Hot rolling mill roll microstructure interpretation: A computational thermodynamics study

The microstructure of the interface of a high chromium cast iron/ductile cast iron composite hot rolling mill roll has been interpreted with the help of Fe-Cr-Si-C laboratory alloys and thermodynamic calculations of their solidification paths. It has been shown that the mill roll presents two limiting microstructures: the ductile cast iron core and the white cast iron shell. While the core is heavily contaminated with chromium, originating from the partial dissolution of the shell during core pouring, the shell shows a typical high chromium white cast iron micro-structure, since it remains solid during processing. The high chromium contents in the core originate a typical “mottled” cast iron microstructure, combining nodular graphite and ledeburite. The region between the two limiting microstructures is called interface. As the distance from the core toward the shell increases in the interface, the content of chromium increases but that of silicon decreases. The resulting microstructural gradient shows ledeburitic M₃C, hexagonal-section M₃C, duplex M₃C/M₇C₃ carbides, and M₇C₃ carbide, which could be well correlated with the calculated solidification paths of the laboratory samples.