A method for forming ultra-thin stainless steel pipe fittings

Abstract

This invention discloses a method for forming ultra-thin stainless steel pipe fittings, relating to the field of stainless steel pipe production and processing technology. It aims to solve the problems of branch pipe rupture and inner wrinkling during the current hydroforming process of ultra-thin stainless steel pipe fittings. The method includes: S1: billet preparation and pretreatment; S2: mold and sealing assembly; S3: staged collaborative forming; S301: pre-expansion and mold-fitting stage, where the internal pressure is increased to 0.8 MPa at a rate of 0.1 MPa / s and maintained stably; S302: main forming precise material replenishment stage, employing a gradient pressure increase strategy; S303: forming stress release stage, maintaining a stable internal pressure of 6-7 MPa for 10 seconds to ensure the billet completely fits the mold cavity, followed by a staged pressure relief method. This invention, through the application of a nano-lubricating film coating on the inner wall of the billet, low-temperature preheating treatment, stress-dispersing mold design, and the collaborative control of gradient pressure increase and real-time monitoring and material replenishment during the main forming stage, achieves effective dispersion of tensile stress concentration at the top of the branch pipe of the ultra-thin stainless steel pipe fitting and precise material replenishment, significantly reducing the risk of rupture.

Description

Technical Field

[0001] This invention relates to the field of stainless steel pipe production and processing technology, and more specifically, to a method for forming ultra-thin stainless steel pipe fittings. Background Technology

[0002] With the continuous improvement of modern industry's requirements for lightweight, integrated, and high-precision components, ultra-thin stainless steel pipe fittings are increasingly widely used in key fields such as transportation equipment, new energy vehicles, and precision instruments due to their advantages of high strength, corrosion resistance, and low weight. Hydraulic / internal high-pressure forming technology, as an advanced integral forming process, has become one of the preferred solutions for manufacturing ultra-thin pipe fittings due to its high material utilization rate (up to 90% or more), simplified process, and ability to form complex closed-section components in one step. Its core principle is to provide forming driving force by filling the tube blank with high-pressure liquid, while simultaneously supplementing material with an axial feeding device, so that the tube blank gradually conforms and forms in the mold cavity, ultimately obtaining a pipe fitting of the required shape and size.

[0003] However, ultra-thin stainless steel (such as 304 and 316L) has material properties such as high yield strength, significant work hardening effect, limited plasticity reserve, and poor thermal conductivity (only 1 / 3 that of carbon steel). Combined with its extremely thin wall thickness and insufficient rigidity, it is prone to the following two fatal defects when using traditional hydraulic / internal high-pressure forming processes, which seriously restrict product quality and production efficiency:

[0004] 1. Tensile stress concentration areas, such as the top of the branch pipe, are prone to cracking. During the forming process of multi-branch pipe fittings, the top of the branch pipe must bear the greatest circumferential and axial tensile stress, becoming a typical stress concentration area. Since the wall thickness of ultra-thin stainless steel is only 0.1-0.5mm, the material thinning rate during forming is sensitive. When the internal pressure loading speed is too fast or the axial feeding is not timely, the local thinning rate at the top of the branch pipe is prone to exceed the material's plastic limit, leading to the initiation and rapid propagation of microcracks, ultimately causing cracking defects. In existing technologies, although attempts have been made to alleviate this problem by optimizing the internal pressure loading path or adjusting the axial feed speed, the actual production failure rate is still as high as 15%-30% due to the fast deformation response speed and extremely narrow parameter window of ultra-thin materials. In addition, the frictional resistance between the ultra-thin pipe wall and the mold is significant, further hindering the axial material flow to the top of the branch pipe, exacerbating the tensile stress concentration, and further increasing the risk of cracking.

