Rayleigh Number Criterion for Formation of A-Segregates in Steel Castings and Ingots

A Rayleigh number-based criterion is developed for predicting the formation of A-segregates in steel castings and ingots. The criterion is calibrated using available experimental data for ingots involving 27 different steel compositions. The critical Rayleigh number above which A-segregates can be expected to form is found to be 17 +/- A 8. The primary source of uncertainty in this critical value is the dendrite arm spacing. The Rayleigh number criterion of the current study is implemented in a casting simulation code and used to predict A-segregates in three case studies involving steel sand castings. By comparing the predictions with observations made in the actual castings, the Rayleigh number criterion is shown to correctly predict the regions where no A-segregates form. However, the regions where A-segregates do form are somewhat over-predicted. Based on the results of the three case studies, the primary reason for this over-prediction is persumed to be the presence of a central zone of equiaxed grains in the casting sections. A-segregates do not form when the grain structure is equiaxed.