Influence of Microstructure on Crack Susceptibility of Large Chilled Iron Mill Rolls

Large chilled iron mill rolls fabricated by permanent-mold gravity suffer from surface macroscopic cracking during iron solidification. This defect is usually intermittent over time, and in some extent, unrelated with the chemical composition and the process parameters used. To study this phenomenon, a metallographic analysis has been carried out on two real 5.5-ton chilled iron mill rolls made of the same chemical composition, manufactured with quite similar process parameters, but giving two very different results in practice: one of the two mill rolls exhibited a macroscopic crack. Because of the different metallurgical structures observed, especially regarding graphite content, a three-dimensional (3D) numerical analysis has been adopted to predict the thermal-mechanical behavior of the casting during the solidification and cooling stages. Special attention has been dedicated to the influence of the graphite expansion on mechanical stresses occurring in the casting skin. Numerical results point out that graphite expansion notably affects the evolution of stress-strain fields within the casting to an extent that it could lead to the formation of macroscopic cracks in the early stages of the casting solidification. Furthermore, the numerical analysis highlights that the thermal deformation of the permanent mold contributes to increase the risk of cracks to occur