[0005] 2. Insufficient internal pressure support leads to wrinkling on the inner side. The key to hydraulic / internal high-pressure forming lies in the coordinated matching of internal pressure and axial feeding. The internal pressure needs to provide sufficient radial support for the tube blank to prevent the material from becoming unstable under axial pressure. The axial feed needs to supplement the material required for tube blank expansion to ensure the integrity of the forming. However, for ultra-thin stainless steel tubes, when the internal pressure loading lags behind the axial feed speed, or when the peak internal pressure is insufficient, the inner side of the tube blank lacks effective radial support and is unable to resist the compressive stress generated by axial feeding, making it prone to instability and wrinkling. This wrinkling defect is irreversible. Once formed, it cannot be eliminated in subsequent processes. This not only leads to the tube shape accuracy exceeding the standard, but also causes stress concentration during service, reducing the fatigue life of the component. In the existing technology, although attempts have been made to improve the stability of internal pressure by increasing the initial pre-pressure or optimizing the sealing structure, the sealing of ultra-thin tube walls is extremely difficult, and liquid leakage is prone to occur, resulting in internal pressure fluctuations exceeding ±5%, making it impossible to form continuous and effective radial support. At the same time, the traditional axial double-end feeding method has poor synchronization accuracy, which further aggravates the uneven material flow, causing the wrinkling rate on the inner side to exceed 20%.

[0006] Furthermore, the work hardening effect of ultrathin stainless steel leads to a continuous increase in the material's deformation resistance during the forming process, further narrowing the process parameter window and reducing the defect control capability of existing solutions. Based on the above problems, we propose a forming method for ultrathin stainless steel pipes. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art, adapt to practical needs, and provide a method for forming ultra-thin stainless steel pipe fittings, so as to solve the problems of branch pipe rupture and inner wrinkling during the current hydroforming process of ultra-thin stainless steel pipe fittings.

[0008] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for forming ultra-thin stainless steel pipe fittings, comprising the following steps:

[0009] S1: Tube blank preparation and pretreatment;

[0010] S101: Select 316L ultra-thin stainless steel tube blanks with a wall thickness of 0.1-0.5mm, cut them to the target length, and reserve 5-10% axial material allowance;

[0011] S102: Ultrasonic degreasing and cleaning of the inner and outer walls of the cut tube blank with ethanol to remove oil and impurities.

[0012] S103: After cleaning, a nano-lubricating film with a thickness of 5-10μm is uniformly coated on the inner wall of the tube blank and then air-dried at room temperature.

[0013] S104: After drying, the tube blank is placed in a constant temperature furnace and preheated at a low temperature of 150-250℃ for 30 minutes, and then cooled to below 80℃ with the furnace.

[0014] S2: Mold and sealing assembly:

[0015] S201: Stress-dispersing mold with a gradual transition design for the branch pipe area. The radius of the mold branch pipe cavity is R≥3t, where t is the wall thickness of the pipe blank. The mold cavity surface is coated with TiN. The mold is preheated to 60-80℃.

[0016] S202: The tube blank treated by S1 is placed into the mold cavity, and adaptive elastic bushing sealing heads made of fluororubber composite carbon fiber reinforced material are installed at both ends. The sealing heads are axially pressed by hydraulic drive with a clamping force of 0.3-0.5MPa.

[0017] S203: An internal pressure supply system consisting of a high-pressure pump station and a pressure sensor, and an axial feed device driven by dual servo motors, wherein the synchronization accuracy of the axial feed device is ≤0.05mm;

[0018] S3: Phased collaborative forming;

[0019] S301: During the pre-expansion and molding stage, the internal pressure is increased to 0.8MPa at a rate of 0.1MPa / s and kept stable. At the same time, the axial feed device is controlled to feed synchronously at both ends at a rate of 0.1mm / s and a feed amount of 30% of the total feed amount, which lasts for 10-15s, so that the tube blank fits the mold cavity wall.

