Metallurgical characteristics of the plasma (ion)-carburized layer of austenitic stainless steel SUS316L

Abstract

The recently developed technique for the low-temperature plasma (ion) carburizing of austenitic stainless steel improves not only wear properties, but also corrosion resistance. Conventional plasma (ion) carburizing of austenitic stainless steel has attracted less attention, however, as the formation of chromium carbides decreases the corrosion resistance of the materials. In this study, we carried out the high-temperature plasma (ion) carburizing of austenitic stainless steel SUS316L, which is the specification of Japanese Industrial Standard on steel for special purpose, at a temperature of 1303 K under a pressure of 346 Pa in an atmosphere of CH 4+H 2 plasma for a duration of 14.4 and 28.8 ks. As a result, two interesting metallurgical phenomena occurred in the plasma (ion)-carburized layer of the SUS316L austenitic stainless steel. First, the volatilization of the manganese from the specimen surfaces decreased the manganese concentration at the surface of the plasma (ion)-carburized layer. In comparing the CH 4+H 2 plasma with the CH 4-free plasma (i.e., the Ar+H 2 plasma), the depth of affected layer of the former in which the manganese concentration is lower than that in the matrix was two times deeper than that of the latter. This suggests that the manganese in the plasma (ion)-carburized layer of austenitic stainless steel SUS316L diffuses more rapidly than that in the matrix. Secondly, measurements taken by Glow Discharge Optical Emission Spectroscopy (GDOES) indicated an “uphill” diffusion phenomenon similar to that reported by L. S. Darken in 1949. According to the carbon distribution profile of the plasma (ion)-carburized specimen moving from the surface to the inner part, the intensity of the carbon initially lowered within the first few microns of depth, reached its nadir, and thereafter rose. The carbon in the layer seemed to be affected by silicon, nickel, and iron, as the intensities of these elements peaked in the plasma (ion)-carburized layer.