Heat Treatment of Steels

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Heat Treatment of Steels[edit]

The properties of steels may be altered greatly by the heat treatment to which the steel is subsequently subjected. These heat treatments bring about a change in the mechanical properties principally by modifying the steel’s structure. Those heat treatments which concern shipbuilding materials are described.

ANNEALING This consists of heating the steel at a slow rate to a temperat- ure of say 850 °C to 950 °C, and then cooling it in the furnace at a very slow rate. The objects of annealing are to relieve any internal stresses, to soften the steel, or to bring the steel to a condition suitable for a subsequent heat treatment.

NORMALIZING This is carried out by heating the steel slowly to a temperature similar to that for annealing and allowing it to cool in air. The resulting faster cooling rate produces a harder stronger steel than annealing, and also refines the grain size.

QUENCHING (OR HARDENING) Steel is heated to temperatures sim- ilar to that for annealing and normalizing, and then quenched in water or oil. The fast cooling rate produces a very hard structure with a higher tens- ile strength.

TEMPERING Quenched steels may be further heated to a temperature somewhat between atmospheric and 680 °C, and some alloy steels are then cooled fairly rapidly by quenching in oil or water. The object of this treat- ment is to relieve the severe internal stresses produced by the original hard- ening process and to make the material less brittle but retain the higher tensile stress.

STRESS RELIEVING To relieve internal stresses the temperature of the steel may be raised so that no structural change of the material occurs and then it may be slowly cooled.