Numerical simulation of solute undercooling influenced columnar to equiaxed transition of Fe-C alloy with cellular automaton

Abstract

The columnar to equiaxed transition (CET) is crucial to the solidification quality and the mechanical property of steels. With the cellular automaton approach, the present work developed a solidification model that consists of dendritic growth and heterogeneous nucleation to investigate the CET behaviors of Fe-C alloys at the microscopic scale, as well as the influencing mechanisms of thermal conditions (thermal gradient and cooling rate) and the initial carbon contents on the CET location, CET type and quantitative characteristics of the inner structure. The results show that the predicted CET map that illustrates whether the CET occurs or not at given thermal gradients and cooling rates for the Fe-0.82C alloy in weight percentage agrees with the analytical result calculated with the Hunt model. Two CET types are observed in the simulation. The columnar-tip type prefers conditions with low thermal gradients and high initial carbon contents, while it is the opposite for the interbranch type. The difference in conditions is an interaction effect of the distributions of the melt undercooling and the solute segregation. The CET is promoted by decreasing the thermal gradient and by increasing the cooling rate and the initial carbon content. Meanwhile, the inner dendritic structure gradually changes from the columnar-equiaxed mixed type to the equiaxed type. The CET location and the average and the mean square deviation of the inner dendritic diameter change sharply within the high thermal gradient, weak cooling rate and low initial carbon content range and become gentler with the decrease in the thermal gradient and the increase in the cooling rate and the initial carbon content. Therefore, the inner dendrites become finer and more uniform. Thermal conditions directly influence the undercooling distribution, whereas the initial carbon content works through the dendritic growth velocity and the solidification time.