Hardening is the heating and subsequent cooling of steel at such a speed that there is a considerable increase in hardness, either on the surface or throughout. In most cases hardening takes place in conjunction with subsequent reheating, the annealing. Depending on the material, hardening improves the hardness and wear resistance or regulates the ratio of toughness to hardness. This is known as tempering. Practically all technically interesting steel alloys, such as spring steels, cold-worked steels, quenched and tempered steels, anti-friction bearing steels, hot-worked steels and tool steels, as well as a large number of high-alloy stainless steels and cast iron alloys, can be hardened.
Shield gas hardening is the hardening of components in an inert atmosphere. This protects the surface of the component against scaling and oxidation and against carburisation and decarburisation. By means of a regulated carbon potential in the shield gas atmosphere, decarburisation and carburisation processes can be reversed.
Vacuum hardening is the hardening of components under a controlled partial pressure, during which temperatures of up to 1,300 °C may be reached. The aim of this process variant is the creation of bright metallic workpiece surfaces which render further mechanical processing unnecessary.