Hollow pipe fitting forming device and forming method

Abstract

The invention provides a hollow pipe fitting forming device and method. The hollow pipe fitting forming device comprises a heating mechanism, a bulging quenching mold and an air source mechanism. The heating mechanism comprises a roller hearth furnace provided with a conveying roller way, a furnace body is sequentially provided with a feeding isolation chamber, a heating chamber and a discharging isolation chamber in the length direction, all the chambers are isolated from one another and communicated with a protective atmosphere system, and the chambers are used for introducing dry air or inert gas into the furnace body and maintaining the dew point temperature below-15 DEG C; hydrogen adsorption on the surface of the hollow pipe blank is effectively inhibited, and the hydrogen embrittlement risk is reduced. After a pipe blank is heated at a high temperature, austenitizing of a material structure is achieved, then the pipe blank is rapidly transferred to a bulging quenching die, gas expansion deformation occurs in a die cavity under the action of set air pressure, the hollow pipe fitting is formed by attaching the die wall, meanwhile, the hollow pipe fitting attached to the die cavity is synchronously cooled through a cooling assembly in the die, and quenching treatment is achieved. The material structure is converted into martensite, and the mechanical property of a formed part is improved.

Description

Technical Field

[0001] This invention relates to the field of tube blank processing technology, specifically to a hollow tube forming device and forming method. Background Technology

[0002] With the continuous advancement of lightweighting trends in the automotive industry, especially in the field of new energy vehicles, lightweighting is of great significance for improving vehicle energy efficiency, driving range, and collision safety. Studies have shown that for every 10% reduction in the weight of a vehicle, its energy consumption can be reduced by 6%-8%. To achieve lightweighting goals, ultra-high strength steel pipes are increasingly widely used in key structural components such as A-pillars, B-pillars, and anti-collision beams.

[0003] Hot gas expansion forming technology is an important process for realizing complex shapes of ultra-high strength steel pipes. Because it can use high-pressure gas to form the billet under high temperature conditions and achieve rapid quenching in the mold to form a martensitic structure, it can balance structural lightweighting and mechanical strength. It is gradually replacing the traditional cold stamping or CBS+PB (roll pressing + stamping) process and has become the preferred process route for OEMs (such as Geely, Xiaomi, Li Auto, etc.) in the development of new models.

[0004] In existing hot gas expansion forming systems, the commonly used heating methods mainly include self-resistance heating and multi-layer chamber furnace heating. Self-resistance heating relies on the workpiece's body resistance for electrothermal conversion, but it has high requirements for the uniformity of the workpiece material and the consistency of its thickness, making it difficult to adapt to irregularly shaped tube blanks with large curvature or non-uniform wall thickness. Furthermore, it is difficult to create a stable dry atmosphere during the heating process, posing a risk of hydrogen embrittlement. While multi-layer box furnaces achieve multi-faceted heat distribution in their structure, the varying distances between the billet and the heat source, coupled with relatively poor gas flow, make temperature uniformity difficult to control. Similarly, the frequent opening and closing of the furnace door makes it difficult to maintain a dry atmosphere, thus posing a risk of hydrogen embrittlement. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the problems in the prior art and provide a hollow tube forming device and forming method.

[0006] To address the aforementioned technical problems, the present invention provides a hollow tube forming apparatus, comprising: A heating mechanism for heating tube blanks includes a furnace body and a first conveying roller conveyor. The first conveying roller conveyor is arranged inside the furnace body along its length and conveys a tray carrying the tube blank along this direction. The furnace body is provided with a feeding isolation chamber, a heating chamber and a discharging isolation chamber in sequence along its length. The three chambers are isolated from each other and are respectively connected to a protective atmosphere system for introducing protective gas into the furnace body to maintain the dew point temperature inside the furnace body below -15°C. An expansion quenching mold includes a mold body and a cooling assembly disposed within the mold body. The mold body is used to perform air expansion forming on a tube blank heated by the heating mechanism within its mold cavity to form a hollow tube. The cooling assembly is used to simultaneously initiate cooling after the hollow tube is fitted into the mold cavity, thereby achieving quenching treatment during the air expansion forming process. The gas source mechanism is used to input gas at a set pressure into the tube blank cavity contained in the mold body, so that the tube blank will expand and deform within the mold body and be formed.

[0007] In one embodiment of the present invention, the heating mechanism further includes a second conveying roller conveyor, which is disposed above the furnace body and arranged parallel to the first conveying roller conveyor, and its conveying direction is opposite to that of the first conveying roller conveyor. The output end and the input end of the first conveyor roller are respectively provided with a first lifting mechanism and a second lifting mechanism. The first lifting mechanism is used to retrieve the empty pallet on the first conveyor roller to the second conveyor roller, and the second lifting mechanism is used to cyclically transfer the pallet on the second conveyor roller to the first conveyor roller.

[0008] In one embodiment of the present invention, the heating mechanism further includes a third conveying roller conveyor, which is arranged parallel above the second conveying roller conveyor and its conveying direction is opposite to that of the first conveying roller conveyor. The first lifting mechanism is also used to transfer empty pallets on the first conveyor rollers to the third conveyor rollers.

[0009] In one embodiment of the present invention, the heating chamber includes a preheating zone, a heating zone and a heat preservation zone arranged sequentially along the length of the furnace body, and an isolation door is provided between the preheating zone and the feeding isolation chamber, and between the heat preservation zone and the discharging isolation chamber.

[0010] In one embodiment of the present invention, the feeding isolation chamber is provided with a feeding furnace door, and the discharging isolation chamber is provided with a discharging furnace door.

