Glow discharge nitriding of AISI 316L austenitic stainless steel: Influence of treatment pressure

The influence of treatment pressure on the characteristics of the modified surface layers produced by low-temperature d.c. glow discharge nitriding on AISI 316L austenitic stainless steel samples is investigated. Glow discharge nitriding treatments were performed at 703 K for 5 h at working pressures in the range of 1.5–20 hPa. Morphology and microstructure of the untreated and nitrided samples were studied by means of microscopy techniques, energy dispersion spectroscopy and X-ray diffraction analysis; microhardness measurements and corrosion resistance tests were also performed. The nitriding treatments produce a hardened surface layer consisting mainly of the so-called S phase. The presence of nitrides and the thickness of the modified layer depend on the used treatment pressure. When treatments are performed at 2.5 hPa, a fairly large amount of nitrides is observed in the modified layer, while when the nitriding pressure is lower or higher than 2.5 hPa, the nitride amount decreases and the layer becomes thinner. When the treatments are performed at 10 or 20 hPa, only a very small amount of chromium nitride is present as small surface precipitates. Metallographic analysis shows that many slip lines are present both at the surface and in the cross-section of the modified layer, presumably due to high stresses occurring during the formation of the layer. X-ray diffraction analysis of the S phase shows that its diffraction peaks are shifted from those of a perfect f.c.c. lattice; the observed shifts may be explained assuming that the S phase has an f.c.c. structure with a high density of stacking faults. Corrosion resistance tests, performed in 5% NaCl aerated solution with the potentiodynamic method, show that with the used treatment parameters nitriding at a pressure of 10 hPa or higher allows to obtain a significant improvement of the corrosion resistance in respect of the untreated alloy, reducing the anodic currents up to about 4 orders of magnitude.