A two-step upset tube end forming method and apparatus with heating assistance

Abstract

The application relates to the technical field of upsetting processes, in particular to a two-step upsetting pipe end forming method and device with heating assistance, wherein the method comprises the following steps: S1, clamping a pipe workpiece in an upsetting die to perform first-pass upsetting to make the workpiece produce plastic deformation; S2, taking out the workpiece subjected to the first-pass upsetting from the upsetting die, heating the deformed area of the workpiece to a predetermined temperature of 1000 DEG C to 1100 DEG C within 2 to 5 seconds, and then cooling to room temperature; and S3, clamping the cooled workpiece in the upsetting die again to perform second-pass upsetting, so that the workpiece reaches a preset final shape and size. The process can significantly reduce the main strain during final forming from a high level of one-time forming to a safe range, effectively inhibits the initiation of macroscopic cracks and microcracks at inner and outer corners, and solves the cracking problem caused by plastic cumulative damage in the room-temperature large-angle bending forming of small-size stainless steel pipe joints.

Description

Technical Field

[0001] This application relates to the technical field of upsetting processes, and in particular to a two-step upsetting tube end forming method and apparatus using heating assistance. Background Technology

[0002] Upsetting pipe ends is one of the core processes in pipe manufacturing, forming flanges, joints, sockets, and other connecting components and functional parts at the ends of pipes through axial compression. With the rapid development of fuel cell systems for new energy vehicles, higher requirements have been placed on the strength, pressure resistance, and sealing of pipe connections. Pipe joint structures are becoming increasingly compact, with more drastic changes in material wall thickness and larger bending angles during the forming process. For small-sized, thin-walled pipe joints (outer diameter 6-26 mm, wall thickness 1-4 mm), represented by 316L stainless steel, large-angle bending deformation occurs at the pipe ends during room temperature upsetting. Material flow is difficult to control, leading to concentrated and severe strain at the inner and outer corners of the pipe. When the strain exceeds the material's forming limit, forming defects such as wrinkling and microcracks occur. These defects are highly susceptible to premature fatigue cracking under cyclic loading, severely affecting the service life and reliability of parts. How to improve the forming quality of small-sized, complex stainless steel pipe joints during room temperature large-angle bending without significantly increasing process complexity is a pressing technical challenge in this field.

[0003] To address the aforementioned problems, various improvement solutions have been proposed in the prior art. For example, patent document CN111531103A discloses an induction heating upsetting device for aluminum alloy cylinders, including an upsetting support ring, an induction coil, and a forming mold, which heats the aluminum alloy cylinder through the induction coil and performs a one-time upsetting. Patent document CN113510212A discloses an upsetting process for the ends of titanium alloy drill pipes, which connects a heating furnace and an upsetting mold to form a sealed space and fills it with CO2 as a protective atmosphere, performing a one-time upsetting after holding at 720℃ for 2 minutes and 1010℃ for 3 minutes. Patent document CN117358868A discloses a heating upsetting device and method for pipe ends, which uses a heating furnace to heat the pipe once, an induction heating coil group to heat the pipe a second time, and after the second heating is completed, directly feeds it into the upsetting mechanism for a one-time upsetting. In addition, multi-step forming processes have also been reported. For example, patent document CN121178764A discloses a forming method for an ultra-large flange hollow part. In the pre-forging stage, the upper end of the tubular billet is extruded to obtain a pre-upset head, and then further upsetting is performed in the final forging stage. Patent document CN114985655A discloses a short-process thickening method for steel pipe ends using two-pass upsetting. After the steel pipe is heated in the first and second heating furnaces, it enters the third heating furnace for uniform temperature treatment, is sent to the first die cavity to complete the first upsetting, and then returns to the third heating furnace for supplementary heating and uniform temperature treatment, and is sent to the second die cavity to complete the second upsetting. Both upsetting processes are completed at high temperatures.

[0004] However, the aforementioned existing technologies still have the following drawbacks: First, while thermoforming solutions (such as CN111531103A, CN113510212A, and CN117358868A) can improve the forming performance of materials, they require the workpiece to be kept at high temperatures for extended periods, resulting in high energy consumption. Furthermore, they necessitate protective atmospheres and other isolation measures to prevent oxidation. Sustained high temperatures also pose a risk of inducing grain coarsening and unfavorable phase transformations, affecting the final product performance. Second, multi-step thermoforming solutions (such as CN114985655A) require switching between multiple mold cavities to complete two upsetting passes. This results in complex mold structures, high equipment requirements, and the need to maintain high-temperature thermoforming throughout the process, leading to lengthy procedures and limited production cycles. Third, in existing cold upsetting processes, the material accumulates significant plastic damage during a single deformation. The strain concentration area is prone to exceeding the forming limit, causing cracks. The conventional method of performing a single overall heat treatment after multiple cold working passes cannot promptly repair the plastic damage incurred between processes to support subsequent deformation. Summary of the Invention

[0005] This application provides a two-step upsetting tube end forming method and apparatus with heating assistance, which can effectively reduce cracking caused by plastic cumulative damage without increasing mold complexity or introducing problems associated with high-temperature thermoforming.

[0006] In a first aspect, this application provides a two-step upsetting method for forming rough tube ends using heat assistance, which employs the following technical solution: A two-step upsetting method for forming the rough tube end with heating assistance includes the following steps: S1: The pipe workpiece is clamped in the upsetting die and the first upsetting is performed to cause plastic deformation of the workpiece; S2: The workpiece that has undergone the first upsetting process is removed from the upsetting mold, and the deformed area of ​​the workpiece is heated to a predetermined temperature of 1000℃~1100℃ within 2~5 seconds and then cooled to room temperature. S3: After cooling, the workpiece is clamped again in the upsetting mold for a second upsetting process, so that the workpiece reaches the preset final shape and size.

[0007] By employing the above technical solution, the first upsetting pass induces the predetermined plastic deformation in the workpiece, decomposing the total deformation required for a single forming process into two parts, thus minimizing strain concentration exceeding the material's forming limit during a single large deformation. After the first upsetting pass, plastic damage and residual stress accumulate inside the workpiece, reducing its formability. At this point, by heating the deformation area to 1000℃~1100℃ in an extremely short time (2~5 seconds), the skin effect of heating rapidly raises the temperature of the deformation area above the recrystallization temperature, causing recrystallization within the material. This significantly reduces dislocation density, repairs plastic damage, eliminates residual stress, and restores the material's formability to near its initial state. The extremely short heating time of 2~5 seconds minimizes the workpiece's residence time in the sensitization temperature range, reducing the risk of intergranular corrosion caused by chromium carbide precipitation along grain boundaries, while also inhibiting grain growth and maintaining a fine-grained structure. After cooling to room temperature, the second upsetting pass is performed. At this point, the material has recovered to a highly plastic state and can withstand the second deformation without cracking. The synergistic effect of the two-step upsetting process significantly reduces the principal strain during final forming from the higher level of single-step forming to a safe range, effectively suppressing the initiation of macroscopic and micro-cracks at the inner and outer corners. While macroscopic cracks are commonly found at the inner corners of single-step cold-formed samples, this process only produces micro-cracks, and the crack length decreases significantly with increasing temperature. Furthermore, this scheme uses the same set of molds for both upsetting passes, eliminating the need to switch mold cavities and minimizing positioning errors and time losses caused by mold changes. It achieves design standards without subsequent cutting or grinding finishing processes, resulting in a simple process with controllable costs. Compared to single-step forming, this process effectively suppresses the initiation of macroscopic cracks at the inner corners, significantly reducing or even eliminating crack length, while requiring no changes to the mold structure, using the same mold for both passes, and eliminating the need for subsequent machining adjustments.

