10-Low Alloy Steel.pdf

Low Alloy Steel

Introduction

Introduction

Steel

Plain

Low Alloy

High Alloy

►Low-alloy steels constitute a category of

ferrous materials that exhibit mechanical properties superior to plain carbon steels as the result of additions of alloying elements such as nickel, chromium, and molybdenum. Total alloy content can range from 2.07% up to levels just below that of stainless steels, which contain a minimum of 10% Cr.

►the total content of alloying elements is < 5

%, and

 Al>0.3%  Cr >0.03%  Co>0.03%  Mo>0.08%  Ni>0.6 %

►The primary function of the alloying

elements in low alloy steels is to increase hardenability in order to optimize mechanical properties and toughness after heat treatment. In some cases, however, alloy additions are used to reduce environmental degradation under certain specified service conditions. ►Wide range of manufacturing methods i.e. welding, casting, machining, forming.

Classification AISI/SAE

Classification Based on application ► Low-carbon quenched and tempered steels combine high yield strength (from 350 to 1035 MPa) and high tensile strength with good notch toughness, ductility, corrosion resistance, or weldability. The various steels have different combinations of these characteristics based on their intended applications. ► Medium-carbon ultrahigh-strength steels are structural steels with yield strengths that can exceed 1380 MPa. Many of these steels are covered by SAE/AISI designations or are proprietary compositions. Product forms include billet, bar, rod, forgings, sheet, tubing, and welding wire.

Classification Based on application ► Bearing steels used for ball and roller bearing applications are comprised of low carbon (0.10 to 0.20% C) casehardened steels and high carbon (-1.0% C) throughhardened steels. Many of these steels are covered by SAE/AISI designations. ► Chromium-molybdenum heat-resistant steels contain 0.5 to 9% Cr and 0.5 to 1.0% Mo. The carbon content is usually below 0.2%. The chromium provides improved oxidation and corrosion resistance, and the molybdenum increases strength at elevated temperatures. They are generally supplied in the normalized and tempered, quenched and tempered or annealed condition. Chromium-molybdenum steels are widely used in the oil and gas industries and in fossil fuel and nuclear power plants.

General Application

Low Alloy Steel Mn steels (13xx) ►Chemical composition : 0,3 - 0,45% C, 0,25

- 1% Mn. To gain higher strength & weldability than mild steel 1,6 – 1,9 % Mn ►High Strength, good hardenability ►Application : axle, shaft, gear, tie rod for automobiles, farm equiptment ►Heat treatment : quenching, tempering, annealing, normalizing

Low Alloy Steel Mn steels (13xx) ►Effect of Mn in strengthening plain carbon

steel :

 Solid-solution hardening  Grain size refinement  Increasing proportion of pearlite

Low Alloy Steel Cr steels (5xxx) ►Divided into 3 groups based on their Cr

contents

 50xx : 0,40% Cr  51xx : 0,8 – 1% Cr  52xxx : 1,03% Cr ►Carbon contents: 0,2 – 1,04% C ►High strength, hard, high wear resistance,

low ductility

Low Alloy Steel Cr steels (5xxx) ►Application : ball & roller bearing, spring

steel ►Susceptible to temper embrittlement ►Heat treatment :quenching, tempering, normalizing, annealing

Low Alloy Steel Mo steels (4xxx) ►Restricted to about 0,25% Mo  optimum value based on experiment ►Good hardenability, high strength, good toughness ►Application : rear-axle, automatic transmission components, pinion gear ►Heat treatment : quenching, tempering

Others (XXXX) Please read : ►Avner’s Introduction to Physical Metallurgy ►William F. Smith’s Structure & Properties of Engineering Alloy

Low Alloy Steel Supplements: special low alloy steels ►Special and modern low alloy steels ►Relatively, it is “better” than common low

alloy steel for the same class or application ►More advance production methods ►HSLA steels ►TRIP steels ►Maraging steels

Low Alloy Steel Supplements: special low alloy steels

Low Alloy Steel HSLA steels ► High-strength low-alloy (HSLA) steels, or

microalloyed steels, are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional/plain carbon steels. ► Good formability, fatigue strength and weldability

Low Alloy Steel HSLA steels ►They are not considered to be alloy steels in

the normal sense because they are designed to meet specific mechanical properties rather than a chemical composition (HSLA steels have yield strengths greater than 275 MPa, or 40 ksi). The chemical composition of a specific HSLA steel may vary for different product thicknesses to meet mechanical property requirements

