METHOD OF NITRIDING METAL RING AND APPARATUS THEREFOR

摘要

A metal ring made of a maraging steel is heated in the presence of a halogen compound gas to remove an oxide film from the metal ring surface and form a halogen compound film. The metal ring is heated in a vacuum or reduced-pressure atmosphere to remove the halogen compound film and then held in an ammonia-containing atmosphere at a temperature of 450 to 500°C for a period of 30 to 120 minutes to conduct a nitriding treatment. The nitriding treatment comprises: a step in which the metal ring is placed in a heating oven and heated to that temperature; a step in which a first mixed gas comprising 50 to 90 vol.% ammonia, 0.1 to 0.9 vol.% oxygen, and nitrogen as the remainder is introduced into the heating oven and the atmosphere in the oven is kept at that temperature to form a nitride layer on the metal ring surface; and a step in which, at the time when 1/3 to 1/2 of that treatment period has passed, the atmosphere in the oven is replaced with a second mixed gas comprising 0 to 25 vol.% ammonia and nitrogen as the remainder and this atmosphere in the oven is kept at that temperature until the remainder of the treatment period passes.