Nucleation and Coarsening Behavior of Aluminum Nitride and Its Effect on Abnormal Grain Growth in High-Temperature Carburizing Process

The onset of abnormal austenite grain growth and precipitation behaviors of aluminum nitride(AlN) during pseudo-carburizing process after solution treatment were investigated for the carburizing steel SCr420H. Thermo-Calc software package was used to calculate the solution temperature and determine the types of equilibrium precipitated phases. The coarsening kinetics of AlN precipitates were simulated based on DICTRA. The critical coarsening temperature of AlN precipitates was 1050 °C and the coarsening behavior of precipitate resulted in abnormal grain growth in this steel. Meanwhile, an approach to predict the starting time of abnormal austenite grain growth occurrence was developed by combining the precipitation thermodynamics, the coarsening kinetics of AlN and the grain growth theory. Microstructure characterization for nitride precipitation behavior during carburizing process indicated that the secondary phases in austenite were mainly hexagonal AlN and a small number of composite precipitates TiN-AlN. The experimental and simulation results of this work can offer a useful reference for avoiding abnormal austenite grain growth in high-temperature carburizing process.