Tempering is a process of heat treating, which is used to increase the
toughness of
iron-based
alloys. Tempering is usually performed after
hardening, to reduce some of the excess
hardness, and is done by heating the metal to some temperature below the
critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard
tools are often tempered at low temperatures, while
springs are tempered to much higher temperatures.