Swelling of 316L austenitic stainless steel induced by plasma nitriding

Swelling of 316L austenitic stainless steel plasma nitrided at 400°C under floating potential has been investigated using electron back scattered diffraction and white-light interferometry. Swelling of individual grains strongly depends on their crystallographic orientation, similarly to the thickness of the nitrided layer. After 1 h of treatment, swelling is maximum for the 〈001〉 oriented grains and minimum for the 〈111〉 oriented grains. After 8 and 33 h of nitriding, the maximum of swelling is observed in the grains having their normal direction at about 15° from the 〈001〉 orientation. These results are discussed on the basis of plastic strain after comparison with calculated swellings of the 〈001〉 and 〈111〉 oriented grains, using the thickness of the nitrided layer deduced from the trapping–detrapping diffusion model and a rough estimation of the plastic strain. The satisfactory agreement between experimental and calculated swellings supports the idea that swelling results from the lattice expansion due to the incorporation of nitrogen plus an elastic strain and a plastic strain. For individual grains of the 316L matrix, nitriding leads to a tensile-like elongation of high magnitude (around 20%) and it might be the origin of the lattice rotations which were previously observed after nitriding.