Failure analysis and metallurgical investigation of diesel engine exhaust valves

摘要

This paper presents a failure analysis of three diesel engine exhaust valves used in a truck which failed in service. The failure had taken place at the plates of the exhaust valves. The plate material is made from 5Cr21Mn9Ni4N steel. The fracture surfaces were investigated by SEM and visual observation. The results show that the cracks initiated from the interface of the tapered plane and the transitional region of the plate and propagated toward the bottom of the valve for two failed exhaust valves. Fractographic studies indicate fatigue as the dominant mechanism of the failure of the two exhaust valves. Burning-through occurred on the plate of another failed exhaust valve. The microstructure and phases of the plates of the failed valves were investigated by SEM and XRD, and compared with those of a new valve. The austenitic matrix decomposed to form the lamellar structure consisting of gamma + Cr_23C_6 at the boundary and within the grains in service, which is the general metallurgical character of plate material for the three failed valves. The lamel_ lar structure separates the matrix to decrease strength, toughness, the loading ability of the matrix, and gas corrosion resistance. Formation of the lamellar structure is mainly responsible for the failure of the exhaust valves. The appear_ ance of the lamellar structure in the plate material of the failed valves in service is related to the original metallurgical structure, and depends on the solution-aging treatment.