Pure Cold Forging Process Design Of Deep Cavity Thin-Walled Heavy-Duty Filter Housing
The role of the filter in the construction machinery is like the kidney to the human body. For the oil in the construction machinery lubrication system maintains a clean state without impurities, it needs filters. This is crucial to the normal use of construction machinery.
In order to give full play to the design function of the hydraulic filter of construction machinery, it is often necessary to pressurize the liquid in the filtration operation to achieve the effect of increasing the flow rate and rapid filtration. Therefore, the pressure-bearing capacity of the filter housing (Figure 1) is critical in the entire filtration system.
Initial Process Plan: Warm/Hot Forging + Cold Forging
In our current mass production products, small and medium-sized filter housing and DCT automotive transmission accumulator shell can be described as the same type of product, after cost accounting and performance tests, the original hot forging + cold forging method is not the optimal solution.
Problems in warm/hot forging
As the name suggests, warm/hot forging requires heating the blank at the beginning of forging. But the versatility of heavy parts heating equipment and energy utilization are relatively worse than the performance of small and medium-sized parts. The weight of this part blank reached 19.5kg. The consumable material processed by the hot forging process reached 23.5kg, taking the bar material of φ120mm as an example. The length of the unloading material needs 264.7mm. The length-to-diameter ratio at this time is 2.2, using the medium frequency heating method. It is difficult to determine whether the center temperature of the bar is consistent with the surface temperature and the frequency of use of such a size if the intermediate frequency furnace is extremely low and the versatility is very poor.
In the hot forging process, limited by a large amount of deformation and equipment tonnage, the heating temperature goes up to about 1200° C. At this time, the surface of the bar will produce a large amount of oxide scale. Although the product design process will consider increasing the descaling process, it can not achieve the effect of no pit on the surface.
In addition to causing part oxidation, high temperature will also increase the wear of the forging mold. To form the upper mold as an example, the mold material used is H13, and heat treatment hardness of 58HRC. Otherwise, after 600 pieces of forging, the degree of mold wear has been unacceptable. There will be a need to rework the mold after reproduction.
Mold life is low, and efficient production work is not possible. In addition, the higher heating temperature will also make the metallographic structure, after forging, abnormal. This need to be improved by normalizing. Each handling of heavy parts will produce higher costs, with the oven temperature at 550 ° C. This is very unfriendly to production operators.
In addition, the blank margin of hot forging products is relatively large. So the material consumption is relatively high.
Technical difficulties in the pure cold forging process
From the structure of the parts, this product belongs to the deep hole type parts, which is suitable for the anti-extrusion process. The part material is 20 steel, suitable for cold forging. The initial process is conceived as a cold reverse extrusion blank (Fig. 2). Then cold elongated (Fig. 3). The following problems arise in the actual process development and trial production.
The mechanical limits of the upper and lower molds
FEA forging force analysis of the product using finite element software, from bar forging to Fig. 2 Blank Product, requires 2115 tons, as shown in Figure 4. The minimum diameter of the upper die of the forging die is 106.6mm. The force per unit area of the upper die is calculated to be 2352MPa, which meets the industry standard of less than 2500MPa.
The surface of the upper and lower molds is a problem
The flow of the blank in the mold will form certain wear on the mold surface. A large amount of deformation heat will be generated at the same time as the flow. The instantaneous high temperature generated after forging is 500 °C, as shown in Figure 5, The internal temperature of the forging after forging is 256 °C, as shown in Figure 6.
Fig. 5 The instantaneous high temperature generated after forging is 500 °C
Fig. 6 The internal temperature of the forging blank after forging is 256 °C
The hardness of the cemented carbide mold is very high, but the toughness is very poor. Therefore, it is necessary to have a better use effect in the case of a large preload on the outer ring. Under the limitation of this mold structure, the cemented carbide material cannot be used to resist the adhesion of the mold and the blank. Under the dual action of deformation heat and large flow rate, it is easy to produce mold punch sticking and brushing phenomenon.
Choice of cold-drawn long forging ratio
After cold forging blank, the surface and heart of the product have different degrees of hardness. The metallographic structure will also change accordingly, such as can not reasonably set the forging ratio of cold pulling. It may cause the product to crack, pull off, as shown in Figure 7. The forging situation under the correct forging ratio is shown in Figure 8. If the forging ratio is set too small, in addition to increasing the number of forging passes, it will be too long due to the long blank before forging. The opening height of the equipment is not enough to realize the designed process.
Figure 7 Improper forging ratio causes product cracking and breaking.
Fig. 8 The suitable forging ratio is set up well.
Trial production situation and problem solving
Select forging raw materials with a diameter of 170mm for spheroidization annealing treatment and metallographic organization, The spheroidized raw materials can meet the requirements of no less than 80% of the spheroidization rate. The hardness is not more than 65HRB, laying a good foundation for cold forging.
In order to prevent the occurrence of brushing during the cold anti-extrusion process, the surface roughness of the upper and lower dies is under strict control. The surface phosphorus saponification of the blank is under strict control.
Figure 9 is the product after cold anti-extrusion. The pressure gauge of the equipment master cylinder is displayed as 19MPa. The conversion tonnage is 2280t, and the difference between the finite element analysis is 4%. So, this is in line with theoretical predictions and has no other abnormal factors.
Fig. 9 Products after cold anti-extrusion
In the follow-up cold forging pull-out, the design process considers a reasonable forging ratio to produce the product smoothly. Figure 10 shows the pull length 1, pull length 2, pull length 3, and pull length 4 product pictures.
Fig. 10 Cold forging elongation process
Under the analysis of the theory, the setting of the forging process parameters is in advance. This is after a reasonable mold surface treatment and bar surface treatment. So, the entire forging process has no abnormal situation. Of course, our equipment diversity has unique conditions. We have from 3000t hydraulic press to 315t large-stroke hydraulic press. With this, we can let go of our hands and feet in process design and strive to make each link consistent with the process idea.
As mentioned earlier, the performance requirements of parts are relatively strict. The tensile strength requirements are not less than 500MPa. The yield strength is not less than 350MPa, and the elongation is not less than 8%.
The optimized design of the process makes the deep cavity steel cylinder products complete the process transformation from hot forging + cold forging to pure cold forging. So, it greatly shortens the length of the process route. Therefore, the process also improves the tensile strength and yield strength of the parts. At the same time, improving management efficiency and reducing production costs. From the perspective of materials science, it is reasonable that the elongation has decreased because tensile strength and yield strength are crucial to customer use.