Study On The Constitutive Equation And Heat Treatment Performance Of 40Cr Steel

40Cr steel is the representative of low alloy quenchung and tempering steel,and also the alloy quenching and tempering steel used the most in China.In this paper,the thermal deformation behavior of 40 Cr steel in the temperature range of900～1200℃ was studied by hot compression experiments,the change law of microstructure was analyzed,and the peak stress constitutive model was established.The stress-strain curves at different strain rates and temperatures were obtained by tensile test and Hopkinson dynamic compression test,and the Johnson-Cook constitutive model was established.The fracture morphology was observed by scanning electron microscope to analyze the internal mechanism of material failure under different tensile stress states.In addition,JMat Pro software was used to simulate the microstructure and properties of 40 Cr steel during the cooling process of heat treatment.The main study results are as follows:The hot deformation behavior of 40 Cr steel was studied by hot compression experiment.The results show that with the increase of deformation temperature and the decrease of strain rate,the flow stress decreases and the microstructure is more prone to dynamic recrystallization.When the deformation temperature is1000℃,dynamic recrystallization occurs in the microstructure,but with the change of strain rate,complete dynamic recrystallization and incomplete dynamic recrystallization occur.When the deformation temperature is 1200℃,the dynamic recrystallization is basically completed at all strain rates.The recrystallized grain size increases with the decrease of strain rate.When the deformation temperature is higher and the strain rate is lower,the recrystallized grain grows in the microstructure.By fitting the true stress-strain curve,the constitutive equation of40 Cr steel rheological curve based on Zener-Hollomon parameter was established.The deformation behavior of 40 Cr steel at quasi-static state and high strain rate was studied by quasi-static tensile test and Hopkinson dynamic compression test.The results show that the stress increases with the increase of strain rate at a certain temperature,showing strain rate strengthening effect,indicating that 40 Cr steel has a certain degree of sensitivity to strain rate.At the same time,the stress increases less and less with the increase of strain rate,indicating that temperature has a certain influence on the strain rate strengthening effect.With the increase of the tensile rate,the dimple of the fracture becomes smaller and the depth also decreases,which further reduces the plasticity of the material.When the tensile speed is constant,the stress decreases obviously with the increase of deformation temperature,which indicates that 40 Cr steel is sensitive to temperature and has thermal softening effect.The size,number and depth of dimples increase with the increase of temperature,and the plasticity also increases with the increase of temperature.Johnson-cook constitutive equation of 40 Cr steel was established by calculating the stress-strain curve.The thermodynamic equilibrium phase hardenability phase transformation of40 Cr steel was simulated by JMat Pro software.The thermodynamic equilibrium phase composition,quenched curves,CCT curves,TTT curves,and mechanical properties parameters of the material were obtained.Theoretical guidance can be provided to practice through analysis.There are 37 figures,8 tables and 81 references in this paper.