[0008] Because the first upsetting pass is performed at room temperature rather than under heat, the formation of oxide scale and thermal shock to the mold in a high-temperature environment are minimized, eliminating the need for isolation measures such as atmosphere protection. Between the two upsetting passes, the workpiece is removed from the mold and heated, decoupling the heating and forming processes spatially and temporally. The heating process does not occupy the mold station and does not affect the mold state between the first and second passes. The mold can cool naturally or be used for processing other workpieces during heating, improving equipment utilization. Furthermore, this solution uses the same set of molds for both upsetting passes, eliminating the need to switch between multiple mold cavities, minimizing positioning errors and time losses caused by mold changes, and achieving design standards without subsequent cutting or grinding finishing processes.

[0009] Optionally, in step S2, induction heating is used to heat the deformed area of ​​the workpiece.

[0010] By adopting the above technical solution, induction heating directly generates heat inside the workpiece using the principle of electromagnetic induction. It boasts high heating efficiency and speed, capable of heating the deformed area of ​​small-sized tubular parts to 1000℃~1100℃ within 2~5 seconds, making it a specific technical means to achieve rapid temperature rise. Compared to resistance heating or furnace heating, induction heating allows for precise control of the heating area. By adjusting the shape and position of the coil, only the deformed parts of the workpiece are locally heated, while non-deformed areas remain at a lower temperature due to being fully enclosed and clamped by the mold, minimizing energy waste and heat-affected zone expansion caused by overall heating. Induction heating requires no preheating time; the target temperature is quickly reached after power-on. Combined with a 2~5 second heating time, the entire intermediate heat treatment process is completed in a very short time, without significantly extending the production cycle. Furthermore, the compact structure of induction heating equipment allows for integration with upsetting dies, facilitating compact design and automation of production lines. Induction heating eliminates the need for sealed connections or atmosphere protection devices between the heating furnace and the upsetting die, resulting in simple equipment configuration and convenient operation and maintenance.

[0011] Optionally, in step S2, the heating time is 3 seconds.

[0012] By employing the above technical solution, the extremely short heating time of 3 seconds allows the deformed area of ​​the workpiece to cross the sensitization temperature range of 450℃~850℃ in a very short time. Carbon atoms do not have enough time to diffuse extensively towards the grain boundaries to form chromium carbide, thus maintaining the chromium content near the grain boundaries at the level required for corrosion resistance. This effectively minimizes the risk of intergranular chromium depletion and intergranular corrosion, an effect difficult to achieve in long-term heating or holding processes. Simultaneously, the extremely short heating time prevents recrystallized grains from growing large enough after nucleation, maintaining the average grain size within the fine-grained range after heating. This minimizes the grain coarsening phenomenon commonly seen in traditional long-term heat treatment, ensuring that the material's strength, plasticity, and corrosion resistance are not degraded by heat treatment. The 3-second heating time combined with the 1000℃~1100℃ heating temperature works synergistically to achieve sufficient recrystallization to repair plastic damage while maintaining a fine-grained structure, achieving a comprehensive recovery of material properties rather than an improvement in a single indicator. From a production perspective, the 3-second heating time is compatible with the cycle time requirements of automated production lines, ensuring that the intermediate heat treatment process does not become a bottleneck restricting the overall production efficiency. Compared with traditional heat treatment methods that require tens of minutes to several hours of heat preservation, it has a significant efficiency advantage.

[0013] Optionally, in step S1, the down pressure of the first upsetting pass is 75% to 80% of the total down pressure.

[0014] By adopting the above technical solution, the determination of the compression range is based on the optimized analysis of the strain distribution in the two-step forming process. Controlling the compression in the first step within 75% to 80% of the total compression ensures that the equivalent strain generated by the first deformation is within the safe range for room temperature forming of the material, preventing cracks from initiating during the pre-upsetting stage. Simultaneously, this compression allows for sufficient plastic deformation and dislocation accumulation in the workpiece, providing adequate recrystallization driving force for subsequent induction heating repair. If the deformation is too small, the dislocation density will be insufficient, the recrystallization process will be incomplete, and the material repair effect will be limited. This range also ensures that the remaining compression in the second forming step (20% to 25% of the total compression) is sufficiently small, ensuring that the strain experienced by the material after induction heating repair in the second deformation step is below its forming limit, thus preventing new cracks. The 75% to 80% compression range, combined with the intermediate induction heating repair characteristics, achieves an optimal distribution of deformation between the two steps. Each step's deformation is within the safe range of the material's corresponding state, fully utilizing the material's plastic deformation capacity while minimizing defects caused by excessive deformation.

[0015] Optionally, the down pressure of the first upsetting pass is 77% of the total down pressure.

[0016] By adopting the above technical solution, 77% is the optimal compression ratio determined through simulation optimization. At this compression ratio, the strain and deformation distribution generated in the first deformation step are most reasonable, and the pre-formed workpiece shape is conducive to material flow in the second forming step. The 77% pre-deformation amount allows sufficient and uniform dislocation density to accumulate inside the workpiece, providing a uniform nucleation driving force for recrystallization during induction heating, resulting in uniform grain size and good microstructure consistency after recrystallization. The remaining 23% of the second-pass deformation amount is within the load-bearing capacity range of the repaired material, ensuring smooth material flow during the second forming step and preventing strain concentration at the inner and outer corners. The optimal value of 77%, together with 3-second induction heating and a heating temperature of 1000℃~1100℃, constitutes a complete optimal combination of process parameters. Samples using this parameter combination show no cracks at the inner corners.

[0017] Optionally, the first upsetting pass and the second upsetting pass are completed using the same set of upsetting dies.

[0018] By adopting the above technical solution, two upsetting passes are completed using the same set of molds, minimizing the complex operations and mold costs associated with switching between multiple mold cavities in multi-step forming schemes. The entire forming process can be completed in a single mold assembly, eliminating the need to change or adjust the mold between passes, thus eliminating secondary positioning errors caused by mold switching and ensuring the coaxiality and consistency of the deformation between the two passes. Using the same mold also means that the cavity contours of the two upsetting passes are completely consistent. The shape of the workpiece after the first pre-forming step matches the cavity contour, providing a good positioning basis for the second forming step and minimizing defects such as uneven wall thickness or burrs caused by differences in mold cavities. The collaborative design of the process where the workpiece needs to be removed from the mold between the two passes for heating, cooling, and then reinstalled ensures that, while guaranteeing the feasibility of intermediate heat treatment, the mold structure remains simple, equipment investment costs are controllable, and the process is highly operable. Compared to the solution that requires switching between the first and second mold cavities within the press, this eliminates the need for a mold switching mechanism and its associated auxiliary processes such as mold blowing, cooling, replacement, and lubrication, reducing auxiliary time and increasing output per unit time.

[0019] Optionally, the first upsetting is performed by axial compression forming upsetting.

[0020] By adopting the above technical solution, axial compression upsetting is a forming method that applies pressure along the pipe axis to induce radial flow and increase wall thickness in the pipe end material. It is a core process for forming pipe end flanges, joints, and other connecting parts. This limitation clearly defines the specific forming type addressed by this process as axial compression upsetting. During axial compression upsetting, the pipe end material undergoes compression and radial flow under axial pressure. The material flow path at the inner and outer corners is complex, and the strain state is a multi-directional compression-tension combination, which easily leads to strain concentration exceeding the material forming limit in local areas. This process effectively solves the problems of strain concentration and cracking caused by large-angle bending and restricted material flow in axial compression upsetting through a two-step forming process plus intermediate induction heating, demonstrating clear process targeting. This limitation focuses the application scope of this process on the specific forming type of axial compression upsetting, distinguishing it from other pipe end forming methods such as flaring and expansion.

[0021] Optionally, the cavity of the upsetting die is matched with the outer diameter of the workpiece to provide a fully enclosed clamping and fixing of the non-deformation area of ​​the workpiece.

