A new application of unified constitutive equations for cross wedge rolling of a high-speed railway axle steel

• Microstructure evolution of 25CrMo4 in hot cross wedge rolling is studied.
• Unified material model Determined for predicting microstructure evolution and viscoplastic flow.
• Process modelling with internal variables and validated with experimental results.
• The error of FE-predicted grain size in hot cross wedge rolling is within 10%.

The mechanical properties of 25CrMo4 steel for high-speed railway axles depend on its microstructural evolution during hot cross wedge rolling (CWR). In this paper a set of mechanism-based unified constitutive equations has been determined for the prediction of elastic-viscoplastic material behaviour and microstructural evolution of the 25CrMo4 steel during hot CWR processes. Hot compression tests were conducted using a Gleeble thermo-mechanical simulator at temperatures in a range of 1223–1433 K and at strain rates in a range of 1.0–10.0/s. A set of equations was determined from experimental data of static grain growth, grain refinement and viscoplastic flow using a genetic algorithm (GA) method. Good agreements between the experimental and predicted data were obtained. The determined constitutive equations were implemented into the commercial FE code, DEFORM-3D, via a user defined subroutine and CWR processes were modelled. CWR tests were also carried out and microstructure examinations were conducted. FE simulation results, such as grain size, were compared with those of CWRed parts. Good agreements have been obtained, which shows that the determined constitutive model enables microstructure evolution in CWR processes to be well predicted.