An important feature of the Rockwell hardness test procedure is the use of the preliminary force as part of the testing cycle. Application of the preliminary force acts to push the indenter through minor surface imperfections and to crush residual foreign particles present on the test surface. By establishing a reference beneath the surface prior to making the first depth measurement, it allows testing of materials with slight surface flaws while maintaining much of the test accuracy. Still, as a general rule, the better a test surface is prepared, the more likely the measurement will represent the true Rockwell hardness value of a material.
For the best results, the test surface and the surface in contact with the support anvil should be smooth, flat, and free of oxides, foreign matter, and lubricants. The test surface should be prepared in a manner that will not alter the properties of the test material such as by overheating or cold-working. The test surface should be representative of the material under test. For that reason, surface effects, such as carburization or decarburization, should be removed prior to testing, unless the purpose of the test is to measure these surface features. Similarly, other types of coatings, such as paint, galvanizing, etc., should also be removed prior to testing.
The degree of surface roughness that can be tolerated depends on the force levels to be applied. A finish ground surface is usually sufficient for the Rockwell C scale and for the Rockwell ball scales that apply a force of at least 980.7 N (100 kgf). In general, lighter test forces require better surface finishes. For the superficial scales that use a total force of 147.1 N (15 kgf), a polished surface is usually required