Cracking damage of diesel engine injector nozzles

A lot of injector nozzles used in locomotive diesel engine cracked. The failed nozzles were made of 38CrMoAl steel. The internal and the external surfaces of the nozzles were nitrided as the specification. Visual inspections indicate that the main damage modes are the longitudinal and the circumferential cracking on the external surface of the head region, and the dent at the head tip. The radial cracks were found on the internal surface of the tip region. The failed nozzles and the nozzles hammered by man-made exhibit similar surface damage features. Although the metallurgical examinations on the nitrided layer indicate that the nitride grade and the nitrided layer depth do not correspond to the technical specification and the intergranular micro-cracks appears in the nitrided layer, analysis results indicate that the present cracking failure of the nozzles is not related to the metallurgical defects. The strike on the heads by other objects is responsible for cracking failure of the injector nozzles.