As a general rule, tempering is always performed directly after hardening and is classified as heat treatment method affecting the entire component, i. e. from the surface to the core areas, with regard to its mechanical properties. Another method combines hardening and tempering into one process.
During tempering, the component is heated up to tempering temperature and subsequently cooled down to room temperature. The temperature value during tempering depends on the required final hardness of the products and the material used, since the chemical composition of the steel is the decisive factor for tempering behavior. The duration of tempering treatment is guided by the component cross sections and the charge size; the minimum dwell time subsequent to full heat-up of the workpieces is 1 hour. The data for tempering parameters are provided in the technical delivery specifications of the individual steel materials.
The purpose of tempering is to increase the toughness of the hardened component so that the ductility required due to the occurring loads is ensured. The loss of hardness occurring at the same time is accepted (Fig. 1).
For some types of steel, e.g. hot-forming tool steel, high-speed tool steel, several tempering processes (up to 4 times) are required to obtain optimum mechanical properties. Tempering may be performed in protective-gas plants, vacuum plants, in salt baths or in air. The type of atmosphere chosen will influence the surface of the components. Partial tempering, e.g. in induction plants, is possible. Oxidation of the surface during tempering of steel leads to occurrence of tempering colors, which are indicative of the tempering temperature.