Study of the S phase formed on plasma-nitrided AISI 316L stainless steel

Some tribological and corrosion resistance properties of austenitic stainless steels are enhanced by the formation of the S phase, also called expanded austenite. This phase is formed on the surfaces of austenitic stainless steels nitrided under certain conditions. In this work, AISI 316L steel was plasma-nitrided at 350, 400, 450, and 500 °C, and the samples were characterized by X-ray diffraction (XRD), conversion electron Mössbauer spectroscopy (CEMS), and wavelength dispersive spectroscopy (WDS) in order to investigate the S phase. XRD analysis identified the presence of a distorted cubic structure phase. The modified layer consists of an austenitic phase with different content of nitrogen, ranging from approximately 10 to 40 at%, and also γ′-Fe 4N and ɛ-Fe 2–3N phases. A diminution in the S phase occurs with the increase in nitriding temperature, and this decrease is related to the transformation of the S phase to the γ′-Fe 4N phase.