Mechanical Materials

Mechanical Materials

Mechanical Engineering Design

Dirk Pons

Mechanical Materials Third Edition, 2011

This book gives properties for various materials that are used in mechanical design. The intention is to give general information on each type of material, with typical strength properties. Basic description of metallurgy is included where relevant, though the main focus of the book is on design.

This material is provided under a Creative Commons license(Attribution Non-Commercial No Derivatives), see below for details. The Author[s] accept no liability for the use or inability to use the material in this book. Published in New Zealand 518 Hurunui Bluff Rd Hawarden New Zealand Copyright © Dirk Pons

About the Author Dirk Pons PhD CPEng MIPENZ MPMI is professional Engineer Tohunga Wetepanga and a Chartered Professional Engineer in New Zealand. Dirk is a Senior Lecturer at the University of Canterbury, New Zealand. He holds a PhD in mechanical engineering and a masters degree in business leadership. The Author welcomes comments and s u g g e s t i o n s [email protected]

Mechanical properties of materials 1 2 2.1 2.2 2.3 2.4 2.4.1 2.4.2 3 3.1 3.2 3.3 3.4 3.5 4 5 5.1 5.1.1 5.2 5.3 5.3.1 5.3.2 5.3.3 5.4 5.5 5.6 5.6.1 5.6.2 5.6.3 5.6.4 5.7 5.8 6 7 7.1 7.2 7.3 7.4 7.5 7.6 7.7 8 8.1 8.2 9 9.1 9.2 9.3 10 10.1 10.2 11 11.1

PHYSICAL PROPERTIES OF MATERIALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 IRON-CARBON METALLURGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Manufacture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Iron - Iron Carbide Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Alloys of Iron and carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Strengthening of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Strain Hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 WROUGHT ALLOY STEELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 General Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Steels to BS970 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Steels to AISI-SAE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Casting Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Structural Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 CAST IRON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 STAINLESS STEELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Ferritic Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Super Ferritic Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Martensitic Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Austenitic Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Heat Resisting Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Austenitic Stainless Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Cast Austenitic Stainless Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Duplex Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Precipitation Hardening Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Available forms of Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Stainless Steel Bar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Stainless Steel Tube and Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Stainless Steel Plate and Sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Stainless Steel Fasteners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Basic Metallurgy of Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Colour Coding for Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 HIGH NICKEL AND SPECIAL ALLOYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 ALUMINIUM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Wrought Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Cast Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 Heat Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 Aluminium Finishes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 General Physical Properties of Aluminium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Mechanical Properties of Aluminium Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Product sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 COPPER ALLOYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Mechanical Properties of Coppers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Mechanical Properties of Copper Based Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . 63 POLYMERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Linear and Cross Linked Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Mechanical Properties of Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Polymers for wear applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 ELASTOMERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Rubber Sheeting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Expanded Rubber and Polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 OTHER MATERIALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 Human bone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

Mechanical materials

Mechanical properties of materials This chapter gives properties for various materials that are used in mechanical design. The intention is to give general information on each type of material, with typical strength properties. Basic description of metallurgy is included where relevant. 1

PHYSICAL PROPERTIES OF MATERIALS

The following table gives some physical properties for general classes of materials. Material

Modulus of elasticity E [GPa]

Modulus of rigidity G [GPa]

Poisson’ s ratio

Density D[ kg. m-3]

Coefficient of thermal expansion [10-6 /oC]

Thermal conducti vity [W. m-1 .oC-1]

Specific heat [ J. kg-1. oC1 ]

Aluminium alloys

72

27

0,32

2800

22

173

920

Beryllium copper

127

50

0,29

8300

17

147

420

Brass, Bronze

110

41

0,33

8700

19

78

420

Copper

121

46

0,33

8900

17

381

420

Iron, grey cast

103

41

0,26

7200

12

50

540

Iron, ductile

172

11-13

25-36

500-700

Magnesiu m alloys

45

17

0,35

1800

26

95

1170

Nickel alloys

207

79

0,30

8300

13

21

500

Steel, carbon

207

79

0,30

7850

12

47

460

Steel, alloy

207

79

0,30

7700

11

38

460

Stainless steel

190

73

0,30

7700

14

21

460

Titanium alloy

114

43

0,33

4400

9

12

500

Zinc alloy

83

31

0,33

6600

27

111

460

Reference:JUVINALL R, MARSHEK K, 1991, Fundamentals of Machine Component Design, John Wiley.

4

Mechanical materials 2

IRON-CARBON METALLURGY

The iron carbon alloys include, in order of increasing carbon content, pure iron, mild (low carbon) steels, high carbon steels, and cast irons. 2.1

Manufacture

Iron ore consists of iron oxides, with other elements. The ore is melted with coke (pure coal, ie carbon), which removes the oxide part as CO2. Limestone is added to separate the rock part of the ore, which then floats off. The iron that is left is called pig iron. It has a high carbon content (eg 10%) and many other impurities. Pig iron is not particularly useful on its own, and is subsequently converted into either cast iron or steel.

M

Cast iron is made by melting pig iron and adding coke, limestone and scrap iron to reduce the carbon content to around 3%.

M

Steel is made by blowing oxygen over or through molten pig iron, which removes the carbon and impurities by oxidisation. Next the oxygen is removed by adding manganese, aluminium or silicon. Alternatively the steel may be melted under a vacuum. After this stage the material is called commercially pure iron, and it has a very low carbon content. The material is soft and unsuitable for structural use. Therefore carbon is re-added in a controlled way, to create steel.

