Flange products are widely used, and their annual consumption is large. The closed-die forging process is used to precision form small and medium-sized flanges, which can save material and provide good performance of forgings, showing the advantages of “finer, cleaner, and more economical” in mass production. This paper introduces a closed-type precision forging flange process suitable for automated production, proposes an optimized precision forging process scheme, and analyzes the key issues in automated production. The flange forgings produced by this process not only have higher precision but also have higher production efficiency than the common production method.
The automated closed-end forging process for flanges introduced in this paper can be used to forge flanges less than 1 inch (flange diameter less than Φ130mm), taking a 1-inch flange as an example (Figure 1). The design forging machining allowance is 0.5-1.0mm (excluding the intermediate hole and the increase of the drawing die allowance), as shown in Figure 2, and the weight of the forging is about 2.5kg. The middle hole of the part is small, and it is not recommended to forge out; you can design 10-15mm pits on the upper and lower end face of the forging, or you can make a flat surface. Generally, the height-diameter ratio of billet for automated production is more reasonable at 2.0-2.5, and the size of the forging billet can be calculated as Φ55mm×136mm.
Fig.1 1 inch flange product
Figure.2 1-inch flange forging diagram
The forging process applied to automated production needs to meet two main conditions.
- (1) The process and die design must meet the requirements of stable production, such as the transfer of each station forging articulated placement, the release mechanism to be stable, etc.
- (2) Die life must meet the requirements of mass production; for production-oriented enterprises, frequent production stoppages and die replacement will cause great economic losses.
- According to Figure 2, flange forgings can be calculated; the volume of the flange part accounts for about 86% of the volume of forgings, the 1-inch flange forgings size is relatively small, and the main forming trend for upsetting. It can be considered first upsetting, then a forging forming, and upsetting after a forging forming simulation results are shown in Figure 3.
Figure 3a shows the schematic diagram of the primary forming dies. The basic structure of the die includes an upper die, a lower die, and a top bar. The shape of the simulated forging is shown in Figure 3b. The forging has a longitudinal burr at the closing die, the bottom is not filled with the cavity, and the forging does not meet the requirements. The forming force curve of the forging is shown in Fig. 3c. The forming force of the forging in the state of Fig. 3b reaches 8440kN, which is because the forging has a longitudinal burr, causing the forming force to rise sharply. The equivalent force distribution of the lower die is shown in Fig. 3d; the equivalent force distribution is between 123-720MPa, the influence range is larger, and the failure form of the die is mainly manifested as cracking.
Figure.3 Simulation results of primary forging forming process after upsetting
(a) Schematic diagram of die structure (b) Shape of forging (c) Forming force curve (d) Equivalent force distribution of lower die
There are three problems with the primary forging and forming process.
- (1) The simple upsetting is not positioned in the die, which is easily placed on the wrong side and affects the stable production of automation.
- (2) The bottom of the forging is not yet full, but the top already has longitudinal burrs, which will make the forging scrap, and the longitudinal burrs have a very big impact on the die life.
- (3) The equivalent force of the die is larger, which will shorten the life of the die.
- The above problems can be optimized by adding pre-forging steps to optimize the forging process and die equivalent force. The optimized process flow is heating, upsetting, pre-forging, and final forging.
Figure.4 Shows the simulated work step diagram
Because the bottom of the forging formed in one forging is not easily filled, the pre-forged part should supplement the metal material at the bottom. Secondly, increase the positioning of the pre-forged part placed into the final forging die, so the base shape of the pre-forged part is shown in the work step 2 of Fig. 4. The pre-forged part is first extruded with a rod section that can supplement the defects at the bottom of the forging formed in one forging. The rod extrusion length is generally 10-20mm according to the forging condition; if the pre-forged extruded rod is too long, it will make the bottom of the final forging appear longitudinal burr, the outer edge of the flange is not filled, and the qualified forging products cannot be forged. The diameter at the step of the pre-forged rod part and the head is matched with the diameter of the flange rod part of the final forging to achieve the purpose of positioning. The diameter of the pre-forging rod can be smaller than the diameter of the corresponding position of the final forging by about 0.5mm, which can ensure that the coaxiality of the pre-forging part and the final forging die is less than Φ0.5mm when the pre-forging part is placed in the final forging die.
