Strain rate-temperature coupling deformation mechanism and constitutive relationship of D2 wheel steel for high-speed EMUs

Abstract

<p indent="0mm">With the increasing speed of trains and the increasingly complex and changeable service conditions, higher demands are forward for the safety and reliability of train components. Wheelsets are the key components in ensuring the safe operation of high-speed trains and may be subjected to significant dynamic impact loads during service due to the existence of wheel-rail defects such as rail corrugation and wheel irregularity. Due to the geographical and climatic characteristics of wide span and large temperature difference in China and the traction/braking of trains, thermal and mechanical stress may exist and superimpose on each other in the wheel-rail contact area, which leads to a reduction in material load-bearing capacity and endangering the safety of train operation. Therefore, clarifying the internal relationship between the dynamic mechanical properties and the microstructural evolution of wheel steel, revealing the strain rate- and temperature-sensitivities and plastic deformation mechanism of the material, and constructing an accurate constitutive model to describe the dynamic mechanical response of the material are still the key problem to be solved urgently in the study of the impact dynamics of the wheel-rail system. In this paper, the compressive mechanical behavior of D2 wheel steel in the temperature range of <sc>293–873 K</sc> and the strain rate range of <sc>0.001–2400 s^–1</sc> was investigated experimentally using the Gleeble-3800 thermal simulation testing machine and the split Hopkinson pressure bar (SHPB) device, the effect of strain rate and temperature on the quasi-static and dynamic mechanical properties of the material was analyzed, the strain rate- and temperature-sensitivities of wheel steel were discussed. In addition, the compression deformation mechanism of D2 wheel steel under different strain rates and temperatures was revealed using scanning electron microscope (SEM) and transmission electron microscope (TEM) technologies. Finally, the modified Johnson-Cook model was obtained to characterize the plastic flow behavior of the material based on the compression test results. The following conclusions are made based on the experimental results: (1) The compression mechanical response of D2 wheel steel displays obvious power-hardening plastic response characteristics, and there is no obvious yield plateau in stress-strain curves. The yield strength and flow stress of wheel steel increase with increasing strain rate and decreasing temperature, which shows significant strain rate strengthening effect and temperature softening effect. The strain rate- and temperature-sensitivities of D2 wheel steel are synthetically influenced by strain rate, temperature and deformation level. (2) The microstructure of D2 wheel steel is composed of lamellar pearlite and pro-eutectoid ferrite. The increase in test temperature will promote the dynamic recrystallization of ferrite inside the material, the decomposition, melting, diffusion of carbon atoms and granular precipitation of cementite under quasi-static loading. As the strain rate increases, the cementite lamellae of pearlite inside the material are sheared, broken and spheroidized, and the ferrite produces large plastic deformation, while the temperature has less effect on the evolution of the microstructure of the material under dynamic loading. (3) By modifying the strain rate and temperature terms of the classic Johnson-Cook model based on the compressive test results of D2 wheel steel, the modified Johnson-Cook model which has higher prediction accuracy for the plastic flow behavior of D2 wheel steel in the temperature range of <sc>293–873 K</sc> and the strain rate range of <sc>10^–3–2400 s^–1</sc> is obtained. The research results can provide important technical support for accurately evaluating the service safety and developing and establishing the service safety evaluation method and standard system of the wheel-rail system.