Nitriding process for maraging steel

Abstract

Provided is a nitriding process for maraging steel, by which the compressive residual stress of maraging steel can be enhanced. On the surface of a work of maraging steel, part of the titanium solid-soluted in the work is oxidized with oxygen to form titanium oxide (TiO2) on the surface of the work positively, whereby the concentration of titanium oxide on the surface is enhanced (solution treatment step); since titanium present in the form of an oxide does not react with nitrogen in a nitriding step, the resulting work is subjected to sufficient reduction pretreatment prior to a nitriding step to conduct the removal of oxygen; thus, the surface concentration of titanium present in an active state is enhanced; and then the work thus pretreated is subjected to nitriding, whereby the reduced titanium and the solid-soluted titanium are bonded to nitrogen to form titanium nitride (TiN). In this nitriding step, the titanium present in the work is in an active state and therefore can be easily nitrided. Further, the surface concentration of titanium present in an active state is high, so that much nitrogen can penetrate into the work.

Description

technical field [0001] The present invention relates to a method for nitriding treatment of maraging steel containing titanium, and in particular to an improvement in the control technology of compressive residual stress. Background technique [0002] In steel, compressive residual stress is imparted to the surface by nitriding for the purpose of increasing the strength. At this time, when an excessive compressive residual stress is applied, the notch susceptibility becomes sensitive and the toughness is lost. Therefore, it is necessary to apply an appropriate residual stress. [0003] When nitriding treatment is applied to hard-to-nitride steel, the oxide film existing on the steel surface hinders nitriding. For example, in maraging steel, when the surface concentration of atoms such as titanium, which are easily bonded to oxygen, is high in the solution step, an oxide film of the atoms is formed. For this reason, since nitriding is inhibited in the nitriding step, suffic...

AI Technical Summary

Problems solved by technology

[0007] However, in the technique of Patent Document 1, sufficient compressive residual stress cannot be imparted to the surface of the maraging steel.

In addition, the reduction treatment performed as a pretreatment of nitriding treatment is not sufficient, so nitriding is carried out in a state where the oxygen on the surface of the maraging steel cannot be sufficiently removed, and as a result, the improvement of the compressive residual stress is limited.

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