A Kind Of Valve Tappet Cold Extrusion Forming Process Research

In this paper,the material of valve tappet produced by Yibin Tiangong Machinery Co.,Ltd.was annealed 20 Gr Mo low-carbon alloy steel.Which was coarse grain microstructure with high hardness,poor plasticity,and low elongation,was difficult to achieve large plastic deformation.So,its specific process flow was as follows: blanking of bar stock-upsetting-heat treatment of complete annealing-shot blasting-phosphating treatment-the first cold extrusion-the second reverse extrusion,which was complex and large waste of human resources.In view of this practical problem,the pre-heat treatment process was replaced by spheroidizing annealing process.Firstly,the blank was put into the furnace at room temperature and heated to 780 ± 10 ? and kept for 4 hours,then cooled to 720 ± 10 ? and kept for 8 h.After that,the blank was cooled to 480 ? with the furnace and followed by cooled to room temperature in air.After annealing,the microstructure of the blank was spherical pearlite,which was consisted of spherical or granular carbide evenly distributed on the ferrite matrix.The hardness of the blank was lower than 120 HBS.Therefore,the blank had good plasticity and machinability,which provided good plasticity for cold extrusion forming,and finally one-step extrusion forming was realized.The improved cold extrusion process was as follows: blanking of bar stock-upsetting-spheroidizing annealing-shot blasting-surface lubrication-one-step reverse extrusion.The simulation software of DEFROM finite element analysis was used to analyze the equivalent stress,equivalent strain,velocity field,extrusion stress,and flow situation during extrusion procedure.The results showed that the stress was concentrated at the bottom and corner of the die,equivalent strain was uniformly distributed,and the distribution of metal flow in extrusion was consistent,which met the product requirements.According to the optimized process,the cold extrusion die of the product was designed,and the extrusion product was verified and analyzed.The product size and quality satisfied the testing and performance requirements at the same time.The research work of this paper has an important guidance for cold extrusion forming and application value of valve tappet.