[0011] In one embodiment of the present invention, the gas source mechanism includes a nitrogen storage tank, a primary compressor, a medium-pressure gas cylinder, a secondary compressor, a high-pressure gas cylinder, and a recovery gas cylinder; the primary compressor is connected to the nitrogen storage tank, the recovery gas cylinder, and the medium-pressure gas cylinder respectively; the secondary compressor is connected to the medium-pressure gas cylinder and the high-pressure gas cylinder respectively; the medium-pressure gas cylinder and the high-pressure gas cylinder are respectively used to input gas at a set pressure into the tube blank cavity contained in the mold body.

[0012] In another aspect, the present invention provides a method for forming hollow tubes, comprising the following steps: Step 1): The hollow tube blank is placed on a tray and sequentially conveyed to the feed isolation chamber, heating chamber, and discharge isolation chamber of the roller hearth furnace. Protective gases are introduced into the feed isolation chamber, heating chamber, and discharge isolation chamber respectively. The heating chamber heats the tube blank to a set high temperature to austenitize its material structure. The protective gases in the feed isolation chamber and discharge isolation chamber form an air wall, which isolates the heating chamber. During the heating process, the protective gas in the heating chamber maintains the dew point temperature in the roller hearth furnace below -15°C. Step 2): Quickly transfer the heated tube blank to the expansion quenching mold and close the mold; inject high-pressure gas at a set pressure into the cavity of the tube blank, and under its expansion, make the outer wall of the tube blank fit against the surface of the mold cavity to achieve air expansion forming; during the air expansion forming process, use the cooling components set in the mold to cool the tube blank simultaneously, so that its material structure undergoes martensite, and the quenching treatment is completed.

[0013] In one embodiment of the present invention, in step 1), the tube blank is heated to a temperature of 850–950°C for 3–8 minutes to austenitize the material structure.

[0014] In one embodiment of the present invention, the tonnage m corresponding to the pressure value required by the bulging quenching mold during the forming process satisfies the following formula: m = n × (α × d × L × P / 9800 + 10), Where n is the number of mold cavities; d is the diameter of the hollow tube blank in mm; L is the axial length of the hollow tube in mm; α is the safety factor, ranging from 1.0 to 1.4; P is the gas pressure required for tube blank forming in MPa; and m is the tonnage corresponding to the value required by the expansion quenching mold in tons.

[0015] In one embodiment of the present invention, during the bulging quenching process, the quenching holding time T satisfies the following relationship:

[0016] Where T is the quenching and holding time in seconds; h is the pipe wall thickness in mm; and P is the internal air pressure of the hollow pipe during the holding time in MPa.

[0017] In one embodiment of the present invention, step 2) of pressurizing the workpiece by supplying gas through a medium-pressure gas cylinder and / or a high-pressure gas cylinder specifically involves: When the workpiece to be formed is an easy-to-form workpiece, gas is directly supplied through a medium-pressure gas cylinder to the target forming pressure of 10-40MPa. When the workpiece to be formed is a difficult-to-form workpiece, it is first pressurized to a pre-forming pressure of 10-40 MPa by supplying gas from a medium-pressure gas cylinder, and then pressurized to the target forming pressure of 40-80 MPa by supplying gas from a high-pressure gas cylinder.

[0018] In one embodiment of the present invention, the gas storage pressure range of the medium-pressure gas cylinder is 42-45 MPa, and the gas storage pressure range of the high-pressure gas cylinder is 88-98 MPa.

[0019] The technical solution of the present invention has the following advantages compared with the prior art: This invention places hollow tube blanks on a tray and sequentially feeds them into the feeding isolation chamber, heating chamber, and discharging isolation chamber of a roller hearth furnace. Protective gases are introduced into each chamber, forming gas walls in the feeding and discharging isolation chambers to isolate the heating chamber. This effectively isolates the heating chamber from moisture and oxygen in the external air, establishing a continuous and stable low-dew-point protective atmosphere. In particular, the introduction of protective gas into the heating chamber allows the heating process to proceed in a dry atmosphere, controlling the dew point temperature below -15°C. This significantly inhibits oxidation reactions and hydrogen permeation during heating, reducing the risk of hydrogen embrittlement at its source. After heating to the set high temperature and holding at that temperature, the material successfully achieves austenitization, providing a microstructure basis for subsequent quenching to form martensite.

[0020] After heating, the hollow tube blank is rapidly transferred to an expansion and quenching mold, which is then closed. High-pressure gas is quickly injected, and under the expansion effect of the internal gas pressure, the tube blank achieves precise air expansion forming to conform to the shape of the mold cavity. Simultaneously, cooling components within the mold absorb heat from the hollow tube, causing its microstructure to rapidly transform into martensite during deformation, thus achieving the quenching treatment. This simultaneous air expansion and quenching method not only significantly shortens the processing time but also avoids the process complexity and dimensional instability problems caused by reheating and quenching after forming.

[0021] Preferably, the medium-pressure gas cylinder is directly connected to the mold body, enabling direct injection of gas into the tube blank cavity for preforming or one-time forming operations. For hollow tubes with thin walls and minimal cross-sectional changes, the medium-pressure gas cylinder can meet the required forming pressure. The medium-pressure gas cylinder serves as a pre-filled gas source, with a secondary compressor connected to its rear end. The secondary compressor further compresses the medium-pressure gas output from the medium-pressure gas cylinder to form high-pressure gas, which is then injected into the high-pressure gas cylinder. When the workpiece to be formed is a difficult-to-form hollow tube, the medium-pressure gas cylinder is first used to supply gas and pressurize it to a preforming pressure of 10–40 MPa, followed by pressurization to the target forming pressure of 40–80 MPa via the high-pressure gas cylinder.

[0022] The above-mentioned graded gas supply scheme realizes a multi-stage compression and gas storage distribution mechanism, which meets the forming pressure requirements under different working conditions; it avoids pressurizing all workpieces with a two-stage booster device, thereby reducing the frequency of use of the two-stage compressor and improving the service life of the process system equipment.

