1.Selection of processing temperature and strain rate
After drawing the hot working diagram, the appropriate processing temperature and strain rate can be selected. In the description of the hot working drawing, there are two areas suitable for the high-temperature thermal deformation of 20CrMnTiH steel. The heating temperature of equal height spiral bevel gear blank is 1200 ° C. after upsetting and punching, it is rolled, and then the final forging is formed. The final forging deformation range is located in the area of 980-11500 ° C, 0.01s-1 or 0.1s-1. At this time, the power dissipation factor is greater than 0.3, and it is in the non unstable area, The material has good hot workability.
2.Material performance parameters of 20CrMnTiH steel
In the modeling of equal height spiral bevel gear, the effects of thermal expansion and elastic factors of equal height spiral bevel gear material are considered. Therefore, it is necessary to obtain the shrinkage and elastic strain of equal height spiral bevel gear material and modify the equal height spiral bevel gear model. Because the hot compression test under forging condition is not carried out, the elastic strain value of equal height spiral bevel gear material cannot be obtained through the test, and the performance parameters can be obtained by searching the literature.
In forging simulation, the definition of material not only needs the stress-strain curve, but also needs to obtain other performance parameters, including elastic modulus, Poisson’s ratio, thermal conductivity and so on. The thermophysical property parameters of 20crivintih steel are given in the table.
| Modulus of elasticity (GPa) | Poisson’s ratio | Thermal conductivity (I / ° C) | Linear expansion coefficient (1 / ° C) | Yield strength (GPa) |
| 207 | 0.29 | 0.0000126 | 0.0000137 | 0.835 |
It can be seen from the table that the average linear expansion coefficient of the material is 0.0000137 (1 / ° C), the final forging temperature during forging is 1050 ° C, and the cold forging temperature under service conditions is 20 ° C. Therefore, the shrinkage in this temperature range can be obtained as 0.014111.
From the table, it is known that the elastic modulus of the material is 207gpa and the yield strength is 0.835gpa, and the elastic strain value is about 0.004.
3.Die material performance parameters
Using H13 (4Cr5MoSiVl) as the die material, the die preheating temperature is 300 ° C, the elastic modulus of H13 at 300 ° C is 185.248gpa and the yield strength is 1.251gpa, the corresponding elastic strain value is 0.00675. At 300 ° C, the average linear expansion coefficient of H13 is 0.00001239 (1 / ° C). At this time, the shrinkage of the die is 0.00347.
4.Influence of uneven elastic deformation of die tooth profile
In the forging process, the die tooth profile will produce uneven elastic deformation under the action of forming load. Because the deformation is complex and belongs to nonlinear elastic deformation, it is impossible to use specific formulas to describe its elastic deformation law. The elastic deformation will be transmitted to the equal height spiral bevel gear forging, which will change the tooth shape of the equal height spiral bevel gear forging and reduce the forming accuracy of the tooth surface of the forging.
At the same time, when adding tooth surface allowance to the tooth profile of equal high tooth spiral bevel gear, the tooth surface allowance at the large and small ends is often equal, or a larger allowance is added at the large end and a smaller allowance is added at the small end. The addition of tooth surface allowance is often used for cutting to meet the surface accuracy of tooth surface. If the tooth profile of gear forging is deformed due to the deformation of die tooth profile in the forging process, the tooth profile of equal height spiral bevel gear forging will no longer maintain its original tooth shape. In order to meet the requirements of subsequent tooth surface cutting, the forging allowance of tooth surface will have to be increased, and the forging accuracy and material utilization rate will be reduced.
Therefore, the influence of uneven elastic deformation of die tooth profile on the accuracy of forging tooth surface can be solved by adding uneven forging allowance to the tooth surface. Firstly, through the analysis of die deformation in the forging process of equal height spiral bevel gear, the law of uneven elastic deformation of die tooth profile is obtained. Then, the addition of tooth surface allowance is adjusted through the reverse compensation of deformation.
5.Modeling of simulation die
In the forging simulation of equal height spiral bevel gear, it is necessary to obtain the simulation die model of equal height spiral bevel gear. At this time, only the module part is considered as the simulation die in the simulation process. In order to facilitate the acquisition of simulation die model, based on the modeling program of precision forging die module, the simulation die model can be modeled separately to obtain the three-dimensional automatic modeling program of simulation die model.
After the completion of the simulation mold program, based on the material performance parameters, take the diameter of the mold as 800mm and the total height of the upper and lower molds as 400mm to obtain the corresponding simulation mold model.
7.Design of blank section shape
In the design of the die, the external flash groove is not set, but the traditional rectangular section blank will produce external flash in the forging process. If the external flash groove is not set on the die for forging, there will be surplus blank between the parting surfaces of the upper and lower dies of the die, which will affect the die clamping.
Therefore, the section shape of equal high tooth spiral bevel gear blank is modified, and the blank with concave section is proposed. The blank of equal high tooth spiral bevel gear can be obtained by rolling or die forging. However, too large side concave will cause folding of medium high tooth spiral bevel gear forgings in the forging process. Therefore, the side concave section must be designed reasonably. The blank section is shown in the figure. Among them, the cone angle of the blank should not be consistent with the cone angle of the tooth top of the die tooth shape, so that the inner and outer sides of the blank of equal height spiral bevel gear can contact the die tooth shape at the same time. S is the inner residual platform, R1 is the fillet radius of the upper end of the side recess, R2 is the fillet radius of the lower end of the side recess, l is the radius of the side recess, and H is the height of the blank, Φ U is the outer diameter of the blank, Φ N is the inner diameter of the blank.