Rockwell scale

The Rockwell scale is used to measure relative hardness of different solids. It is the measure of the depth of penetration of a hard indenter pushed by a weight in the tested material compared to the penetration of the same indenter in some reference material.

The B scale for softer materials (such as aluminum, brass, and softer steels) uses a hardened steel ball as the indenter and a 100kg weight while the C scale for harder materials use a diamond cone as the indenter and a 150kg weight.

For purposes of reference, very hard steel (e.g. a good knife blade) might have a hardness of HRC 55 to HRC 62 or so. Axes, chisels, etc. would be more in the range HRC 40 - 45. (N.B. Readings below HRC 20 are considered invalid, as are readings much above HRB 100.)

Special correction factors are used for the effect of test-piece curvature ("round work correction factor"); other special scales test the hardness of surface layers (case hardening). The test method is based on precise measurement of the indenter's displacement, therefor paint, rust or other materials on the top or bottom of the test surface can cause erroneous (low hardness) readings.

Another scale for expressing hardness is the Mohs scale of mineral hardness but as this is not really convenient and hard to measure, the Rockwell scale is widely used in engineering.