Alloy steels are carbon steels containing less than 1% carbon but to which other metals have been added in sufficient quantities to alter the properties of the steel significantly. The more important alloying elements are as follows:

Aluminium Up to 1% aluminium in alloy steels enables them to be given a hard, wearresistant skin by the process of nitriding.

Chromium The presence of small amounts of chromium stabilizes the formation of hard carbides. This improves the response of the steel to heat treatment. The presence of large amounts of chromium improves the corrosion resistance and heat resistance of the steel (e.g. stainless steel). Unfortunately, the presence of chromium in a steel leads to grain growth (see nickel ).

Cobalt Cobalt induces sluggishness into the response of a steel to heat treatment. In tool steels it allows them to operate at high-level temperatures without softening. It is an important alloying element in some high-speed steels.

Copper Up to 0.5% copper improves the corrosion resistance of alloy steels.

Lead The presence of up to 0.2% lead improves the machinability of steels, but at the expense of reduced strength and ductility.

Manganese This alloying element is always present in steels up to a maximum of 1.5% to neutralize the deleterious effects of impurities carried over from the extraction process. It also promotes the formation of stable carbides in quench-hardened steels. In larger quantities (up to 12.5%) manganese improves the wear resistance of steels by spontaneously forming a hard skin when subject to abrasion (self-hardening steels).

Molybdenum This alloying element raises the high-temperature creep resistance of steels; stabilizes their carbides; improves the high-temperature performance of cutting tool materials; and reduces the susceptibility of nickel–chrome steels to ‘temper brittleness’.

Nickel The presence of nickel in alloy steels results in increased strength and grain refinement. It also improves the corrosion resistance of the steel. Unfortunately it tends to soften the steel by graphitizing any carbides present. Since nickel and chromium have opposite properties they are frequently combined together (nickel–chrome steels). Their advantages are complementary, whilst their undesirable effects are cancelled out.

Phosphorus This is a residual element from the extraction process. It causes weakness in the steel, and usually care is taken to reduce its presence to below 0.05%. Nevertheless, it can improve machinability by acting as an internal lubricant. In larger quantities it also improves the fluidity of cast steels and cast irons.

Silicon The presence of up to 0.3% silicon improves the fluidity of casting steels and cast irons without the weakening effects of phosphorus. Up to 1% silicon improves the heat resistance of steels. Unfortunately, like nickel, it is a powerful graphitizer and is never added in large quantities to high-carbon steels. It is used to enhance the magnetic properties of ‘soft’ magnetic materials as used for transformer laminations and the stampings for electric motor stators and rotors.

Sulphur This is also a residual element from the extraction process. Its presence greatly weakens steel, and every effort is made to refine it out; in addition, manganese is always present in steels to nullify the effects of any residual sulphur. Nevertheless, sulphur is sometimes deliberately added to low-carbon steels to improve their machinability where a reduction in component strength can be tolerated (sulphurized free-cutting steels).

Tungsten The presence of tungsten in alloy steels promotes the formation of very hard carbides and, like cobalt, induces sluggishness into the response of the steel to heat treatment. This enables tungsten steels (high-speed steels) to retain their hardness at high temperatures. Tungsten alloys form the bases of high-duty tool and die steels.

Vanadium This element enhances the effects of the other alloying elements present and has many and varied effects on alloy steels:

(a) Its presence promotes the formation of hard carbides.

(b) It stabilizes the martensite in quench-hardened steels and thus improves hardenability and increases the limiting ruling section of the steel.

(c) It reduces grain growths during heat treatment and hot-working processes.

(d) It enhances the ‘hot hardness’ of tool steels and die steels.

(e) It improves the fatigue resistance of steels.