Cyclic Deformation Characteristics And Strain Strengthening Behavior Of 34CrNiMo6 High Strength Steel

Ultra-high pressure vessels（pressure vessels with a working pressure above 100MPa）are widely used in chemical,light industry,food,and other industries.With the development of technology,ultra-high pressure vessels tend to be larger,and in the face of harsh working environments such as cyclic loads,high pressure,and corrosive media,failure problems also occur.In the process of container design,in order to fully exploit the many properties of the material,in order to improve the bearing capacity of the container and prolong its fatigue life,the designer usually strengthens the container.However,after the material is strengthened,the strength will increase and the toughness will be greatly lost,which will affect the service life of the container.At present,there are few reports on the mechanical properties and cyclic fatigue properties of materials strengthened by 34CrNiMo6 steel for ultra-high pressure vessels.In view of this,it is of great significance to carry out research on the cyclic deformation characteristics and strain strengthening behavior of 34CrNiMo6 steel to ensure the structural integrity and safe use of ultra-high pressure vessels.In this thesis,taking domestic 34CrNiMo6 steel for ultra-high pressure vessel as the research object,tensile and impact tests,room temperature low-cycle fatigue tests and tensile pre-strain strengthening tests under symmetric strain control were carried out,and the cyclic deformation characteristics of 34CrNiMo6 steel under different load conditions were analyzed.The effect of strain strengthening on the macroscopic mechanical properties of the material was investigated,and the cyclic stress-strain relationship of 34CrNiMo6 steel was predicted by establishing the Chaboche cyclic plasticity model at room temperature.The main contents and conclusions of the research are as follows:（1）Tensile and impact tests at room temperature and low-cycle fatigue tests under symmetrical strain control were carried out on 34CrNiMo6 steel.The cyclic deformation characteristics of 34CrNiMo6 steel under different cyclic loads were compared,and the effects of strain amplitude and strain rate on the cyclic deformation response were analyzed.The results show that the cyclic stress response of 34CrNiMo6 steel exhibits the characteristics of cyclic softening under different cyclic loads.With the increase of the strain level,the cycle times when the cyclic stability is reached are less;as the loading rate increases from 0.001s^-1 to 0.007 s^-1,the number of fracture cycles of the material increases significantly;when the loading rate increases from 0.007s^-1 to 0.014s^-1,the number of fracture cycles of the material decreases.（2）The Chaboche cyclic plasticity model of 34CrNiMo6 steel at room temperature was established,and the initial parameters of the model material were estimated by the tensile-compression test data.Based on the MATLAB platform,the optimization program was developed using the principle of least squares to improve the accuracy of the initial parameters,and the results were obtained.Embedded in ABAQUS finite element software,the cyclic stress-strain relationship of 34CrNiMo6 steel is predicted.The results show that the Chaboche cyclic plasticity model can accurately predict the cyclic deformation behavior of 34CrNiMo6 steel,but does not include the third stage of cyclic softening,which corresponds to the crack propagation stage.（3）The room temperature tensile test under different tensile pre-strain levels and the room temperature low cycle fatigue test under the symmetrical strain control condition were carried out.The results show that,compared with the original specimen,the yield strength and tensile strength of the material increase with the increase of the prestrain in the range of6%prestrain,but the ductility and area shrinkage decrease continuously and monotonically.The pre-strain significantly changed the stress amplitude and shape of the hysteresis curve,and the material showed obvious tensile-compression asymmetry and cyclic softening/hardening characteristics before and after the pre-strain.The low cycle fatigue life showed a trend of increasing first and then decreasing.