Failure Analysis of Grey Iron Engine Flywheel

Grey iron flywheels were often found to have been fractured during shakeout, surface cleaning, finishing, packing, dynamic balance measuring and so on. Among them, a failed flywheel was subjected to forensic failure analysis by visual inspection, chemical composition analysis, scanning electron microscopy, microstructure analysis, hardness measurements, etc. The results show that the internal crack was initiated near the centre of the flywheel and propagated toward its outer edge. Some inclusions containing Si, S, and Ca resulted in the fracture of the flywheel. The graphite plate length in the vicinity of the crack zone is longer than the range specified, and its ferrite content exceeds the one in the material as a whole. The average hardness near the crack zone is lower than the lowest limit of HB187-250 (the relevant material standard). The causes for the formation of the inclusions have been discussed and some measures such as: controlling the temperature of the molten iron, applying a ceramic filter, etc., have been implemented thus avoiding future flywheel fractures in transit.