Computer simulations of cementite dissolution in austenite

Abstract

Diffusion-controlled dissolution of spherical cementite particles in an FeC alloy and an FeCCr alloy is simulated on a computer. The simulations are based on the assumption of thermodynamic equilibrium locally at the carbide-austenite phase interface and involve the simultaneous calculation of the diffusion inside the carbide particle and in the austenite matrix as well as the equilibrium and the flux balances at the moving interface. The results are in reasonable agreement with the experimental information available. The simulation shows that the reaction consists of three stages: (a) a stage which is rate controlled by carbon diffusion in austenite; (b) chromium diffusion in cementite; (c) chromium diffusion in austenite. The first stage is completed in less than a second whereas the second and third stages require hours. The calculation makes use of a detailed description of the thermodynamic and kinetic properties of the system. In order to obtain reasonable agreement with experimental information it was necessary to apply a diffusivity for chromium in cementite of the same magnitude as that in austenite. Some numerical problems connected with the difficulty of maintaining the mass balance and connected with the convergence of the calculation are discussed.