Kinetic model of anisotropic stress assisted diffusion of nitrogen in nitrided austenitic stainless steel

摘要

The influence of elastic anisotropy of austenite on the transport of nitrogen in austenitic stainless steel (ASS) during the expanded austenite formation is analyzed by the presented kinetic model. The model is based on the thermodynamics involving the anisotropic lattice expansion due to nitrogen insertion. Thus, a diffusion behavior of nitrogen is affected by not only nitrogen concentration but also the anisotropic stress induced by nitrogen atom expanding the lattice of the steel. Developed model describes well known experimental results for polycrystalline and single crystal ASSs. Using presented model, we studied the effect of crystallographic orientations of single crystal on nitrogen penetration depth and surface concentration during formation of the expanded austenite. It was shown, that the thickness of the nitrided layer depends on the crystalline orientation and the diffusion rate is found to be highest for (100) orientation. Furthermore, anisotropic surface energies of different oriented crystals lead to anisotropy of nitrogen adsorption process. As a result, the (100) crystalline orientation is most favorable for nitrogen diffusion.