[0023] Furthermore, when the pressure of the residual gas after molding exceeds 42 MPa, this high-pressure gas is preferentially recovered to a medium-pressure gas cylinder for pre-filling of subsequent workpieces or direct molding. For medium-pressure gas between 5 MPa and 42 MPa, it is recovered and temporarily stored in a recovery gas cylinder, then compressed by a primary compressor and returned to the medium-pressure gas cylinder. If the residual gas pressure is below 5 MPa, considering its low recovery efficiency and long recovery time, which would significantly affect the molding cycle of the next workpiece, it is directly vented and not recovered.

[0024] This graded recycling system can effectively improve the reuse efficiency of medium-pressure gas cylinders, reduce the frequent start-stop operations of the primary and secondary compressors, reduce equipment fatigue wear, and extend the overall service life of the system.

[0025] Preferably, the side-push core slides inward to the boundary of the mold cavity, fitting snugly with the upper and lower mold cores to form a closed mold cavity. After the gas expansion forming and quenching processes are completed, the side-push core can slide out in the opposite direction, facilitating the opening between the upper and lower mold cores and thus completing the demolding of the workpiece. A gas channel is provided through the side-push core, which can be connected to an external gas source mechanism to rapidly inject gas at a set pressure into the cavity of the tube blank during the forming process, achieving high-pressure gas expansion forming. This expansion quenching mold has a simple and reasonable structure and is easy to operate.

[0026] In addition, the tonnage corresponding to the pressure value required by the bulging quenching mold during the air expansion forming process, m, satisfies the following formula: m=n×(α×d×L×P / 9800+10); This facilitates the rapid estimation of the required pressure value during the mold design and equipment selection stages, thereby effectively ensuring the stability of the mold cavity closure and avoiding forming failure or mold damage due to insufficient clamping force. Attached Figure Description

[0027] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0028] Figure 1 This is a schematic diagram of the heating mechanism in a preferred embodiment of the present invention; Figure 2 This is a schematic diagram of the bulging quenching mold in a preferred embodiment of the present invention; Figure 3 This is a schematic diagram of the gas source mechanism in a preferred embodiment of the present invention; Figure 4 This is a schematic diagram of the hollow tube forming device in a preferred embodiment of the present invention; Figure 5 This is a flowchart of a hollow tube forming method according to a preferred embodiment of the present invention.

[0029] Explanation of reference numerals in the accompanying drawings: 100, pipe bending machine; 200, pipe bending hopper; 300, feeding mechanism; 400, preforming mechanism; 500, heating mechanism; 510, furnace body; 520, first conveyor roller conveyor; 530, feeding isolation chamber; 540, heating chamber; 550, discharging isolation chamber; 560, protective atmosphere system; 570, first isolation door; 580, second isolation door; 591, feeding furnace door; 592, discharging furnace door; 5110, tray; 600, gas source mechanism; 610, nitrogen. Gas storage tank; 620, primary compressor; 630, medium-pressure gas cylinder; 640, secondary compressor; 650, high-pressure gas cylinder; 660, recovery gas cylinder; 700, expansion quenching mold; 710, upper mold base; 720, upper mold core; 730, lower mold base; 740, lower mold core; 750, side-push core; 760, press; 800, unloading mechanism; 810, first lifting mechanism; 820, second lifting mechanism; 830, second conveyor roller conveyor; 840, third conveyor roller conveyor; w, hollow tube blank. Detailed Implementation

[0030] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0031] See Figures 1 to 3 As shown, one embodiment of the present invention provides a hollow tube forming device, which mainly includes three parts: a heating mechanism 500, a gas source mechanism 600, and an expansion quenching mold 700.

[0032] The heating mechanism 500 is used to heat the tube blank and includes a furnace body 510 and a first conveyor roller conveyor 520 disposed inside the furnace body 510. The first conveyor roller conveyor 520 is arranged along the length of the furnace body 510 and is used to convey the tray 5110 carrying the tube blank along this direction. Structurally, the furnace body 510 is provided with a feed isolation chamber 530, a heating chamber 540 and a discharge isolation chamber 550 in sequence from the feed end to the discharge end. The three chambers are structurally isolated to prevent atmosphere cross-flow. Each chamber is connected to a protective atmosphere system 560, which can introduce dry air or inert gas into the furnace body 510 to maintain the dew point temperature inside the furnace body 510 below -15°C.

[0033] This invention places a hollow tube blank on a tray 5110 and sequentially conveys it to the feed isolation chamber 530, heating chamber 540, and discharge isolation chamber 550 of a roller hearth furnace. Dry air or inert gas is introduced into each chamber. The protective gas in the feed isolation chamber 530 and discharge isolation chamber 550 forms an air wall, isolating the heating chamber 540 and effectively isolating it from external air moisture and oxygen, establishing a continuous and stable low dew point protective atmosphere. In particular, the introduction of protective gas into the heating chamber 540 allows the heating process to proceed in a dry atmosphere, maintaining the dew point temperature below -15°C, significantly inhibiting oxidation reactions and hydrogen permeation during heating, thus reducing the risk of hydrogen embrittlement at the source. After heating to the set high temperature, the material microstructure successfully achieves austenitization, providing the microstructure basis for subsequent quenching to form martensite.

[0034] The expansion quenching mold 700 includes a mold body and a cooling assembly disposed within the mold body. The mold body is used to perform air expansion forming of a hollow tube blank heated to the austenitizing temperature within its mold cavity. Its working principle is as follows: when the hot tube blank is placed into the mold cavity and the mold is closed, the air source mechanism 600 injects gas at a set pressure into the tube blank cavity, causing the tube blank to undergo plastic deformation along the shape of the mold cavity under the action of gas expansion force and adhere tightly to the mold cavity wall to complete the forming operation. During this process, the cooling assembly in the mold is simultaneously activated to rapidly cool the hollow tube fitting against the mold wall, promoting a rapid transformation of its material structure from austenite to martensite, achieving online quenching treatment. This cooling method significantly shortens the time path of traditional post-quenching after air expansion forming, reduces thermal distortion, and improves the dimensional accuracy and forming strength of the hollow tube fitting.

