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What is heat treatment?


Heat treatment is a broad term covering any operation involving heating a metal to a certain temperature and then cooling it at a certain rate. Heat-treatment processes have one of two main purposes. They either release internal stresses caused by prior working, or they increase hardness and strength. Such processes are particularly applicable to steel, which is a complex alloy.

Annealing consists of heating the steel above a certain critical temperature, which depends on the carbon content of the steel. After being held above this temperature for some time, it is allowed to cool slowly at a controlled rate in a furnace. This relieves internal stresses, refines the grain structure and confers softness and ductility. Annealing is also carried out to relieve stresses in many other metals, and also in glass. Normalizing is a similar process, but cooling takes place in still air rather than in a furnace.

Hardening is accomplished by first heating the steel above its critical temperature. It is then cooled rapidly by plunging it into water or oil, a treatment called quenching. This makes the steel very hard, but it is also very brittle and of little use for engineering. Another heat treatment process called tempering is therefore carried out.

Tempering consists of heating the quenched steel to a temperature somewhat lower than the critical temperature and then allowing it to cool slowly. This has the effect of considerably reducing the brittleness, although there is also some reduction in hardness.

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