Tempering is a heat-treatment process in metallurgy, used to harden (or, in some cases, soften) an alloy, notably steel. The metal is heated is slowly to the desired temperature, held there while stresses are relieved and excess solution precipitates out from the supersaturated solid solution, and then cooled, usually by rapid quenching. The temperature determines the properties produced, and may be chosen to retain hardness.
Tempering alters the crystal structure of the alloy. The effect produced depends on the composition of the alloy, the temperature to which it is heated, and the rate at which it is cooled. Usually, the metal is heated slowly to a specific temperature, then cooled rapidly. Tool steel is hardened in this way.