If the steel part has been surface treated by the following, one cannot always grind into the finished surface to prepare for hardness testing.

• Carburizing
• Carbonitriding
• Austenitic nitrocarburizing
• Ferritic nitrocarburizing (FNC)
• Nitriding
• Boronizing

Grinding will cause the treated surface to begin to be depleted in carbon, nitrogen and boron as well as other induced surface-treatment elements. The only remedy is to lightly polish the area that is to be tested with a fine-grit paper.

Care must now be given to the load and indenter selection, particularly if the formed case is thin, which occurs with nitriding and FNC processes. A heavy load will penetrate the formed case and produce a false, incorrect reading. A simple rule of thumb is “the shallower the case, the lighter the hardness testing load and smaller the indenter.”

If the objective of the hardness test is to measure the depth of case, it will be necessary for the test operator to understand the definition of case depth. Case depth is the formed case that has occurred to whatever depth was specified. There are two further definitions that we will identify: effective case and total case.