Failure analysis of diesel engine cylinder head bolts

This paper presents a failure analysis on diesel engine cylinder head bolts used in truck. The fracture occurred at the smooth rod and threaded end of the bolts when initial tightening. Fractographic and metallurgical studies indicate that the fracture surfaces and microstructure of two failed bolts exhibit similar features. The fracture surfaces show cleavage and quasi-cleavage features. Observations on longitudinal section of the failed bolts show that tens of radial cracks are distributed periodically in the centre zone of the rod along the axial orientation, which should exhibit plate-like morphology of three dimensions. The appearance of the radial plate-like cracks on the rod makes the effective loading area of the bolt decrease so that the bolts occurred the brittle fracture at weakest region with the longest crack when initial tightening. The presence of the carbon segregation and metallurgical inclusions cluster regions in the centre zone of the rod of the bolt should be responsible for the formation of the cracks. Unsuitable smelting and casting technology resulted in the formation of the carbon segregation and inclusion cluster regions.