The Collaborative International Dictionary of English v.0.48:

Self-hardening \Self`-hard"en*ing\, a. (Metal.)
   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.
   -- Self`-hard"en*ed, a.
   [Webster 1913 Suppl.]