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 disc harge plasma nitriding and low pressure plasma arc source ion nitriding on AISI 304 austenitic stainless steel were studied using X-ray diffraction (X RD), transmission electron microscopy (TEM) and microhardness testing. The surface nitrogen content was determined by electron probe microanalysis (EP MA) and energy dispersive X-ray analysis (EDX). The nitrogen content in the nitrided layer obtained using a low pressure plasma arc source is higher t han that in the nitrided layer obtained by d.c. glow discharge plasma nitri ding at 420 degreesC. Microstructural analyses by XRD show that both treatm ents at the same temperature of similar to 400 degreesC lead to predominant formation of the f.c.c. nitrogen [N] solid solution phase gamma (N). Howev er, the concentrations of N and the layer thickness of this phase are clear ly different for the various treatments. There are substantial differences in microstructures and phases detected by TEM, which showed that the pure e xpanded 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 + gamma mixture was present in the nitrided layer even when nitriding at temperatures below 450 degreesC. The reason for this is n ot clear. When the temperature was increased to 480-500 degreesC, there was no evident difference in the microstructures and phases in nitrided layers obtained by both treatments. The nitrided layers all consisted of CrN + al pha -Fe phases. Both nitriding methods are able to harden the surface of au stenitic stainless steel by nitrogen diffusion forming nitrided layers. (C) 2001 Elsevier Science B.V. All rights reserved.