Cracking Failure of Locomotive Diesel Engine Injector Nozzles

Abstract

Two locomotive diesel engine injector nozzles fractured during service. The fractured injector nozzles are fabricated with 18CrNi8 steel, and the external and the internal surfaces of injector nozzles are required to be carburized. Visual inspections indicate that the fracture occurred just at the transition fillet root between the large excircle and the secondary excircle of injector nozzle. Fractographic investigation indicates that the crack origin is situated at the oil-filled groove with a thin wall thickness and the brittle intergranular fracture is the main failure mode of the two injector nozzles. Metallurgical examinations show that case depths through the entire wall thickness are present at the fracture regions. The brittle microstructure typical of carburized layer was produced in the fracture regions, which was confirmed by the occurrence of intergranular micro-cracks. The presence of case depth through the entire wall thickness in the fracture region made the toughness at the region decrease intensely. The crack initiated at the weakened location and propagated from the internal surface toward the external surface under the impact of the oiling pressure with high-frequency pulsation to lead to transverse brittle fracture of the two injector nozzles. Additionally, the presence of many brittle Al₂O₃ complex inclusions in the vicinity of the fracture region induced tensile stress in the matrix around inclusions and the stress concentration around the inclusions to promote initiation of the crack.