Failure analysis of the rotating shaft in the rail grinding car

Abstract

Rail grinders belonging to special vehicles have fewer accidents than general railroad vehicles and parts, and the causes are diverse. Therefore, there is a lack of basic data for inspection methods and preventative measures necessary for accident prevention, and the collection and analysis of accident cases are also necessary. In this paper, a failure analysis of the recently derailed rail grinding car's rotating shaft was conducted. Various analyses, including fractography, chemical analysis, metallography and hardness measurement, were performed on the failed shaft. In the fracture surface of the shaft, ratchet marks and fatigue striations indicative of fatigue failure were observed. Consequently, it was confirmed that the failure of the shaft was attributed to fatigue, with the primary cause being identified as the cracks found on the fillet area surface. Chemical analysis of the observed inclusions in the sample revealed manganese (Mn) at 6.16 wt% and sulfur (S) at 1.35 wt%. These are attributed to non-metallic MnS inclusions, which are presumed to have adversely affected the fatigue strength of the material. Additionally, hardness deficiency due to heat treatment defects on the shaft surface was identified. The surface hardness measured approximately HV400, suggesting that it is desirable to achieve a surface hardness of HV600 or HRC55 or higher during high-frequency heat treatment. Lastly, the finish state of the fillet area surface was not smooth. Given that the fillet area is a region where stress concentration occurs, the presence of such machining traces raises the possibility of progressing into fatigue cracks. Finite element analysis revealed that the von-Mises stress was less than the yield strength of the material, and a safety factor based on the yield strength was calculated to be 3.5. The results confirmed that fatigue cracks in the fillet area were caused by the hardness deficiency due to the poor heat treatment, the rough surface finish, and the presence of non-metallic inclusions.