Microstructural characteristics of prematurely failed cold-strip mill work-rolls: Some observations on spalling susceptibility |
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Authors: | Amitava Ray M S Prasad P K Barhai S K Mukherjee |
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Affiliation: | (1) Metallurgical Services Laboratory Division, R&D Centre for Iron & Steel, Steel Authority of India Limited, 834 002 Ranchi, India;(2) Birla Institute of Technology, 835 215 Mesra, Ranchi, India |
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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. |
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Keywords: | cold-strip mill roll failure spalling work-rolls |
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