Tempering is a heats treatment that improves the hardness of hard, brittle steels so that they hold during processing. Tempering requires that the metal be heated below a temperature called the low critical temperatures – depending on the alloy, these temperatures can range from 400–1,300˚F.
Once the low critical temperature of the steel is achieved, that temperature is maintained for a predetermined time. It is the type of steel that is being processed that determines what the critical temperature is & how long the materials need to be kept at that temperature.