austenite. These steels have to be cooled below Mf temperature immediately without time delay to convert

theTi retained austenite and this as cooling. Tim me del elay ay in sub subto zero zemartensite ro tr trea eatm tmen ent t does do esisnknown ot resu result lt subzero in as com co mplete lete a tran transf sfor orma mati tion on to mart marten ensi site te,, whic which h is call called ed as st stab abil iliz izat atio ion. n. The The retained austenite upon tempering will remain as either austenite or transforms to softer products depending on the alloying elements present. In

 

48

 

Introduction to Steels

some cases, the retained austenite may convert to martensite on sudden or impact loading. This will increase the toughness of the steel. This phenomenon is used in some special steels for some particular applications. When the cooling of a steel is arrested in the Ms −Mf range, the degree of  stabilization increases to a maximum with time, and as the temperature approaches Mf, the extent of stabilization increases. However stabilization occurs only when a small amount of martensite is present in the matrix. This is because the formation of martensite plates leads to accommodating plastic deformation in the surrounding matrix, resulting in high concentration tionss of disl disloc ocat atio ions ns in th thee aust austen enit ite. e. Time Time dela delay y at an inte interm rmed edia iate te temperature gives time for plastic relaxation, that is, movement of dislocations, as well as the locking of interfacial dislocations by carbon atoms. Interaction of some of these dislocations with the glissile dislocations in the martensite plate boundary makes martensite plate immobile so that the plate cannot grow further.

5.13 Effec Effectt of Alloying El Elements ements on Harden Hardenability ability All alloying elements increase hardenability except cobalt. There is no clear mechanism why cobalt decreases the hardenability, probably because cobalt addition increases the free energy difference between austenite and ferrite and hence the nucleation kinetics is fast enough to form the softer products. Steel with high alloying elements can form martensite even if they are held isother isot hermal mally. ly. Such Such type type of marten martensit sitee is called called as isother isothermal mal marten martensit sitee witnessed in maraging steels. The effect of alloying element on hardenability is shown in   Chapter 2   (see   Figure Figure 2.7). 2.7). For the same amount of carbon, addition of alloying element has increased the hardenability. But it should  be noted that t hat alloying alloying elements elements increase increase hardenabili hardenability ty but not the hardness. hardness. Only carbon has greater in󿬂uence of increasing hardness of the martensite. Carbon is the only element that increases both hardness and hardenability. Ther Th eree is a comp comple lexx effe effect ct of carb carbid idee fo form rmer erss on hard harden enab abil ilit ity. y. The The carbide formers increase hardenability if they are present in solid solution of austenite or the carbides should dissolve completely in austenite or else hardenability decreases with the increase in concentration of the carbide formers, unless austenitizing temperatures are raised. This is due to the formation of respective carbides that lowers the carbon concentration in austen aus tenite ite.. Furthe Further, r, undiss undissolv olved ed carbid carbides es re󿬁ne th thee grai grain n si size ze,, and and th this is decreases hardenability all the more.

5.13.1 Carbo Carbon n Equivalence Equivalence

Carbon is the most important contributor to hardenability, hardness and strength of steels. Even when other alloying elements are not present, high

 

Effect of Alloying Elements in Steel

 

49

carbon content can result in high hardness and hardenability. However, other alloying elements also contribute to the overall hardenability of the steel. This effect can be generally quanti 󿬁ed by the determination of the carbon equivalence (CE) of the steel. CE is de 󿬁ned by several formulae, and it is important that close attention be paid to the formula being used. The following formula is used in most ASME applications: CE = C + (Mn + Si)/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 It is important that any CE determination is calculated using the actual chemical analysis rather than the maximums speci󿬁ed in materials speci󿬁cations. If this is not done, the calculati calculation on will result in an unrealisti unrealisticall cally y high CE. 5.13.2 Facto Factors rs Affecting Hardenabilit Hardenabilityy

Apart from the alloying elements, there are three factors that affect the hardenability; (i) grain size, (ii) undissolved inclusions and carbides and (iii) inhomogeneity of austenite. Coarse austenite grains result in better hardenability. In the   󿬁ne-grained austenite, the grain boundaries can act as the nucleation site for the transformation of softer products. In similar way the undissolved inclusions and/or the carbid carbides es also also act act as nucl nucleat eation ion site site for for the forma formatio tion n of of softe softerr prod produc ucts. ts. The The austenitizing temperature has to be very high to dissolve the carbides especially of vanadium, niobium and titanium. If the austenitizing temperature is raised with a view to dissolve the carbides, grain growth will result. Although the dissolution of carbides and coarse grains is favorable for better hardenability, the martensite formed will be coarse. The desired properties may not  be achieved achieved.. Therefore, Therefore, a balance balance between between these two has to be achieve achieved. d. The third factor is the inhomogeneity in austenite. On heating to the austenitizing temperature, the austenite formed may not have the same amount of carbon. The austenite grain having lower carbon content may transform to softer product and ends up with ferrite martensitic structure. Thee stee Th steell samp sample le shou should ld be held held at th thee aust austen enit itiz izin ing g temp temper erat atur uree to homogenize the austenite. The holding time is very critical in steels with high amounts of tungsten, vanadium or niobium. Longer holding time to achieve homogeneous austenite may decrease the hardness and hardenabil ab ilit ity. y. Init Initia iall lly, y, carb carbon on di diff ffus uses es fr from om carb carbid ides es and and fo form rmss aust austen enit itee at moderate velocities, then the alloying elements dissolve in ferrite and try to stabilize the ferrite ending up with ferrite and pockets of austenite and after hardening result in ferrite and martensite having lower hardness.

5.14 5.1 4 Eff Effect ect of Alloyin Alloying g Elemen Elements ts on Temper Tempering ing In plain carbon steels, as the tempering temperature increases, the hardness ne ss decr decrea ease sess but but in allo alloy y steel steelss with with carb carbid ides es (esp (espec ecia iall lly y ch chro romi mium um,,

 

50

 

Introduction to Steels

molybdenum), the hardness decreases up to certain temperature and some sudden increase in hardness is observed which is called secondary hardening. Between 200 and 300°C, the rejected carbon from martensite forms Fe2.4C (epsilon carbide), which grows up to 400°C to a length of 300A°. Above 400°C, the epsilon carbide dissolves and forms new alloy carbides of 50A° in size, which increases the hardness. With further increase in temperature, the alloy carbides grow larger and hardness starts decreasing again. Thes Th esee al allo loyi ying ng elem elemen ents ts are are adde added d to th thee st stee eell to incr increa ease se spec specii󿬁c properties of steels. Based on the alloy content, the alloy steels are classi󿬁ed as low and high alloy steel. There is no strict rule to classify the low and high alloy steels.

 

6 Low Alloy Steels

6.0 Int Introd roduct uction ion Althou Alth ough gh it is a misn misnom omer er,, allo alloy y st stee eels ls cont contai ain n sign signii󿬁ca cant nt amou amount ntss of  alloyi all oying ng eleme elements nts.. The alloy alloy steels steels are genera generally lly cla classi ssi󿬁ed as low alloy steel ste elss and and high high allo alloy y steel steels. s. In gene general ral if th thee amou amount nt of to tota tall allo alloyi ying ng elements exceeds 5% it is called as high alloy steels and below 5% it is referred to as low alloy steels. In this chapter, only low alloy steels used for common applications are discussed. The Society of Automotive Engineers (SAE (S AE), ), the the Amer Americ ican an Ir Iron on and and St Stee eell In Insti stitu tute te (AIS (AISI) I),, and and th thee Amer Americ ican an Society for Testing and Materials (ASTM) are responsible for the classi 󿬁cation tion and and spec specii󿬁ca cati tion on of st stee eels ls as well well as ot othe herr allo alloys ys.. The The AISI AISI/S /SAE AE designation for these steels consists of a four-digit number, the   󿬁rst two digits indicating the alloy content, while the last two indicate the carbon concentration. For plain carbon steels, the  󿬁rst two digits are 1 and 0 while alloy steels are designated by other initial two-digit combinations (e.g., 13, 41, 43). The third and fourth digits represent the weight percent carbon multiplied by 100. For example, 1060 steel is a plain carbon steel containing 0.60% C. Various low alloy steels designated as per AISI classi󿬁cation are given below: Carbon steels 1XXX Plain carbon steels 10XX Free machining steels 11XX Resulfurized and rephosphorized Plain carbon steels 12XX Manganese Steels 13XX

Nickel steels 2XXX 3.5% Ni 23XX 5% Ni 25XX Ni-Cr steels 3XXX 1.25% Ni- 0.65% Cr 31XX  51  

 

52

3.5% Ni- 1.6% Cr 33XX Mo steel 4XXX C0.25% Mo 40XX Cr-Mo 41XX Ni-Cr-Mo 43XX C-0.4%Mo 44XX C-0.55%Mo 45XX 1.8% Ni-Mo 46XX 1.05% Ni-Cr-Mo 47XX 3.5% Ni-Mo 48XX Cr steels 5XXX Low Cr (0.35%) 50XX Medium Cr (0.9% Cr) 51XX High Cr (1.45% Cr) 52XX Cr (0.75%)   –  V 61XX W steel 7XXX Triple alloy steels 0.3% Ni-0.4% Cr-0.12% Mo 81XX 0.55% Ni-0.5% Cr- 0.2 Mo 86XX 0.55% Ni-0.5% Cr   –  0.25% Mo 87XX 0.55% Ni -0.5%Cr   –  0.35% Mo 88XX 3.25% Ni-1.2% Cr- 0.12% Mo 93XX 1% Ni- 0.8% Cr- 0.25% Mo 98XX Boron Steels (0.0005% B   –  5 ppm) C TS14BXX 0.5% Cr 50BXX 0.8% Cr 51BXX 0.3% Ni-0.45% Cr-0.12% Mo 81BXX 0.55% Ni-0.5% Cr-0.2% Mo 86BXX 0.45% Ni-0.4% Cr-0.12% Mo 94BXX

Introduction to Steels

6.1 Pla Plain in Carb Carbon on Stee Steels ls (1XX (1XXX X Series) Series) The plain carbon steels are classi󿬁ed as low carbon steels in which carbon% is less than 0.25%; in medium carbon steel carbon content varies between

 

Low Alloy Steels

 

53

0.25 and 0.45%; in high carbon steel the carbon content varies between 0.45 and 0.6%; eutectoid steels have 0.8% carbon and hypereutectoid steels have more than 0.8% carbon. In the plain carbon steels, as the carbon content increases, the strength increases but the ductility and the fabrication properties such as weldability, formability decrease. The low carbon steels have reasonable strength but cannot be hardened  by heat treatment. The low carbon steel is widely used as constructional material and is the only alloy used in hot   󿬁nished condition. The medium and high carbon steels are hardenable and have better strength than low carbon steels. In 1XXX series there is one classi󿬁cation 11XX, which is called as free mach ma chin inin ing g steel steels. s. Thes Thesee st stee eels ls have have slig slight htly ly high higher er am amou ount ntss of sulf sulfur ur,, which increase the machinability of steels by forming elongated manganese ne se sulp sulphi hide de (MnS (MnS)) incl inclus usio ions ns.. Howe Howeve ver, r, th thes esee type typess of st stee eels ls have have problems during welding and hot forming. These types of steels are used where machining is required. Thee extensive Th othe otherr cl clas assi si󿬁ca cati tion on in 1XXX 1XXX se seri ries es is 13XX 13XX call called ed as mang mangan anes esee steels. Generally, in all steels about 1% manganese is always present to counteract the ill effect of sulfur. The sulfur forms iron sul󿬁de (FeS)   –   a low meltin melting g eutect eutectic ic   –   whic which h affe affect ctss th thee hot hot work workin ing g pr prop opert ertie iess and and weldability by cracking in hot condition (hot cracking). Addition of manganese forms manganese sulphide (MnS) in preference to FeS. MnS will not melt during hot working and hence does not affect the hot working and welding properties. Moreover, MnS has higher surface tension and high higher er melt meltin ing g poin pointt th than an FeS FeS and and th thus us impr improv oves es th thee weld weldab abil ilit ity y by forming isolated pockets of liquid during welding instead of continuous liquid   󿬁lm as in the case of FeS. It is well established that if the surface tension between solid grains and grain boundary liquid is low it forms continuous liquid   󿬁lm and leads to high crack sensitivity, whereas if the surf su rfac acee te tens nsio ion n is high high,, only only isol isolat ated ed pock pocket etss of liqu liquid id fo form rms, s, whic which h reduces the sensitivity to hot cracking. About 1.6–1.9% manganese is used in low alloy steels to increase strength and ductility, whereas above 2% imparts brittleness.

6.2 Nic Nickel kel S Stee teels ls (2X (2XXX) XX) Nickel is the   󿬁rst element alloyed with iron, which is now removed from

the series of low alloy steels due to the high cost of nickel. However, for few special critical series ofs alloys nick nickel el is anand aust austen enit itee applications st stab abil iliz izer er,, it this st stre reng ngth then ens ferr ferrit iteeare byused. soli solid dAlthough so solu luti tion on stre streng ngth then enin ing. g. Addi Additi tion on of nick nickel el in incr crea ease sess st stre reng ngth th with withou outt much much decrease in ductility. It also increases impact strength and toughness as well as improves toughness at low temperatures when added in small

 

54

 

Introduction to Steels

amounts. And so, Nickel steel has good low temperature strength and high Notch Tensile Strength (NTS). Nickel addition improves steel ’s resistance to oxidation, corrosion and abrasive resistance. Nickel is heat-resistant, and when wh en al allo loye yed d with with st stee eel, l, it in incr crea ease sess th thee heat heat resi resist stan ance ce of th thee st stee eel. l. Becaus Bec ausee nickel nickel reduc reduces es eutect eutectoid oid temper temperatu ature re (appro (approxim ximate ately ly 100 100°C °C for 1% of Ni), low temperature quenching is suf 󿬁cient to get full austenite during heat treatment and hence distortion and thermal stresses are less in the components after forming martensite on quenching. Nick Ni ckel el is a weak weak hard harden enab abil ilit ity y agen agentt and and amon among g th thee stand standar ard d allo alloy y steels, those containing nickel alone as the principal alloying constituent are rare. Instead, nickel is used in combination with other alloying elements such as chromium, molybdenum, or vanadium to produce steels with excellent combinations of strength and toughness in the quenched and tempered condition. Most of these steels contain around 0.5% nickel, although over 3% nickel is found in some grades. The alloy 2317 can be case carburized to have surface tough and isfor used for pins, high strength bolts and highstrong tensile bolts. and Alloy 2515core is used wrist kingpins requiring high hardness and fatigue strength.

6.3 Nickel Nickel-Chrom -Chromium ium Steels Steels (3XXX) (3XXX) Nickel Nick el chro chrom miu ium m st stee eels ls were were deve develo lope ped d as an alte altern rnat atee to nick nickel el st stee eels ls.. Part Partia iall substitution of chromium for nickel gives properties of the nickel steels with lowerr cost. Nickel gives lowe gives toughness and chromium chromium hardens hardens the alloy. Care has to be take taken n whil whilee addin adding g chro chromi mium um and and nicke nickell sin since ce they they ha have ve oppo opposi site te effec effectt on the eutecto eutectoid id (auste (austenit nitizi izing) ng) tempe temperat rature. ure. Normal Normally ly the ratio ratio between between nickel and chromium is maintained below 2.5. Above this ratio, narrows the austenitic range and hence heat treatment will become critical.

6.4 Moly Molybde bdenum num St Steels eels (4XXX (4XXX)) In gene genera ral, l, moly molybd bden enum um is used used to enha enhanc ncee the the pr prop opert ertie iess im impa part rted ed by other alloying elements especially nickel and chromium as a synergic effect. These steels normally have molybdenum in the range of 0.15–0.6%. Addition of molybdenum resists softening during tempering and hence can be

used for slight slightly ly elevat elevated ed temper temperatur aturee applic applicati ations ons.. It increa increases ses ductil ductility ity,, toughness, machinability and hardenability. The major advantage is that it eliminates temper brittleness. Molybdenum containing steels require high temperature and longer heating time for austenitizing since the dissolution of MoC in austenite is slow. Among the various grades of molybdenum steels, 41XX is more popular and used in pressure vessels. The grades 43XX,

 

Low Alloy Steels

 

55

46XX, 47XX 46XX, 47XX,, and and 48XX 48XX have have ni nick ckel el in addi additi tion on to moly molybd bden enum um.. Due Due to nickel, these grades have good ductility and toughness.

6.5 Chr Chromi omium um Ste Steels els (5XXX) (5XXX) Addition of chromium increases hardenability, strength, hardness, and wear resistance. However, the ductility is reduced. Chromium steels, if tempered abovee 540°C abov 540°C suffer suffer temper temper embrit embrittle tleness ness.. The case case carbur carburize ized d chr chromi omium um steels have hard case but tough core is missing unlike nickel steels.

6.6 Van Vanadi adium um St Steels eels (6XXX (6XXX)) Like molybdenum, vanadium is used to enhance the properties imparted  by other alloying elements especially chromium. But the vanadium car bides are stronger than molybdenum carbides. These grades of steels normally have vanadium in the range of 0.15 –0.6%. Vanadium carbides are insoluble in normal austenitizing temperature and retards the grain growth of austenite. Like molybdenum steels, vanadium steels also resist softening and have good creep resistance.

6.7 Tun Tungst gsten en Steels Steels (7X (7XXX) XX) Tungsten was the   󿬁rst among the alloying elements systematically used as early as in the middle of the 19th century to improve steel properties. Tungsten forms very stable carbides. These grades of steel have both wear resistance and temperature resistance. Importance of tungsten in steel has stead ste adil ily y incr increa ease sed, d, with with th thee st stee eell indu indust stry ry bein being g th thee larg larges estt tu tung ngste sten n cons co nsum umer er.. The The use use of tu tung ngste sten n in struc structu tural ral st stee eels ls decl declin ined ed sinc sincee 19 1940 40  because alloying with molybdenum and chromium as well as with vanadium and nickel has given better performance at lower cost. Today, the series itself is removed from the list like nickel steels. Although tungsten is nott popu no popula larr al allo loyi ying ng el elem emen entt in low low allo alloy y st stee eels ls,, tu tung ngst sten en is th thee main main alloying element in tool and die steels due to its excellent properties.

6.8 Tri Triple ple Alloy Alloy Steels Steels (8XXX (8XXX and 9XXX) 9XXX) Triple alloys are hardenable chromium, molybdenum, and nickel low alloy stee steels ls ofte often n used used for for carb carbur uriz izin ing g to deve develo lop p a case case hard harden ened ed pa part rt at

 

56

 

Introduction to Steels

cheaper cost. It is a balanced alloy and has good welding qualities. Case hardening of this alloy will result in good wear characteristics. The nickel imparts good toughness and ductility while the chromium and molybdenum contribute increased hardness penetration and wear. These grades are used where a hard, wear-resistant surface and a ductile core is required. In a few grades, lead is added to have better machinability and surface   󿬁nish. These grades are available in aircraft quality and bearing quality.

6.9 Sil Silico icon n Steel Steelss Silicon steels are also called as electric steels. Silicon promotes a ferritic microstructure and increases strength. Silicon reduces the hysteresis loss in electrical applications. About 3% silicon is used in these grades of steels and worldwide are working make electric steels with about 6% silicon, whichresearchers will have high magnetictoproperties. Silicon also increases resistance to oxidation and used for moderate temperature applications. Care must be taken while using these grades of steel at high temperatures  because silicon is a graphatizer and causes graphatization of cementite.

6.10 6.1 0 Bor Boron on Steels Steels Like other alloying elements, boron has no effect on Ms temperature and retai retaine ned d aust austen enit ite. e. It does does not not chan change ge th thee   󿬁ne nene ness ss of pear pearli lite te,, no norr it prod pr oduc uces es any any soli solid d solu soluti tion on stren strengt gthe heni ning ng in ferri ferrite te.. It show showss no effec effectt on tempering response but the outstanding feature of boron is the improvement in hardenability by the addition of even a minute (0.0015% to 0.0030%) quan qu anti tity ty of boro boron. n. In case case an exce excess ssiv ivee amou amount nt of boro boron n (gre (great ater er th than an 0.00 0.0030 30%) %) is pres presen ent, t, the the boro boron n cons consti titu tuen ents ts beco become me segre segrega gate ted d in the auste austeni nite te grain boundaries, which not only lowers hardenability but also may decrease toughness, cause embrittlement and produce hot shortness. Boron delays the beginning of the transformation of ferrite by suppressing the nucleation of proeutectoid ferrite on austenitic grain boundaries but it has no effect on growth, that is, end of transformation, which facilitates isothermall proc ma proces esse sess such such as ausfo ausform rmin ing g to form form to toug ugh h bain bainit ite. e. In addi additi tion on,, it increa inc reases ses harden hardenabi abili lity ty by preven preventin ting g the formati formation on of softer softer structu structures res

during cooling from the austenization temperature, after annealing or hot working. boron onof hardenability also depends on the amount of  carbon inThe the effect steels.ofThe effect boron is inversely proportional to carbon. Boron is much more effective at low carbon levels. Boron contribution falls to zero as the eutect eutectoid oid carbon carbon conten contentt is approa approach ched. ed. An interes interesting ting sideli sidelight ght of  this phenomenon is that the carburized boron steels are marked by a high

 

Low Alloy Steels

 

57

core hard core harden enab abil ilit ity y but but a low low case case hard harden enab abil ilit ity. y. Bo Boro ron n has has lon long been been used used as a replacement for other alloying elements in heat treatable steels, especially when these constituents were in short supply. Boron does not retard grain growth but reduces grain boundary diffusion hence the creep properties. Boron Boro n steels steels are becom becoming ing increas increasing ingly ly popul popular ar and their their app applic licati ation on is  becoming  becom ing more diverse. diverse. Their Their higher higher properties, properties, at a reasonable reasonable price, price, are achiev ach ieved ed through through advanc advanced ed manufac manufactur turing ing techno technolog logy. y. Altho Although ugh boron boron steels were originally designed mainly for the hard, wear resistant elements, now they are also being used for other applications. Boron is not added as a separate alloying element in plain carbon steels. The letter B is introduced after the   󿬁rst two digits. For example, 51BXX indicates that the steel has about 0.8% chromium along with boron. Boro Bo ron n steel steelss are used used when when th thee base base comp compos osit itio ion n meet meetss mech mechan anic ical al property requirements (toughness, wear resistance, etc.), but hardenability is in insu suf  f 󿬁cie cient nt for the intend intended ed sectio section n size. size. Carbon Carbon-ma -manga nganes nese-b e-boro oron n (C-Mn-B) steels are generally speci󿬁ed as replacements for alloy steels for reasons of cost as C-Mn-B steels are far less expensive than alloy steels of  equiva equ ivalen lentt harden hardenabi abilit lity. y. Boron Boron is someti sometimes mes used used in non-he non-heatat-tre treate ated d steels stee ls as of nitrog nitrogen en scaven scavenger ger.. By avoidi avoiding ng inters interstit titial ial nit nitrog rogen, en, boron boron make ma kess the the stee steell more more form formab able le and and el elim imin inate atess th thee need need fo forr st stra rain in age age suppressing anneals especially in steels used for automotive strip stock. Boron is added to some steels for the nuclear industry since it has a high neutron absorption capability. Levels of 4% boron or more have been used,  but due to the lack of hot ductility and weldability in these steels, boron contents of 0.5% to 1.0% are more common for neutron absorption applications. For this application of boron, the Fe-B has to be of the highest purity. Great care has to be taken while melting the boron steels since boron reacts readily with oxygen nitrogen is not useful to steels if it is in combined form. Boronand must be in itsand atomic state to improve hardenability. If this precaution is not taken then it can lead to erratic heat treatment response. Care has to be taken while heat treating since boron may also  become ineffective if its state is changed by incorrect heat treatment. For exam ex ampl ple, e, high high auste austeni niti tizi zing ng te temp mper erat atur ures es must must be avoi avoide ded d as well well as temper tem peratu ature re ranges ranges where where certai certain n precip precipita itatio tion n of boron boron occurs occurs.. Also, Also, while heat treating, oxidizing and nitriding atmospheres are to be avoided. Boron steels are not to be carbonitrided. Typical mechanical properties of most commonly used low alloy steels are listed in Table in  Table 6.1

6.11 6.1 1 Int Inters erstiti titial al Free St Steels eels The low carbon steels (0.2% C) combine moderate strength with excellent ductility and are used extensively for their fabrication properties in the

 

 

58

Introduction to Steels

TABLE 6.1 Mechanical properties of selected low alloy steels S. No AI AISI SI Cond Condit ition ion

1

 

2

 

3

 

4

 

5

 

1010   normalized

cold drawn 1020   as rolled normalized annealed 1030   normalized hardened and tempered 1040   hot rolled cold drawn hardened and tempered at

1050  

6

 

1080  

7

 

1117  

8

 

9

 

705°C hardened and tempered at 204°C hot rolled cold drawn hardened and tempered at 705°C hardened and tempered at 204°C as rolled normalized annealed hot rolled

cold rolled 1213   hot rolled cold rolled 1340   annealed hardened and tempered at 705°C hardened and tempered at 538°C hardened and tempered at 370°C hardened and tempered at

UTS (MPa)

YS (MPa)

Elongation %

Hardness (HB)

324 365 448 441 393 490 540 496 552 607

179 305 346 330 294 260 440 290 490 421

28 20 36 35 36 21 12 18 18 12 33 33

95 105 143 131 111 75 HRB 149 144 160 183

779

600

19 19

262

620 690 662

338 579 421

15 15 10 30 30

180 200 192

986

758

10 10

321

1010 965 615 430

586 524 375 230

12 12 11 24 23 23

293 293 174 121

480 379 517 703 690

400 228 340 434 517

15 25 25 10 26 25 25

137 110 150 207 235

993

910

17 17

363

1520

1360

13

461

1960

1610

8

578

204 C 10   3140   annealed hardened and tempered at 705°C hardened and tempered at 538°C

655

462

25

187

792

648

23 23

233

1050

920

17

311

(Continued )

 

Low Alloy Steels

 

59

TABLE 6.1   (Cont.) S. No AI AISI SI Cond Condit ition ion

hardened and tempered at 370°C hardened and tempered at 204°C 11   4130   annealed hardened and tempered at 705°C hardened and tempered at 538°C hardened and tempered at 370°C hardened and tempered at 204°C 12   4140   annealed hardened and tempered at 705°C hardened and tempered at 538°C hardened and tempered at 370°C hardened and tempered at 204°C 13   4150   annealed hardened and tempered at 705°C hardened and tempered at 538°C hardened and tempered at 370°C hardened and tempered at 204°C 14   4340   annealed hardened and tempered at 705°C hardened and tempered at 538°C hardened and tempered at

UTS (MPa)

YS (MPa)

Elongation %

Hardness (HB)

1520

1380

13

461

1930

1710

11

555

558 676

359 614

28 28 28

156 202

986

910

16 16

302

1430

1240

13

415

1610

1360

12

461

655 807

414 690

26 23 23

197 235

1160

1050

17

341

1590

1460

13

461

2000

1730

11

578

731 880

379 800

20 20 20

197 262

1360

1250

11

401

1700

1580

10

495

2070

1710

10

578

745 965

469 827

22 23 23

217 280

1180

1090

16

363

1590

1420

12

461

370°C hardened and tempered at 204°C 15   5140   annealed hardened and tempered at 705°C

1950

1570

11

555

572 717

290 572

29 27 27

167 207

(Continued )

 

60

 

Introduction to Steels

TABLE 6.1   (Cont.) S. No AI AISI SI Cond Condit ition ion

hardened and tempered at 538°C hardened and tempered at 370°C hardened and tempered at 204°C 16   5150   annealed hardened and tempered at 705°C hardened and tempered at 538°C hardened and tempered at 370°C hardened and tempered at 204°C 17   5160   annealed hardened and tempered at 705°C hardened and tempered at 538°C hardened and tempered at 370°C hardened and tempered at 204°C 18   6150   annealed hardened and tempered at 705°C hardened and tempered at 538°C hardened and tempered at 370°C hardened and tempered at 204°C 19   8650   annealed hardened and tempered at 705°C hardened and tempered at

UTS (MPa)

YS (MPa)

Elongation %

Hardness (HB)

1000

896

18

302

1520

1380

11

429

1900

1560

7

534

676 800

359 700

22 22 22

197 241

1100

1030

15

321

1650

1520

10

461

2150

1720

8

601

724 793

276 690

17 23 23

197 229

1170

1040

14

341

1810

1630

9

514

2220

1790

4

627

662

407

23

197

814

738

21 21

241

1260

1190

12

375

1700

1540

10

495

2170

1860

7

601

717 841

386 779

22 21 21

212 255

1210

1070

14

363

538°C hardened and tempered at 370°C hardened and tempered at 204°C

1650

1530

12

495

1940

1720

11

555

(Continued )

 

Low Alloy Steels

 

61

TABLE 6.1   (Cont.)

S. No AI AISI SI Cond Condit ition ion 20   8740   annealed

hardened and tempered at 705°C hardened and tempered at 538°C hardened and tempered at 370°C hardened and tempered at 204°C 21   9255   annealed hardened and tempered at 705°C hardened and tempered at 538°C hardened and tempered at 370°C hardened and tempered at 204°C

UTS

YS

Elongation

Hardness

(MPa) 690

(MPa) 414

% 22

(HB) 201

820

690

25 25

241

1210

1150

15

363

1570

1460

12

461

2000

1650

10

578

780 896

490 703

22 21 21

229 262

1250

1100

14

352

1790

1650

5

534

2140

1980

2

601

annealed or normalized condition for bridges, buildings, cars and ships. However, even 0.2% carbon, has limited ductility for deep drawing operations, and brittle fracture becomes a problem, particularly for welded thick sections. Improved low carbon steels < 0.2% are produced by deoxidising or   “killing”  the steel with aluminium or silicon or by adding manganese to re󿬁ne the the grai grain n size size.. It is now now more more comm common on,, howe howeve ver, r, to add add smal smalll amou am ount nt of (< 0.1% 0.1%)) of niob niobiu ium, m, whic which h redu reduce cess th thee carb carbon on cont conten entt by forming NbC particles. These particles not only restrict grain growth but also give rise to strengtheni strengthening ng by precipitati precipitation on hardening hardening within within the ferrite grains. Other carbide formers, such as titanium, may be used but because niobium does not deoxidize, it is possible to produce a semikilled steel ingot having reduced ingot pipe, giving increased tonnage yield per ingot cast. These types of steels are called Interstitial Free (IF) steels. Due to their excell exc ellent ent formab formabili ility ty and nonagi nonaging ng chara characte cteris ristic ticss are widely widely pre prefer ferred red

material for automotive applications. From early 1970s, carbon contents close to 0.01% became readily available and it was then necessary to add suf 󿬁cient alloying addition (titanium and niobium) to be able to combine with all the carbon and nitrogen in the steel and to leave a small surplus in order to obtain very good formability. More recently, with improvements in vacuum degassing techniques, ultralow carbon (ULC) contents below 0.003% have become easily available and

 

62

 

Introduction to Steels

with these ultra-low carbon contents, it has been possible to lower the alloy addition (close to 0.01% of niobium) while still achieving high formability. However, a number of different combinations of niobium and titanium are also in use.

