I'm no expert, but the differences between Rockwell testing methods are not always straightforward and do not always easily lend themselves to simple comparisons (aka all is sometimes not equal) as each one utilizes a slightly different method of measuring the hardness. Each testing method (Rockwell A, B, C, etc.) is used for a specific metal need/application. There are tables that allow for the comparison of hardness values such that they may be cross-referenced, and one can compare values of two or more testing methods.
Rockwell Process: A minor load of 10 kgf is applied first to the metal under test, thus causing an initial indentation/penetration; then the reader is zeroed. Following, the major load (60, 100, or 150 kgf as listed below) is applied. The major load is removed and the minor load (10 kgf) is left on and the reading is taken, thus giving the dimensionless hardness number.
Rockwell A - For tungsten carbide & other extremely hard materials; it's also used for thin, hard sheets. This test uses a 60-kgf load on a diamond indenter.
Rockwell B - B For materials of medium hardness such as low & medium carbon steels in the annealed condition. This test uses a 100-kgf on a 1/16" ball indenter.
Rockwell C - For materials harder than Rockwell B 100. This test uses a 150-kgf load on a diamond indenter.
Rockwell D - Used where a somewhat lighter load is desired than on the Rockwell C scale, such as on case-hardened pieces. This test uses a 100-kgf load on a diamond indenter.
Rockwell E - For very soft materials such as bearing metals. This test uses a 100-kgf on a 1/8" ball indenter.
I hope helps and doesn't muddy the water.<img src="/ubbthreads/images/graemlins/grin.gif" alt="" />
