Heating a metal to a temperature below its melting point for one or more of the following purposes:
(1) Softening of a cold-worked structure by re-crystallisation and/or grain growth
(2) Softening of an age-hardened alloy by causing an almost complete precipitation of the second phase in relatively coarse form
(3) Softening of certain age-hardenable alloys by dissolving the second phase and cooling rapidly enough to obtain a supersaturated solution (this usage is generally applied to nickel-base and copper-base alloys; the treatment should be more precisely called a 'solution heat treatment'
(4) Relief of residual stress and change in properties induced during the thermal flattening of a sheet at slightly elevated temperatures.