Failure analysis of diesel engine cranks

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I. Analysis 1. Fracture analysis: The macroscopic morphology of the fracture surface of this failed crankshaft is shown in Figure 1. Obvious fatigue beading patterns can be seen on the fracture surface; based on the positions of the centers of these beading patterns, it can be inferred that the source of fatigue lies on the side of the main journal. The expansion zone accounts for a large proportion of the fracture surface area, while the instantaneous fracture zone covers a smaller area. Under a scanning electron microscope, the fracture surface showed that the cracks originated at multiple points on the surface of the transition radius between the main journal and the crank. 2. Chemical composition (wt%): C S Si Mn P V; measured values: 0.50, 0.015, 0.40, 1.21, 0.020, 0.10; technical requirements: 0.45–0.51, 0.008–0.035, 0.17–0.37, 0.9–1.2, ≤0.035, 0.05–0.10. Microstructural structure: Samples taken from vertical cross-sections; the microstructure of the surface layer at the crankshaft’s rounded areas is medium-carbon martensite, while the microstructure of the core area is pearlite + bulk ferrite (Figure 2) ； The depth of the effectively hardened layer at the corner is 3.8 mm. A fatigue crack originating from the surface can be seen at the crankshaft radius (Figure 3). 4. Core hardness: 280 HBW10/1000. II. Conclusions 1. The crankshaft suffered from fatigue fracture. 2. The chemical composition and microstructure of the crankshaft are normal. 3. The causes of crankshaft failure are unrelated to the quality of the blank.