Modelling surface thermal damage to hot mill rolls

Surface thermal damage to hot mill work roll materials was investigated using twin-disc high-temperature laboratory test rig under conditions that are relevant to those of the early stands of a hot strip rolling mill. Two typical roll grades were tested: high speed tool steel and high-carbon high-chromium steel. Mill data were scaled down to suit the test sample dimensions and used to calculate the thermal cycle, contact times and contact pressure. A coupled thermomechanical model based on the finite element method was developed to predict the temperature distribution within the core and at the surface of the roll test samples. The predictions of the model showed good agreement with the temperatures measured using embedded thermocouples at several depths in the sub-surface of the roll test sample. After high temperature testing, the test samples were sectioned and prepared by conventional metallography for microscopic analysis. The thermally induced surface damage in the test samples were found to exhibit similar features to those observed in actual mill work rolls during service, especially the high-carbon-high-chromium steel, which showed a greater extent of surface thermal damage than the high speed tool steel test samples. Based on the results, it is concluded that the new test rig was proven capable of producing thermal damage in a consistent and reproducible manner and, thus, it can be regarded as a valuable off-line test resource to assist in further understanding the work roll surface deterioration phenomena.