Mechanical Materials

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Mechanical Engineering Design

Mechanical

Materials

Dirk Pons

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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

About the Author

Dirk Pons PhD CPEng MIPENZ MPMI is professional Engineer Tohunga Wetepanga a n d a C h a r t e r e d 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 A u t h o r w e l c o m e s c o m m e n t s a n d s u g g e s t i o n s [email protected]

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1 PHYSICAL PROPERTIES OF MATERIALS . . . 4

2 IRON-CARBON METALLURGY . . . 5

2.1 Manufacture . . . 5

2.2 Iron - Iron Carbide Diagram . . . 6

2.3 Alloys of Iron and carbon . . . 7

2.4 Strengthening of materials . . . 9

2.4.1 Strain Hardening . . . 9

2.4.2 . . . 10

3 WROUGHT ALLOY STEELS . . . 13

3.1 General Properties . . . 13 3.2 Steels to BS970 . . . 16 3.3 Steels to AISI-SAE . . . 23 3.4 Casting Steel . . . 24 3.5 Structural Steel . . . 24 4 CAST IRON . . . 26 5 STAINLESS STEELS . . . 30

5.1 Ferritic Stainless Steels . . . 31

5.1.1 Super Ferritic Stainless Steels . . . 31

5.2 Martensitic Stainless Steels . . . 33

5.3 Austenitic Stainless Steels . . . 35

5.3.1 Heat Resisting Stainless Steels . . . 37

5.3.2 Austenitic Stainless Alloys . . . 38

5.3.3 Cast Austenitic Stainless Alloys . . . 39

5.4 Duplex Stainless Steels . . . 41

5.5 Precipitation Hardening Stainless Steels . . . 42

5.6 Available forms of Stainless Steels . . . 43

5.6.1 Stainless Steel Bar . . . 43

5.6.2 Stainless Steel Tube and Pipe . . . 45

5.6.3 Stainless Steel Plate and Sheet . . . 47

5.6.4 Stainless Steel Fasteners . . . 48

5.7 Basic Metallurgy of Stainless Steels . . . 49

5.8 Colour Coding for Stainless Steels . . . 51

6 HIGH NICKEL AND SPECIAL ALLOYS . . . 52

7 ALUMINIUM . . . 55

7.1 Wrought Alloys . . . 55

7.2 Cast Alloys . . . 56

7.3 Heat Treatment . . . 56

7.4 Aluminium Finishes . . . 57

7.5 General Physical Properties of Aluminium . . . 57

7.6 Mechanical Properties of Aluminium Alloys . . . 57

7.7 Product sections . . . 60

8 COPPER ALLOYS . . . 62

8.1 Mechanical Properties of Coppers . . . 62

8.2 Mechanical Properties of Copper Based Alloys . . . 63

9 POLYMERS . . . 65

9.1 Linear and Cross Linked Polymers . . . 65

9.2 Mechanical Properties of Polymers . . . 66

9.3 Polymers for wear applications . . . 73

10 ELASTOMERS . . . 75

10.1 Rubber Sheeting . . . 75

10.2 Expanded Rubber and Polymer . . . 75

11 OTHER MATERIALS . . . 76

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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_/o_C] Thermal conducti vity [W. m-1 .o_C-1_] Specific heat [ J. kg-1 . o C -1_] 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

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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.

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Iron-iron carbide diagram

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

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.

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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 910o_{C, some}

of the austenite crystal structure changes to ferrite. On further cooling to below 720oC the remaining austenite microstructure changes to ferrite and cementite,

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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 720o_{C, 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.

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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.

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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

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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 300o_{C). 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 450o_{C and 650}o_C,

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 550o_{C and 600}o_{C. The materials should be cooled fast through this}

danger zone.

* Blue brittleness occurs in mild steels exposed to 300o_C.

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 500o_{C, strain hardening (cold work),}

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Maraging

This heat treatment can be applied to Iron-nickel alloys (no carbon). On slow cooling the microstructure is martensite. When tempered at about 500o_{C for several hours,}

precipitates form, and these strengthen the material. This is also called precipitation hardening. The effect also occurs with aluminium alloys.