[0022] By adopting the above technical solution, the mold cavity matches the outer diameter of the workpiece and provides full-wrap clamping for the non-deformation zone. This ensures that the workpiece will not experience axial movement or radial displacement during upsetting, guaranteeing forming accuracy. Especially in a two-step forming process, where the workpiece requires two clamping operations, reliable clamping is a prerequisite for ensuring coaxiality and dimensional consistency during both deformations. Secondly, the full-wrap clamping ensures that the non-deformation zone of the workpiece is tightly fitted to the mold during induction heating. The metal mold, acting as a heat conductor, quickly dissipates heat, keeping the temperature of the non-deformation zone at a low level. Heat is confined within the deformation area, achieving the localized heating effect of induction heating. This reduces the total heat input to the workpiece, thus lowering energy consumption, and minimizes structural changes or performance degradation in the non-deformation zone due to heating, ensuring that the original mechanical properties and corrosion resistance of the pipe's non-deformation zone are unaffected by the thermal process. Furthermore, the lower mold adopts a split structure with bolt fastening, making the removal and re-clamping of the workpiece between passes convenient and efficient. The split structure ensures both full-wrap clamping effectiveness and ease of assembly and disassembly.

[0023] Secondly, the upsetting forming device provided in this application adopts the following technical solution: An upsetting forming apparatus, comprising: Upsetting die, used to clamp tubular workpieces and upset them, including: The upper mold and the lower mold, wherein the lower mold is a split structure, consisting of a left half mold and a right half mold assembled together, and fastened with bolts through mounting holes, and the cavity matches the outer diameter of the workpiece; Induction heating equipment includes induction heating coils, special clamps for induction heating of pipes, and AC power supply box.

[0024] By adopting the above technical solutions, the split structure design of the lower die makes the removal of the workpiece after the first upsetting pass and the re-clamping operation after induction heating convenient and efficient, without the need for complex auxiliary tools, providing a structural foundation for rapid transfer between the two passes. The matching of the cavity and the outer diameter of the workpiece ensures clamping stability and forming accuracy, preventing workpiece movement during the upsetting process. The induction heating coil in the induction heating equipment can be arranged around the deformation area of ​​the workpiece. By changing the coil shape and number of turns, it adapts to different pipe diameters and deformation zone lengths, ensuring that the heating area precisely corresponds to the deformation area. The special induction heating fixture for pipes ensures that the workpiece is within the effective heating range of the coil and in a fixed position during the heating process, guaranteeing the uniformity and repeatability of heating. The AC power supply box provides high-frequency AC power, enabling induction heating to heat small-sized pipes to the target temperature within seconds, which is the foundation for achieving rapid heating. The device has clearly defined functions and a simple structure, and can be used with existing general equipment such as presses. It does not require a sealing connection or atmosphere protection device between the heating furnace and the upsetting die, nor does it require multiple sets of mold cavities and their switching mechanisms. It has good industrial applicability and is easy to modify.

[0025] Optionally, the edge of the upper mold is provided with two extended guide sections, which are fitted to the outer edge of the lower mold.

[0026] By adopting the above technical solution, the extended guide section set on the edge of the upper die plays an automatic guiding and geometric limiting role during die assembly and upper die descent, which is a key structure for improving forming accuracy. The guide section fits against the outer edge of the lower die, and the relative position of the upper and lower dies is determined through geometric contact in the early stage of the upper die descent, ensuring the concentricity of the upper die upsetting cone surface and the lower die cavity, and minimizing die misalignment caused by initial positioning deviation. During the upsetting process, the guide section slides along the outer edge of the lower die, continuously constraining the movement trajectory of the upper die, so that the upper die descends vertically along the set axis, ensuring that the upsetting force is applied along the axis of the tube, and minimizing defects such as uneven wall thickness or single-sided flash caused by eccentric loading. In the two-step forming process, the workpiece needs to undergo two clamping and two upsetting processes. The presence of the guide section ensures that the relative position of the upper and lower dies remains consistent during the two upsetting processes, thereby ensuring the coaxiality of the two deformation processes, and the dimensional accuracy and geometric tolerance of the final product are effectively controlled. The guide structure works in conjunction with the split lower mold structure. The split structure ensures the convenience of clamping and removal, while the guide structure ensures the accuracy of closing and upsetting. Together, they achieve a balance between convenient clamping and high-precision forming.

[0027] In summary, this application includes at least one of the following beneficial technical effects: 1. The first upsetting pass is performed at room temperature, decomposing the total deformation required for one-time forming into two parts, minimizing strain concentration exceeding the material's forming limit during a single large deformation. After the first upsetting pass, plastic damage and residual stress accumulate inside the workpiece, reducing the material's formability. At this point, induction heating is introduced between the two passes to rapidly raise the temperature of the deformation area above the recrystallization temperature, causing recrystallization within the material, significantly reducing dislocation density, repairing plastic damage, eliminating residual stress, and restoring the material's formability to near its initial state. After cooling to room temperature, the second upsetting pass is performed. At this point, the material has recovered to a highly plastic state and can withstand the second deformation without cracking. This process significantly reduces the principal strain during final forming from the higher level of one-time forming to a safe range, effectively suppressing the initiation of macro-cracks and micro-cracks at the inner and outer corners, and solving the cracking problem caused by accumulated plastic damage during room temperature large-angle bending of small-sized stainless steel pipe joints. 2. Induction heating utilizes the principle of electromagnetic induction to directly generate heat inside the workpiece. It boasts high heating efficiency and speed, capable of heating the deformation area of ​​small-sized pipes to above the recrystallization temperature within seconds. The extremely short heating time ensures that the workpiece remains within the sensitization temperature range for a very short period, preventing carbon atoms from diffusing to the grain boundaries in large quantities to form chromium carbide. This maintains the chromium content near the grain boundaries at the level required for corrosion resistance, effectively minimizing the risk of intergranular chromium depletion and intergranular corrosion. Simultaneously, the extremely short heating time prevents recrystallized grains from growing rapidly after nucleation, maintaining the average grain size within the fine-grained range after heating. This minimizes the grain coarsening phenomenon commonly seen in traditional long-term heat treatment, ensuring that the material's strength, plasticity, and corrosion resistance do not deteriorate due to heat treatment. 3. The split structure and guide section work together to ensure the coaxiality of the two deformation stages and the dimensional accuracy of the final product while ensuring convenient clamping. This process does not require isolation measures such as atmosphere protection, the equipment is simple in structure, and it has good compatibility with existing presses and other general equipment. Attached Figure Description

[0028] Figure 1 This is an overall structural diagram of the upsetting die in the embodiments of this application; Figure 2 This is a cross-sectional view of the upsetting die in the embodiments of this application; Figure 3 This is a diagram illustrating the cavity in an embodiment of this application; Figure 4 This is a diagram illustrating the right half of the model in an embodiment of this application; Figure 5 This is the first step of the compression equivalent deformation polygonal diagram in the embodiments of this application; Figure 6 This is a metallographic image of the overall left and right contours of the part in Embodiment 1 of this application; Figure 7 This is an enlarged metallographic view of the left and right inner corners of the part in Embodiment 1 of this application; Figure 8 This is a metallographic image of the overall left and right contours of the part in Comparative Example 1 of this application; Figure 9 This is an enlarged metallographic view of the left and right inner corners of the part in Comparative Example 1 of this application; Figure 10 This is a metallographic image of the overall left and right contours of the part in Comparative Example 2 of this application; Figure 11 This is an enlarged metallographic view of the left and right inner corners of the part in Comparative Example 2 of this application; Figure 12 This is a metallographic image of the overall left and right contours of the part in Comparative Example 3 of this application; Figure 13 This is an enlarged metallographic view of the left and right inner corners of the part in Comparative Example 3 of this application; Figure 14 This is a metallographic image of the overall left and right contours of the part in Comparative Example 4 of this application; Figure 15 This is an enlarged metallographic view of the left and right inner corners of the part in Comparative Example 4 of this application; Figure 16 This is a metallographic image of the inner corner of the part in Comparative Example 8 of this application, and a partial enlarged view of the inner corner.

[0029] Reference numerals: 100, upper mold; 110, extended guide section; 120, forming groove; 200, lower mold; 210, left half mold; 220, right half mold; 230, cavity; 231, rounded chamfer. Detailed Implementation

[0030] The following combination Figures 1 to 5 This application will be described in further detail.

