Our discussion on hardness-testing principles continues. In part 2 we will classify some hardness-testing methods.
Scratch tests and plowing tests are similar. Early hardness testing made use of the scratch method, and this goes back to the early 1700s. The idea of the scratch test is that one material is capable of scratching another. The Mohs and file hardness tests are examples of this type.
Similarly, for the plowing test, a blunt element (usually diamond) is moved across the surface of the workpiece being tested under controlled conditions of load and shape. The width of the groove is the measure of hardness. The Bierbaum test is an example.
For static indentation tests, a ball, cone or pyramid is forced into the surface of the metal being tested. The relationship of load to the area or depth of indentation is the measure of hardness. These tests include Brinell, Knoop, Rockwell and Vickers.
The Brinell method of hardness testing came into use around the end of the 19th century, and the Rockwell test came into use around approximately 1920. Figure 1 gives an indication (please note that it is only an indication) of other mechanical conditions that the hardness testing can indicate (e.g., tensile, impact and ductility).
In order of load magnitude, the static indentation tests are:
- Vickers (micro and macro)
- Knoop (micro)
- Rockwell superficial (light-load macro)
- Rockwell (macro, A scale and B Scale)
- Brinell macro (heavy load)
Macro-hardness testing gives a rapid response to the results of the heat-treatment procedure because time-consuming sample preparation is required for microhardness testing. All hardness tests present the heat treater with an indication of the success (or failure) of the heat-treatment procedure. Figure 2 is a simple “tree diagram” that illustrates the methods of macro-hardness testing and includes the working load weight ranges for each of the macro-hardness testing methods.