What is hardness testing? Hardness testing is perhaps the most applied and widely used of all of the mechanical-testing procedures in order to define the quality of the heat-treatment procedure.

Hardness testing is a unique test method because it gives us an approximate comparison of other mechanical properties of the material being tested. There are correlations and comparisons of hardness to tensile strength. But these are only comparisons and should be seen only as comparisons.

Many definitions have been applied many times over a period of 1,000 years. The internationally accepted definition of hardness as it refers to the thermal transformation of metals is simply: “The resistance to permanent indentation.” Hardness testing can now be defined as a measurement of the resistance of a material to indentation by a fixed geometrical shape applied under a static and consistent load.

The indentation made by the constant-geometry indenter is measured either by depth or area. The two keys for accurate hardness testing are cleanliness (part and test machine) and squareness of the sample to the test indenter.

A known load is applied, but that load will vary according to the hardness scale and method being used to measure the resistance to indentation. Figure 1 shows the various hardness-testing methods of Brinell and Rockwell (macro), Knoop and Vickers (micro) and rebound tests (scleroscope).