Study of Expanded Austenite Formed in Plasma Nitrided AISI 316L Samples, Using Synchrotron Radiation Diffraction

AISI 316L stainless steel samples nitrided at different conditions of temperature and time, showing different properties, such as nitrogen concentration (C-N) and nitrided layer thickness, were studied. Expanded austenite (gamma(N)) diffraction peaks up to the (222) reflection were observed using suitable wavelength synchrotron radiation. XRD patterns were fitted by Le Bail method, using a special triclinic crystal structure (with a lattice distortion eta) for gamma(N), whose peaks were decomposed in a few subpeaks, to consider C-N gradient across the nitrided layer. Our results indicate that regarding gamma(N) magnetic behavior, which was observed for the samples nitrided at 450 degrees C, it seems to be correlated not only to high C-N (>= 31at.%), but also to higher eta (>= 2.4 degrees), which reaches up to 5.6 degrees. This distortion eta decreases when C-N increases, consequently, with its minimum close to the surface. On the other hand, for paramagnetic samples (350 degrees C), eta increases up to 1.4 degrees when C-N increases up to 30 at.%.