SURFACE HARDENING TREATMENT METHOD FOR ENGINE VALVE

PROBLEM TO BE SOLVED: To provide a surface hardening method for an engine valve harmless for the environment instead of a salt bath nitriding method (Tufftride TF-1) accompanied by cyanogen waste water and cyanogen-containing industrial waste and a fluoride film activation type gas soft nitriding method requiring harmful exhaust gas treatment equipment. SOLUTION: In the surface hardening treatment method for an engine valve, a gaseous mixture comprising a carbon feeding compound (s) and a nitrogen feeding compound (s) being gas at 150°C under the atmospheric pressure as essential component is heated to ≥200°C under the atmospheric pressure; owing to the catalytic action of an engine valve as the metallic material to be treated, a metallic inner wall of a furnace and/or a metallic catalyst installed in the furnace, HCN is produced in the above atmosphere; the produced HCN is made to act on the surface of the engine valve to modify and activate a passivated film on the surface; and, successively, gas nitriding and/or gas nitriding-carburizing is performed at 400 to 600°C. COPYRIGHT: (C)2005,JPO&NCIPI