Cementite Dissolution Kinetics of High Carbon Chromium Steel During Intercritical Austenitization

The cementite dissolution behavior of a hyper-eutectoid high carbon chromium bearing steel has been studied using the theory of local equilibrium assumption at interface. The dissolution mode was established and the process was calculated using DICTRA software. Heat treatment was performed on DIL 805 formastor, and microstructure was observed by ZEISS-SUPRA-55 SEM. Fraction and size of cementite particles were counted by ImageJ software. The results indicate that dissolution rate of cementite is fast in the beginning and then becomes slowly when holding at 840, 860 and 880 degrees C respectively. The simulation results are in good agreement with statistical results of SEM graphs. The fast dissolution stage is controlled by diffusion of C atoms, and the subsequent sluggish stage is governed by diffusion of Cr. Microscopically, dissolution of cementite depends upon the temporary destruction and instant recovery of local equilibrium at austenite/cementite interface. Diffusion rate of carbon is high, leading to the rapid distribution between cementite and austenite. C content in austenite increases significantly with the holding time, while little change happens to Cr content. The increase of Cr content in cementite obviously enhances its stability and reduces subsequent dissolution rate.