Definition of surface hardness depth SHD
- Surface hardening is a heat treatment process that does not change the chemical composition of the surface layer, in contrast to nitriding (nitriding hardness depth) and case hardening (case hardness depth), for example. The objective 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.
Effective hardening depth determination
- Determination of the effective hardening depth (DS) after surface hardening is specified in the DIN EN 10328 standard. The effective hardening depth (surface hardness depth SHD) is the distance between the specimen surface and the point in the layer where the Vickers hardness is 0.8 times the minimum value of the surface hardness, see graph.
- The surface of the specimen on which the measurement is performed must be finely machined.
- The hardness indentations are arranged on one or more lines perpendicular to the specimen surface with a width defined in the standard. The test force to be used is 9.807 N (HV). From the curve representing the hardness according to the distance from the surface, we obtain the effective hardening depth by measuring the distance from the surface to the calculated hardness limit.
