Effect of surface martensite on the early stage of nitrocarburizing of AISI 316L

Very early-stage formation of expanded austenite is observed to be inhibited in regions thermodynamically favourable to the formation of α'-martensite. In this study, therefore, gas phase HCN-surface reaction as a short-term afterglow plasma nitrocarburizing treatment is conducted on mechanically polished AISI 316 L austenitic stainless steel. Analysis is performed by electron backscatter diffraction (EBSD), energy dispersive X-ray spectroscopy (EDS), light optical microscopy, and X-ray diffraction (XRD). As a result of grinding and polishing, strain-induced α'-martensite is observed in the remains of the primary ferritic microstructure due to thermodynamically favourable segregation left over from initial solidification. Following short-term plasma nitrocarburizing (at 400 °C for only 30 min), a very thin, discontinuous expanded austenite layer is observed; regions where it is absent are correlated with elemental segregation congruent with that resulting in α'-martensite formation. The presence of expanded austenite is assessed by detection of N using EDS, and the appearance of a white layer after etching.