self-hardening

 Designating, or pert. to, any of various steels that harden when heated to above a red heat and cooled in air, usually in a blast of cold air with moderate rapidity, without quenching. Such steels are alloys of iron and carbon with manganese, tungsten and manganese, chromium, molybdenum and manganese, etc. They are chiefly used as high-speed steels.  [Webster 1913 Suppl.]