Alloying elements The effects of the major elements in steels and cast iron are as follow: * Carbon strengthens iron by forming different crystal structures to pure iron. Higher carbon content increases hardness, but reduces ductility. * Manganese removes oxygen during steel formation. Also promotes the formation of pearlite. * Sulphur is a deoxidiser. It has the useful property of making the steel easier to machine. However it can reduce high temperature ductility unless manganese is present. * Silicon is a major component in the ore, and is also added as a deoxidiser. The inclusions which remain in the steel cause weakness. * Hydrogen is responsible for hair line cracks, also called hydrogen embrittlement. This can be a problem in forging and in steels used in space. Hydrogen is removed by melting the steel under a vacuum.

5

Mechanical materials 2.2

Iron - Iron Carbide Diagram

The iron - iron carbide diagram shows the phases (molecular lattice structure) of various compositions of iron and carbon, and their temperature dependence. The diagram is valid for slow cooling only, such that diffusion can occur even in the solid states. Iron and carbon form an intermediate iron-carbide compound called

Iron-iron carbide diagram

cementite, with composition Fe3C, at 6,7% mass Carbon. Higher carbon contents are not of practical interest. The top lines show the transformation of liquid to solid. Regions just below the top lines are where the material is partly molten and partly solid. Internal lines show changes in crystal structure of the solid (called phase or polymorphic changes). The important phases are ferrite and austenite. The upper region of delta phase is not significant in this discussion.

6

Mechanical materials 2.3

Alloys of Iron and carbon

The main types of iron-carbon alloys are : PURE IRON Commercial purity iron (not the same as cast iron which has a high carbon content) consists of only ferrite grains. Non-metallic inclusions may also be present between the grains. Pure iron is not really used as a structural material. MILD STEEL For example take a composition of 0,5% carbon, as shown on the diagram below. On cooling from the molten state, austenite starts to solidify in small nuclei. The solid granules have a composition richer in Fe, and the remaining liquid is poorer in Fe. The exact compositions are given by drawing a horizontal line at the temperature concerned: where this line meets the boundaries represents the compositions. As the temperature drops, so the compositions change, by means of diffusion. When the temperature intersects the solidus austenite line, then all the remaining liquid transforms into austenite. The entire structure is now austenite, and if cooling is slow then diffusion evens out the composition. At some temperature below 910oC, some of the austenite crystal structure changes to ferrite. On further cooling to below 720oC the remaining austenite microstructure changes to ferrite and cementite,

7

Mechanical materials which are in microscopic layers. This combination of ferrite and cementite is called pearlite. The final state at room temperature is thus ferrite grains mixed with pearlite. In practice the steels with more than 0,4% carbon are usually fast cooled rather than slowly, and microstructure is different to that described above. This forms martensite, a hard material. If the nominal carbon content had been 0,83%C, then the final state would be pearlite only. HYPEREUTECTOID STEEL This is steel with a very high carbon content, between 0,83% and 1,7%. For example, follow the changes for a steel with 1,5% carbon. On cooling from the molten state, austenite starts to solidify. At the solidus line the remaining liquid solidifies to austenite too. As the temperature drops further it leaves the pure austenite region, and cementite starts to form. Just below 720oC, all the remaining austenite changes to ferrite and cementite, which are layered together as pearlite. The final state at room temperature is thus cementite grains mixed with pearlite. Steels have a maximum of 1,7% Carbon. Higher carbon content materials are called cast irons. CAST IRON The term “cast iron” makes most people think of pure iron. However cast iron is far from being pure iron: instead it contains very high carbon content. A typical composition might be 3% carbon, as shown on the diagram below. On cooling from the molten state, austenite starts to solidify. Just above 1130oC, the remaining liquid has the eutectic composition of 4,3% C. Further cooling results in the eutectic liquid solidifying into austenite and cementite. Just below 720oC, all the austenite changes to ferrite and cementite (pearlite). The final state at room temperature is thus cementite grains mixed with pearlite. This is called white cast iron. Cementite is brittle, and its high concentration in white cast iron makes this a weak material.

8

Mechanical materials

Cast irons have carbon contents of 2 to 5%. Melting temperature is lower than for steels, as shown on the iron-iron carbide diagram. This makes the cast irons easier to cast than steels. Cast irons have a high content of cementite, which is brittle. However the cementite can be encouraged to decompose to ferrite and chunks of carbon. The several types of cast iron are distinguished by the state of the cementite.

2.4

Strengthening of materials

There are two ways of increasing the strength of a material. The one is by strain hardening, and the other is by heat treatment. The comments below apply to materials generally, but to steels in particular.

2.4.1

Strain Hardening

Strain hardening is plastic deformation of the material, which causes the yield strength to be increased (but ductility to decrease). The mechanism is that dislocations are driven to their limits by the deformation, such that they are run up against barriers (eg grain boundaries, inclusions, carbides) and cannot move further. Dislocations are imperfections in the lattice structure of a material. 9

Mechanical materials Strain hardening is also called cold work. It is usually applied during the rolling or extrusion of the material. It is an important hardening mechanism for pure materials which cannot be heat treated. Aluminium is a typical material that is routinely strain hardened. Strain hardening may be undone by annealing.

