PURPOSE: To harden the surfaces of steel contg. metals such as Ni, Co and others in which N, B are difficult to penetrate by forming thin layers of Ce, Cs, etc. on said surface and using the same as catalysts.

CONSTITUTION: In hardening the surfaces of steel contg. Ni and Co such as stainless steel and high speed steel by penetrating B and N therein, thin layers are formed on the surfaces of the steel by the vapor of Ce or Cs and the steel is put in a vacuum furnace. While the steel is heated to high temp. together with boron or boron compd., gaseous ammonia is introduced into the furnace. The boron compd. is reduced by the gaseous ammonia and penetrates in the surfaces of the steel with the Ce and Cs layers as catalysts, thus forming the hardened layer of high abrasion resistance contg. boron nitride as a lubricant.