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Mechanical Engineering Design Notes

Materials Contents

Alloy and Stainless Steels

1. Introduction:
Plain carbon steels are relatively cheap, but have a number of Property limitations. These include:

(i) Cannot be strengthened above about 690 MN/m2 without loss of ductility and impact resistance.
(ii) Not very hardenable i.e. the depth of hardening is limited.
(iii) Low corrosion and oxidation resistance.
(iv) Must be quenched very rapidly to obtain a fully martensitic structure, leading to the possibility of quench distortion and cracking.
(v) Have poor impact resistance at low temperatures.
Alloy steels containing a number of alloying elements have been developed to overcome these deficiencies, albeit at extra cost. Plain carbon steels contain only iron and carbon and less than 0.5% Mn and less than 0.5% Si.
Low carbon steels contain less than 0.25% carbon, medium carbon between 0.25% and 0.6% carbon.
High carbon steels between 0.6% to 1.4% carbon.
Small additions of other alloying elements give high strength low alloy (HSLA) steels and some tool steels, while higher additions produce tool steels, heat resisting steels and stainless steels.

The principal alloying elements used are: manganese (Mn), nickel (Ni), chromium (Cr), molybdenum (Mo), tungsten (W), vanadium (V), cobalt (Co), silicon (Si), boron (B), copper (Cu), aluminium (Al), titanium (Ti) and niobium (Nb).

2. General Effects of Alloying Elements:
1. Increase hardness in solid solution (in ferrite) without decreasing the ductility as much as carbon does.
2. Reduce the critical cooling velocity (except Co) by making the transformation to the equilibrium phase slower. Alloy steels may therefore be hardened by an oil or even air quench, reducing the risk of cracking or distortion that can result from a rapid water quench. Most elements also lower the Ms and Mf temperatures to below room temperature, leading to some 'retained austenite' in the quenched structure.

Carbon content % Alloy content % Critical cooling velocity Deg C per sec
0.42 0.55 Mn 550
0.40 1.60 Mn 50
0.42 1.12 Ni 450
0.40 4.80 Ni 85
0.38 2.64 Cr 10

3. Either increase or decrease the a to y transition temperature. Elements are either ferrite stabilising e.g. Cr (BCC), W (BCC), V (BCC), Mo (BCC), Al (FCC), Si (diamond cubic) or austenite stabilising e.g. Ni (FCC), Mn (diamond cubic), Co (HCP/FCC>690K), Cu (FCC) and also carbon.

4. Some form hard, stable carbides. E.g. Cr, W, V, Mo, Ti, Nb, and others such as Mn help stabilise carbides. The carbides may be of the type Cr7C3, W2C, Mo2C and VC or more complex like Fe4W2C etc.

5. Some cause graphitisation of iron carbide (cementite) e.g. silicon, Ni, Co, Al. For this reason these elements are not added to high carbon steels unless counteracted by a strong carbide former.

6.They confer the characteristic property of the alloying element on the steel. E.g. chromium confers corrosion resistance when more than about 12% is added to a steel, rendering it a stainless steel.

7. They affect the rate of grain growth at high temperature. Cr speeds up grain growth rate, so it is important not to overheat high chromium steels as coarse grains can give brittle properties. Elements like V, Ti, Nb, Al and Ni slow down grain growth rates and so are used in case hardening steels.

8. They alter the eutectoid composition and temperature.

9. They improve mechanical properties such as tensile strength. Hardness is improved due to carbides present. Strength increased by elements dissolved in ferrite and toughness improved by finer grain structures. Walters devised approximate multiplication factors for the effect on ferrite with a basic strength of 250 MN/m2

3. Types of Alloy Steels: Alloy steels are generally classified as low-alloy steels or high-alloy steels. Low-alloy steels have similar microstructures and heat treatment requirements to plain carbon steels and contain up to 3 or 4 % of alloying additions in order to increase strength, toughness or hardenability. High-alloy steels have structures and heat treatments that differ considerably from plain carbon steels. A surumary of a few selected alloy steels is given below.

3.1 Low alloy constructional steels: As well as carbon, these contain additions of Mn, Ni, Cr, Mo etc. Nickel strengthens ferrite in solution but also causes graphitisation of carbides. For this reason it is usually accompanied by strong carbide stabilisers such as chromium, which also strengthens ferrite and increases hardenability. The Ni is usually in the majority, with maximum amounts 4.25% Ni and 1.25%Cr, often resulting in air hardenable steels. Tempering in the range 250oC -4000C can result in 'temper brittleness', but this can be minimised by additions of 0.3% Mo giving 'nickel-chrome-moly' steels, used in axles, shafts, gears, con-rods etc. Some Mn can be substituted for more expensive Ni. (See Table for more details).

3.2 Alloy tool and die steels: (B5970 and B54659). These acquire hardness and wear resistance by incorporating carbides that are harder than cementite, while retaining strength and some toughness. They also have high hardenability and the ability to resist the tempering effects of use in hot working dies and from frictional heating in high speed machining operations. Alloying additions include Cr, W, Mo and V, which are strong carbide formers and also stabilise ferrite and martensite.

