Surface hardening is a heat treatment method that involves rapidly heating a part to the quenching temperature and then quickly cooling it, so that only the surface layer obtains a quenched microstructure.

For example, components such as gears, cams, and crankshafts operate under alternating loads such as torsion and bending, and are subjected to friction and impact. As a result, their surfaces experience higher stresses than their cores. Therefore, the surfaces of these components must possess high strength, hardness, and wear resistance. The surface quenching process can achieve this requirement for a hard surface and a tough core.