Tempering is not to be confused with tempers on rolled stock-these tempers are an indication of the degree of cold work performed. The mechanism of tempering depends on the steel and the tempering temperature. The prevalent Martensite is a somewhat unstable structure. When heated, the Carbon atoms diffuse from Martensite to form a carbide precipitate and the concurrent formation of Ferrite and Cementite, which is the stable form. Tool steels for example, lose about 2 to 4 points of hardness on the Rockwell C scale.
Tempering is done immediately after quench hardening.
In this region a softer and tougher structure Troostite is formed. This has less strength than Troostite but more ductility and toughness. Heating in a bath also ensures that the entire part has the same temperature and will undergo the same tempering. Regardless of the bath, gradual heating is important to avoid cracking the steel. After reaching the desired temperature, the parts are held at that temperature for about 2 hours, then removed from the bath and cooled in still air.