After increasing the pre-forging station, the forging forming force and the equivalent force of the die are improved, and the simulated forgings are shown in work step 3 of Fig. 4, and the forgings are formed better. The pre-forging forming force curve is shown in Fig. 5a, and the maximum forming force is about 2180 kN. The final forging forming force curve is shown in Fig. 5b, and the maximum forming force is reduced from 8440 kN in one forging to 4790 kN. The distribution of die equivalent force is shown in Fig. 5c, and the maximum equivalent force is reduced from 740 MPa in one forging to about 500 MPa, and the coverage is greatly reduced. The maximum equivalent force is at the positioning of the pre-forged rod, and the main form of failure is shown as wear. The distribution of equivalent forces in the die is significantly improved, and the die life can be doubled.
Figure.5 Simulation results after process optimization
(a) Pre-forging forming force curve (b) Final forging forming force curve (c) Equivalent force distribution of the die
In summary, the equipment of the production line can be determined as follows: high speed sawing machine, medium frequency furnace, 1000t hot die forging press, stepping beam manipulator, cooling and lubrication system, and transfer device. The main equipment is shown in Figure 6.
Figure.6 Main equipment of the production line
The multi-station dies applied in automated production differ from the common die.
- (1) The life of the die used in automated production should meet the requirements of mass production, and the life of the hot forging die should be above 10,000 pieces, not to affect the normal production of the line.
- (2) The forging parts must be stable and reliable when taken off the die and transferred between the stations.
As shown in Figure 7, the die structure of 3 stations is designed one by one according to the work step diagram, and the die cavity is designed according to the forging work step diagram. The die design should pay attention to the size of the header and the pre-forging die and the size of the pre-forging and the final forging die. Generally, the difference between the header’s outer diameter and the pre-forging die’s inner diameter is about 0.5mm. This difference should be achieved by adjusting the header height. The clearance between the pre-forged part and the final forging die at the associated positioning is about 0.5mm, which can be ensured by die processing. Whether the design of each station forging size is reasonable is directly related to the size and die life of the closed finish forgings.
At the same time, we should pay attention to the design of the reset structure of the ejector rod of each station to be stable and reliable. The ejector rod can be mechanically reset or reset; automatic reset connects the die ejector rod with the equipment ejector rod. Spring reset is to pull back the ejector rod with a spring, occupying a certain amount of die space.
Fig.7 3-station mold
(a) Upper and lower molds of stations 1, 2, and 3 (b) Lower molds and jaws of stations 1, 2 and 3
The production process is as follows: After the billet is discharged from the high-speed sawing machine, it is automatically fed into the material frame of the IF furnace. The automatic feeding device feeds the billet into the IF furnace and heats the forging to (1150±20)℃. After the heating is completed, the billet is fed into the forging station from the chute, and then it is clamped by the stepping beam for forging in 3 stations upsetting, pre-forging, and final forging. The automatic cooling and lubricating system cool and lubricate the die during the station transfer. After the forging is completed, the conveyor belt sends it to the material frame. 3 stations of die can be forged simultaneously or at intervals.
The pre-forged parts are shown in Fig. 8a, and the final forgings are shown in Fig. 8b, Fig. 8c, and Fig. 8d. The forgings are consistent with the simulated situation and meet the processing requirements.
Analysis and solution of key problems of automated production
There needs to be more than the optimized design of forming process and die to realize automated production, and the cooperation between each production line equipment is also important. Two other points in the production process have a greater impact on the application of the process.
