Bending fatigue crack propagation behavior of U75V heavy rail steel

In order to clarify the fatigue crack propagation behavior of pearlitic rail, the three-point bending fatigue crack propagation rate of U75V heavy rail steel in rolling state and heat treatment state was measured. The microstructure, lamella, fracture morphology and fatigue crack propagation trajectory of rail were observed by optical microscope, scanning electron microscope and EBSD. The results show that the average distance between fatigue striation of rolled and heat-treated rails is 252.5 nm and 215. 4 nm, the fracture surface of rolled rail presents cleavage steps and river patterns. and the river patterns tend to merge. However, the fatigue fracture surface of the heat-treated rail presents a large number of cleavage steps, more microcracks and tear edges, and the river pattern is dominated by tributaries. The fatigue crack growth rate of heat-treated rail is much lower than that of rolled sample, and it is also slow to reach the stage of crack instability. The fatigue crack propagation mode of rolled and heat-treated rail is the mixed propagation mode of transgranular fracture and intergranular fracture dominated by transgranular fracture, the pearlite lamellae spacing of rolled and heat-treated rails is 272. 2 nm and 148. 4 nm respectively. The pearlite lamellae of heat-treated rails is fine and has various directions, while there are significant pearlite clusters in the heat-treated rail. There are many branch cracks and crack bridges in the crack growth path, which hinders the crack propagation in the heat-treated U75V heavy rail steel, which is an important reason why the fatigue crack propagation rate of the heat-treated rail is lower than that of the rolled rail.