Annealing
Metals are annealed to relieve stresses and strains in the "grain" or structure of the metal which are due to distortion set up during the course of manufacture. It operates by providing conditions under which the molecules can recover from the effect of whatever distorting process had been applied. Cold bending and hammering are examples of distorting processes.
When a metal is annealed, it is first heated strongly and then allowed to cool. The process affects different metals in different ways, but generally it may be said that ferrous metals need to be cooled slowly, whereas rapid cooling is permissible with brass or copper.
See Tempering.