Large gear is an important mechanical transmission part in mining machinery, steam turbine, generator and other large equipment. Its quality directly affects the performance level of related products. Large gear blanks formed by die forging are widely used because of their dense and uniform structure, continuous streamline, high dimensional accuracy, easy to forge material internal defects and broken dendritic structure, and gradually replace coarse free forgings and castings with low strength.
In the forging process, there are mainly defects such as excessive forming force, insufficient filling and low die life. This is due to the large contact area between the gear blank and the die, fast cooling and excessive friction when the blank is filled into the die cavity. Therefore, how to reduce the contact area between blank and die, control the cooling rate of gear blank and reduce the forming force is an urgent problem to be solved in large gear forging.
(1) The rational design of the shunt decompression groove can make the gear blank hub, spoke and rim step transition fully filled at the same time; The forming force can be greatly reduced by about 30% compared with the original process, which is a typical “big work with small equipment”.
(2) By analyzing the metal flow law in the forging process of gear blank, a theoretical basis is provided for the selection of forging process. By adopting the asymmetric design of upper and lower dies, uniform metal flow can be obtained, so as to reduce the uneven distribution of deformation stress in gear blank and lay a foundation for future heat treatment.
(3) In view of the good hardenability of the improved 42grmo4v, the step quenching process is formulated to avoid the cracking and deformation of the gear blank. At the same time, metallographic analysis shows that after normalizing + quenching and tempering heat treatment process, the grain size of the material can reach grade 7, and the grade of non-metallic inclusions is also very high.
(4) With the help of DEFORM-3D finite element software simulation, the actual die test times can be reduced and the cost can be saved. The simulation results are basically consistent with the actual production. Therefore, it has certain guiding significance for the improvement of forging process scheme and die design.
With the help of finite element simulation technology, the original process is improved from the two aspects of reducing forming force and improving filling incompleteness. The forging process of large gear based on the principle of shunt decompression and asymmetric design of die cavity is proposed. The purpose is to make the metal flow uniformly and reduce the deformation stress. The new process provides a theoretical basis for practical production. In order to increase the hardenability layer of large gear blank, a high hardenability material – improved 42crmo4v is used (that is, on the basis of European standard 42grmo4v, adjust the content of Mn, Cr and other elements, and reduce the content of harmful elements such as P and S. at the same time, in order to refine the grain and prevent cracking and deformation, the heat treatment process of normalizing + step quenching is adopted. The feasibility of the heat treatment process is verified by metallographic structure analysis and hardenability experiment.