[0020] S302: In the main forming precision feeding stage, a gradient pressure increase strategy is adopted, increasing the internal pressure by 0.5MPa every 5s, eventually reaching 6-7MPa, while simultaneously accelerating the axial feed rate to 0.2-0.3mm / s; the forming height of the top of the branch pipe is monitored in real time by a displacement sensor, and when the height reaches 80% of the target value, the feed rate is reduced to 0.1mm / s; the thinning rate of the top of the branch pipe is monitored in real time by an ultrasonic thickness gauge, and when the thinning rate approaches 12%, the internal pressure increase is paused, the feed is maintained for 3-5s, and then the pressure is increased again, lasting for 20-30s;

[0021] S302: During the stress release stage of forming, maintain a stable internal pressure of 6-7MPa for 10s. At the same time, finely adjust the feed amount through the axial feed device to ensure that the tube blank completely fits the mold cavity. Then, adopt a segmented pressure relief method, reducing the pressure by 1MPa every 5s to complete the release of residual stress for 15s.

[0022] S4: Unloading and Retrieving Components;

[0023] S5: Quality Inspection.

[0024] Preferably, in S103, the nano-lubricating film is composed of polytetrafluoroethylene and molybdenum disulfide, and the mass ratio of polytetrafluoroethylene to molybdenum disulfide in the nano-lubricating film is 7:3-8:2; in S104, the low-temperature preheating temperature is adjusted according to the wall thickness of the tube blank: when the wall thickness of the tube blank is ≤0.2mm, the preheating temperature is 150-180℃; when the wall thickness of the tube blank is >0.2mm and ≤0.5mm, the preheating temperature is 200-250℃.

[0025] Preferably, the preheating method of the mold in S201 is embedded preheating with electric heating wire, and the temperature fluctuation of each area of ​​the mold cavity is ≤±2℃.

[0026] Preferably, the final internal pressure in the main forming precision feeding stage described in S302 is dynamically adjusted according to the billet wall thickness. Specifically, when the billet wall thickness increases by 0.1 mm, the final internal pressure increases by 1-1.2 MPa compared to 6 MPa, and the maximum internal pressure does not exceed 8 MPa.

[0027] Preferably, during the segmented pressure relief process described in S302, when the internal pressure drops below 2 MPa, the pressure relief rate is adjusted to decrease by 0.5 MPa every 5 seconds to further reduce residual stress.

[0028] Preferably, the inner wall of the adaptive elastic bushing sealing head in S202 is provided with 3-5 annular grooves, the depth of which is 0.2-0.3 mm and the width of which is 1-1.5 mm, for storing lubricating medium and enhancing sealing performance.

[0029] Preferably, the specific process of S4 is as follows: after the internal pressure is completely released, loosen the sealing head and the mold, take out the formed pipe fitting, and remove the sealing pressure edge at both ends; use a neutral cleaning agent in combination with high-pressure spraying to clean the residual nano-lubricating film on the surface of the pipe fitting.

[0030] Preferably, S5 specifically includes the following steps:

[0031] S501: Dimensional inspection, including branch pipe height, wall thickness distribution, and ellipticity;

[0032] S502: Defect detection, confirming that the pipe fittings are free of cracks, wrinkles, and microcracks through visual inspection and ultrasonic testing;

[0033] S502: Mechanical property testing, sampling tests of pipe fittings for tensile strength and yield strength to ensure that more than 90% of the performance of the raw materials is maintained.

[0034] Compared with the prior art, the beneficial effects of the present invention are:

[0035] 1. This invention achieves effective dispersion of tensile stress concentration at the top of the branch pipe of ultra-thin stainless steel pipe fittings and precise material supply by setting up a nano-lubricating film coating on the inner wall of the pipe blank, low-temperature preheating treatment, stress-dispersing mold design, and coordinated control operation of gradient pressure increase and real-time monitoring and material replenishment during the main forming stage, significantly reducing the risk of cracking. The 5-10μm thick polytetrafluoroethylene and molybdenum disulfide composite nano-lubricating film can reduce the friction coefficient between the pipe blank and the mold. Combined with low-temperature preheating of 150-250℃ adapted to the wall thickness, it can reduce the yield strength of the material and alleviate the problem of increased deformation resistance caused by cold work hardening. The gradual transition design of the rounded corners of the mold branch pipe cavity can disperse the stress concentration at the top of the branch pipe, and the TiN coating further enhances the effect. To reduce frictional resistance and prevent material flow obstruction, a gradient pressure increase strategy of 0.5 MPa every 5 seconds is adopted in the main forming stage. Combined with real-time feedback from displacement sensors and ultrasonic thickness gauges, the material feeding speed is reduced when the forming height of the top of the branch pipe reaches 80% of the target value, and the pressure increase and material feeding are paused when the thinning rate approaches 12%. This ensures that axial material is continuously supplied to the stress concentration area, preventing the local thinning rate from exceeding the plastic limit. This design works simultaneously from four dimensions: drag reduction, hardness reduction, stress dispersion, and precise material feeding. This reduces the scrap rate of the branch pipe breakage, while ensuring that the thinning rate of the top of the branch pipe is ≤15%, and that the tensile strength, yield strength, and other mechanical properties are maintained at more than 90% of the raw material, meeting the usage requirements of high-end fields such as transportation equipment and precision instruments.

[0036] 2. This invention achieves effective suppression of wrinkling on the inner side of ultra-thin stainless steel pipe fittings and a significant improvement in forming dimensional accuracy through the design of an adaptive elastic bushing sealing head, precise mold preheating, coordinated operation of internal pressure and axial feed during the pre-expansion molding stage, and a stress release design with segmented pressure relief. The adaptive elastic bushing sealing head, reinforced with fluororubber and carbon fiber, has an annular groove on its inner wall to store lubricating medium and enhance sealing fit. It adapts to 0.1-0.5mm ultra-thin pipe walls with a clamping force of 0.3-0.5MPa, ensuring internal pressure fluctuation ≤±1%, thus solving the problems of easy pressure leakage and insufficient internal pressure support in traditional seals. The mold uses an electric heating wire. The embedded preheating method, with a preheating temperature of 60-80℃, reduces the thermal stress between the tube blank and the mold, avoiding uneven material flow caused by thermal expansion and contraction. During the pre-expansion and mold-fitting stage, the pressure is increased to 0.8MPa / s and maintained at a stable rate, while simultaneously completing the axial feed of 30% of the total material replenishment. This allows the tube blank to fit into the mold cavity wall in advance, establishing effective radial support and preventing compressive stress instability and wrinkling during subsequent material replenishment. The segmented pressure relief design can slowly release residual stress, further suppressing the generation of micro-wrinkles. The shape accuracy meets the assembly requirements of high-end equipment, while reducing stress concentration during service and significantly improving the fatigue life of the tube fittings. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the process flow of the present invention;

[0038] Figure 2 This is a schematic diagram of the overall process structure of the present invention;

[0039] Figure 3 This is a schematic diagram of the stress-dispersing mold structure of the present invention. Detailed Implementation

[0040] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0041] Example 1

[0042] The following uses a 316L ultra-thin stainless steel tee fitting with a wall thickness of 0.2mm and a diameter-to-thickness ratio of t / D=0.015 as an example to illustrate the implementation process of this invention in detail:

[0043] S1. Tube blank preparation and pretreatment

[0044] S101: Select a 316L ultra-thin stainless steel tube blank with a wall thickness of 0.2mm, cut it to the target length of 100mm, and leave an 8% axial material allowance (8mm). The final length after forming is 92mm. S102: Place the tube blank in an ethanol solution for ultrasonic degreasing and cleaning for 15min to remove oil and impurities from the inner and outer walls, and let it air dry naturally. S103: Prepare a composite nano-lubricant with a mass ratio of polytetrafluoroethylene and molybdenum disulfide of 7.5:2.5, and apply it evenly to the inner wall of the tube blank by spraying. Control the film thickness to 8μm, and let it air dry at room temperature for 30min. S104: Place the dried tube blank in a constant temperature furnace, set the preheating temperature to 180℃, hold it at that temperature for 30min, and then remove it after cooling it to 75℃ in the furnace to avoid surface oxidation.