6.12 Applic Applications ations a ass Engineerin Engineering g Materia Materiall As nick nickel el has has th thee abil abilit ity y to impa impart rt high high to toug ughn hnes ess, s, es espe peci cial ally ly at low low temperatures, nickel steels (2XXX) led to the development of cryogenic steels having important applications in the transportation and storage of  lique󿬁ed gases. A structural steel for lower service temperature without the risk of brittle failure must contain more nickel. Thus, a low carbon 2.5% nickel steel can be used down to   –60°C while 3.5% nickel lowers  –

the allowable temperature to 100°C and 9% Ni steels are useable up to –196°C. The nickel-chromium steels (31XX) have good properties at low cost and used for drive axle shafts, connecting rods. The 33XX are used for heavy duty applications since they have good hardenability. Among the the vari variou ouss grad grades es of moly molybd bden enum um st stee eels ls,, 41XX 41XX is more more popu popula larr an and d used in pressure vessels. The grades 43XX, 46XX, 47XX and 48XX are used us ed main mainly ly wher wheree high high fati fatigu guee and and tens tensil ilee pr prop opert ertie iess are are requ requir ired ed.. Chromium steels with high carbon is used for making knifes and for keen ke en cutt cuttin ing g edge edges. s. The The 5210 521000 grad gradee is used used fo forr maki making ng roll roller erss of  antifriction bearings due to high hardness and wear resistance. Vanadium steels are used when the molybdenum steels could not with stand the the temp temper eratu ature re or st stre ress ss.. Trip Triple le allo alloy y st stee eels ls are used used as gear gears, s, ring ring gears, shafts, pinions, spline shaft, piston pins, oil pumps, piston rods and an d li line ners, rs, cams cams,, oil oil to tool ol slip slips, s, jigs jigs,, gaug gauges es,, plas plasti ticc mold molds, s, jaws jaws,, and and crankshafts or similar applications. The main application of silicon steels is in making sheets for transformer cores and for magnetic applications since the hysteresis loss is minimum in silicon steels. Silicon steels are also used in automobile valves called as valve steels having 3 –4% silicon si sinc ncee it is re resi sist stan antt to oxid oxidat atio ion, n, both both at high high temp temper erat atur ures es and and in stron str ongl gly y oxid oxidiz izin ing g solu soluti tion onss at lo lowe werr temp temper erat atur ures es.. Boro Boron n st stee eels ls are used us ed prim primar aril ily y in punc punchi hing ng to tool ols, s, spad spades es,, kniv knives es,, saw saw blad blades es,, and and safe sa fety ty beam eams in veh vehic iclles, and and so on, where here high igh hard arden enab abil ilit ity y is required. Boron is also used in deep drawing steels, where it removes

interstitial nitrogen and allows lower hot rolling temperatures. Applications for carbon-manganese-boron steels include earth scraper segments, track links, track links, roller rollers, s, drive drive sprock sprockets ets,, axle axle com compon ponent entss and cranks crankshaf hafts. ts. Interstitial free steels are used in making automotive bodies due to its excellent deep drawing properties. The other categories of low alloy steels are high strength low alloy steels (HSLA), dual phase steels, and transformation induced plasticity (TRIP)

 

Low Alloy Steels

 

63

steels. steel s. They They al also so have have low low allo alloyi ying ng el elem emen ents ts,, but but th thee fo form rmul ulat atio ion n and and treatment are different. The metallurgy of these alloy steels are dealt in subsequent chapters. Stainless steels, maraging steels, and tool steels fall under the category of high alloys steels because they have alloying elements more than 10%. The physical metallurgy aspects of these steels are also described in later chapters of this book.

 

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7  High Strength Steels

7.0 Int Introd roduct uction ion The lowmany strength inathe previous chapter have been in use for yearssteels sinceconsidered they exhibit number of useful properties such as very good formability, suf 󿬁cient strength, good weldability, corrosion resistance or suitable for corrosion protection, good surface quality, and so on. In the last decade, a clear interest for the development of high strength steels has emerged. With stronger steels, thinner parts can be fabricated and an d weig weight ht savi saving ngss can can be obta obtain ined ed.. With Within in th thee fami family ly of low low carb carbon on steels, several variants like Al killed steel, interstitial free (IF) steel, and so on, have been developed and were discussed in the previous chapter.

7.1 De󿬁nition and Need of High Strength Steels The de󿬁nition was framed by automotive manufacturers as they are the main users. The de󿬁nition for high strength steels (HSS) and advanced high strength steels (AHSS) is arbitrary because the terminology used to cl clas assi sify fy stee steell prod produc ucts ts vari varies es cons consid ider erab ably ly th thro roug ugho hout ut th thee worl world. d. It is gene ge nera rall lly y acce accept pted ed th that at th thee tr tran ansi siti tion on from from mild mild st stee eell to high high stren strengt gth h steels (HSS) occurs at a yield strength of about 270 MPa. For yield strength leve levels ls betw betwee een n 280 280 to 350 350 MPa, MPa, ty typi pica call lly y a simp simple le ca carb rbon on mang mangan anese ese (C-Mn) steel is used. The composition of these steels is similar to low carb ca rbon on mild mild st stee eels ls,, exce except pt th they ey have have more more carb carbon on and and ma mang ngan anes esee to

increase the strength to the desired level. This is also known as conventional high strength steels. This approach usually is not practical for yield strengths greater than 350 between MPa due280 to drop offMPa in elongation andinweldability. The yield strengths and 550 are achieved high strength low alloy (HSLA) steels, also known as microalloyed (MA) steels. This family of steels usually has a microstructure of   󿬁ne grained ferrite that has been strengthened with carbon and/or nitrogen precipitates of 

65  

 

66

Introduction to Steels

titanium, vanadium, or niobium (columbium). Adding manganese, phosphorus, or silicon further increases the strength. These steels can be formed successfully whentrade usersoff. are aware of the limitations of the higher strength, lower formability Stee Steels ls with with yi yiel eld d st stre reng ngth th le leve vels ls in exce excess ss of 700 700 MPa MPa are are call called ed as advanc adv anced ed high high streng strength th steels steels (AHSS) (AHSS) and steels steels with with tensil tensilee str streng engths ths exce ex ceed edin ing g 780 780 MPa MPa are call called ed   “ultra ultrahighhigh-streng strength th steels. steels.”   AHSS AHSS with with tensile strength of at least 1000 MPa are often called   “Giga Pascal steel” (1000 MPa = 1GPa) as represented in   Figure 7.1. 7.1. AHSS have excellent strength combined with excellent ductility, and thus meet many functional requirements. Dual phase (DP), transformation induced plasticity (TRIP), and martensitic steels are some of the grades collectively referred to as AHSS. The following two grades of steels are not classi 󿬁ed under HSS or AHSS: (i) Austenitic stainless steel having strength level of above 700 MPa with el elon onga gati tion on of abou aboutt 60% 60% and and (ii) (ii) mara maragi ging ng st stee eels ls with with stren strengt gth h abov abovee 1900MP 190 0MPaa and about about 40% elonga elongatio tion,. n,. These These steels steels due due to high high all alloyi oying ng content are expensive choices for many components especially in automo bile sector and are not normally preferred for automobile applications. The austenitic grade stainless steel is used where good corrosion resistance is required and maraging steels are candidate material for aerospace applications and are discussed in Chapter in  Chapter 8. 8. The need for HSS and AHSS, mainly in the automobile market, arose due to the oil crises of 1973 and 1979. The oil crisis provided the initial stimuli for weight reduction. The car body is assembled from large body panels and constitutes about 25–30% of the total weight of a medium size car. It is, therefore, the heaviest vehicle component. Therefore reducing the

70 60     ) 50    %     (   n 40   o    i    t   a   g 30   n

Low Strength Steels (700MPa)

Conventional HSS 

IF-HS Mild

ISO BH

 AHSS  TRIP

  o     l    E 20

C  M   n  H S L  L A

 

10 0

D P  M ARTEN SITE

0

300

600 900 Tensile Strength (MPa)

1200

1600

FIGURE 7.1 Representation of low, high, and ultra-high strength steels

 

 High Strength Steels

 

67

weight of the body could have a signi 󿬁cant impact on the total weight of  the complete car. It was planned that this could be done by the substitution of steel bySubsequently, aluminium but ythe penalty would have increased incre ased cost. Subsequen tly,orit plastics, was generally generall accepted acce pted that the use of been high strength steel, which would enable component performance to be maintained at a reduced thickness, was the only way to achieve both weight and cost savings. Although the initial application is in automobile industries, the HSS has captured the market in other engineering   󿬁elds also.

7.2 Typ Types es and Problems Problems in Develop Developing ing High Strengt Strength h Ste Steels els The   󿬁rst high strength steels were the mild steels with a higher degree of  temper rolling to increase the yield strength. The main problem, however, was the low formability which restricted their use. Nevertheless, strengthening by cold reduction is an effective way of strengthening when a very low degree of formability is acceptable. Cold reduced steels continue to be used, therefore, for many strapping applications and in the zinc coated condition for corrugated roo󿬁ng panels. Attempts made to increase the strengt stre ngth h and ductil ductility ity (forma (formabil bility ity)) in steels steels led to the develo developm pment ent of  following grades of steels. 7.2.1 7.2 .1 Mod Modii󿬁ed Standard Low Alloy Steels

The modi󿬁cation was carried out to AISI 4340 and equivalent medium carbon nickel, chromium, and molybdenum steels. These steels can be oilquenched tempered toum any ar desired strength hardness. can ca n be impr imand prov oved ed by vacu vacuum arc c or el elec ectro trosl slag agand reme remelt ltin ing g to Properties lowe lowerr th thee hydrogen, nitrogen, and oxygen contents and to reduce the number of  nonme non metal tallic lic inclus inclusion ions. s. An impro improvem vement ent is obtain obtained ed by increa increasin sing g the silicon content of 4340 from 0.15 –0.30% to 1.45–1.80% to take advantage of the the well well-k -kno nown wn effe effect ct of sili silico con n in inhi inhibi biti ting ng th thee grow growth th of carb carbid idee particles. This steel, called 300 M, can be tempered at a higher temperature than than 4340 4340 to deve develo lop p th thee same same hard hardne ness ss,, th there ereby by redu reduci cing ng quen quench chin ing g stres str esse ses. s. The The addi additi tion on of si sili lico con n also also move movess th thee temp temper er embr embrit ittl tlem emen entt range to higher temperatures. Tempering at 315°C produces a maximum

in notch impact toughness and near maximum yield strength. The additional silicon also increases hardenability and adds a component of solid solution strengthening. A small addition of vanadium may provide   󿬁ne V(C, N) particles during tempering. Hy-Tuf is a modi󿬁cation of AISI 4130 with increased manganese and silicon with an addition of nickel. In D6AC the chromium and molybdenum contents are increased over those in 4340. Thes Th esee stee steels ls are also also call called ed as marte martens nsit itic ic st stee eels ls (MS (MS or Mart Mart). ). Seri Seriou ouss limitations in producing these conventional high strength steels are: (i) the

 

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associated reduction in fracture toughness and hydrogen embrittlement. Carbon is one of the strongest elements that can increase the strength by fo form rmin ing g le mart mato rten ensit siteerogen but bu affe afrittle fect cts sment th theet to toug ughn hnes Marte Maproble rtens nsit ite e isrelated mo re suscep sus ceptib tible hydrog hyd ent it embrit emb tlemen and also alsess. os.has pro blems ms relmore ated with fabricatio fabrication n   –   form formin ing, g, weld weldin ing, g, and and so on; on; (ii) (ii) th thee allo alloyi ying ng eleelement me ntss in incr crea ease se stren strengt gth h but but in incr crea ease se aust austen enit itiz izin ing g temp temper eratu ature ress and and alli allied ed prob proble lems ms duri during ng heat heat tr treeat atme ment nt;; (iii (iii)) the the co conv nven enti tion onal al high high strengt stre ngth h steels steels have have limite limited d formab formabili ility ty becaus becausee their their hig high h strengt strength h is developed prior to the forming process; and (iv) attention must be paid when wh en the the high high st stre reng ngth th st stee eels ls ar aree el elec ectr trop opllat ated ed,, as th ther eree have have been been many man y failur failures es of high high streng strength th steels steels into into which which hydrogen hydrogen was intro intro-duced during electroplating of protective surface layers. Concentration of a few parts per million is often suf 󿬁cient to cause failure. Although much hydrogen escapes from steel in the molecular form during treatment me nt,, some some can can re rem mai ain n and and prec precip ipit itat atee at inte intern rnal al surf surfac aces es suc such as inclus inc lusion ion/ma /matri trixx and carbi carbide/ de/mat matrix rix interf interfac aces, es, where where it forms forms voids voids or cracks. Hence, it is necessary in high strength steels to reduce carbon as low a level as possible, consistent with good strength. Developments in the technology of high alloy steels have produced high strengths in steels with very low carbon contents. The loss in strength due to carbon is compensated sat ed by othe otherr st stre reng ngth then enin ing g mech mechan anis isms ms such such as subs substi titu tuti tion onal al so soli lid d solution, grain re󿬁nement, precipitation hardening, and so on. The susceptib cep tibili ility ty to hydrog hydrogen en is overco overcome me by avoidi avoiding ng vulner vulnerabl ablee mar marten tensit sitic ic structu stru cture, re, as high high strengt strength h steels steels have have more more resi resistan stantt bainit bainitic, ic, ferrit ferritic, ic, and sphero spherodiz dized ed microst microstruc ructur turee by thermo thermomec mechan hanica icall tre treatm atment entss and controlled heat treatments. 7.2.2 Repho Rephosphor sphorized ized Steel

The main emphasis was the development of steels in which the loss in ductil duc tility ity with with increa increasin sing g streng strength th was minimi minimized zed.. Substi Substitut tution ional al solid solid solution strengthened steels were developed in which the loss in elongation per unit increase in strength is less for a solid solution strengthened steel. The   󿬁rst possibility to increase the strength of an alloy is to increase the content of atoms of solid solution elements such as phosphorus, silicon

and mang and mangan anes esee whic which h will will st stre reng ngth then en th thee steel steel.. Bu Butt th thee last last two two will will reduce the formability to an unacceptable level. However, the strength increase that could be obtained with a rephosphorized addition was limited by the of phosphorus on welding. Hence only small amounts ofdetrimental phosphoruseffect (between 0.04 and 0.08 %) can be tolerated which will increase the strength roughly by 40 –80 MPa. These steels were restricted to relatively modest strength increases with minimum yield stresses up to 300 MPa. The steels were highly formable. Li Limi mite ted d amou amount ntss of phos phosph phor orus us were were adde added d to IF st stee eel. l. Th Thes esee grad grades es

 

 High Strength Steels

 

69

called IF-HSS (high strength steels) have an ultimate tensile strength (UTS) of 360–400 MPa and good formability. 7.2.3 Micro Alloy Alloyed ed or High Strength Low Alloy (HSLA (HSLA)) Steels

By the middle of 1960s, the higher strength steels, based on micro alloy additions, had become established and each of the classi 󿬁cation societies introduced speci󿬁cations with yield stress values as a substitute to cold rolled high strength steels. The higher strengths are achieved by grain re󿬁nement and precipitation strengthening while the carbon content is main ma inta tain ined ed as lo low w as poss possib ible le and and also also th thee carb carbon on equi equiva vale lent nt (CE) (CE) is restricted to 0.41 % max for ease of welding and forming. Grain re 󿬁nementt and men and preci precipit pitati ation on harde hardeni ning ng is achie achieve ved d throug through h the additi addition on of  small amounts of carbide or carbonitride forming elements like niobium and titanium. During hot rolling, undissolved   󿬁ne particles restrict the grain growth of austenite grains. In the last hot rolling passes, austenite will wi ll not not re reccryst rystal alli lize ze anym anymor oree and and th thee grai grains ns will will be   󿬂att atten ened ed out. out. Durin Dur ing g subseq subsequen uentt cool cooling ing many many ferrit ferritee nuclei nuclei wil willl be activ activate ated d and and will provide a   󿬁ne ferrite grain size, which is a   󿬁rst important contribution to strengthening. Additional precipitates are also formed which can furth fu rther er stren strengt gthe hen n th thee ferr ferrit ite. e. A ty typi pica call HS HSLA LA steel steel ca can n have have a yiel yield d strength (YS) of about 320 MPa and an ultimate tensile strength (UTS) of 440 MPa, but the formability is too low for car body panels. HSLA stee steels ls can can howe howeve verr be used used fo forr struc structu tura rall ap appl plic icat atio ions ns,, fo forr exam exampl ple, e,  beams. Various types of HSLA were developed based on the needs and requirement. 7.2.4 Bake Hard Hardening ening (BH) Steels Steels

The steel The steel must must be re rela lati tive vely ly soft soft and and high highly ly de defo form rmab able le to make make in to desired shape. However, after forming, the part should be as strong as possible. The yield point phenomenon and strain aging help to solve this dilemma by strengthening the material during strain aging. When a plain low low carb carbon on stee steell is st strai raine ned d pl plas asti tica call lly y to a parti particu cula larr st strai rain n show showin ing g serrations (region A of  Figure   Figure 7.2) 7.2) called as yield point phenomenon due to lockin locking g of moving moving di dislo slocat cation ionss by the interst interstiti itial al carbon carbon or nitrog nitrogen en

atoms. If it is unloaded and reloaded again without any appreciable time delay or any heat treatment yield point does not occur since the dislocations have been torn away from the carbon and nitrogen atoms (region B  Figure   Figure 7.2). 7.2 ). ature If itreis or unloaded after aging for temper several days at of  room roo m temper tem peratu severa severall and hours houreloaded rs at slight slightly ly elevat ele vated ed tem peratu atures res (120–170°C), then yield point reappears and moreover, the yield point will  be increased (region C of  Figure   Figure 7.2). 7.2). This is due to diffusion of carbon and nitrogen atoms to dislocations during the aging to form new solute atmospheres anchoring the dislocations.

 

 

70

Introduction to Steels

 Z  Y   X 

  s   s   e   r    t    S  A

B

C 

Strain

FIGURE Loading 7.2 and unloading of plain low carbon steels showing yield point phenomenon and strain aging.

This phenomenon is used in bake hardening (BH) steels. The steel is formed in the region B and so-called   “paint baking cycle,”  is carried out at abo ab out 170º 170ºC C for for 20–30 minu minute tess es espe peci cial ally ly in car car bo body dy manu manufa fact ctur urin ing g leading to higher yield strength (point Z in  Figure 7.2 7.2). ). This increases the yield strength roughly by 50 MPa after paint baking cycle. The important caution is to eliminate strain aging during forming (region A in Figure in  Figure 7.2) 7.2)  because the yield point can lead to dif 󿬁culties with surface markings or stretcher strains due to localized heterogeneous deformation. The steels are subjected to about 2% reduction after the   󿬁nal pass of the rolling (skin pass rolling), which eliminates yield point serration and push the steel to region B so that the yield point phenomenon (region A in   Figure 7.2 7.2)) will not occur while making the actual components. Nitrogen plays a more important role in the strain aging of iron than carbon because it has a higher solubility and diffusion coef 󿬁cient and produces less complete precipitation during slow cooling. 7.2.5 Dual Phase (DP) and Trans Transforma formation tion Induced Plasti Plasticc (TRIP (TRIP)) Steels

Dual phase steels are low carbon steel grades with carbon content around 0.1%. These steels have soft ferrite and 10 –20% dispersed hard martensite islands giving tensile strengths and above 600 MPa. This microstructure is obtained by soaking in up thetointercritical range (~800ºC), followed  by rapid cooling. During intercritical intercri tical annealing, annealing , part of the austenite austen ite transforms into ferrite. DP steels have relatively high strain hardening values for their strength but the formability is low. One way to increase formability is to have small amount of austenite, which on straining is

 

 High Strength Steels

 

71

converted as martensite contributing to higher strength and formability. Based on this line of thought, transformation induced plasticity (TRIP) steels developed, which have some retained austenite in addition to the were dual phase (ferrite + martensite). 7.2.6 High Streng Strength th Steels by Thermo Mechanica Mechanicall Treatment Treatmentss

There is a limit to the strengthening that can be achieved by a combination of grain re󿬁nement, solid solution and precipitation effects. Thermomechanical nic al proces processin sing g has permit permitted ted the develo developm pment ent of high high strengt strength h steels steels with low carbon contents and this has contributed greatly to improved weldability and, therefore, employed to obtain values of tensile strength abov ab ovee 500 500 MPa, MPa, depe depend ndin ing g on whet whethe herr th thee st stee eell is in th thee hot hot roll rolled ed condition or has also been cold rolled and annealed. 7.2.6.1 7.2. 6.1 Patenting Patenting of Steel Wire Wiress

Steel wires are used extensively throughout the world for many critical engineering applications such as high strength cables for bridges, cable and ski lifts, general haulage, for example, ship moorings, and, on a large scale, for reinforcing radial tires. They are also widely used as piano and vi viol olin in stri string ngs. s. In all all case cases, s, th thei eirr prop proper erti ties es of very very high high st stre reng ngth th and and toughness are just about unique. The high strength steel wire is basically a fully eutectoid 0.8%C steel containing some manganese and silicon designed to develop the maximum amou am ount nt of pear pearli lite te,, th that at is is,, abou aboutt 100% 100%.. The The wire wire is st stre reng ngth then ened ed by a process called patenting. To produce   󿬁ne wire it starts as coiled rod of  diameter around 5 mm. In this condition, the strength is about 1100–1200 MPa and the aim of the patenting process is to multiply this by a factor of  about 3. The rod is   󿬁rst given a preliminary drawing reduction down to 1–2 mm diameter without lubrication. It is then passed into a furnace and heat he ated ed in the the rang rangee 950 950–10 1000 00ºC ºC,, th that at is is,, in th thee sing single le ph phas asee aust austen enit itee doma do main in for for tr tran ansf sfor orma mati tion on into into homo homoge gene neou ouss aust austen enit itee wi with th all all th thee carbon in solid solution. It is then very carefully cooled to transform into 󿬁ne pearlite. This can be done isothermally by cooling in liquid lead or in a   󿬂uidised bath or even by controlled air cooling and then drawn into thin

wire. The resulting work hardened pearlite is extremely strong   –  probably the the stro strong nges estt mate materi rial al know known n poss posses essi sing ng some some duct ductil ilit ity y an and d th ther eref efor oree toughness. The one of in the process. Also dur during ingcooling coolin cooling grate the isausten aus tenite itethe to critical pearli pearlite, te,parameters the exothe exothermi rmic c tra transf nsform ormati ation on give givess off off suf  suf 󿬁ci cien entt heat heat to in incr crea ease se th thee temp temper erat atur uree in th thee rang rangee of  transformation. The cooling rate allows for the heat of transformation to foll fo llow ow a   󿬁ne nely ly judg judged ed path path into into th thee pear pearli lite te doma domain in th that at lead leadss to th thee formation of a very   󿬁ne pearlite, that is, in the range of 500–600ºC (and

 

 

72

Introduction to Steels

without any bainite). The microstructure is then basically 100% pearlite with a spacing of about 0.25 micron. Patented wires are usually drawn up to strengths of about 3000 MPa for usesteel in tyre reinforcements. However, there is a trend to develop even higher strength wires by increasing the drawing strain and, in some cases, the carbon content. These wires are aimed to attain 4000 MPa. 7.2.6.2 7.2. 6.2 Severe Plastic Plastic Deforma Deformation tion of Steels

Patenting is limited to only thin wires and cables. Thick sections or sheets could not be made by patenting. Alternatively, severe plastic deformation meth me thod ods, s, espe especi cial ally ly equa equall chan channe nell angu angula larr pres pressi sing ng (ECA (ECAP) P) is unde underr research to develop high strength coupled with ductility in low carbon steels used in the construction industry.   Table 7.1   shows the properties achieved by the low carbon steel (Fe 450 grade) after ECAP. In ECAP, true shear strains more than 1 is possible. At these levels of  strain, and even before, the cementite layers break up and, surprisingly  begin to dissolve locally so that excess carbon goes into solution in the ferrite. And it is possible to obtain duplex microstructure of ferrite (about 13%) and martensite (about 87%) in the ECAPed low carbon steels upon quenching from intercritical temperature as shown in Figure in  Figure 7.3 achieving 7.3  achieving tensile strength of 638 MPa and 28% elongation. These types of steels show  better combination of strength and ductility than the as ECAPed samples due to dual phase structure of ferrite and martensite. In the dual phase microstructure, the ferrite contributes to the maximum elongation and the martensite contributes to the strength. However, this behavior is in the research level only and becoming popular now. These thermomechanical processes have limitation in industrial scales for the following reasons:

TABLE 7.1 Tensile strength and % elongation of annealed, as received and as ECAPed mild steel (Fe 450 grade) S.No .No

Sample condition

Tensile strength gth (MPa)

% elo lon ngation

1 2 3

 

as received

 

Annealed

 

ECAP at 200°C

4 5 6 7 8

   

ECAP at 250°C ECAP at 300°C

     

 

459 325 683

26 51 13

649 630 ECAP at 350°C 619 ECAP at 200°C –  2 passes   707 Intercritical annealed   638

12 13 13 12 28

 

 

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73

FIGURE 7.3 Microstructure of quenched ECAPed steel from intercritical temperature showing ferrite and martensitic structure.

1. L Larg argee redu reduct ctio ions ns in cr cros osss se sect ctio ion n must must be made made,, so larg largee sect sectio ions ns cannot be treated. 2. At the temperatures involved, loads on equipment will be severe. 3. The steel must be used in the shape in which it is formed since the fabricability is limited. 4. Joining is dif 󿬁cult; fusion welding is almost impossible. 7.2.7 Nanos Nanostruct tructured ured Steels

It has been possible for some time to produce iron in which the space 󿬁lling crystals are just 20 atoms wide. These samples were prepared from the vapour phase followed by consolidation. Although of limited engineering value, work of this sort inspired efforts to invent methods of making large quantities of steels with similar   󿬁ne grain structures. Modern technologies allow steels to be made routinely and in large quantities with grai gr ain n si size zess limi limite ted d to a mini minimu mum m of abou aboutt 1 micr micron on by reco recoal ales esce cenc ncee effects. Processes generally involving severe thermomechanical processing have been developed to achieve nanostructured ferrite grains in steel, in

the range of 20–100 nm. Although the nanostructured steels are strengthened en ed as expe expect cted ed from from th thee Hall Hall–Pe Petc tch h equa equati tion on,, th they ey tend tend to exhi exhibi bitt unstable plasticity after yielding. The plastic instability occurs in both tension and in compression testing, shear bands causing failure in the latter case. Also, the steels lack with ductility due to nanograins, which dimini dim inish sh work work harden hardening ing capaci capacity. ty. The conven conventio tional nal mechan mechanism ismss of  dislocation multiplication fail in nanograined steels because of the proximity of the closely spaced boundaries which then become impossible to accumulate dislocations during deformation. Grain boundaries are also

 

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good sinks for defects. These effects cause nanostructured materials not to work harden. 7.2.8 Bainiti Bainiticc Stee Steels ls

There are large markets for steels with strengths less than 1000 MPa, and where the total alloy concentration rarely exceeds 2 %. Bainitic steels are well suited for applications within these constraints. The bainitic structure has better hardness, strength and ductility comparable to hardened and tempered martensite. If a steel is cooled rapidly from an austenite state to a temperature close to 450°C, the ferrite and pearlite reactions are suppressed and the austenite will transform to a lower temperature transformation product called bainite and is called austempering explained in the previous chapter. However, alloy design must be careful in order to obtain the right right micro microstr struct ucture ures. s. Steels Steels with with inadeq inadequat uatee harden hardenabi abilit lity y ten tend d to transform to mixtures of allotriomorphic ferrite and bainite. Attempts to improv imp rovee harden hardenabi abilit lity y usuall usually y lead lead to partia partially lly mar marten tensit sitic ic micro microstru strucctures. The solution solution therefore lies in low alloy, low carbon carbon steels, steels, containin containing g small amounts of boron and molybdenum to suppress allotriomorphic ferrite formation. Boron increases the bainitic hardenability. Other solute addi ad diti tion onss can, can, in the the pres presen encce of boro boron, n, be kept kept at suf  suf 󿬁cie cientl ntly y low concentrations to avoid the formation of martensite. A typical composition (in wt%) might be Fe–0.1C–0.25Si–0.50Mn–0.55Mo   –   0.003B. 0.003B. Steels Steels like these are found to transform into virtually fully bainitic microstructures with very little martensite using normalizing heat treatments. The reduced alloy concentration not only gives better weldability, but also a higher strength due to the re󿬁ned bainitic microstructure. These steels are made with very low impurity and inclusion contents, to avoid the format for mation ion of cement cementite ite partic particles les.. Some Some typica typicall all alloy oy compos compositi itions ons are given in Table in  Table 7.2. 7.2. Medium Med ium strengt strength h steels steels with with the same same mic micros rostru tructu cture re but somewh somewhat at reduced alloy content have found applications in the automobile industry as crash reinforcement bars to protect against sidewise impact. Another major advance in the automobile industry has been in the application of   bainitic forging alloys to manufacture manufact ure components such as cam shafts. shafts . These were previously made of martensitic steels by forging, hardening,

tempering, straightening and   󿬁nally stress relieving. All of these operati tion onss are are now now re repl plac aced ed by contr ontrol olle led d cool coolin ing g from from th thee die die fo forg rgin ing g temperature, to generate the bainitic microstructure, with cost savings which occasions for the on entire unit. have made the difference between pro󿬁t and loss By inocul inoculati ating ng molten molten steel steel with with contro controlle lled d add additi itions ons of nonmet nonmetall allic ic part pa rtic icle les, s, bain bainit itee can can be in indu duce ced d to nucl nuclea eate te intra intragr gran anul ularl arly y on th thee inclusions, rather than from the austenite grain surfaces. This intragranularly nucleated bainite is called   “acicular ferrite.”   It is a much more

 

 High Strength Steels

 

75

TABLE 7.2 Chemical composition, wt%, of typical bainitic steels Alloy

C

Si

Mn

Early bainitic steel Ultra-low carbon Ul Ultr traa-h high igh stren treng gth Creep resistant Forging alloys Inoculated Nanostructured

0.10 0.02 0. 0.20 20 0.15 0.10 0.08 1.00

0.25 0.20 2.0 .000 0.25 0.25 0.20 1.50

0.5 2.0 3. 3.00 0.5 1.0 1.4 1.9

Ni

Mo

–

–

0.3

0.30

 

–

–

 

–

 

  1.00 0.5 1.00

 

–

–

 

–

  0.26

Cr

 

V

0.003

  –

B

Nb

Other

–

–

–

  –

–

  0.01

–

–

–

–

–

–

–

–

–

  –

–

–

–

–

–

1.26

0.1

2.30

 

  –

   

0.05

0.10 0. 0.110 –

  –

  –

0. 0.01 0122 Ti –

󿬂

di diso sorg rgan aniz ized ed micr micros ostr truc uctu ture re with with commercially a larg larger er ab abil ilit ity y to ectt used ec crac cracks ks. Inoculated steels are now available and arede being in. demanding structural applications such as the fabrication of oil rigs for hostile environments. Advances in rolling technology have led to the ability to cool the steel plate rapidly during the rolling process, without causing undue distortion. This has led to the development of   “accelerated cooled steels,”  which have a bainitic microstructure that are highly formable and compete with conventional control rolled steels. It would be nice to have a strong material with good toughness without requ requir irin ing g mech mechan anic ical al proc proces essi sing ng or ra rapi pid d cool coolin ing g to reac reach h th thee desi desire red d properties. The following conditions are required to achieve this: (i) The material material must not rely on perfec perfectio tion n to achiev achievee its proper propertie ties. s. Stre Streng ngth th can can be gene genera rate ted d by in inco corp rpor orat atin ing g a larg largee nu numb mber er of  defects such as grain boundaries and dislocations, but the defects must not be introduced by deformation if the shape of the material is not to be limited. (ii) Defects Defects can be introduced introduced by phase transformat transformation, ion, but to disperse disperse them on a suf 󿬁ciently   󿬁ne scale requires the phase change to occur at large under cooling (large free energy changes). Transformation at

low temper temperatu atures res also also has the advant advantage age that that the micros microstru tructu cture re  becomes re󿬁ned. (iii)) A strong material (iii material must be able to fail in a safe manner. manner. It should be tough. (iv)) Recale (iv Recalesce scenc ncee (a tempor temporary ary rise rise in temper temperatu ature re durin during g coo coolin ling g of  a metal, caused by a change in crystal structure) limits the undercooling that can be achieved. Therefore, the product phase must be such that it has a small latent heat of formation and grows at a rate which allows the ready dissipation of heat.