[0035] The gas source mechanism 600 is mainly used to input gas at a set pressure into the tube blank cavity contained in the expansion quenching mold 700 to achieve gas expansion forming of the tube blank. The gas source mechanism 600 may include: a nitrogen storage tank 610, a primary compressor 620, a medium-pressure gas cylinder 630, a secondary compressor 640, a high-pressure gas cylinder 650, and a recovery gas cylinder 660.

[0036] Nitrogen storage tank 610 is used to store dry nitrogen, providing a stable gas supply basis for subsequent gas sources. Nitrogen can be compressed by an external air compressor 760 and then enter the storage tank. A connecting pipeline is provided between nitrogen storage tank 610 and primary compressor 620.

[0037] The primary compressor 620 is connected to the nitrogen storage tank 610, the medium-pressure gas cylinder 630, and the recovery gas cylinder 660. Its main function is to compress the nitrogen in the nitrogen storage tank 610 or the residual gas in the recovery gas cylinder 660 into the medium-pressure gas cylinder 630, forming a medium-pressure gas supply source. The gas pressure after primary compression is suitable for control within the range of 42-45 MPa, which is applicable to the direct gas expansion molding of easily formable hollow tubes. When the workpiece to be molded is an easily formable hollow tube, gas is directly supplied through the medium-pressure gas cylinder 630 to the target molding pressure of 10-40 MPa.

[0038] The medium-pressure gas cylinder 630 is directly connected to the mold body, enabling direct injection of gas into the tube blank cavity for preforming or one-time forming operations. For hollow tubes with thin walls and small cross-sectional changes, the medium-pressure gas cylinder 630 can meet the forming pressure requirements.

[0039] In the forming operation of difficult-to-form workpieces, the medium-pressure gas cylinder 630 serves as a pre-filled gas source, with a secondary compressor 640 connected to its rear end. The secondary compressor 640 is used to further compress the medium-pressure gas output from the medium-pressure gas cylinder 630 to form high-pressure gas, which is then forced into the high-pressure gas cylinder 650.

[0040] The high-pressure gas cylinder 650 is also connected to the gas input channel of the mold body and is used to supplement the gas supply during the molding process that requires higher pressure support. Its output pressure can reach 88-98MPa, which is suitable for the gas expansion molding of difficult-to-form hollow tubes with complex cross sections, large wall thickness or long dimensions. When the workpiece to be molded is a difficult-to-form hollow tube, it is first pressurized to a pre-forming pressure of 10-40MPa by supplying gas through the medium-pressure gas cylinder 630, and then pressurized to the target forming pressure of 40-80MPa by supplying gas through the high-pressure gas cylinder 650.

[0041] The above-mentioned graded gas supply scheme realizes a multi-stage compression and gas storage distribution mechanism, which meets the forming pressure requirements under different working conditions; it avoids pressurizing all workpieces with the secondary booster equipment 640, thereby reducing the frequency of use of the secondary compressor 640 and improving the service life of the process system equipment.

[0042] Furthermore, when the pressure of the residual gas after molding exceeds 42 MPa, this high-pressure gas is preferentially recovered to the medium-pressure gas cylinder 630 for pre-filling of subsequent workpieces or direct molding. For medium-pressure gas between 5 MPa and 42 MPa, it is temporarily stored in the recovery gas cylinder 660, and then compressed again by the primary compressor 620 before flowing back to the medium-pressure gas cylinder 630. If the residual gas pressure is below 5 MPa, considering its low recovery efficiency and long time requirement, which would significantly affect the molding cycle of the next workpiece, it is directly vented and not recovered.

[0043] This graded recycling system can effectively improve the reuse efficiency of medium-pressure gas cylinder 630, reduce the frequent start-stop operations of primary compressor 620 and secondary compressor 640, reduce equipment fatigue wear, and extend the overall service life of the system.

[0044] like Figure 1 As shown, in this embodiment, the heating chamber 540 in the heating mechanism 500 is divided into a preheating zone, a heating zone and a heat preservation zone along the length of the furnace body 510. The three zones are arranged in sequence to form a temperature progression and heat preservation transition layout from the feeding end to the discharging end.

[0045] A first isolation door 570 is installed between the preheating zone and the feeding isolation chamber 530 to seal off and isolate the atmosphere exchange between the feeding section and the heating section; a second isolation door 580 is installed between the insulation zone and the discharge isolation chamber 550, similarly used to block gas convection between the discharge side and the insulation zone. These isolation doors open and close rapidly after the workpiece passes through, thereby maintaining the stability of the atmosphere inside the heating chamber 540, reducing oxygen and water vapor content, and further facilitating dew point temperature control.

[0046] In addition, the feeding isolation chamber 530 is equipped with a feeding furnace door 591, and the discharging isolation chamber 550 is equipped with a discharging furnace door 592, for controlling the entry and exit of pipe fittings. The furnace doors open and close vertically, cooperating with the first conveying roller conveyor 520 inside the furnace body 510, enabling them to open and close quickly in a short time during the entry and exit of the billet, minimizing the leakage of protective gas, helping to maintain the air wall effect between the isolation chamber and the heating chamber 540, and enhancing the atmosphere isolation effect.

