Cellular automaton simulation on cooperative growth of M<sub>7</sub>C<sub>3</sub> carbide and austenite in high Cr cast irons

Abstract

M<sub>7</sub>C<sub>3</sub> carbide’s amount, size, morphology and distribution in the microstructure contribute much to the wear resistance of high chromium cast irons. In the present paper, a two-dimensional microscopic cellular automaton model for the growth of the faceted M<sub>7</sub>C<sub>3</sub> carbide together with the austenitic dendrite grains in an Fe-4%C-17%Cr ternary alloy is developed to obtain the evolution of M<sub>7</sub>C<sub>3</sub> carbide grain morphology, the concentration redistribution and their interaction during the growth of M<sub>7</sub>C<sub>3</sub> carbide and austenite grains, and also the total influence on the final M<sub>7</sub>C<sub>3</sub> carbides’ size. The model includes the effect of latent heat release on the temperature drop. The grain growth velocity is determined by both the diffusion of C solute and the diffusion of Cr solute at the S/L interface. The equilibrium concentration in liquid cells is interpolated from the tablulated solidification path which is prescribed by Gulliver-Scheil approximation coupling with the thermodynamic equilibrium calculation. The morphology of the faceted M<sub>7</sub>C<sub>3</sub> carbide is maintained through setting its neighborhood relations and optimizing its shape factor at grain growth. The results show that the individual grain growth velocity for M<sub>7</sub>C<sub>3</sub> carbide and austenite increases with the increase of the supersaturation and Peclet number of solute C and Cr. The austenite precipitation and grain growth obviously speed up the growth velocity of M<sub>7</sub>C<sub>3</sub> carbide grains. While with the austenite grains gradually touching and enveloping the M<sub>7</sub>C<sub>3</sub> carbide grain, the growth velocities for both kinds of grains decrease. The rejection of solute C and Cr during austenite grain growth complements the absorption of solute C and Cr during M<sub>7</sub>C<sub>3</sub> carbide grain growth, thus promoting their growth. The predicted cooling curve fits with the evolution tendency of the experimental one. The predicted final solidification microstructure and M<sub>7</sub>C<sub>3</sub> carbide amount in volume fraction are in agreement with the experimental ones. Furthermore, both C solute concentration distribution and Cr solute concentration distribution in both residual liquid and austenite are consistent with the predictions by the Gulliver-Scheil, partial equilibrium and lever rule model.