The Effect of Grain Boundary Ferrite on Fatigue Crack Propagation in Pearlitic Rail Steels

Fatigue crack growth has been examined in pearlitic rail steels to determine the effect of grain boundary ferrite and nonmetallic inclusions on the rates of crack propagation under Mode I loading at ambient temperature. The influence of alternating and maximum stress intensity on fatigue crack propagation in these materials has also been evaluated. The data were obtained using single-edge notch specimens machined from the head of rails and the web of wheels. Cyclic loading was performed on a hydraulic test machine at various R ratios. As the maximum stress intensity, K m a x , approaches the fracture toughness, K 1 c , increasing amounts of cleavage bursts occur and the crack growth rates increase. In steels containing ferrite at the prior austenite grain boundary, the amount of cleavage is reduced at a given K level and the growth rates are reduced by an order of magnitude. The results show that traces of grain boundary ferrite (∼2 to 4 percent by volume) can reduce crack growth rates by a factor of two at intermediate and low growth rates (<5 × 10 - 4 mm/cycle). Increasing R ratio is shown to increase significantly the fatigue crack growth rate near K 1 c , due to the increase in percent cleavage, while little effect of R ratio is observed at intermediate growth rates. Stable fatigue growth is observed above K 1 c and is explained in terms of a pop-in model. Inclusions were observed occasionally at the initiation site of a stable cleavage burst that occurred during fatigue. However, nonmetallic inclusions do not significantly affect the fatigue crack growth rates in rail steels within the range of inclusion content from 0.097 to 0.318 percent by area.