Role of Alloying and Microstructure on the Strength and Toughness of Experimental Rail Steels

Abstract

Previous studies on eutectoid steel have demonstrated that strength and toughness are essentially independently varying parameters, with the former primarily controlled by the pearlite interlamellar spacing and the latter primarily controlled by the prior austenite grain size. The work has now been extended to study the effect of modest compositional variations on strength and toughness of 17 experimental rail alloys. Carbon, manganese, and silicon levels were varied over ranges to yield eutectoid or hypoeutectoid microstructures. Vanadium was added to some of these compositions, primarily as a grain refiner. These steels were heat treated to produce varying austenite grain sizes and a reasonably constant, fine pearlite spacing. Instrumented impact tests on precracked Charpy bars were performed to determine both the dynamic fracture toughness (KId), and the Charpy transition temperature. Tensile tests were used to monitor strength and ductility. Mechanical test data were correlated with observed microstructural variations, particularly the austenite grain size and pearlite spacing as well as fractographic studies of the variation of fracture facet size with austenite grain size. These studies led to a general description of the fracture process in such steels.