Phase field simulation analysis of nitrogen absorption-phase transformation behavior in the high temperature stable ferritic stainless steel

Synopsis : The behavior of diffusion and phase transformation in a high temperature stable ferritic stainless steel (SUS444) nitrogen-absorbed at 1450 K for 4 hr under 0.4 MPa N₂ gas was investigated with EPMA mapping. While Cr content in the nitrogen absorption layer (NA layer: γ phase) increased, Mo discharged from NA layer was enriched in the grain boundary of α/γ. Using a 1D-phase field simulation (PFS), the phenomenon during the NA treatment in the Fe-19Cr-2Mo-0.5Mn-N alloy was discussed. In the case of PFS with impurity diffusion (lattice diffusion) coefficients in Fe, since the diffusion velocity in γ phase was smaller than that of a phase, concentration distribution between NA layer and matrix a phase is caused by local phase equilibrium in depending on the composition of NA layer. Additionally, diffusion of substitutional atoms is slower than the growing velocity of NA layer that is depended on nitrogen diffusivity. This is the reason why Cr content in the NA layer fluctuates and why the average compositions in a phase and NA layer has the opposite trend with thermodynamic calculated and experimental results. On the other hand, in the case of the simulation under Dγ > Dα in consideration of the grain boundary diffusion of substitutional elements, the segregation behavior of solute elements was in good agreement with the results of EPMA mapping. Thus, it was confirmed that not only the diffusion of N atoms but also the grain boundary diffusion contributes greatly to the growing phenomenon of NA layer in a high stable ferritic stainless steel.