Nitriding of an austenitic stainless steel in plasma torch at atmospheric pressure

Experiments with a nitrogen torch at atmospheric pressure have been performed in order to identify the role of surface processes in the mechanism of nitrogen transport during nitriding of stainless steel AISI 304. Unusually thick (∼175 μm) layers of supersaturated N solid-solution f.c.c. phase have been obtained for 10 min at 450 °C. Samples treated at 550 °C have a radically different structure. The scanning electron microscopy (SEM) surface and cross-sectional micrographs reveal that surface topography is an indicator of the degree of modification occurring in the nitrided layer. Surface vacancies generated by surface instabilities move deeply into the bulk at elevated temperatures and form a highly defective layer with pores and microcracks. The transport of nitrogen in austenitic stainless steel is driven by the fluxes of matrix atoms directed to stabilize surface instabilities. Nitrogen depth profiles simulated on the basis of a model with a surface-atom relocation process and activation energy of 1.15 eV, and including balanced fluxes of atoms in the bulk for relaxation of surface energy, are in qualitative agreement with experimental results.