[0047] See Figure 1 As shown in this embodiment of the invention, in order to realize the recycling and reuse of the tray in the roller hearth furnace, the heating mechanism 500 is additionally provided with a second conveying roller 830. The second conveying roller 830 is disposed above the furnace body 510 and is arranged parallel to the first conveying roller 520. The conveying direction of the second conveying roller 830 is opposite to that of the first conveying roller 520.

[0048] Furthermore, a first lifting mechanism 810 and a second lifting mechanism 820 are respectively provided on the output and input sides of the first conveyor roller 520. The first lifting mechanism 810 is used to transfer empty pallets after the heating and conveying task is completed from the first conveyor roller 520 to the second conveyor roller 830 above it; the second lifting mechanism 820 is used to transfer the pallets on the second conveyor roller 830 back to the input end of the first conveyor roller 520, realizing the recycling and reuse of pallets. The above-mentioned lifting mechanisms can be electric lifting platforms or cylinder guide rail lifting devices, which have mature structures and stable operation.

[0049] During operation, the first conveyor roller 520 carries the tray carrying the hollow tube blank from the input end through the feeding furnace door 591, the feeding isolation chamber 530, the heating chamber 540, and the discharging isolation chamber 550, until it reaches the output end. After heating is completed, the unloading mechanism or robotic arm at the output end removes the heated hollow tube blank from the tray, at which point the tray is empty. The empty tray is then lifted to the upper second conveyor roller 830 by the first lifting mechanism 810, transported by the second conveyor roller 830 to the corresponding side of the input end of the first conveyor roller 520, and then vertically lowered back into the input end of the first conveyor roller 520 by the second lifting mechanism 820, thus completing the closed-loop circulation of the tray.

[0050] Through the coordinated operation of the above structures, the pallet can circulate between the upper and lower conveyor rollers, avoiding the cycle time loss caused by frequent pallet replacement or manual handling, realizing the automated recycling of pallets in the roller hearth furnace, and improving production efficiency.

[0051] Furthermore, the heating mechanism 500 also includes a third conveying roller conveyor 840, which is arranged parallel above the second conveying roller conveyor 830 and its conveying direction is opposite to that of the first conveying roller conveyor 520; the first lifting mechanism 810 is also used to transfer the empty pallet on the first conveying roller conveyor 520 to the third conveying roller conveyor 840.

[0052] In this embodiment, it is considered that the molding and processing of different types of hollow tubes often requires matching with pallets of different structures. If the heating device needs to be interrupted every time the pallet is changed, it will inevitably affect the production cycle and the continuity of the production line. Therefore, an independent third conveyor roller conveyor 840 is added above the second conveyor roller conveyor 830 for the temporary storage of old pallets, so as to realize the online replacement of pallets.

[0053] Specifically, after a batch of billets completes its heating and conveying process, the old pallets are lifted by the first lifting mechanism 810 and placed sequentially on the third conveyor roller conveyor 840. During this process, operators or the automatic system can simultaneously add new pallets matching the next batch of billets sequentially through the feed furnace door 591 at the input end of the first conveyor roller conveyor 520, until all old pallets are lifted to the third conveyor roller conveyor 840 and all new pallets are in place. This completes the switching between old and new pallets without interrupting the operation of the roller hearth furnace, ensuring continuous operation of the heating mechanism 500 and improving production efficiency.

[0054] See Figure 2As shown, in an embodiment of the present invention, the bulging quenching mold 700 includes an upper mold base 710, an upper mold core 720, a lower mold base 730, a lower mold core 740, and a side-push core 750. The upper mold base 710 is connected to a press 760, the upper mold core 720 is fixedly installed on the upper mold base 710, and the lower mold core 740 is fixedly installed on the lower mold base 730; a side-push core 750 that can move radially is respectively provided on both sides of the upper mold core 720 and the lower mold core 740, and multiple components cooperate with each other to form a mold cavity space for forming a hollow tube blank w.

[0055] To achieve sealing during the air-expansion molding process and subsequent demolding, the side-push core 750 can slide in the radial direction: after the mold is closed, the side-push core 750 slides inward to the boundary of the mold cavity, and fits with the upper and lower mold cores 740 to form a closed mold cavity; after the air-expansion molding and quenching process is completed, the side-push core 750 can slide out in the opposite direction, which facilitates the opening of the upper mold core 720 and the lower mold core 740, thereby completing the demolding of the workpiece.

[0056] In addition, in order to allow high-pressure gas to be smoothly injected into the inner cavity of the tube blank, a gas channel is provided through one of the side push cores 750. This gas channel can be connected to an external gas source mechanism 600 to quickly inject gas at a set pressure into the inner cavity of the tube blank during the molding process, thereby achieving high-pressure gas expansion molding.

[0057] Furthermore, the cooling mechanism includes cooling water pipes respectively disposed inside the upper mold core 720 and the lower mold core 740. The cooling water pipes are arranged along a preset cooling path inside the mold core, and can introduce cooling water flow during the air expansion molding process to achieve rapid cooling of the mold cavity wall and the hollow tube blank attached to its surface.

[0058] In practical applications, after the high-temperature tube blank is conveyed into the expansion and quenching mold 700 and the mold cavity is closed, the control system can activate the cooling system, allowing cooling water to flow rapidly through the cooling water pipes. Because the pipes are close to the mold cavity surface, the cooling water carries away heat from the mold cavity wall, causing the hollow tube fittings attached to its surface to cool down rapidly as well. This achieves simultaneous quenching during the expansion process, promoting a rapid transformation of the tube blank material's microstructure from austenite to martensite, effectively improving the strength of the hollow tube fittings. Simultaneously, the internal integrated cooling method avoids temperature fluctuations caused by external spraying or other structures.

