rapid cooling, as by immersion in oil or water, of a metal object from the high temperature at which it has been shaped. This usually is undertaken to maintain mechanical properties associated with a crystalline structure or phase distribution that would be lost upon slow cooling. The technique is commonly applied to steel objects, to which it imparts hardness. On the other hand, copper objects that have become hardened by hammering or other deformation at ordinary temperatures can be restored to the malleable state by heating and quenching. Compare annealing; tempering.