Determination of the hardness penetration depth after surface hardening is defined in standard DIN EN 10328. The hardness penetration depth (Surface Hardness Depth, SHD) is the distance between the specimen surface and the location in the layer where the Vickers hardness equals the limit of 0.8 times the minimum value of the surface hardness; see illustration (Shd).
Surface hardening is a heat treatment process that does not change the chemical composition of the surface layer, in contrast to nitriding and case hardening, for example. The aim of surface hardening is to achieve a completely martensitic structure in the surface layer and leave the remaining cross-section in its original condition.
Surface hardening increases both strength and hardness in the workpiece surface layer. Hardening also creates a residual compressive stress in the surface layer, which leads to an improvement in vibration resistance.
Surface hardening is conducted in three stages. First the surface layer is heated to the necessary treatment temperature. Then it is maintained at this temperature. Finally the workpiece is cooled to room temperature. This takes place very quickly, as only the surface layer of the workpiece is affected.