Ductile damage inadvertently exists in the steel during hot tension. The ductile damage during hot forming directly influences the mechanical properties of 25CrMo4 steel for high-speed railway axle. To investigate the grain growth/refinement rule and damage features of 25CrMo4 in hot forming, grain growth test and grain refinement test were conducted using the thermal mechanical simulator Gleeble-1500. In the grain growth test, the specimens were compressed to ensure that the initial austenitic grain size was small enough, then held at the deformation temperatures (1223K, 1273K, 1323K and 1373K) for 0min, 10min, 20min and 30min, respectively, to study the grain growth rule. In the grain refinement test, the specimens were stretched to different strain level at three temperatures (1313K, 1373K and 1433K) with two strain rates of 1.0/s and 10.0/s to study the grain refinement rule. The micro-voids and micro-cavities were found in tensile specimens during grain refinement test. Based on damage evolution mechanisms, damage constitutive equations are formulated to model the evolution of micro-voids and micro-cavities for 25CrMo4 under hot forming conditions. Partial experiment data were used to determine the material constants in damage constitutive equations by using the Genetic Algorithm (GA) method. To validate the model, the experimental data and computed curves of effective stress and grain size were compared. Close agreements were found between the experimental and prediction results. The developed viscoplastic damage equations are able to characterize the deformation behaviour of 25CrMo4 in hot tension process.
Predicting the onset of cracks in bulk metal forming by ductile damage criteria