Research On Thermal Fatigue Performance Of Bainitic Wheel Steel

The practice indicates that the thermal fatigue damage caused by the train braking process seriously threatens the safety of train operation,and the new Mn-Si-Cr series bainite steel is expected to become a good substitute for new wheel materials because of its better comprehensive properties.However,in the actual line service test,the new type of bainite wheel also has thermal fatigue damage and there are few studies on the thermal fatigue performance of bainite steel.For the Mn-Si-Cr bainitic wheel steel,this paper studies its thermal fatigue damage law and mechanism,explores the internal temperature field of wheel,stress field distribution law as well as the law of the internal temperature variation of wheel during the braking process of the train brake shoe by using the finite element simulation,and analyzes the maximum temperature along with the stress level at the tread as the wheel is braking.At the same time,based on the simulation results,this paper studies the process of crack initiation as well as the mechanism of crack growth during the thermal fatigue cycles by performing thermal fatigue experiments.Observing the changes of microstructure and its properties during thermal fatigue,this paper explores that tempering treatment influences on the thermal fatigue properties of bainitic wheel steels at different temperatures of 300 ? and 600 ?.The results of finite element simulation calculation with regard to the temperature field and stress field distribution during the braking process of the wheels show that the maximum temperature of the tread surface can reach about 593 ? during the braking process of the long ramp and the maximum thermal stress at the wheel tread surface can reach 178 MPa.Based on evaluating the damage of force on the tread,the force on the tread is in the B-grade which is the rare braking zone under the braking condition of a long ramp and damage may occur at the tread.Based on the maximum temperature obtained by simulation,the upper limit temperature of thermal fatigue was set to 600 ?.The results show that thermal fatigue cracks gradually become larger with the increase of the thermal fatigue cycles.This paper defines the thermal fatigue crack(C_TF)to quantitatively describe the fatigue damage degree of the material after the thermal fatigue.The process of thermal fatigue cycles has the characteristics of high temperature tempering.During the thermal fatigue cycles,there are variations in the combination of bainite laths,decomposition of retained austenite,precipitation of carbides,and decrease in hardness.The thermal fatigue test of the samples after tempering at different temperatures of300 ? and 600 ? shows that although the comprehensive mechanical properties of 300? tempered bainite steel are better than that of 600 ? tempering,the damage degree from thermal fatigue of 300 ? tempered bainite steel is significantly greater than that tempered at 600 ?.At 300 ? tempering condition,there was filmy retained austenite in the microstructure of the material,while the retained austenite was basically completely decomposed accompanying by carbide precipitation at 600 ?.Due to the presence of retained austenite in 300 ? tempered bainite steel,the transformation of retained austenite and the dynamic precipitation of carbides during the subsequent thermal fatigue cycles may accelerate the generation and propagation of cracks.In the tempering and holding stage of the tempering process at 600 ?,the retained austenite has been basically completely decomposed and some carbides has been precipitated at the same time.