Experimental Study and Computer Simulation on Multicomponent Diffusion in Multiphase Dispersions During Solid Carburizing of HP40Nb and KHR45A Alloys

To simulate solid carburizing processes of HP40Nb and KHR45A alloys at 1273 K and 1373 K (1000 °C and 1100 °C), Thermo-Calc (Thermo-Calc Software, Inc., Stockholm, Sweden) and DICTRA (Thermo-Calc Software, Inc.) software analyses were performed. A model to treat multicomponent diffusion in multiphase dispersions was applied, where carbides are assumed to be distributed in face-centered cubic matrix and diffusion only occurs in the matrix. The time-dependent carbon flux determined by weight gain measurement was used as boundary condition in diffusion simulations. The calculated carbides are NbC, M₂₃C₆, and M₇C₃ (“M” stands for metal atoms), where M₂₃C₆ first precipitates, then M₇C₃ appears as carbon increases, and NbC covers the largest area of carbon content. The results show that carburization resistance is much better for KHR45A than HP40Nb due to the addition of elements (Si, Cr, Ni, and Nb). Microhardness measurements were also conducted to obtain the carburized case depths, and the results agreed well with the predicted calculations.