A method for mechanical expansion of a low residual stress pressure vessel

Abstract

The application discloses a kind of low residual stress pressure vessel mechanical expansion methods, comprising: fine pretreatment and adaptability detection are carried out to workpiece, and obtain qualified pressure vessel tube sheet and heat exchange tube, sealant is coated on the tube sheet cooperation surface and carries out tube hole initial positioning, the modeling of all tube holes of tube sheet is carried out, realize coaxial alignment calibration;Drive ball tube expander to enter heat exchange tube at uniform speed, and expansion is connected using staged rolling expansion process and is matched with helium circulation cooling;Real-time acquisition expansion parameter calculates residual stress, when estimated residual stress is overproof or deformation is uneven, adjust the rolling expansion process parameter of next expansion section;After expansion, low-frequency micro-vibration aging treatment is carried out and is matched with low-temperature constant temperature preservation, to constant speed Slowly retreat sword, detect treated finished product, qualified product is stored, and unqualified product is marked for repair.The application can realize the double precision control of whole shape control stress, guarantee the low value of expansion residual stress and stress field homogenization.

Description

Technical Field

[0001] This invention relates to the field of heat exchanger technology, and in particular to a method for mechanical expansion jointing of a pressure vessel with low residual stress. Background Technology

[0002] Shell-and-tube pressure vessel heat exchangers are indirect heat exchangers that use the walls of tube bundles enclosed in a shell as the heat transfer surface. They are widely used in petroleum, chemical, energy, food, pharmaceutical, and other fields. The heat exchange tubes and tube sheets are connected by mechanical expansion joints, which are the core structure for pressure bearing, heat exchange, and sealing. The residual stress level of the joint directly determines the fatigue resistance, corrosion resistance, and service life of the equipment. Traditional mechanical rolling expansion processes often use a single, high-speed, large-deformation, one-time rolling expansion molding process. While simple, this process has significant drawbacks: the deformation rate during rolling is fast, the metal rheology is intense and uneven, and severe stress concentration occurs locally on the inner wall of the heat exchange tubes and the tube sheet hole walls. After molding, the joint has high residual tensile stress values ​​and uneven distribution.

[0003] Under long-term high-pressure, high-low temperature alternating, and corrosive media conditions, high residual stress areas in pressure vessels are highly susceptible to failures such as microcracks, stress corrosion cracking, joint springback loosening, and interface leakage. These are among the main causes of leakage and scrapping of pressure vessel heat exchangers. Existing improved expansion joint processes mainly focus on improving expansion joint accuracy and sealing performance, lacking a systematic process solution for actively controlling and homogenizing residual stress. They cannot suppress processing stress or eliminate stress peaks at the source, making it difficult to meet the manufacturing standards for high-pressure, long-life, and fatigue-resistant pressure vessels.

[0004] Therefore, there is an urgent need for an automated mechanical expansion joint process that can actively control the deformation process, significantly reduce and homogenize residual stress, ensure stable forming quality, and be compatible with various pipe materials, in order to solve the current stress failure problem of expansion joints in pressure vessels and improve equipment reliability. Summary of the Invention

[0005] The present invention aims to provide a mechanical expansion joint method for pressure vessels with low residual stress that can achieve precise control of both shape and stress throughout the entire process, in order to solve the problems of high residual stress, stress concentration, easy corrosion and cracking of joints and leakage failure in existing expansion joints. The present invention achieves low residual stress and uniform stress field of expansion joints, and is suitable for precision machining of pressure vessels of multiple materials and pressure levels while ensuring connection strength and sealing performance.

