Hardening is a heat treatment that makes steels hard and wear resistant. Above all, tools and components subject to wear are hardened. Hardening consists of several operations. First, the tool is heated to hardening temperature and, if necessary, maintained at this temperature. Then it is quenched, i.e. immersed in water or oil. This makes the steel very hard, but also brittle and fragile. For this reason, the workpiece is subsequently tempered, i.e. heated to annealing temperature. In general, the aim of annealing treatment is to increase the deformation capacity of hardened components and to reduce the risk of cracking. The workpiece is then allowed to cool in the air.
When heated, the cubic space-centered ferrite lattice is transformed into the cubic surface-centered austenite lattice. The free space in the middle of the crystal is occupied by a C atom. If the austenitic steel is cooled (quenched) very quickly, the cubic surface-centered austenite lattice suddenly turns into the cubic space-centered ferrite lattice. The C atom in the middle has no time to migrate out of the lattice. There is now a C atom and additionally an iron atom in the middle of the lattice. This strongly distorts the crystal lattice. The result is a needle-like structure called martensite. Martensite is very hard, but brittle and only occurs if the steel contains at least 0.2% carbon. With certain high-alloy steels, cooling in the air already leads to the formation of martensite.