2.4.2

Heat Treatment

Heat treatment is the controlled heating and cooling of a material so as to change the microstructure. There a number of terms which describe different aspects of this process. Annealing In this heat treatment the material is heated (but not melted), so that all the alloying elements including precipitates are taken back into solution. Dislocations are also smoothened out by high temperature diffusion. Then the material is cooled slowly. Final mechanical properties are low strength but high ductility. Annealing is also called solution heat treatment, or normalisation. Low carbon steels are usually used in the normalised condition, since they are practically impossible to harden. Medium carbon (mild) steels are usually used in the quenched and tempered condition. High carbon steels are used in the heat treated condition, that is quenched and controlled tempering. Quench hardening This is a process whereby the high temperature microstructure is cooled so quickly that it does not have time to change into the usual phase, but must instead go into another form. When austenite is cooled slowly, so that diffusion can occur, it changes to ferrite and cementite. However this is suppressed if the austenite is cooled fast, and it changes suddenly into a different crystal structure called martensite. The microstructure of martensite is fine needles or plates. These fine hard particles of Martensite strengthen a material by preventing dislocation movement. It is important to quench the austenite fast enough, otherwise diffusion will allow pearlite to form. This information is shown on a temperature-time-transformation (TTT) diagram. Steels with carbon contents less than 0,4% require very fast cooling rates to form martensite. The fast quench is very difficult with the thick sections often used in engineering. Increasing the carbon content (or other alloying elements, eg Ni) of the steel causes martensite to form at lower cooling rates, which are more easily obtained. For any given steel there exists a critical cooling rate, and the steel should be cooled faster than this if martensite is required. Quenching is done in water or oil. Water gives the faster quench. The limiting ruling section of a steel is the maximum diameter of round bar that can be heat treated successfully all the way through. Hardenability refers to the ease with 10

Mechanical materials which a steel may be fully quenched to martensite, and is typically measured by the Jominy end-quench. Martensite has high internal residual stresses, and a slightly larger volume than the austenite. Furthermore there may be stresses due to the different cooling rates between the inside and the surface. Consequently the material may crack during quenching. Martensitic reactions occur in steels, and also many other metal alloys. Martempering is a two stage quenching process, where the steel is cooled fast to a temperature just above that at which martensite starts to form (about 300oC). The material is held at this temperature for a while, to relieve the stresses. After a few minutes at this temperature, bainite would begin to form, but before that the steel is quenched again to form martensite. Tempering Martensite is formed by rapid quenching, but thereafter a tempering heat treatment is usually applied. Tempering involves reheating the material (so that some of the martensite converts to other structures), and then slow cooling. The effect is to reduce the hardness and strength, but to increase the ductility. The tempering temperature is less than the annealing temperature. The higher the temperature the greater the ductility. For steels, tempering is usually done between 450oC and 650oC, and the material may be held at the temperature for an hour. The martensite converts into cementite and ferrite, in a fine microstructure called sorbite. Tempering is not without problems, as brittleness can occur in the following conditions: * Tempering steel between 250oC and 350oC causes loss of notch toughness, called brittle tempering. The mechanism is that residual austenite converts to bainite, expanding in the process. * Alloyed steels may also show temper brittleness if exposed to temperatures between 550oC and 600oC. The materials should be cooled fast through this danger zone. * Blue brittleness occurs in mild steels exposed to 300oC. Austempering Under slow cooling, austenite would transform to pearlite. However under suitable cooling rates the austenite changes to bainite. This has the same composition as pearlite, but the microstructure is slightly different. The process of forming bainite is called austempering, and it is used in some steels. Ausforming This is a special process for increasing strength of steels. It involves heating the material to the austenite phase, cooling to about 500oC, strain hardening (cold work), quenching to form martensite, and finally tempering.

11

Mechanical materials Maraging This heat treatment can be applied to Iron-nickel alloys (no carbon). On slow cooling the microstructure is martensite. When tempered at about 500oC for several hours, precipitates form, and these strengthen the material. This is also called precipitation hardening. The effect also occurs with aluminium alloys.

12

Mechanical materials 3

WROUGHT ALLOY STEELS

Steel is one of the most familiar materials in mechanical engineering. This section describes the steels that are alloys of iron and carbon, together with small amounts of other elements. The other large group of steels are the stainless steels, and these are left to the next chapter. Design choices The designer has to specify a steel according to three basic parameters: * alloy composition * shape of section (eg round vs channel) * size (eg diameter) In theory any grade of steel is available in any section at any size, providing that you are prepared to pay for it. In practice only certain commonly used combinations are readily available. From the view of the designer, the easiest way to put some order into all the many combinations is to classify a steel according to one of the following basic applications: * Wrought steel in the form of round and rectangular bars. This material is used to fabricate parts by metal removal processes (machining). * Casting steel, which is poured into moulds. * Structural steel, in various sections, is used for fabricating structures (columns, beams etc). Sometimes these are large structures, like factories. * Flat products: strip, sheet, and plate. Each of these categories may have its own particular favourite alloy compositions, shape and size combinations, and these are not usually available in the other categories. The steels are described below according to these categories.

3.1

General Properties

Hardness Hardness is used to check on heat treatment. It is also sometimes used to distinguish different steels, although chemical analysis is better for this (spectroscopic analysis is usually used). However mechanical designers usually find strength properties more useful than hardness. Hardness is quicker, easier, and less destructive to measure than ultimate tensile strength, and therefore it is often used to estimate tensile strength. The relationship between Brinell hardness Hb and ultimate tensile strength Rm for steels is approximately as follows:

13

Mechanical materials There are a number of measurements of hardness. Some hardness equivalents are shown in the table and figure below.