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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 H_b and ultimate tensile strength Rm for steels is approximately as follows:

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Hardness equivalents

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

Brinell Vickers Rockwell C 120o diamond cone with 150 kg load Rockwell B 1/16" steel ball indenter with 100 kg load 675 598 540 57 53 50 401 494 454 430 47 45 42

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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 20 102 99 97 212 202 192 212 202 192 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

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.o_{C) at 300}o_C 13

:

m/(m.o_{C) between 0}o_{C and 300}o_C Source: Stainless steel buyers guide 1992, SASSDA, Johannesburg.

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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 R_m R_e Rp0.2 Rf HB HV HRC tensile strength yield strength proof strength

uncorrected fatigue strength Brinell hardness

Vickers hardness

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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.

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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 Other properties E: Elastic Mod. [MPa] J: Mod. Rigidity [MPa] Comments 070M20 normalised P 400 200 355 21% En3A, 3C 070M26 normalised P Q 216 355 417 20% 080M30 normalised P Q 231 340 417 19% En5 080M36 normalised Q R 247 401 463 18% 080M40 normalised Q R 510 247 386 463 17% En8 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 600 309 417 478 571 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

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Designation Condition Rm Tensile strength [MPa] Re Yield strength [MPa]

Elong-ation Other properties E: Elastic Mod. [MPa] J: Mod. Rigidity [MPa] Comments 150M28 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.

strength [MPa] Re Yield strength [MPa] Elong-ation Other properties E: Elastic Mod. [MPa] J: Mod. Rigidity [MPa] Comments 216M28 normalised P Q -355 432 En8M 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 360 215 En 1A

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DIRECT HARDENING ALLOY STEELS

Including alloy steels capable of surface hardening by nitriding, designation 500 to 999.

strength [MPa] Re Yield strength [MPa] Elong-ation Other properties E: Elastic Mod. [MPa] J: Mod. Rigidity [MPa] Comments 503M30 Q R S 432 525 587 17% 17 15 503M40 = En12 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

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Designation Condition Rm Tensile strength [MPa] Re Yield strength [MPa] Elong-ation 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

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Designation Condition Rm Tensile strength [MPa] Re Yield strength [MPa] Elong-ation 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

strength [MPa] Re Yield strength [MPa] Elong-ation 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% En39B 045M10 quenched 432 18% 080M15 quenched 463 16% 210M15 quenched 463 16% 130M15 quenched 649 14%

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214M15 quenched 649 12%

3.3 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 plain carbon

11xx free cutting, with sulphur 12xx free cutting, with sulphur and

phosphor 13xx manganese up to 1,9% 23xx nickel 3,5% 25xx nickel 5 % 31xx nickel 1,25% chromium 0,6% 32xx nickel 1,75% chromium 1,0% 33xx nickel 3,5% chromium 1,5 % 34xx nickel 3,0% chromium 0,8 %

303xx corrosion and heat resisting

40xx molybdenum 0,25% 41xx molybdenum 0,20%, chromium 1% 43xx molybdenum 0,23%, chromium 0,8%, nickel 1,8% 46xx molybdenum 0,25%, nickel 1,75% 51xx chromium 0,8%

514xx corrosion and heat resisting 515xx corrosion and heat resisting

52xx chromium 1,5%

61xx chromium 0,78%, vanadium

0,13%

86xx nickel, chromium, molybdenum

92xx manganese, silicon

93xx 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

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Designation Condition Rm Tensile strength [MPa] Re Yield strength [MPa] Elong-ation 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 Carburising steels 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

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

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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.

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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 620o_{C), 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 850o_{C 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,

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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 g-ation Other properties E: Elastic Mod. [GPa] J: Mod. Rigidity [GPa] Comments

WHITE CAST IRONS

2,75% C 250 - 300 0 400-550HB.

Hard, brittle, used for wear resistant surfaces

3,25% C 300-450 0

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 0,5-1,0 100-150 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

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Designation Condition Rm Tensile strength [MPa] Re Yield strength [MPa] or 0,2% proof Elon g-ation 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 DUCTILE IRON SABS 936/937 (1970) 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 International standard ISO 1086 (1976)

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Designation Condition Rm Tensile strength [MPa] Re Yield strength [MPa] or 0,2% proof Elon g-ation Other properties E: Elastic Mod. [GPa] J: Mod. Rigidity [GPa] Comments 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

German standard DIN 1693

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

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.

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

An excellent South African reference on all stainless steel matters is the Southern Africa Stainless Steel Development Association (SASSDA), which produces technical literature and an annual supplier guide on behalf of the industry. Their address is PO Box 4479, RIVONIA 2128. Telephone (011) 803-5610.

