An extensive metallurgical investigation was carried out on samples of a failed roller bearing from the support and tilting
system of a basic oxygen furnace (BOF) converter used in the steel melting shop of an integrated steel plant. The converter
bearing was fabricated from low-carbon, carburizing grade steel and had failed in service within a year of fitting to a repaired
shaft. Microscopic observations of both the broken roller and inner-race samples revealed subsurface cracking and preponderance
of brittle oxide and other macroinclusions. Electron probe microanalysis (EPMA) studies confirmed that the brittle oxides
that formed stringers were alumina, and the other macroinclusions were complex silicates. Both the alumina and silicate inclusions
were deleterious to contact-fatigue properties. Microstructurally, the carburized regions of the broken roller and of inner-race
samples contained high-carbon tempered martensite. Microhardness measurements revealed that although the core hardness of
the roller and the inner-race samples were similar, the surface hardness of the roller was approximately 8.5 HRC units harder
than that of the inner-race. Scanning electron microscope (SEM) observations of the roller fracture surface revealed striations
indicative of fatigue, and energy-dispersive spectrometric (EDS) analyses corroborated a high incidence of silicate inclusions
at crack sites. The study suggests that the failure of the bearing occurred because the hardness difference between the roller
bearing and the inner-race surfaces resulted in wear of the inner-race. The wear led to shaft misalignment and play during
service. The misalignment, coupled with the presence of inclusions, caused fatigue failure of the roller bearing.
Keywords bearing failure - fatigue - nonmetallic inclusions - rolling-element bearing - tempered martensite