| Heavy-duty gears with large modulus are the essential key part in heavy equipment such as locomotive,heavy trucks,vessels and wind turbine generators,of which quality,performance and life are directly relative to the working properties and service life of the whole machine.Due to the difficulty of the maintenance of the gears,the reliability and life of the gears are much higher than that of the general machinery.Moreover,with the rapid development of the automotive industry and the increase in demand for heavy machine equipment,the demand for efficient gear forming technology is urgent and booming.Therefore,in this thesis hot-gear rolling with high frequency induction heating process was applied to achieve high-efficiency precision forming of large-modulus gears.To get a satisfying temperature distribution,longitudinal magnetic flux(LMF)coil and transverse magnetic flux(TMF)coil were designed.The influences of current density and current frequency on the heating efficiency and the temperature distribution of the workpiece corresponding to two coil structures were investigated.From the numerical results,a conclusion can be made that compared with TMF coil,the LMF coil can reduce the inner hole temperature and improve the heating efficiency.With proper current density and current frequency,the workpiece heated by LMF coil can obtain a better temperature distribution than that heated by TMF coil.In order to understand and model the deformation behavior and microstructure evolution,SAE 8620H,is selected as the object and the flow behavior and microstructure evolution are observed by an isothermal hot compression tests at 1000-1100 ? with a strain rate of 0.1-10 s-1.Depending on the experimental results,a set of internal-state-variable based unified constitutive equations is put forward to modeling the flow behavior and microstructure evolution of SAE 8620H.Moreover,the evolution of the dislocation density and the fraction of dynamic recrystallization based on the theory of thermal activation is modeled and reincorporated into the constitutive law.The material parameters in the constitutive model are calculated based on the experimental flow stress.The predicted flow stress under different deformation conditions agreed well with the experimental results.The principle of the stress,strain and metal flow laws during the teeth forming process was studied using Finite simulation method.The reason resulting the defects of metal folding on the teeth top,inner hole enlarging and the asymmetry of the left and right teeth profile were analyzed.And the influence of process parameters on the defects was studied.Moreover,the solution to decrease the defects was put forward.The microstructure evolution during the gear rolling process was studied using the secondary developmental subroutine of DEFORM software.The effects of rolling temperature and the feeding rate on the microstructure evolution during the forming process was investigated.Based on the external gear rolling program,the experimental machine was designed and developed.A new gear rolling machine driven by two servomotors was designed and carried out debugging and testing.Moreover,the lower part of the idler gear was selected for precision rolling forming,the measured gear tooth profile deviation,the tooth orientation and pitch deviation meet the level 12.The microstructure of the workpiece after rolling was analyzed.It was found that the degree of dynamic recrystallization of the metal near the tooth profile was very high and the grains were smaller,especially near the tooth root.The distribution of recrystallization at the tooth roots helps to improve the contact fatigue strength and bending fatigue strength of gear teeth. |
PDF Full Text Request