This method means that the steel is heated in a furnace atmosphere with a high carbon potential, resulting in a carburized surface which gets hard after quenching.
Hardening depths are typically between 0.2 to 1.5 mm and is controlled by temperature and holding time in the furnace. Case hardening depth is defined as the depth below the surface where the hardness decreased to 550 HV. Common surface hardness after hardening and tempering is 58-62 HRC. The steels used for case hardening has low carbon content. This, together with the hard and durable surface of a strong improvement in fatigue strength. The reason is the hardened surface layer greater volume than the underlying material, which creates compressive stresses in the surface.