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.

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

R_m ultimate tensile strength R_e yield strength Rp0.2 proof strength R_f uncorrected fatigue (endurance) strength HB Brinell hardness 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.

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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: 3Cr12, 430. Always used in annealed condition.

Description Designation Condition (1) Rm [MPa] Re [MPa] Elong-ation [%] 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.

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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

Density [kg/m3] ₇₈₀₀ ₇₇₀₀ ₇₈₀₀ ₇₈₀₀

Modulus of Elasticity [GPa] 200 207 200

Torsion modulus of elasticity [GPa]

65

-Max continuous temperature [o_C]

750 600

Specific heat capacity [J/(kg.o_C)] ₄₆₀ ₄₆₀

Thermal conductivity [W/(m.o_C)

at 300o_C]

23 (24)

Coefficient of thermal expansion [:m/(m.o_{C) between 0}o_{C and}

300o_C]

11 11,3 10,6 (11,7)

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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

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MARTENSITIC STAINLESS STEELS

AISI wrought 410 Q&T P 540-695 370 20 152-207 HBN 13Cr 0,12C AISI wrought 420 Q&T R Q&T S 695-850 850-925 525 585 15 13 201-255 HBN 223-277 HBN AISI wrought 420 Q&T R Q&T S 695-850 850-925 525 585 15 13 201-255 HBN 223-277 HBN AISI wrought 416 Q&T R Q&T S 695-850 850-925 525 585 11 10 201-255 HBN 223-277 HBN AISI wrought 431 Q&T T 925-1000 680 11 248-302 HBN AISI wrought 441 Q&T T 925-1000 680 11 248-302 HBN

ASTM cast CA-15 annealed 620 450 18 170-240 Equivalent to AISI 410.

Rotor blades, pumps, valves

ASTM cast CA-40 annealed 690 485 15 Equivalent to BS420C29

ASTM cast CB-30 annealed 450 205 - Equivalent to

DINX-22CrNi17

ASTM cast CB-7Cu quenched - - - Equivalent to 17-4PH.

Pistons, valve seats,.

ASTM cast CA-6NM annealed 760 550 15 220 Equivalent to BS425C11.

Water turbine casings.

ASTM cast CC-50 annealed 880 - - Equivalent to BS452C11.

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Physical properties

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 [o_C]

Specific heat capacity [J/(kg.o_C)] Thermal conductivity [W/(m.o_{C) at 300}o_C] Coefficient of thermal expansion [:m/(m.o_C) between 0o_{C and} 300o_C] 11,4 11,0 10,8 11,0

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 <0,08%. Molybdenum may be added (2-3%) for additional corrosion resistance. Low carbon grades (max 0,03% C) are denoted by L. Ti or Nb may be used as stabilisers

Properties: Excellent corrosion resistance, easily weldable (MMA, MIG, TIG, SAW), cold work hardenable, good cryogenic prop, easily

cleananable, tolerates up to 925oC. Non-magnetic when in fully annealed condition, low hardness, medium strength, excellent to good ductility, excellent impact resistance, excellent scaling resistance, high strength at elevated temperatures

BUT don't like acids or halide ions (Cl)

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Applications: Widely used in food processes, cryogenic, chemical processes. high temperature heat exchangers, low temperature gas

storage. Often cold worked, which increases strength but decreases ductility.

Common grades: Common grades 304, 316.

AUSTENITIC STAINLESS STEELS

AISI wrought

304 & 304L softened 465 170 40 183 HBN 241 MPa fatigue. Used in fasteners, grade DIN K18-8. 18Cr 10Ni 0,06C (L: 0,03C) AISI wrought 312 softened 495 195 40 183 HBN AISI wrought

316 & 316L softened 465 170 40 183 HBN 269 MPa fatigue. Used in fasteners, grade DIN K18-8-2. 17Cr 11Ni 2,5Mo 0,07C (L: 0,03C) AISI

wrought

317 softened 465 170 40 183 HBN 18Cr 13Ni 3,5Mo

AISI wrought

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5.3.1 Heat Resisting Stainless Steels

This is a special group of austenitic stainless steels, suitable for higher temperature duty (up to 1100o_C).

