Abstract
In order to develop a method of gas nitriding of austenitic stainless steels without surface treatment such as pickling, three types of steels, SUS304, SUS316 and SUS310, were pre-treated 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 m3/h at 843 K for 20 h. The influence of the mechanical pre-treatment 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 enhanced markedly 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 induced readily by the mechanical pre-treatment, 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 pre-treatment enables the gas nitriding of austenitic stainless steels without pickling, when the surface roughness and the amount of strain-induced martensite are controlled properly.