[0045] S2. Mold and sealing assembly

[0046] S201: Install the stress-dispersing tee mold, with a branch pipe cavity radius R=0.6mm (3×0.2mm), and a TiN coating hardness HV=2200 on the cavity surface. Activate the embedded preheating device with electric heating wire to raise the mold temperature to 70℃. Monitor the temperature using a temperature sensor to ensure that temperature fluctuations in each area are ≤±2℃. S202: Place the pipe blank treated in S1 into the mold cavity. Install fluororubber composite carbon fiber reinforced adaptive elastic bushings at both ends. The inner wall of the sealing head has four annular grooves (depth 0.25mm, width 1.2mm). Drive the sealing head through the hydraulic system to axially press the two ends of the pipe blank with a clamping force of 0.4MPa, ensuring a tight seal without damage. S203: Connect the high-pressure pump station and pressure sensor (accuracy ±0.01MPa) to form an internal pressure supply system. Connect the axial feed device driven by dual servo motors, and debug to confirm that the feed synchronization accuracy is 0.03mm, meeting the design requirements.

[0047] S3. Phased Collaborative Forming

[0048] S301: Pre-expansion and die-fitting stage: The internal pressure supply system is activated, raising the internal pressure of the tube blank to 0.8MPa at a rate of 0.1MPa / s and maintaining stable pressure; simultaneously, the axial feed device is activated, feeding synchronously at both ends at a feed rate of 0.1mm / s, with a feed amount of 30% of the total feed amount (total feed amount = branch pipe volume × 1.1 = 12.56mm³ × 1.1 ≈ 13.82mm³, corresponding to a feed amount of 2.6mm), continuing for 12s, until the tube blank completely fits the mold cavity wall, establishing stable radial support; S302: Main forming precision feeding stage: A gradient pressure increase strategy is adopted, raising the internal pressure by 0.5MPa every 5s, eventually reaching 6.5MPa (due to the tube blank wall thickness of 0.2mm, this is an increase of 0.5MPa compared to 6MPa); the axial feed rate is simultaneously accelerated to 0.25mm / s, and the forming height of the top of the branch pipe (target height) is monitored in real time by a displacement sensor. 10mm), when the height reaches 8mm (target value 80%), the feed rate is reduced to 0.1mm / s; the thinning rate of the top of the branch pipe is monitored in real time by an ultrasonic thickness gauge. When the thinning rate reaches 11.5% (close to the 12% warning threshold), the internal pressure increase is paused, the feed is maintained for 4s and then the pressure is increased again. The entire stage lasts for 25s to ensure sufficient material supply and avoid excessive thinning; S303: Forming stress release stage: a stable internal pressure of 6.5MPa is maintained for 10s. The feed amount is finely adjusted by 0.3mm (≤5% of the total material supply) through the axial feed device to ensure that the tube blank completely fits the mold cavity; then the segmented pressure relief program is started. When the internal pressure drops to above 2MPa, it is reduced by 1MPa every 5s. When the internal pressure drops to below 2MPa, the pressure relief rate is adjusted to reduce by 0.5MPa every 5s until the internal pressure is completely relieved. This stage lasts for 15s to effectively release residual stress.

[0049] S4. Unloading and Retrieving

[0050] After the internal pressure is completely released, the hydraulic system is controlled to loosen the sealing head, open the mold, and take out the formed tee fitting. The sealing edges at both ends (about 2mm wide) are removed by mechanical cutting. Then the fitting is placed in a neutral cleaning agent and the surface residual nano-lubricating film is cleaned by high-pressure spraying (pressure 0.8MPa). After cleaning, it is allowed to air dry naturally.

