It is the ability of a metal to become hard (and inevitably brittle) under a given cooling rate. Therefore, the higher the hardenability of an alloy, the lower the cooling rate required to harden the metal. We have more heard about the hardenability in steels rather than other alloys. This is because in steels, there are different phases which can appear subject to the applied cooling rate below the critical temperature (the eutectoid temperature) as well as the alloying elements present in that grade of steel. Most of alloying elements in steel improve its hardenability by decreasing the critical cooling rate required to obtain the hardest possible phase i.e. martensite.