Correlation of microstructure and thermal fatigue property of three work rolls

Abstract

This is a study of the thermal fatigue property in three centrifugally cast work rolls, i.e., a nickel-grain cast-iron roll (Ni-grain roll), a high-chromium cast-iron roll (Hi-Cr roll), and a high-speed steel roll (HSS roll). The thermal fatigue mechanism was investigated with a focus on the roll microstructure and the increase in tensile stress which led the specimen to fracture when it reached the tensile strength. The thermal fatigue test results indicated that the thermal fatigue property was best in the HSS roll, followed by the Hi-Cr roll and the Ni-grain roll, respectively, and that the thermal fatigue life of each roll decreased with the increase of the mean temperature or of the temperature range of the thermal fatigue cycle. The results were then interpreted based on the amount of primary carbides and the cyclic softening phenomenon associated with the exposed time to elevated temperatures. The coarse primary carbides on the specimen surface acted as fatigue crack initiation sites, as they cleaved at a low stress level to form cracks. The HSS-roll, having the highest tensile strength and the smallest amount of primary carbides, thus showed better thermal fatigue property than the other rolls. For the improvement of the thermal fatigue property of the rolls, this study suggests a homogeneous distribution of primary carbides by reducing the carbide segregation formed along the solidification cell boundary and by optimizing of the roll-casting process.