 

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Introduction to Steels

Recent discoveries have shown that carbide free bainite can satisfy these criteria. Bainite and martensite are generated from austenite without diffusion by a displacive mechanism. Not only does this lead to solute trapping  but also a huge strain energy term, both of which reduce the heat of  transformation. The growth of individual plates in both of these transformations is fast, but unlike martensite, the overall rate of reaction is much smaller for bainite. This is because the transformation propagates by a sub unit mechanism in which the rate is controlled by nucleation rather than growth gro wth.. This This dimini diminishe shess recale recalesce scence nce.. This This leads leads to the develo developm pment ent of  nanostructured bainitic steels. 7.2.8.1 7.2. 8.1 Nanostructu Nanostructured red Bainitic Bainitic Steels

The major setback in the nanograined material is the lack of ductility due to the loss of work hardening capacity. This can be resolved by introducing retai retaine ned d aust austen enit itee in th thee micr micros ostr truc uctu ture re.. The The strai strain n or st stre ress ss indu induce ced d martensitic transformation of this austenite enhances the work hardening coef 󿬁cient, making it possible to get substantial ductility in nanostructured steels. However, the amount of austenite must be above a threshold level, whic wh ich h is esti estima mate ted d to be abou aboutt 10 vol. vol. %. The The same same meth method odol olog ogy y is adopted in nanostructured bainitic steels. However, there is a possibility of form formati ation on of coar coarse se ceme cement ntit itee parti particl cles, es, whic which h are de detri trime ment ntal al fo forr toughness. The precipitation of cementite during bainitic transformation can be suppressed. This is done by alloying the steel with about 1.5 % of  silicon, which has a very low solubility in cementite and greatly retards its growth gro wth.. An intere interesti sting ng micros microstru tructu cture re result resultss when when this this silico silicon n alloye alloyed d steel is transformed into upper bainite. The carbon that is rejected into the residua resi duall austen austenite ite,, instea instead d of precip precipita itatin ting g as cement cementite ite,, remain remainss in the austen aus tenite ite and stabili stabilizes zes it down down to ambien ambientt temper temperatu ature. re. The result resulting ing micros mic rostru tructu cture re consis consists ts of   󿬁ne pl plat ates es of bain bainit itic ic ferr ferrit itee se sepa para rate ted d by carbon enriched regions of austenite. With these precautions an alloy has been designed with the approximate composition Fe–1C–1.5Si–1.9Mn–0.25Mo–1.3Cr–0.1V %, which on transformati ma tion on at 200° 200°C, C, le lead adss to bain bainit itee pl plat ates es th that at are are on only ly 20–40 nm th thic ick k separated by carbon enriched   󿬁lms of retained austenite. This is the hard-

est ever bainite, that can be manufactured in bulk form, without the need for rapid heat treatment or mechanical processing. It is important to note that that it cons consis ists ts only only of two two phas phases es,, slen slende derr plat plates es of bain bainit itic ic ferr ferrit itee in a matrix of carbon-enriched austenite. The slend slender er pl plate atess of bainit bainitee are disper dispersed sed in stable stable car carbon bon enrich enriched ed austenite austen ite,, with with its face face centre centred d cubic cubic lattic lattice, e, buffer bufferss the propag propagati ation on of  cracks. The bainite obtained by transformation at very low temperatures is the hardest ever (700HV, 2500 MPa), has considerable ductility (5 –30%) and is tough (30–45 MPa m1/2) and does not require mechanical processing or rapid cooling. The steel after heat treatment therefore does not have

 

 High Strength Steels

 

77

long range residual stresses. It is very cheap to produce and has uniform properties in very large sections. The reason for the high strength is well understood from the scale of the micros micr ostr truc uctu ture re and and the the deta details ils of th thee comp compos osit itio ions ns and and fra fract ctio ions ns of th thee phases. However, the stress versus strain behavior is fascinating in many respects. Virtually all of the elongation is uniform, with hardly any necking. Indeed, the broken halves of each tensile specimen could be neatly   󿬁tted together. It is not clear what determines the fracture strain. In effect, the hard  bainite  baini te has achieved all of the essential objectives objectives of structural structural nanomaterial nanomaterialss that are the subject of so much research, in large dimensions. The potential advantages of the mixed microstructure of bainitic ferrite and austenite can be listed as follows: 1. Cementite is responsible for initiating fracture in high strength steels. Its the microstructure more resistant to cleavage failureabsence and voidmakes formation. 2. The bainitic ferrite is almost free of carbon. 3. The microstructure derives its strength from the ultra󿬁ne grain size of  the the ferr ferrit itee pl plat ates es,, whic which h are are le less ss th than an 1μm in th thic ickn knes ess. s. It is th thee thickness of these plates that determines the mean free slip distance, so that the effective grain size is less than a micrometer. This cannot  be achieved by any other commercially viable process. It should be  borne in mind that grain re󿬁nement is the only method available for simultaneously improving the strength and toughness of steels. 4. The ductile   󿬁lms of austenite that are intimately dispersed between the plates of ferrite have a crack blunting effect. They further add to toug toughn hnes esss by in incr crea easi sing ng th thee work work of frac fractu ture re as th thee aust austen enit itee is induced to transform to martensite under the in󿬂uence of the stress 󿬁eld of a propagating crack. 5. The diffusion of hydrogen in austenite is slower than in ferrite. The pres pr esen ence ce of auste austeni nite te can, can, th there erefo fore re,, impr improv ovee th thee stres stresss corr corros osio ion n resistance of the microstructure. 6. Steels with the bainitic ferrite and austenite microstructure can be obtained without the use of any expensive alloying additions. All that

is required is that the silicon concentration should be large enough to suppress cementite. High strength nanobainitic steels are not as popular as quenched and tempered martensitic steels, because, in spite of these appealing features, the the bain bainit itic ic ferr ferrit ite/ e/au auste steni nite te micr micros ostru truct ctur uree do does es not not alwa always ys give give th thee expected good combination of strength and toughness. This is because the relatively relati vely large   “ blocky”   regi region onss of aust austen enit itee be betw twee een n th thee shea sheave vess of   bainite readily transform into high carbon martensite under the in󿬂uence

 

78

 

Introduction to Steels

of stress. This untempered, hard martensite embrittles the steel. This high carbon austenite can be stabilized by adding manganese and/or nickel so that that the the marte martens nsit itic ic trans transfo form rmat atio ion n ceas ceases es.. Thus Thus,, a Fe–4Ni–2Si–0.4C% (3.69Ni, 3.85Si at %) alloy has been developed.

7.3 Met Metallu allurgy rgy of High Str Streng ength th Stee Steels ls High strength steels (HSS) are complex, sophisticated materials, with carefully ful ly select selected ed chemi chemical cal compos compositi itions ons and multi multiph phase ase micro microstr struct ucture uress resulting from precisely controlled heating and cooling processes. Various strengthening mechanisms are employed to achieve a range of strength, ductility, toughness, and fatigue properties. These steels are not the mild steels of yesterday; rather they are uniquely lightweight and engineered to meet the challenges of today’s veh vehicl icles es for stringe stringent nt safety safety regula regulatio tions, ns, re redu duce ced d emis emissi sion ons, s, and and soli solid d perf perfor orma manc nce, e, at affo afford rdab able le co cost st.. It is a continuous challenge for the steel industry to develop steel grades that combine high strength with an acceptable ductility. HSS are produced by controlling the chemistry and cooling rate from the austenite or austenite plus ferrite phase, either on the run out table of the hot mill (for hot rolled products) or in the cooling section of the continuous anneal ann ealing ing furnac furnacee (conti (continuo nuousl usly y anneal annealed ed or hot dip coated coated produc products). ts). Research has provided chemical and processing combinations that have created many additional grades and improved properties within each type of AHSS. Conven Con ventio tional nal low to high high streng strength th steels steels (mild, (mild, interst interstiti itial al fre free, e, bake bake hardenable, and high strength low alloy steels) have simpler structures. AHSS are different from the conventional HSS because they were developed for increased strength and ductility for enhanced formability. The principal difference between conventional HSS and AHSS is their microstructure. struct ure. Conventional Conventional HSS are single single phase ferritic steels with a potential potential for for som some pear pearllit itee in C-Mn C-Mn st stee eels ls.. AHSS HSS are are prim primaarily rily st stee eels ls with a microstructure containing a phase other than ferrite, pearlite, or cementite   –  for example, martensite, bainite, austenite, and/or retained austenite

in quanti quantitie tiess suf 󿬁cie cient nt to produc producee uniqu uniquee mechan mechanic ical al proper propertie ties. s. Some Some type typess of AHSS AHSS have have a high higher er st stra rain in hard harden enin ing g ca capa paci city ty resu result ltin ing g in a strength ductility balance superior to conventional steels. In martensitic steels (MS), nearly all austenite is converted to martensite.  󿬁

The Th eiteeresu result ing gormart ma ensi siti c matr ma trix cont coisntai ains ns aure smal sm am amou ount of ve ne fe ferr rrit and/ anltin d/or bain barten init ite e ticphas ph ases es. .ix This Th st stru ruct ctur e all typi tyl pica call lly yntfo form rmsvery s ry duri during ng a swift quench following hot rolling, annealing, or a post forming heat treatment. Increasing the carbon content increases strength and hardness. The resulting structure is mostly lath (platelike) martensite. Careful combination nat ionss of silico silicon, n, chrom chromium ium,, mangan manganese ese,, boron, boron, nickel nickel,, mo molyb lybden denum, um,

 

 High Strength Steels

 

79

and/or vanadi and/or vanadium um can increa increase se harden hardenabi abilit lity. y. The result resulting ing marten martensit sitic ic steel is best known for its extremely high strength; UTS from 900 to 1700 MPaa have MP have been been obta obtain ined ed.. MS has has re rela lati tive vely ly low low elon elonga gati tion on,, but but post post quench tempering can improve ductility, allowing for adequate formability considering its extreme strength. Often used where high strength is critical, MS steel is typically roll formed and may be bake hardened and electrogalvanized for applications requiring corrosion resistance, but heat treating decreases its strength. Because MS steel has such high strength to weight ratio, it is weight and cost-effective. Achieving the combination of both high strength and high ductility in the same steel has long been elusive metallurgically. Existing adva ad vanc nced ed high high st stre reng ngth th steel steelss re requ quir iree th that at custo custome mers rs make make a trade tradeof off  f   between high strength parts that are designed using more limited geometries or using more expensive production methods such as hot stamping. The processing of HSS and steels. AHSS grades are somewhat novelmetallurgy and uniqueand compared to conventional Table 7.3   lists few most commonly used high strength and advanced high strength steels. Unlike low carbon steels, each high strength steel grade is identi󿬁ed by metallurgical type, minimum yield strength (in MPa), and mini mi nimu mum m te tens nsil ilee st stre reng ngth th (in (in MPa) MPa).. As an exam exampl ple, e, DP 500/ 500/80 8000 mean meanss a dual phase steel with 500 MPa minimum yield strength and 800 MPa minimum ultimate tensile strength.

TABLE 7.3 Mechanical properties most common high strength and advanced high strength steels. Steel grade

YS, MPa

     

BH 210/340 BH 260/370 DP 280/600 IF 300/420 DP 300/500

   

210 260 280 300 300

UTS, MPa

340 370 600 420 500

Elongation, %

34–39 29–34 30–34 29–36 30–34

 

 

350 350 400 450

450 600 700 800

23–27 24–30 19–25 26–32

   

500 700 70 700 950 1250

800 800 1000 1200 1520

14–20 10–15 12–17 5–7 4–6

HSLA 350/450 DP 350/600 DP 400/700

   

TRIP 450/800 DP 500/800 CP 700/800 DP 700/1000 Mart 950/1200 Mart 1250/1520

     

 

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Introduction to Steels

7.4 Applic Applications ations P Particu articularly larly for Automotiv Automotives es The major application of high strength steels being framed by automotive manufacturers is in automotives. The family of high strength steels (HSS) continues to evolve and grow in application, particularly in the automotive industry. New steel types are already being used to improve the performance of vehicles on the road, and emerging grades will be increasingly employed. By 1975, the average vehicle contained 3.6% medium and high strength steels for a total vehicle content of 61%, mostly mild steel. In the 1980 19 80s, s, the the use use of in inte ters rsti titi tial al free free (I (IF) F) and and galv galvan aniz ized ed st stee eels ls grew grew fo forr complex parts. By 2007, the average vehicle contained 11.6% medium and high strength steels, for a total of 57%. The use of HSS in automobiles is quickly expanding with more research. High strength low alloy (HSLA) steels, which had been used for major construction projects such as the Alaska Arctic Line Pipe Project in the 1970s, were increasingly developed and selected for automotive applications through the 1990s for their consistent strength, toughness, weldabilit ity, y, and and lo low w cost cost.. Many Many auto automo moti tive ve anci ancill llar ary y pa part rts, s, body body st stru ruct ctur ure, e, suspension and chassis components, as well as wheels, are made of HSLA steel. stee l. Bake Bake harden hardening ing (BH) (BH) steels steels are most most commo commonly nly used used for making making closure panels like door outers, hoods, and decklids, as it can be hardened during painting cycle itself and have high dent resistance. Dual phase (DP) steels are excellent in the crash zones of the car for their high energy absorption. These steels are used for automotive body panels, wheels, bumpers, crash boxes, support components, A, B, C, and D pillars in cars and box girders for chassis as they can be engineered or tailored to provide excellent formability for manufacturing complex parts for automobiles. DP is increasingly used by automakers in current car models. For example, in the 2011 Chevrolet Volt, the overall upper body structure is 6% DP by mass, and the lower structure is 15%, including such parts as the reinforcement for the rocker outer panel. Transformation induced plasticity (TRIP) steels are some of the newest in development, but steel companies are quickly offering a greater variety of TRIP steels for automotive applications. They boast of its wide applic-

ability, abilit y, especi especiall ally y in compli complicat cated ed parts, parts, and its high high pot potent ential ial for mass mass savi sa ving ngs. s. It can can now now be obta obtain ined ed in a vari variet ety y of grad grades es.. Auto Automo moti tive ve appl ap plic icat atio ions ns of TRIP TRIP incl includ udee body body struc structu ture re and and anci ancill llar ary y pa part rts. s. With With high energy absorption and strengthening under strain, it is often selected for components that require highAcrash such member mem bers, s, longit longitud udina inal l beams, beams, and energy B pillar pillarmanagement, reinfo reinforce rcemen ments, ts, sills, silas ls,cross and  bumper reinforcements. For example, in 2007 Honda introduced the use of  TRIP in the frame and side structure of the MDX, RDX, and CRV. Patented steel wires are used in new generation tyres for heavy duty trucks and personal vehicles. Martensitic steels (MS) often used for body

 

 High Strength Steels

 

81

structures, ancillary parts, and tubular structures due to its high strength to weight ratio and cost effectiveness. MS grades are recommended for  bumper reinforcement and door intrusion beams, rocker panel inners and reinforcements, side sill and belt line reinforcements, springs, and clips. For example, in the 2007 Honda Acura MDX, the rear martensitic bumper  beam assists in rear crash protection. In the past past severa severall years, years, autom automake akers rs have have increa increasin singly gly inc incorp orpora orated ted HSS and AHSS into vehicles, especially structural and safety components. Research at Ducker Worldwide predicts that 50% of the average body in white (BIW) will be converted to HSS in this decade. Proposed regulations for 2017–2025 will impose much stricter Corporate Average Fuel Economy (CAFE) standards and, if passed, would potentially boost HSS use signi󿬁cantly. Current research aims to continue to expand the broad spectrum of  HSS. HS S. One One ar area ea of part partic icul ular ar inte intere rest st is th thee   “third generation generation steels”   to  bridge the and gap the of strength-elongation balance conventional and HSS steels austenitic-based steels. Somebetween materials under development include nanosteels, bainitic steels, and so on.

 

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8  High Alloy Steels

8.0 Int Introd roduct uction ion The steels with more than 10% alloying elements are called high alloy steels. The most commonly used high alloy steels are maraging steels, stainless steels, and tool steels. This chapter describes these steels.

8.1 Mar Maragi aging ng S Stee teels ls Conventionally, steels with yield strength higher than 700 MPa are called ultra-high strength steels. This level of strength can be achieved in normal lo low w al allo loy y stee steels ls with with high high carb carbon on and and allo alloy y cont conten ents ts as disc discus usse sed d in previous chapters. However these steels will have problems such as: (i) The strength depends mainly on carbon content. As carbon content increases weldability, machinability and formability are decreased (ii) Special controls are needed to avoid decarburization (iii) Distortion due to quenching (iv) Will require high austenitizing temperature and (v) Hydrogen embrittlement is a serious problem.

Many comp Many compon onen ents ts in aero aerosp spac acee and and spac spacee ve vehi hicl cles es need need st stee eels ls with with strength levels more than 1500 MPa and also without the problems mentioned above and this resulted in the discovery of MARtensite AGING steels called maraging steel, classi󿬁ed under ultra-high strength steels. The development of the nickel maraging steels began in Inco research la labo bora rato tori ries es in th thee la late te 1950 1950ss and and was was base based d on th thee conc concep eptt of usin using g substitutional elements to produce age hardening in a low carbon nickeliron martensitic matrix. Balanced additions of cobalt and molybdenum to iron nickel martensite gave a combined age hardening effect appreciably greater than the additive effects of these elements separately. The iron-nickel83  

 

84

Introduction to Steels

FIGURE 8.1 Microstructure of maraging steels

870

760 Heating  γ

650

90 percent transformed

10 percent transformed

540    C    °  ,   e   r   u 430    t   a   r   e   p   m   e    T 320

210

α α+γ

 γ 90 percent transformed

10 percent transformed α

Cooling

100

α+γ

0 0

5

10

15

20

25

30

35

Nickel, percent

FIGURE 8.2 Iron-nickel phase diagram showing hysteresis loop

 

 High Alloy All oy Steels

 

85

cobalt-molybdenum matrix was amenable to supplemental age hardening by small addition of titanium and aluminium. This resulted in the development of nickel-cob nickel-cobalt-m alt-molyb olybdenum denum family of maraging maraging steels. First commerci commercial al heat of maraging steel was made in December 1960. 8.1.1 Metall Metallurgy urgy of Maraging Maraging Steels

These steels have low carbon (less than 0.03%) and 20 –25% nickel and other additions such as titanium, aluminium, chromium, cobalt, and so on. The iron nickel martensite (Figure (Figure 8.1) 8.1) formed is very ductile due to the low carbon content and tempering is not required. The transformation temperature of austenite to martensite (α) upon heating and cooling has wide wide differ differenc encee (hystere (hysteresis sis)) as shown shown in   Figure 8.2. 8.2. So on reheating the iron nickel martensite, the martensite to austenite transformation will not occur at the same temperature at which austenite to martensite transformation occurred. This temperature difference, called as hysteresis, facilitates itat es for preci precipit pitati ation on hardeni hardening ng (aging) (aging) and thus thus the maragin maraging g stee steell is strengt stre ngthen hened ed by both both martens martensite ite and precip precipita itates tes result resulting ing in ultraultra-hig high h strength. If nickel content is more than 20%, austenite is retained upon cooling and martensite will not form. These type of steels can also be aged, that are called as ausaged steels. However, the strength of the ausaged steels are less than that of maraging steels and applications of ausaged steels are limited. 8.1.2 Effect of Com Composit position ion 8.1.2.1 Cobalt 8.1.2.1 Cobalt and Molybdenum Molybdenum Coba Co balt lt stren strengt gthe hens ns th thee matri matrixx by soli solid d solu soluti tion on st stre reng ngth then enin ing g wh whil ilee moly mo lybd bden enum um di disso ssolv lves es in th thee matri matrixx as well well as fo form rmss carb carbid ides es.. The The in incr crea ease se in stre streng ngth th of marag maragin ing g steel steelss is much much high higher er if bo both th coba cobalt lt and molybdenum are added together rather than individually as shown in  Figure 8.3. 8.3. Thee reas Th reason on fo forr th thee syne synerg rgic ic effe effect ct is as fo foll llow ows: s: Coba Cobalt lt fo form rmss soli solid d

solution with iron and nickel and does not form either intermetallics or carb ca rbid ides es to re resp spon ond d for for agin aging. g. On th thee ot othe herr ha hand nd,, moly molybd bden enum um fo form rmss carbides as well as strengthens the matrix by solid solution strengthening. When molybdenum alone is added in maraging steels, to some extent it dissolves in austenite and not available for forming carbide/intermetallic prec pr ecip ipit itat ates es.. Inte Intere resti sting ngly ly,, addi additi tion on of coba cobalt lt redu reduce cess th thee solu solubi bili lity ty of  molybdenum in austenite and releases molybdenum for precipitation. The addi ad diti tion on of moly molybd bden enum um and and coba cobalt lt incr increa ease sess notc notch h tens tensil ilee st stre reng ngth th,, ensures high value of reduction in area, and also reduces the tendency to stress corrosion cracking.

 

86

 

Introduction to Steels

FIGURE 8.3 Synergic effect of cobalt and molybdenum in maraging steels

8.1.2.2 8.1. 2.2 Titanium Titanium

Titanium is the main precipitation hardener in maraging steels. The addition of titanium between 0.6–0.7% does not affect notch tensile strength (NTS) and melting characteristics. However, more than 0.7% titanium affects air melting characteristics. Titanium also reduces carbon and nitrogen by forming TiC or TiN from martensite making martensite more ductile and tough. 8.1.2.3 8.1. 2.3 Silicon Silicon and Manganes Manganesee

Both silicon and manganese are detrimental elements in maraging steels since both decrease NTS. In many grades of steels nickel is replaced with manganese to stabilize the austenite but it is not used in maraging steels  because it has a negative effect on NTS.

8.1.2.4 8.1. 2.4 Carbon Carbon

The carbon content is restricted to a maximum of 0.03% as more than that reduces NTS although it is an economical hardener in steels. 8.1.2.5 8.1. 2.5 Aluminiu Aluminium m

Aluminium is not added intentionally as alloying element but up to 0.1% is allowable as it is used as deoxidizer during steel melting. Higher than 0.1% of aluminium decreases toughness.

 

 High Alloy All oy Steels

 

87

8.1.2.6 8.1. 2.6 Calcium Calcium

Maximum allowable limit is 0.02% as an impurity. 8.1.2.7 8.1. 2.7 Boron Boron and Zirconiu Zirconium m

Boron and zirconium are added about 0.003% and 0.01%, respectively, as  both retard grain boundary precipitation and thus increase toughness and reduce the tendency of stress corrosion cracking. 8.1.2.8 8.1. 2.8 Chromium Chromium

Chromi Chro mium um is adde added d in mara maragi ging ng steel steelss wher wheree ox oxid idat atio ion n resi resista stanc ncee is required. Maraging steels alloyed with chromium have substantially greater resistance to oxidation in air at 540°C than a 5% chromium tool steel. 8.1.3 Types and Compo Composition sition of Maraging Maraging Steels

Maraging steels are basically iron nickel alloys with titanium. The other alloying elements such as cobalt, molybdenum, and chromium are added to improve various properties. The types are (i) Ni-Ti, (ii) Ni-Co-Mo-Ti-Al, (iii) Ni-Cr, and (iv) low alloy maraging steels. Like low alloy steels, there is no standardized designation. The numbers ascribed to the various grades of mara maragi ging ng st stee eels ls ar aree mostl mostly y base based d on proo prooff stres stresse sess in SI unit units. s. For For example, 18Ni1400 indicates the grade has 18% nickel and yield strength is 1400 MPa. It varies from country to country: for example, in the United States, the numbering refers to the nominal 0.2% proof stress values in kilo pounds per square inch (18Ni 250). Many grades are also numbered by the manufacturers.   Table 8.1 manufacturers. 8.1 shows  shows composition and yield stress of the most commonly used maraging steels. 8.1.4 Mecha Mechanical nical Prope Properties rties

Maraging steels have ultra-high strength combined with high ductility and

high fracture toughness. Further, the strength of the maraging steels can be increased by cold working and subsequent aging. This type of process is called marfor mar formi ming. ng. The maragi maraging ng steels steels have have good good impac impact, t, fatigu fatigue, e, and fractur fracturee toughness. The impact fatigue limit of maraging steels ranges from 400 to 500 MPa. Thefatigue major requirements for aerospace andtorocket cases are high strength to density and highlanding tensile gears strength density ratios and these are well satis󿬁ed by maraging steels. Table steels.  Table 8.2 shows 8.2  shows the fatigue strength to density and tensile strength to density ratios of maraging steels along with other candidate materials.

 

 

88

Introduction to Steels

TABLE 8.1 Composition and its yield stress of the most commonly used maraging steels Nominal Composition, wt % Grade

Proof stress, MPa

Ni-Ti type 25 Ni  

1900 20 Ni   1900 Ni-Co-Mo-Ti-Al type 18Ni1400   1400 18Ni1700   1700 18Ni1900   1900 18Ni2400   2400

Ni

25 20

Co

Mo

 

–

–

 

–

–

T Tii

   

1.5 1.5

Cr

Al

Others

 

–

–

–

 

–

–

–

0.1   0.1   0.1   0.15  

–

18 18 18 17.5

8.5 8 9 12.5

3 5 5 3.75

0.2 0. 0 .4 0. 0 .6 1.8

 

–

 

–

 

–

 

–

       

17Ni1600 (cast)   1600 Ni-Cr type

17

10

4.6

0.3

 

–

  0.05

  1270 IN 733   1562 15 IN 736   1270 IN 833   920 Low alloy maraging steels IN 863   994 IN 335   994 IN 787   631 IN 866   631

12 10 10 7

3 2

0.2 0.3 0. 0.2

–

–

–

12–5–3

3 3 0.8 1

 

–

 

 

–

–

 

–

  0.5   0.2   0.2

   

– –

 

 

–

 

–

 

–

3 12 12 10   12        

0.3 0.7 0.3  

3   3   0.6   0.5  

– – –

  –

 

–

 

–

 

–

–

 

1 Si

–

       

0.5 Si 0.5 Si 0.3 Si, 1.1Cu, 0.03Nb 0.4 Si, 1.3 Cu, 1.8 Mn

– – –

TABLE 8.2 Fatigue strength to density and tensile strength to density ratios of  candidate materials Allo loy y

Fatigue strength/densi sitty

Tensile st strrength/d th/de ensity

 

Al 2024 Beta Ti alloy

 

  Maraging steels  

8620 steels

30 32 21 25

68 120 57 99

Though the best material is beta titanium alloys, it is not economical for commercial vehicles due to its high processing cost. The next best material is Al 2024 but the cross section area required to withstand the fatigue load re resu sult ltss in bulk bulky y comp compon onen entt and and more moreov over er,, th thee tens tensil ilee st stre reng ngth th to dens densit ity y ratio ratio is less. Hence the best alternative is maraging steels for aircraft landing gears.

