Premature failure of an automobile brake disc: Effect of nonmetallic inclusions

Abstract

A metallurgical failure analysis was conducted on a grey cast-iron (GCI) automotive brake disc to unravel the root cause of its premature failure. The brake rotor had been in service for about one year and three months and covered a total mileage of 17 173 km before the failure was reported. However, this investigation aims at identifying the causes of the failure and proffer solutions to forestall similar future occurrences. Chemical composition analysis on the disc suggested that its elemental composition is within the required standard for such a component. The disc swan-neck thickness, hardness and microstructure analysis conformed adequately with the material specifications. SEM-EDS study of the fracture surface showed the presence of admixture oxides of Al, Mg, Si, Na, Ca, Ti and Mn, suggesting nonmetallic (slag and sand) inclusions. A fractographic study of the broken surface indicated that the failure was caused by these oxide inclusions embedded in the swan neck region of the rotor during the casting process. Microcracks were observed to have been initiated within the inclusion particles and propagated through the graphite-pearlite interface, eventually leading to the disc failure. Recommendations are also provided to guide the operation of metal-casters to prevent the recurrence of nonmetallic inclusions, which can significantly diminish the service life of cast products.