Microstructure and Nanosize Precipitate of Nitrided 316L Stainless Steel

The microstructure and nanosize precipitates of nitrided 316L stainless steel were investigated mainly by transmission electron microscopy (TEM) and X-ray diffraction analysis. Nitriding treatment performed in a molten salt bath followed by cooling down to ambient temperature and further diffusion treated. Produced layer consist of surface layer at the top, compound layer in the middle and final diffusion layer. Supersaturated phase is γ′N not γ-Fe₄N. The lattice parameter of γ′N is expanded about 16% by nitrogen supersaturation. Precipitates size of few nm were observed in the compound layers. These nanosize precipitates were coarsening up to ~ 40 nm by further heat treatment at 450 °C for 1 h. It was identified as Si₃N₄ by energy dispersive X-ray spectrometer, selected area electron diffraction patterns at two different zone axes [110], [114] and high resolution TEM analysis.