Surface Spalling Causes of Working Roll with High Cr and Ni in Rear Unit of Hot Rolling Finishing Mill

The spalling on the surface of improved high Ni and Cr working rolls happened continuously in the rear unit of a hot rolling finishing. The causes of spalling were found out by site investigation, and analyes of fracture, microstructure, inclusion and fracture characteristics. The results show that the tail-out accident easily happed on F7 stand to result in microcracks on the surface of working roll, and the microcracks would be crack source to cause spalling if the roll was not changed or was poor ground. The carbides in working roll was fairly big in size and acted as inclusions, and the microcracks propagation rate along carbide boundaries or across carbides was fairly fast to cause spalling. The effective ways to prevent accidental spalling were improving the working roll matrix structure, carbide morphology, use and maintenance conditions, increasing the fatigue and thermal-shock resistance ability, and completely eradicating the remained cracks.