On the formation of expanded austenite during plasma nitriding of an AISI 316L austenitic stainless steel

Plasma nitriding of an AISI 316L austenitic stainless steel at low (400 °C) and high temperatures (550 °C) was performed under different nitriding gas mixtures. Nitrided surfaces were characterized by XRD using the Rietveld method. Expanded austenite “γN” with a special triclinic (t) crystalline structure was formed during the low-temperature nitriding treatment. Minor volume fractions of Fe3N, Fe4N and Cr2N nitrides were also found. The expanded austenite phase showed a distortion ε of the lattice angles due to a very high nitrogen content dissolved in austenite, supersaturating the solid solution and leading to a 10% lattice distortion and to high compressive residual stresses at the surface.After nitriding the specimens at 550 °C the case was composed primarily by a high volume fraction of Fe4N, Cr2N and CrN nitrides, leading to a low distortion of the parent austenitic phase, maintaining the original cubic lattice.