Further investigation of the structure and properties of austenitic stainless steel after plasma nitriding

Abstract

A series of plasma nitriding experiments has been conducted on AISI 304L austenitic stainless steel at temperatures ranging from 375 to 475°C using pulsed-DC plasma with different pulse duty cycles, N2–H2 gas mixtures and treatment times. It is shown that a wide range of treatment parameters exist that allow the formation of the S-phase. The formation and growth of this surface layer depend strongly on the treatment parameters, such as nitrogen partial pressure and duty cycle. Within the parameter range investigated, the layer growth appears to be diffusion controlled with an activation energy about 107 kJ/mol. The formation of CrN precipitates during plasma nitriding is not accompanied by the formation of bcc iron, which might be expected due to the loss of free chromium. However, the S-phase transforms into CrN and bcc iron following a heat treatment at 450°C or higher for 25 h. The wear rate after plasma nitriding is greatly reduced compared with the untreated material.