Research On The Hot Deformation Behavior And Processing Map Of SAE8620RH Gear Steel

As an alloy steel of Cr-Ni-Mo series,SAE8620RH gear steel has been widely used in high-grade gear products.With the development of new energy vehicles,the high speed gears matched with the high speed motors have put forward higher requirements on the accuracy and performance of gears.The traditional gear processing may destroy the symmetrical metal fiber streamline along the tooth profile,which will affect the final mechanical property of the gear.Hot die forging,as a precision forging technology,not only improves production efficiency,but also maintains the continuity of the metal streamlines on the gear teeth.Therefore,selecting the thermal deformation parameters properly to control the mechanical properties and microstructure evolution has become the study focus of gear steel thermoplastic forming.In this dissertation,the hot deformation behavior of SAE8620RH gear steel at different deformation temperatures with a wide range of strain rates was studied by using a Gleeble-3800thermal simulator.The constitutive equation,processing map and dynamic recrystallization critical strain model as well as volume fraction model of SAE8620RH gear steel were derived and constructed,which provides a basis for the selection and optimization of thermal processing parameter in SAE8620RH gear steels in production practice.The specific research results are as follows:?1?Single-pass thermal compression tests on SAE8620RH gear steel were carried out on a Gleeble-3800 thermal simulator to obtain their flow stress curves at temperatures of 850?1000?and strain rates of 0.02?8 s^-1.The effects of thermal deformation parameters on the flow stress and microstructure were analyzed,and a strain-compensated constitutive model was constructed by expanding the material constant to the dependent variable polynomial.The average absolute error is 4%and the correlation coefficient is 0.978,indicating that the model can accurately predict the high temperature deformation behavior and the changing law of flow stress in the SAE8620RH gear steel.?2?Based on the dynamic material model?DMM?,the processing maps of the tested steel were established using the Prasad criterion and the Murty criterion,and the relevant size and distribution of power dissipation efficiency?were further analyzed.Combined with the corresponding microstructure analysis,it proves that the processing map of SAE8620RH steel based on Murty theory is more reliable than that based on Prasad theory.Besides,the power dissipation efficiency factor of the tested steel,?,gradually increases with the increase of deformation degree and the decrease of the Z parameter.There are three instability areas that need to be avoided in the processing map,two of which exist in the high temperature area and the other one exists in the low temperature area with medium strain rate.?3?Using the method of work hardening rate,the critical strain of dynamic recrystallization in SAE8620RH gear steel was obtained,and the relationship between it and the Z parameter was also established.As the Z parameter decreases,both critical strain and critical stress decrease.Based on the modified Avrami equation,the dynamic recrystallization volume fraction model of SAE8620RH gear steel was established,and thus the dynamic recrystallization volume fraction under different thermal deformation conditions were obtained.