Failure analysis of a modern high performance diesel engine cylinder head

This paper presents a failure analysis on a modern high performance diesel engine cylinder head made of gray cast iron. Cracks appeared intensively at the intersection of two exhaust passages in the cylinder head. The metallurgical examination was conducted in the crack origin zone and other zones. Meanwhile, the load state of the failure part of the cylinder head was determined through the Finite Element Analysis. The results showed that both the point of the maximum temperature and the point of the maximum thermal-mechanical coupling stress were not in the crack position. The excessive load was not the main cause of the failure. The large cooling rate in the casting process created an abnormal graphite zone that existed below the surface of the exhaust passage (about 1.1 mm depth), which led to the fracture of the cylinder head. In the fractured area, there were a large number of casting defects (dip sand, voids, etc.) and inferior graphite structure (type D, type E) which caused stress concentration. Moreover, high temperature gas entered the cracks, which caused material corrosion, material oxidization, and crack propagation. Finally, premature fracture of the cylinder head took place.