Failure Investigation and Crack Characterization of Spalled Work Roll in Hot Strip Mill

In hot rolling mills, failure of work rolls is a major concern as it adversely affects mill operations and impacts production. These rolls fracture or spall due to many different reasons. Analysis of failed or spalled roll materials and actual rolling conditions in service are therefore necessary to understand the roll failure mechanism. The hot strip mill referred here consists of two stands in a roughing mill and seven stands in a finishing mill. HSS cast iron rolls are used in the primary strands of finishing mill for the rolling of cast slabs into HR sheets. This present case study describes the spalling of one of the finishing stand non-forged work rolls, and the analysis carried out using destructive as well as non-destructive testing techniques. The spalled roll showed fatigue arrest marks in a smooth continuous path along the circumferential direction. Characteristic features of the fracture surface were like those of surface-initiated spalling which is known as ribbon fatigue spalling. The fracture or spalled region was analysed using ultrasonic tests to identify the origin of the crack. A sample was also collected from the crack origination location where multiple cracks/fire cracks were observed. A detail micro-structural as well as SEM/EDS characterization of the fire cracks was done to understand the mechanism of spalling. The analysis revealed that fire cracks originated from the development of localized thermal stresses on the roll surface. These cracks resulted the roll spalling in operation.