Calculation of compositional distribution in nitrided iron by analytical modelling

In this research nitrogen concentration depth profile in the nitriding process of pure iron has been analytically modelled. The compound region was considered as two separated layers including epsilon (ε) and gamma prime (γ') nitrides over an alpha (α) diffusion zone. The analytical model was constructed based on the binary Fe-N phase diagram system below 590oC and up to 11 wt% nitrogen. Considering relevant initial and boundary conditions, the Fick's second diffusion law was applied for three separated ε, γ' and α zones in a semi-infinite domain. Nitrogen depth profile equations were obtained related to nitriding temperature, time and diffusion distance from the surface. The modelling results were compared with the experimental data from literature and a good agreement was confirmed. Application of the present models indicated that increasing nitriding temperature from 500 to 580oC leads to increasing the maximum nitrogen concentration at the γ'/α interface, higher nitrogen distribution in α diffusion zone and deeper nitrogen diffusion in the substrate. At a certain nitriding temperature, increasing nitriding time up to 10h has a significant effect on the nitrogen distribution, but longer periods have no considerable influence.