Abstract: A nitriding process for a maraging steel is provided in order to increase the compressive residual stress of the maraging steel. A workpiece is made of maraging steel in which titanium is solid solved, and a partial amount of the titanium is oxidized at the surface of the workpiece so that titanium oxide is positively generated and concentrated at the surface. Then a reducing treatment is sufficiently performed as a pretreatment of a nitriding step, thereby removing oxygen. As a result, surface concentration of the titanium in an activated state is increased. Next, by performing the nitriding treatment, the reduced titanium and the solid solved titanium combine with nitrogen as titanium nitride. The titanium is in the activated state and is thereby easily nitrided. Moreover, the surface concentration of the titanium in the activated state is high, whereby a large amount of nitrogen infiltrates into the workpiece.

Abstract: A metal ring made of maraging steel is heated in the presence of a halogen compound gas, so as to eliminate an oxide film from the surface of the metal ring and to form a halogenous compound film. Thereafter, the metal ring is heated under a vacuum or reduced pressure atmosphere to eliminate the halogenous compound film, and then it is maintained under an atmosphere comprising ammonia at a processing temperature ranging between 450° C. and 500° C. for a processing time ranging between 30 and 120 minutes, so as to carry out a nitriding processing. The above nitriding processing comprises the steps of: placing the above metal ring into a heating furnace and raising the temperature inside the heating furnace to the above processing temperature; introducing a first mixed gas consisting of 50% to 90% ammonia byvolume, 0.1% to 0.