[0059] See Figure 4As shown in other embodiments, since some hollow tubes have complex structures, such as bending structures or irregular cross-sectional shapes, pre-bending and cross-sectional shaping are required during the overall forming process. Therefore, the hollow tube forming device can also be equipped with a tube bending machine 100 and a tube bending chute 200, or even a pre-forming mechanism 400, depending on the actual situation. For convenient loading and unloading, a loading mechanism 300 and a unloading mechanism 800 can also be provided.

[0060] The feeding mechanism 300, pre-forming mechanism 400, heating mechanism 500, expansion and quenching mold 700, and unloading mechanism 800 are arranged horizontally in sequence, which facilitates the linkage between the mechanisms and thus achieves stable and efficient operation of the production line. To reduce the transportation distance of high-pressure gas, the gas source mechanism 600 is placed close to the expansion and quenching mold 700. Since the bending cycle is much lower than the stamping cycle, it is difficult to achieve a balance between the supply and consumption of bent pipes if directly connected. This production line has a bending pipe storage unit 200 to pre-store bent pipe parts to compensate for the cycle time difference. The bending machine 100 and the bending pipe storage unit 200 can be arranged relatively independently to realize the processing and storage of bent pipes.

[0061] See Figures 1 to 3 As shown, another aspect of this invention provides a method for forming hollow tubes, comprising the following steps: Step 1): The hollow tube blank is placed on a tray and sequentially conveyed to the feed isolation chamber 530, heating chamber 540, and discharge isolation chamber 550 of the roller hearth furnace via the first conveyor roller 520. The feed isolation chamber 530, heating chamber 540, and discharge isolation chamber 550 are each connected to a protective atmosphere system 560, through which dry air or inert gas is introduced to establish a stable protective atmosphere environment within the furnace. During heating, the protective gases in the feed isolation chamber 530 and discharge isolation chamber 550 form "gas walls" at their interfaces with the heating chamber 540, effectively isolating the high-temperature environment inside the heating chamber 540 and preventing the intrusion of external moisture or oxygen. Under the action of the protective gas in the heating chamber 540, the dew point temperature inside the roller hearth furnace is stably maintained below -15°C, thereby suppressing hydrogen absorption on the surface of the tube blank at high temperatures, reducing the risk of hydrogen embrittlement, and ensuring the stability of heating quality.

[0062] Meanwhile, heating chamber 540 can heat the entire tube blank to a set high-temperature range, which is 850°C to 950°C, for 3 to 8 minutes, ensuring that the material microstructure fully completes the austenitic transformation, laying the microstructure foundation for subsequent gas expansion forming and quenching treatment. It should be noted that dew point temperature is positively correlated with air humidity; controlling the dew point temperature controls the atmospheric humidity, ensuring the air is sufficiently dry, reducing moisture content, and thus effectively suppressing the risk of hydrogen embrittlement.

[0063] The heating time needs to be set reasonably based on the material type, wall thickness, and target heating temperature of the hollow tube blank to ensure that the tube blank material can be fully austenitized. The following is Table 1, showing the preferred heating times: Table 1

[0064] In Table 1 above, "bare plate" refers to steel plate material without coating treatment; "Al-Si coated plate" mainly refers to hot-formed steel plate with aluminum-silicon coating on the surface, and the influence of its melting point characteristics on the matching of heating temperature and time needs to be considered. Using the above parameters to set the heating time can effectively ensure the degree of austenitization of the material structure, while avoiding the melting loss of Al-Si coating.

[0065] Step 2): The hollow tube blank, under high temperature, is rapidly transferred by a robotic arm to the expansion and quenching mold 700, and the mold is closed. The entire transfer process is controlled within 5-12 seconds to reduce heat loss. With the mold fully closed, high-pressure gas (nitrogen) of 5-90 MPa is injected into the tube blank cavity through the medium-pressure gas cylinder 630 and / or high-pressure gas cylinder 650 in the gas source mechanism 600. The gas expansion force causes the tube blank to expand radially and fit tightly against the inner wall of the mold cavity, completing the air expansion forming operation. During the air expansion forming process, the cooling water circulation in the cooling water pipes inside the upper and lower mold cores is rapidly activated. The cooling water flows at high speed in the pipes, quickly removing heat from the mold cavity wall, causing the hollow tube that has been fitted against the mold cavity wall to cool down rapidly and complete the quenching treatment.

[0066] The tube blank undergoes rapid cooling during gas expansion forming, enabling its material microstructure to quickly transform from austenite to martensite, thereby improving the strength and dimensional stability of the hollow tube fitting. This cooling method significantly shortens the time path of post-quenching in traditional gas expansion forming, reduces thermal distortion, and improves the dimensional accuracy and forming strength of the hollow tube fitting.

[0067] In embodiments of the present invention, to ensure stable and reliable air expansion forming of the hollow tube blank in the bulging and quenching mold 700, the pressure provided by the mold during the forming process needs to be reasonably set according to the structural dimensions and process requirements of the formed part. Therefore, the tonnage m corresponding to the pressure value required by the bulging and quenching mold 700 during the air expansion forming process satisfies the following formula: m = n × (α × d × L × P / 9800 + 10); Where m represents the tonnage corresponding to the pressure value required by the expansion quenching mold 700, in tons (t); n is the number of mold cavities, i.e., the number of workpieces accommodated by the mold in the same forming process; d is the outer diameter (mm) of the hollow tube blank to be formed; L is the axial length (mm) of the hollow tube; P is the internal gas pressure (MPa) required for gas expansion forming, which is usually selected according to the complexity of the workpiece and the yield characteristics of the material; α is the safety factor, selected in the range of 1.0 to 1.4, used to compensate for the load margin of the system under the fluctuation of working conditions; 9800 is the unit conversion constant, which converts the pressure into the unit of mass force; the constant term 10 is a fixed offset value, used to consider the mechanical clamping force and structural stiffness requirements when the mold is opened / closed.