[0006] To achieve the above objectives, the basic solution of the present invention is as follows: a method for mechanical expansion jointing of a pressure vessel with low residual stress, comprising the following steps: S1. Perform fine pretreatment and compatibility testing on the workpieces to obtain qualified workpieces, including pressure vessel tube sheets and heat exchange tubes. S2. Apply sealant to the mating surfaces of the pressure vessel tube sheet; S3. Obtain the tube sheet drawings and heat exchange tube dimensions of the pressure vessel and perform initial positioning of the tube holes; S4. Perform three-dimensional scanning modeling of all tube holes in the pressure vessel tube sheet, and correct the spatial position of the tube expansion mechanism based on the scanning results to achieve coaxial alignment between the ball tube expander and the heat exchange tube. S5. Drive the ball expander to extend into the heat exchange tube at a uniform speed to the preset expansion depth. Use a staged expansion process to expand the pressure vessel tube sheet and heat exchange tube in sections, and use helium for circulating cooling. S6. Real-time acquisition of rolling torque fluctuation, pipe wall deformation and expansion uniformity, calculation of estimated residual stress, and adjustment of rolling process parameters for the next expansion section when the estimated residual stress exceeds the standard or the deformation is uneven. S7. After the pressure vessel is expanded, the pressure vessel is subjected to low-frequency micro-vibration aging treatment at a set frequency, and residual stress is relieved by low-temperature constant temperature insulation treatment. S8. After the expansion joint is shaped, control the servo mechanism to slowly retract the tool at a constant speed within the feed speed range. S9. Perform performance and stress compliance tests on the finished products after processing. If the products are qualified, they will be put into storage; if they are not qualified, they will be marked for rework.

[0007] Further, step S1 specifically includes: polishing and grinding the inner wall of the tube hole of the pressure vessel tube sheet and the outer wall of the heat exchange tube so that the roughness of the inner wall of the tube hole and the roughness of the outer wall of the heat exchange tube are respectively within the corresponding roughness range, and wiping and cleaning with anhydrous ethanol; detecting the roundness of the tube hole of the tube sheet, the diameter and wall thickness of the heat exchange tube, screening qualified workpieces, calculating the single-sided assembly gap between the tube hole and the heat exchange tube, and selecting qualified workpieces within the gap range for pre-assembly and positioning.

[0008] Further, step S2 specifically includes: using a micro-scale uniform coating process to uniformly coat an ultra-thin sealant layer on the mating area of ​​the inner wall of the tube sheet and the mating area of ​​the outer wall of the heat exchange tube, with the coating thickness controlled within a set thickness range. The sealant is a modified polytetrafluoroethylene high-temperature resistant sealant suitable for pressure vessel conditions. After coating, it is pre-cured at room temperature and allowed to stand to ensure uniform film coverage.

[0009] Furthermore, step S4 specifically includes: using machine vision combined with a laser ranging module to perform three-dimensional scanning modeling of all tube holes in the tube sheet, identifying the tube hole coordinates, perpendicularity and hole depth parameters, and correcting the spatial position of the tube expansion mechanism according to the scanning results, so that the coaxiality deviation between the ball tube expander and the heat exchange tube axis is less than or equal to the deviation threshold.

[0010] Further, step S5 specifically includes: driving the rolling expander to extend into the heat exchange tube at a uniform speed to the preset expansion depth, dividing the rolling expansion process into four stages, and performing graded expansion of the pressure vessel tube sheet and the heat exchange tube; simultaneously activating the helium cooling system during the entire rolling expansion process, filling high-purity, high-pressure helium from the air inlet of the rolling expander, so that the helium in the central air channel is continuously injected and circulated back, and the temperature of the expansion area is stably controlled within the normal temperature range.

[0011] Furthermore, step S6 specifically includes: using a high-precision torque sensor and a laser deformation sensor to collect data in real time on the rolling torque fluctuation, pipe wall deformation, and expansion uniformity, and to calculate the estimated residual stress in real time; when the estimated residual stress is detected to exceed the standard or the deformation is uneven, the rolling process parameters of the next expansion segment are adjusted.

[0012] Furthermore, step S7 specifically includes: after the pressure vessel is expanded, it is subjected to low-frequency micro-vibration aging treatment and low-temperature constant temperature insulation in conjunction with a set temperature; the residual stress of mechanical rolling deformation is gradually released through a composite process, so that the residual stress dissipation rate is greater than or equal to the dissipation rate threshold, and the residual stress of the joint is controlled within the set value of the material yield strength.

