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Microstructure of Plasma-Nitrided 316 Austenitic Stainless Steel

Austenitic stainless steels plasma-nitrided at low temperature can have a tremendously improved surface hardness without affecting their excellent corrosion resistance. The surface layer of these nitrided materials was considered to be composed of an uncharacterized, so called “S-phase”. During the past decade, several research groups have studied “S” phase using X-ray diffraction and transmission electron microscopy (TEM), but its microstructure is not yet well understood and is still a topic of debate. In this paper, we characterize the microstructure of 316 stainless steel nitrided by intensified plasma-assisted processing (IPAP) using cross-sectional TEM. Samples of 316 austenitic stainless steel were nitrided for 1 hour by IPAP at a temperature of ∼ 400 °C. Plasma-nitrided samples were cross-sectioned and glued face-to-face by joining the nitrided surface. Crosssectional specimens for TEM observations were prepared by mechanical grinding, polishing and dimpling followed by Ar+ milling. TEM studies were carried out in a newly installed JEOL JEM 2010 electron microscope in the Materials Characterization Facility at the Louisiana State University.