A common term for the evaluation of the heat treatment process is the Case Hardness Depth (CHD) of surface hardened materials. This is done by means of performing a series of hardness impressions from the edge of the sample towards the centre, findind the Hardness Limit.

Hardness Limit and Case Hardness Limit are depending on the heat treatment process and are efined as following:

1. Carburised or carbonitrided parts (EN ISO 2639)
Hardness Limit = 550 HV
CHD (Eht) = Distance from surface to point where hardness is 550 HV

2. Induction or flame hardened parts (EN 10328, ISO 3754)
Hardness Limit = 80% x (Minimum) surface hardness.
CHD (Rht) = Distance from surface to point where hardness is 80% of (minimum) surface hardness.

3. Nitrided or nitrocarburized parts (DIN 50190-3)
Hardness Limit = Core Hardness + 50 HV.
CHD (Nht, NCD) = (Max.) Distance from the surface to the point where hardness is 50HV1 above core hardness