[0031] The following will refer to the accompanying drawings in the embodiments of this application ( Figures 1 to 5 This application provides a clear and complete description of the technical solutions in its embodiments. Obviously, the described embodiments are merely a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0032] In the description of this application, the terms "first pass" and "second pass" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. The terms "installation," "connection," "joining," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0033] like Figures 1 to 4 As shown, this application provides a two-step upsetting pipe end forming device with heating assistance, including an upsetting die and an induction heating device. The upsetting die is used to clamp the pipe workpiece and perform upsetting forming on the workpiece. It includes an upper die 100 and a lower die 200. The upper die 100 has a forming groove 120. The lower die 200 is a split structure, which is composed of a left half die 210 and a right half die 220 assembled to form a cavity 230. The cavity 230 has an arc-shaped chamfer 231 at the end near the upper die 100. The left half die 210 and the right half die 220 are fastened to the lower die 200 by mounting holes and bolts. The cavity 230 of the lower die 200 matches the outer diameter of the workpiece and is used to fully enclose and fix the non-deformation area of ​​the workpiece. The upper die 100 has two extended guide sections 110 along its edge, which fit against the outer edge of the lower die 200. These extended guide sections 110 are used for automatic guidance and geometric positioning during the downward movement of the upper die 100, ensuring the concentricity of the upsetting tapered surface of the upper die 100 and the cavity 230 of the lower die 200. The induction heating equipment includes an induction heating coil, a pipe induction heating fixture (which can be fixed using any clamping device such as calipers, expansion shafts, or robotic arms), and an AC power supply box. The induction heating coil is arranged around the deformation area of ​​the workpiece, and the pipe induction heating fixture is used to fix the workpiece within the effective heating range of the coil. The upper and lower dies are fixed to the corresponding fixtures of the forming testing machine via lead-out ends.

[0034] In practical use, first, place the extended fixed end of the lower mold 200 into the lower fixture of the testing machine and clamp it. Place the upper mold 100 on top of the lower mold 200. Use the geometric limit of the guide section of the upper mold 100 to determine the relative position of the upper and lower molds 200. Then, start the upper fixture of the testing machine to clamp the upper mold 100, completing the mold installation and positioning. Place the pipe workpiece into the cavity 230 of the lower mold 200, ensuring that the bottom surface of the workpiece is tightly overlapped with the supporting surface of the cavity 230. Then, assemble the inner mold of the lower mold 200 into the installation position so that the cavity 230 fits against the workpiece. Lock it with bolts to complete the clamping and centering of the workpiece. Start the press to drive the upper mold 100 downward to perform the first upsetting, causing the workpiece to produce the predetermined plastic deformation. After the first upsetting pass, the workpiece is removed from the upsetting die and installed onto a special induction heating fixture for pipes (the fixture can be a vise-like structure, placing the processed end of the pipe at the axis of the induction coil). This ensures the deformation area of ​​the workpiece is within the effective heating range of the induction heating coil. The AC power supply is then turned on, and the deformation area of ​​the workpiece is heated to the predetermined temperature within a predetermined time using induction heating. The workpiece is then allowed to cool naturally to room temperature in air. After cooling, the workpiece is clamped again into the same set of upsetting dies, and the press is started to drive the upper die 100 downwards for the second upsetting pass, bringing the workpiece to the preset final shape and size.

[0035] The heating method can be direct electric heating, laser heating, or direct heat conduction heating. In this embodiment, induction heating is preferred.

[0036] The present application will be further described in detail below with reference to specific embodiments.

[0037] Example 1: Refer to Figure 5 and Figure 6 This embodiment describes the upsetting of the pipe end of a 316L stainless steel pipe with an outer diameter of 12.7 mm and a wall thickness of 1.24 mm.

[0038] S1: The tubular workpiece is clamped in the upsetting die, which includes an upper die and a lower die. The lower die is a split structure, composed of a left half and a right half, which are fastened to each other by mounting holes and bolts. The cavity of the lower die matches the outer diameter of the workpiece, providing a full-wrap clamping and fixing of the non-deformation area of ​​the workpiece. The edge of the upper die has two extended guide sections that fit against the outer edge of the lower die, used for automatic guidance and geometric limiting during the downward movement of the upper die, ensuring the concentricity of the upsetting cone surface of the upper die and the cavity of the lower die. The press is started to drive the upper die downward for the first upsetting pass. The downward pressure of the first upsetting pass is 77% of the total downward pressure (i.e., 3.1 mm), the die downward speed is 10 mm / s, and the maximum load is approximately 80 kN, causing plastic deformation of the workpiece.

[0039] S2: Remove the workpiece after the first upsetting pass from the upsetting die and install it onto the pipe induction heating fixture, ensuring that the deformation area of ​​the workpiece is within the effective heating range of the induction heating coil. Turn on the AC power supply box and heat the deformation area of ​​the workpiece to 1050°C within 3 seconds using induction heating, then allow it to cool naturally to room temperature in the air.

[0040] S3: After cooling, the workpiece is clamped again in the same set of upsetting dies. The press is started to drive the upper die downward to perform the second upsetting. The down pressure of the second upsetting is 23% of the full down pressure (i.e., 0.9mm). The die downward speed is 10mm / s, and the maximum load is about 80kN, so that the workpiece reaches the preset final shape and size.

[0041] In this embodiment, the first upsetting pass is performed at room temperature, during which the workpiece undergoes 77% pre-deformation, accumulating sufficient dislocations and plastic damage. Subsequently, induction heating rapidly heats the deformed area to 1050°C within 3 seconds, quickly crossing the sensitization temperature range of 450°C to 850°C. This prevents carbon atoms from diffusing to the grain boundaries in large quantities to form chromium carbide, effectively minimizing the risk of intergranular chromium depletion and intergranular corrosion. Simultaneously, 1050°C is above the recrystallization temperature of 316L stainless steel, causing recrystallization within the material, significantly reducing dislocation density, repairing plastic damage, eliminating residual stress, and restoring formability to near its initial state. The extremely short heating time of 3 seconds also inhibits the growth of recrystallized grains, maintaining the grain size within the fine-grained range. After cooling to room temperature, the second upsetting pass is performed. At this point, the material has recovered to a highly plastic state, capable of withstanding the remaining 23% deformation without cracking.

[0042] Testing revealed that the pipe fittings manufactured using the process described in this embodiment exhibited no cracks at the inner corners, maintained a complete outer corner profile, and met the dimensional accuracy requirements. Finite element analysis revealed the equivalent strain of the pipe fittings. The maximum equivalent strain of the two-step induction heating upsetting process in this embodiment decreased from 3.01 to 1.27 compared to a single-step forming process. This indicates that the synergistic effect of the two-step forming process plus intermediate induction heating ensured that the final forming strain fell within the safe range of the material's forming limit.

[0043] Example 2: The difference between this example and Example 1 is that the pressing amount of the first upsetting pass is 75% of the total pressing amount (i.e., 3.0 mm), and the pressing amount of the second upsetting pass is 25% of the total pressing amount (i.e., 1.0 mm).

[0044] In S1, the initial upsetting pressure is set to 3.0 mm, and the die descending speed is 10 mm / s, causing plastic deformation of the workpiece. In S2, the deformation area of ​​the workpiece is heated to 1050°C within 3 seconds using induction heating, and then naturally cooled to room temperature in air. In S3, the second upsetting pressure is set to 1.0 mm, and the die descending speed is 10 mm / s, bringing the workpiece to the preset final shape and size.

[0045] In this embodiment, the initial upsetting pressure is 75% of the total initial upsetting pressure. This pressure induces sufficient plastic deformation and dislocation accumulation in the workpiece, providing adequate recrystallization driving force for subsequent induction heating repair. After induction heating to 1050°C and cooling, the material's formability recovers to near its initial state. The second upsetting pressure is 25% of the total initial upsetting pressure. The repaired material can withstand this deformation without cracking.

