Simulation of carburization and oxidation of a Ni-Cr-Fe based high-temperature austenitic alloy using ThermoCalc, DICTRA, and phase-field modeling

Thermo-Calc and DICTRA databases TCFE9 and MOBFE4 are used to simulate carburization in a Ni-Cr-Fe based high-temperature austenitic steel at 900°C, 1000°C, and 1100°C. To simulate carburization, long-range diffusion through the multiphase structure was handled by a disperse phase model for multicomponent diffusion. The simulation revealed a transformation from M₂₃C₆ to the more thermodynamically stable M₇C₃ with increasing carbon content, consistent with literature and experiment. As the carburization resistance of this high Ni-Cr-Fe based alloy relies on the formation and retention of a stable chromia scale, modeling and understanding the physics driving this scale’s microstructural evolution is important. Thermo-Calc calculations on chromia show that the stability of the protective scale depends on the temperature and chromium concentration. As the Cr content of the alloy increases, the calculated maximum mole fraction of chromia increases and the width of the chromia stable phase region increases. Temperature has the effect of both widening and shifting the chromia stable phase region to higher O₂ partial pressures. Thermo-Calc equilibrium calculations revealed kinetics to play an important role in oxide formation and growth. To simulate oxide microstructural evolution, a binary multi-phase field model is developed, informed by experimental results. Preliminary simulations show good agreement with experiment.