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Mechanical Engineering Design Notes

Materials Contents

Metals Selection and Specifications:
Ferrous Metals

Iron is the metal with the highest atomic weight that can be created in the cores of massive stars:

How incredibly fortunate is that?

These notes provide a brief introduction to UK and US specifications for ferrous metals, their properties and applications.

Most steels used in engineering have very similar Young's modulus, 210 GPa; density, 7800 kg per cubic metre; and Poisson's ratio, 0.28. The shear modulus is about 80 GPa, but there is some variation in this figure. Stainless steels and tool steels have slightly different values.

UK and US specifications are both used in the UK and are outlined below.

The coding system used for steels in the UK is given in British Standard 970. Originally 'EN' numbers were used, and can still be found in some older books, but the current standard, developed in the 1970s and 1980s, uses a six-digit designation, for example 070M20. The first three digits of the designation denote the family of steels to which the alloy belongs:

000-199 Carbon and carbon-manganese steels. The first three figures indicate 100 times the mean manganese content.

200-240 Free cutting steels where the second and third digits represent 100 times the minimum or mean sulphur content.

250 Silicon-manganese spring steels.

300-399 Stainless, heat resisting and valve steels.

500-999 Alloy steels in groups according to alloy type.

The fourth digit of the designation is a letter (A, H or M ) denoting that the steel will be supplied to meet the following requirements:

A: The steel will be supplied to close limits of chemical composition (no mechanical or hardenability properties specified).

H: A combination of hardenability and chemical analysis.

M: A combination of mechanical properties and chemical analysis.

Where the fourth letter is 'S', it denotes a stainless steel specification.

The fifth and sixth digit represent 100 times the mean carbon content of the steel (this does not apply to stainless steel specifications). For example 080A40 denotes a carbon-manganese steel, supplied to close limits of chemical composition, containing 0.7-0.9 per cent manganese and 0.4 per cent carbon.

It should be noted that the specifications 251A60 and 251H60 differ in the chemical composition tolerance bands which are narrower in 251A60 than in 251H60. This is the typical difference between A and H series steels.

Steels can be divided into the following groups:

1. Low-carbon steels or mild steels. These are used for lightly stressed components in general engineering, often in sheet form where processing includes: welding, bending and forming. For sheet metal components formed by drawing, bending and stretching, fine grain size assists in maintaining a good surface. Some of the popular specifications are 040A10, 045Mb, 070M20, 080A15 and 080M15. Yield strengths are typically in the region of 200 MPa.

2. Low-carbon free cutting steels. These are the most popular type of steel for the production of turned components where machineability and surface finish are important. Applications include automotive and general engineering. A commonly used specification is 230M07.

3. Carbon and carbon-manganese case hardening steels. These steels are suitable for components that require a wear resisting surface and tough core. Specifications include 045A10, 045Mb, 080M15, 210M15, 214M15.

4. Medium-carbon and carbon-manganese steels. These offer greater strength than mild steels and respond to heat treatment. Tensile strengths in the range 700-1000 MPa can be attained. Applications include gears, racks, pinions, shafts, rollers, bolts and nuts. Specifications include 080M30, 080M40, 080A42, 080M50, 070M55, 150M36.

5. Alloy case-hardening steels. These are used when a high load carrying capacity combined with a hard wear resisting surface is required. The alloying elements provides superior mechanical properties in comparison with carbon and carbon-manganese case hardening steels. Typical applications include gears, cams, rolled and transmission components. Types include 635M15, 655M13, 665M17, 805M20 and 832M13.

6. Alloy direct hardening steels. These steels include alloying elements such as Ni, Cr, Mo and V and are used for applications where high strength and shock resistance are important. Types include 605M36, 708M40, 709M40, 817M40 and 826M40.

7. Spring steels. These usually contain 0.6-0.9 % carbon together with similar quantities of manganese to provide appropriate hardenability (US specs. 1065, 1085). About 0.35% silicon is also included to resist 'sag'. About 1% Vanadium is also included in some specifications and small additions of chromium and /or molybdenum may also be made. (US specs. 6150, 8660, 9260).

8. High Strength Low Alloy (HSLA) or microalloyed steels. A wide range of these steels is available offering yield strengths between 200 and 1000 MPa, according to alloy content and processing. These steels contain less than 0.1% each of elements such as niobium, vanadium and titanium to promote grain refinement or precipitation hardening. Increasingly used in automotive, offshore, pressure vessels and pipeline applications, typically with a tensile strength of about 500 MPa.

9. Stainless steels. There are three types:

(a) Martensitic stainless steels (12% Cr) which can be hardened and tempered to give tensile strengths in the range 550-1000 MN/m2. Applications include fasteners, valves, shafts, spindles, cutlery and surgical instruments. Specifications include 410S21, 420S29, 420S45, 431S29,416S21, 416S41, 416S37 and 441S49.

(b) Ferritic stainless steels (17% Cr) which are common in strip and sheet form. Applications include domestic and automotive trim, catering equipment and exhaust systems. They have good ductility and are easily formed. Specifications include 403S17 and 430S17.

(c) Austenitic stainless steels (18% Cr, 8% Ni) which offer the highest resistance to corrosion. While these steels can not be hardened by heat treatment, they do undergo considerable work hardening. Applications include the food, chemical, gas and oil industries as well as medical equipment and domestic appliances. Specifications include 302S31, 304S15, 316S1 I, 316S31, 320S31, 321S31, 303531, 325S31, 303S42 and 326S36.

Link to British Stainless Steel Association

Below is a Comparison of some old and new BS steel specifications.

