Low pressure plasma arc source ion nitriding compared with glow-discharge plasma nitriding of stainless steel

The nitrided layers produced by low temperature conventional d.c. glow discharge plasma nitriding and low pressure plasma arc source ion nitriding on AISI 304 austenitic stainless steel were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM) and microhardness testing. The surface nitrogen content was determined by electron probe microanalysis (EPMA) and energy dispersive X-ray analysis (EDX). The nitrogen content in the nitrided layer obtained using a low pressure plasma arc source is higher than that in the nitrided layer obtained by d.c. glow discharge plasma nitriding at 420°C. Microstructural analyses by XRD show that both treatments at the same temperature of ∼400°C lead to predominant formation of the f.c.c. nitrogen [N] solid solution phase γN. However, the concentrations of N and the layer thickness of this phase are clearly different for the various treatments. There are substantial differences in microstructures and phases detected by TEM, which showed that the pure expanded austenite phase with a f.c.c. structure was formed in the nitrided layer for the low pressure plasma arc source, but for glow discharge plasma nitriding, the CrN+γ mixture was present in the nitrided layer even when nitriding at temperatures below 450°C. The reason for this is not clear. When the temperature was increased to 480–500°C, there was no evident difference in the microstructures and phases in nitrided layers obtained by both treatments. The nitrided layers all consisted of CrN+α-Fe phases. Both nitriding methods are able to harden the surface of austenitic stainless steel by nitrogen diffusion forming nitrided layers.