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Under service conditions, final failure of rails is usually caused by impact. To elucidate the phenomenon, the mechanism of impact-initiated cleavage fracture has been studied in fully pearlitic rail steel. The study has demonstrated that the early stages of cracking probably occur by strain localization in the ferritic lamellae. By direct correlation studies of fracture surface and microstructure, it has been further shown that a crack usually does not change direction at pearlite colony boundaries. Rather, the prior austenite grain size is the controlling agent for the more effective obstacle to crack propagation, namely, the cleavage facet size. The data show that the facet size is dependent on the prior austenite grain size, although it is always somewhat less, particularly for the larger grain size materials. The reasons why it is an effective parameter to represent the toughness of eutectoid steels are discussed.
steels, railroad tracks, microstructure, fractures (materials), surfaces, cleavage, pearlite, austenite, grain boundaries, crack initiation, crack propagation, toughness
Association of American Railroads Research Center, Chicago, Ill
Assistant professor, Carnegie-Mellon University, Pittsburgh, Pa