Modeling of growth and dissolution of grain boundary cementite in vacuum carburizing process

In order to predict microstructures during vacuum carburizing, the model which simulates not only the carbon(C) diffusion but also growth/dissolution of cementite(theta) is required. For development of a new model we applied vacuum carburizing to low alloy steels and analyzed the distribution of C and theta by GD-OES and image analysis of microstructures. The C in retained austenite(gamma) phase after carburizing was also measured by lattice constants obtained from XRD. We also simulated multi-component diffusion with gamma matrix and theta layer to analyze a velocity of the moving interface. The new carburizing model was proposed based on the findings, which suggest that C in gamma phase at the carburizing surface is supersaturated and corresponds to C concentration for metastable equilibrium condition to graphite. The growth and dissolution of the 0 follow a square root of time with the coefficients controlled by diffusion of Si in gamma and Cr in theta respectively. The required parameters such as diffusivity coefficients are obtained by the CALPHAD method. The calculated C distributions and volume fractions of theta represent the experimental results.