[0068] This calculation formula, combined with specific molding conditions and workpiece dimensional characteristics, facilitates rapid estimation of the required pressure value during mold design and equipment selection. It should be noted that in practical applications, this pressure m is typically provided by the hydraulic press 760 connected to the mold, and therefore can also be considered as the minimum pressure reference that the press 760 must apply during the molding process. This solution effectively ensures the stability of the mold cavity closure, preventing molding failure or mold damage due to insufficient clamping force.

[0069] In this embodiment, to ensure that the hollow tube blank achieves the desired microstructure transformation after forming in the expansion and quenching mold 700, rapid quenching is required simultaneously with the gas expansion forming. Therefore, this invention sets a quenching and holding time T that conforms to the material phase transformation law based on the wall thickness of the hollow tube and the internal gas pressure level during quenching and holding, ensuring that the cooling rate reaches or exceeds 27°C / s, thereby promoting a full transformation of the material microstructure from austenite to martensite.

[0070] Specifically, the quenching holding time T (unit: seconds) can be estimated using the following formula:

[0071] Where h is the wall thickness of the hollow tube (in mm), and P is the air pressure inside the tube during quenching and pressure holding (in MPa).

[0072] The pressure holding time T calculated using this piecewise function formula, combined with the design of an efficient cooling water circuit, can effectively control the material cooling rate to be no less than 27°C / s, thereby improving the strength and hardness of the molded parts, reducing incomplete martensitic transformation or uneven microstructure, and ensuring the molding quality of hollow tubes.

[0073] In this embodiment of the invention, a medium-pressure gas cylinder 630 and a high-pressure gas cylinder 650 are used to provide molding gases at different pressure levels to the hollow tube blank, in order to adapt to the different requirements of molding gas pressure for different workpiece structures. Specifically, for easily moldable workpieces with simple structures, small wall thicknesses, and gradual changes in cross-sectional shape, the tube blank can be directly filled with gas through the medium-pressure gas cylinder 630 to reach the target molding pressure of 10-40 MPa to complete the gas expansion molding operation.

[0074] For complex workpieces with large abrupt changes in cross-section, large wall thickness, or high molding deformation resistance, a segmented pressurization method is adopted. First, the medium-pressure gas cylinder 630 provides a pre-forming gas pressure of 10-40 MPa to complete the initial bonding and pre-forming of the hollow tube blank. Then, the high-pressure gas cylinder 650 continues to pressurize to the target molding pressure of 40-80 MPa to ensure that the tube can fully fit the mold cavity.

[0075] Among them, the medium-pressure gas cylinder 630 has a gas storage pressure range of 42-45 MPa, which can stably output the gas pressure required for preforming or direct forming; the high-pressure gas cylinder 650 has a gas storage pressure range of 88-98 MPa, which is suitable for providing subsequent high-pressure supplementation to support the entire gas expansion process of difficult-to-form workpieces. Through the above-mentioned differentiated gas supply methods, not only can the adaptability requirements of easy / difficult-to-form workpieces be met, but the operating conditions of the gas source equipment are also improved, and the service life of equipment such as compressors and cylinders is extended.

[0076] See Figure 4 and Figure 5 As shown, in other embodiments of the present invention, the hollow tube forming method includes a pre-forming process before step 1) above. This process performs necessary pretreatment for hollow tubes of different shapes and complex structures, such as bending and cross-sectional shaping, and may also include loading and unloading processes. Specifically, it includes the following steps: Step a): Pipe bending. A CNC pipe bending machine 100 is used to bend the straight pipe blank to form a prefabricated bent pipe structure with the required curvature. The axis of the pipe after bending is controlled to be close to the axis of the final product, and the maximum deviation is required not to exceed 2mm. Since the bending process is relatively slow, in order to alleviate the conflict with the main production line, the bent pipe is temporarily stored in the bending silo 200 to ensure the continuous supply of materials to the main production line.

[0077] Step b): Loading. The bent pipe fittings are transported to the loading mechanism 300 by AGV unmanned vehicles or forklifts for positioning and attitude adjustment to ensure that the pipe fittings are in a uniform clamping state and process benchmark in the subsequent processing stage.

[0078] Step c): Pre-forming. For hollow tubes requiring irregular cross-sections, a pre-forming stamping die is used to cold-stamp the tubes, initially achieving the cross-sectional shape and providing a good forming foundation for subsequent air expansion forming.

[0079] Step 1): Heating. The pre-formed hollow tube is picked up by a robotic arm from the feeding device and placed into a roller hearth furnace, passing sequentially through the feeding isolation chamber 530, the heating chamber 540, and the discharging isolation chamber 550. A protective atmosphere is established by introducing dry air or inert gas into each chamber, ensuring the dew point temperature inside the furnace is controlled below -15°C, thereby inhibiting hydrogen adsorption on the material surface and reducing the risk of hydrogen embrittlement. The tube is heated to a set temperature (preferably 850–950°C) in the heating chamber 540, causing its material microstructure to austenitize, providing the metallographic basis required for subsequent forming and quenching. More specific details are as described above and will not be repeated here.

[0080] Step 2): Air expansion forming and simultaneous quenching. The heated fitting is rapidly transferred to the air expansion and quenching mold 700 within 5-12 seconds to complete mold closure. Depending on the complexity of the workpiece, a medium-pressure gas cylinder 630 and / or a high-pressure gas cylinder 650 are selected to inflate the fitting cavity, causing its outer wall to fit against the mold cavity to achieve air expansion forming. The mold is equipped with cooling channels, which perform cooling operations simultaneously with forming, causing the material microstructure to rapidly transform into martensite, completing in-mold quenching. More specific details are as described above and will not be repeated here.

[0081] Step d): Unloading. After the formed and quenched pipe fittings are molded, they are taken out by the unloading mechanism 800 or a robot, and then proceed to the next process or be directly shipped out of the warehouse.

