Ribbon Fatigue Spalling of a Forged Work Roll Used at a Cold Rolling Mill

Catastrophic failure of a forged work roll used in a cold rolling mill was investigated. The spalled roll showed fatigue arrest marks in a smooth continuous path along the circumferential direction. Characteristic features of the fracture surface were similar to that of surface initiated spalling which is known as ribbon fatigue spalling. The investigation consisted of visual observation, profiling of crack using ultrasonic testing, microstructural characterization by optical and scanning electron microscopy, XRD analysis, and hardness measurements. Ultrasonic evaluation confirmed the initiation of root crack on the roll surface followed by an extensive sub-surface propagation (~815 mm length) of fatigue crack. Microstructural analysis revealed the presence of dendritic columnar structure and coarse irregular carbides indicating improper manufacturing process. Retained austenite content (~12%) as measured using XRD was higher than the usual desirable limit for such application. These microstructural abnormalities deteriorated the fatigue resistance of the roll. Since the roll failed within few hours of the running campaign and there was no operational abnormality reported in the campaign, it can be inferred that the initiation of the fatigue crack was conceived prior to the running campaign. It was an inspection deficiency that the crack was not detected prior to the start of the campaign. Initiation of fatigue crack which could not be detected during inspection led to the premature failure of roll in the early stage of campaign.