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:
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.
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