Hardness equivalents Brinell

401

Vickers

Rockwell C 120o diamond cone with 150 kg load

675 598 540

57 53 50

494 454 430

47 45 42

Rockwell B 1/16" steel ball indenter with 100 kg load

14

Mechanical materials Brinell

Vickers

Rockwell C 120o diamond cone with 150 kg load

Rockwell B 1/16" steel ball indenter with 100 kg load

375 352 331 311

389 363 339 316

40 37 35 33

293 277 262

296 279 263

31 29 26

248 235 223

248 235 223

24 22 20

212 202 192

212 202 192

96 94 92

183 174 166

183 174 166

90 88 86

159 153 146

159 153 146

84 82 80

140 134 128 124

140 134 128 124

78 76 73 71

102 99 97

Modified from Stainless steel buyers guide 1992, SASSDA, Johannesburg.

Physical properties Typical physical properties for steels are: Density: Modulus of Elasticity: Torsion modulus of elasticity: Specific heat capacity: Thermal conductivity: Coefficient of thermal expansion:

7870 kg/m3 200 GPa 65 GPa 455 J/(kg.oC) 70 W/(m.oC) at 300oC 13 :m/(m.oC) between 0oC and 300oC

Source: Stainless steel buyers guide 1992, SASSDA, Johannesburg.

15

Mechanical materials 3.2

Steels to BS970

There are many name systems for steels, and several are in use. Some of the main systems are the British (BS), the German (DIN) and the American (ASME). The system of an En number for each steel is an old British one, which was widely used. It is now obsolete, but remnants of it may still be found. It has largely been replaced with the new British standard, as follows. Steels are classified according to BS 970, as xxxAyy, where: xxx Plain carbon steels and carbon manganese steels use 000 to 199, which is 100x the Mn content. Free cutting steels use 200 to 240, where the xx of 2xx is 100x the sulphur content. Direct hardening alloy steels, including alloy steels capable of surface hardening by nitriding, 500 to 999. Stainless steels use 300 to 449 A Supply requirements: A Analysis (some spring steels) H Hardenability (some spring steels) M Mechanical properties S Stainless steel (wrought) C Stainless steel (cast) yy Represents 100x carbon content for the carbon steels, otherwise arbitrary. Abbreviations are as follow. SYMBOLS

DESCRIPTION

Rm Re Rp0.2 Rf HB HV HRC

tensile strength yield strength proof strength uncorrected fatigue strength Brinell hardness Vickers hardness Rockwell hardness, C scale

16

Mechanical materials Tensile strength ranges Reference symbols that are used for the condition or tensile ranges of hardened steel are: Symbol

Tensile Strength Range [MPa]

Hardness range, Brinell HB

P

540 - 695

Q

617 - 772

179-229

R

695 - 850

201-255

S

772 - 927

223-277

T

850 - 1004

248-302

U

926 - 1081

269-331

V

1004 - 1158

293-352

W

1081 - 1235

311-375

X

1158 - 1313

Y

1235 - 1390

Z

1544 min

The same letters always represent the same lower limit of the tensile range. In order to get a particular steel to a given condition, it will be necessary to follow a particular heat treatment procedure. These procedures and the milestone temperatures are given in the standards. These details are not included here. From a design perspective it is important to note that the composition determines the hardenability, and that not every tensile strength range can be attained by a given steel. Most of the steel alloys listed in the tables are available in the form of round bar. A (limited) range of diameters will be available from any one supplier. Typical applications for the better alloys are for shafts and for relatively small machine parts, and the round section is usually suitable. Some of the grades for which there is sufficient demand may also be available in other sections, such as rectangular. The designer may have a choice of condition within the round sections, between "as rolled" (also called "black") and "bright bar". The former has scale on it from the hot rolling process and this gives it a dark grey appearance. "Bright bar" looks shiny since this scale has been removed, and the bar has been gauged (eg to h11). "Bright bar" may be sufficiently accurate for use in less critical machine parts, but not "as rolled" bar. Note that the B designation after some of the old En numbers does not refer to the bright condition but to the alloy composition.

17

Mechanical materials

PLAIN CARBON STEELS The lower carbon grades, up to 0,20% (xxxM20) are used for cold formed products, rivets, stampings, machine parts. They can be carburised. Carbon contents up to 0,4% (xxxM40) give stronger steels (l\although with less ductility), which are suitable for shafts, gears, forged parts. They can be carburised, and heat treatment is also possible. Designation

Condition

Rm Tensile strength [MPa]

Re Yield strength [MPa]

Elong-ation

070M20

normalised P

400

200 355

21%

070M26

normalised P Q

216 355 417

20%

080M30

normalised P Q

231 340 417

19%

080M36

normalised Q R

247 401 463

18%

080M40

normalised Q R

247 386 463

17%

080M46

normalised Q R S

278 370 448 525

15%

080M50

normalised R S T

278 432 494 571

14%

En43A

070M55

normalised R S T

309 417 478 571

13%

En9

120M19

normalised P Q R

262 355 448 510

19%

En14A

150M19

normalised P Q R

293 340 432 510

17%

En14A

120M28

normalised Q R

309 417 510

17%

En14B

510

600

Other properties E: Elastic Mod. [MPa] J: Mod. Rigidity [MPa]

Comments

En3A, 3C

En5

En8

18

Mechanical materials Designation

Condition

150M28

Rm Tensile strength [MPa]

Re Yield strength [MPa]

Elong-ation

Other properties E: Elastic Mod. [MPa] J: Mod. Rigidity [MPa]