[0051] S5. Quality Inspection

[0052] S501: Dimensional Inspection: The height of the branch pipe was measured to be 10.02mm using a laser rangefinder (meets the target requirement). The thinning rate at the top of the branch pipe was measured to be 12.3% (≤15%) using an ultrasonic thickness gauge, and the thickening rate at the root was measured to be 18.5% (≤20%). The ellipticity was 0.08mm (≤0.1mm). All dimensional parameters were qualified. S502: Defect Inspection: The surface of the pipe fittings was visually inspected and found to have no obvious wrinkles or scratches. Ultrasonic flaw detector (frequency 5MHz) was used for inspection and no defects such as cracks or microcracks were found. S503: Mechanical Performance Inspection: Three pipe fittings were sampled and tested. The tensile strength was 520MPa and the yield strength was 240MPa, which respectively maintained 92.9% and 94.1% of the raw material (tensile strength 560MPa, yield strength 255MPa), meeting the design requirements.

[0053] detection indicators Traditional hydraulic / internal high-pressure forming process The forming method of the present invention Process improvement effect Branch pipe top breakage scrap rate 15%～30% 2% The rate of fracture defects was reduced by more than 86.7%. Wrinkling rate on the inside of pipe fittings ＞20% 1.5% The wrinkling defect rate was reduced by more than 92.5%. Overall molding pass rate ≤65% 97.5% The pass rate increased by more than 32.5 percentage points.

[0054] Based on the data comparison in the table above, in this embodiment, the pipe fitting forming qualification rate reached 97.5%, with a breakage scrap rate of 2% and a wrinkling rate of 1.5%, which is a significant improvement compared to the traditional process, verifying the effectiveness and practicality of the present invention.

[0055] The embodiments disclosed in this invention are preferred embodiments, but are not limited thereto. Those skilled in the art can easily understand the spirit of this invention based on the above embodiments and make different extensions and variations, but as long as they do not depart from the spirit of this invention, they are all within the protection scope of this invention.

Claims

1. A method for forming ultra-thin stainless steel pipe fittings, characterized in that, Includes the following steps: S1: Tube blank preparation and pretreatment; S101: Select 316L ultra-thin stainless steel tube blanks with a wall thickness of 0.1-0.5mm, cut them to the target length, and reserve 5-10% axial material allowance; S102: Ultrasonic degreasing and cleaning of the inner and outer walls of the cut tube blank with ethanol to remove oil and impurities. S103: After cleaning, a nano-lubricating film with a thickness of 5-10μm is uniformly coated on the inner wall of the tube blank and then air-dried at room temperature. S104: After drying, the tube blank is placed in a constant temperature furnace and preheated at a low temperature of 150-250℃ for 30 minutes, and then cooled to below 80℃ with the furnace. S2: Mold and sealing assembly: S201: Stress-dispersing mold with a gradual transition design for the branch pipe area. The radius of the mold branch pipe cavity is R≥3t, where t is the wall thickness of the pipe blank. The mold cavity surface is coated with TiN. The mold is preheated to 60-80℃. S202: The tube blank treated by S1 is placed into the mold cavity, and adaptive elastic bushing sealing heads made of fluororubber composite carbon fiber reinforced material are installed at both ends. The sealing heads are axially pressed by hydraulic drive with a clamping force of 0.3-0.5MPa. S203: An internal pressure supply system consisting of a high-pressure pump station and a pressure sensor, and an axial feed device driven by dual servo motors, wherein the synchronization accuracy of the axial feed device is ≤0.05mm; S3: Phased collaborative forming; S301: During the pre-expansion and molding stage, the internal pressure is increased to 0.8MPa at a rate of 0.1MPa / s and kept stable. At the same time, the axial feed device is controlled to feed synchronously at both ends at a rate of 0.1mm / s and a feed amount of 30% of the total feed amount, which lasts for 10-15s, so that the tube blank fits the mold cavity wall. S302: In the main forming precision feeding stage, a gradient pressure increase strategy is adopted, increasing the internal pressure by 0.5MPa every 5s, eventually reaching 6-7MPa, while simultaneously accelerating the axial feed rate to 0.2-0.3mm / s; the forming height of the top of the branch pipe is monitored in real time by a displacement sensor, and when the height reaches 80% of the target value, the feed rate is reduced to 0.1mm / s; the thinning rate of the top of the branch pipe is monitored in real time by an ultrasonic thickness gauge, and when the thinning rate approaches 12%, the internal pressure increase is paused, the feed is maintained for 3-5s, and then the pressure is increased again, lasting for 20-30s; S302: During the stress release stage of forming, maintain a stable internal pressure of 6-7MPa for 10s. At the same time, finely adjust the feed amount through the axial feed device to ensure that the tube blank completely fits the mold cavity. Then, adopt a segmented pressure relief method, reducing the pressure by 1MPa every 5s to complete the release of residual stress for 15s. S4: Unloading and Retrieving Components; S5: Quality Inspection.

