Effect of microstructure on the cleavage fracture stress of high-speed railway wheel steel

The microstructures with different grain sizes and pearlite interlamellar spacings of high-speed railway wheel steel with a carbon mass fraction of 0.54% were achieved by heat treatment at different austenitizing temperatures and cooling rates. Notched 3-point bending(3PB) bars and tensile specimens with different microstructures were tested at temperatures from -120 ℃ to 20 ℃. The stress distribution ahead of the notch tip was calculated by a two-dimensional plane strain finite element method. The fracture surfaces of the 3PB specimens were observed by scanning electron microscopy and the cleavage initiation site was located. The cleavage fracture stress of specimens with different microstructures was measured. It is shown that the cleavage fracture stress depends on the microstructure when the cleavage critical event is propagation-controlled. The characteristic size of un-propagated microcracks decreases by grain refinement,the decrease of pearlite interlameller spacing is be propitious to increase the effective surface energy of the pearlite microstructure,so the cleavage fracture stress increases with decreasing grain size and pearlite interlameller spacing.