The P21 low carbon mould steels are carburized or nitrided. These can be warmth treated to a hardness of 30 to 60 HRC, which guarantees that the steel can be easily machined into large, intricate molds and dies.
P21 is a precipitation-solidifying steel that incorporates aluminum. It is usually supplied after prehardening to 32 to 36 HRC. The P21 steel is especially suited for basic completion molds because of its superior polishability.