Microstructure Origin of Strength and Toughness of a Premium Rail Steel

摘要

In this paper, the mechanical properties, fracture toughness, fracture surface morphology and failure mechanisms of different layers in a premium rail steel are presented. Three layers were sliced from representative locations of the rail head. One of these is the top layer, which underwent head hardening due to air quenching in the manufacturing process; the other two layers are below the top crust at different depths away from the top of the rail head. One was sliced from the middle of the head and another was at the bottom of the head which is very close to the web. It has been found that the microstructures and mechanical properties of the top layer are different from those of the inner layers, while the middle layer and the layer near the web demonstrated similar mechanical properties, microstructure and fracture toughness. The top layer displayed 15% higher tensile strength than the other two layers. However, the strain to failure of the top layer, 11%, is only about 60% of that of the inner layers, 17.5% This indicates that the heat treatment decreases the ductility of the top layer of the rail head. Also found is the decrease in fracture toughness due to the head hardening. The top layer has a fracture toughness, K_(1C), 75MPa m~(1/2). This value for the inner layers is about 95MPa m~(1/2). Microscopic examination of the fracture surface morphology of these layers revealed a transition from a brittle-like fracture mechanism for the top layer to a more ductile mechanism for the inner layers.