Plasma nitriding of sintered AISI 316L at several temperatures

Samples of sintered AISI 316L stainless steel were plasma nitrided in a mixture of H2–20% N2, for 3 or 4 h. The treatment temperature was selected in 400–550 °C interval, in steps of 50 °C. X-ray diffraction (glancing angle geometry-GAXRD), conversion electron Mössbauer spectroscopy (CEMS), optical microscopy and Vickers microhardness were used as analytical techniques. For T≥500 °C and t=4 h, a 40-μm layer is formed. The GAXRD results showed a transformation of the austenite γ phase to the martensite in the sinterization process and showed as well, that the γ′ (Fe4N) phase is the predominant nitride besides small amounts of ε-Fe2N, γN CrN, Cr2O3 and the fcc nitrogen supersatured solid phase γN. The CrN phase seems to decrease with temperature while the γN phase fraction is almost ≤10%, independently on the temperature. The CEMS results indicated that while the γN fraction decreases with temperature of plasma nitriding, the γ′ fraction increases proportionally.