Composition: High chromium 24%, nickel eg 20%,

Properties: Resist oxidisation at high temps, good hi temp strength Forms: Available as plate

Applications: Typically furnace parts. Common grades: Common grades 309, 310.

HEAT RESISTING STEELS

cast (2) 309 515 240 10 23Cr 14Ni

AISI wrought

310 softened 540 215 40 207 HBN 217 MPa fatigue.

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5.3.2 Austenitic Stainless Alloys

This is another group of special austenitic stainless steels. The steels are highly alloyed, giving greater corrosion resistance than the conventional austenitics. Composition: Chromium 20-27%, nickel 25-42%, molybdenum 3-6%, low

carbon, highly alloyed Fe< 50%

Properties: Properties as for austenitic ss, weldable, resist pitting corrosion & SCC

Forms: Available as sheet, plate, tube.

Applications: Generally for high corrosion resistance, eg petrochemical industries.

Common grades: Proprietary alloys

AUSTENITIC STAINLESS ALLOYS

DIN G-NiMo30 negotiable 495 320 6 DIN G-NiMo16CrW negotiable 495 320 40 DIN NiCr15Fe as cast 485 195 30

The family of Austenitic stainless steels, and their derivatives consists of the following. The main characteristic or niche application of each alloy is given.

302 general purpose 302B scaling resistance 202 general purpose 205 less Ni 201 less Ni 304 corrosion resistance

304L resists sensitization by low carbon

304N mechanical properties

304LN mechanical properties

308 welding material

321 resists sensitization by Ti 347 resists sensitization by Ta and

Nb

348 limited Co and Ta for nuclear use

316 corrosion resistance

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316N strength

317 corrosion resistance

309 heat resistance, 309S similar

310 more heat resistance, 310S

similar

314 even more heat resistance

329 resists SCC

330 resists thermal shock

305 reduced work hardening

384 less work hardening

301 reduced work hardening

303 machinability

303Se surface texture

Physical properties

304 310 316 321

Density [kg/m3] ₇₉₀₀ ₇₉₀₀ ₈₀₀₀ ₇₉₀₀

Modulus of Elasticity [GPa] 195 205 195 195

85 70 70 72

Max continuous temperature [o_C]

925 1150 925 950

Specific heat capacity [J/(kg.o_C)] ₅₀₃ ₅₀₃ ₅₀₃ ₅₀₃

at 300o_C]

17,4 15,2 16,4 (18)

300o_C]

17,8 16,5 17,5 17,8

5.3.3 Cast Austenitic Stainless Alloys

Stainless steels may be cast, but a remelted bar of say 316, will not show the same mechanical properties of the original wrought material. In order to get the mechanical properties of a particular wrought steel ina casting, it is necessary to change the composition. There is an AISI designation for cast stainless steels. However (in South Africa) the cast steels rather follow the ASTM system.

Composition: Chromium 20-27%, nickel 25-42%, molybdenum 3-6%, low carbon, highly alloyed Fe< 50%

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Properties: Properties as for austenitic ss, weldable, resist pitting corrosion & SCC

Forms: Available as sheet, plate, tube.

Applications: Generally for high corrosion resistance, eg petrochemical industries.

CAST AUSTENITIC STAINLESS STEELS

ASTM A743 CF-8 quenched 450 195 35 130-200 HB pumps, stirrers ASTM CF-8M quenched 485 205 30 130-200 HB ASTM CF-3 quenched 450 195 35 ASTM CF-3M quenched 485 205 30 ASTM CG-8M quenched 520 240 25 130-200 HB chloride resistance ASTM CN-7M quenched 430 170 35 130-200 HB

sulphuric acid resistance ASTM

A297

HH as cast 515 240 10 chain links, furnace parts

ASTM HK as cast 450 240 10

ASTM HT as cast 450 - 4 furnace parts

ASTM HU as cast 450 - 4

ASTM HX as cast - - - furnace parts, grates

ASTM HC as cast 380 - - furnace nozzles

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5.4 Duplex Stainless Steels

These stainless steels are hybrids of the austenitic and ferritic stainless steels. They have a mixed ferritic/austenitic (i.e. duplex) structure.

Composition: Chromium 18-26%, nickel 5-7%, molybdenum 3% Properties: Highly resist SCC, high strength, good weldability Forms: Available as plate, sheet, tube.

