Mechanisms of the Hydrogen Influence on the Diffusivity of Nitrogen During Plasma Nitriding Austenitic Stainless Steel

The influence of hydrogen on the diffusivity of nitrogen in plasma nitriding process of austenitic stainless steel (ASS) is analyzed by the presented theoretical calculations. Both processes in the bulk and on the surface are taken into account and involved into calculations. The internal stress assisted hydrogen and nitrogen diffusion in ASS taking place during plasma nitriding using various mixtures of nitrogen, and hydrogen is concerned as a key element of elucidation of nitrogen transport in expanded austenite. A systematic model for nitrogen transport in ASS that takes into account the hydrogen actions at steel surface, hydrogen, and nitrogen diffusion with concentration-dependent diffusion coefficient and stress interaction is proposed. It is shown that diffusion behavior of nitrogen in steel is affected by not only hydrogen and nitrogen concentration but also, the stress induced by interstitial solutes atoms expanding the lattice of the alloy. Increase of total hydrogen and nitrogen concentration in steel leads to increase of a gradient in the compositionally induced stress and, as result, the driving force for the diffusion of nitrogen increases, i.e., the nitrogen atoms diffuse deeper into the steel. Moreover, it was shown that the addition of hydrogen in H-2-N-2 mixture flux with concentrations in the range - (30 to 40) pct enhances nitrogen penetration into steel due to the hydrogen actions at steel surface.