Fatigue Damage Tolerance of Bainitic and Pearlitic Rail Steels

The microstructure-properties relationships, fatigue crack growth, and fracture surface morphology of J6 bainitic and premium pearlitic rail steels are studied. Specimens are cut from the middle of each railhead along the longitudinal direction of the rail using electrical discharge machining. Fatigue crack propagation tests are conducted under load control conditions using a servo hydraulic material testing system. A simple form of the fatigue power law is used to rank the fatigue crack growth kinetics of the two materials. The results show that bainitic steel has superior fatigue damage tolerance as represented by the fatigue lifetime, fatigue crack propagation kinetics, and fatigue fracture surface morphological features. Fracture surface analysis of fatigue failed specimens revealed the various mechanisms by which bainitic steel acquired its superior resistance to fatigue crack growth. Bainitic rail steel displayed more ductile fracture features such as tearing, and extensive ridge formation during the stable crack propagation process than pearlitic steel. These features are responsible for the crack deceleration and indicate a considerably high energy consuming process associated with the crack propagation of bainitic steel. Pulled-up pearlitic lamella, limited microcracks, and microvoid coalescence are found on the fracture surface of pearlitic rail steel during the stable crack process. The unstable crack propagation region of bainitic steel exhibits both large and small dimples indicative of high resistance to material separation. On the other hand, cleavage and intergranular separation are associated with the unstable crack region in pearlitic steel.