Applications: Heat exchanger, vessels (especially Chlorides) Common grades: Proprietary alloys

DUPLEX STAINLESS STEELS

cast DIN X2CrNiMoN 22.5 annealed 680 450 30 cast ASTM CD-4MCu or DIN X8CrNiMo27 .5 quenched 689 483 16 190-230 HBN Physical properties

DIN X2CrNiMoN22.5

Density [kg/m3] ₆₈₀₀

Modulus of Elasticity [GPa] na Torsion modulus of elasticity

[GPa]

na Max continuous temperature

[o_C]

na Specific heat capacity [J/(kg.o_C)] _na

at 300o_C]

na Coefficient of thermal expansion [:m/(m.o_{C) between 0}o_{C and}

300o_C]

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5.5 Precipitation Hardening Stainless Steels

These are stainless steels that produce a microscopic precipitate on aging. This precipitate strengthens the material. The aging heat treatment determines the mechanical properties.

Composition: 15,5% Chromium, 3% Ni, max 0,07% Carbon, plus other elements

Properties: Hardenable, weldable, good corrosion resistance (similar to 304), high strength, good impact resistance, resists SCC

Forms: Available in forged bar (round, hexagonal, square) and forgings. Applications: Can be forged or cast. Machinable before and after age

hardening. Marine applications, gas turbine blades

Common grades: Common alloy is 17-4PH. Usually supplied in solution heat treated condition, and requires age hardening to develop desired strength. The higher the aging temperature the low the strength. The metallurgical process should be obtained from the supplier if necessary.

Description Designation Condition (Heat treatment) Rm [MPa] Re (0,2%) [MPa] Elon g-ation [%] Ultimate shear strength [MPa] Bending fatigue strength 10^7 & 10^8 cycles [MPa] Brinell hardness

PRECIPITATION HARDENING STAINLESS STEELS

wrought 17-4PH H900 H925 H1025 H1075 H1100 H1150 1379 1310 1172 1138 1034 1000 1276 1207 1138 1034 931 862 14 14 15 16 17 19 1179 972 931 855 621 & 503 607 & 510 572 & 538 & & -621 & -621 420 409 352 341 332 311

ASTM cast CB-7Cu quenched - - - Equivalent to

17-4PH

Physical properties

PH13-8Mo

15-5PH 17-4PH 17-7PH

Density [kg/m3] ₇₈₀₀ ₇₈₀₀ ₇₈₀₀ ₇₈₀₀

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Max continuous temperature [o_C]

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

at 300o_C]

300o_C]

11,2 11,4 11,6 11,6

5.6 Available forms of Stainless Steels The common forms of stainless steel are

* plate, sheet, strip * round bar, rod wire * pipe

* sections

5.6.1 Stainless Steel Bar Availability

Bar refers to shapes including round, square, hexagon, angles, tees, channels. Bar is produced according to various ASTM standards, depending on the usage of the product. The standard for general engineering applications is ASTM A484. A wide variety of products and sizes are produced, including:

* round bar (hot rolled, rough turned, turned, precision ground) * square bar (hot rolled)

* flat bar (hot rolled, cold drawn, ground) Tolerances

For products produced according to standards, the tolerances on dimensions depend on the shape and the size. Values are tabulated by suppliers.

Surface finish

The surface finish for bars is as follows. 1 Hot worked only

(a) scale not removed

(b) rough turned (round bar only) (hardenable steels may be annealed beforehand)

(c) pickled (possibly with blast cleaning) 2 Heat treated (annealed)

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(a) scale not removed

(b) rough turned (round bar only) (c) pickled, possibly with blast cleaning (d) cold worked (drawn or rolled)

(e) centre-less ground (round bar only) (f) polished (round bar only)

3 Annealed and cold worked (Available for alloys 302, 303Se, 304, 316) (d) cold worked (drawn or rolled)

(e) centreless ground (round bar only) (f) polished (round bar only)

Hollow bar is produced in 316 to the following nominal sizes.