A typical composition is 18%W, 4%Cr, 1%V, 0.8%C. Quenching from high temperatures (13000C) is necessary, in order to dissolve as much W and C in austenite, for maximum hardness and heat resistance, followed by heating to 3000C - 4000C to transform any retained austenite to martensite then to 5500C to relieve internal stresses and produce carbide particles in a toughened martensite matrix. This martensite is then temper resistant up to 7000C.

3.3 Stainless steels: The addition of more than about 12% Cr renders a steel 'stainless' or corrosion resistant because of a passive layer of chromium oxide Cr2O3 on the surface. Steels containing large amounts of Cr are ferritic, as Cr is a ferrite stabiliser. Stainless steels can be classified into three main types.

3.3.1 Ferritic stainless steels: These contain 12% to 25% Cr and less than 0.1% carbon. They are ferritic up to the melting point, i.e. austenite never forms, and therefore cannot be quench hardened to give martensite. They can be work hardened but are oniy ductile above the ductile- brittle transition temperature found in BCC metals. Prolonged overheating can cause precipitation of an embrittling sigma phase.

3.3.2 Martensitic stainless steels: These contain 12% to 25% Cr and 0.1% to 1.5%C. The higher carbon content restores the alpha to gamma transition temperature by making the gamma loop larger. This means that the steel can be heated into the austenite region and quenched to give a martensitic structure. Hardenability is generally high enough that hardening can be achieved by air-cooling. Uses include knives, cutting tools, dies etc.

3.3.3: Austenitic stainless steels: These contain both Cr and Ni, and since Ni has a greater effect on the a to 7 transition temperature this can be reduced to below room temperature, and the austenitic FCC phase is retained. This gives a stainless non-magnetic steel that, being FCC, is more ductile and can be worked to produce deep shapes used in chemical plant, kitchenware and architectural work. Nickel assists the ductility by resisting the grain growth promoted by Cr, although severe cold work can produce martensite from the austenite. Fast cooling depresses the alpha to gamma transition temperature, giving austenite for as low as 7% Ni, while fast heating raises it, (thermal hysteresis).

A common alloy is 18% Cr, 8% Ni. Depending on the presence of other ferrite stabili sing and austenite stabili sing elements and cooling rates, a variety of microstructures can result. Schaeffler diagrams were introduced to help predict the structure around a weld in stainless steel, but can be used to predict structure in an alloy subjected to similar thermal treatment such as air cooling.

The welding of stainless steels can lead to the problem of 'weld decay' unless the steel is 'stabilised' by additions of about 1% Nb or Ti. In the heat affected zone (HAZ), where the steel is subjected to temperatures in the range 550oC to 850oC, the carbon present can react with the Cr to form chrome carbide Cr36C6 precipitates on the grain boundaries. This depletes the chromium oxide Cr2O3 on the surface and in corrosive conditions the area around the grain boundaries become anodic and corrodes. This problem can be alleviated in one of three ways:

  • (i) Resolutionise the precipitates by heating above 930oC and cool quickly through the critical range 5500C to 850oC to allow insufficient time for the transformation to the carbide to start.
  • (ii) Only use very low carbon stainless steel (e.g. 304L, 316L), since the time to transformation is much longer, or
  • (iii) Use stabilised steels containing 1% to 2% Nb or Ti, both of which are stronger carbide formers than Cr, thus leaving the Cr in solution and as Cr2O3 on the surface.

3.4: Marageing Steels: These are iron-nickel alloys, a typical example being 18%Ni, 8%Co, 4%Mo and up to 0.8%Ti, with less than 0.05% carbon. Heat treatment involves solution treatment at 8000C - 8500C followed by quenching of the austenite to give a BCC martensitic structure. This is less brittle than the BCT martensite found in plain carbon steels because of the low carbon. Ageing at 4500C - 5000C for 2 hours produces finely dispersed precipitates of complex intermetallics such as TiNi3 resulting in tensile strengths around 2000 MN/m2. Alter solutionising, they are soft enough to machine cheaply, before ageing, which can compensate for higher materials cost. They are relatively tough, with good corrosion resistance and good weldability since they do not air harden so rapidly as some steels. Uses include aircraft undercarriage components, dies, tools, engine parts etc.

3.5: Hadfields manganese steel: This is a high alloy steel that contains 12%-14% Mn and 1% C. It is austenitic at all temperatures and therefore non-magnetic. It has a unique property in that when the surface is abraded or deformed, it greatly increases surface hardness while retaining a tough core. For this reason it is used in pneumatic drill bits, excavator bucket teeth, rock crusher jaws, ball mill linlngs and railway points and switches. Water quenched from 10500C to retain carbon in solution, the soft core has a strength of 849 MN/m2, ductility of 40% and a Brinell hardness of 200, but after abrasion this rises to 550 BHN. The reason for the rapid rise in surface hardening is uncertain, though martensite formation or, more likely, work hardening have been proposed.

4. References:
John, V. B., Introduction to Engineering Materials, Macmillan.
Higgins, R.A., Properties of Engineering Materials, Hodder and Stoughton.
Higgins, R.A., Engineering Metallurgy Vol.1, Hodder and Stoughton.
Smith, W. F., Foundations of Materials Science and Engineering, McOraw-Hill.
Askeland, D. R., The Science and Engineering of Materials, Wadsworth.

David Plane, February 2003.

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