Influences of plasma nitriding edge effect on properties of 316 L stainless steel

Some rings of different colors are formed when cylindrical stainless steel samples are plasma nitrided. They appear because of a phenomenon known as the edge effect associated with this technique. In this paper, we discuss some of the characteristics of these rings formed in nitrided AISI 316 L austenitic stainless steel: dimension, microhardness, composition, and corrosion resistance. Two cylindrical samples with different diameters were plasma nitrided in a 80% H2–N2 gas mixture, at a pressure of about 4 torr, for 4 h, at TN = 400 °C. One of the samples was surrounded by another ring-shaped sample, whose dimensions were chosen to contain the rings formed during the nitriding process. Consequently, rings of different dimensions were produced. The composition of samples was identified by conversion electron Mössbauer spectroscopy (CEMS) and Glancing angle X-ray diffraction (GAXRD). Regarding their relative dimensions, the rings showed similar characteristics, in spite of their different diameters. The samples displayed higher values of hardness in their rings than in their central regions. GAXRD detected peaks of Cr2N on the rings and of the expanded austenitic γN phase in the central region. CEMS results demonstrated the presence of the paramagnetic γN and ε phases. The proportion of these phases was consistent among the samples, regardless of their diameters. Concerning corrosion, the samples with a ring showed a poorer corrosion resistance than those without a ring. All results are in agreement with the presence of chromium nitride Cr2N on the ring of each sample.