(1) A general term used to describe heating applied as a preliminary to some further thermal or mechanical treatment. (2) A term applied specifically to tool steel to describe a process in which the steel is heated slowly and uniformly to a temperature below the hardening temperature and is then transferred to a furnace in which the temperature is substantially above the preheating temperature. (3) Nonfer. met.-Heating a metal to a relatively high temperature for a relatively long time in order to change the structure before working. Ingots are homogenized by preheating.