Evaluation of Hot Tearing in Large Mottled Iron Rolls by Microstructural and FEM Casting Analyses

Large hollow chilled iron mill rolls produced by means of permanent-mold gravity casting may suffer from hot tearing. This paper investigates the causes of this phenomenon by examining a real production lot of 5.5 ton mill rolls. Two representative mill rolls, one with and one without a large hot tear, are selected for a detailed microstructural examination. On the basis of the results of this analysis, FEM casting simulations are set up to predict the temperature and the stress fields on the mill roll surfaces during solidification, as well as to evaluate the effects of graphite precipitation and mold deformation. The microstructural examination shows that the cracked mill roll displays a much lower amount of graphite than the undamaged mill roll. The FEM analysis predicts that mold deformation is responsible for a non-uniform temperature and stress fields on the mill roll surfaces, and that graphite precipitation is able to attenuate Von Mises stresses during iron solidification and cooling, thereby reducing the likelihood of macroscopic hot tears.