Curve measurements

NHD - Nitriding Hardness Depth

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Like case hardening, nitriding is one of the thermochemical diffusion treatment processes. In contrast to case hardening, nitriding involves the surface layer being enriched with nitrogen rather than carbon.

Workpieces made of ferrous materials exhibit better wear, strength and corrosion properties after nitriding. The increase in nitrogen content also leads to an increase in hardness.

Nitriding is customarily conducted preferably in a temperature range of 500 to 550°C. As long as it is thermally stable, the original structure remains unchanged. Then the nitrogen donor is transported to the workpiece surface and adsorbed. This results in a release of nitrogen atoms at the specimen surface. The nitrogen atoms are absorbed and diffuse further into the workpiece interior. The nitriding layer is created.

Determination of the nitriding hardness depth is defined in standard DIN 50190-3. This is ascertained from the hardness curve and lies at a hardness limit of 50 HV below the core hardness, see illustration (Nhd).

Nitriding hardness depth curve

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

To the general tips

First, the core hardness must be determined. This is ascertained from a minimum of three hardness test indents and allows the hardness limit to be obtained. Subsequently, hardness indents are applied from the edge to the workpiece interior at precisely defined distances. The test method to be used is Vickers in the low-load range.

The nitriding hardness depth is derived from the curve representing the hardness over distance from the surface by measuring the distance from the surface to the calculated hardness limit.