[0013] Furthermore, the low-temperature constant temperature insulation is achieved using a trolley-type gas-fired heat treatment furnace, which heats up at a set heating rate and maintains the temperature according to a set holding time.

[0014] Furthermore, step S8 specifically includes: after the expansion joint is shaped, controlling the servo mechanism to slowly retract the tool at a constant low speed below the feed speed set range, with the retraction speed controlled within the set speed range, so that the expansion joint size and stress state are stable.

[0015] Furthermore, the performance and stress compliance testing includes residual stress testing, dimensional re-inspection, and airtightness and water pressure resistance testing.

[0016] Compared with the prior art, the advantages and beneficial effects of this invention are as follows: Qualified pressure vessel tube sheets and heat exchange tubes are obtained through refined preprocessing and compatibility testing; sealant is applied to the mating surfaces of the pressure vessel tube sheets to meet the requirements of low-stress mechanical expansion molding; pressure vessel tube sheet drawings and heat exchange tube dimensions are obtained for initial tube hole positioning; three-dimensional scanning modeling is performed on all tube holes of the pressure vessel tube sheet to correct the spatial position of the expansion mechanism, achieving high-precision coaxial alignment between the ball expander and the heat exchange tube; a graded rolling expansion process is used to drive the rolling expander to uniformly extend into the heat exchange tube to the preset expansion depth, and helium is used for circulating cooling throughout the rolling expansion process to avoid stress accumulation and thermal stress superposition during single large deformation; real-time acquisition of rolling torque fluctuations, tube wall deformation, and expansion uniformity is used to calculate the estimated residual stress, and... When residual stress is estimated to be excessive or deformation is uneven, the rolling expansion process parameters are adjusted to ensure stress consistency in batch processing. After expansion, the pressure vessel is subjected to low-frequency micro-vibration aging treatment at a set frequency, combined with low-temperature constant-temperature insulation treatment to release residual stress, thereby homogenizing and eliminating residual stress. After expansion and shaping, the servo mechanism is controlled to slowly retract the tool at a constant speed below the set feed speed to ensure the stability of the expansion joint size and stress state. Finally, the finished product is subjected to performance and stress compliance testing. If the product is qualified, it is put into storage; if it is unqualified, it is marked for rework and awaits rework. This achieves dual precise control of shape and stress throughout the process, ensuring low residual stress and uniform stress field, and is suitable for precision machining of pressure vessels of multiple materials and pressure levels. Attached Figure Description

[0017] Figure 1 This is a schematic flowchart of a mechanical expansion joint method for a pressure vessel with low residual stress in one embodiment.

[0018] Figure 2 This is a schematic diagram of a four-stage progressive expansion joint in one embodiment.

[0019] Figure 3 This is a schematic diagram of a tube expander with helium cooling in one embodiment.

[0020] In the attached diagram, the components are: 1. tube expander; 2. helium inlet; 3. tube sheet; 4. heat exchange tube; 5. expansion shell; 6. expansion rod; 7. expansion ball; 8. end cap nut; 9. inlet connector; 10. thrust bearing; and 11. square connector. Detailed Implementation

[0021] To make the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] like Figure 1 As shown, a method for mechanical expansion jointing of a pressure vessel with low residual stress is provided, comprising the following steps: Step S1 involves performing fine pretreatment and compatibility testing on the workpiece to obtain qualified workpieces, including pressure vessel tube sheets and heat exchange tubes.

[0023] Specifically, step S1 includes: polishing and grinding the inner wall of the tube hole of the pressure vessel tube sheet and the outer wall of the heat exchange tube so that the roughness of the inner wall of the tube hole and the roughness of the outer wall of the heat exchange tube are within the corresponding roughness range, and wiping and cleaning with anhydrous ethanol; detecting the roundness of the tube hole of the tube sheet, the diameter and wall thickness of the heat exchange tube, screening qualified workpieces, calculating the single-sided assembly gap between the tube hole and the heat exchange tube, and selecting qualified workpieces within the gap range for pre-assembly and positioning.