BS 970 designation UTS(MN/m2) Yield stress(MN/m2) Old BS EN number Approx. Eqiv. AISI US 4 fig Spec.
230M07 475
080M15 450a 330a 32C
070M20 560a 440a 3B
210M15 490
215M15 720
080M30 620a 480a 6 1030
080M40 660a 530a 8 1040
635M15 770 min
655M13 1000 min
665M17 770 min
34 4620
605M36, 1.5%Mn 0.25%Mo 700-850 a 540 a

709M40, 1%Cr 0.3%Mo 700-1200 a 550 - 950 a 19 4140
817M40, 1.2%Cr 0.3%Mo 850 - 1000 a 700 a 24 4340
410S21 700 - 850a a 56A
431S29 850 - 1000 a 57
403S17 400 min 280

430S17, 17% Cr 400 min 280

304S15, 10%Ni 18%Cr 500a 250a 58E
316S11, 11Ni 17Cr 3Mo +Ti 500a

303S31 500a

a Varies according to processing / heat treatment.

Reference: British Standards Institute, BS 970, various dates.

US Specifications

In the USA a number of 4 digit code systems (mainly identical) were developed by interested organisations: SAE (Society of Automotive Engineers), AISI (American Iron and Steel Institute). These have been brought together in a Unified Numbering System for Metals and Alloys (UNS) by the ASTM (American Society for Testing Materials).

The UNS uses a letter prefix to designate the material, eg: G for carbon and alloy steels, A for aluminium alloys, C for copper-base alloys and S for stainless or corrosion-resistant steels.

For the steels, the first two numbers following the letter prefix indicate the composition, excluding the carbon content. The various compositions used are as follows.

The second number pair refers to the approximate carbon content. Thus, G10400 is a plain carbon steel with a carbon content of 0.37 to 0.44 percent. The fifth number following the prefix is used for special situations. For example, the old designation AISI 52100 represents a chromium alloy with about 100 points of carbon. The UNS designation is G52986.

The old 4 - digit AISI type specifications (omitting the initial 'G' and the final digit) are still in use.

The UNS designations for stainless steels, prefix S, utilize the older AISI designations for the first three numbers following the prefix. The next two numbers are reserved for special purposes. The first number of the group indicates the approximate composition. Thus 2 is a chromium-nickel-manganese steel, 3 is a chromium-nickel steel, and 4 is a chromium alloy steel. Sometimes stainless steels are referred to by their alloy content. Thus S30200 is often called an 18-8 stainless steel, meaning 18 percent chromium and 8 percent nickel.

Some of the main groups of steels and their SAE - AISI specifications are given in the table below:

Spec. Alloying elements Spec. Alloying elements
10xx Plain carbon, non sulphurised and non phosphorised 46xx Nickel molybdenum: 1.75% Ni, 0.25% Mo
11xx Free cutting carbon steel, re-sulphurised 48xx Nickel molybdenum: 3.5% Ni, 0.25 Mo
13xx Manganese steels: 1.75% Mn 51xx Medium chromium: 0.8 - 1.0% Cr
23xx Nickel steels: 3.5% 52xx Chromium, high carbon: 1.45% Cr, 1.00% C min.
25xx Nickel steels: 5% 61xx Chromium vanadium steels: 0.8%Cr, 0.1%V
31xx 1.25% Ni, 0.6% Cr 86xx Chromium nickel molybdenum steels: 0.55% Ni, 0.5% Cr, 0.2% Mo
32xx 1.75% Ni, 1% Cr 87xx Chromium nickel molybdenum steels: 0.55% Ni, 0.5% Cr, 0.25% Mo
33xx 3.5% Ni, 1.5% Cr 92xx Manganese silicon: 0.8% Mn, 2.0% Si
40xx Molybdenum: 0.2% & 0.25% 93xx Nickel chromium molybdenum: 3.25% Ni, 1.2% Cr, 0.12 Mo
41xx Chromium molybdenum 94xx Manganese nickel chromium molybdenum: 0.95 - 1.35% Mn, 0.45% Ni, 0.4% Cr, 0.12% Mo
43xx Nickel chromium molybdenum: 1.82% Ni, 0.5 and 0.8% Cr, 0.25% Mo 97xx Nickel chromium molybdenum: 0.55% Ni, 0.2% Cr, 0.2% Mo
XXBXX.B Boron steels 98xx Nickel chromium molybdenum: 1.00% Ni, 0.8% Cr, 0.25% Mo

Commonly used classes are 23xx, 25xx, 31xx, 32xx, 33xx, 41xx, 43xx, 46xx and 48xx.

The 4140 and 4340 are two classes that have been developed for use at high strength levels, UTS values of up to 2000 MPa can be achieved by appropriate heat treatment.

The ASTM numbering system for cast iron is in widespread use. This system is based on the tensile strength. Thus ASTM No.30 cast iron has a minimum tensile strength of 30 kpsi

Suggested reading:
'Selection and Use of Engineering Materials', by J A Charles, F A A Crane and J A G Furness, Butterworth Heinemann, 1997.

'The Alloy Tree - A Guide to Low Alloy Steels, Stainless Steels and Nickel-base Alloys', by J C M Farrar, CRC, Woodhead Publishing Ltd., 2004, ISBN 1 85573 766 3.

'Carbon and Alloy Steels', ASM Speciality Handbook, 1995, ISBN: 0-87170-557-5.
'Stainless Steels', ASM Speciality Handbook, 1994, 0-87170-503-6.
'Microalloyed Steels 2002', ASM, 2002, ISBN: 0-87170-773-X.

David Grieve, Revised: 7th February 2010, 30th March 2007, 12th August 2005, 1st August 2005, 11th May 2005, 1st April 2005, 28th September 2004, 8th September 2004. Original: 12th December 2002,

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