Low Alloy Steel HSLA steels ►The HSLA steels in sheet or plate form have

low carbon content (0.05 to 0.25% C) in order to produce adequate formability and weldability, and they have manganese content up to 2.0%. Small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium, and zirconium are used in various combinations

Low Alloy Steel HSLA steels – Application ►Oil and gas pipelines, heavy-duty highway and offroad vehicles, construction and farm machinery, industrial equipment, storage tanks, mine and railroad cars, barges and dredges, snowmobiles, lawn mowers, and passenger car components. Bridges, offshore structures, power transmission towers, light poles, and building beams and panels, structural parts of the vehicle ►Material savings by using HSLA for automotive application = approx. $23/vehicle. Total = $11,101,714/yr. (AISI 2004)

Low Alloy Steel HSLA steels – Categories ►Weathering steels, which contain small amounts of alloying elements such as copper and phosphorus for improved atmospheric corrosion resistance and solid-solution strengthening ►Microalloyed ferrite-pearlite steels, which contain very small (generally, less than 0.10%) additions of strong carbide or carbonitrideforming elements such as niobium, vanadium, and/or titanium for precipitation strengthening, grain refinement, and possibly transformation temperature control

Low Alloy Steel HSLA steels – Categories ►As-rolled pearlitic steels/pearlite-reduced steels, which may include carbon-manganese steels but which may also have small additions of other alloying elements to enhance strength, toughness, formability, and weldability ►Acicular ferrite (low-carbon bainite) steels, which are low-carbon (less than 0.05% C) steels with an excellent combination of high yield strengths, (as high as 690 MPa, or 100 ksi) weldability, formability,and good toughness

Low Alloy Steel HSLA steels – Categories ►Dual-phase steels, which have a microstructure of martensite dispersed in a ferritic matrix and provide a good combination of ductility and high tensile strength ►Inclusion-shape-controlled steels, which provide improved ductility and through-thickness toughness by the small additions of calcium, zirconium, or titanium, or perhaps rare earth elements so that the shape of the sulfide inclusions is changed from elongated stringers to small, dispersed, almost spherical globules

Low Alloy Steel TRIP steels ►TRIP (TRansformation Induced Plasticity)

steels are one of the newest family of advanced high strength steels (AHSS) developed for the automotive industry. ►The steels have a microstructure of soft ferrite grains with bainite and retained austenite. The retained austenite transforms into martensite (a hard phase) during plastic deformation (like stamping or crash event) stage.

Low Alloy Steel TRIP steels ►The hard martensite delays the onset of necking resulting in a product with high total elongation, excellent formability and high crash energy absorption. In addition, TRIP steels also exhibit extremely high fatigue endurance limit, thereby providing excellent durability performance ►TRIP steels can therefore be engineered or tailored to provide excellent formability for manufacturing complex parts. In addition, these steels can be designed into the automotive body structure to provide excellent crash energy absorption.

Low Alloy Steel TRIP steels ►Application: Structural, automotive

Low Alloy Steel TRIP steels – Characteristics ► Work hardening – As compared with other high strength steels, TRIP steel displays higher work hardening rate in entire range of plastic deformation. ► Yield point elongation (YPE) - Tested as delivered TRIP steels usually show YPE; however, some grades may have no YPE. ► Formability – Due to high work hardening rate TRIP steel behaves in a stable way in stamping processes (resistance to onset necking) and displays remarkably high formability (high potential to form parts of complex geometry). ► Bendability – TRIP steel demonstrates good bendabilty. As a result, product and process design solutions leading to springback control are easier to implement

Low Alloy Steel TRIP steels – Characteristics ►Bake hardening – TRIP steels have an excellent bake-hardening capacity. The increase in the yield strength in typical paint baking cycle is approximately 10 ksi (70 MPa). ►Product mass reduction capacity – TRIP steels have high potential for part downgauging and weight reduction. ►Fatigue performance – TRIP steels have higher fatigue strength than equivalent conventional HSLA steels.

Low Alloy Steel TRIP steels – Characteristics DUAL-TEN® Steel (Dual Phase Steel)

Topic Area

TRIP Steel

Ferrite-martensite

Microstructure

None Good

Yield Point Elongation Formability for strength level Bake Hardening Strain Rate Sensitivity Fatigue Weldability

Ferrite-bainite-austenite Yes Very Good

Excellent Good Good OK

Excellent Excellent Good More difficult than DUAL-TEN®