Abstract
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. The unusually thick (∼175 μm) layers of supersaturated nitrogen solid solution fcc phase were obtained after treatment for 10 min at 450°C. A radically different structure occurs in specimens treated at 550°C. Scanning electron microscopy (SEM) surface and cross-sectional micrographs reveal that the surface topography indicates 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 defected layer with pores and microcracks. The transport of nitrogen in austenitic stainless steel is driven by the flux es of matrix atoms directed to stabilise surface instabilities. Nitrogen depth profiles simulated on the basis of the model with the surface atom relocation process and an activation energy of 1·1-1·5 eV, and including balanced flux es of atoms in the bulk for relax ation of surface energy are in quantitative agreement with ex perimental results.
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