The quotient of R consisting of achieved (HRCgeh) and theoretically possible hardness(HRCmax) according toeq. 1 is referred to as degree of hardening and is used as measure for hardenability.
R > 0.95 for highly-stressed parts and R > 0.70 for medium stressed parts (ca. 50 % of martensite) should be reached.
In this relation found empirically by Spies, the maximum hardness can be determined for heat-treated steels between 0.20% and 0.65% of C .
Rockwell hardness test