Thermodynamics and kinetics of solution nitriding

In order to clarify the thermodynamics and kinetics of solution nitriding, the non-uniform nitrogen distribution formed by solution nitriding was investigated in austenitic (18Cr–10Ni) and ferritic (25Cr) stainless steels, and then it was simulated with DICTRA. The main issue for austenitic stainless steel is the surface reaction determined by the mass transfer from gas to solid, while that for ferritic stainless steel is the ferrite to austenite transformation and the austenite growth kinetics induced by nitrogen absorption. DICTRA simulations gave close agreement with the experimental results and demonstrated that solution nitriding is obviously controlled by mass transfer of nitrogen from gas to solid as well as diffusion in the solid, and austenite growth is controlled by nitrogen diffusion in austenite. In addition, the analysis suggests that nitrogen should have a stronger interaction with chromium than that treated in the thermodynamic database and austenite nucleation induced by nitrogen absorption takes place without any change in Cr composition.