OD [mm] 32 36 40 45 50 56 63 71 75 80 85 90 95 100 ID [mm] 20 16 25 20 16 28 25 20 32 28 20 36 32 25 40 36 28 50 40 36 32 56 45 40 36 40 63 50 45 40 45 71 63 56 50 50 80 71 63 56 OD [mm] 106 112 118 125 132 140 150 160 170 180 ID [mm] 80 71 63 56 90 80 71 63 90 80 71 63 100 90 80 71 106 90 80 71 112 100 90 80 125 106 95 80 132 122 112 140 130 118 150 140 125 OD [mm] 190 200 212 224 236 250 ID [mm] 160 150 132 160 150 140 170 130 170 130 190 150 200

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There are a number of ways in which pipes may be joined together: (1) welded mitre joints

(2) butt welding pipes to appropriate fittings (elbow, tee, reducing, flange etc)

(3) screwed fittings which mate with appropriate threads cut on the pipe

(4) fittings with ferrules, where the ferrules seal onto the pipe All the of fittings are available in stainless steel. They are available in several alloys. The fittings are usually made by shaping operations such as forging.

5.6.2 Stainless Steel Tube and Pipe

Tube refers to thin walls, and pipe to thicker walls. There are several types of production process.

* hot worked seamless tube/pipe, produced by extrusion

* cold worked tube/pipe, which uses extruded raw material. Cold worked products have better surface texture, and finer tolerances. A wide range of sizes is available from OD 3 mm to 610 mm.

* centrifugal casting, for outer diameters greater than 65 mm. Length is limited to about 5 m.

* continuously welded: a continuous strip is folded over and welded (usually TIG) longitudinally. The outside weld bead is removed, but not the inner bead. * fabrication welded: pieces of plate/sheet are pressed to shape and welded

together

* spiral welded: a continuous strip is formed and welded into a helix. Welding could be both inside and outside, or just

outside.

Pipe/tube is typically used for carrying fluids where duty involved one or more of temperature, corrosion, and hygiene. Historically pipes have been designated by the bore (inside) diameter, which then determined outer diameter. With improvement in materials, the wall thickness could be reduced, allowing larger bores. The nominal bore size is still used, but it now bears little relation to any actual pipe dimension! There are also several standards for pipe dimensions, the principal of which has been the British Standard Pipe (BSP).

Seamless and longitudinally Welded pipe is produced in 304 and 316 to the following nominal sizes.

Nominal bore [mm or inches]

Schedule Outer diameter [mm] Inner diameter [mm] 3 mm or 1/8" 10 40 80 10,29 10,29 10,29 7,8 6,83 5,46 6 mm or 1/4" 10 BSP std 40 BSP heavy 80 13,72 13,36 13,72 13,36 13,72 10,4 9,3 9,25 8,07 7,67 10 mm or 3/8" 10 40 80 17,15 17,15 17,15 13,84 12,52 10,74

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Schedule Outer diameter [mm] Inner diameter [mm] 15 mm or 1/2" 5 10 40 ISO heavy 80 160 XX heavy 21,34 21,34 21,34 21,34 21,34 21,34 21,34 18,03 17,12 15,8 14,9 13,9 11,8 6,4 20 mm or 3/4" 5 10 40 BSP std 80 160 XX heavy 26,67 26,67 26,67 26,67 26,67 26,67 26,67 23,4 22,5 20,9 20,5 18,9 15,6 11,0 25 mm or 1" 5 BSP light 10 ISO std 40 BSP std 80 160 XX heavy 33,4 34,0 33,4 33,7 33,4 34,0 33,4 33,4 33,4 30,1 28,7 27,9 27,7 26,6 26,7 24,3 20,7 15,1 32 mm or 1 1/4" 5 10 40 80 160 42,16 42,16 42,16 42,16 42,16 38,9 36,7 35,1 32,5 29,5 40 mm or 1 1/2" 5 10 ISO std 40 BSP heavy 80 160 48,3 48,3 48,4 48,3 48,2 48,3 48,3 45,0 42,7 41,9 40,9 39,3 38,1 34,0 50 mm or 2" 5 10 40 BSP std 80 160 XX heavy 60,33 60,33 60,33 60,33 60,33 60,33 60,33 57,0 54,8 52,5 51,4 49,3 42,9 38,2 80 mm or 3" 5 10 40 80 160 XX heavy 88,9 88,9 88,9 88,9 88,9 88,9 84,7 82,8 77,9 73,7 66,7 58,4 95 mm or 3 1/2" 5 10 40 80 101,6 101,6 101,6 101,6 97,4 95,5 90,1 85,4 100 mm or 4" 5 10 BSP std 40 80 160 114,3 114,3 114,3 114,3 114,3 114,3 110,1 108,2 107,0 102,3 97,2 87,3

