The Deformation Mechanism And Dynamic Constitutive Relationship Of D2 Wheel Steel In Wide Temperature Range

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,it is important to clarify the internal relationship between the dynamic mechanical properties and the microstructural evolution of wheel steel,reveal the rate temperature sensitivity and plastic deformation mechanism of the material and construct an accurate ontological model to describe the dynamic mechanical response of the material.In this thesis,the tensile and compressive mechanical behavior,deformation mechanisms and constitutive relationship of D2 wheel steel at different temperatures and strain rates are investigated experimentally.The main research contents and results are as follows:（1）The quasi-static tensile and compression mechanical tests of D2 wheel steel were carried out,the relationship of the stress-strain responses was analyzed,the compressive deformation mechanism and tensile fracture mechanism of the material were revealed.The results show that the quasi-static tensile and compression mechanical responses of D2 wheel steel displayed obvious elastic-plastic characteristics,the rim steel exhibits higher plastic flow than that of web steel;the microstructure of the wheel steel is composed of lamellar pearlite and proeutectoid ferrite,and the space of pearlite lamella in rim steel is smaller than that of web steel;the probability of the dynamic recrystallization of ferrite and the decomposition,dissolution of cementite and granular precipitation of cementite in the material increase with the increasing of temperature,leading to the more significant thermal softening effect of material;there are a lot of dimples and microvoids in the fracture of wheel steel under quasi-static tensile loading,the material displays obvious ductile fracture characteristics.（2）The dynamic compressive mechanical properties of D2 wheel steel were studied under the temperature range of 293～873 K and strain rate range of 800～2400 s^-1,the stress-strain response curves of the material under the coupling effect of strain rate and temperature were obtained,the microstructure evolution law of the material under compressive loading was revealed,and the dynamic constitutive relationships considering temperature and strain rate of material was established.The results show that the compressive stress-strain responses of wheel steel are elastic-plastic,the material shows obvious strain rate strengthening and temperature softening effect,the strain rate sensitivity and temperature dependence are affected by the coupling of strain hardening,strain rate strengthening and thermal softening of the material;with the increase of strain rate,the ferrite display severe plastic deformation and the cementite fragmentation and spheroidization intensified,while there are little effects of temperature on the microstructure evolution;based on the Johnson-Cook constitutive model,the constitutive model coupling strain rate and temperature of D2 wheel steel was established,and the dynamic compression mechanical behavior of wheel steel can be better predicted by the model.（3）The dynamic tensile mechanical properties of D2 wheel steel under the strain rate range from 800 s^-1 to 2400 s^-1 were studied,the asymmetry on tensile and compressive mechanical responses was found,the dynamic tensile fracture mechanism was revealed and the dynamic constitutive relationship considering the influence of tensile damage of the wheel steel was established.The results show that the wheel steel exhibits a strain rate strengthening effect under dynamic tensile loading,and the tensile strength and plastic flow of rim steel is higher than that of web steel;the tensile strength and plastic flow of the wheel steel are higher than the compression test results under the same strain rates,the stress-strain responses of the material show obvious tension and compression asymmetry;a large number of dimples and local cleavage surfaces appear in the fracture of wheel steel under dynamic tensile loading,and the fracture mechanism is a quasi-ductile fracture;a strain rate-dependent constitutive model for D2 wheel steel was established,the dynamic tensile behavior of the material can be described accurately by the model.In this thesis,the dynamic compression and tensile mechanical properties of D2 wheel steel under a wide range of temperatures were studied.The microstructure evolution and tensile fracture mechanism of the material were revealed,and the dynamic constitutive relationship considering the influence of temperature and tensile damage of the wheel steel was established.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.