Alternated high- and low-pressure nitriding of austenitic stainless steel: Mechanisms and results

A combined surface modification treatment consisting of ion nitriding at high pressure and high temperature, followed by a cycle at low pressure, both cycles using a gas mixture of (N-2/H-2):(50/50) in pressure, was applied to stainless-steel AISI 304. In the first cycle, in a glow discharge at 4x10(-1) mbar and temperatures of 400- 450 degreesC, high-pressure nitriding was applied to the samples. In the second cycle, in a glow discharge at 8x10(-4) mbar, low-pressure nitriding was applied to the samples. Applying this sequential hybrid treatment alternately, good nitriding was obtained. X-ray diffraction (XRD) measurements showed the effects of this hybrid ion nitriding in the AISI 304 surface, indicating thick nitrided layers, confirmed by Auger electron spectroscopy (AES). Conversion electron Mossbauer spectroscopy, combined with AES and XRD, showed phases and compounds induced by such treatments. Vickers hardness measurements showed great enhancement in the surface hardness. Applying other combinations of gas mixtures and cycles, produced diverse results in the surface, like the induction of alpha- and epsilon-phases.