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Schedule Outer diameter [mm] Inner diameter [mm] 125 mm or 5" 5 10 40 80 141,3 141,3 141,3 141,3 135,8 134,5 128,2 122,3 150 mm or 6" 5 10 40 80 160 168,28 168,28 168,28 168,28 168,28 162,7 161,5 154,1 146,3 131,8 200 mm or 8" 5 10 20 40 80 219,08 219,08 219,08 219,08 219,08 213,5 211,6 206,4 202,6 193,7 250 mm or 10" 5 10 20 40 80S 273,05 273,05 273,05 273,05 273,05 266,5 264,7 260,4 254,5 247,7 300 mm or 12" 5 10 20 40 80S 323,85 323,85 323,85 323,85 323,85 315,5 314,7 311,2 304,8 298,5 350 mm or 14" 10 355,6 342,9 400 mm or 16" 10 20 406,4 406,4 393,7 390,6 450 mm or 18" 10 457,2 444,5 500 mm or 20" 10 508,0 495,3

5.6.3 Stainless Steel Plate and Sheet Availability

Sheet is material less than 3,5 mm in thickness. The common dimensions are: Thickness: 0,55/ 0,7/ 0,9/ 1,2/ 1,5/ 2,0/ 2,5/ 3,0 mm

Area Size: 1000x2000/ 1250x2500/ 1500x3000/ mmxmm All combinations of thickness and size are possible.

Plate is material of at least 3,5 mm in thickness. The common dimensions are: Thickness: 3,5/4,5/6/8/10/12/16/20/25/30mm

Area Size: 1000x2000/ 1250x2500/ 1500x3000/ 1500x5000/ 1500x6000 mmxmm All combinations of thickness and size are possible.

Tolerances

The form tolerances are followed according to standards such as ASTM, BS 1449 Part 4, and DIN 17440.

Surface finish

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Mill finish Rough-ness [:m CLA]

Appearance Process

0 Scaly black Hot rolled, annealed. No pickling or passivating.

Not advised to be used in this form.

1 Frosty. Hot rolled, annealed, pickled, passivated. For

industrial applications

2D 0,4-0,1 Dull matt Cold rolled after: annealed, pickled, passivated. Used for deep drawing.

2B 0,1-0,5 Reflective Additional cold roll of 2D material. Used for drawing. Polishes easier than 2D.

2BA 0,05-0,1 Polished, near mirror Termed "Bright Annealed". Additional cold roll of 1 material with polished rollers.

3 0,4-1,5 abrasive direction

apparent

Ground in one direction with 80-100 grit abrasive. Used where surface is to be polished afterwards.

4 0,2-1,0 smooth unidirectional

grinding

Ground in one direction with 150 grit abrasive. Used where reflectivity is not required.

6 satin texture

(multidirectional grinding marks)

Ground with abrasive grit on a rotating mop.

7 0,02 reflective Buffed surface

8 0,02 mirror Buffed

5.6.4 Stainless Steel Fasteners

The commonly used stainless steel fasteners are those of 304 and 316 alloys.

Fasteners in other special alloys are less easily available. Fasteners are produced to DIN standards, and are designated:

Alloy type Designation 304/305 K18-8 (or A2)

316 K18-8-2 (or A4)

Standard parts produced in these grades include nuts, bolts, cap screws, set screws, washers.

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5.7 Basic Metallurgy of Stainless Steels

Stainless steels are corrosion resistant because they form a strong, adherent layer of chrome oxide on the exposed surfaces, and this prevents further corrosion. This passive film forms naturally, and it can also be made to form by chemical treatment (passivation). However to form the film, the steel needs to have at least 11% chromium, and enough oxygen in the environment. Although the passive film regenerates when damaged, corrosion will occur if the passive layers are removed continuously, or prevented from forming (eg lack of oxygen).

A stainless steel which otherwise should resist corrosion may become locally

sensitive to chemical attack. This is called sensitisation, and it is due to precipitation of chrome carbides between the grains, at a critical temperature. In those regions the chromium is therefore no longer available to resist corrosion. Inside the grains the chromium is usually unaffected, as the atom is too large to diffuse quickly. Therefore the corrosion is limited to the grain boundaries. It is called intergranular corrosion, or knife edge corrosion.

