Curve measurements

SHD - Surface Hardness Depth

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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.

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).

Hardness penetration depth curve

The surface of the specimen to be measured must be finely machined.

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The hardness indents 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). The hardness penetration depth is derived from the curve representing the hardness over distance from the surface by measuring the distance from the surface to the calculated limit hardness (GH).