 

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89

The wear resistance of maraging steels are inferior to hardened/casehardened steels in low stress abrasion conditions. However, case-nitrided maraging steels are better for high stress abrasion conditions because of its hard case and soft core. For gouging wear applications, maraging steels are  better than austenitic stainless steels and Had󿬁eld manganese steels due to its high toughness. The fatigue strength of maraging steels varies between 0.41 and 0.53 of  its tensile strength like conventional steels. However, maraging steels does not show any de󿬁nite endurance limit like ferrous alloys and titanium. 8.1.5 Effect of Solution Solutionising ising and Aging Temperature Temperature on Mechanical Properties

Thee achi Th achiev evab able le st stre reng ngth th of th thee mara maragi ging ng steel steel wi with th resp respec ectt to th thee solu soluti tion onizi izing ng temperature is schematically shown in Figure in  Figure 8.4. 8.4. The trend is more or less similar for most of the grades and types of maraging steels. As the solutionizing temperature increases, the tensile strength increases till a critical temperature above above which there there is drop in strength. strength. Below the the critical tempe temperature rature,, the precipitation strengtheners such as carbides, nitrides, and carbonitrides of  titanium do not completely dissolve in the matrix resulting in coarse carbides and inhomogeneous distribution of the precipitates. Above the critical temperature, it is observed that the austenite is not converted to martensite comp co mple lete tely ly le leav avin ing g some some amou amount nt of re retai taine ned d auste austeni nite te that that reduc reduces es th thee strength of the maraging steel. The critical temperature is about 760°C that may vary slightly depending on the alloying additions. However, the properties drastically come down if heated above 950°C due to uncontrollable grain growth. Care has to be taken in this regard.

    h    t

  g   n   e   r    t    S   e     l    i   s   n   e    T

Critical temperature

Solutionising Temperature

FIGURE 8.4 Effect of solutionizing temperature on mechanical properties

 

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Similar to solutionizing temperature, aging temperature has signi󿬁cant role in achieving the best properties.  Figure 8.5  8.5   schematically represents the effect of aging temperature on the mechanical properties of maraging steel. Most grades of maraging steels have the same trend and may slig slight htly ly chan change ge base based d on th thee comp compos osit itio ion n and and type type of marag maragin ing g st stee eell grades. In maraging steel, it is interesting to note that slight under aging or over aging does not affect the mechanical properties to a great extent. 8.1.6 Corro Corrosion sion Behavior Behavior of Maraging Steels

In normal atmospheric condition, maraging steels corrode uniformly like normal steels and are covered with rust. However, the pits are smaller than conventional steels and corrosion rate is about half of the conventional steels. In marine atmosphere, the steels and maraging steels have the same corrosion rate but after about six months the corrosion rate of  maraging steel reduces signi󿬁cantly. In general, maraging steels need to  be protected under corrosive environments like other conventional steels. stee ls. Maragi Maraging ng steels steels can be el elect ectrop roplat lated ed with with chrom chromium ium,, nicke nickell or cadmium where even slight corrosion is not tolerable. However, hydrogen that forms during plating operations may be absorbed by the steel caus ca usin ing g emb embri ritt ttle leme ment nt and and cause ause dela delaye yed d crac cracki kin ng (o (or) r) hyd hydroge rogen n induc ind uced ed crack cracking ing as shown shown in   Figure Figure 8.6. 8.6. The The effe effect ct due due to abso absorb rbed ed hydrogen can be eliminated by baking for 24 hours in the temperature range of 200–320°C. Mara Ma ragi ging ng stee steels ls ar aree pron pronee to st stres resss corr corros osio ion n crac cracki king ng (SCC (SCC)) unde underr aqueous environments with or without sodium chloride. However, maraging steels are better than other high strength steels offering better fracture

FIGURE 8.5 Schematic representation of effect of aging temperature on mechanical properties

 

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FIGURE 8.6 Delayed cracking due to hydrogen embrittlement in maraging steels

toughness values in SCC conditions i.e. can tolerate larger cracks without crac crack k prop propag agat atio ion n unde underr give given n stati staticc stres stress. s. SCC SCC main mainly ly depe depend ndss on solutionizing temperature. Above 760°C, as the temperature increases, the time for cracking is reduced due to (i) grain growth and (ii) precipitation of  TiC in the grain boundaries. This can be eliminated by suitable change in the composition so that the precipitation of TiC at grain boundaries is avoided. 8.1.7 Advan Advantages tages of Maraging Maraging Steels

1. Ultra high strength strength with goo good d ductility. ductility. 2. Insens Insensiti itive ve to notche notchess and the notch notch tensil tensilee stre strengt ngth h ratio ratio is close close to 1. 3. Less Less pron pronee to cr crac acki king ng and and prop propag agat atio ion n of crac cracks ks due due to high high 0.5 fracture toughness approx. 5500 MPa/mm 4. Tempering Tempering does not occur upon heating heating and maintains maintains strength strength up

to 350°C 5. Air cool cooling ing is is suf 󿬁cient to form martensite (good hardenability) and for precipitation hardening hence distortion due to cooling is less. 6. No decarb decarburi urizat zation ion proble problem m due due to ultraultra-low low carbon carbon (0.003 (0.003%) %) and henc he ncee no need need for for spec specia iall at atmo mosp sphe here ress fo forr heat heat trea treatm tmen entt th that at reduces furnace and handling cost. 7. Dimensiona Dimensionall changes changes are minimum minimum due to marten martensitic sitic transformatransformation because of very low carbon content as shown here: c = 2.861+ 0.116% C = 2.861348 = 0.012% a = 2.861–0.013% C = 2.860961 = -0.00136%

 

 

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8. Have good machinab machinability ility,, weldabili weldability, ty, formability. formability. 9. Can be surface hardened hardened or electroplat electroplated. ed. 10. Corrosion Corrosion resistance resistance is much better than many high strengt strength h steels. 8.1.8 Applic Application ationss in Aeros Aerospace pace

Because of high strength to weight ratio with good fracture toughness, good fabrication properties and simple heat treatment, maraging steels are widely used in critical areas such as aerospace   –  rocket motor cases, pressu pre ssure re hull hull in hydros hydrospac pace, e, aircra aircraft ft forgin forgings, gs, solid solid propel propellan lantt missil missilee cases, jet engine starter impellers, aircraft arrestor hooks, torque transmission shafts, aircraft ejector release units, landing gears, and so on. It is also used in structural engineering and ordnance   –   lightweight portable military ordnance components, and fasteners. Maraging   –  punches are used bridges, for tooling and machinery and die bolsters forsteels cold forgin for ging, g, extrus extrusion ion press press rams rams and mandre mandrels, ls, extrus extrusion ion dies dies for alumi alumi-nium, cold reducing mandrels in tube production, die casting dies for aluminium and zinc base alloy, machine components like gears, index plates, lead screws, and so on. Because the physical metallurgy and properties of maraging steels are unique, they have many commercial advantages and from the   󿬁rst day of  production to till date the applications for maraging steels have steadily grown and widening. However, in automotive industries, maraging steels are not used due to its high cost; however, low alloy maraging steels may substitute in future some of the components of automobiles.

8.2 Sta Stainl inless ess Ste Steels els Stainless steels are iron-based alloys containing suf 󿬁cient amount of chromium mi um to form form a chrom chromiu ium m oxide oxide layer layer (Cr (Cr2O3) of 1–3 nm thic thick. k. The The chrom chromium ium oxide layer is adherent to base material and self-healing. Thus, the steel

achieves stainless character and hence got its generic name as stainless steel. It is also known as inox steel or inox, derived from the French word “inoxydable.”  The initial work started in England and Germany in 1910 and commercial production of stainless steels started in 1920 in the United States. Production of precipitation hardenable stainless steel started in 1945. Stainless steels require a minimum of 10.5% of chromium to form the adherent selfhealing chromium oxide layer to achieve its stainless properties. However, practically more chromium is added since chromium forms carbides   󿬁rst and and chro chromi mium um oxid oxidee is form formed ed later later.. Suf  Suf 󿬁cien cientt amou amount nt of  chromium is added to form the chromium oxide layer after forming the carbides.

 

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Stainless steel is used for many critical applications and alloying elements such suc h as nick nickel el,, molyb molybden denum um,, copp copper, er, ti titan tanium ium,, niob niobiu ium, m, alumi alumini nium um,, and and so on are added to improve speci󿬁c properties demanded for speci󿬁c applications. 8.2.1 Effect of Alloyin Alloyingg on Struc Structure ture and Prope Properties rties 8.2.1.1 8.2. 1.1 Chromium Chromium

It is the the majo majorr allo alloyi ying ng el elem emen entt in st stai ainl nles esss st stee eels ls.. Mini Minimu mum m of 10.5 10.5% % chromium is required for the formation of the protective layer of chromium oxide on the steel surface. The strength of this protective (passive) layer increases with increasing chromium content. Apart from this, chromium prompts the formation of ferrite within the alloy structure being a ferrite stabilizer. It also forms carbides and strengthens the steels. 8.2.1.2 8.2. 1.2 Nickel  Nickel 

Nickel improves general corrosion resistance and prompts the formation of  austenite. Stainless steels with 8–9% nickel have a fully austenitic structure. Increa Inc reasin sing g nickel nickel conten contentt beyond beyond 8–9% further improves both corrosion resistance (especially in acid environments) and workability. 8.2.1.3 8.2. 1.3 Molybdenum Molybdenum and Tungsten Tungsten

Molybdenum increases resistance to both local (pitting, crevice corrosion, etc.) and general corrosion. Molybdenum and tungsten are ferrite stabilizers and when usedtoinmaintain austeniticthe alloys, must be balanced with austenite stabilizers in order austenitic structure. Molybdenum is added to martensitic stainless steels to improve high temperature strength. 8.2.1.4 8.2. 1.4 Nitrogen Nitrogen

Nitrogen increases strength and enhances resistance to localized corrosion.

It is an austenite former. 8.2.1.5 8.2. 1.5 Copper  Copper 

Copper increases general corrosion resistance to acids and reduces the rate of work hardening. 8.2.1.6 8.2. 1.6 Carbon Carbon

Carbon enhances strength (especially, in hardenable martensitic stainless steel ste els) s),, but but may may have have an adve advers rsee effe effect ct on corr corros osio ion n resi resista stanc ncee by th thee formation of chromium carbides. It is an austenite stabilizer.

 

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8.2.1.7 8.2. 1.7 Titanium, Titanium, Niobi Niobium um and Zirconium Zirconium

Where it is not desirable or, indeed, not possible to control carbon at a low level, titanium or niobium may be used to stabilize stainless steel agains aga instt interg intergran ranula ularr corros corrosion ion.. As titani titanium um (niob (niobium ium and zircon zirconium ium)) have greater af 󿬁nity for carbon than chromium, titanium (niobium and zirconium) carbides are formed in preference to chromium carbide and thus localized depletion of chromium is prevented. These elements are ferrite stabilizers. 8.2.1.8 8.2. 1.8 Sulfur  Sulfur 

Sulfur is added Sulfur added to impro improve ve the machin machinabi abilit lity y of stainle stainless ss steels steels.. Sulfur Sulfur  bearing stainless steels exhibit reduced corrosion resistance. 8.2.1.9 8.2. 1.9 Cerium Cerium

Cerium, a rare earth metal, improves the strength and adhesion of the oxide   󿬁lm at high temperatures. 8.2.1.10 8.2. 1.10 Manganese Manganese

Manganese is an austenite former, that increases the solubility of nitrogen in the steel and may be used to replace nickel in nitrogen-bearing grades. 8.2.1.11 8.2. 1.11 Silicon Silicon

Silicon improves resistance to oxidation and is also used in special stainless steels exposed to highly concentrated sulfuric and nitric acids. Silicon is a ferrite stabilizer. 8.2.2 8.2 .2 Cla Classi ssi󿬁cation

Based on the room Based room temper temperatu ature re micro microstru structu cture, re, the stainle stainless ss steels steels are classi󿬁ed as; 1. 2. 3. 4. 5.

Martensitic Martensitic stainless stainless steels steels Ferritic Ferritic stainle stainless ss steels steels Austenitic Austenitic stainless stainless steels steels Duplex Duplex (ferritic-auste (ferritic-austenitic nitic)) stainless steels steels Precipitat Precipitation ion hardenable hardenable stainless stainless steels steels

The AISI has classi󿬁ed the wrought and cast stainless steels with three di digi gitt numb number ers. s. The The 400 400 se seri ries es is assi assign gned ed fo forr mart marten ensi siti ticc and and ferr ferrit itic ic

 

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95

stainless steels. Austenitic stainless steels are speci󿬁ed with 300 series and 200 series. In 200 series austenitic stainless steels, nickel is replaced by manganese. The duplex stainless steels and precipitation hardenable stainless steels are mostly designated by the manufacturers. 8.2.2.1 8.2. 2.1 Martensiti Martensiticc Stainless Stainless Steel 

Martensitic stainless steels contain chromium about 10.5 to 18% and carbon of maximum 1.2%. Chromium is a ferrite stabilizer and carbon is austenite stabilizer. These elements should be balanced in martesitic stainless steel in such a way that on heating it should form austenite and only then upon quenching it will form BCT martensite at room temperature (Figu ( Figure re 8.7 8.7). ). If  it is not balanced, chromium will stabilize ferrite even at high temperature and austenite austenite bay will be completely completely vanished vanished as shown shown in Figure in Figure 8.8. 8.8. And also it should be ensured that chromium availability after forming carbide should be at least 10.5% to form adherent self-healing protective layer to achieve stainless properties. Essentially, the chromium content should be higher if the carbon amount is increased. Mart Ma rten ensi siti ticc st stai ainl nles esss st stee eels ls are are heat heat trea treata tabl blee and and have have more more we wear ar resistance but corrosion resistance is less than the other types. Chemical composition and mechanical properties of most commonly used martensitic grade stainless steels are given in Table in  Table 8.3. 8.3. Oill quenc Oi quenchi hing ng th these ese alloy alloyss from from tempe temperat rature uress betwe between en 982–1066°C produces the highest strength and/or wear resistance as well as corrosion resistance. The as quenched structure of fresh martensite must be tempered to restore some ductility. The values given in   Table 8.3   are for for samp sample less te temp mper ered ed at te temp mper erat atur ures es from from 204° 204°C C to 649° 649°C C fo forr two two hours.

FIGURE 8.7 Microstructure of martensitic stainless steel

 

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FIGURE 8.8 Iron-chromium phase diagram

8.2.2.2 8.2. 2.2 Ferritic Ferritic Stainles Stainlesss Steels Steels

Ferritic stainless steels contain 10–30% chromium. At room temperature, it has BCC ferrite (Figure (Figure 8.9 8.9). ). Its corrosion resistance is better than martensitic stainless steel. It gives very good surface   󿬁nish and hence used for architectu archi tectural ral purpose. purpose.   Table 8.4   shows shows the compos compositi ition on and mechan mechanica icall properties of the most important ferritic stainless steels. Ferritic stainless steels are used for their anticorrosion properties rather than tha n for their their mechan mechanica icall proper propertie tiess (streng (strength, th, ductil ductility ity,, or toughn toughness ess). ). Initia Ini tially lly,, ferrit ferritic ic stainl stainless ess steels steels were were used used in non-we non-welde lded d con constr struct uction ionss (riveted and bolted assemblies, etc.).

The ferritic stainless steels are generally classi󿬁ed as (i)   󿬁rst-generation, (ii (ii)) second second-ge -gener nerati ation, on, and (iii) (iii) thirdthird-gen genera eratio tion. n. The   󿬁rst genera generatio tion n in incl clud udes es main mainly ly th thee fo foll llow owin ing g grad grades es:: 430, 430, 434, 434, 436, 436, 442 442 and and 446. 446. In addition to their high chromium content, those containing 0.12% to 0.20% of carbon are vulnerable to a drop in ductility and to corrosion due to the formation of martensite and intermetallic precipitates in the ferritic phase in the as welded condition. The   󿬁rst-ge rst-generati neration on steels steels were supposed to be nonweldable, particularly for thicker sections that have a high inclination to cold cracking and exhibit a drastic drop in ductility in the as welded condition. The low weldability of these alloys is mainly due to the high carbon car bon and chromi chromium um conten contents. ts. The dif 󿬁cul culty ty in obtain obtaining ing alloys alloys with with a lo low w le leve vell of impu impuri riti ties es and and unde undesi sira rabl blee resi residu dual al elem elemen ents ts sulf sulfur ur,,

 

 High Alloy All oy Steels

 

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97

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FIGURE 8.9 Microstructure of ferritic stainless steel

phosphorus, carbon, and so on have restricted the weldability of ferritic stainless steels. The invention invention of the AOD (argon oxygen decarburiza decarburization) tion) process in the 1960s has enabled the development of high purity iron-chromium alloys containing negligible traces of carbon. Since then, the development of other generation gener ation of ferritic ferritic stainless stainless steels had been started. The second second generation generation incl includ udes es low low carb carbon on allo alloys ys deve develo lop ped from from th thee modi modi󿬁ca cati tion on of   󿬁rstgeneration grades. They contain other ferrite forming elements like titanium, niobium, and aluminum. In addition to promoting the ferritic microstructure, aluminum also improves the oxidation resistance at high temperatures by stabili stab ilizin zing g the chr omium um oxide oxiand de the layer. layer. This This generat genof eration ion alloys includ includes esnoticeably 405, 409, 409Cb, 439, and chromi so on grades, weldability these is  better than the   󿬁rst generation grades. The third generation is of recent alloys called superferritic. The forming process of these alloys experiences some dif 󿬁culties related to the elevated chro ch romi mium um leve levels ls.. Thes Thesee ar aree used used for for appl applic icat atio ions ns subj subjec ecte ted d to seve severe re

corrosion corrosi on condi conditio tions ns such such as in chlor chloride ide enviro environm nment ents. s. The grades grades 444, 444, 29-4, 29-4-2 are the most typical ones in this class. These are high purity alloys in which the amount of interstitial elements, carbon and nitrogen in particular, are reduced to a minimum in order to improve the ductility of  the alloy. Howe Ho weve ver, r, vari variou ouss ty type pess of hot hot embr embrit ittl tlem emen entt (los (losss of duct ductil ilit ity) y) are are asso associ ciat ated ed with with ferri ferriti ticc st stai ainl nles esss st stee eels ls th that at shou should ld be ta take ken n care care wh when en used at high temperatures. 1. Embrittlement at 475°C : This phenomenon is proportional to the chromium content and occurs in the temperature range of 425 –550°C. It is the re ressult of the decomposition of the ferrite into two phases: a chromium rich phase (α') and an iron rich phase ( α). The more the

 

 High Alloy All oy Steels

 

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99

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 ,     h     t    g    n    e    r     t    s    e     l     i    s    a    n     1     3     0     4     6     6     9     P     4    e    M     T

    0     4     2     0     5     9     1     5     1     4     8     4     1     5     5     1     8     6     1     0     5     0     4

 ,     h     t    g    n    e    r     t    s     d     l    a    e    P     1     5     4     i     2     2     Y    M     2     7     1     3     C     %     X     9    +     3  .     0    =     b     N  ,     5  .    –     0     )     1     %  .     t     0      w     C     i     T     3    n  .     i  ,     0     %    3     (    –    x     0    r     1    n  .  .     6     0    e     0    o     i     h     l   -     i     t    =    =     t     i    s     O     A    T     N    o    p

    2     0     0     7     1     5     1     0     5     4     4     4     0     1     7     1     5     1     2     3     3     2     3     2     3     4     4     )     N    +     C     (     4    +     2  .     0    =     i     T     5  ,     1  .     0    =     l     A  ,     9  .     3  .     8     0  .     0   -     0   -     0    o    o    =    –     M    M    N

    )     N    +     C     (     4    +     2  .     0     2    =  .     2  .     i     4     4     T    –    –  ,  .     5  .     1     5     5     3     2    =  .    =     3    =     b     0   -    o    o    –     N    N    M    M

   a    c     i    n    a     h    c    e    m     d    n    a    n    o     i     t     i    s    o    p    m    o    c       4   .     l       8    a    c       E     i       L    m       B    e       A     h       T     C

   m    o     C     l    a    c     i    m    e     h     C

    i     S     1     1     1    n     M     1     1     1     i     5  .     N     0    –     1     5  .     7  .     5  .     4     1     9     1     1     1    –

   r     C

    C    e    p    y     T

   –

   –

    5  .     5  .     5  .     1     0     7     1     1     1

    5     5     2     2  .     2  .  .     2  .     1     0     0     1   -     1     1     0     0     4     5  .     4  .  .     3  .     3  .     1     0     0     1   -     1     1     0     0     5  .     5     2     5     5     5     5     1  .  .  .  .  .    –     0    –     0     0    –     1     0     0     2     5  .     9     1     8     9     0     0    –     7     1     1     2     3     3    –    –    –    –    –  .     3     8     8     4     7     7     7     0     5     7     1     1     1     1     2     1     2     2     2

    5     2     5     5     3     2     2     1     1     1  .     0  .     0  .     0  .     1  .     0  .     2  .     0  .     0  .     0     0     0     0     0     0     0     0     0     2       4     5     9     9    b     0     4     6     9     2     4     6    –     4    –     0     0     0     3     3     3     3     4     4     4     9     9     4     4     4    C    4     4     4     4     4     4     4     2     2     6     3     3     0  .     0  .     0  .     0     0     0

 

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alloy is rich in chromium, the faster will be the reaction. The consequences this consist essentially selective corrosion of iron iron rich richofphas ph ase. e.reaction In orde order r to prev pr even entt th this isintype tyape of embr embrit ittl tlem emen ent, t, the th thee service temperatures of the part must always be limited to 400°C max. 2. Precipitation of embrittling phases and intergranular corrosion Sigma Phase: Sigma (σ) phase (a chromium-rich and very brittle phase) is the product of transformation of the ferritic delta phase occurring when the microstructure is subjected to a long exposure time in the temperature range of 540–870°C, as shown in Figure in  Figure 8.10 8.10.. This new phase (σ) forms rapidly in high chromium grades and molybdenum also promotes its formation. The presence of this new phase alters  both theonizin corrosion andatductility. Dissolving requires a soluti solutioni zing g heat hearesistance t treatme treatment nt temper temperatu atures res above abovethis 900°C. 900phase °C. Howeve How ever, r, ferri fe rriti ticc stain stainle less ss steel steelss ar aree limi limite ted d to 400° 400°C C maxi maximu mum m and and henc hencee th thee possibility of formation of sigma phase is remote during service. 8.2.2.3 8.2. 2.3 Austenitic Austenitic Stainless Stainless Steels

Austenitic stainless steels have FCC austenite phase at room temperature and contai contain n typica typically lly 16–25 25% % chro chromi mium um and and 8–26 26% % nick nickel el.. Th Thes esee also also contai con tain n nitrog nitrogen en in soluti solution on as austen austenite ite stabil stabilize izer. r. Non Nonava availa ilabil bility ity of  nick ni ckel el duri during ng 1950 1950ss due due to ci civi vill war war in Afri Africa ca and and Asia Asia a new new se seri ries es called 200 series evolved that had manganese instead of nickel to stabilize austenite. Due to less cost and good strain hardening, the 200 series are widely used for noncritical applications. Austenitic stainless steels make up over 70% of total stainless steel production and is more expensive than

FIGURE 8.10 Microstructure showing formation of sigma ( σ) phase

 

 High Alloy All oy Steels

 

101

the other types. Type 304 is the most common type among all austenitic stainless and the mostprocesses. Table weldable  Table one and it can be welded byand all fusion andsteels, resistance welding processes. 8.5 shows 8.5  shows composition mechanical properties of most commonly used austenitic stainless steels. Austenitic stainless steels are nonmagnetic and are extremely formable and weldable. They can be successfully used from cryogenic temperatures to the red hot temperatures. Austenitic (FCC) structure is very tough and ductile down to absolute zero as they have no ductile to brittle temperature (DBTT) like ferritic structure. Austenite is a high temperature phase and more densely packed (FCC) and hence austenitic stainless steels do nott lo no lose se thei theirr st stre reng ngth th at elev elevat ated ed te temp mper erat atur ures es as rapi rapidl dly y as ferr ferrit itee (conve (co nventi ntiona onall steels) steels).. So, austen austeniti iticc stainl stainless ess steel steel can be used used for high high temperature applications also. In addition, they are soft enough (i.e., with a yield strength about 200 MPa) to be easily formed by the same tools that work with carbon steel and they can also be made incredibly strong by cold work, up to yield strengths of over 2000 MPa. These properties make the austenitic steel a versatile material for varying applications. But for the cost of the nickel these alloys would be widely used. 8.2.2.4 8.2. 2.4 Lean and Richer Richer Austeniti Austeniticc Stain Stainless less Steels:

Lean alloys have less than 20% chromium and 14% nickel and constitute the largest portion of all stainless steel produced. These are principally 201, 301, and 304. The main difference among the lean austenitic alloys lies in their work hardening rate. The leaner the alloy, the lower the austenite stability. As unstable alloys are deformed, they transform from austenite to the much harder martensite. This increases the work hardening rate and enhances ductility. Rich Ri cher er al allo loys ys,, such such as 305, 305, have have lo lowe west st work work hard harden enin ing g rate rate.. Th Thee austenite is more stable and will not transform to martensite on deformation tion like like lean lean allo alloys ys.. Thes Thesee grad grades es ar aree pref prefer erre red d fo forr th thos osee appl applic icat atio ions ns where corrosion predominate.

8.2.2.5 Austenitic 8.2.2.5 Austenitic Stainless Stainless Steels for High Temp Temperatu erature re Oxid Oxidation ation Resistance:

High temper High temperatu ature re oxidat oxidation ion resist resistanc ancee can be enhanc enhanced ed by additi addition on of  silicon and rare earths. If the application requires also strength at high temper tem peratu ature, re, car bon, , nitrog nit rogen, en, proprietary niobiu niobium, m, and molyb mo lybden denum umin are added. added. 302B, 309, 310,carbon 347, and various alloys are found this group. Chromium, molybdenum, nickel, and nitrogen alloyed austenitic stainless steel is used when corrosion resistance is the main objective. Alloys such as silicon, molybdenum, nitrogen and copper are added for resistance to speci󿬁c environments. This group includes 316L, 317L, 904L, and many proprietary grades.

 

 

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Introduction to Steels

    )     N     H     B     (    s    e    n     d    r     0     0     0     0     0     0     0     0     0    a     1     1     8     8     8     8     8     0     0     H     2     2     1     1     1     1     1     2     2

   s     l    e    e     t    s    s    s    e     l    n     i    a     t    s    c     i     t     i    n    e     t    s    u    a     d    e    s    u    y     l    n    o    m    m    o    c     t    s    o    m     f    o

   s    e     i     t    r    e    p    o    r    p     l    a    c     i    n     h    a    c    e     M

    0     0     0     0     0     0     2     2     2

    0     5     5     0     0     0     0     0     1     8     2     2     2     2     1

    )     %     (    n    o     i     t    a    g    n    o     0     0     0     0     0     0     0     0     0     0     0     0     0     l     E     4     4     5     5     4     4     4     4     4     5     4     4     4

    0     5     5     0     5     4     5     5     5     3

   a     P     M  ,     h     t    g    n    e    r     t    s    e     l     i    s    n     3     0     0     2     5     8     8     5     8     1     8     8     8    e     9     9     5     1     1     1     1     1     7     1     1     1     T     9     7     6     6     5     5     5     5     5     5     5     5     5     5

    5     5     7     1     0     1     8     8     2     9     5     5     5     6     4

   a     P     M  ,     h     t    g    n    e    r     t    s     d     l    e     6     6     2     7     5     7     7     5     7     7     7     7     7     i     7     4     0     0     0     0     0     0     4     0     0     0     Y     7     2     2     2     2     2     2     2     2     2     2     2     2     2

    5     0     2     2     0     0     4     4     4     2     2     2     2     2     2

   r    e     h     t     O     )     %     t     i    w     S    n     i

    N    N     5     5     2  .  .     2     0     0

   –

   –

   –

   –

   –

   –

   –

   –    –     3    –    o    o     b    o    u     0     2     3     2     2    3     N    3  .     M    M    i     T     N    M    C     3    o    o    o    0     5    2     4   -     4     0  .  .    –    –  .     8    –    –     4    –     0     M    M    M     N    3     3     0     0     4    1

    3    –     1     1     1     1     2     1     1     1     1     1     1     1     1

    1     1     1     1     1

       e     i     t    r    e    p    o    r    p     l    a    c     i    n    a     h    c    e    m     d    n    a    n    o     i     t       5   .     i       8    s    o       E    p       L       B    m       A    o       T     C

    (    n    o     i     t     i    s    o    p    m    o     C     l    a    c     i    m    e     h     C

    5  .     0     7     1    –    –    n     5     5  .  .     M     5     7     2     2     2     2     2     2     2     2     2     2     2

    2     2     2     2     2

    5  .     3     5     2     4     4     4     5     1     1     2     1     1     1     0     0    –    –    –    –    –    –    –     6     8     1     1     i     5  .    –    –    –    –     0     0     0     2     9     0     0     0     N     3     4     6     8     8     1     1     1     1     1     1     1     1

    5     5     8     1     1     2    –    –    –     1     1     0     1     3     1     1     1     1     2

   r     C

    8     9     8     9     9     9     4     6     8     8     8     1     1     1     1     1     1     2     2     1     1     1    –    –    –    –    –    –    –    –    –    –    –     6     7     6     7     7     9     9     7     2     4     6     6     6     1     1     1     1     1     1     1     1     2     2     1     1     1

    0     0     3     2     2     2    –    –    –     8     8     8     8     9     1     1     1     1     1

    C

    5     5     5     5     5     8     3     2     5     8     3     3     1  .     0  .  .     1  .     1  .     1  .     1  .     0  .     0  .     2  .     0  .     0  .     1  .     2     0     0     0     0     0     0     0     0     0     0     0     0     0

    8     3     8     8     2     0  .     0  .     0  .     0  .     0  .     0     0     0     0     0

   e    p    y     T

    N     B     L     L     L     1     2     1     2     2     4     4     5     9     0     6     6     6     0     0     0     0     0     0     0     0     0     1     1     1     1     2     2     3     3     3     3     3     3     3     3     3     3     3

    L     7     L     1     7     4     1     7     1     2     4     3     3     3     3     0     9

 

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103

The austenitic stainless steels that require enhanced machinability have a high high conten con tentt of contro controlle inclus inclusion ions, sulph sulphide ides, s, or oxy sulphi sul phides des, , to improve machinability at lled thed expense ofs,corrosion resistance. Carbon kept  below 0.03% and designated as L grade is used when prolonged heating due to multipass welding of heavy section (greater than about 2 mm) or when welds requiring a postweld stress relief are anticipated. Austenitic stainless steels are available as both wrought and cast alloys. The composition of wrought and cast alloys are almost the same except that the cast counterparts differ primarily in silicon content. 8.2.2.6 8.2. 2.6 Austenite Austenite Stability  Stability 

The formation of martensite at room temperature is thermodynamically possible in the case of austenitic stainless steels, but the driving force for its formation may be insuf 󿬁cient for it to form spontaneously. As it is known that martensite forms from austenite by diffusion- less shear mechanism, it can also occur if that shear is provided mechanically by external forces. This is called as strain induced martensite. The transformation temperature and the degree to which it occurs varies with composition of the alloy. The following equation relates the temperature at which 50% of the austenite transforms to martensite with 30% true strain (Md30). Md30 (°C) = 551–462(%C + %N)   –  9.2(%Si)   –  8.1(%Mn)   –  13.7(%Cr)   –   29 (%Ni + Cu)   –  18.5(%Mo)   –  68(%Nb)   –  1.42 (grain size in microns   –   8) All the alloying elements (ferrite and austenite stabilizers) decrease the Md30 temperature and delay martensite formation i.e. if austenitic stainless steel is deformed above Md30 temperature, the austenite will be stable and will not form any martensite and if the deformation temperature is lower the austenite transforms to martensite when the true strain is 30%. Figure Fig ure 8.11 8.11   represe represents nts schema schematic ticall ally y the transf transform ormati ation on of austen austenite ite to martensite at different strain and temperature.

