Hardening is the process of heat treatment in which steel is kept at suitable temperature till it gets transformed into austenite and then quenching it rapidly. Based on the carbon content present in the steel, the temperature for austenitizing is decided. By increasing the heating time, the core gets completely transformed to austenite. The final microstructure of the hardened steel would be ferrite or martensite.

Parts where core hardening is specified shall be tempered at a temperature not less than 280°C above the nitrocarburizing temperature. In the process of metal hardening, the mechanical properties of the components are imparted to make if fit for use.