Comments

normalised Q R S

324 401 479 571

16%

En14B

120M36

normalised Q R S

340 417 510 571

16%

En15B

150M36

normalised Q R S

355 401 324 556

15%

En15

PLAIN CARBON STEELS: FREE CUTTING STEELS These steels are alloyed to provide greater ease of machinability. Otherwise increasing strength generally means greater difficulty of machining. Designation

Condition

Rm Tensile strength [MPa]

216M28

normalised P Q

355 432

212M36

normalised P Q R

340 401 494

225M36

normalised Q R

401 479

216M36

normalised P Q R

340 401 479

212M44

normalised Q R S

401 463 540

225M44

normalised R S T

448 525 602

220M07

normalised

360

Re Yield strength [MPa]

215

Elongation

Other properties E: Elastic Mod. [MPa] J: Mod. Rigidity [MPa]

Comments

En8M

En 1A

19

Mechanical materials

DIRECT HARDENING ALLOY STEELS Including alloy steels capable of surface hardening by nitriding, designation 500 to 999. Designation

Condition

503M30

Rm Tensile strength [MPa]

Re Yield strength [MPa]

Elongation

Other properties E: Elastic Mod. [MPa] J: Mod. Rigidity [MPa]

Comments

Q R S

432 525 587

17% 17 15

526M60

T V

617 741

En11

530M40

R S T

525 587 680

En18

605M30

R S T U V

525 587 680 757 849

605M36

R

494

En16

606M36

R S T

525 587 680

En16M

608M38

R S T U V

494 556 680 757 849

En17

640M40

R S T U

525 556 680 757

En111

653M31

S T U

556 680 757

En23

708M40

R S T U

525 556 680 757

En19A

709M40

R S T U V

494 556 680 757 850

En19

722M24

T

680

suitable for nitriding, En40B

785M19

Q

448

En13

503M40 = En12

20

Mechanical materials Designation

Condition

Rm Tensile strength [MPa]

Re Yield strength [MPa]

Elongation

Other properties E: Elastic Mod. [MPa] J: Mod. Rigidity [MPa]

Comments

816M40

S T U V

556 680 757 850

En110

817M40

T U V W X Z

649 757 850 942 1019 1235

En24

823M30

T U V W X Z

649 741 850 942 1019 1235

En24

816M40

S T U V

556 680 757 850

En110

817M40

T U V W X Z

649 757 850 942 1019 1235

En24

823M30

T U V W X Z

649 741 850 942 1019 1235

En24

826M31

T U V W X Z

649 741 850 942 1019 1235

En25

826M40

U V W X Y Z

741 833 927 1019 1097 1235

En26

830M31

T U V W

649 757 850 942

En27

835M30

Z

1235

En30B

897M39

Z

1235

suitable for nitriding, En40C

905M31

R S

525 587

suitable for nitriding, En41A

21

Mechanical materials Designation

Condition

Rm Tensile strength [MPa]

Re Yield strength [MPa]

Elongation

Other properties E: Elastic Mod. [MPa] J: Mod. Rigidity [MPa]

Comments

905M39

R S T

525 587 680

suitable for nitriding, En41B

945M38

R S T U V

494 587 680 757 850

suitable for nitriding, En100

STEELS FOR CASE HARDENING Designation

Condition

Rm Tensile strength [MPa]

Re Yield strength [MPa]

Elongation

Other properties E: Elastic Mod. [MPa] J: Mod. Rigidity [MPa]

Comments

523M15

quenched

618

13%

En206

527M20

quenched

772

12%

En207

635M15

quenched

772

12%

En351

637M17

quenched

927

10%

En352

655M13

quenched

1004

9%

En36A

659M15

quenched

1313

8%

En39A

665M17

quenched

772

12%

En34

665M20

quenched

850

11%

En34

665M23

quenched

927

10%

En35

805M17

quenched

772

12%

En361

805M20

quenched

850

11%

En362

805M22

quenched

927

10%

En363

805M25

quenched

1004

9%

En363

815M17

quenched

1081

8%

En353

820M17

quenched

1158

8%

En354

822M17

quenched

1313

8%

En355

832M13

quenched

1081

8%

835M15

quenched

1313

8%

045M10

quenched

432

18%

080M15

quenched

463

16%

210M15

quenched

463

16%

130M15

quenched

649

14%

En39B

22

Mechanical materials 214M15

3.3

quenched

649

12%

Steels to AISI-SAE

The American steel naming system has four (or five) digits. The first digit is for the main alloying element (1 carbon, 2 nickel, 3 Ni + Cr, ...), the second digit is the percentage of that alloying element, and the last two (or three) digits give 100 times the carbon content. 10xx 11xx 12xx 13xx 23xx 25xx 31xx 32xx 33xx 34xx 303xx 40xx 41xx 43xx 46xx 51xx 514xx 515xx 52xx 61xx

plain carbon free cutting, with sulphur free cutting, with sulphur and phosphor manganese up to 1,9% nickel 3,5% nickel 5 % nickel 1,25% chromium 0,6% nickel 1,75% chromium 1,0% nickel 3,5% chromium 1,5 % nickel 3,0% chromium 0,8 % corrosion and heat resisting molybdenum 0,25% molybdenum 0,20%, chromium 1% molybdenum 0,23%, chromium 0,8%, nickel 1,8% molybdenum 0,25%, nickel 1,75% chromium 0,8% corrosion and heat resisting corrosion and heat resisting chromium 1,5% chromium 0,78%, vanadium

