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.

Description

TECHNICAL FIELD [0001]The present invention relates to a nitriding process for a maraging steel containing titanium, and specifically, the present invention relates to an improvement in a technique for controlling the degree of compressive residual stress.BACKGROUND ART[0002]In order to improve strengths of steels, compressive residual stress may be generated at the surface of the steels by a nitriding treatment. In this case, if excessive degrees of the compressive residual stress are generated, there may be cases in which notch sensitivity is increased or toughness is decreased. Therefore, it is necessary to generate an appropriate degree of the compressive residual stress.[0003]When a nitriding treatment is performed on a steel with low nitridability, an oxide layer existing at the surface of the steel prevents nitriding of the steel. For example, in a case of maraging steel, if surface concentrations of atoms that easily combine with oxygen, such as titanium, are high, oxide lay...

AI Technical Summary

Benefits of technology

[0013]According to the nitriding process for the maraging steel of the present invention, the surface concentration of the titanium oxide is increased by the solution heat treatment. Moreover, by performing the reducing treatment before the nitriding treatment, the surface titanium concentration is increased. Therefore, nitriding of the maraging steel is facilitated. As a result, high compressive residual stress is obtained, whereby the strength of the maraging steel is further improved.

Problems solved by technology

In this case, if excessive degrees of the compressive residual stress are generated, there may be cases in which notch sensitivity is increased or toughness is decreased.

However, according to the technique disclosed in Japanese Unexamined Patent Application Laid-open No. 2004-162134, compressive residual stress cannot be sufficiently generated at the surface of a maraging steel.

Moreover, a reducing treatment is used as a pretreatment of a nitriding treatment, but it is not sufficiently performed.

As a result, improvement of the compressive residual stress is limited.

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