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Warm Forging Die Cooling System Design
Plastic forming techniques at different temperatures have different requirements for the mold. In cold extrusion, because the contact stress between the blank and the mold wall is very large, it is indispensable to fully lubricate the mold, which is also a necessary condition for cold extrusion, especially the cold extrusion of ferrous metal, and the mold The cooling is generally not considered.
In hot forging, the working environment of the mold is very bad, in addition to being subjected to large loads, it is also affected by the alternating temperature. The mold will heat up rapidly due to the high forming temperature of the workpiece. At this time, sufficient cooling of the mold is necessary to maintain the normal service life of the mold. Generally, lubrication is less considered.
Warm forging has the characteristics of cold and hot forming. Therefore, when warm forging, not only the lubrication of the mold but also the cooling of the mold must be fully considered. Practice has proved that: warm forging, especially in medium and high temperature warm forging, the design of the cooling system of the mold is not reasonable or insufficient cooling, which will lead to early failure of the mold. When the warm forging forming temperature is above 600 °C, if the cooling is insufficient, even if a good mold material is used, under the continuous action of high temperature and high load of forging processing, the tempering softening of the mold, especially the punch, cannot be caused. Take advantage of the potential life of the mold.
1. Cooling method for warm forging die
The heating temperature of the billet during warm forging is generally below 800 ° C. The general-purpose crank press is generally used in China for slow forming speed. It takes 3-5s from the blank to the forging to form the mold. The heating time of the mold is also relatively long and the temperature rise is relatively fast. At this time, if a large amount of cooling water is used for jet cooling, the billet will be too cold, and the lubricant between the mold wall and the billet will be washed or diluted. The deformation resistance is sharply increased, and the warm forging cannot be performed normally. In addition, warm forging is often different from hot forging. After a fire is heated, several processes are continuously formed. If the cooling is improper, the forgings are lower than the final forging temperature in the final forming process, even in the blue brittle region of the metal, which will affect Formability and mechanical properties of forgings.
How to make the mold fully cooled during warm forging without excessively lowering the billet is a problem that must be seriously considered when designing the warm forging process and the mold structure. Especially in warm forging forming at medium and high temperature temperatures, the design of the mold cooling system is particularly important.
1.1 Punch cooling method
In the forming mold, the punch can be divided into a cavity type punch (upper mold) and a press-in type punch due to the shape and performance of the forging.
The inner shape of the cavity type punch is substantially close to the die, and its outer shape is relatively large. The circulating cooling water tank can be arranged in the mold with reference to the die cooling method. Press-in punches are primarily used to form forged or extruded parts with holes. In addition to transmitting pressure, the punch also functions to control the flow of metal, and the unit pressure is relatively high. Limited by the dimensions of the forging and the structure of the forging, the shape of the press-in punch is generally small and has a shaft shape. If the circulating cooling water tank is opened on the punch, the strength thereof is greatly reduced. If the cooling water is sprayed by the cooling water, the cooling water may be dropped into the die or directly dropped on the blank in the die to make the billet cold and warm forging impossible. When designing a warm forging die, we used spray cooling for the press-in punch.
The cooling spray is a kind of water vapor which is a mixture of cooling water and compressed air. The device is installed above the punch and arranged in a ring shape. When the mold is opened, the mist is ejected from a plurality of nozzles to cool the punch. The advantages are as follows: (1) After the cooling mist encounters the punch, it will evaporate immediately after cooling the punch, so that the billet is not too cold. (2) The ratio of cooling water to compressed air can be controlled by adjusting the steam and liquid valves to achieve the best cooling effect. (3) The cooling spray is sprayed through a plurality of nozzles on the copper pipe, so that the cooling of the punch is uniform, and no large temperature stress is formed, thereby prolonging the service life of the punch.
1.2 concave mold cooling mechanism
The die cooling mechanism is to open a plurality of annular water tanks on the inner wall of the prestressing ring (outer ring and inner ring), and to cut through the vertical groove and open the water outlet hole so that the water surrounds the die in the outer ring and the inner ring respectively for several weeks. The heat then flows into the cooling unit. In order to facilitate the installation of the cooling system and not affect the strength of the prestressing ring, when designing the prestressing ring, the external dimensions should be adjusted according to the experience in actual production, taking the upper limit of the recommended value.
During warm extrusion, the cooling water is injected from the inlet of the outer ring, and under the action of pressure, surrounds the water channel through the outer ring, reaches the lower water hole of the inner ring and flows into the inner ring water tank; in the inner ring, the cooling water passes through two circles. The annular water tank surrounds the die for two weeks to take away the heat of the die and then flows back to the outer ring water tank through the water hole in the upper part of the inner ring; after circulating again in the outer ring water tank, it flows out from the water outlet of the outer ring and is input into the cooling device through the pipeline. Recycle. It is worth mentioning here that the cooling water must be a closed circuit, that is, the cooling water pipe should be closed, because the cooling water channel is opened on the prestressing ring, and the sealing of the pipe can be ensured after the prestressing ring is pressed. It is desirable to use a cooling water circulation method for mold cooling.
2. Mold cooling and die life
In the actual production, the warm forging die using the above-mentioned convex and concave die cooling device is a tapered bearing roller having a diameter of 27 mm and a height of 59 mm, and the forming temperature is 700 ° C, and the cooling is enabled when the mold material is 3Cr2W8V. After the device is operated for several hours, its temperature rise is less than 100 °C. After batch testing, the average life of the mold can reach more than 3x102 pieces. Under the same working conditions, the convex and concave mold cooling devices are not activated, and after the mold has continuously formed hundreds of forgings, the punching mold has a softened piercing, and the female die has difficulty in drawing the die. phenomenon. Both convex and concave molds fail early.
It is worth mentioning that the timing of opening the cooling device for different mold materials is different. For some materials that are particularly sensitive to temperature, such as high-speed steel Wl8Cr4V, W6Mo5Cr4V2, etc., preheating must be carried out before the mold works, and the preheating temperature is generally around 200 °C. After the mold temperature rises to 250 ° C - 300 ° C after forming, the cooling water device is turned on again. Otherwise, opening the cooling water device too early will affect the preheating temperature of the mold, and the mold is prone to cracks during forming. Reasonable control of the preheating and cooling timing of high speed steel can greatly improve the service life of high speed steel molds.
3. Conclusion
In summary, the cooling device of the warm forging die must be highly valued in the design of the mold. For forgings and different mold materials of different materials and sizes, the cooling degree of the mold should be adjusted by controlling the amount of water in the cooling water and the timing of opening to maximize the potential life of the mold material.