86xx 92xx 93xx

0,13% nickel, chromium, molybdenum manganese, silicon nickel, chromium, molybdenum

In front of the number is placed a letter, which specifies how the steel is to be produced: A basic open hearth alloy steel B acid Bessemer carbon steel C basic open hearth carbon steel D acid open hearth carbon steel E electric furnace steel (carbon or alloy) If there is no prefix, then it is taken to be C. If letters B or L appear in the middle of the steel’s number, then this shows that Boron or Lead have been included. Suffix letters (after the number) refer to specifications for: A analysis (chemical composition) H hardenability

23

Mechanical materials Designation

Condition

Rm Tensile strength [MPa]

Re Yield strength [MPa]

Elongation

Other properties E: Elastic Mod. [MPa] J: Mod. Rigidity [MPa]

Comments

1002 A

290

131

1010A

303

200

1018A

341

221

1020HR

455

290

1045 HR

638

414

1212 HR

424

193

4340 HR

1041

910

52100A

1151

903

1015

503

317

32

HB 149

1022

565

324

27

HB 163

1117

662

407

23

HB 192

1118

779

524

17

HB 229

4320

1006

648

22

HB 293

4620

793

531

22

HB 235

8620

896

531

22

HB 262

E9310

1165

952

15

HB 352

Carburising steels

Reference: JUVINALL R, MARSHEK K, 1983, Fundamentals of Machine Component Design. John Wiley.

3.4

Casting Steel

Foundries have stock of certain grades of steel, and may also be able to cast proprietary alloy compositions. No data are given here for cast steels as there is a large choice of materials, many of them proprietary. Remember that ductile iron is a serious contender for casting, with mechanical properties in many cases better than cast steels.

3.5

Structural Steel

Structural steel is pre-formed steel used for the fabrication of structures. One application is in buildings (typically factories and warehouses), for which the steel framework is constructed (fastened or welded) on a concrete foundation and covered with cladding (steel, aluminium or other sheets, usually with some profile). Internal architecture may also be made from structural steel. Another application is the

24

Mechanical materials fabrication of bases for machines. There are several types of section used for structural purposes. * hot rolled sections include angles, channels, H and I sections, plates, flats, squares and rounds. These tend to be relatively thicker than the other sections. * cold formed sections. These include various angles, C and S shapes. They have uniform thickness throughout, being made from sheet material. Lips are typically provided at the edges. * hollow sections, including round, square and rectangular tubes. These are fabricated by rolling and welding processes, and may have an internal seam. * plate and sheet There are several standards to which structural steel is produced. Different structures are produced in different standards. For applications where mechanical properties are non-critical, steel may be ordered as “commercial” or "mild steel". Geometry and Material properties are given in a separate chapter.

25

Mechanical materials 4

CAST IRON

Cast irons are available in a number of types: white, grey, malleable, ductile (also called nodular, or spheroidal graphite), and austenitic. See the section on the iron carbide diagram for details of the metallurgy of the cast irons. White cast iron The microstructure at room temperature is cementite mixed with pearlite. White cast iron is brittle, but hard and wear resistant. The material is not usually used on its own in castings. A typical use is to form a hard surface layer on a casting. This is done by placing metal chill plates in the mould, next to which white cast iron will form, while the rest of the casting will be in the grey cast iron state. White cast iron may be transformed to malleable cast iron. Grey cast iron This cast iron contains silicon, which causes the cementite to change into ferrite (pure iron) and graphite flakes. The graphite flakes make the material softer, easier to machine, and somewhat sound absorbent. However the tensile strength is relatively low. There a several forms of grey cast iron, with different degree of dissociation of the cementite * pearlitic grey cast iron: the cementite in the pearlite is left as it is, but that in the primary grains of cementite is converted * ferritic grey cast iron: all the cementite, in the pearlite and the primary grains of cementite, is converted Grey cast iron may be heat treated to change the structure from pearlitic to ferritic or the other way. Heat treatment is also used to remove residual stresses (at about 620oC), for annealing and hardening. Small amounts of phosphorus lower the freezing temperature, giving fluidity in casting, and less shrinkage. Malleable cast iron This is a white cast iron that is heat treated to change the microstructure. White cast iron is heated to about 850oC for several days, during which the cementite changes to ferrite and blobs of carbon. This gives ductility. A variant is to create pearlite instead of ferrite. The carbon can also be oxidised out of the surface layer to create whiteheart cast iron. Ductile iron This has spheres of carbon in ferrite or pearlite, like malleable cast iron. However this state is created during solidification (by adding magnesium) rather than by heat treatment. This is a major advantage to the foundry. Other names are nodular cast iron, and spheroidal graphite cast iron. The material has relatively high strength and ductility. As cast the matrix around the carbon will be pearlite, but this can be heat treated into ferrite or martensite. The material is widely used for engineering components, even those that are relatively highly stressed, eg crankshafts, gears,

26

Mechanical materials brake drums, machine parts. Larger wall thicknesses are possible than with malleable iron. Austenitic cast iron These materials contain alloying elements that allow austenite to exist down to room temperatures (instead of changing into pearlite). Corrosion resistance is good. Mechanical properties follow. Designation

Condition

Rm Tensile strength [MPa]

Re Yield strength [MPa] or 0,2% proof

Elon gation

Other properties E: Elastic Mod. [GPa] J: Mod. Rigidity [GPa]

