Microstructure origin of strength and toughness of a premium rail steel

The mechanical properties, fracture toughness, fracture surface morphology, and failure mechanisms of different layers in a premium railhead were stud ied. Correlation between the mechanical properties and the failure mechanis ms for each of the layers was made. It has been found that the microstructu re and mechanical properties of the top layer are different from those of t he inner layers, while the middle layer and the layer near the web demonstr ated similar mechanical properties, microstructure, and fracture toughness. The top layer displayed 15% higher tensile strength than the other two lay ers, However, the strain to failure of the top layer, 11%, is only about 60 % of that of the inner layers, 17.5%. The top layer has a fracture toughnes s, K-lc, of 75 MPa m(1/2). This value for the inner layers is about 95 MPa m(1/2). Thus, the heat treatment decreases the ductility and fracture tough ness of the top layer of the railhead, Transition from the brittlelike frac ture mechanism of the top layer into a more ductile mechanism of the inner layers was also found.