Failure Analysis of S55C Medium Carbon Steel for the Pickup Crankshaft

Abstract

There have been reports from service centers of important problems of a diesel engine crankshaft in mini trucks. All crankshafts crack from the same region, the fracture occurred in the first crankpin of the crankshaft. The crankshaft is made from JIS-S55C hardened medium carbon steel. A series of experiments including chemical analysis, macro and microstructural analysis, mechanical properties test, and numerical stress analysis. The results of fracture surface of this crankshaft were cause by fatigue with the detection of beach marks, ratchet marks, river marks, cleavage fracture and fatigue striations which these results indicate that the fracture mode of brittle fracture. The microstructure is composed of perlite-ferrite on the outer surface showed some coating nitriding layers, most hard coating is not found, the hardness test revealed an uneven hardness distribution, the maximum hardness on the center of the shaft measuring 309 HV, which is more than compared to the outer surface is 259 HV for these applications. It was found that the hardness value was abnormal. This is consistent with the experimental results of the surface microstructure which showed some coating layers, while the hard coating is not found. Besides, the numerical results also revealed that the first crankpin fillet was the most vulnerable to breakage, which is consistent with the mechanical experimental results. Finally, the summary analysis results are consistent with the hypothesis of this research and as an information for a prevent such failures crankshaft in the future.