A two-dimensional model for the description of the columnar-to-equiaxed transition in competing gray and white iron eutectics and its application to calender rolls

A two-dimensional (2-D) microstructural model has been developed for the description of the solidification of gray and white iron eutectics at the scale of a casting process. The model deals with the competition that can occur between the white and gray iron microstructures and between the columnar and equiaxed morphologies. The evolution of the volume fraction of gray and white iron and the respective proportions of columnar and equiaxed morphologies are calculated at each node of a finite-element (FE) mesh of the casting. The model is coupled with the commercial FE software ABAQUS, which provides the solution of the heat-flow problem. Columnar solidification is described with a new front tracking algorithm that allows the undercooling at the interface and the non-steady-state stage of growth to be calculated accurately, Equiaxed solidification is described with a deterministic model based on nucleation and growth laws. The model is first applied to a casting of a simple geometry in order to investigate possible numerical problems associated with the front tracking algorithm. The model is then applied to the solidification of a reduced-scale cast-iron calender roll. The results of the simulation are compared with measurements performed in an instrumented casting of 0.4 m in diameter and 1 m in length. A comparison between calculated and measured volume fractions of white and gray iron is presented as a function of the radial position in the casting. It shows that the model is able to predict properly the transition from white to gray iron, which occurs at approximately 20 me from the external surface of the roll. Comparisons between calculated and measured temperature evolutions and equiaxed grain densities of equiaxed grains are also presented and show satisfactory agreement.