This tempe This temperat ratur uree is the comm common on inde indexx of auste austeni nite te stabi stabili lity. ty. The The lean lean allo alloys ys obviously have the Md30 temperature higher than room temperature and hence it forms martensite even during deforming at room temperature, that contributes for the high strain hardening. The actual Md30 temperature can  be found out for each alloy based on the compositio composition n and thus formation formation of  strai str ainn-in indu ducced marte martens nsit itee can can be avoi avoide ded. d. Beca Becaus usee the the carb carbon on leve levels ls of  austenitic stainless steels are always relatively low, strain induced martensite is selftotempered not brittle. Martensite formed is, of course, susceptible hydrogenand embrittlement. The strainthus induced martensite can be eliminated by heatin heating g the austenitic austenitic stainless stainless steels to high high temperature temperaturess (normally (normally above 800°C) and subsequent quenching is called as quench annealing. Care Ca re has has to be ta take ken n whil whilee usin using g th thee regr regres essi sion on equa equati tion on sinc sincee th this is regression analysis was generated for homogeneous alloys. If alloys are inhomogeneous such as sensitized or solute segregated, due to welding,

 

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Introduction to Steels

FIGURE 8.11 Variation of martensite formation with temperature and true strain (Schematic)

then the equation applies on a microscopic scale. Sensitized zones (i.e., the region reg ionss near near grain grain bound boundari aries es where where chrom chromium ium carbid carbides es have have precip precipiitated) will have a much higher tendency to transform to martensite. 8.2.2.7 8.2. 2.7 Mechanical Mechanical Properties Properties

The tensile properties in the annealed state related to composition as given  by the empirical equation. Yield Yie ld stren strength gth (MPa) (MPa) = 15.4 15.4 [4.4+ [4.4+23( 23(%C %C)+3 )+32(% 2(%N)] N)]+0. +0.24 24 (%Cr (%Cr)+0 )+0.94 .94 (%Mo (%Mo)) + 1.3 0.46(%Si) (d-0.5+ ) 1.2(%V)+0.29(%W)+2.6(%Nb)+1.7(%Ti)+0.82(%Al)+0.16(% ferrite)+ Tensile strength (MPa) = 15.4 [29+35(%C)+55(%N)]+ 2.4(%Si)+ 0.11(%Ni)+ 1.2(%Mo)+ 5(%Nb)+3(%Ti)+1.2(Al%)+0.14(% ferrite)+0.82 (d-0.5) where percentage of elements refer to weight percentage and d is the

grain diameter in millimeters. There are several empirical relations used to calcul cal culate ate the mecha mechanic nical al proper propertie ties. s. In genera generall as the alloyi alloying ng elemen elements ts increase, the strength increases. The tensile strength relationship does not obey for lean alloys, such as 301, in which tensile strength increases with decreasing alloy content due transformation of austenite to martensite, that produces higher tensile strengths in austenitic stainless steels. Onee impo On import rtan antt poin pointt th that at desi design gn engi engine neer erss shou should ld ta take ke ca care re is th that at austenitic steels dostrength not havetoa some clear yield point canofbegin to deform at stainless 40% of the yield extent. As abut rule thumb,  behavior at less than half the yield strength is considered fully elastic and stresses below two thirds of the yield strength produce negligible plastic deformatio defor mation. n. This quasi-elastic quasi-elastic behavior behavior is due to many active slip systems systems in the FCC structure. The tensile properties of austenitic stainless steels with unstable austenite (lean alloys), are very strain rate dependent due to

 

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105

the in󿬂uence of adiabatic heating during testing increasing the stability of  the austenite. Austenitic stainless steels exceptional The ambient temperature impact strength of have austenitic stainlesstoughness. steels is quite high high.. Abse Absenc ncee of a trans transit itio ion n te temp mper erat atur uree (D (DBT BTT) T) make make th thee aust austen enit itic ic stainless steel suitable for cryogenic applications. 8.2.2.8 8.2. 2.8 Precipitat Precipitation ion of Carbides Carbides

The solubility of carbon is very little at room temperature and even the 0.03% of L grade is mostly in a supersaturated solution. Because carbon has a great thermodynamic af 󿬁nity for chromium and forms chromium carb ca rbid ides es of M23C6   form. form. Format Formation ion of chrom chromium ium carbid carbides es causes causes local local chromi chr omium um deplet depletion ion adjace adjacent nt to carbid carbides es such such that that the chromi chromium um level level can ca n beco become me lo low w enou enough gh not not to be st stai ainl nles esss resu result ltin ing g in much much lowe lowerr corrosion resistance than the surrounding area. Intere Int eresti stingl ngly y in normal normal condit condition ions, s, austen austeniti iticc stai stainle nless ss steels steels are not having chromium carbides due to the slow diffusion of carbon and the even slower diffusion of chromium in austenite. However, on reheating and prolon prolonged ged holdin holding g at high high temper temperatu atures res will will pre precip cipita itate te chrom chromium ium carb ca rbid ides es and and caus causee depl deplet etio ion n of chro chromi mium um near near carb carbid ides es resu result ltin ing g in locali loc alized zed corros corrosion ion is called called   “sensitization.”   In gene genera rall at low low temp temper eraatures, grain boundary diffusion is much more rapid than bulk diffusion, and grain boundaries provide excellent nucleation sites and hence precipitation of chromium carbides occurs along grain boundaries. The temperature and incubation time for chromium carbide formation (t (tim imee fo forr sens sensit itiz izat atio ion) n) is depe depend nded ed on th thee amou amount nt of carb carbon on in th thee austenitic stainless steels. For example, in austenitic stainless steel with 0.06% 0.06 % C (norma (normall 304 grades) grades),, the chrom chromium ium carbid carbides es start start precip precipita itatin ting g from 475 to 850°C and the time varies between 1 min to 100 h depending on the the te temp mper erat atur uree whic which h is cal called led as sens sensit itiz izaation tion zone zone wher wheree as fo forr austenitic stainless steel with 0.02% C sensitization zone is between 475

to 580°C and time is above 100 h. Care has to be taken while using the austenitic stainless steel at high temperatures and also during welding  by ensuring that the alloy is not cooled slowly in the sensitization sensit ization te temp mper erat atur uree rang range. e. Even Even the the aust austen enit itic ic st stai ainl nles esss st stee eels ls shou should ld be quench que nched ed from from anneal annealing ing temper temperatu ature re even even for anneal annealing ing is called called as quench annealing. Much Mu ch long longer er heat heat treat treatme ment nt is re requ quir ired ed to elim elimin inat atee th thes esee de depl plet eted ed zones by reishomogenization ofthem. slowlyAlloying diffusingelements chromium only short time required to form canalthough have a major in󿬂uence on carbide precipitation by their in 󿬂uence on the solubility of  carbon in austenite. Molybdenum and nickel accelerate the precipitation by diminishing the solubility of carbon. Chromium and nitrogen increase the solubility of carbon and thus retard and diminish precipitation. Nitrogen is especially useful in this regard.

 

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8.2.2.9 Stabilization

The composition of austenitic stainless steels are adjusted in such a way that the material will not be subjected to sensitization is called stabilized austenitic stainless steels or stabilization. The carbon is lowered to harmless levels, so that free carbon is not available to form carbides. These grades are mentioned as L grade. 304 L refers to the reduced carbon level of 304 grade. Normally L grade has carbon lower than 0.03%. The other way is to add nitrogen, that incr increa ease sess the the solu solub bil ility ity of carb carbon on in aust austen enit itee an and d are are me ment ntio ione ned d as LN grades, that is, 304 LN. It was found that adding more powerful carbide formers than chromium could preclude the precipitation of chromium car bides.  bide s. Titanium Titanium and niobium niobium are the most useful useful elements elements in this regard. regard. Titanium above four times the carbon and niobium eight times the carbon avoid the formation of chromium carbide precipitation and sensitization. 8.2.2.10 8.2. 2.10 Weaknesses Weaknesses

Austenitic stainless steels are less resistant to cyclic oxidation than ferritic grades because their greater thermal expansion coef 󿬁cient tends to cause the protective oxide coating to spall. They can experience stress corrosion cracking (SCC) if used in an environment for which they have insuf 󿬁cient corrosion resistance. The fatigue endurance limit is only about 30% of the tensile strength (vs. ~50 to 60% for ferritic stainless steels). This, combined with wi th thei theirr high high th ther erma mall expa expans nsio ion n coef  coef 󿬁cien cients ts,, make make th them em es espe peci cial ally ly susceptible to thermal fatigue. 8.2.2.11 Duplex 8.2.2.11 Duplex Stainless Stainless Steels The duplex stainless steels have both ferrite and austenite at room temperat pe ratur ure. e. The The ferri ferrite te st stab abil iliz izer er chro chromi mium um abou aboutt 23–28 28% % and and aust austen enit itee stabilizer nickel about 2.5–5% are added to achieve the duplex structure

shown show n in   Figure Figure 8.12 8.12. The The aust austen enit itee is isla land ndss (lig (light ht)) are are embe embedd dded ed in a continuous ferrite matrix. The duplex structure contains about 45 –65% austenite and the rest is ferrite. The amount of ferrite and austenite can be adjusted by balancing chromium and nickel contents. Dupl Du plex ex stai stainl nles esss st stee eels ls were were cr crea eate ted d to comb combat at corr corros osio ion n prob proble lems ms caused by chloride bearing cooling waters and other aggressive chemical process   󿬂uids. The   󿬁rst-generation duplex stainless steels were developed more than 70 years ago in Sweden for use in the paper industry. The corrosion performance is evaluated by resistance to pitting and termed as pitting resistance equivalent, that depends on their alloy content. Pitting resistance equivalent = Cr% + 3.3Mo% + 16N% The term   “Super Duplex”   was   󿬁rst used in the 1980’s to denote highly alloyed, high-performance duplex stainless steel with a pitting resistance equivalent of >40. With its high level of chromium, super duplex steel

 

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107

FIGURE 8.12

Microstructure of duplex stainless steel

provides outstanding resistance to acids, acid chlorides, caustic solutions and other environments in the chemical/petrochemical, pulp and paper indu indust stri ries es,, ofte often n repla replaci cing ng 300 300 se seri ries es st stai ainl nles esss steel steel,, high high nick nickel el su supe perr austenitic steels and nickel-based alloys. The chemical composition based on high contents of chromium, nickel and molybdenum improves intergranular and pitting corrosion resistance. Additi Add itions ons of nitrog nitrogen en promot promotee structu structural ral harden hardening ing by inters interstit titial ial sol solid id solution mechanism, that raises the yield strength and ultimate strength values without impairing toughness. Moreover, the two-phase microstructure guarantees higher resistance to pitting and stress corrosion cracking in comparison with conventional stainless steels. Dupl Du plex ex stai stainl nles esss st stee eels ls ar aree grad graded ed fo forr th thei eirr corr corros osio ion n perf perfor orma manc ncee depending on their alloy content. •   Lean

Duplex such as 2304, that contains no deliberate molybdenum

addition; •   2205, has molybdenum, the work-horse grade accounting for more than 80% of duplex usage; •  25 Cr duplex such as Alloy 255 and DP-3; •  Super-Duplex; with 25–26 Cr and increased molybdenum and nitrogen ge n comp compar ared ed with with 25 Cr grad grades es,, incl includ udin ing g gr grad ades es such such as 2507 2507,, Zeron 100, UR 52N+, and DP-3W. Most of the duplex stainless steels are proprietary grades but few grades had ha d been been iden identi ti󿬁ed by UNS UNS numb number er.. Chem Chemic ical al comp compos osit itio ion n of most most commonly used duplex stainless steels are given in  Table 8.6. 8.6. The bene󿬁ts of duplex stainless steels are as follows: (i) high strength, (ii) high hig h resist resistanc ancee to pittin pitting, g, crevic crevicee corros corrosion ion,, stress stress corros corrosion ion crack cracking ing,,

 

 

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TABLE 8.6 Chemical composition (in wt%) of most commonly used duplex stainless steels Chemical Composition, wt% UNS number num ber Typ Type e C

Mn Si

Cr

Ni

Mo

N

Cu

Other

  0.03   0.03   – S31803   –   0.03 S32001   –   0.03 S32205   2205 0.03 S32304   2304 0. 0 .03 S32520   –   0.03

2 1 2 4–6 2 2.5 1.5

24–26 24–26 21–23 22–23 19.5–21.5 21.5 21.5–24.5 24.5 24–26

5.5–6. 6.55 5.5–7. 7.55 4.5–6. 6.55 1–3 4. 4.55–6.5 3–5.5 5.5–8

1. 1.22–2 2. 2.55–3.5 2. 2.55–3.5 0.6 3–3.5 0.05–0. 0.66 3–4

0.14–0.2   0.1–0.2 0.08–0.2   0.05–0. 0.17 17 0.14–0.2   0. 0.05 05–0.2 0.2–0.35

–

–

0.2–0.8

W-0.1–0.2

–

–

S31200 S31260

  –

S32550   255 S32750   2 25507 S32760   –  

0.04 0.03 0.03 S32900   3 3229 d 0.06

1.5 1.2 1 1

1 0.75 1 1 1 1 0. 0.8 1 0.8 1 0.75

–

–

–

24 27 24–26 24–26 23–28

4.5 6. 6.55 6–8 6–8 2.5–5

2. 2.99 3.9 3–5 3–4 1–2  

1

 

–

– –

0.06–0.6   – 0.5–2   –

–

–

  –

0.1 0.25 1.5 2.5 0.24–0. 0.32 32 0. 0.55   0.2–0.3 0.5–1  

–

–

–

–

–

corrosi corro sion on fati fatigu guee and and er eros osio ion, n, (iii (iii)) high high th ther erma mall co cond nduc ucti tivi vity ty and and low low coef 󿬁ci cient ent of therma thermall expans expansion ion,, (iv) (iv) good good worka workabil bility ity and weldab weldabili ility, ty, and (v) high energy absorption. 8.2.2.12 Precipitation Hardenable Hardenable Stainless Steel Steelss

The precipitation hardenable steels as the name indicates are precipitation hard ha rden enab able le.. Copp opper er,, molyb olybd denu enum, and and alu aluminiu inium m are are add added to strengthen by precipitation hardening the austenitic phase or martensite phas ph ase. e. The The prec precip ipit itati ation on hard harden enab able le st stai ainl nles esss steel steelss are of th thre reee type types: s: martensitic, semiaustenitic, and austenitic. Table austenitic.  Table 8.7 8.7 shows  shows the composition

of most commonly used precipitation hardenable stainless steels. Martensitic Marten sitic group steels are solutioni solutionized zed at 1040°C and air cooled cooled so that aust au sten enit itee is tran transf sfor orme med d to marte martens nsit ite. e. Agin Aging g in temp temper eratu ature re rang rangee of  455–56 565° 5°C C caus causes es prec precip ipit itat atio ion n effe effect ct.. Thes Thesee grad grades es can can have have tens tensil ilee strength of 1345 MPa and yield strength of 1241 MPa with 13% elongation in the the full fully y aged aged cond condit itio ion. n. Thes Thesee allo alloys ys have have poor poor cold cold fo form rmin ing g an and d shearing characteristics. The compositions of the semiaustenitic type pre–

–

cipitation hardenable stainless such 17t 7(Ms) PH PH 15 are adju ad just sted ed in such such a way wa y th that at steels mart marten ensi site te as st star art (M s) and temp temper erat atur uree7 Mo is well we ll  below room temperature. When these stainless steels are quenched from solutionizing temperature, the austenite is stable and martensite will not form. At this condition they are soft and ductile and can be easily formed. The formed parts are subjected to conditioning treatment, that is, they are heated to a temperature between 760 and 950°C, so that the carbon comes

 

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TABLE 8.7 Composition of most commonly used precipitation hardenable stainless steels Composition Compos ition in wt % Alloy

C

Martensitic PH 13–8 Mo   0.05 15–5 PH

 

0.07

  0.07 Stainless W   0.07 Semiaustenitic 17–7 PH   0.07 PH 15–7 Mo   0.09 AM 350   0.1 AM 355   0.13 0. Austenitic 17–10 P   0.12 HNM   0.3 17–4 PH

Cr

Ni

Mo

Al

Mn

Si

o otthers

12.25–13.25 14–15.5

7.5–8.5 3.5–5.5

2–2.5

0.9–1.3 –  

0.1 1

0.1 1

16.5 17

4 7

1

1

–

–

0.01N 2.5–4.5 Cu, 0.14–0.45 Nb Cu-2.75 Ti- 0.7

17 17 15 16.5 15.5

7 7 4.3 4.3

0.6 1 0.8 0.95

0.4   – 1   – 0.25 N- 0.1 0.25 N- 0.1

17 17 18.5

10 9.5

–

–

  –

 

–

 

–

 

0.2

 

–

 

1.15 1

2.5 2.75 2.75

–

 

–

 

–

 

–

–

 

–

–

   

   

 

3.5

 

–

 

   

P- 0.25 P- 0.23

out from austenite and forms respective carbides and brings the Ms and Mf temperature above room temperature. Upon cooling, the austenite transforms in to martensite. Hence they are called as   “semiaustenitic.”   If the conditioning temperature is ifatthe theconditioning lower side (around 760°C), be above room temperature but temperature is atMf the will higher side (around 950°C), Mf temperature will be below zero and in such cases subzer sub zero o coolin cooling g (or) refrig refrigera eratio tion n is requir required ed to comple complete te the marten martensit sitic ic transformation. The martensite obtained due to high temperature condition-

ing is harder due to high carbon content. Alternatively, the austenitic to martensitic transformation can also be achieved by cold working. However in this, the subsequent aging is carried out between 455 and 565°C for 1 to 3 h. In case case of auste austeni niti ticc prec precip ipit itat atio ion n hard harden enab able le sta stain inle less ss steel steels, s, th thee Ms temperature is well below zero and thus the transformation of austenite is not feasible. Aging is carried out in the austenitic matrix itself to get its optimal mechanical properties. The aging kinetics of the austenitic type is much mu ch sl slow ower er as com compare pared d to se sem mi aust austen enit itic ic or mart marten ensi siti ticc type type.. The The interesting fact about the alloys AM 350 and 355 is that they do not respond to precipitation hardening but the heat treatment cycle is similar to that of  semiaustenitic type and hence they are also classi󿬁ed under semi austenitic precipitation hardenable stainless steels. However, the martensite produced in AM 350 and 355 are harder than other alloys due to high carbon and thus the strength of these alloys are higher than other alloys.

 

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Thus the required mechanical properties can be obtained in precipitation hard ha rden enab able le stain stainle less ss st stee eels ls by se sele lect ctio ion n of prop proper er heat heat trea treatm tmen entt cycl cyclee rather than adjusting its composition. 8.2.3 Weldab Weldability ility of Stainles Stainlesss Steel

The unique properties of the stainless steels are derived from the addition of alloying elements, principally chromium and nickel, to steel. Typically, more than 10% chromium is required to produce a stainless iron. The   󿬁ve grades of stainless steel have been classi󿬁ed according to their microstructure. The   󿬁rst three consist of a single phase but the fourth group contains  both ferrite and austenite in the microstructure and the   󿬁fth fth grou group p is precipitation hardenable. As nickel (plus carbon, manganese and nitrogen) promot pro motes es austen austenite iteferrite and formation, chrom chromium ium the (plus (plus silic silicon, on,ofmolybd mol ybdenu enum m and nionio bium) encourages structure welds in commercially available stainless steels can be largely based on their chemical composition tion.. Beca Becaus usee of th thee di diff ffer eren entt micr micros ostru truct ctur ures es,, th thee allo alloy y gr grou oups ps have have differ dif ferent ent weldin welding g charac character teristi istics cs and suscep susceptib tibili ility ty to defect defects. s. The propro blems due to welding and their remedial actions are discussed very brie󿬂y. 8.2.3.1 8.2. 3.1 Martensiti Martensiticc Stainless Stainless Steels

The main problems during welding of martensitic stainless steel are (i) susceptible to cold cracking and (ii) hydrogen pick up due to martensitic structure. The simple solution is to maintain carbon below 0.25%. High carbon martensitic stainless steels are pre heated to 250°C and cooled slowly in Ms and Mf range to avoid thermal stresses so as to avoid cold cracking. The details regarding slow cooling in Ms and Mf temperatures were discussed in Chapter in  Chapter 2. 2.

To avoi avoid d hydr hydrog ogen en pi pick ck up use use (i) (i) pure pure argo argon n atmo atmosp sphe here re duri during ng welding, (ii) low hydrogen electrodes iii) baked electrodes to avoid moisture. Because the   󿬁nal microstructure required is martensite, postweld heat treatment (tempering) is necessary for getting optimum properties. Care should be taken to transform all the austenite to martensite before post weld heat treatment or else austenite will convert to untempered martensite after postweld heat treatment. 8.2.3.2 8.2. 3.2 Austeniti Austeniticc Stainless Stainless Steel 

As compared to other grades of stainless steels, austenitic stainless steels are easy to weld. Preheat or postweld heat treatment is not necessary and even ev en stre stress ss re reli liev evin ing g anne anneal alin ing g is not not requ requir ired ed fo forr auste austeni niti ticc stain stainle less ss steel ste els. s. Howe Howeve ver, r, th thes esee grad grades es face face majo majorr prob proble lems ms of (i) (i) soli solidi di󿬁cation cracking and (ii) sensitization. The solidi󿬁cation cracking is due to high

 

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111

thermal expansion of austenite, that is about 2.5 times higher than ferrite and the austenitic stainless steels are subjected to interdendritic cracking. Introduction of small amount (5–10%) of delta ferrite or free ferrite solves this problem. The weld chemistry is adjusted to have desired delta ferrite as per Schaffer (Figure (Figure 8.13) 8.13) or Delong diagram, that is a plot between austenite and ferrite promoting elements in terms of nickel and chromium equivalents. In the Delong diagram, the effect of nitrogen is included to calculate the nickel equivalent otherwise it is much similar to the Schaffer diagram. The chemistry of   󿬁ller metal or the electrode should be adjusted based on the Schaffer diagram to have desired amount of delta ferrite in weld pool to avoid avo id solidi solidi󿬁ca cati tion on cr crac acki king ng.. In shor short, t, matc matchi hing ng elec electr trod odes es with with base base material is not used for welding the austenitic stainless steels. For example, 309 grade electrode is of used for weldingis304 The other problem sensitization duegrade. to segregation of chromium carbides at grain boundaries depleting the chromium content in the near zones as discussed in Section in  Section 8.2.2.8. 8.2.2.8. This leads to intergranular corrosion. Sensitization can be avoided by (i) using extra low carbon electrodes, and (ii) (i i) by stab stabil iliz izin ing g th thee aust austen enit itee by addi adding ng tita titani nium um or niob niobiu ium m so th that at

Austenite   n     h    M   ×    5  .    0   +

  e   s   a   e   r   c   n    i    t

   C   +    0    3   +    i    N   =   q    i  e    N

  n   e    t   n   o   c   e    t    i   r   r   e    F

A+M A+F

Martensite A+M+F F + M

Ferrite

M+F

Creq = Cr + Mo + 1.5 × Si + 0.5 × Nb

FIGURE 8.13 Schaffer diagram

 

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chromium carbide does not form. The amount of titanium and niobium to  be added to avoid the sensitization is calculated from the following empirical equations: Ti = 5 x (C+N) % Nb = 10 x (C+N) % 8.2.3.3 8.2. 3.3 Ferritic Ferritic Stainle Stainless ss Steel 

The major problems in welding of ferritic stainless steels are (i) uncontrollable grain growth (ii) hot tearing and (iii) hydrogen embrittlement. The ferritic ferrit ic stainless stainless steels are   “stabilized”  by adding titanium and/or niobium to stabilize against grain growth during welding. Hot tearing is due to the presence of impurities such as sulfur, phosphorus, carbon, and its compounds forming unwanted phases mainly during the last stage of solidi󿬁cation. Preheating to 150–200°C helps to reduce the hot tearing by reducing heat input and the holding time at high temperatures thus avoiding precipitation of unwanted phase. The best solution is to eliminate the impurities which is achieved in second and third generation of ferritic stainless steels and these are easily weldable as compared to   󿬁rst-generation ferritic stainle less ss st stee eels ls.. Hydr Hydrog ogen en embr embrit ittl tlem emen entt can can be preve prevent nted ed by usin using g (i) (i) pu pure re argon atmosphere during welding, (ii) low hydrogen electrodes, and (iii)  baked electrodes electrodes to avoid avoid moisture. moisture. 8.2.3.4 8.2. 3.4 Duplex Duplex Stainles Stainlesss Steels

Duplex stainless steels have a two phase structure of almost equal proportions of austenite and ferrite. Modern duplex steels are readily weldable  but the procedure, procedure, especially especially maintain maintaining ing the heat input input range, range, must be strictly followed to obtain the correct weld metal structure. Although most weld we ldin ing g proc process esses es can can be used used,, low low heat heat inpu inputt weld weldin ing g pr proc oced edur ures es are are

usually avoided. Preheat is not normally required and the maximum interpass temperatur temperaturee must be controlled controlled.. Choice of 󿬁ller or electrode is important as it is designed to produce a weld metal structure with a ferrite-austenite  balance  balan ce to match match the parent parent metal. metal. The nitrogen nitrogen loss during during welding welding is compensated by using the   󿬁ller or electrode with nitrogen or the shielding gas itself may contain a small amount of nitrogen. 8.2.3.5 Precipitation Hardenable Hardenable Stainless Steel Steelss Precipi Prec ipitat tation ion harden hardening ing stainl stainless ess steels steels are catego categorize rized d into three three grou groups: ps: martensitic, semi austenitic, and austenitic. It is important that proper   󿬁ller metals are used if it is intended that the welds have the same heat treatment response as the base material. Commonly   󿬁ller materials, having composition close to the welded parts, are used for welding precipitation hardening

 

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113

stainless stainl ess steels. steels. Weldabil Weldability ity of austenitic austenitic precipitat precipitation ion hardening hardening stainless stainless steelss is poor steel poor beca becaus usee of their their susc suscep epti tibi bili lity ty to hot hot crac cracks ks.. Limi Limite ted d he heat at input and welding of parts in solution treated condition are required for diminishing risk of cracks. Nickel alloys (nickel-chromium-iron) are used as 󿬁ller materials for welding austenitic precipitation hardening stainless steels. In genera general, l, weldin welding g of the precip precipita itatio tion n harden hardening ing stainl stainless ess steels steels is similar to the more common non hardenable stainless steels. When proper techniques are used, joints of excellent quality and high strength can be produced in the precipitation hardening alloys. The   󿬁nal heat treatment (postweld heat treatment) is carried out either during or after the joining operation. However, to obtain optimum properties, it is important that the recommended heat treating procedures is followed. 8.2.4 Applic Application ation for Corrosio Corrosion n Resistance and High Tempe Temperatur ratures es

Stainless steels are used extensively in industries for their corrosion resistance to both aqueous, gaseous and high temperature environments, for their mechanical properties at all temperatures from cryogenic to the very high high,, and and occa occasi sion onal ally ly for for ot othe herr spec specia iall phys physic ical al prop proper erti ties es.. Unif Unifor orm m corrosion is common in unprotected carbon steels but this does not occur on stainless steels in normal environments and stainless steel will provide unli un limi mite ted d se serv rvic icee life life with withou outt main mainte tena nanc nce. e. St Stai ainl nles esss steel steel is used used fo forr  buildings for both practical and aesthetic reasons because of its excellent corro co rrosi sion on re resi sist stan ance ce.. St Stai ainl nles esss st stee eell was was in vogu voguee du duri ring ng th thee art deco deco peri pe riod od.. Some Some di dine ners rs and and fast fast fo food od re rest stau aura rant ntss use use larg largee or orna name ment ntal al panels and stainless   󿬁xtures and furniture. Because of the durability of  the material, many of these buildings still retain their original appearance. Stainless steel is used today in building construction because of its durability and being a weldable building metal it can be made into aesthetically pleasing shapes. An example of a building in which these properties

are exploited is the Art Gallery of Alberta in Edmonton, that is wrapped in stai stainl nles esss stee steel. l. Type Type 316 316 st stai ainl nles esss is used used on th thee exte exteri rior or of both both th thee Petronas Twin Towers and the Jin Mao Building, two of the world ’s tallest skyscrapers. The Parliament House of Australia in Canberra has a stainless steel   󿬂agpole weighing over 220 metric tons. The aeration building in the Edmonton Composting Facility, the size of 14 hockey rings, is the largest stainless steel building in North America. Stainless steel is a modern trend for roo󿬁ng material for airports due to its low glare re󿬂ectance to keep pilots from being blinded, also for its properties that allow thermal re󿬂ectance in order to keep the surface of  the roof close to ambient temperature. The Hamad International Airport in Qatar was built with all stainless steel roo 󿬁ng for these reasons, as well as the Sacramento International Airport in California. Some   󿬁rearms incorp inc orpora orate te stain stainles lesss steel steel compo compone nents nts as altern alternati atives ves to blued blued steel. steel. Some handgun models, such as the Smith & Wesson Model 60 and the

 

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Colt M191 Colt M19111 pi pisto stol, l, are are made made enti entire rely ly from from st stai ainl nles esss st stee eel. l. Th This is give givess a high high lust luster er   󿬁nish nish,, simi simila larr in appe appear aran ance ce to nick nickel el plat platin ing. g. Unli Unlike ke platin pla ting, g, the   󿬁nish is not not subj subjec ectt to   󿬂aki aking, ng, peelin peeling, g, wearwear-off off from from rubbing, or rust when scratched. Martensitic stainless steels have superior wear resistance of high carbon alloys with the excellent corrosion resistance of chromium stainless steels. Martensitic stainless steels have good corrosion resistance to low concentrations of mild organic and mineral acids. Their exposure to chlorides in everyday type activities (e.g., food preparation, sport activities, etc.) is generally satisfactory when proper cleaning is performed after use. These alloys are used where strength, hardness, and/or wear resistance must be combined with corrosion resistance such as cutlery, dental and surgical instruments, nozzles, valve parts, hardened steel balls and seats for oil well pumps, separating screenssteels and strainers, springs, shears, and strip, wear surfaces. Martensitic stainless are available as plate, sheet, and   󿬂at bars. The ferritic stainless steels can be made with superior surface   󿬁nish and with wit h differ different ent surfac surfacee   󿬁nish nishes es and and ar aree most most suit suitab able le fo forr arch archit itec ectu tural ral applications due to its resistance to general corrosion. The re󿬁ned grades (secon (se cond d and third third genera generatio tion) n) ferrit ferritic ic stainle stainless ss steels steels are used used in many many 󿬁elds and applications such as elements and accessories for kitchens and  bathrooms, roofs, architectural objects, appliances, safeguards, metallic doors, elevators, storage tanks, and so on. Because of their good thermal conductivity and low thermal expansion these alloys are also used in the manufacturing of chimneys, muf 󿬂ers, exhaust systems, fasteners, fasteners, as well as heating elements used in molten salt baths for heat treatments, and so on. Corrosion of unprotected carbon steel occurs even inside reinforced concrete structures as chlorides present in the environment (marine/deicing) di diff ffus usee thro throug ugh h th thee conc concre rete te.. Corr Corros osio ion n prod produc ucts ts (rus (rust) t) have have a high higher er volume than the metal and create internal tensions causing the concrete

cover to spall. Among the various techniques used to mitigate the corrosion of steel reinforcing bar in concrete, use of ferritic stainless steels or martensitic stainless steels rather than carbon steel is one of the better methods. However, it is not popular due to the high cost. Stainless steels especially austenitic stainless steels have a long history of  applic app licati ation on in contac contactt with with water water due due to their their exc excell ellent ent corrosi corrosion on resisresistance. Applications include plumbing, potable and waste water treatment to desalination. Types 304 and 316 stainless steels are standard materials of  constr con struct uction ion in contac contactt with with water. water. Howeve However, r, with with increa increasin sing g chlori chloride de cont co nten ents ts high higher er allo alloye yed d st stai ainl nles esss st stee eels ls such such as Type Type 2205 2205 and and supe superr austenitic and super duplex stainless steels are preferred. Austenitic (300 series) stainless steel, in particular Type 304 and 316, are the materials of choice for the food and beverage industry. Stainless steels do not affect the taste of the product are easily cleaned and sterilized to prevent bacterial contamination of the food, and are durable. However,

 

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acidic foods with high salt additions, such as tomato sauce, and highly salted condiments, such as soya sauce may require higher alloyed stainless steels such as 6% molybdenum superaustenitic stainless steel to prevent pitting corrosion by chloride. Surgical tools and medical equipment are usually made of stainless steel, because of its durability and ability to be sterilized in an autoclave. In addition, surgical implants such as bone reinforcements and replacements (e.g., hip sockets and cranial plates) are made with special alloys formulated to resist corrosion, mechanical wear, and biological reactions in vivo. Stainless steel is used in a variety of applications in dentistry. It is comm co mmon on to use use st stai ainl nles esss st stee eell in many many inst instru rume ment ntss that that need need to be sterilized, such as needles, endodontic   󿬁les in root canal therapy, metal posts po sts in root root cana canall tr trea eate ted d te teet eth, h, te temp mpor orary ary crow crowns ns and and crow crowns ns fo forr deciduous arch wires orthodontics. The of surgical stainlessteeth, steeland alloys (e.g., 316and L) brackets have alsoinbeen used in some the early dental implants. Cook Co okwa ware re and and bake bakewa ware re may may be cl clad adde ded d in st stai ainl nles esss st stee eels ls fo forr easy easy clea cleani ning ng,, dura durabi bili lity ty and and fo forr use use in in indu duct ctio ion n cook cookin ing g (thi (thiss requ requir ires es a magnetic grade of stainless steel, such as 432). Because stainless steel is a poor conductor of heat, it is often used as a thin surface cladding over a core of copper or aluminium to conduct heat more readily. Stainless steel is used for jewelry and watches, with 316L being the type commonly used for such applications. Super austenitic stainless steels with 6% molybdenum are used in bleach plant and Type 316 is used extensively in the paper machine. It can be re󿬁nished by any jeweler and will not oxidize or turn  black. Valadium, a stainless steel and 12% nickel alloy, is used to make class and military rings. Valadium is usually silver-toned, but it can be electroplated to give it a gold tone. The gold tone variety is known as Sunlit Sun litee Valadi Valadium um.. Other Other   “Valadium”   type typess of allo alloys ys are are trad tradee na name med d differently, with names such as   “Siladium”   and   “White Lazon.”

