A steel which because of its high alloy content will form martensite when cooled in air from the austenitic state.

Often called "self-hardening" steel. It is a steel that achieves full hardness when air-cooled from above its critical point. It does not need rapid quenching by oil or water. Air hardening greatly reduces the risk of distortion.

A steel containing sufficient carbon and other alloying elements to harden fully during cooling in air or other gaseous mediums from a temperature above its transformation range. Such steels attain their martensitic structure without going through the quenching process. Additions of chromium, nickel, molybdenum and manganese are effective toward this end. The term should be restricted to steels that are capable of being hardened by cooling in air in fairly large sections, about 2 in. or more in diameter.