Analysis And Optimization For Bending Defect In The Forming Roll Forging Process Of A Coupler Yoke

Coupler yoke is one of the most important parts of coupler draft gear in the railwaywagon. It transfers the force in longitudinal direction of the rail-engine, when carriagesare drawn, coupled and buffered. With the requirement of high speed heavy haulstrategy, the performance of the train is developed, which requires higher quality of theproduction technology. The quality of the coupler yoke part is a key factor which candetermine the safety of the train. So, more and more research is made on thereplacement of casting process for the production of coupler yoke by forging process. Itshows that the guarantee period of the part in forging process can be increased from8years to25years. However, the complicated structure of the part needs huge tonnageequipment, which leads to difficulty in forging process. To reduce the force needed inthe process and make it feasible, the process that the part is formed in die forging andthen bended in bending die is adopted before the welding process for connecting platesto the two ends.13B type coupler yoke consists of two ends, two connecting plates and middle bulk.The connecting plates have the minimum size in height direction. And the cross sectionsalong longitudinal direction are quite different. To decrease the force in die forging,forming roll forging process, in which the connecting plate is completely formed to therequired dimension, is designed. And then the ends as well as the middle bulk of thepart will be formed in die forging process.Although roll forging process is a process with lower requirement for equipmentand higher material utilization, the quality of the part can be affected by factors such astemperature, friction condition, rolling speed, etc, which results in some defects such asunder-filling, folds. So it needs a long adjustment period. In this paper, the13B typecoupler yoke part with defect of bending formed in the roll forging process was studied.The bending defect can lead to flexure in connecting plate and even ruin the part. Withthe purpose of solving the problem, the four-pass roll forging process was researched.The main content in this paper as following:①Areliable FEM model for the initial roll forging process of the13B type coupleryoke was established on the basis of much investigation and analysis on previousreports. By the comparison with the production data, it shows that the FEM is accuratewith production results. In addition, the temperature field, strain field and the load in the four-pass roll forging process were analyzed by numerical simulation software FORGE.From the analysis the overall realization for previous process was obtained.②The deformation and distribution of the metal in process was analyzed to obtainthe causes of the defect. The analysis shows that difference of elongation at the top andbottom surface of the middle bulk as well as the mismatch of the mold with the part inpreceding pass are the major causes of the defect. While the mismatch problem resultsfrom under-filling defect at connecting plate in2thand3thpass, slipping at middle bulkin3thand4thpass, and slipping at transitional section between front connecting plateand middle bulk. And different elongation of the upper and lower surface of the billet iscaused by asymmetric structure of the middle bulk in height direction.③Based on the research results above, the optimization for process and mold wasproposed. Firstly, the forming of single characteristic of connecting plate and middlebulk is optimized. For connecting plate, an off-round pass was modified in the2thpass,which improved stable forming in the2thpass and solved the under-filling defect in2thand3thpasses. While for middle bulk, an asymmetric construction of middle bulk in3thpass was proposed. On one hand it can decrease the asymmetric elongation in the upperand lower surface of the billet, which can improve the stress in X direction in4throllforging process. On the other hand, it can solve slipping in middle bulk. Then, the formbetween connecting plate and middle bulk is replaced by circular arc, so that it canslove the slipping problem at the transition. When calculating for the arc length of thedies, the filling percent of99%was taken into account, and the value of slip calculatedby the finite element analysis software was adopted. Moreover, to avoid the interferenceof the end of the billet and the dies, guide grooves were designed for3thand4thpass.The optimized result shows that the quality of the part in roll forging process can meetthe forming in die forging.The research achievements of the paper solved the problems of the bending defectof coupler yoke and provided basis for production of other part in roll forging process.