Duplex stainless steels are used in heat exchangers, tubes and pipes for prod pr oduc ucti tion on and and hand handli ling ng of gas gas and and oi oil, l, heat heat exch exchan ange gers rs and and pipe pipess in desalinati desal ination on plants, plants, mechanic mechanical al and structural structural componen components ts for corrosive corrosive environme envir onments, nts, pipes pipes in process process industries industries handling handling solut solutions ions containin containing g chlorides, utility and industrial systems such as rotors, fans, shafts and press rolls where high corrosion fatigue strength is required   –  cargo tanks, vessels, piping, and welding consumables for chemical tankers. A wide range of stainless steels are used throughout the paper making process. For exam ex ampl ple, e, dupl duplex ex stain stainle less ss st stee eels ls ar aree bein being g used used in dige digeste sters rs to conv conver ertt wood chips into wood pulp. The stainle stainless ss steel, steel, howeve however, r, satis satis󿬁es the de󿬁nit nition ion for high high stre strengt ngth h steels in addition to corrosion resistance, they are not widely used because of cost cost due due to th thee high high amou amount nt of allo alloyi ying ng addi additi tion ons. s. Howe Howeve ver, r, so some me automotive manufacturers use stainless steel as decorative items in their vehicles. The use of corrugated stainless steel panels popular during the

 

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1960s and 1970s, declined due to high cost and are again picking up in metro trains due to the availability of lean grade austenitic stainless steels such as 200 series, 301, and 304. In addition to good appearance, the lean alloys strain harden rapidly and thus gain strength and wear resistance. Thee coac Th coache hess buil builtt with with th thes esee lean lean grad gradee au auste steni niti ticc st stai ainl nles esss steel steels, s, on accidents due to impact load, will strain harden and gain strength and will not get crushed ensuring the safety of the passengers. Stainless steels are used in aircrafts during 1930s but the use of stainless steel in mainstrea str eam m ai airc rcra raft ft is hind hinder ered ed by it itss exce excess ssiv ivee weig weight ht comp compar ared ed to ot othe herr materials, such as aluminium. Important considerations to achieve optimum corrosion performance are: choose the correct grade for the chloride content of the water; avoid crevices when possible by good design; follow good fabrication practices, particularly removing weld heat tint and sensitization; avoid formation of strain induced martensite in austenitic stainless steels since even small amount of martensite in austen austenit itic ic stain stainle less ss steel steelss (espe (especi ciall ally y in 316 grades grades)) will will un unde dergo rgo stress stress corrosion cracking; drain promptly after hydro testing. Selection of appropriate stainless steel for speci󿬁c corrosion environment is briefed in Chapter in  Chapter 9. 9. Hardness and strengths change very little with increases in tempering temperature up to 480°C and so martensitic stainless steels can be used up to temperature range of 400°C. Similarly, the ferritic stainless steels can also be used up to 400°C. The austenitic stainless steels can be used up to a temperature of 600°C. The other advantage in austenitic grade is the absence of ductile to brittle temperature and hence can be used at cryogeni ge nicc tempe tempera ratu ture re also also.. The The stain stainle less ss steel steelss are most most suit suitab able le ma mate teri rial al where moderate temperature and corrosion coexist.

8.3 8.3 To Tool ol Stee Steels ls As the name indicates, steels used for making tools for cutting and forming are called   “tool steels.”   The tool should have better properties than the workpiece materials to serve the intended purpose. 8.3.1 Prope Properties rties Requ Required ired for Tool Steels

The properties required are: (i) high room temperature temperature hardn hardness ess to withstand withstand against against wear (ii) good hardenabil hardenability ity for through through hardening hardening of thick thick secti sections ons (iii) resistance resistance to thermal thermal softening softening to withstand the temperature temperature produced during machining (iv) good toughness toughness and ductility ductility to withstand withstand shock loads

 

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(v) high stiffness stiffness for dimensional dimensional tolerances tolerances (vi) good wear resistanc resistancee for long life (vii) resistance resistance to chemical chemical reactions reactions with the workpiece workpiece and lubricant lubricantss (viii)) high thermal (viii thermal conducti conductivity vity to dissipate dissipate the heat (ix) low thermal expansion expansion for maintaini maintaining ng accuracy accuracy (x) (x) good good mach machin inab abil ilit ity y and and grin grinda dabi bili lity ty to shap shapee th thee to tool olss th that at are are normally complicated (xi) availabil availability ity at cheaper cheaper cost. In order to achieve these properties, more numbers of alloying elements in high amounts are added to steels and the necessity and effect of each element is brie󿬂y given here. 8.3.2 Effect of Allo Alloying ying Elements Elements

Carbon is the element that increases both hardness and hardenability and thus increases abrasion (wear) resistance. Carbon is the cheapest alloying element in tool steels. Chromium increases hardenability, wear resistance  by forming carbides and corrosion corros ion resistance resist ance by forming chromium oxide layer. Chromium also increases resistance to molten metal and oxidation useful in making die steels for melting low temperature metals such as aluminium. It also increases scaling resistance. Five percent chromium can ca n resi resist st scal scalin ing g up to 650° 650°C, C, 8% chro chromi mium um is requ requir ired ed fo forr sc scal alin ing g resistance up to 750°C. About 10 –12% chromium is required to resist te temp mper erat atur uree of 850° 850°C. C. Addi Additi tion on of Si (0.8 (0.8–1% 1%)) incr increa eases ses stren strengt gth h of  chromium oxide layer and can further increase the resistance to oxidation and scalin scaling. g. Chrom Chromium ium also also protec protects ts from from chemic chemical al attack attack and liqui liquid d metal attack. Vanadium and molybdenum form primary carbides and act

as grain re󿬁ners apart from increasin increasing g hardenabi hardenability. lity. Silicon Silicon increases increases hardenability and high temperature strength. Nickel increases hardenabil ab ilit ity y and and toug toughn hnes esss and and impa impart rtss shoc shock k resi resist stan ance ce to to tool ol steel steelss and and reduces cracking while quenching by decreasing austenitizing temperature tu re.. Coba Cobalt lt impr improv oves es re red d hard hardne ness ss (o (or) r) hot hot hard hardne ness ss (resi (resista stanc ncee to thermal softening) but it decreases hardenability. Normally, reduction in harden har denabi abilit lity y due due to cobalt cobalt is compen compensate sated d by adding adding other other alloyi alloying ng elements that increase hardenability. Tungsten apart from forming primary carbides improves hot hardness. 8.3.2.1 8.3. 2.1 Role of Carbides Carbides in Tool Steels

As discussed in   Chapter 55,, the carbide forming elements form carbides such as M7C3, MC, M6C, or M23C6. The properties, especially the wear resistance in tool steels, are decided by the type of carbides present. The

 

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role of carbi carbides des is very very critic critical al and its its impact impact on hardn hardness ess and and wear wear resistan resistance ce is shown in Table in  Table 8.8. 8.8. Thee tool Th tool stee steell liste isted d in se seri rial al numb number er 3 has has low low hard hardne ness ss and and wear wear rate rate due due to high volume ume of carb arbides an and d type ype of carb arbides. It fo form rmss MC typ type and and M7C3 type of car arbi bid des. es. The The to tool ol st stee eell list listed ed in se seri rial al numb number er 7 has th thee high highes estt hard hardne ness ss but but the wear resistance is low although the total carbide content is high. It has low amount of MC type of carbides and forms more M6C and M23C6  type of carbides. Increasing M7C3  from 11–12% to 14–15% the wear resistance can be increased by a factor of two. However, further increase in M 7C3 causes decrease in malleability. Increasing MC content can further increase the wear resistance but more than 25% decreases grindability. In case of  tool steels used for elevated temperatures, wear rate mainly depends on size and distribution of carbides. At high temperature the carbides coalesce and form form coarse coarse preci precipit pitate atess decrea decreasin sing g the proper propertie ties. s. M7C3   carbides easi ea sily ly coale oalesc scee th than an MC ty type pe and and so fo forr elev elevat ated ed temp temper erat atur ures es MC carbides are preferred for better properties. The carbide forming elements should be cautiously selected for the intended application. 8.3.3 8.3 .3 Typ Typee of Tool Tool Steels

Theree ar Ther aree many many ty type pess of to tool ol st stee eels ls.. They They are are na name med d ba base sed d on th thei eirr hardening harde ning condition conditions, s, properties, properties, and/or and/or applicati applications. ons. A few important important types of tool steels are briefed in this section. They are:  “

”

(i) (i) Wate Wa terr ha hard rden enin ing g tool toTheir ol steel steels s   arecontent re repr pres esen ente ted d as W   0.6 followed follow by –W5). numerals (W1 carbon varies between anded1.4% with chromium of maximum 0.5% and vanadium 0.25%   –  simple high carbon steel. These grades are hardened by quenching in water and

TABLE 8.8 Relation between type and volume of carbides with hardness and wear resistance at room temperature S. No

Hardness (HRc)

Total carbide content (% Vol)

61 61.5 60 60 61.5 62 63.5

16 17 17 23–25 16–18 16–18 22 22

–

1 2 3 4 5 6 7

             

   

M7C3

 

MC

M6C and M23C6

–

 

–

–

–

–

16 17 17   19–20 9–10 9–10 –   –  

4–5 7–8 7–8 3 3

 

–

 

–

 

–

19 19

         

Volume of worn off, mm3

0.268 0.263 0.117 0.228 0.223 0.277 0.271

 

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hencee call henc called ed wate waterr hard harden enin ing g to tool ol steel steels. s. They They are are ve very ry hard hard and and strong but lack in ductility and toughness. (ii) Shock resisting tool steels   are denoted as   “S”   followed by numerals (S1-S7). The nominal composition in % is: C   –  0.45–0.65, Si   –   1–2, Cr   – 1.5–3.5 and Mo   –   0.5–1.4. They have good toughness due to relatively low carbon than water hardening tool steels and can resist shocks or repeated loading and hence known as shock resisting tool steels. (iii) Cold work tool steels (or) Oil hardening steels  are symbolized as   “O” fol follow lowed by nume umeral rals (O1(O1-O O7). 7). Thes Thesee grade radess have ave manga angan nese ese (1–1.6 1.6%,) %,),, silico silicon n (1%), (1%), chrom chromium ium (0.5 (0.5–0.7 0.75%) 5%),, tungst tungsten en (0.5 (0.5–1.75%), and molybdenum (0.25%), along with carbon (0.9–1.2%). These grades of stee steels ls have have good good hard harden enab abil ilit ity y and and as mart marten ensi site te fo form rmss upon upon quenching in oil are called oil hardening tool steels. (iv) Air hardening tool steels   are indicated as   “A”  followed by numerals (A2   –  A4, A6- A10). These grades of tool steels contain high amount of  carb ca rbon on (1–1.3 1.35%) 5%),, manga manganes nesee (1.8 (1.8–2% 2%), ), sili silico con n (1.2 (1.25% 5%), ), chro chromi mium um (1–5% 5%), ), nick nickel el (1.5 (1.5–1.8 1.8%), %), vanadi vanadium um (1–4.7 4.75%) 5%),, tungst tungsten en (1–1.25%), and molybdenum (1–1.5%). They can be hardened by cooling in air to very high hardenability due to the alloying elements and hence called ai airr hard harden enin ing g to tool ol steel steels. s. Thes Thesee grad gradee of to tool ol st stee eels ls have have exce excell llen entt stiffness, good wear resistance, and moderate red (or) hot hardness  but they have poor fabricability. (v) High Carbon High Chromium steels  are indicated as   “D”  followed by numerals (D2   –  D5, D7). These grades have high carbon (up to 2.25%), high chromium along vanadium (4%), molybdenum (1%), and cobalt (3%).(12%) Due to highwith carbon and chromium they are called high carbon high chromium (HCHCR) tool steels. These grades have exce ex cell llen entt wear wear re resi sista stanc nce, e, good good abra abrasi sion on resi resist stan ance ce,, and and mini minima mall

dimensional changes during hardening. (vi) Hot working tool steels  are speci󿬁ed as   “H”  followed by numerals. These grade tool steels are used for making tools for high temperature appl ap plic icat atio ions ns such such as di dies es for for hot hot fo forg rgin ing, g, die die ca cast stin ing g dies dies,, plast plastic ic moul mo uldi ding ng di dies es,, and and so on, on, and and henc hencee are are ca call lled ed hot hot work workin ing g to tool ol steels. Based on their composition, the hot working tools steels are subd su bdiv ivid ided ed as chro chromi mium um base base (10 (10–19 19), ), tung tungst sten en base base (21 (21–26 26)) and and molybdenum base (41–43). The chrome base hot working tool steels have about 3.25 to 4.25% chromium along with carbon   –   0.35–0.4%,  tungsten   –   1.5–5% and molybdenum   –   1.5–2.5%. vanadium   –   0.4–2%, 2%, tungsten The chrome base tool steels have low thermal expansion, good red hardness, hardn ess, good toughness, toughness, low distortion distortion while while hardening hardening,, medium medium resistance to corrosion and oxidation and good fabrication characteristi istics cs.. Tung Tungste sten n base base hot hot work workin ing g to tool ol st stee eels ls have have tu tung ngst sten en abou aboutt 9–18 18% % al alon ong g with 2–12 12% % chro chromi mium um and and 0.25 0.25–0.5% 0.5% carb carbon on.. Thes Thesee

 

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grades hav grades havee low dis distor tortio tion n dur during ing que quench nching ing,, goo good d hot har hardne dness, ss, good toughness and fair wear resistance. These grades are costly as compared to chromium base and fabrication properties are inferior to chrome chr ome bas basee but can wi with th sta stand nd hig higher her tem temper peratu atures res tha than n chrom chromee  base. In molybdenum base hot working tool steels, tungsten is partially replaced by molybdenum. The nominal composition (in wt %) of  the molybdenum based tool steels are: C   –  0.55–0.65, Cr   –   4, V   –   1–2, W   –   1.5–6 and Mo   –   5–8. These steels are cheaper than tungsten-based hot working tool steels. (vii) High speed steels (HSS)  are identi󿬁ed as   “T”   and   “M”   based on the major alloying element tungsten or molybdenum based respectively. Advent of HSS in around 1905 made a break through at that time in the cutting speed. It can cut four times faster than the carbon steels and are so known as high speed steel. They are complex iron-base alloys of carbon, chromium, vanadium, molybdenum, or tungsten, or combin com binati ations ons th there ereof, of, and wit with h sub subst stant antial ial amount amountss of cob cobalt alt.. The carbon and alloy contents are balanced to give high attainable hardening response, high wear resistance, high resistance to the softening effect of heat, and good toughness for effective use in industrial cutting operations. Commercial practice has developed two groups of cutting materials as tungsten base (T1, T2, T4 –T6, T8, T 15) and molybdenum  base (M1–M4, M6–M7, M 30, M 33, M 34, M 36, M 43–M 47). The nominal composition (in wt %) of tungsten base HSS are: C: 0.7 –1.5, Cr: 4, V: 1–5, W: 12–20, Co: 5–12. Among the various grades T1 is most widely used that has 18% tungsten, 4% chromium and 1% vanadium –

–

and more commonly referred to as 18 4 1. Thee ma Th mate teri rial al sh shor orta tage gess an and d high high co cost stss ca caus used ed by Wo Worl rld d War II spurred the development of less expensive alloys substituting molybdenum for tungsten. The advances in molybdenum-based high speed

steel during this period put them on par with and in certain cases  better than tungsten-b tungsten-based ased high speed steels resulting in the use of M2 steel steel ins instea tead d of T1 ste steel. el. Mol Molybd ybdenu enum m abo about ut 4.5–9.5 9.5% % rep replac laced ed the tungsten from 12–20% to 1.5–6.75%. Both tungsten and molybdenum  base HSS have good red hardness and wear resistance. Now a days advanced tools like carbide tools, cemented carbide tools, that can cut 4 to 12 times faster than HSS have superseded HSS, HSS   󿬂ourish still due to its lower cost and good fabrication properties as compared to the advanced cutting tools. (viii) (vi ii) Mo Mould uld steels steels   speci󿬁ed as   “P”   follow followed ed by nu numer merals als (P1–P39) are used for making casting dies to handle the liquid metal requiring good shock sh ock res resist istanc ancee rat rather her th than an har hardn dness ess.. Hen Hence, ce, th thee car carbon bon con conten tentt is kept low (0.07–0.1%). The other alloying elements such as chromium (0.6–5%), nickel (0.5–3.5%), and molybdenum (0.2% max.) are added to increase the sticking resistance and toughness for making moulds.

 

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(ix) Special purpose tool steels  are of two types, low alloy type (L) and carb ca rbon on-t -tun ungs gste ten n ba base sed d (F (F). ). Th Thee lo low w al allo loy y type type ha hass abou aboutt 0.5 0.5–1.1% carbon, 0.75–1.5% chromium, maximum of 1.5% nickel, maximum of  0.2% 0.2 % van vanadi adium um and mo molyb lybden denum um of 0.2 0.25% 5% ma maxim ximum um.. The These se ste steels els havee goo hav good d wea wearr res resist istanc ance, e, tou toughn ghness ess and fai fairr res resist istanc ancee to dim dimenensional sio nal tol tolera eranc nces. es. The car carbon bon-tu -tungs ngsten ten typ typee has tungs tungsten ten 1.2 1.255–3.5% and an d ca carb rbon on 1–1. 1.25 25%. %. Th Thes esee gr grad ades es ar aree br brit ittl tlee an and d ha have ve hi high gh we wear ar resistance.

8.3.4 8.3. 4 Class Classii󿬁cation Based on Properties Based on thermal stability, the tool steels are classi 󿬁ed as (i) nonthermo stable (W and S type), (ii) semithermo stable (D, O and A types), and (iii) thermo stable (T and M based HSS). Nonthermo stable tool steels achieve high hardness and strength by forming martensite which in turn depends on the amount of carbon. Mostly nonthermo stable tool steels are plain carbon and with low alloying elements. The properties fall if the temperature is increased above 200°C due to tempering of martensite. Semithermo stable tool steels will have high carbon and high carbide forming elements such su ch as ch chrom romium ium,, mo molyb lybden denum, um, van vanadi adium, um, tun tungst gsten, en, and so on. The hard ha rdne ness ss an and d st stre reng ngth th is du duee to ma mart rten ensi site te an and d ca carb rbid ides es.. Th Thee al allo loy y carbides resist the softening of martensite during tempering and can be used for slightly elevated temperatures (about 400°C). For Thermo stable tool steels, the hardness and strength are due to martensite, carbides, and preci pr ecipit pitati ation on har harden dening ing (or (or)) int interm ermeta etalli llics. cs. The car carbid bidee for former merss suc such h as tungsten, molybdenum, and vanadium also along thei theirr re resp spec ecti tive ve ca carb rbid ides es. . He Henc nce, e, mo more reform am amou ount ntssintermetallics of thes thesee elem elemen ents tswith ar aree added in the thermo stable tool steels. The strength, hardness, and other prope pr opert rties ies mai mainly nly dep depend end on car carbid bides, es, the their ir siz sizee and dis distr tribu ibutio tion. n. The

thermo stable can with stand high temperatures.

8.3.5 8.3. 5 Heat Treat Treatment ment of Tool Steels Heat treatment of tool steels differs slightly from the general heat treatment discussed in   Chapter 22.. The tool steels are used in hardened and tempered condition and are stress relieved after rough machining to the required shape. The work sequence is: rough machining, stress relieving, semi󿬁nis nish h ma machi chinin ning, g, har harden dening ing and tem temper pering ing,, and   󿬁na nall grin grindi ding ng to required dimensions. Stress relieving:  This treatment is done after rough machining by heating to 550–650°C. The material is heated until it has achieved uniform temperature all the way through and then cooled slowly, in a furnace.  Heating to hardening (austenitizing) temperature:  The fundamental rule for hea heatin ting g sh shoul ould d be slo slow w to har harden dening ing (au (aust steni enitiz tizing ing)) tem temper peratu ature re to minimize distortion. Normally, vacuum furnaces or furnaces with controlled

 

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protective gas atmosphere or molten salt baths are used to avoid decarburization and oxidation that result in low surface hardness with a risk of  cracking. Holding time at hardening temperature cannot be generalized to cover all heating situations and the composition of the tool steel. It varies  between  betwe en 30 minutes minutes to few minutes. minutes. Especially Especially in HSS, the soaking soaking time should shou ld be minimu minimum m since since the carbid carbides es can dissoc dissociat iatee an and d carbon carbon enters enters austenite and the alloying elements (ferrite stabilizers) go in to solution and try to st stab abil iliz izee fe ferri rrite te.. Afte Afterr quenc quenchi hing ng it ends ends up with with marte martens nsite ite and and retained ferrite that not only reduces the properties and cannot be restored. Quenching rate (critical cooling rate) is decided based on the TTT diagram as discussed in  in   Chapter 2. 2. The choice between a fast and slow quenching rate rate is usua usuall lly y a comp compro rom mise; ise; to get get th thee best best micr micros ostr truc uctu ture re an and d to tool ol performance, the quenching rate should be rapid; to minimize distortion, a slow quenching rate, but higher than critical cooling rate, is recommended. Appropriate quenching medium needs to be selected for correct results and great care has to be taken while quenching through the martensite range, as martensite formation leads to an increase in volume inducing stresses in the material. Martempering is one of the best of all hardening methods. New concepts have been introduced with modern types of furnaces, and the the te tech chni niqu quee of quen quench chin ing g at a cont contro roll lled ed rate rate in a prot protec ecti tive ve gas gas atmosphere or in a vacuum furnace with gas is becoming increasingly widespread. The cooling rate is roughly the same as in air for protective gass at ga atmo mosp sphe here re,, and and th thee prob proble lem m of oxid oxidiz ized ed surf surfac aces es is elim elimin inat ated ed.. Inco In comp mple lete te form formati ation on of mart marten ensi site te resu result ltss in reta retain ined ed auste austeni nite te on quenching decreasing the properties. Complete formation of martensite shou sh ould ld or be oilen sure red dened befo be reedte temp mper erin ing treat treolin atme ment Tool Tove ol st stee th that atnsit are ar wa wate terroiensu l-ha hard rden edfore need ne subz subzer ero og cool co ing g nt. to. have ha full fueels ll lsma marte rtens iteee  because in most cases the Mf temperature temperat ure is below room temperature. temperat ure. In case case of ai airr-ha hard rden ened ed to tool ol steel steels, s, te temp mper erin ing g itse itself lf wi will ll co conv nver ertt th thee

retained austenite to martensite and this needs double or triple tempering discussed here brie󿬂y. Tempering:  The material should be tempered immediately after quenching. If it is not possible, the material must be kept warm, in a special   “hot cabinet,” awaiting tempering. The choice of tempering temperature is often determ det ermine ined d by experi experienc encee and needs. needs. If maximu maximum m hardn hardness ess is desire desired, d, tempering must be done at about 200°C, but never lower than 180°C. High speed steel is normally tempered at about 20°C above the peak of the secondary hardening temperature. If a lower hardness is desired, higher te temp mper erin ing g te temp mper erat atur uree is empl employ oyed ed.. Norm Normal ally ly,, do doub uble le temp temper erin ing g is recommended for tool steel that are air hardened and triple tempering is done for high speed steel with a high carbon content, over 1%. After quenching (hardened in air), certain amount of austenite remains untransformed when the material is to be tempered. During   󿬁rst tempering, most of the austenite is transformed to martensite and untempered. A second tempering gives the material optimum toughness and required

 

 High Alloy All oy Steels

 

123

hardne hard ness ss.. The The same same li line ne of re reas ason onin ing g can can be appl applie ied d wi with th rega regard rd to retained austenite in high speed steel. In this case, however, the retained aust au sten enit itee is high highly ly allo alloye yed d and and tr tran ansf sfor orma mati tion on to marte martens nsit itee is slow slow.. During tempering, some diffusion takes place in the austenite, secondary carbides are precipitated resulting in low alloyed austenite that is more easily transformed to martensite when it cools after tempering. So, third tempering can be bene󿬁cial in driving the transformation of the retained austenite further to martensite. Holding times in connection with tempering is also critical. After the tool is heated through, hold the material for at least two hours at full temperature each time. Apar Ap artt from from th thee re regu gula larr heat heat tr trea eatm tmen ent, t, surf surfac acee trea treatm tmen ents ts such such as nitriding, nitro carburizing are carried out to increase the surface hardness and an d wear wear prop proper erti ties es.. Surf Surfac acee coat coatin ing g of to tool ol st stee eell is beco becomi ming ng more more common. The hard coating normally consists of titanium nitride and/or titanium carbide. These coatings have very high hardness and low friction giving a very wear resistant surface, minimizing the risk of adhesion and sticki sti cking. ng. The two most most commo common n coatin coating g method methodss are: are: physic physical al vapour vapour deposition (PVD) coating and chemical vapour deposition (CVD) coatings. Generally, the tool steels used for cutting operation will have carbon more than 0.6% carbon and are hardened to high hardness (59 –60 HRc) values but the tool steels used for making dies will have carbon less than 0.6% and are hardened to low value of hardness (42 –50 HRc). The die steels stee ls need need some some toughn toughness ess to arrest arrest the propag propagati ation on of cracks cracks formed formed during operation while the same is not required for cutting grade tool steels as they are under the state of triaxial compressive stresses as against die steels, that experience tensile stresses also. 8.3.6 Applic Application ationss for Machining Machining and Forming

Water-hardening tool steels with 0.6–0.75% carbon are used where some toughness is required such as hammers and concrete breakers. For waterhardening harde ning tool steels with carbon content content of 0.75–0.95% used for tools such as punches, chisels, dies, and shear blades hardness is of prime importance rather than toughness. In case of wood working tools, drills, taps, reamers, turning tools, and so on, where wear resistance is more important, waterhard ha rden enin ing g tool tool st stee eels ls with with 0.95 0.95–1.4% 1.4% carb carbon on are are used used.. Howe Howeve ver, r, th thee applic app licati ations ons are limite limited d to soft soft materi materials als like like wo wood, od, brass, brass, alumi aluminiu nium, m, and soft steels that require low cutting speed and low cost. Shock resisting tool steels are mainly used as forming tools, punches, chisels, pneumatic tools, and shear blades. Cold work tool steels (or) oil-hardening grade tool steels are used for making tools for cold working operations such as taps, form tools, and expansion reamers. These types of tool steels have good wear resistance. Air-hardening tool steels are used for making blanking, forming, trimming, and thread rolling dies. High carbon high chromium steels are used for making blanking, piercing dies, wire drawing dies, bar

 

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and tubes, thread rolling dies, and master gauges. Hot working tool steels (chromium-based) are used for making hot extrusion dies, die casting dies, hot forging dies, mandrels, and hot shearing dies. Hot working tool steels (tungs (tu ngsten ten-ba -based sed)) are used used for making making mandre mandrels ls and extrus extrusion ion dies dies for  brass, nickel alloys, and steels. The applications of hot working molybdenum-based tool steels are same as that of tungsten-based tool steels but are lower cost than tungsten-based tool steels. High speed steels are used for making cutting tools, tool bits, drills, reamers, broaching taps, milling cutters, hobs, saws, and wood working tools. Mould steels are used for making master hub, low temperature die castin cas ting g dies, dies, and inject injection ion and compre compressi ssion on mouldi moulding ng dies dies for plasti plastics. cs. Spec Sp ecia iall purp purpos osee to tool ol steel steelss (low (low allo alloy) y) are are us used ed fo forr maki making ng beari bearing ngs, s, rollers, clutch plates, cam collets and wrenches of the machine tools and also in gauges, knurls, and so on. Special purpose tool steels (C-W) are used for paper cutting knives, wire drawing dies, plug gauges, forming, and   󿬁nishing tools where toughness is not the major requirement.

 

9 Selection of Materials

9.0 Int Introd roduct uction ion Most problems originate from improper selection of work, material, men, process, tools/equipment, place, and time. If any one of these is wrong the result is undesirable. Material selection is an important step in the process of designing any physical object in the context of product design, safe and reliable functioning of a part or component at minimum cost while meeting product performance goals. Systematic selection of the best material for a given application begins with properties, cost, and other nontechnical parameters of candidate materials. For example, a thermal blanket must have ha ve poor poor ther therma mall cond conduc ucti tivi vity ty in orde orderr to mini minimi mize ze heat heat tran transf sfer er fo forr a give given n te temp mper erat atur uree di diff ffer eren ence ce.. The The mate materi rial al sele select cted ed fo forr th thee blan blanke kett should be affordable. The other nontechnical factors such as availability, ordering time, skill, and/or facility available to fabricate the blanket in the local conditions decide the material for making a thermal blanket.

9.1 Too Tools ls Used for Select Selection ion of Materials Materials Normal methods of selection of materials are: (i) consultin consulting g experts since they have developed developed the expertise over long experience and experimentations (ii) based on the establishe established d handbooks handbooks (iii) searching searching in the Internet. Internet. The main problems in these methods can be: (i)dif 󿬁cult to get experts, or too costly for small tasks (ii) details details in handbook handbookss may be obsolete obsolete in the fast-changin fast-changing g mater materials ials world 125  

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(ii (iii) i) very very often often local local condit condition ionss may not match match with with the search search results results from the Internet or dif 󿬁cult to avoid bogus information. The other best way is to systematize the process of selection such that even a novice will be able to follow. Objective of systemizing is certainly not to suggest   “the best” material for any component:   “Name a component  – here is the suitable material.”   But to help arrive at the most appropriate material logically. As the proverb goes,   “It is a lot better to teach   󿬁shing rather than gifting a pound of  󿬁  󿬁 sh.”

