| After carburizing and quenching,the surface of the gear is mainly hardened martensite,which improves the hardness and strength of the workpiece surface,but also greatly increases the internal stress of the gear,even cracks and fracture.Therefore,the quenched parts need to be tempered in time to improve the toughness and further stabilize the structure of hardened layer to meet the design requirements.Accurate numerical simulation of tempering process depends not only on the reliability of phase transformation model and model parameters,but also on the integrity of thermophysical and mechanical parameters.Therefore,the main model parameters of phase transformation in tempering process are determined by using phase transformation dilatometer and JMat Pro software.Secondly,based on the simulation of carburizing and quenching,the tempering process is simulated by Deform software,and the distribution results of microstructure field,stress field and hardness field are obtained and verified by experiments.Finally,the effects of different tempering processes on the hardness and yield strength of carburized layer were discussed.The main work is as follows:(1)The model parameters of martensite decomposition and retained austenite decomposition were determined.Martensite samples with three kinds of carbon content were prepared.The tempering degree method and isothermal expansion method were used to determine the parameters in the decomposition model of martensite.It is found that the kinetic parameters determined by isothermal expansion method are in the range of solid phase transformation parameters,while those determined by tempering degree method are smaller.Therefore,in the numerical simulation of tempering,the decomposition amount of martensite is calculated with the parameters determined by isothermal expansion method.At the same time,JMat Pro software was used to calculate the transformation kinetic parameters of retained austenite decomposition with three kinds of carbon content.(2)Two tempering processes of 400℃ for 3h and 200℃ for 3h were simulated.It is found that the decomposition amounts of retained austenite and martensite increase significantly with the increase of tempering temperature.When tempered at400℃,part of retained austenite transforms into bainite,while no austenite transforms into bainite when tempered at 200℃.In addition,the creep rate of18 CrNiMo7-6 alloy steel under different stress states at 200℃ and 400℃ was calculated by JMat Pro software,and the effect of creep strain on residual stress was studied.The results show that the transformation strain is the main factor of stress relaxation when tempering at low temperature.When tempered at medium and high temperature,the effect of transformation strain on stress relaxation gradually weakens,and creep strain is the main factor.(3)By analyzing the hardness values of 18 CrNiMo7-6 alloy steel tempered at200℃ and 450℃ for different times,a tempering hardness model suitable for18 CrNiMo7-6 alloy steel is proposed and verified.In addition,the effects of tempering time and tempering temperature on yield strength were studied. |