Comments

WHITE CAST IRONS 2,75% C

250 - 300

0

3,25% C

300-450

0

400-550HB. Hard, brittle, used for wear resistant surfaces

GREY CAST IRON 3,25%C as cast

150-250

100-200

0,5

180-240 HB Common usage

3,25%C annealed

125-200

85-140

0,51,0

100-150 MB

2,75%

300-400

200-275

0,5

210-320HB

–

–

Ultimate Compre ssive strength [MPa]

Ultimat e shear strength [MPa]

ASTM 20

152

572

179

E 66-97 J 27-39

HB156. Endurance 69 MPa. Soft iron castings

ASTM 25

179

669

220

E 79 - 102 J 32 - 41

HB 174. Endurance 79 MPa.. Housings, IC engine blocks

ASTM 30

214

752

276

E 90 - 113 J 36 - 45

HB 210. Endurance 97 MPa. Brake drums

ASTM 35

252

855

334

E 100-119 J 40 - 48

HB 212. Endurance 110 MPa. Brake drums

27

Mechanical materials Designation

Condition

Rm Tensile strength [MPa]

Re Yield strength [MPa] or 0,2% proof

Elon gation

Other properties E: Elastic Mod. [GPa] J: Mod. Rigidity [GPa]

Comments

ASTM 40

293

965

393

E110 - 138 J 44 - 54

HB 235. Endurance 128 MPa. Cylinder liners, camshafts

ASTM 50

362

1130

503

E 130-157 J 50 - 55

HB 262. Endurance 148 MPa. High strength

ASTM 60

431

1293

610

E 141-162 J 54 - 59

HB 302. Endurance 169 MPa . High strength

•

•

MALLEABLE CAST IRON 2,5%C blackheart

350-400

260-300

10-20

110-140HB Black & white used for engineering parts, vehicle castings

2,5%C whiteheart

400-450

280-320

5-20

120-220HB

SABS SG38

375

245

17

E: 172

HB180 Endurance limit 0,55xRm

SABS SG42

410

275

12

E: 172

HB200 Endurance limit 0,54xRm

SABS SG50

490

345

7

E: 172

HB170-240 Endurance limit 0,49xRm

SABS SG60

590

390

4

E: 172

HB210-280 Endurance limit 0,45xRm

SABS SG70

685

440

3

E: 172

HB230-300 Endurance limit 0,44xRm

SABS SG80

785

490

2

E: 172

HB260-330 Endurance limit 0,44xRm

DUCTILE IRON SABS 936/937 (1970)

International standard ISO 1086 (1976)

28

Mechanical materials Designation

Condition

Rm Tensile strength [MPa]

Re Yield strength [MPa] or 0,2% proof

Elon gation

ISO 800-2

800

480

2

ISO 700-2

700

420

2

ISO 600-3

600

370

3

ISO 500-7

500

320

7

ISO 400-12

400

250

12

ISO 370-17

370

230

17

GGG-40

400

250

15

GGG-50

500

320

7

GGG-60

600

380

3

GGG-70

700

440

4

GGG-80

800

500

2

GGG-35,3

350

220

22

GGG-40,3

400

250

18

Other properties E: Elastic Mod. [GPa] J: Mod. Rigidity [GPa]

Comments

German standard DIN 1693

References:

JUVINALL R, MARSHEK K, 1991, Fundamentals of Machine Component Design, John Wiley. KARSAY SI, Ductile iron castings, Ferrous Casting Centre PO Box 785711 Sandton 2146 South Africa.

29

Mechanical materials 5

STAINLESS STEELS

A stainless steel is a ferrous steel with at least 11% chromium. The materials have good corrosion resistance because a layer of chrome oxide naturally forms on the exposed surfaces, and prevents further corrosion. If this passive layer is damaged, then a new layer forms. However corrosion will occur if the passive layers are removed continuously, or prevented from forming. There is a common belief that stainless steels are much stronger than carbon steels. This is generally wrong: the ordinary stainless steels have mechanical properties which are similar and even less than those for ordinary carbon steels. Designers usually use stainless steels not so much for strength but for corrosion resistance.

A common misconception is that stainless steels are non-magnetic. In fact only the austenitic 300 series alloys are non-magnetic.

The designations used for wrought steels generally follow the USA AISI system, which is basically similar to the British and Canadian. The German DIN system has more limited use. Cast stainless steels follow the USA ASTM system, which differs from the British. SYMBOLS Rm Re Rp0.2 Rf

ultimate tensile strength An excellent South African reference on all stainless yield strength steel matters is the Southern Africa Stainless Steel proof strength Development Association (SASSDA), which produces technical literature and an annual supplier uncorrected fatigue guide on behalf of the industry. Their address is PO (endurance) strength Box 4479, RIVONIA 2128. Telephone (011) 803HB Brinell hardness 5610. HV Vickers hardness HRC Rockwell hardness, C scale Note (1): Q denotes quenching, T tempering, P,R,S,T refer to strength range as per conventional steels Note (2): The AISI steels are sometimes not used for casting.

30

Mechanical materials 5.1

Ferritic Stainless Steels

These are the conventional ferritic stainless steels. Composition: Chromium eg 18%, no nickel, low carbon Properties:

Magnetic, non hardenable, poor welding (TIG may be best), moderate corrosion resistance, low hardness, medium strength, good ductility, moderate impact resistance, good scaling resistance, medium strength at elevated temperatures

Forms:

Available in sheet, coil, tube, plate. Generally thin gauge material, up to plates in the case of 3Cr12.