9.2 System Systemized ized S Selectio election n of Materials Materials The rudiments of selection are the same whether the selection is engineering material or men or process   –  or, for that matter, anything. If the basics of selection are understood, the best possible material, process, or men can  be selected for every work. In order to systematize, three groups of data are required: (i) data on availability, (ii) data on requirements, and (iii) data on nonfunctional aspects. The data data on availa availabil bility ity includ includes es the differ different ent engin engineer eering ing materi materials als available in that location, their technologically relevant properties, manufacturing properties and supply conditions such as minimum quantity, forms of  avai av aila lab bilit ility y (she (sheet etss or rods rods or powd powder ers, s, and and so on.) on.),, deli delive very ry sc sch hedu edules, les, and and so on. Engineering materials can be broadly classi󿬁ed as metals such as iron, copp co pper, er, al alum umin inum um,, and and their their allo alloys ys,, and and so on, on, and and nonm nonmet etal alss such such as ceramics ceram ics (e.g., (e.g., alumina alumina and silica silica carbi carbide), de), polymers polymers (e.g. (e.g. poly vinyl chloride), chloride), natural natu ral materi materials als (e.g., (e.g., wood, wood, cotton cotton,, 󿬂ax ax,, and and so on.) on.),, comp compos osiites tes (e.g (e.g.., carb carbon on

󿬁 bre

reinforce reinforced d polymer, polymer, glass   󿬁 bre reinforced reinforced polymer, polymer, and so on.), on.), and foams. Each of these materials is characterized by a unique set of physical, mechan mec hanic ical, al, and chemic chemical al prope propertie rties, s, that can be treated treated as attrib attribut utes es of  a speci󿬁c material. The selection of material is primarily dictated by the speci󿬁c set of attributes required for an intended service. In particular, the selection of a speci󿬁c engineering material for a part or component is guided  by the function it should perform perform and the constraints constraints imposed imposed by the properproperties of the material. For example, electrical wires need good electrical conduct du ctiv ivit ity y to avoi avoid d el elec ectri trica call loss losses es.. Si Sinc ncee meta metals ls are good good cond conduc ucto tors rs of  electricity metals can be considered for selection. Among the metals silver, copper, aluminium, steel, or nickel can be considered. The data on manufacturing properties needed to shape the selected material for the desired shape or contour as well as the manufacturing properties such as machinability, formability, weldability, and castability will be required. Next Ne xt data data re requ quir ired ed will will be fu func ncti tion onal al requ require ireme ment ntss such such as st stre reng ngth th,, hardness, wear resistance, and so on, service life expected, shaping required, cost co st of se serv rvic icee fail failur uree and and se serv rvic icee env environ ironme ment nt such such as tem tempera peratu ture re,,

 

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corrosion, humidity, and so on. The technical requirements are guided by the design. The service life expected is more crucial in selection as it varies from few seconds to few years. The material selection is mainly based on the service life. For example, the passenger car’s service service life is about 󿬁 fteen years. The material for body in white should be selected accordingly. High corrosion resistant materials like stainless steels and titanium although suitable, these metals will increase the cost of the automobile. A coated/painted steel can comfortably withstand   󿬁fteen years and, hence, it is better to select any of the suitable grade of steel based on other requirements. The shape and size of the component is decided by the design and the selected material should have the manufacturing properties. Cost of service failure plays a major role in the selection of material and the cost of service failure is taken care by the factor of safety in design. For example, failure of the aerospace components during service will result in heavy damages. With a view to avoid service failure the factor of safety can be increased, that will increase the initial cost and an d runn runnin ing g (fuel (fuel)) cost cost.. The The best best alter alterna nativ tivee is to use use the the mater materia iall wi with th stringe stri ngent nt norms norms in the require required d propert properties ies,, normal normally ly cal called led as aerosp aerospace ace grad gr ades, es, will will reduc reducee the the prob probab abil ilit ity y of fail failur uree wi with thin in th thee fact factor or of sa safe fety ty.. Service environment is another important parameter to consider. For example, in urban conditions, weathering steels is best against corrosion resistance  but it cannot be used in indu industrial strial or marine conditio conditions ns due to the presence presence of  hi high gh amou amount nt of sulf sulfur ur and and chl chlor orin inee in the the indu indust stri rial al and and mar marin inee en envi viro ronm nmen ent. t. The third data required are on aspects such as cost, legality, conventional/traditional preferences, and aesthetics. Cost is more important especially in comparison with the market competitors. The purchasing power of the people has great impact on the cost. Legality and environmental issu issues es shou should ld also also be cons consid ider ered ed whic which h vary vary from from plac placee to plac placee an and d countr cou ntry y to countr country. y. Conven Conventio tional nal/tr /tradi aditio tional nal prefer preferenc ences es and aesthe aesthetic tic

tastes of the clients are more important in the consumer products. For example, colored stainless steel though technically success, the products made of colored stainless steel have not succeeded commercially mainly  because of tradition that stainless steels must have a shiny white surface. A judicious combination of all these three factors leads us to the selection of appropriate materials. Very often more than one material may meet the requirements. In such cases, selection may be based on the ranking of  the functional and nonfunctional requirements.

9.3 Cas Case e Studie Studiess 9.3.1 Case I: Manufa Manufacturin cturingg of Gears Data on availability:   Assume practically all types of materials are available in the region and also their technical properties, supply conditions, and so on, are known. So the   󿬁rst data group is ready.

 

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Dataof ontorque requirements:  Theshould gears are for torque magnitude transmitted be known. If ittransmission. is minimal as The in clocks, even polymeric (plastic) gears would be tolerable. For slightly higher level as in car windshield wiper motor, medium carbon steel will be suitable. For the torque for driving a car, low alloy steel in heat treated condition may be required and so on and so forth. The service life expected depends on the product, say for clocks,   󿬁ve years is suf 󿬁cient whereas for automobiles it should be about   󿬁fteen years. The dimensions and shape of the gear, decided by the design engineer, depends on facility available in local conditions. Cost of service failure is high for automobiles but for clock it is low.. Servic low Servicee enviro environm nment ent like like local local temper temperatu ature re condi conditio tions, ns, corros corrosive ive environments should also be considered. While selecting the gear material for winter conditions and marine atmospheres, the material behavior with respect to temperature and its corrosion resistance need to be prioritized. Data on nonfunctional aspects:  Cost is the major in 󿬂uencing factor in this group. Based on the cost (raw material and manufacturing) plastics, mild steel, gun metal, and alloy steels are suitable in the increasing order. Based on the cost of failure and service environments, the material can be selected. If these factors are low, cheaper material such as plastics can be selected. No other nontechnical factor is serious in this example.

9.3.2 Case II: Selectio Selection n of Reinfo Reinforcing rcing Mate Material rial for Cement Concr Concrete ete in Marine Environments Data on availability: Assume practically all types of materials are avail-

able in the region and also their technical properties, supply conditions, and so on, are known. So the   󿬁rst data group is ready. Data on requirements: The reinf reinforce orcemen mentt should should withs withstan tand d the static static load, load,

the the ther therma mall expa expans nsio ion n shou should ld matc match h with with conc concre rete te and and it shou should ld resis resistt corrosion due to chlorides; the service life and maintenance of the concrete. Data on nonfunctional aspects: Cost is the major in󿬂uencing factor in the selection. No other nontechnical factor is serious in this example. Selected Select ed materials materials: Based on the static strength, thermal expansion and cost, most suitable material is mild steel. But the corrosion of unprotected carbon steel occurs even inside reinforced concrete structures as chlorides present in the environment (marine) diffuse through the concrete. Corrosion products (rust) having a higher volume than the metal create internal tension causing spall. Mitigating the corrosion steel rein reinfo forc rcin ing g bar bar the in concrete conc concre rete tecover is a to must mu st.. Vari Va riou ouss tech techni niqu ques es are are of reco recommmended: mend ed: thicker thicker concrete concrete cover; cover; cathodic cathodic protection protection;; membranes membranes,, epoxy coatings, and so on. The other alternative is to use stainless steel rather than carbon steels. Stainless steel provides both strength and corrosion resistance inside the concrete, providing a long, maintenance free service life of the structure  but the cost is higher as compared to protected mild steel. It should be

 

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note no ted d that th auste aunsio steni niti c ing st stai ainl ess steel stedel half shou shlfould nohigh t gher beersele seth lect cted edconc be caus use itlls ther th erma mal l at expa expans ion n tic bein be g nles two tw os and an ha time tild menot hi than an cobeca ncre rete te ewill wiits damage the concrete from inside due to temperature   󿬂uctuations. Among these two materials, the selection is based on which factor is prioritized: cost or the maintenance-free service life. If it is cost, coated mild steel is suitable material; if it is maintenance-free service life, stainless steel is the appropriate material. 9.3.3 Case III: Selection of Stainless Stainless Steel for Architectur Architectural al Applicatio Applications ns

In general, all grades of stainless steel have good resistance to general corrosion compared to carbon steels. But for speci󿬁c environments, correct grade of stainless steel has to be selected for maintenance-free long service life with a low life cycle cost and excellent sustainability. It is important to note no te that that this this sele select ctio ion n is only only fo forr appe appear aran ance ce and and not not fo forr st stru ruct ctur ural al integrity. Data on availability: Assume practically all types of stainless steels are available in the region and also their technical properties, supply conditions, and so on are known. So the   󿬁rst data group is ready. Data on requirements: The main requirement is maintenance free, sustainable grade of stainless steel at low cost for the given environment. So, the following factors have to be considered: (i) environmental pollution, (ii) coastal exposure or deicing salts exposure, (iii) local weather pattern, (iv) design considerations, and (v) maintenance schedule. The criteria required for this application should be evaluated based on the score for each of the conditions as given in Table in  Table 9.1 9.1.. Selected materials: Based on the scores, the grade of stainless steels is selected. If the score is between 0 and 2, 304 or 304 L will be an economical

choice. Type 316/316L or 444 is the most economical choice if the score is 3. Type 317L or a more corrosion resistant stainless steel is suggested for the score 4. If the score is 5 and above, more corrosion resistant stainless stee steell such such as 4462 4462,, 317L 317LMN MN,, 904L 904L,, supe superd rdup uple lex, x, supe superf rfer erri riti tic, c, or a 6% molybdenum superaustenitic stainless steel is to be selected. Systematic selection for applications requiring multiple criteria is more comp co mple lex. x. For For exam exampl ple, e, a rod rod th that at shou should ld be st stif ifff and and ligh lightt requ requir ires es a material with high Young’s modulus and low density. If the rod will be pulled in tension, the speci󿬁c modulus, or modulus divided by density, ’

will determine the best material. Butmaterial because for a plate scales as its thickness cubed, the best a stiffs bending and lightstiffness plate is determined by the cube root of stiffness divided by density. An Ashby plot, named for Michael Ashby of Cambridge University, is a scatter plot that that di disp spla lays ys two two or more more prop proper erti ties es of many many mate materi rial alss or clas classe sess of  materials. These plots are useful to compare the ratio between different prop pr oper erti ties es.. For For th thee exam exampl plee of th thee stiff stiff/l /lig ight ht part part disc discus usse sed d abov abovee an Ashby plot will have Young’s modulus on one axis and density on the

 

 

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TABLE 9.1 Points for each functional requirements Points Poin ts Criter Criteria ia –  Environmental pollution 0 1 2 3 3 4

           

Very low or no pollution Low (urban conditions) Moderate (urban conditions) High (urban conditions) Low or moderate (industrial conditions) High (industrial conditions)

Criteria –  Coastal exposure 1

 

Low (1.6 to 16 km from salt water)

3 4

       

Moderate (30 m to 1.6 km from salt water) High (< 30 m from salt water) Marine (some salt spray or occasional splashing) Severe marine (continuous splashing) Severe marine (continuous immersion)

5 8

 

10

Deicing salts exposure 0

 

1 3

     

No salt salt was was detected detected on a sample from the site and and no change in exposure conditions is expected or traf 󿬁c and wind levels on nearby roads are too low to carry chlorides to the site and no deicing salt is used on sidewalks Very low salt exposure (≥ 10 m to 1 km) Low salt exposure(< 10 to 500 m) Moderate salt exposure (< 3 to 100 m)

4

 

High salt exposure (< 2 to 50 m)

2

Local weather pattern

-1 0 1 2

       

Cold climates, regular heavy rain, humidity below 50% Tropical or subtropical, wet, regular or seasonal very heavy rain Regular very light rain or frequent fog, humidity above 50% Hot, humidity above 50%, very low or no rainfall

Design Considerations 0 -2 -1 1

       

Boldly exposed for easy rain cleaning and vertical surfaces Surface  󿬁 nish is pickled, electro polished, or roughness   ≤ Ra0.3  μ m Surface  󿬁 nish roughness Ra 0.3   μm < X   ≤ 0.5  μ m Surface  󿬁 nish roughness Ra 0.5   μm < X   ≤ 1  μ m

     

Surface nish roughness Ra > 1  μ m Sheltered location or unsealed crevices Horizontal surfaces

 󿬁

2 1 1

Maintenance schedule 0 -1 -2 -3

       

Not washed Washed at least naturally Washed four or more times per year Washed at least monthly

 

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other axis, withitone datato point for material each candidate On such a plot, is easy   󿬁ndon outthe notgraph only the with thematerial. highest stiffness, or that with the lowest density but also with the best ratio. Using a log scale on both axes facilitates selection of the material with the best plate stiffness.

 

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Subjective Questions

Short Answers 1. Why is steel known as a moving standard? 2. What are the four allotropes of iron? 3. Steel is an enigma   –  it rusts easily, yet it is the most important of all metals. Justify this statement. 4. Why are phase transformations involving austenite very important in the heat treatment of steels? 5. Why is the iron-carbon diagram not a true equilibrium diagram? 6. Although the iron-carbon diagram is not a true equilibrium diagram, it is useful for practical applications. Why? 7. Write down the peritictic reaction in the iron-carbon diagram. Mention the concentration of carbon and temperature at which it occurs. 8. Write down the eutectic reaction in the iron-carbon diagram. Mention the concentration of carbon and temperature at which it occurs.

9. Write down the eutectoid reaction in the iron-carbon diagram. Mention the concentration of carbon and temperature at which it occurs. 10. What is pearlite? 11. De󿬁ne heat treatment. 12. What are the four basic types of heat treatment? 13. Why is the strength of the normalized steel higher than annealed steel of the same composition? 14. De󿬁ne hardening. 15. Why won’t the phase formed due to hardening be found in the iron carbon diagram? 16. What is meant by transformation diagrams? What are the two types of  transformation diagrams? 17. De󿬁ne critical cooling rate. 18. What are the two types of martensite? State the relation between the amount of carbon and the type of martensite formed. 19. Schematically represent the relationship between carbon content and the maximum obtainable hardness in steels.

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󿬁

Subjective Questions

20. De ne 21. List thehardenability. quenchants used for hardening the steels. 22. What is the purpose of tempering the hardened steels? 23. What are the types of isothermal heat treatments? 24. What is meant by surface hardening? 25. What are the two types of surface hardening? 26. What is the purpose of adding alloying elements in steel? 27. List the ferrite and austenite stabilizers in steels. 28. Which are the alloying elements that stabilize ferrite without forming carbides? 29. Why cobalt is called as a neutral stabilizer? 30. Phosphorous is the most effective solid solution hardener but it will be normally restricted below 0.25% in steels. Why? 31. What are the elements present in steel as metallic particles? 32. What is meant by subzero cooling? Why it is necessary? 33. De󿬁ne carbon equivalent. 34. What is the amount of carbon in 1060 steel? 35. Nickel is the   󿬁rst element alloyed with iron, but has been removed now from the series of low alloy steels. Why? 36. Why has the use of tungsten in low alloy steels declined since 1940? 37. Why are silicon steels called electric steels? 38. Why does boron facilitate isothermal processes in steels? 39. De󿬁ne high strength steels. 40. What are the different types of conventional high strength steels? 41. What are the different types of advanced high strength steels? 42. What is Giga Pascal steel? 43. Why aren’t austenitic stainless steel and maraging steels listed as high

strength steel though they have high strength and ductility? 44. What is the need for high strength steels? 45.. Why 45 Why aren aren’t conv conven enti tion onal al hi high gh st stre reng ngth th st stee eels ls reco recomm mmen ende ded d fo forr electroplating? 46. What is meant by patenting of steel wires? 47. Why do thermomechanical processes have limitations in industrial scale production? 48. The major setback in the nanograined material is the lack of ductility due to the loss of work hardening capacity. How it can be solved? 49. What are the potential potential advantages advantages of the t he mixed microstruct microstructure ure of bainitic bainitic and’austenite? 50. ferrite Why aren t high strength nanobainitic steels as popular as quenched and tempered martensitic steels? 51. Why are maraging steels classi 󿬁ed as ultra-high strength steel? 52. Which element is added in maraging steels to induce precipitation hardening?

 

Subjective Questions

 

135

53. is meant by marforming? ’t maraging steels used in automotive industries? 54. What Why aren 55. How do stainless steels achieve their   “stainless”  properties? 56. More chromium is added in stainless steels than required amount for forming self-healing chromium oxide layer. Why? 57. How are stainless steels classi󿬁ed and what are they? 58. Ferritic stainless steels are used for their anticorrosion properties rather than for their mechanical properties: true or false? Justify your answer. 59. What are the types of embrittlement associated with ferritic stainless? 60. Which type of stainless steel(s) can be used for cryogenic applications? Why? 61. What are the classi󿬁cations of austenitic stainless steels? 62. The formation of martensite is thermodynamically possible in austenitic stainless steels, but it is not forming. Why? 63. Why are austenitic stainless steels quasielastic? What are its implications? 64. What is meant by sensitization in austenitic stainless steels? 65. Why chromium carbides normally precipitate along grain boundaries in austenitic stainless steels? 66. What is meant by stabilization in austenitic stainless steels? 67. In general, what are the weaknesses of austenitic stainless steels? 68. How duplex stainless steels have both ferrite and austenite at room temperature? 69. What are the bene󿬁ts of duplex stainless steels? 70. What are the three types of precipitation hardenable stainless steels? 71. What are the alloying elements added in precipitation hardenable stainless steels to achieve precipitation hardening characteristics? 72. What What are the main main problems problems during during weld welding ing of martens martensiti iticc stainl stainless ess steel? steel?

73. What are the main problems during welding of austenitic stainless steel? 74. What are the main problems during welding of ferritic stainless steel? 75. Why electrodes electrodes matching matching with the base materi material al are not used for welding welding austenitic stainless steels? 76. What are the advantage and limitation in using ferritic stainless steels or martensitic stainless steels instead of carbon steel in reinforced concrete? 77. Why aren’t austenitic stainless steels used in reinforced concrete? 78. Although stainless steels satisfy the de 󿬁nition for high strength steels in addi ad diti tion on to corro corrosi sion on re resi sist stan ance ce,, th they ey are are no nott clas classi si󿬁ed unde underr high high strength steels. Why not? 79. List the properties fortools tool such steels. 80. Although currently,required advanced as carbide tools and cemented carbide tools have superseded HSS, still HSS is used. Why? 81. Classify tool steels based on their thermal properties. 82. In heat treatment of high speed steels, soaking time at the austenitic region should be minimum. Why?

 

136

 

Subjective Questions

83. Generally, tool steels forwhile cutting to high hardness (59-60 HRc) used values tooloperations steels usedare forhardened making dies are hardened to low value of hardness (42-50 HRc). Why? 84. What are the normal methods of selection of materials? 85. What are the rudiments of selection of materials? 86. For most of application steel is the preferred material. Why? 87. De󿬁ne   “Cost of service failure”?

Long Answers 1. Draw the iron-carbon diagram, label the phases and lines, and explain the three invariant reactions of the system. 2. Explain the solubility of carbon and properties of ferrite, austenite and cementite phase in iron carbon diagram. 3. Explain the microstructure developed during slow cooling of eutectoid steel. 4. Explain the microstructure developed during slow cooling of hypo eutectoid steel. 5. Explain the microstructure developed during slow cooling of hyper eutectoid steel. 6. What is annealing? Explain various types of annealing with respect to steel? 7. What is normalizing? Explain why the amount of phases differ from the equilibrium diagram when normalized? 8. Represent various types of annealing and normalizing for hypo and

hyper eutectoid steels schematically and explain. 9. Explain hardening with respect to TTT diagram. 10. With a schematic sketch, explain how hardenability is measured using the Jominy end quench test? 11. Explain with schematic sketch (i) martempering, (ii) ausforming, and (iii) austempering. 12. Explain the mechanism of precipitation hardening. 13. Explain the various case hardening techniques. 14. Explain case carburizing? Narrate its advantages over other chemical surface hardening methods. 15. Explain case nitriding? Detail its advantages over other chemical surface hardening methods. 16. Explain carbonitriding? Detail its advantages over other chemical surface hardening methods. 17. Explain boriding? What are its advantages over other chemical surface hardening methods? 18. Explain the effect of carbon on mechanical properties of steel.

 

Subjective Questions

 

137

19. What are the applications, properties, and microstructure of low carbon steels. 20. Expl Explai ain n the the applic applicati ation ons, s, prope properti rties es,, and and micr microst ostru ruct cture ure of medi medium um carbon carbon steels. 21. What are the applications, properties, and microstructure of high carbon steels. 22. Enumerate the effects of the alloying elements on iron carbon diagram. 23. What are the effects of manganese, nickel, and nitrogen on steels? 24. It is necessary to carefully control, not only the nitrogen content, but also the form in which it exists, in order to optimize impact properties. Explain the statement. 25. Discuss the effect of cobalt addition in steels. 26. Discuss the type of carbides formed and their properties in steels when chromium, molybdenum, tungsten, tantalum, vanadium, niobium, zirconium, and titanium added as alloying elements. 27. Summarize the effect of various types of carbides on properties of low alloy steels. 28. What are the elements present in steel as metallic particles? Explain the purpose of adding those elements. 29. Explain the role of manganese, tantalum, titanium, and zirconium in controlling the sulphide and oxysulphides inclusions and its in 󿬂uence on the properties of steel. 30. Explain the effect of alloying elements on hardenability. 31.. Expla 31 Explaiin why why a ti time me dela delay y in subz subzer ero o tr trea eatm tmen entt does does not not resu result lt in comp comple lete te transformation of retained austenite to martensite? 32. Explain the factor affecting hardenability.

33. Explain the effect of alloying elements on tempering. 34. Write short notes on any two types of low alloy steels. 35. Narrate the microstructure and applications of interstitial free steels. 36. Schematically explain the high strength and advanced high strength steels. 37. Write brie󿬂y about any two types of high strength steels and problems in developing those steels. 38. How standard low alloy steel (AISI 4340) is modi 󿬁ed as high strength steels? What are its limitations? 39. Write brie󿬂y about rephosphorized steel. 40. Write short notes on micro alloyed or high-strength low alloy (HSLA) steels. 41. Enumerate about bake hardening (BH) steels. 42. Explain how patented steel wires achieve their high strength? 43. Write brie󿬂y about nanostructured steels. 44. Write short notes on bainitic steels. 45. Explain the conditions required to achieve strong and tough bainitic steels.

 

138

 

Subjective Questions

46. Explain the metallurgy of maraging steels. 47. Explain the effect of composition on maraging steels. 48. The increase in strength of maraging steels is much higher if both cobalt and molybdenum are added together rather than individually. Why? 49. Write brie󿬂y about types of maraging steels. 50. Write short notes on the mechanical properties of maraging steels. 51. Explain the effect of solutionizing and aging temperature on mechanical properties of maraging steels. 52. Narrate about the corrosion behavior of maraging steels. 53. Write brie󿬂y the advantages and applications of maraging steels. 54. Write brie󿬂y on the effects of alloying elements on structure and properties of stainless steels. 󿬁

55. How stainless stainless are type classiofed? Explain microstructure, properties and applications ofsteels any one stainless steels. 56. Explain microstructure, properties, and applications of any one type of  heat treatable stainless steels. 57. Write brie󿬂y about three generations of ferritic stainless steels. 58. Explain the types of embrittlement associated with ferritic stainless. 59. Explain the different types of austenitic stainless steels? 60. Write brie󿬂y about strain induced martensite in stainless steels. 61. Discuss the precipitation of carbides in austenitic stainless steels and its effect on properties. 62. What is sensiti sensitizati zation? on? Explai Explain n how how it can can be avoid avoided ed in in au austen steniti iticc stainl stainless ess steels. 63. Explain the heat treatment (precipitation hardening) sequence for semiaustenitic stainless steels.

64. Discuss brie󿬂y about weldability of stainless steels. 65. Write an essay about application of stainless steels with respect to its corrosion resistance and high temperatures. 66. Discuss the effect of alloying elements on tool steels. 67. Explain the role of carbides in tool steels. 68. Write brie󿬂y on the different types of tool steels. 69. Write brie󿬂y the complexities of the heat treatment of tool steels. 70. Why is double tempering recommended for tool steels that are air hardened, while triple tempering is done for high speed steel with a high carbon content, for example, over 1% ? Explain brie󿬂y. 71. Discuss the applications of tool steels. 72. What are the normal methods of selection of materials? Explain their relative limitations in brief. 73. What are the rudiments of selection of materials and explain them brie󿬂y. 74. What are the implications of a cost of service failure in the selection of  materials? 75. Explain with an example the role of nonfunctional (nontechnical) aspects in the selection of materials.

 

Objective Questions

 

139

Objective Objecti ve Questions Questions 1. Solubility of carbon in FCC-gamma iron is more than in alpha-BCC iron, though the packing factor of FCC is more than BCC (a) lower free energy (b) larger void size (c) (c) smal smalle lerr at ato omic rad radii (d) (d) more va vaccanc ancy 2. Which of the following get diffused in the surface layer of the steel parts subjected to nitriding (a) (a) Mono Monoat atom omic ic nitr nitrog ogen en (b) (b) mole molecu cula larr nitro nitroge gen n (c) Ammonia (d) none of these 3. The maximum solubility of Carbon (in wt %) in FCC iron at 1147°C is (a) 0.8 % (b) 2.1% (c) 0.025% (d) 6.67% 4. Eutectoid temperature in iron carbon diagram occurs at (a) 723°C (b) 1333°F (c) 996 K (d) all 5. From the following microstructures ( microstructures  ( 󿬁gure 2.2) of 2.2)  of pearlite, identify normalized and annealed (both the steels are having the same composition and at the same magni󿬁cation) (a) I is annealed and II is normalized (b) I is normalized and II is annealed (c) both are normalized (d) both are annealed 6. The order of decreasing weldability among the following steels is P) Fe- 0.6 C Q) Fe-0.4 C R) HSLA a) R-Q-P

b) P-Q-R

c) Q-P-R

d) Q-R-P

7. In steels, which of the following element does not form carbide and stabilizes ferrite (a) Ni (b) Mn (c) Si (d) Hf   8. Sample A is austenitized at 900°C and sample B is austenitized at 1300°C. Both the samples are having the same composition and quenched in the same medium. Which will have more depth of  hardness? (a) Sample A (b) sample B (c) both both will will have have same same depth depth of hardn hardness ess (d) cannot cannot say 9. Of the following which element will increase the austenitising temperature without forming carbides in steels? (a) Cr (b) Si (c) Ni (d) Mn 10. During heat treatment: (a) Microstructure of a given material is modi 󿬁ed, and consequently engineering properties are modi󿬁ed. (b) There is no change in microstructure, but properties are modi 󿬁ed. (c) There are changes only in dimensions. (d) None of the above

 

140

 

Objective Questions

11. Pearlite is a (I) (I) sin singl glee pha phase se (II) (II) mixt mixtur uree of of two two phas phases es (III (III)) eut eutec ecto toid id mi mixt xtur uree (a) I is correct (b) I and II are correct (c) (c) II and and III ar aree corre orrect ct (d) (d) II is corre orrect ct 12. The strength of normalized steel is_______________ annealed steel of  the same composition (a (a)) lower ower than than (b (b)) sam same as (c (c)) hig igh her (d) (d) cann cannot ot say say 13. The purpose of tempering (a) to conv conver ertt re reta tain ined ed auste austeni nite te to martensite (c) to har arde den n th thee marte artens nsit itee 14. Cobalt is aite stabili (a) austen austenite stabilizer zer (c) (c) neut neutral ral st stab abil iliz izer er

(b (b)) to soft soften en the the mart marten ensi site te (d) (d) to conv onvert ert aust austen eniite to ferr ferriite. te.

(b) ferrite ferrite stabil stabilize izerr (d) (d) carb carbid idee fo form rmer er

15. In steels, copper is present as a (a) carbide (b) solid solution in ferrite (c) solid solid solu solutio tion n in in auste austenit nitee (d) metall metallic ic ele elemen mentt 16. The average amount of carbon in 1060 steel is (a) 0.6 at% (b) 0.6 wt% (c) 1.06 at%

(d) 1.06 at%

17. Of the following, which is not a high strength steel? (a) TRIP ste teeels (b) dual phase st steeels (c) stainl stainless ess steels steels (d) marten martensit sitic ic steels steels 18. The element hardening is added in maraging steels to induce precipitation

(a) manganese

(b) molybdenum

(c) cobalt

(d) titanium

19. The element added in stainless steels to achieve its   “stainless” property is (a) nickel (b) chromium (c) molybdenum (d) titanium 20. Of the following, which type of type of stainless steel can be used for cryogenic applications? (a) ferritic (b) martensitic (c) duplex (d) austenitic 21. In the heat treatment of high speed steels, soaking time at austenitic region should be (a) minimum (b) maximum 22. The amount of pearlite in 0.6% carbon steel is (a) 75% (b) 9% (c) 25% (d) 91% 23. The amount of ferrite in 0.2% carbon steels is 75%. If the steel is normalized, the amount of pearlite will be (approx.) (a) 25% (b) 75% (c) 35% (d) 65%

 

Objective Questions

 

141

24. Annealing temperature of hypereutectoid steels is 50°C above (a) (a) A1 li line ne (2) (2) A2 li line ne (3) (3) A3 li line ne (4) (4) Acm Acm line line 25. Final structure of austempered steel (a) (a) pea pearl rlit itee (b) (b) fer ferri rite te + grap graphi hite te (c (c)) bai baini nite te (d) (d) mar marte tens nsit itee 26. Which of the following element makes steel strong at cheaper cost and effectively? (a) chromium (b) carbon (c) calcium (d) chlorine 27. Annealing improves (a) grain grain size size (b) ducti ductilit lity y (c) electr electrica icall prop propert erties ies (d) All of abov abovee 28. A peritectic reaction is de󿬁ned as (a) (a) two two soli solids ds re reac acti ting ng to form form (b) (b) two two soli solids ds reac reacti ting ng not not to fo form rm a liquid (c) (c) li liqu quid id and and soli solid d re reac acti ting ng to form another solid

a liquid (d) (d) two two soli solids ds reac reacti ting ng to fo form rm a third solid

29. Austempering is the heat treatment process used to obtain higher (a (a)) hard ardness ness (b (b)) to tou ughn ghnes esss (c (c)) britt rittle len ness ess (d) (d) duc ductili tility ty 30. The hardness obtained by hardening process does not depend upon (a) carbon content (b) work size (c) atmosp atmospher heric ic temper temperatu ature re (d) quench quenching ing rate rate 31. If the alloy steel at room temperature is magnetic, which phase should not be present? (a) ferri rrite (b) pearlite (c) aust steenite (d) cementite 32. Which of the following affects the hardenability of steel?