[0046] Testing revealed that the pipe fittings manufactured using the process described in this embodiment exhibited no cracks at the inner corners. Finite element analysis yielded an equivalent strain of 1.32. Compared to Embodiment 1, the first-step deformation was slightly smaller (75% vs. 77%), while the remaining deformation in the second step was slightly larger (25% vs. 23%). Although no cracks were found at the inner corners, the outline integrity at the outer corners was slightly inferior to that of Embodiment 1. This indicates that effective forming can be achieved within a compression range of 75% to 80%, with 77% being the preferred value.

[0047] Example 3: The difference between this example and Example 1 is that the pressing amount of the first upsetting pass is 80% of the total pressing amount (i.e., 3.2 mm), and the pressing amount of the second upsetting pass is 20% of the total pressing amount (i.e., 0.8 mm).

[0048] In S1, the initial upsetting pressure is set to 3.2 mm, and the die descending speed is 10 mm / s, causing plastic deformation of the workpiece. In S2, the deformation area of ​​the workpiece is heated to 1050°C within 3 seconds using induction heating, and then naturally cooled to room temperature in air. In S3, the second upsetting pressure is set to 0.8 mm, and the die descending speed is 10 mm / s, bringing the workpiece to the preset final shape and size.

[0049] In this embodiment, the initial upsetting pressure is 80% of the total initial upsetting pressure. This pressure causes significant plastic deformation in the workpiece during the pre-upsetting stage, but it remains within the material's safe room temperature forming range and does not trigger cracks. After induction heating to 1050°C and cooling, the material's formability is restored. The second upsetting pressure is only 20% of the total initial upsetting pressure, resulting in smaller deformation. The repaired material can easily withstand this deformation.

[0050] Testing revealed that the pipe fittings manufactured using the process described in this embodiment exhibited no cracks at the inner corners. Finite element analysis revealed the equivalent strain of the pipe fittings, with a maximum equivalent strain of 1.33. Compared to Embodiment 1, the first-step deformation was larger (80% vs. 77%), resulting in slightly higher strain concentration during the pre-forming stage and placing higher demands on mold alignment accuracy and the original quality of the workpiece. Under the same equipment and material conditions, Embodiment 1 exhibits a wider process window and better stability.

[0051] Example 4: The difference between this example and Example 1 is that the heating time of induction heating in S2 is 2 seconds and the heating temperature is 1050℃.

[0052] S1 is the same as in Example 1, with the first upsetting pass having a compression amount of 77% of the total compression amount (i.e., 3.1 mm). In S2, the workpiece after the first upsetting pass is removed from the upsetting die and installed on a special induction heating fixture for pipes. The deformed area of ​​the workpiece is heated to 1050°C within 2 seconds by induction heating, and then naturally cooled to room temperature in air. S3 is the same as in Example 1, with the second upsetting pass having a compression amount of 23% of the total compression amount (i.e., 0.9 mm).

[0053] In this embodiment, the heating time of 2 seconds is shorter than the 3 seconds in Example 1, further shortening the residence time of the workpiece in the sensitization temperature range and further reducing the risk of intergranular corrosion. Simultaneously, the shorter heating time results in a shorter recrystallization grain growth time and finer grain size. However, the 2-second heating time places higher demands on the power density of the induction heating equipment, requiring higher output capacity of the power supply box and higher coupling efficiency of the coil.

[0054] Testing revealed that the pipe fittings manufactured using the process described in this embodiment exhibited no cracks at the inner corners, and the grain size was slightly smaller than that of Embodiment 1. Finite element analysis revealed the equivalent strain of the pipe fittings, with a maximum equivalent strain of 1.27. Provided the equipment capacity meets the requirements, a 2-second temperature rise can achieve an even finer grain structure.

[0055] Example 5: The difference between this example and Example 1 is that the heating time of induction heating in S2 is 5 seconds and the heating temperature is 1050℃.

[0056] S1 is the same as in Example 1, with the first upsetting pass having a compression amount of 77% of the total compression amount (i.e., 3.1 mm). In S2, the workpiece after the first upsetting pass is removed from the upsetting die and installed on a special induction heating fixture for pipes. The deformed area of ​​the workpiece is heated to 1050°C within 5 seconds by induction heating, and then naturally cooled to room temperature in air. S3 is the same as in Example 1, with the second upsetting pass having a compression amount of 23% of the total compression amount (i.e., 0.9 mm).

[0057] In this embodiment, the heating time of 5 seconds is longer than the 3 seconds in Example 1, and the residence time of the workpiece in the sensitization temperature range of 450°C to 850°C is extended. However, since the total heating time is still only 5 seconds, which is much shorter than the tens of minutes to several hours of traditional heat treatment, the amount of carbon atoms diffusing to the grain boundaries to form chromium carbide is still limited, and the risk of intergranular corrosion is still within a controllable range. At the same time, the 5-second heating time makes the growth time of recrystallized grains slightly longer than that in Example 1.

[0058] Testing revealed that the pipe fittings manufactured using the process described in this embodiment exhibited no cracks at the inner corners. The grain size was slightly larger than that of Example 1, but still within the fine-grained range. Finite element analysis yielded an equivalent strain of 1.27. Compared to Example 1, the 5-second heating time places lower demands on equipment power density, making it a viable alternative when equipment capacity is limited.

[0059] Example 6: The difference between this example and Example 1 is that the predetermined temperature for induction heating in S2 is 1000℃.

[0060] S1 is the same as in Example 1, with the first upsetting pass having a compression amount of 77% of the total compression amount (i.e., 3.1 mm). In S2, the workpiece after the first upsetting pass is removed from the upsetting die and installed on a special induction heating fixture for pipes. The deformed area of ​​the workpiece is heated to 1000°C within 3 seconds by induction heating, and then naturally cooled to room temperature in air. S3 is the same as in Example 1, with the second upsetting pass having a compression amount of 23% of the total compression amount (i.e., 0.9 mm).

[0061] In this embodiment, the heating temperature was 1000℃, which reached the recrystallization temperature of 316L stainless steel. Testing showed that after induction heating treatment at 1000℃, the elongation of the material recovered to near its initial level, indicating that recrystallization had occurred and plastic damage was effectively repaired. Compared to treatment at 1050℃, the elongation of the material after treatment at 1000℃ was slightly lower, but still within an acceptable range, meeting the forming requirements of the second upsetting pass.

[0062] Testing revealed that the pipe fittings manufactured using the process described in this embodiment exhibited only minute capillary cracks at the inner corners, with crack lengths significantly shorter than those in the one-step cold-formed sample. Finite element analysis yielded an equivalent strain of 1.27. Compared to Example 1, the recrystallization degree of the 1000℃ treatment was slightly lower than that of the 1050℃ treatment, and the recovery of material formability was also slightly lower. The microscopic defects at the inner corners of the product were slightly more numerous than in Example 1, but no visible cracks were observed macroscopically.

[0063] Example 7: The difference between this example and Example 1 is that the predetermined temperature for induction heating in S2 is 1100℃.

[0064] S1 is the same as in Example 1, with the first upsetting pass having a compression amount of 77% of the total compression amount (i.e., 3.1 mm). In S2, the workpiece after the first upsetting pass is removed from the upsetting die and installed on a special induction heating fixture for pipes. The deformed area of ​​the workpiece is heated to 1100°C within 3 seconds by induction heating, and then naturally cooled to room temperature in air. S3 is the same as in Example 1, with the second upsetting pass having a compression amount of 23% of the total compression amount (i.e., 0.9 mm).

[0065] In this embodiment, the heating temperature is 1100℃, which is higher than 1050℃. At 1100℃, the recrystallization process is more complete, the solubility of carbon in austenite increases, the amount of chromium carbide precipitated decreases, and the diffusion rate of chromium atoms increases, which can replenish the consumption at grain boundaries in a timely manner. Therefore, sensitization is less likely to occur at this temperature. Testing showed that after induction heating treatment at 1100℃, the elongation of the material recovered to near its initial level, and the recrystallized grains were uniform with good microstructure consistency.

