Metallurgical and Mechanical Failure Analysis of an Aftermarket Flywheel

Abstract

A failure analysis investigation was performed on the remnants of an aftermarket gray cast iron flywheel that catastrophically fractured during operation in a vehicle after 24 miles of operation. Light microscopy, 3D X-ray micro-computed tomography (micro-CT), scanning electron microscopy (SEM), metallography, and hardness/tensile testing techniques were utilized to characterize manufacturing quality, mode(s) of failure, microstructural variation, fracture surfaces, and mechanical properties of the failed component. Light microscopy examination of the remnant surfaces showed that the flywheel shattered with signs of radial heat checking fissure cracks. A metallurgical examination of the flywheel showed that it was manufactured from cast gray iron, with evidence of microstructural changes near heat-affected zones from graphite flakes in a ferrite/pearlite matrix to needle and lath formations similar to bainite or martensitic phases. The CT scan slices and fracture surface examination in the SEM showed signs of porosity and dendritic formations along a fracture surface believed to be the crack initiation location. The analysis suggests manufacturing flaws found within the flywheel were a likely contributing factor leading to premature failure during service.