Fracture and fatigue crack growth analysis of rail steels

Low carbon baintic steel shows promising potential, especially in critical components such as frogs and switches. Microstructural analysis of J6 bainitic rail steel was performed and compared with the microstructure of premium pearlitic rail steel. The bainitic microstructure revealed a mixture of tempered martensite and ferrite associated with intralath carbides. Typical pearlitic microstructure with a fine lamellar aggregate of very soft and ductile ferrite and very hard carbide cementite was observed. The mechanical properties, plane stress fracture toughness, K-Ic, and the fatigue crack growth behavior of the two steels were evaluated. Test specimens were machined from railheads of each material using electrical discharge machining (EDM). Rectangular unnotched and notched specimens were used for the mechanical properties and fatigue evaluation respectively. 1/2T compact tension specimens were used for the K-Ic evaluation according to ASTM E399. The J6 bainitic steel has ultimate strength, yield strength, and elongation to failure of about 1500 MPa, 1100MPa, and 13% respectively. These values are higher than those for pearlitic steel. It was found that the average K-Ic for the bainitic rail steel is 52MPa root m, while that of the premium pearlitic steel is 41MPa root m. Fatigue studies showed that the crack speed for the bainitic steel is lower than that for the pearlitic steel over the entire range of the energy release rate. The bainitic steel exhibits a higher rate of crack deceleration in the second stage, as indicated by the lower slope of the fatigue crack propagation kinetics curve in comparison with the pearlitic steel. This attests to the superior fatigue damage tolerance of the bainitic rail steel and provides evidence to support the superior rolling fatigue damage tolerance of the bainitic rail steel reported in the literature.