[0066] Testing revealed that the pipe fittings manufactured using the process described in this embodiment exhibited no cracks at the inner corners, and the inner and outer corner contours remained intact. Finite element analysis yielded an equivalent strain of 1.27. Compared to Example 1, the 1100℃ treatment resulted in a higher degree of recrystallization and more complete recovery of the material's formability. However, due to the higher heating temperature, the power requirements for the induction heating equipment were also higher, and the surface oxidation tendency of the workpiece was slightly greater than that of the 1050℃ treatment.

[0067] Example 8: The difference between this example and Example 1 is that the pipe workpiece is a 316L stainless steel pipe with an outer diameter of 6mm and a wall thickness of 1mm.

[0068] In step S1, a 316L stainless steel pipe workpiece with an outer diameter of 6mm and a wall thickness of 1mm is clamped in an upsetting die. The first upsetting pass applies 77% of the total upsetting amount, and the die descends at a speed of 10mm / s. In step S2, the workpiece's deformation area is heated to 1050℃ within 3 seconds using induction heating, and then naturally cooled to room temperature in air. In step S3, the second upsetting pass applies 23% of the total upsetting amount, and the die descends at a speed of 10mm / s, bringing the workpiece to the preset final shape and size.

[0069] In this embodiment, the pipe has an outer diameter of 6mm and a wall thickness of 1mm, classifying it as a small-sized, thin-walled pipe fitting. Due to its smaller size, lighter weight, and lower heat capacity, the induction heating process achieves a faster heating rate and better temperature uniformity. Testing revealed that the pipe fitting manufactured using this embodiment's process exhibited no cracks at the inner corners, and the product's dimensional accuracy met design requirements. Finite element analysis yielded an equivalent strain of 1.55. Furthermore, the thinner wall thickness results in a shorter material flow path during forming, leading to a more uniform strain distribution and excellent forming quality.

[0070] Example 9: The difference between this example and Example 1 is that the pipe workpiece is a 316L stainless steel pipe with an outer diameter of 26mm and a wall thickness of 4mm.

[0071] In step S1, a 316L stainless steel pipe workpiece with an outer diameter of 26mm and a wall thickness of 4mm is clamped in an upsetting die. The first upsetting pass applies 77% of the total upsetting amount, and the die descends at a speed of 10mm / s. In step S2, the workpiece's deformation area is heated to 1050℃ within 3 seconds using induction heating, and then naturally cooled to room temperature in air. In step S3, the second upsetting pass applies 23% of the total upsetting amount, and the die descends at a speed of 10mm / s, bringing the workpiece to the preset final shape and size.

[0072] In this embodiment, the pipe has an outer diameter of 26mm and a wall thickness of 4mm, classifying it as a relatively large thin-walled pipe fitting. Due to the larger size, greater mass, and higher heat capacity of the workpiece, the induction heating rate is slightly slower than that of smaller workpieces, but it still reaches the target temperature within 3 seconds. Testing showed that the pipe fitting manufactured using this embodiment's process exhibited no cracks at the inner corners, the product's dimensional accuracy met design requirements, and the maximum equivalent strain was 1.60. Compared to Embodiment 1, the larger workpiece wall thickness results in greater material flow resistance during the forming process, placing higher demands on mold strength and equipment tonnage. However, the two-step forming process combined with intermediate induction heating effectively suppresses crack formation.

[0073] Example 10: The difference between this example and Example 1 is that no extended guide section is provided on the edge of the upper mold.

[0074] In S1, the upper die edge of the upsetting die does not have an extended guide section; the concentricity between the upper and lower dies is only ensured by the positioning of the testing machine fixture. The first upsetting pass has a compression amount of 77% of the total compression amount (i.e., 3.1 mm). In S2, the deformation area of ​​the workpiece is heated to 1050°C within 3 seconds by induction heating, and then naturally cooled to room temperature in air. In S3, the second upsetting pass has a compression amount of 23% of the total compression amount (i.e., 0.9 mm).

[0075] In this embodiment, since the upper die does not have an extended guide section, the concentricity of the upper and lower dies depends entirely on the positioning accuracy of the testing machine fixture. If there is a deviation in the fixture positioning during the two clamping processes of the workpiece, the deformation axes of the two upsetting passes may not completely coincide, resulting in uneven product wall thickness or burrs on one side.

[0076] Testing revealed that some samples of the pipe fittings manufactured using the process described in this embodiment exhibited slight wall thickness unevenness, and the number of cracks at the inner corners was slightly increased compared to Example 1. Finite element analysis revealed the equivalent strain of the pipe fittings, with a maximum equivalent strain of 1.27. This indicates that the extended guide section of the upper die plays a crucial role in ensuring the coaxiality and forming accuracy of the two upsetting passes, and the guide section structure of Example 1 is the preferred solution.

[0077] Example 11: The difference between this example and Example 1 is that the lower mold is an integral structure, not a split structure.

[0078] In S1, the lower die of the upsetting mold is an integral structure, with the cavity matching the outer diameter of the workpiece. During clamping, the workpiece is inserted into the cavity from above the lower die. The first upsetting pass involves a compression of 77% of the total compression (i.e., 3.1 mm). In S2, the workpiece after the first upsetting pass is removed from the integral lower die, and the deformed area is heated to 1050°C within 3 seconds using induction heating, followed by natural cooling to room temperature in air. In S3, the cooled workpiece is reinserted into the integral lower die cavity for the second upsetting pass, with a compression of 23% of the total compression (i.e., 0.9 mm).

[0079] In this embodiment, the integral lower mold has a simpler structure and lower processing cost. However, because the lower mold is an integral structure, the removal and reinstallation of the workpiece between two passes is not as convenient as with a split structure. In particular, for workpieces with pre-upset heads, interference may occur when removing them from the integral cavity, increasing the difficulty of operation.

[0080] Testing revealed that although no cracks were generated at the inner corners of the pipe fittings manufactured using the process described in this embodiment, and the equivalent strain of the pipe fittings obtained through finite element simulation analysis showed a maximum equivalent strain of 1.27, the operation time for clamping and removing the workpiece was significantly longer than that in Embodiment 1, and repeated clamping may cause scratches on the workpiece surface. The split-type lower mold structure has a significant advantage in ensuring the convenience of clamping, and the split-type structure in Embodiment 1 is the preferred solution.

[0081] The metallographic structure of Examples 2-11 is not significantly different from that of Example 1, so the metallographic diagrams of Examples 2-11 are not shown.

[0082] Examples 1-11, and the actual effects are compared in Table 1 below: Table 1 Comparison of Test Results 1-11 Example 1 Approximately 115% 1.27 Approximately 19.0 No cracks Preferred solution Example 2 Approximately 115% 1.32 Approximately 19.0 No cracks The integrity of the outer corner profile is slightly inferior to that of Example 1. Example 3 Approximately 115% 1.33 Approximately 19.0 No cracks Higher precision is required for mold alignment. Example 4 Approximately 115% 1.27 Slightly less than 19.0 No cracks The grain size is slightly smaller than that in Example 1. Example 5 Approximately 115% 1.27 Slightly greater than 19.0 No cracks The grain size is slightly larger than that in Example 1. Example 6 Approximately 109% 1.27 Approximately 19.0 Extremely small capillary cracks The degree of recrystallization is slightly lower than that of treatment at 1050℃. Example 7 Approximately 112% 1.27 Approximately 19.0 No cracks Recrystallization is more complete, and the surface oxidation tendency is slightly increased. Example 8 Approximately 115% 1.55 Approximately 19.0 No cracks For smaller workpieces, better temperature uniformity is achieved. Example 9 Approximately 115% 1.60 Approximately 19.0 No cracks Large workpieces require higher tonnage equipment. Example 10 Approximately 115% 1.27 Approximately 19.0 Some samples had slight wall thickness unevenness The deformation axes of the two upsetting passes may not completely coincide, resulting in uneven wall thickness or burrs on one side of some samples. Example 11 Approximately 115% 1.27 Approximately 19.0 No cracks, but low clamping efficiency. High difficulty of operation In the table, the elongation rate is the total elongation rate of the material obtained by the "pre-stretching-induction heating-secondary stretching" test of the tubular workpiece, which is only related to whether induction heating treatment is performed, whether multi-step stretching is performed, and the treatment temperature.

