Effect of Mechanical Pretreatment on Gas Nitriding Behavior of Austenitic Stainless Steels

Abstract

In order to develop a method of gas nitriding of austenitic stainless steels without chemical treatment such as pickling, three types of steels, SUS304, SUS316 and SUS310, were pretreated under various mechanical processes (as-cut with resinoid blade, polishing with emery papers, grinding with CBN wheel, and shot peening), followed by nitriding in NH3 gas with a flow rate of 2.8 m 3 /h at 843 K for 20 h. The influence of the mechanical pretreatment on nitriding behavior was examined in connection with two factors of surface roughness and strain-induced martensite. Nitriding reaction depends primarily on surface roughness in SUS310 and SUS316. This nitriding reaction is markedly enhanced with increasing surface roughness when its value exceeds 6 and 1 μm for SUS310 and SUS316, respectively, while no reaction is recognized for the surface roughness less than these values. In SUS304, martensitic transformation is readily induced by the mechanical pretreatment, and not only the surface roughness but also the strain-induced martensite contribute to nitriding behavior. It is confirmed from the present results that mechanical pretreatment enables the gas nitriding of austenitic stainless steels without pickling, when the surface roughness and the amount of strain-induced martensite are controlled properly.