2. The method for forming ultra-thin stainless steel pipes according to claim 1, characterized in that, In S103, the nano-lubricating film is composed of polytetrafluoroethylene (PTFE) and molybdenum disulfide (MoD), and the mass ratio of PTFE to MoD in the nano-lubricating film is 7:3-8:

2. In S104, the low-temperature preheating temperature is adjusted according to the tube blank wall thickness: when the tube blank wall thickness is ≤0.2mm, the preheating temperature is 150-180℃; when the tube blank wall thickness is >0.2mm and ≤0.5mm, the preheating temperature is 200-250℃.

3. The method for forming ultra-thin stainless steel pipes according to claim 2, characterized in that, The preheating method of the mold described in S201 is embedded preheating with electric heating wire, and the temperature fluctuation of each area of ​​the mold cavity is ≤±2℃.

4. The method for forming ultra-thin stainless steel pipes according to claim 3, characterized in that, The final internal pressure in the main forming precision feeding stage described in S302 is dynamically adjusted according to the billet wall thickness. Specifically, when the billet wall thickness increases by 0.1 mm, the final internal pressure increases by 1-1.2 MPa compared to 6 MPa, and the maximum internal pressure does not exceed 8 MPa.

5. The method for forming ultra-thin stainless steel pipes according to claim 4, characterized in that, During the segmented pressure relief process described in S302, when the internal pressure drops below 2 MPa, the pressure relief rate is adjusted to decrease by 0.5 MPa every 5 seconds to further reduce residual stress.

6. The method for forming ultra-thin stainless steel pipes according to claim 5, characterized in that, The inner wall of the adaptive elastic bushing sealing head described in S202 is provided with 3-5 annular grooves, the depth of which is 0.2-0.3 mm and the width of which is 1-1.5 mm, for storing lubricating medium and enhancing sealing performance.

7. The method for forming ultra-thin stainless steel pipes according to claim 6, characterized in that, The specific process for S4 is as follows: After the internal pressure is completely released, loosen the sealing head and the mold, take out the formed pipe fitting, and remove the sealing pressure edges at both ends; use a neutral cleaning agent in conjunction with high-pressure spraying to clean the residual nano-lubricating film on the surface of the pipe fitting.

8. The method for forming ultra-thin stainless steel pipes according to claim 7, characterized in that, S5 specifically includes the following steps: S501: Dimensional inspection, including branch pipe height, wall thickness distribution, and ellipticity; S502: Defect detection, confirming that the pipe fittings are free of cracks, wrinkles, and microcracks through visual inspection and ultrasonic testing; S502: Mechanical property testing, sampling tests of pipe fittings for tensile strength and yield strength to ensure that more than 90% of the performance of the raw materials is maintained.

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