[0083] In summary, a first-pass compression of 75%~80% (i.e., 3.0~3.2mm / 4.0mm) can ensure that the strain of the two forming processes does not exceed the forming limit of the material, with 77% (3.1mm / 4.0mm) being the preferred value; the total elongation of the material after heating at 1050℃ is close to 115%, and the elongation after heating at 1000℃ is about 109%; after heat treatment at 1000℃-1100℃, the average grain size is 19.0μm, and the grain size is basically maintained at the same level.

[0084] Comparative Example 1: This comparative example uses a one-step cold forming process, without induction heating treatment or two-step forming.

[0085] A 316L stainless steel pipe workpiece with an outer diameter of 12.7mm and a wall thickness of 1.24mm is clamped in an upsetting die. The press is started to drive the upper die downward, completing the entire pressing amount (4.0mm) in one go, so that the workpiece reaches the preset final shape and size.

[0086] Testing revealed cracks on both the inner and outer walls of the pipe fittings manufactured using a one-step cold forming process. The largest crack on the inner wall of the fitting was approximately 250 μm, and the largest on the outer wall was approximately 78 μm, with an elongation of 63%. Finite element analysis showed that the equivalent strain of the fittings was 2.5 μm. This is because during a single large deformation, the pipe end underwent a large-angle bend, making material flow difficult to control and exceeding the material forming limit, leading to crack formation. Such cracks are highly susceptible to premature fatigue cracking under cyclic loading, severely impacting the service life of the components.

[0087] Comparative Example 2: This comparative example uses a two-step molding process but does not involve intermediate induction heating.

[0088] S1: A 316L stainless steel pipe workpiece with an outer diameter of 12.7 mm and a wall thickness of 1.24 mm is clamped in an upsetting die for the first upsetting pass, with a pressing amount of 77% of the total pressing amount (i.e., 3.1 mm). S2: The workpiece after the first upsetting pass is removed from the upsetting die and placed at room temperature for the same duration as in Example 1 (approximately 3 seconds of heating time plus cooling time), but without any heat treatment. S3: The workpiece is clamped again in the same set of upsetting dies for the second upsetting pass, with a pressing amount of 23% of the total pressing amount (i.e., 0.9 mm).

[0089] Testing revealed that pipe fittings formed in a two-step process without intermediate induction heating still exhibited cracks at the inner corners. While the crack length was reduced compared to single-step cold forming, it remained clearly visible, measuring approximately 103 μm; the elongation was 80%. Finite element analysis revealed the equivalent strain of the fitting, with a maximum equivalent strain of 1.27. This is because plastic damage and residual stress accumulated within the material after the first upsetting pass. Without heat treatment to repair these issues, the material's formability was not restored, leading to further cracking during the second deformation step. Compared to Example 1, intermediate induction heating is a crucial step in eliminating cracks; its role in repairing plastic damage and restoring material formability through recrystallization is irreplaceable.

[0090] Comparative Example 3: This comparative example uses a two-step forming process plus intermediate induction heating, but the predetermined temperature for induction heating is 850°C.

[0091] S1 is the same as in Example 1, with the first upsetting pass having a pressing amount of 77% of the total pressing amount (i.e., 3.1 mm). In S2, the deformed area of ​​the workpiece is heated to 850°C within 3 seconds by induction heating, and then naturally cooled to room temperature in air. S3 is the same as in Example 1, with the second upsetting pass having a pressing amount of 23% of the total pressing amount (i.e., 0.9 mm).

[0092] Testing revealed that the pipe fittings treated with intermediate induction heating at 850℃ exhibited cracks at the inner corners, with crack lengths ranging from approximately 65μm to 72μm and an elongation of 81%. Finite element analysis revealed the equivalent strain of the fittings, with the maximum equivalent strain being 1.27, consistent with Example 1. This is because while the 850℃ heating temperature can eliminate some residual stress (resulting in recovery), it does not reach the recrystallization temperature of 316L stainless steel. The dislocations and plastic damage within the material cannot be fully repaired through recrystallization, resulting in limited recovery of the material's formability. Therefore, cracks still occur during the second upsetting process. Compared to Example 1 (heating at 1050℃), a heating temperature above 1000℃ is required for sufficient recrystallization to effectively repair plastic damage.

[0093] Comparative Example 4: This comparative example uses a two-step forming process plus intermediate induction heating, but the predetermined temperature for induction heating is 950℃.

[0094] S1 is the same as in Example 1, with the first upsetting pass having a pressing amount of 77% of the total pressing amount (i.e., 3.1 mm). In S2, the deformed area of ​​the workpiece is heated to 950°C within 3 seconds by induction heating, and then naturally cooled to room temperature in air. S3 is the same as in Example 1, with the second upsetting pass having a pressing amount of 23% of the total pressing amount (i.e., 0.9 mm).

[0095] Testing revealed that pipe fittings treated with intermediate induction heating at 950℃ still exhibited cracks at the inner corners, although the crack length was reduced compared to those treated at 850℃, ranging from approximately 53μm to 66μm; the elongation was 88%. Finite element analysis revealed the equivalent strain of the fittings, with the maximum equivalent strain being 1.27, consistent with Example 1. While 950℃ is close to, but not fully reaches, the recrystallization temperature of 316L stainless steel; some recrystallization occurred, but the degree was insufficient, leaving some residual plastic damage within the material and incomplete recovery of formability. Compared to Example 1 (heated at 1050℃), a heating temperature above 1000℃ is required to achieve sufficient recrystallization and complete recovery of the material's formability.

[0096] Comparative Example 5: This comparative example uses a two-step forming process plus intermediate induction heating. The predetermined temperature for induction heating is 1050℃, but the heating time is 20 seconds.

[0097] S1 is the same as in Example 1, with the first upsetting pass having a pressing amount of 77% of the total pressing amount (i.e., 3.1 mm). In S2, the deformed area of ​​the workpiece is heated to 1050°C within 20 seconds by induction heating, and then naturally cooled to room temperature in air. S3 is the same as in Example 1, with the second upsetting pass having a pressing amount of 23% of the total pressing amount (i.e., 0.9 mm).

[0098] Testing revealed that the pipe fitting product heated to 1050℃ in 20 seconds, while showing no macroscopic cracks at the inner corners, exhibited slight blackening on the material surface (a clear oxidation issue, which was not investigated metallographically). A noticeable oxide scale was observed on the surface, and the grain size was larger than in Example 1. This is because the 20-second heating time prolongs the workpiece's residence time within the sensitization temperature range of 450℃ to 850℃, increasing the amount of chromium carbide precipitated along grain boundaries, exacerbating chromium depletion between grains, and reducing resistance to intergranular corrosion. Simultaneously, the longer heating time allows more time for recrystallized grains to grow. Compared to Example 1 (3-second heating), the heating time needs to be controlled within the range of 2 to 5 seconds, preferably 3 seconds, to achieve rapid heating while minimizing sensitization and grain growth.

[0099] Comparative Example 6: This comparative example uses a two-step forming process plus intermediate induction heating, but the pressing amount of the first upsetting pass is 72.7% of the total pressing amount (i.e., 2.9 mm), and the pressing amount of the second upsetting pass is 27.3% of the total pressing amount (i.e., 1.1 mm).

[0100] In S1, the downforce of the first upsetting pass is set to 2.9 mm. In S2, the deformation area of ​​the workpiece is heated to 1050°C within 3 seconds by induction heating, and then naturally cooled to room temperature in air. In S3, the downforce of the second upsetting pass is set to 1.1 mm.

[0101] Testing revealed that pipe fittings using a 72.7% pre-forming compression amount exhibited cracks at the inner corners. Finite element analysis revealed the equivalent strain of the fittings, with a maximum equivalent strain of 1.40 and an elongation of 115%. This was due to insufficient compression in the first step (below 75%), resulting in inadequate plastic deformation and dislocation accumulation, thus insufficient driving force for induction heating recrystallization. Simultaneously, the remaining deformation in the second step was excessive (30%). Even after induction heating repair, the strain experienced by the material in the second deformation step still exceeded its forming limit, leading to crack formation. Compared to Example 1 (77% pre-forming compression amount), the first compression amount needs to be controlled within the range of 75% to 80% to ensure optimal distribution of deformation between the two steps.

