Evaluation of Fatigue Limit Characteristics of Lamellar Pearlitic Steel in Consideration of Microstructure
Shigeru HAMADA, Yuu SAKODA, Daisuke SASAKI, Masaharu UEDA and Hiroshi NOGUCHI
Abstract:In this study, tensile and fatigue tests were performed to evaluate the fatigue limit of lamellar pearlitic steel used for manufacturing railroad rail. The fatigue ratio of lamellar pearlitic steel was lower than that of general steels, the reason for which is unknown. Fatigue cracks initiated in pearlitic steel at a very early stage of the fatigue test. Then, it was speculated that the steel should be treated as that with initial defects. In order to determine the initial defect size in the ultra-low cycle fatigue test, tensile tests were performed. The tensile test results clarified that the crack initiation size depends on the crystal structure. In order to predict the fatigue limit of the pearlitic steel, the prediction method proposed by Murakami was applied to the steel. The measured defect sizes were applied to the prediction method, and fatigue tests were performed. The predicted fatigue limit and the test results were in good agreement. In addition, in order to evaluate the crystal structure at the location of the fatigue crack initiation, electron backscatter diffraction pattern (EBSD) analyses were performed. The analysis results revealed that the crack initiation depends on the pearlite block. Then, we concluded that the fatigue limit of pearlitic steel can be predicted by Murakamifs method and the defect size is pearlite block size. Then, decreasing the pearlite block size would cause an improvement in the fatigue limit of the pearlitic steel. Key Words:Fatigue limit, Lamellar structure, Pearlitic steel, Pearlite block, Pearlite colony, Crack initiation, Initial defect, EBSD