If the steel part has been surface treated by carburizing, carbonitriding, austenitic nitrocarburizing, ferritic nitrocarburizing, nitriding or boronizing, one cannot always grind into the finished surface. If this occurs, it most likely means that the treated surface will begin to be depleted in carbon, nitrogen, boron or other induced surface treatment elements. The only thing that can be done is to lightly polish the area that is to be tested with a fine-grit paper.

Care must now be given to the load selection, particularly if the formed case is thin. A heavy load will penetrate the formed case and will result in a false/incorrect reading. A simple rule of thumb might be “the shallower the case, the lighter the hardness testing load should be.”

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 case that forms at whatever depth was specified. There are two additional definitions that will identify effective case and total case.

The effective case is the depth at which the hardness is 513 hardness Vickers (or approximately 50 HRC scale). Total case is the case that has been formed to a depth of core hardness plus 50 hardness Vickers (or approximately 5 HRC scale).

Test Method

The method of testing is usually applied to a sample that has been cut, either from a test coupon or from a sample from the part being tested.

The hardness-test loading is usually a microhardness, which can be varied from 10- up to 1,000-gram load mass. The thickness of the diffused case will determine the load that is selected. Generally, it is a 200- to 300-gram load that is used to determine a total case depth.

The application of the test is performed on the test coupon across the formed case through to the core hardness. Another hardness-testing method for microhardness (and some macro) is to use the Knoop hardness test unit. This will give the operator the tested values in Knoop hardness units. Unlike the Vickers test (which makes a pyramidal square diamond impression and the measurement is made across the two axes), the Knoop method   impression is an elongated diamond. The hardness value is ascertained by measuring the longitudinal axis. 

Conclusion

Whichever procedure is used for the process of hardness testing, it is necessary that the test method is chosen carefully and that the test is conducted in an accurate and meaningful manner. It is this test that will determine the effectiveness of the heat-treatment process that has been performed. It is also very important that the quality-assurance technician/heat-treatment associate has a good understanding of the test methods, the load/indenter selection and the interpretation of the results to produce accurate hardness values.