Annealing is a heat treatment that fully softens the steel. The steel is heated to a certain temperature (less than melting), at which all compounds re-dissolve into solution. Thereafter the steel is cooled in such a way that the elements remain in solution. For most steels the cooling needs to be slow, but for austenitic stainless steels the cooling must be rapid in order to retain the austenitic microstructure. Quenching rates from fastest to slowest are: brine, water, oil, moving air, still air. The heat affected zone (HAZ) is a strip alongside a weld, and its characteristic is that it has been subject to a range of temperatures from melting (at the weld) to ambient (at some distance away). Somewhere in the HAZ will be a line of grains that were subjected to say 600o_{C, and etc. The problem is that certain temperatures cause}

changes in the grain structure (depending on the alloy). Thus the mechanical and corrosion properties of the steel will not be uniform. Annealing could be used to restore the grain structure.

Pickling is the chemical removal of scale from stainless steel, which would otherwise interfere with the formation of the passive layer. The scale usually forms on welding or other high temperature processes. Pickling is done with hydrofluoric and nitric acids. It is usually followed by a passivating process.

Passivation is a chemical process whereby a stainless steel is subject to nitric acid (usually paste or solution). The nitric acid promotes the formation of the passive layer of chrome oxide on the surface of the steel. This layer would form naturally, but not necessarily as well. The acid also cleans the surface.

While stainless steels have fair to excellent corrosion resistance, they are not totally immune to corrosion. Types of corrosion include:

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Abrasive corrosion

This is mechanical abrasion from moving surfaces or particles. Chemical attack occurs at the newly exposed surfaces. Also called Fretting (typically when small oscillating movements occur between two surfaces).

Intergranular corrosion

At critical a temperature, Chrome carbides form at the grain boundaries, thereby deactivating the Cr, and also causing brittleness. Corrosion occurs between the grains. To avoid this problem

* reduce the time that the steel spends at the critical temperature (usually in the range 450o_{C to 850}o_C).

* use stabilisers, that is elements that soak up the carbon more readily than chromium, thus leaving the Cr to provide the corrosion resistance. Typical stabilisers are Titanium (Ti), Niobium (Nb), Tantalum (Ta).

* use low carbon alloys (L designations in stainless steels), so that there is less carbon around in the first place.

The critical temperature usually occurs somewhere in the heat affected zone (HAZ) alongside a weld. Thus the HAZ is vulnerable to corrosion and

embrittlement. Pitting corrosion

This is a highly localised form of corrosion. Pits are created in the material while the rest of the surface may be undamaged. Chlorides cause this type of corrosion in stainless steels.

Crevice corrosion

Small closed volumes do not get adequate oxygen diffusion, and this causes protective oxide films to corrode. Bacteria can also cause this type of

corrosion if their numbers are sufficient to block of an area of the surface. This is called microbiologically induced corrosion (MIC). The wastes from the

bacteria may also be corrosive. Stress corrosion cracking (SCC)

A combination of stress and aggressive chemicals is very harmful to all materials, for the following reasons. Small cracks are basically lattice imperfections, and exist in practically all engineering materials. The cracks grow when the local deformation is sufficient to overcome the bonds between the lattice molecules. The deformation may be related to the applied stress. Also, the lattice bonds may be weakened by chemicals that have affinity for the lattice molecules. Furthermore chemical reaction rates increase with temperature. Thus SCC is worsened by

* higher stress

* more aggressive media * increased temperature

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The functions of the alloying elements in stainless steels are basically: Cr forms a stable chromium oxide film on the surface, which halts further

oxidisation.

Ni promotes the formation of an austenitic grain structure

C provides heat treatability if used in sufficient quantities (martensitic ss) Mo promotes the stability of the chromium oxide film

Ti stabiliser (forms carbides in preference to Cr)

Mn promotes the formation of an austenitic grain structure S increases machinability, but decreases corrosion resistance Se increases machinability, especially surface finish

Nb stabiliser Ta stabiliser

N promotes the formation of an austenitic grain structure

Si decreases viscosity of molten steels for casting, also resists high temperature oxidation of wrought steels

5.8 Colour Coding for Stainless Steels

Stainless steel pipe, tube and sections are colour coded with painted ends or end plugs. Grade Colour 303 304 304L 316 316L Midnight Blue White Medium Yellow Brilliant Green Signal Red 409 410 420 430 431 Light Blue Golden Brown Black Eau-de-Nil Mines grey