The heating performance of medium frequency furnace
Medium frequency furnace is commonly used in automated production, but also the first step of the hot forging process and directly affects the forging forming. Currently, the quality of IF furnaces of various manufacturers in the market varies, and it is necessary to put forward some notes on the selection of IF furnaces in this paper.
(a) pre-forged parts, (b) the bottom of the final forging, (c) the front of the final forging, (d) cold forgings
The problem often encountered in using medium-frequency furnaces is the uneven heating temperature of forgings. Uneven heating temperature generally has two cases.
- (1) Uneven temperature at both ends of the billet; the first end of the furnace temperature is low.
- (2) Uneven heating on both sides of the billet axially, with low temperature on the side of the billet in contact with the pushing track.
The former case of upsetting parts will appear as shown in Figure 9a, where one side of the header has a large head, and the other has a small head, affecting the forming in the next station. In the latter case, the temperature difference does not significantly affect the header. Still, in the pre-forging, the situation shown in Fig. 9b will occur, where the upper plane of the pre-forged part is high on one side and low on the other, and the longitudinal burr is very high on one side and not full on the other side.
The billet’s temperature difference greatly impacts the forgings in each step, which may cause the forgings to be scrapped in the forging process. This kind of unqualified semi-finished forgings cannot be judged by signal processing, likely leading to serious production accidents. Therefore, the selection of IF furnace equipment and the adjustment of relevant parameters are very important. According to the parameters of the diameter, volume, and production beat of the billet, the parameters of the IF furnace are set reasonably, as well as the heating interval and the equalizing temperature interval section in the furnace chamber.
Clamping stability of the stepper beam manipulator
The main reason for choosing the stepper beam manipulator operation is: that the forging transfer speed is fast, which can significantly improve production efficiency. However, the stepper beam manipulator is prone to thermal failure when involved in the hot forging process. The frequent problems are heat deformation of the clamp and signal feedback failure.
Under hot forging conditions, as the ambient temperature rises, the clamping jaws may not be able to hold the billet when they are deformed by heat, especially when forgings such as round flanges and have almost vertical gripping surfaces. Usually, it can be solved by increasing the clamping angle, changing clamping to bracketing, forging clamping steps, etc.
Normally, when the robot can’t clamp the billet or the billet is dropped, a stop signal should be issued to protect the equipment and dies of the production line. However, the clamping sensing signal system may fail to heat, a relatively common problem for stepper beam manipulators in the hot forging process. When the sensing signal system is subjected to heat failure, it may appear as the repeated forging of one product or stacked forging of two products, which may break the clamp, break the die or even damage the equipment. At present, the solution to this problem is to choose reliable equipment and, secondly, to cool the key parts of the manipulator together with the die to reduce the ambient temperature. At the same time, the clamp material should be resistant to high temperatures, and the signal-sensing device should be as far away from the hot state environment as possible.
Figure.9 Effect of heating on forgings
(a) The effect of the temperature difference between the two ends of the billet on the forgings (b) The effect of the temperature difference between the two sides of the billet axially on the forgings
The multi-station forging process and die design applied to automated production should consider all factors; it is the basis of forging qualified products and the key link to automated production. Attention should be paid to optimizing the shape of the pre-forged parts, considering the correlation between the stations and the stability of the pick and place forging parts, and extending the die life as much as possible. The function of each piece of equipment on the production line should be stable and reliable, especially attention should be paid to the heating temperature uniformity of the medium-frequency furnace, and the clamping stability and signal reliability of the manipulator are also important. Producing flanges of less than 1 inch (diameter <Φ130mm) using the automated method of 3-station forging with a stepper beam manipulator has improved both the accuracy of the forgings and the production efficiency. The machining allowance of forgings is 0.5-1.0mm (excluding the allowance added by intermediate holes and drawing dies), and the stable production efficiency is 300-400 pieces per hour. This process has been verified by production practice.
Author: Yan Hongyan