Nitriding process for maraging steel

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.

1 . A nitriding process for a maraging steel containing titanium, comprising:
 a solution heat treating step for generating and concentrating titanium oxide at the surface of the maraging steel by a solution heat treatment;   a reducing step for reducing the titanium oxide so as to concentrate the titanium at the surface of the maraging steel; and   a nitriding step for nitriding the maraging steel, in which the titanium is concentrated at the surface, and thereby applying compressive residual stress at the surface of the maraging steel.   
     
     
         2 . The nitriding process for the maraging steel according to  claim 1 , wherein the concentration of the titanium at the surface of the maraging steel after the reducing step is set to be not less than 13.0 at %. 
     
     
         3 . The nitriding process for the maraging steel according to  claim 1 , wherein the reducing step is performed by using reducing gas at a flow rate of not less than 24.7 L/m 3 .