Failure analysis of a truck diesel engine injector nozzle-valve

The longitudinal cracking took place on the head of an injector nozzle-valve used truck diesel engine during the engine trial test. The fatigue fracture is the dominant failure mechanism of the nozzle-valve. A lot of ovoid and elongated dents were found on the cone of internal cavity of nozzle-valve and the dents were associated to debonding of the ovoid compound inclusions composed of calcium aluminate and MnS, and MnS stringers on the surface from the matrix. Debonding of inclusions was most likely to occur in the process of the carburization-heating of the nozzle-valve. Due to non-coherence between the inclusions and the matrix, debonding of inclusions maybe took place during the process of operating under the action of alternative operational load on the nozzle-valve cone. The size of these dents was enlarged further owing to the cavitation damage during the operating. The crack origin was just initiated from a large ovoid dent appearing at the seal interface between the nozzle-valve cone and the needle-valve, which acted as stress raiser. The edge of the large ovoid dent at the specific location was subjected to two kinds of stresses in the operating, the circumferential tensile stress resulting from the internal pressure of fuel-oil and the circumferential tensile stress resulting from the contact load between the nozzle-valve cone and the needle-valve. Under the action of the alternative superimposing stress, the fatigue crack initiated from the edge of the large dent and propagated outward along the radial to lead to longitudinal cracking throughout the entire wall thickness of nozzle-valve. The upper cone of nozzle-valve had been almost carburized throughout, which would strongly embrittle the cone with thin wall thickness to promote crack propagation.