Failure analysis of a diesel engine crankshaft

A failure investigation has been conducted on a diesel-engine crankshaft used in a truck, which is made from 42CrMo forging steel. The crankshaft was nitrided. The fracture occurred in the web between the 2nd journal and 2nd crankpin. The depth of the nitrided layer in various regions of the crankshaft particularly in the fillet region close to the fracture was determined by SEM observation and micro-hardness (HV0.1) measurement, combined with nitrogen content analysis by EDAX. The mechanical properties of the crankshaft including tensile properties, marohardness (HB) and surface hardness (HV1) were evaluated. Fractographic studies indicate that fatigue is the dominant mechanism of failure of the crankshaft. The partial absence of the nitrided layer in the fillet region close to the fracture makes fatigue strength decrease to lead to fatigue initiation and propagation in the weaker region and premature fracture. The partial absence of the nitrided layer may result from over-grinding after nitriding. In order to prevent fatigue initiation in the fillet the final grinding has to be done carefully and the grinding amount controlled to avoid grinding down the nitrided layer.