Applications: Sinks, architectural trim, conveyors, fume extractors. Usually used as corrosion resistance sheet. Common grades:

Description

Designation

3Cr12, 430. Always used in annealed condition.

Condition (1)

Rm [MPa]

Re [MPa]

Elongation [%]

Other properties

Comments

FERRITIC STAINLESS STEELS 3Cr12

weldable (MMA, MIG, TIG) with 309L filler

AISI wrought

403

softened

415

280

20

170 HBN

13Cr 0,12C

AISI wrought

430

softened

430

280

20

170 HBN 276 MPa fatigue

general purpose 17Cr

5.1.1

Super Ferritic Stainless Steels

These steels substitute for austenitic stainless steels where stress corrosion cracking (SCC) and pitting are problems. Composition: Chromium 18%, molybdenum 2% (or 26/1) Properties:

Resist pitting and stress corrosion cracking, properties similar to ferritic. Poor weldability.

Forms:

Available as sheet, tube.

31

Mechanical materials Applications: Sheet products: heater panels, solar heaters, heat exchanger tubing. Welded products are made from thichnesses less than about 5mm. Common grades:

Common grades 444. Proprietary alloys are also available.

The family of Ferritic stainless steels, and their derivatives consists of the following. The main characteristic or niche application of each alloy is given. 430 general purpose 446 scaling resistance 442 scaling resistance 444 SCC resistance 429 weldability 405 resistant to hardening 409 automotive exhausts 430F machinability 430FSe machine texture 434 auto trim 436 heat and corrosion resistance Physical properties Typical physical properties for select steels are: 430

3Cr12

444

409

7800

7700

7800

7800

Modulus of Elasticity [GPa]

200

207

200

Torsion modulus of elasticity [GPa]

65

-

Max continuous temperature [oC]

750

600

Specific heat capacity [J/(kg.oC)]

460

460

Thermal conductivity [W/(m.oC) at 300oC]

23

(24)

Density [kg/m

3]

Coefficient of thermal expansion 11 11,3 10,6 (11,7) [:m/(m.oC) between 0oC and 300oC] Modified from Stainless steel buyers guide 1992, SASSDA, Johannesburg.

32

Mechanical materials 5.2

Martensitic Stainless Steels

There is only one group of martensitic stainless steels. All the other stainless steels have low carbon, except the martensitic group. Here the carbon is used to give hardenability through the formation of martensite. Composition: Chromium eg 18%, high carbon, Properties:

Hardenable, poor welding, moderate corrosion resistance, magnetic, medium to high strength, good to fair ductility, moderate to poor impact resistance, fair scaling resistance, medium strength at elevated temperatures

Forms:

Available in bar and strip.

Applications: Heat treatment is used to control strength and hardness: eg for blades, shafts, springs, cutlery Common grades: Common alloys are 410, 420, 431. The family of Martensitic stainless steels, and their derivatives consists of the following. The main characteristic or niche application of each alloy is shown. 410 general purpose 414 corrosion resistance 431 corrosion resistance 422 mechanical properties at higher temperatures 403 turbine parts 420 mechanical properties 420F machinability 416 machinability 416Se machined texture 440C hardness 440B toughness 440A additional toughness

33

Mechanical materials

Description

Designation

Condition (1)

Rm [MPa]

Re [MPa]

Elongation [%]

Other properties

Comments

13Cr 0,12C

MARTENSITIC STAINLESS STEELS AISI wrought

410

Q&T P

540695

370

20

152-207 HBN

AISI wrought

420

Q&T R Q&T S

695850 850925

525 585

15 13

201-255 HBN 223-277 HBN

AISI wrought

420

Q&T R Q&T S

695850 850925

525 585

15 13

201-255 HBN 223-277 HBN

AISI wrought

416

Q&T R Q&T S

695850 850925

525 585

11 10

201-255 HBN 223-277 HBN

AISI wrought

431

Q&T T

9251000

680

11

248-302 HBN

AISI wrought

441

Q&T T

9251000

680

11

248-302 HBN

ASTM cast

CA-15

annealed

620

450

18

170-240

ASTM cast

CA-40

annealed

690

485

15

Equivalent to BS420C29

ASTM cast

CB-30

annealed

450

205

-

Equivalent to DINX22CrNi17

ASTM cast

CB-7Cu

quenched

-

-

-

Equivalent to 17-4PH. Pistons, valve seats,.

ASTM cast

CA-6NM

annealed

760

550

15

ASTM cast

CC-50

annealed

880

-

-

220

Equivalent to AISI 410. Rotor blades, pumps, valves

Equivalent to BS425C11. Water turbine casings. Equivalent to BS452C11. Construction parts.

34

Mechanical materials

Physical properties Typical physical properties for select steels are: 410

416

420

431

Density [kg/m3]

7800

7800

7800

7800

Modulus of Elasticity [GPa]

200

200

200

200

Torsion modulus of elasticity [GPa] Max continuous temperature [oC] Specific heat capacity [J/(kg.oC)] Thermal conductivity [W/(m.oC) at 300oC] Coefficient of thermal 11,4 11,0 10,8 11,0 expansion [:m/(m.oC) between 0oC and 300oC] Modified from Stainless steel buyers guide 1992, SASSDA, Johannesburg.

5.3

Austenitic Stainless Steels

The conventional austenitic stainless steels are a large group. All the austenitic steels contain chromium and nickel. Composition:

Chromium 18%, nickel 8%, low carbon