Special Alloy Steel

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.

 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.

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 250^oC -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).

 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.

Types of Alloy 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).

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.

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.

Alloy steels containing a number of alloying elements have been developed to overcome these deficiencies, albeit at extra cost.