(a) (a) amou amount nt of of car carbo bon n in aust austen enit itee (c) (c) ins insol olub uble le part partic icle less in auste austeni nite te

(b) (b) auste austeni niti ticc grai grain n si size ze (d) (d) all all of tthe he abov abovee

33. The minimum carbon percentage required in steel so as to respond to hardening by heat treatment is (a) 0.02% (b) 0.08% (c) 0.2% (d) 0.8% 34. The crystal structures of two alloy steels having nominal carbon content (i) 18%Cr (ii) 18%Cr8%Ni. (a) (a) FCC FCC and and BCC BCC (b) (b) bot both h BCC BCC (c (c)) bot both h FCC FCC (d) (d) BCC BCC and and FCC FCC 35. Which alloy steel having nominal carbon content (i) 18%Cr and (ii) 18% Cr8%Ni can be hardened by conventional quenching from high temperatures? (a) i only (b) ii only (c) (c) i and and ii (d) (d) neit neithe herr can can be hard harden ened ed by quen quench chin ing g 36. The hardness of martensite in normal steel is primarily due to (a) carbon (b) austenite stabilizers (c) (c) ferri ferrite te st stab abil iliz izers ers (d) (d) neut neutral ral stabi stabili lize zers rs

 

142

 

Objective Questions

37. The crystal structure of martensite in maraging steels is (a) BCT (b) BCC (c) FCC (d) HCP 38. Melting point of plain carbon steel is normally (a) less than melting point (b) more than melting point of pure iron of pure iron (c) same as pure iron (d) depends on carbon content 39. Which one of the following techniques does NOT require quenching to obtain   󿬁nal case hardness? (a)   󿬂am amee hard harden enin ing g (b) (b) in indu duct ctio ion n hard harden enin ing g (c) nitriding (d) carburizing 40. A 0.4% plain carbon steel sheet is heated and equilibrated in the inter-critical region followed by steel instant water quenching. The microstructure of the quenched sheet consists of  (a (a)) full fully y mar arte tens nsiite (b (b)) proeu roeute teccto toid id ferri errite te + marte arten nsi site te (c) (c) mart marten ensi site te + pear pearli lite te (d) (d) mart marten ensi site te + au aust sten enit itee 41. The intergranular corrosion can be prevented using (a) stabilized grade of stainless steel containing (b) low carbon grade of  titaniu tita nium m and niobiu niobium m as alloyi alloying ng elemen elements ts stainl stainless ess steel steel (c) both a. and b. (d) none of the above 42. Which element precipitates at the grain boundaries, when austenitic stainless steel is heated at 900°C? (a (a)) al alu uminiu inium m car arb bid idee (b (b)) chrom romium ium carb arbide ide (c) (c) magn magnes esiu ium m carb carbid idee (d) (d) moly molybd bden enum um carb carbid idee 43. Which of the following is the last phase obtained after completing heat

treatment cycle in patenting process? (a) (a) bain bainit itee (b) (b) ma mart rten ensi site te (c (c)) pear pearli lite te

(d) (d) none none of th thee abov abovee

44. Which of the following statements is false for heat treatment processes? (a) mart martem empe peri ring ng proc proces esss is desi design gned ed to over overco come me limi limita tati tion onss of quen quench chin ing g (b) pearlite is obtained as the   󿬁nal phase in martempering process (c) water is used as quenching medium in the Jominy end quench test (d) martensite in maraging steels can be cold rolled 45. Which of the following factors increase hardenability of a steel? (a) alloying elements (b)   󿬁ne grain size (c) high high nitr nitroge ogen n conten contentt in steel steel (d) all all of the above above 46. In which of the following methods, surface of a steel component  becomes hard due to phase transformation of austenite to martensite? (a) nitriding (b)   󿬂ame hardening (c) both a and b (d) none of the above 47. The process of decomposing martensitic structure, by heating martensitic steel below its critical temperature is called as (a) (a) auste austeni niti tisi sing ng (b) (b) quen quench chin ing g (c (c)) tempe temperi ring ng (d) (d) subz subzer ero o cool coolin ing g

 

Objective Questions

 

143

48. What is the crystal structure of   δ-ferrite? (a) body-center body-centered ed cubic structure structure (b) face-centered face-centered cubic cubic structure structure (c) orthorhom orthorhomic ic crystal structure structure (d) face-cent face-centered ered tetragonal tetragonal structure structure 49. Austenite phase in Iron-Carbon equilibrium diagram (a) is is face-c face-cent entere ered d cubic cubic structu structure re (b) is magne magnetic tic phas phasee (c) exists below 727°C (d) is hardest phase 50. The best and economical method for selection of material is (a) consulting experts since experts have developed the expertise over long experience and experimentations (b) based on established handbooks (c) searching the Internet (d) to systematize the process of selection Q.No

1

2

3

4

5

6

7

8

9

10

Answer

b

a

b

d

a

a

c

b

b

a

Q.No

11

12

13

14

15

16

17

18

19

20

Answer

b

c

b

c

d

b

c

d

b

d

Q.No

21

22

23

24

25

26

27

28

29

30

Answer

a

a

c

a

c

b

d

c

b

c

Q.No

31

32

33

34

35

36

37

38

39

40

Answer

c

d

c

d

d

a

b

a

c

b

Q.No

41

42

43

44

45

46

47

48

49

50

Answer

c

b

c

b

a

a

c

 

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a

a

d

 

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Engineering. Butterworth-Heinemann, Oxford. Engineering. Physical Metallurgy 9. 9.Abb Abbasc aschia hian, n, R., L. Abb Abbasc aschia hian n and R. E. Ree Reed-H d-Hill ill.. 200 2000. 0.   Physical Principles.. Cengage Learning, Stamford, CT. Principles 10.Prabha 10. Prabha,, B., P. Sundaramoort Sundaramoorthy, hy, S. Suresh, S. Manimozhi and B. Ravisankar. Ravisankar. 2009. Studie Stu diess on stress stress cor corros rosion ion cra cracki cking ng of supe superr 304 304H H Austen Austeniti iticc sta stainl inless ess ste steel. el.  Journal of Materials Engineering Performance (December). Performance (December). DOI: 10.1007/s11665-0089347-9.   https://link.springer.com/article/10.1007/s11665-008-9347-9 9347-9. https://link.springer.com/article/10.1007/s11665-008-9347-9.. 11 11.. Aj Ajit ith, h, P. M. M.,, P. Sath Sathiy iya, a, K. Gudim Gudimet etla la an and d B. Rav Ravis isan anka kar. r. 20 2013 13.. Mech Mechan anic ical al,, metallurgical characteristics and corrosion properties of equal channel angular Research 717: 9–14. pressing of duplex stainless steel. Advanced steel. Advanced Materials Research 717: 12.Mahata, A. K., U. Borah, A. Davinci, S. K. Albert and B. Ravisankar. 2014. Room temperature temper ature torsional behavior of 15-Cr15-Cr-15Ni 15Ni titani titanium um modi󿬁ed austen austenitic itic stainless steel. Procedia steel. Procedia Engineering 86: Engineering  86: 166–172. 13.Kondaveeti, C. S., S.2017. P. Sunkavalli, D. Undi, V. Hanuma Kumar, K. and B. Ravisankar. Metallurgical and L. mechanical properties ofGudimetla mild steel processed by Equal Channel Angular Pressing (ECAP). Transactions (ECAP).  Transactions of the Indian Institute of Metals 70: Metals 70: 83–87. 14.Ravisankar, B. and A. Rajadurai. 1992. Pack boronising of AISI 304 & 410 stainless steel. International Convention on Surface Engineering, INCOSURF 92, Indian Institute of ScienceBangalore. 15. 15. Ar Ariv ivaz azha haga gan, n, R., R., B. Pr Prab abha ha,, S. Mani Manimo mozh zhi, i, S. Su Sure resh sh,, G. Um Umas asha hank nker er,, R. Nagalakshmi and B. Ravisankar. 2007. Metallurgical studies in super 304H 145

 

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 Index

A

Acicular ferrite, 74 ferrite,  74 Advanced High Strength Steels, 65 Steels, 65,, 66  66,, 79 Age hardening, 23 hardening, 23,,  46  46,, 83  83,,  84 Allotropes, 1 Allotropes,  1,,  2 Alloy carbides, 40 carbides,  40,,  50,  50, 121 Alpha ferrite, 4 ferrite, 4,, 5 Annealing, Annealing, 11 , 12,  12 ,  13,  13,, 110  14  14,,  16  16,, 33  33,,  56  56,,  70,  70, 78 78, 11, ,  103,  103 ,  105,  105 Ausaged steels, 84 steels,  84 Ausformimg, 21 Ausformimg,  21,,  22,  22,  23  23,,  32  32,, 56 Austempering, 21 Austempering,  21,, 22,  22,  74 Austenite stabilizers, 36 stabilizers, 36,,  37  37,,  93,  93,  103 Austenite stability, 101 stability, 101,,  103 Austenitic stainless steels, 6 steels, 6,,  38,  38, 42  42,,  43  43,, 89 89,,  94,  94, 95,  95,  100–106 106,,  110  110,,  111  111,, 114–116 201, 101 201,  101,, 102 202, 102 202,  102 301, 101 301,  101,, 102,  102,  104,  104,  116 302, 102 302,  102

BH 260/370, 79 260/370,  79 Boriding (or) Boronizing, 30 Boronizing, 30 Boron, 27 Boron,  27,,  28  28,, 30  30,,  56  56,,  57  57,, 62  62,,  74  74,, 78  78,,  87 Boron steels, 45 steels, 45,,  52  52,, 56  56,,  57  57,, 62 C

Calcium, 86 Calcium, 86 Carbonitrides, 39 Carbonitrides,  39,,  43  43,, 57  57,,  69  69,, 89 Carbonitriding, 11 Carbonitriding,  11,,  30 Carbon equivalent, 69 equivalent, 69 Carbon steels, 3 steels, 3,,  11  11,, 12  12,,  19  19,, 21  21,,  23  23,, 27  27,,  28  28,, 31–33 33,,  38,  38,  43  43,,  46  46,, 47  47,,  49  49,, 51  51,,  53  53,, 57 57,, 61,  61, 65  65,, 70  70,, 72  72,, 74  74,, 79  79,, 113  113,, 120  120,, 128,,  129 128 Carburizing, 3 Carburizing,  3,, 11,  11,  28–30 30,,  55,  55, 123 Case depth, 27 depth, 27,,  28 Case hardening, 27 hardening, 27–29 29,,  56 Cementite, 4 Cementite,  4–9,  13–15 15,,  20,  20, 21  21,,  37  37,, 39  39,,  41  41,, 43 43,, 46,  46,  56  56,, 72  72,,  74  74,,  76–78 Chemical surface hardening, 27 hardening, 27,,  28

Chromium, 23,,  30 Chromium, 23  30,,  32  32,, 33  33,,  36  36,,  37  37,, 41 –44 44,, 47 47,, 49,  49,  54–57 57,,  67,  67,  78  78,, 84  84,,  87  87,, 90  90,, 92–96 96,,  98,  98, 100  100,,  101,  101, 104–107 107,, 110–114 114,,  117–121 121,, 123,  123,  124 Chromium carbide,  carbide,   41 41,,   42 42,,   93 93,,   94 94,, 104 –106 106,,   111 111,,   112 Chromium steels, 41 steels, 41,,  55 Cobalt, 30 Cobalt,  30,, 39  39,,  40  40,, 44  44,,  46–48 48,,  83–87 87,,  117,  117, 119,, 120 119 Continuous Cooling Transformation, 16 Transformation, 16 Copper, 29 Copper,  29,,  45  45,,  47  47,, 93  93,,  101  101,, 108,  108,  115  115,,  126 Critical Cooling Rate, 16 Rate, 16,,  19–21 21,,  28,  28,  39  39,, 122 Cyaniding, 11 Cyaniding,  11

302 B, 101 B,  101 304, 101 304,  101,, 102,  102,  105,  105,  106  106,,  111  111,, 114  114,,  116  116,, 129 304L, 102 304L,  102,, 106,  106,  129 305, 101 305,  101,, 102 309, 101 309,  101,, 102,  102,  111 310, 101 310,  101,, 102 316, 102 316,  102,, 113,  113,  114,  114,  115  115,,  116  116,, 129 316L, 101 316L,  101,, 102,  102,  115,  115, 129 316LN, 102 316LN,  102 317, 102 317,  102 317L 101, 317L  101, 102  102,,  129 321, 102 321,  102 347, 347, 101  101, , 102 904L, 101 904L,  101, , 102,  102,  129

D B

Delong diagram, 111 diagram, 111 Dispersed metallic particles, 44 particles,  44 Dual phase steels, 62 steels,  62,, 70 DP 280/600, 79 280/600,  79 DP 300/500, 79 300/500,  79

Bainite, 16,,  21–23, Bainite, 16 23,  56  56,, 72  72,, 74  74,,  76–78 Bainitic steels, 74 steels, 74–76, 76,  81 Bake hardening steels, 69 steels, 69,, 70  70,,  78–80 BH 210/340, 79 210/340,  79

149

 

150

DP 350/600, 79 350/600, 79 DP 400/700, 79 400/700, 79 DP 500/800, 79 500/800, 79 DP 700/1000, 79 700/1000,  79 Ductile to Brittle temperature, 101 temperature,  101,, 116 Duplex stainless steels, 45 steels,  45,, 95  95,,  106–108 108,, 112,,  114, 112  114,  115 S31200, 108 S31200,  108 S31260, 108 S31260,  108 S31803, 108 S31803,  108 S32001, 108 S32001,  108 S32205, 108 S32205,  108 S32304, 108 S32304,  108 S32520, 108 S32520, 108 S32550, 108 S32550,  108 S32750, 108 S32750,  108 S32760, 108 S32760,  108 S32900, 108 S32900,  108 E

Embrittlement, 39,,  56, Embrittlement, 39  56, 67  67,,  68  68,,  83,  83,  90  90,,  91  91,, 98 98,,  100,  100, 103,  103,  112 Epsilon carbide, 50 carbide, 50 Equal Channel Angular Pressing, 72 Pressing, 72 Eutectic, 6 Eutectic,  6,, 53 Eutectoid, 6 Eutectoid,  6–9,  36 –40, 40,  46  46,, 53  53,,  54  54,, 56  56,, 71

 

Index

Flame hardening, 11 hardening, 11,, 28  28,,  29 Free machining steels, 51 steels, 51,, 53 Full annealing, 12 annealing, 12,,  13 G

Giga Pascal steel, 66 steel, 66 H

Hardenability, 18–20 Hardenability, 18 20,,  30,  30, 32  32,,  33  33,, 35  35,,  39  39,, 40 40,,  48,  48,  49  49,, 54–57 57,, 62,  62,  67  67,, 74  74,,  79  79,, 91 91,,  116,  116, 117,  117,  119 Hardening, 11,, 12 Hardening, 11  12,,  14–16 16,,  20,  20, 21  21,,  25  25,, 27  27,, 28 28,, 30,  30,  31  31,, 35  35,,  39  39,, 40  40,,  46  46,,  49  49,, 50  50,, 73 73,, 74,  74,  76  76,, 78  78,,  93  93,, 97  97,,  100  100,, 101,  101, 103,,  107, 103  107, 116  116,,  118–123 High carbon steel, 31 steel, 31–33 33,, 53,  53,  118 High speed steels (HSS), 40 (HSS),  40,, 41  41,,  120  120,, 121,  121, 122,,  124 122 High Strength Low Alloy (HSLA) steels, 62 steels,  62,, 65  65,,  69  69,, 78  78,,  80 HSLA 350/450, 79 350/450,  79 High strength steels (HSS), 65 (HSS), 65–67 67,, 68,  68,  69  69,, 71 71,, 76–81 81,, 90,  90,  82  82,,  115 D6AC, 67 D6AC,  67 300 M, 67 M,  67

High temperature oxidation resistance, 101 resistance,  101 Hy Tuf, 67 Tuf,  67 Hydrogen embrittlement, 68 embrittlement, 68,,  83  83,,  91  91,, 103,, 112 103 Hyper eutectoid, 4 eutectoid, 4

F

Ferrite stabilizers, 36 stabilizers,  36,,  37  37,,  46  46,, 93  93,, 94 94,,  122 Ferritic stainless steels, 38 steels, 38,, 94  94,, 96,  96, 98–100 100,, 106,,  112, 106  112, 114,  114,  116 29-4, 98 29-4,  98,, 99 29-4-2, 98 29-4-2,  98,,  99 405, 98 405,  98,,  99 409, 98 409,  98,,  99 409 Cb, 98 Cb,  98,, 99 430, 96 430,  96,,  99

I

Inclusions, 45,,  46 Inclusions, 45  46,, 49  49,,  53  53,,  67  67,, 74  74,,  103 Induction hardening, 11 hardening, 11,, 28–30 Inoxydable (or) inox steels, 92 steels, 92 Intergranular carbides, 43 carbides, 43

434, 96,,  99 434, 96 436, 96 436,  96,,  99 439, 98 439,  98,,  99 442, 96 442,  96,,  99 444, 98 444,  98,,  99,  99, 129 446, 96 446,  96,,  99,  99,129 generation, 96,,  98,  98, 112 󿬁rst generation, 96 second generation, 96 generation, 96,, 98 third generation, 96 generation, 96,,  98,  98, 112  112,,  114

Intermetallic compounds, 43 compounds, 43,, 46  46,,  47 Interstitial free steels, 57 steels, 57,,  61  61,,  62  62,,  65  65,,  66  66,, 68 68,, 80 IF 300/420, 79 300/420, 79 Iridium, 39 Iridium,  39 Iron carbide, 4 carbide, 4,,  20  20,,  23 Iron carbon diagram, 1 diagram, 1,, 3  3,, 4  4,, 6  6,, 8  8,, 9  9,, 12  12,, 21  21,, 35 35,, 36 Iron nickel martensite, 83 martensite, 83,,  84

 

 Index

Isoforming, 23 Isoforming, 23 Isothermal heat treatment, 21 treatment, 21  J

 Jominy end quench test, 18 test, 18,,  19 L

Lath martensite, 18 martensite, 18,,  78 Lead, 45 Lead,  45,, 47,  47,  56,  56, 71 Lean Duplex Stainless steel, 107 steel, 107 Ledeburite, 4 Ledeburite,  4,,  6 Low alloy steels, 11 steels,  11,,  39  39,,  42  42,,  51,  51, 53  53,,  55  55,, 57  57,, 58 58,,  62,  62, 67,  67,  83,  83, 87 AISI 1010, 58 1010,  58 AISI 1020, 58 1020,  58 AISI 1030, 58 1030,  58 AISI 1040, 58 1040,  58 AISI 1050, 58 1050,  58 AISI 1080, 58 1080,  58 AISI 1117, 58 1117,  58 AISI 1213, 58 1213,  58 AISI 1340, 58 1340,  58 AISI 3140, 58 3140,  58 AISI 4130, 59 4130,  59

 

151

18Ni1900, 88 18Ni1900, 88 18Ni2400, 88 18Ni2400,  88 Ni-Cr type, 87 type,  87,,  88 12-5-3, 88 12-5-3,  88 IN733, 88 IN733,  88 IN 736, 88 736, 88 IN 866, 88 866, 88 IN 833, 88 833, 88 Ni-Ti type, 88 type,  88 20 Ni, 88 Ni,  88 25 Ni, 88 Ni,  88 Low alloy Maraging steels, 87 steels, 87,, 88  88,,  92 IN 335, 88 335, 88 IN 787, 88 787, 88 IN 863, 88 863, 88 Marquenching, 21 Marquenching,  21 Martempering, 21 Martempering,  21,,  22  22,, 122 Martensitic stainless steels, 93 steels, 93–95 95,, 110,  110, 114,,  116 114 AISI 410, 97 410, 97 AISI 410 HC, 97 HC, 97 AISI 420, 97 420, 97 AISI 420 HC, 97 HC, 97 AISI 425 Mod, 97 Mod,  97 AISI 440 A, 97 A, 97 Martensitic steels (MS), 43 (MS), 43,,  66  66,, 67  67,,  74  74,,

AISI 4140, 59 4140,  59 AISI 4150, 59 4150,  59 AISI 4340, 59 4340,  59,, 67 AISI 5140, 59 5140,  59 AISI 5150, 60 5150,  60 AISI 5160, 60 5160,  60 AISI 6150, 60 6150,  60 AISI 8740, 61 8740,  61 AISI 9255, 61 9255,  61 Low carbon steels, 28 steels, 28,,  32,  32,  33  33,, 51  51,,  52  52,, 53  53,, 57 57,,  61,  61, 65,  65,  70,  70, 72  72,,  74  74,, 79 Lower bainite, 21 bainite, 21,,  76 M

77 81 Mart 1250/1520, 79 1250/1520,  79 Mart 950/1200, 79 950/1200,  79 Martensite, 14 Martensite,  14–23 23,,  27,  27, 28  28,,  31  31,,  32  32,, 43  43,, 47–50 50,,  54,  54,  66  66,,  68  68,, 70–72 72,, 74,  74, 76–78 78,,  83,  83, 84  84,,  86  86,, 89  89,,  91  91,, 95  95,,  96  96,, 101,,  103, 101  103,  104  104,,  108–111 111,,  116,  116,  119,  119, 121–123 Martensite deformation temperature (Md), 103 (Md),  103,,  104 Martensite  󿬁 nish temperature (Mf), 16 (Mf), 16,, 18 18,, 22,  22,  46–48 48,,  109,  109, 110  110,,  122 Martensite start temperature (Ms), 17 (Ms),  17,, 21–23 23,,  46–48 48,,  56,  56, 108–110

Manganese, 31,,  36–38, Manganese, 31 38,  45–47, 47,  51  51,,  53  53,,  57  57,, 61 61,,  62,  62, 65–68, 68, 71  71,,  78,  78, 86  86,,  89  89,,  94  94,, 95 95,,  100,  100, 110,  110,  119 Maraging steels, 11 steels, 11,,  38  38,, 40  40,,  46  46,, 48  48,,  63,  63,  66  66,, 83–92 17Ni1600 (cast), 88 (cast),  88 18Ni1400, 87 18Ni1400,  87,, 88 18Ni1700, 88 18Ni1700,  88

Medium carbon steel, 27 steel,  27,,  32  32,,  33  33,, 52 52,,  128 Micro alloyed steels (MA), 42 (MA),  42,,  43  43,,  65 Mild steel, 3 steel, 3,, 31  31,, 65  65,, 67  67,, 72  72,, 78  78,, 80  80,, 128  128,, 129 Molybdenum, 23 Molybdenum,  23,, 32  32,,  33  33,,  36–38 38,,  41,  41, 42  42,, 44 44,,  47,  47, 50  50,,  54–56 56,,  67,  67, 74  74,,  78  78,, 83–87 87,,  93,  93,  100  100,,  101  101,, 105  105,,  107,  107, 108,,  110, 108  110,  115  115,, 117,  117,  119–121 121,, 124,,  129 124

 

152

 

Index

Molybdenum carbide (MoC), 41 (MoC), 41,, 54 54,,  55 Molybdenum steels, 54 steels, 54,,  55,  55,  62  62,,  67

17-10 P, 109 P, 109 HNM, 109 HNM,  109 Martensitic precipitation hardenable, 108,,  109, 108  109,  112 15-5 PH, 109 PH,  109 17-4 PH, 109 PH,  109 PH 13-8 Mo, 109 Mo,  109 Semi Austenitic precipitation hardenable, 109 hardenable,  109 17-7 PH, 108, PH,  108, 109  109 AM 350, 109 350,  109 AM 355, 109 355,  109 PH 15-7 Mo, 108, Mo,  108, 109  109

N

Nanostructured steels, 73 steels, 73,,  74,  74, 76 Nanostructured bainitic steels, 76 steels, 76 Neutral stabilizers, 39 stabilizers, 39 Nickel, 30 Nickel,  30,, 32,  32, 36–38, 38, 41  41,, 44,  44, 46  46,, 47  47,, 53–56, 56, 62 62,,  67,  67, 78,  78,  84–86, 86,  90  90,,  93–95, 95,  100  100,, 101,,  105–107 101 107,, 110  110,,  111  111,, 113–115 115,, 117,,  119–121 117 121,, 124  124,,  126 Nickel chromium steels, 51 steels, 51,,  54,  54, 62 Nickel steels, 51 steels,  51,, 53–55, 55, 62  62,,  67 Niobium (Nb), 23 (Nb),  23,,  36,  36, 37  37,,  39  39,, 42  42,,  44  44,, 47  47,, 49 49,,  61,  61,  62,  62,  66  66,,  69  69,, 75  75,,  88  88,, 93  93,,  94,  94, 98 98,,  99,  99,  101–104 104,, 106  106,,  109–112 Niobium carbide (NbC), 42 (NbC), 42,,  47  47,, 61 Nitrides, 30 Nitrides,  30,, 39,  39,  43,  43,  44,  44, 46  46,,  47  47,, 89 Nitriding, 11 Nitriding,  11,,  29,  29, 30,  30,  37,  37, 57  57,,  123 Nitrocarburizing, 30 Nitrocarburizing,  30,,  123 Normalizing, 11 Normalizing,  11,, 13,  13,  14–16, 16,  74 Notch tensile strength (NTS), 54 (NTS), 54,,  85,  85, 86 86,,  91

Precipitation hardening, 11 hardening, 11,, 23  23,,  24  24,,  43  43,, 46 46,, 61,  61,  68  68,, 69  69,,  84  84,,  91  91,,  108  108,, 109,  109, 112,,  113, 112  113,  121 Proeutectoid ferrite, 8 ferrite,  8,,  9  9,, 14  14,,  56 Process annealing, 12 annealing, 12 Proeutectoid cementite, 9 cementite, 9 R

Rephosphorized Steel, 68 Steel, 68 Rhodium, 39 Rhodium,  39 Ruthenium, 39 Ruthenium,  39

O

S

Osmium, 39 Osmium,  39

Palladium, 39 Palladium, 39 Patenting, 71 Patenting,  71,,  72 Pearlite, 4 Pearlite,  4,, 6  6,, 8  8,, 9  9,, 12–14, 14, 16  16,, 21,  21, 22  22,, 27,  27, 28  28,, 31 31,,  37,  37,  38,  38, 41  41,,  43  43,, 46  46,,  56  56,,  71,  71,  72  72,, 74 74,,  78 Peritectic reaction, 6 reaction, 6 Pitting resistance equivalent, 106 equivalent, 106 Plain carbon steels, 11 steels, 11,,  19  19,,  31,  31, 33  33,,  38  38,, 43  43,,

Schaffer diagram, 111 diagram, 111 Sensitization, 105 Sensitization,  105,,  106  106,,  110–112 112,,  116 Severe Plastic Deformation of steels, 72 steels,  72 Sigma phase, 44 phase, 44,,  46  46,, 100 Siladium, 115 Siladium,  115 Silicon steels, 56 steels, 56,,  62 Silicon (Si), 30 (Si), 30,, 31  31,,  36  36,, 37  37,,  39  39,, 41  41,,  46  46,,  47  47,, 49 49,, 56,  56,  61  61,, 62  62,,  66–68 68,,  71,  71,  74–78 78,, 86 86,, 88,  88,  94  94,, 97  97,,  99  99,,  101–104 104,, 108–111 111,, 117,  117,  119 Special purpose tool steels, 121 steels,  121,,  124 Spherodise annealing, 13 annealing, 13

46 46,,  49,  49,  51–53, 53,  57 Plate martensite, 18 martensite, 18,, 78 Platinum Precipitation hardenable stainless steel, 45 steel,  45,, 92,  92,  94  94,, 95  95,,  108–110 110,, 112 Austenitic precipitation hardenable, 109

Stabilization, 42,, 43 Stabilization, 42  43,,  47  47,,  48  48,, 106 Stainless steels, 3 steels, 3,,  6  6,,  11  11,, 38  38,,  41–46 46,,  63,  63, 83  83,, 89 89,, 92–116 116,, 127,  127,  129 Strain aging, 43 aging, 43,,  69  69,,  70 Strain induced martensite, 103 martensite, 103,,  116 Stress Corrosion Cracking (SCC), 85 (SCC), 85,,  87  87,, 90 90,, 91,  91,  106  106,, 107  107,,  116 Stress relieving annealing, 12 annealing, 12,,  110

P

 

 Index

Sub critical annealing, 12 annealing, 12 Super Duplex Stainless steels, 114 steels, 114 T

Tantalum (Ta), 36 (Ta), 36,, 37,  37,  41,  41, 42  42,,  44  44,,  45  45,, 47 Tantalum carbide (TaC), 41 (TaC), 41,, 47 Tempering, 11 Tempering,  11,, 20–22, 22, 25  25,,  30  30,, 32  32,,  33  33,,  43,  43, 47 47,,  49,  49, 54,  54,  56,  56, 67  67,,  74  74,,  79  79,, 84  84,,  91,  91, 110,,  116, 110  116,  121–123 Thermo mechanical treatments, 71 treatments, 71 Time Temperature Transformation (TTT), 16 (TTT),  16–19, 19,  21  21,, 122 Titanium (Ti), 36 (Ti), 36,, 37,  37, 39,  39, 42–47, 47, 49  49,, 61  61,, 62  62,, 66 66,,  69,  69, 75,  75,  84–89, 89,  93  93,,  94  94,, 98  98,,  99  99,, 102,,  104, 102  104,  106,  106,  109  109,,  111  111,, 112  112,,  123  123,, 127 Titanium carbide (TiC), 43 (TiC), 43,, 47  47,, 86  86,, 91,  91, 123 Titaniu Tita nium m carbo carbo nitr nitride ide (Ti (Ti(CN (CN)), )), 43, 43, 47, 47, 89 Titanium nitride (TiN), 43 (TiN), 43,,  47  47,, 86  86,,  123 Tool steels, 40 steels, 40,,  42,  42, 63,  63,  83  83,,  116–124 Air hardening tool steels, 119 steels, 119,,  123 Cold work tool steels, 119 steels,  119,, 123 High Carbon High Chromium steels, 119,,  123 119 High speed steels (HSS), 40 (HSS),  40,, 41  41,,

 

153

(TRIP) steels TRIP 450/800, 79 450/800, 79 Triple alloy steels, 52 steels, 52,,  55  55,,  62 Tungsten, 33 Tungsten,  33,,  36  36,, 37  37,,  41  41,, 42  42,, 44  44,,  47  47,, 49  49,, 55  55,, 93 93,,  117,  117, 119,  119,  120  120,, 121,  121,  124 Tungsten carbide, 41 carbide, 41 Tungsten steels, 55 steels, 55 U

Ultra High Strength Steels, 66 Steels, 66,, 83 Upper bainite, 21 bainite, 21,, 76 V

Valadium, 115 Valadium, 115 Vanadium, 23 Vanadium,  23,, 30  30,,  33  33,, 36  36,,  37  37,,  39  39,,  41  41,,  42  42,, 44 44,,  46,  46,  47  47,, 49  49,,  54  54,, 55  55,,  66  66,, 67  67,,  79  79,, 117–121 Vanadium carbide, 42 carbide, 42,, 55 Vanadium steels, 55 steels, 55,, 62 W

Weldability, 31,,  53 Weldability, 31  53,,  57  57,, 65  65,, 71  71,,  74  74,, 80 80,,  83,  83,  92  92,, 96  96,,  98  98,, 108  108,,  110,  110,

120–124 Hot working tool steels, 119 steels,  119,, 120  120,,  124 Mould steels, 120 steels,  120,, 124 Nonthermo stable, 121 stable, 121 Oil hardening steels, 119 steels, 119,, 123 Semithermo stable, 121 stable, 121 Shock resisting tool steels, 119 steels, 119,,  123 Special purpose tool steels, 121 steels,  121,,  124 Thermo stable, 121 stable, 121 Water hardening tool steels, 118 steels, 118,, 119,,  123 119 Tools used for selection of materials, 125 materials,  125 Transformation diagrams, 15 diagrams, 15,,  16 Transformation induced plasticity, 42 plasticity, 42,, 62 62,,  66,  66,  70,  70,  71  71,,  80

113,  114 113, White Lazon, 115 Lazon, 115 Wootz steel, 3 steel, 3 Y

Yield point phenomenon, 69 phenomenon, 69,, 70 Z

Zirconium, 36,, 37 Zirconium, 36  37,,  42–47 47,,  87,  87,  94 Zirconium carbide (ZrC), 42 (ZrC), 42,, 47 47,, 94 Zr (C,N), 42 (C,N),  42,,  47 Zirconium nitride (ZrN), 42 (ZrN), 42,, 47