The properties of steels can be greatly modified by thermal treatments, which change the internal crystalline structure of the alloy. Hardening of steel is based on the fact that iron undergoes a change in crystal structure when heated above its “critical” temperature.

Above this critical transformation temperature, the structure is called austenite, a phase capable of dissolving carbon up to 2%. Below the critical temperature, the steel transforms to ferrite, in which carbon is insoluble and precipitates as an iron carbide compound, FeeC (sometimes called cementite).

If a steel is cooled rapidly from above the critical temperature, the carbon is unable to diffuse to form cementite, and the austenite transforms instead to an extremely hard metastable constituent called martensite, in which the carbon is held in supersaturation. The hardness of the martensite depends sensitively on the carbon content.

Low-carbon steels (below about 0.20%) are seldom quenched, while steels above about 0.80% carbon are brittle and liable to crack on quenching. Plain carbon steels must be quenched at very fast rates in order to be hardened. Alloying elements can be added to decrease the necessary cooling rates to cause hardening; some alloy steels will harden when cooled in air from above the critical temperature.

It should be noted, however, that it is the amount of carbon that primarily determines the properties of the alloy; the alloying elements serve to make the response to heat treatment possible.

Normalizing is a treatment in which the steel is heated over the critical temperature and allowed to cool in still air. The purpose of normalizing is to homogenize the steel. The carbon in the steel will appear as a fine lamellar product of cementite and ferrite called pearlite.

Annealing is similar to normalizing, except the steel is very slowly cooled from above the critical. The carbides are now coarsely divided and the steel is in its softest state, as may be desired for cold-forming or machining operations.

Process annealing is a treatment carried out below the critical temperature designed to recrystallize the ferrite following a cold-working operation. Metals become hardened and embrittled by plastic deformation, but the original state can be restored if the alloy is heated high enough to cause new strain-free grains to nucleate and replace the prior strained structure. This treatment is commonly applied as a final processing for low-carbon steels where ductility and toughness are important, or as an intermediate treatment for such products as wire that are formed by cold working.

Stress-relief annealing is a thermal treatment carried out at a still lower temperature. No structural changes take place, but its purpose is to reduce residual stresses that may have been introduced by previous nonuniform deformation or heating.

Tempering is a treatment that always follows a hardening (quenching) treatment. After hardening, steels are extremely hard, but relatively weak owing to their brittleness. When reheated to temperatures below the critical, the martensitic structure is gradually converted to a ferrite-carbide aggregate that optimizes strength and toughness.

When steels are tempered at about 260°C, a particularly brittle configuration of precipitated carbides forms; steels should be tempered above or below this range. Another phenomenon causing embrittlement occurs in steels particularly containing chromium and manganese that are given a tempering cycle that includes holding at or cooling through temperatures around 567 to 621°C. Small molybdenum additions retard this effect, called temper brittleness. It is believed to be caused by a segregation of trace impurity elements to the grain boundaries.

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