[0102] Comparative Example 7: This comparative example uses a two-step forming process plus intermediate induction heating, but the pressing amount of the first upsetting pass is 85% of the total pressing amount (i.e., 3.4 mm), and the pressing amount of the second upsetting pass is 15% of the total pressing amount (i.e., 0.6 mm).

[0103] In S1, the down pressure of the first upsetting pass is set to 3.4 mm; in S2, the deformation area of ​​the workpiece is heated to 1050°C within 3 seconds by induction heating, and then naturally cooled to room temperature in air. In S3, the down pressure of the second upsetting pass is set to 0.6 mm.

[0104] Testing revealed that pipe fittings using 85% pre-forming compression exhibited microcracks immediately after the first upsetting pass. This was due to excessive compression in the first step (exceeding 80%), causing deformation during the pre-upsetting stage to exceed the material's safe room temperature forming range, thus initiating cracking during the pre-forming stage. Although subsequent induction heating treatment was performed, the existing cracks could not be completely repaired through recrystallization, resulting in crack defects in the final product. Comparison with Example 1 (77% pre-forming compression) demonstrates that the first-pass compression should not exceed 80%; otherwise, irreparable cracks may occur during the pre-forming stage.

[0105] Comparative Example 8: This comparative example uses one-step upsetting, with induction heating performed before upsetting. Other steps are the same as in Example 1.

[0106] Testing revealed that the cracks from the induction heating treatment in front of the pier mostly exhibited an extended pattern, with crack lengths ranging from approximately 150μm to 200μm and an elongation rate of 90%. Finite element simulation analysis yielded the equivalent strain of the pipe fitting, with the maximum equivalent strain being 2.5, the same as that of one-step cold forming.

[0107] The actual effects of Example 1 and Comparative Examples 1-8 are compared in Table 2 below: Table 2 Comparison of detection results between Example 1 and Comparative Examples 1-8 Example 1 Approximately 115 1.27 0 (No cracks) Approximately 19.0 No cracks / Comparative Example 1 Approximately 63 2.5 The inner wall is approximately 250 mm, and the outer wall is approximately 78 mm. / Cracks were found on both the inner and outer walls, with the largest crack on the inner wall measuring approximately 250 μm and the largest on the outer wall measuring approximately 78 μm. The strain is concentrated during a single large deformation, with the principal strain of 3.01 far exceeding the forming limit; the elongation decreases while the principal strain increases. Comparative Example 2 Approximately 80 Approximately 1.27 Visible but not quantified Deformed grains (cannot be quantified) Cracks are still visible at the inner corners, though reduced compared to one-step cold forming. Maximum strain 1.58; without heat treatment to repair plastic damage, elongation is lower than in Example 1. Comparative Example 3 Approximately 81 Approximately 1.27 65~72 Approximately 19.1 Crack at the inner corner Only recovery occurred (residual stress disappeared), but the recrystallization temperature was not reached; the elongation decreased. Comparative Example 4 Approximately 88 Approximately 1.27 53~66 Approximately 19.0 Cracks still exist at the inner corner, though they have decreased compared to 850℃. Partial recrystallization but incomplete; elongation decreased. Comparative Example 5 / / / Approximately 21 blackened surface / Comparative Example 6 Approximately 115 Approximately 1.40 exist / Cracks exist at the inner corner. The initial compression was 72.7%, lower than 75%, indicating insufficient recrystallization driving force. The second step resulted in excessive deformation, with strain reaching 1.23. Comparative Example 7 Cracking occurs during the preforming stage. / Microcracks appear during the preforming stage / Microcracks appear during the preforming stage The initial compression was 85%, exceeding 80%, which exceeded the safe zone during the preforming stage and caused cracks that could not be repaired by subsequent heating. Comparative Example 8 Approximately 90 Approximately 2.5 The crack length is approximately between 150 μm and 200 μm. / Extended cracks appeared Heating before forming only reduces the resistance to deformation and cannot repair the plastic damage that occurs during the forming process. In summary, this application employs a two-step upsetting process with intermediate induction heating, decomposing the total deformation required for a single forming process into two parts. Rapid induction heating between the two passes repairs plastic damage and restores the material's formability. The first upsetting pass is performed at room temperature, minimizing oxidation and grain coarsening risks associated with high-temperature thermoforming. Both upsetting passes use the same set of molds, eliminating the need to switch mold cavities and minimizing positioning errors and time losses associated with mold changes. Induction heating can rapidly heat the workpiece's deformation area to 1000°C to 1100°C within 2 to 5 seconds, causing recrystallization and repairing plastic damage. The extremely short heating time effectively minimizes intergranular corrosion and grain growth. The initial compression in the first pass is controlled within 75% to 80% (preferably 77%) of the total compression, ensuring optimal distribution of deformation between the two steps, with each step's deformation falling within the safe range corresponding to the material's condition. The extended guide section of the upper die fits snugly against the outer edge of the lower die, ensuring the concentricity of the upper and lower dies and the coaxiality of the two deformation passes. The split lower die structure ensures convenient workpiece clamping and removal. Experimental verification shows that no cracks occur at the inner corners of the pipe fitting products using the technical solution of this application, effectively solving the cracking problem caused by plastic cumulative damage during room temperature large-angle bending forming of small-sized stainless steel pipe fittings, and significantly improving the structural integrity and fatigue life of the product.

[0108] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A two-step upsetting method for forming the end of a tube using heat-assisted upsetting, characterized in that, Includes the following steps: S1: The pipe workpiece is clamped in the upsetting die and the first upsetting is performed to cause plastic deformation of the workpiece; S2: The workpiece that has undergone the first upsetting process is removed from the upsetting mold, and the deformed area of ​​the workpiece is heated to a predetermined temperature of 1000℃~1100℃ within 2~5 seconds and then cooled to room temperature. S3: After cooling, the workpiece is clamped again in the upsetting mold for a second upsetting process, so that the workpiece reaches the preset final shape and size.

2. The two-step upsetting tube end forming method with heating assistance according to claim 1, characterized in that, In step S2, induction heating is used to heat the deformed area of ​​the workpiece.

3. The two-step upsetting tube end forming method with heating assistance according to claim 1, characterized in that, In step S2, the heating time is 3 seconds.

4. The two-step upsetting tube end forming method with heating assistance according to claim 2, characterized in that, In step S1, the down pressure of the first upsetting pass is 75% to 80% of the total down pressure.

5. The two-step upsetting tube end forming method with heating assistance according to claim 4, characterized in that, The pressing amount of the first upsetting pass is 77% of the total pressing amount.

6. The two-step upsetting tube end forming method with heating assistance according to claim 1, characterized in that, The first and second upsetting passes are completed using the same set of upsetting molds.

7. The two-step upsetting tube end forming method with heating assistance according to claim 1, characterized in that, The first upsetting process uses axial compression forming upsetting.

8. The two-step upsetting tube end forming method with heating assistance according to claim 1, characterized in that, The cavity of the upsetting die matches the outer diameter of the workpiece, and is used to fully enclose and fix the non-deformation area of ​​the workpiece.

9. An upsetting forming device, characterized in that, include: Upsetting die, used to clamp tubular workpieces and upset them, including: The upper mold (100) and the lower mold (200) are a split structure, consisting of a left half mold (210) and a right half mold (220) assembled together and fastened with bolts through mounting holes. The cavity (230) matches the outer diameter of the workpiece. Induction heating equipment includes induction heating coils, special clamps for induction heating of pipes, and AC power supply box.

10. The upsetting forming apparatus according to claim 9, characterized in that, The upper mold (100) has two extended guide sections (110) on its edge, and the extended guide sections (110) are in contact with the outer edge of the lower mold (200).

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