Heat treatment is defined as an operation or combination of operations, involving heating and cooling of a metal or alloy in the solid state to obtain desirable properties.
Heat treatment consists of three phases.
(a) Heating of the metal
(b) Soaking of the heat into metal
(c) Cooling of the metal
Heating. Heating temperature of metal for the purpose of heat treatment depends upon its grade, grain size, type and shape of a metal or alloy. Generally, the metal is never heated much beyond its upper critical temperature. Plastic deformation takes place because of heating. This deformation depends upon the chemical composition of the metal or alloy and the temperature at which it is heated.
Soaking – Metal or alloy to be given heat treatment, is held at the specified temperature for a specified period so that there is a uniformity of temperature throughout the mass. The period of soaking depends upon the size and shape of the component.
Cooling – Main changes in the properties of metal or alloy take place in the cooling process. Change in properties or transformation depends mostly on the rate at which the cooling takes place. The different media used for cooling are,
(a) Caustic Soda solution.
(b) Brine solution (NaCl).