Microstructural Characteristics of Prematurely Failed Cold-Strip Mill Work-Rolls: Some Observations on Spalling Susceptibility

Abstract

A comprehensive metallurgical investigation was carried out on samples of prematurely failed cold-strip mill work-rolls used in an integrated steel plant to study the influence of microstructural characteristics on failure propensity and roll life. The samples pertained to 3 wt.% Cr-base forged steel work-rolls, which exhibited variations in roll life despite operation under similar mill environments. Optical and SEM revealed that while a uniform dispersion of fine globular carbides was conducive to higher roll life, carbides of angular and/or elongated morphologies acted as stress-raisers, induced microcracking of the tempered martensite matrix, and accentuated eventual spalling. From the standpoint of nonmetallic inclusions, higher life rolls were cleaner. Low/poor life rolls showed higher incidences of MnS and angular TiN inclusions, which often provided sites for the precipitation of undesirable elongated carbides. Although microprobe analysis indicated that carbides in these rolls were essentially M7C3, quantitative metallography revealed that, apart from morphology, roll performance was profoundly influenced by carbide content and count. Interestingly, while higher life rolls were characterized by carbide contents of >4.5 vol.% and counts of >200,000 number/mm2, rolls exhibiting low and poor lives showed significantly lower values of these features. X-ray diffractometry of spalled roll specimens indicated that while higher life rolls contained minimal retained austenite, rolls exhibiting lower lives inevitably contained >10% retained austenite. The deleterious effect of excessive retained austenite on the spalling susceptibility of cold-strip mill work-rolls was attributed to its possible transformation to martensite under imposed rolling stresses.