Numerical prediction of ductile damage evolution of 40CrNiMo railway axle steel during hot cross wedge rolling

Ductile damage formed by hot forming seriously affects the railway axle fatigue. It is vital to study the damage evolution mechanism of the 40CrNiMo steel during the high-temperature cross wedge rolling for the manufacturing of the high-speed railway axles. In the current study, the ductile damage model was established and verified. The specimens of the 40CrNiMo steel were subjected to the high-temperature tensile test using a Gleeble-3800 thermal simulation testing machine. Three deformation temperatures (1223 K, 1323 K, and 1423 K) and four strain rates (0.5 s -1 , 1.0 s -1 , 5.0 s -1 , and 10 s -1 ) were set. The deformation temperature, the strain rate, and the stress state were quantified by the Z parameter, the stress triaxiality, and Lode parameter, respectively. These were considered in the Bai & Wierzbicki (B&W) fracture strain equation to establish the fracture strain expression for the 40CrNiMo steel, and then the relevant parameters were solved. The secondary development route was used to embed the model into the Deform-3D software to simulate the distribution characteristics of the ductile damage as well as the evolution of the stress state and the Z parameter when the 40CrNiMo steel was rolled into the high-speed rail axle at the high-temperature. The simulation results were studied, and the direction of the process improvement was proposed. The results show that the damage could be decreased by reducing the forming angle, increasing the broadening angle, maintaining the section shrinkage at 35%, machining the hollow axle, and setting the roll speed and the rolling temperature at 8 r/min and 1273–1323 K, respectively.