[0082] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A hollow tube forming device, characterized in that: include A heating mechanism for heating tube blanks includes a furnace body and a first conveying roller conveyor. The first conveying roller conveyor is arranged inside the furnace body along its length and conveys a tray carrying the tube blank along this direction. The furnace body is provided with a feeding isolation chamber, a heating chamber and a discharging isolation chamber in sequence along its length. The three chambers are isolated from each other and are respectively connected to a protective atmosphere system for introducing protective gas into the furnace body to maintain the dew point temperature inside the furnace body below -15°C. An expansion quenching mold includes a mold body and a cooling assembly disposed within the mold body. The mold body is used to perform air expansion forming on a tube blank heated by the heating mechanism within its mold cavity to form a hollow tube. The cooling assembly is used to simultaneously initiate cooling after the hollow tube is fitted into the mold cavity, thereby achieving quenching treatment during the air expansion forming process. The gas source mechanism is used to input gas at a set pressure into the tube blank cavity contained in the mold body, so that the tube blank will expand and deform within the mold body and be formed.

2. The hollow tube forming device according to claim 1, characterized in that: The heating mechanism further includes a second conveying roller conveyor, which is disposed above the furnace body and arranged parallel to the first conveying roller conveyor, and its conveying direction is opposite to that of the first conveying roller conveyor. The output end and the input end of the first conveyor roller are respectively provided with a first lifting mechanism and a second lifting mechanism. The first lifting mechanism is used to retrieve the empty pallet on the first conveyor roller to the second conveyor roller, and the second lifting mechanism is used to cyclically transfer the pallet on the second conveyor roller to the first conveyor roller.

3. The hollow tube forming device according to claim 2, characterized in that: The heating mechanism further includes a third conveying roller conveyor, which is arranged parallel above the second conveying roller conveyor and its conveying direction is opposite to that of the first conveying roller conveyor. The first lifting mechanism is also used to transfer empty pallets on the first conveyor rollers to the third conveyor rollers.

4. The hollow tube forming device according to claim 1, characterized in that: The heating chamber includes a preheating zone, a heating zone, and a heat preservation zone arranged sequentially along the length of the furnace body. Isolation doors are respectively provided between the preheating zone and the feeding isolation chamber, and between the heat preservation zone and the discharging isolation chamber; and / or, the feeding isolation chamber is provided with a feeding furnace door, and the discharging isolation chamber is provided with a discharging furnace door.

5. The hollow tube forming apparatus according to claim 1, characterized in that: The gas source mechanism includes a nitrogen storage tank, a primary compressor, a medium-pressure gas cylinder, a secondary compressor, a high-pressure gas cylinder, and a recovery gas cylinder; the primary compressor is connected to the nitrogen storage tank, the recovery gas cylinder, and the medium-pressure gas cylinder respectively; the secondary compressor is connected to the medium-pressure gas cylinder and the high-pressure gas cylinder respectively; the medium-pressure gas cylinder and the high-pressure gas cylinder are used to input gas at a set pressure into the tube blank cavity contained in the mold body.

6. A method for forming hollow tube fittings, characterized in that: Includes the following steps: Step 1): The hollow tube blank is placed on a tray and sequentially conveyed to the feed isolation chamber, heating chamber, and discharge isolation chamber of the roller hearth furnace. Protective gases are introduced into the feed isolation chamber, heating chamber, and discharge isolation chamber respectively. The heating chamber heats the tube blank to a set high temperature to austenitize its material structure. The protective gases in the feed isolation chamber and discharge isolation chamber form an air wall, which isolates the heating chamber. During the heating process, the protective gas in the heating chamber maintains the dew point temperature in the roller hearth furnace below -15°C. Step 2): Quickly transfer the heated tube blank to the expansion quenching mold and close the mold; inject high-pressure gas at a set pressure into the cavity of the tube blank, and under its expansion, make the outer wall of the tube blank fit against the surface of the mold cavity to achieve air expansion forming; During the air-expansion forming process, the tube blank is simultaneously cooled by the cooling components set in the mold, causing the material structure to martensitize and completing the quenching treatment.

7. The hollow tube forming method according to claim 6, characterized in that: In step 1), the tube blank is heated to 850–950°C for 3–8 minutes to austenitize its material structure.

8. The hollow tube forming method according to claim 6, characterized in that: The tonnage m corresponding to the pressure value required by the bulging quenching die during the forming process satisfies the following formula: m = n × (α × d × L × P / 9800 + 10), Where n is the number of mold cavities; d is the diameter of the hollow tube blank in mm; L is the axial length of the hollow tube in mm; α is the safety factor, ranging from 1.0 to 1.4; P is the gas pressure required for tube blank forming in MPa; and m is the tonnage corresponding to the value required by the expansion quenching mold in tons.

9. The hollow tube forming method according to claim 6, characterized in that: During the expansion quenching process, the quenching holding time T satisfies the following relationship: ， Where T is the quenching and holding time in seconds; h is the pipe wall thickness in mm; and P is the internal air pressure of the hollow pipe during the holding time in MPa.

10. The hollow tube forming method according to claim 6, characterized in that: Step 2) involves supplying gas to pressurize the workpiece using a medium-pressure gas cylinder and / or a high-pressure gas cylinder. Specifically, this involves: When the workpiece to be formed is an easy-to-form workpiece, gas is directly supplied through a medium-pressure gas cylinder to the target forming pressure of 10-40MPa. When the workpiece to be formed is a difficult-to-form workpiece, it is first pressurized to a pre-forming pressure of 10-40 MPa by supplying gas from a medium-pressure gas cylinder, and then pressurized to the target forming pressure of 40-80 MPa by supplying gas from a high-pressure gas cylinder.

Images