| EA4T alloy steel is a commonly used railway axle material and is widely used in high-power locomotives,high-speed trains and subways at home and abroad.The axle mainly bears cyclic load in the process of use.The fatigue failure caused by cyclic load is the most common failure mode of axles.The forging process is one of the key steps in the production process of the axle.The forging process parameters affect the final performance of axles.Therefore,this paper takes EA4T materials as the research object,and uses experiment and theoretical analysis to study the effects of different forging process parameters on the microstructure and fatigue property of EA4T materials.The main research contents of the thesis include:(1)In this paper,EA4T low carbon alloy steel billet was hot forged and the microstructure and performance test of forged EA4T materials were carried out.EA4T materials before and after forging were analyzed after selecting three forging process parameters of forging ratio,initial forging temperature and post-forging cooling method.The metallographic structure and fatigue fracture morphology of EA4T material specimens were observed by metallographic microscope and scanning electron microscope respectively.(2)Static tensile testing,fatigue testing and data processing.Static tensile test and fatigue test of EA4T alloy steel under different forging process parameters were conducted,and the test data was sorted out.The fatigue test data was fitted by the least squares method,and the material constants were obtained according to the fitted curve.The S-N curve of EA4T materials under different forging conditions was obtained.(3)The influence of different forging process parameters on the fatigue life of EA4T materials was discussed and analyzed.The change of fatigue life of EA4T material specimens was explained by microstructure and static tensile test.The effects of different forging ratios,initial forging temperature and post-forging cooling method on the fatigue properties of EA4T alloy steel specimens were analyzed.(4)The fatigue crack initiation mechanism of EA4T materials was analyzed,which based on dislocation theory and fatigue fracture theory.According to the fatigue fracture morphology of EA4T materials under different forging parameters,the fatigue crack initiation under different forging parameters was analyzed.Based on the formula of fatigue life calculation,the threshold value of stress intensity factor which was the key factor affecting fatigue life was investigated,and the relationship between fatigue life,threshold value,yield strength and grain size were analyzed.Based on the above research,it can be observed that the fatigue life of EA4T materials increases first and then decreases with the increase of forging.ratio,and EA4T materials with forging ratio 4.0 has the best fatigue life value.The fatigue life of EA4T materials increases with the increase of the initial forging temperature,reaching the maximum at 1250?,and the fatigue life of EA4T materials reaches the optimal value under the sand cooling condition.The change of fatigue life of EA4T materials was discussed by analyzing the grain size,phase transition and fatigue fracture.These results and conclusions provide guidance for the production practice and application of EA4T low carbon alloy steel. |
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