[0024] Specifically, the inner walls of the tube holes in the pressure vessel tube sheet and the outer walls of the heat exchange tubes are finely polished to thoroughly remove oxide scale, micro-burrs, oil stains, and rust. Anhydrous ethanol is used for wiping and cleaning to ensure that the mating surfaces are clean and free of impurities. The roundness of the tube holes, the diameter tolerance, the diameter of the heat exchange tubes, and the wall thickness are checked using precision testing equipment. The single-sided assembly gap between the tube holes and the heat exchange tubes is strictly controlled to be 0.10–0.30 mm, the inner wall roughness of the tube holes is Ra1.6–Ra3.2 μm, and the outer wall roughness of the heat exchange tubes is Ra2.0–Ra3.6 μm. Qualified workpieces are selected for precise pre-assembly and positioning to reduce initial stress defects from the assembly foundation.

[0025] Step S2: Apply sealant to the mating surfaces of the pressure vessel tube sheet.

[0026] Specifically, step S2 includes: using a micro-scale uniform coating process to uniformly coat an ultra-thin sealant layer on the mating area of ​​the inner wall of the tube sheet and the mating area of ​​the outer wall of the heat exchange tube, with the coating thickness controlled within a set thickness range. The sealant is a modified polytetrafluoroethylene high-temperature resistant sealant adapted to pressure vessel conditions. After coating, it is pre-cured at room temperature and allowed to stand to ensure uniform film coverage.

[0027] Specifically, before the workpiece pretreatment is completed and pre-positioning is completed, a special high-temperature and high-pressure resistant sealant is used to coat the mating surfaces of the pressure vessel tube sheet to meet the requirements of mechanical low-stress expansion joint forming.

[0028] A micro-scale uniform coating process is adopted to uniformly coat an ultra-thin adhesive layer on the mating area of ​​the inner wall of the tube sheet and the mating area of ​​the outer wall of the heat exchange tube. The coating thickness is controlled between 0.03 and 0.06 mm. The special high-temperature and high-pressure resistant sealant is a modified polytetrafluoroethylene high-temperature resistant sealant adapted to pressure vessel conditions.

[0029] After coating, allow it to stand at room temperature for 3-5 minutes to ensure that the adhesive layer is evenly coated without accumulation, bubbles, or breaks. This not only fills the tiny gaps in the micro-assembly and improves the sealing performance of the interface after subsequent expansion, but also provides a small amount of lubrication and friction reduction during the rolling process, further reducing mechanical rolling friction resistance, reducing frictional heat and deformation stress, and assisting in achieving low-stress molding. At the same time, it prevents over-expansion and poor adhesion caused by excessively thick adhesive layers.

[0030] Step S3: Obtain the pressure vessel tube sheet drawing and heat exchange tube dimensions, and perform initial positioning of the tube holes.

[0031] Step S4: Perform 3D scanning modeling of all tube holes in the pressure vessel tube sheet, and correct the spatial position of the tube expansion mechanism based on the scanning results to achieve coaxial alignment between the ball tube expander and the heat exchange tube.

[0032] Specifically, step S4 includes: using machine vision combined with a laser ranging module to perform three-dimensional scanning modeling of all tube holes in the tube sheet, identifying the tube hole coordinates, perpendicularity and depth parameters, and correcting the spatial position of the tube expansion mechanism based on the scanning results, so that the coaxiality deviation between the ball tube expander and the heat exchange tube axis is less than or equal to the deviation threshold.

[0033] Specifically, machine vision combined with a laser ranging module is used to perform three-dimensional scanning and modeling of all tube holes in the tube sheet, automatically identify tube hole coordinates, perpendicularity, and hole depth parameters, and servo automatically correct the spatial position of the tube expansion mechanism to ensure that the coaxiality deviation between the ball tube expander and the heat exchange tube axis is ≤±0.02mm, thus completely eliminating the problem of uneven deformation on one side and local stress concentration caused by eccentric rolling.

[0034] Step S5: Drive the ball expander to extend into the heat exchange tube at a uniform speed to the preset expansion depth. Use a staged expansion process to expand the pressure vessel tube sheet and heat exchange tube in sections, and use helium gas for circulating cooling.

[0035] Specifically, step S5 includes: driving the rolling expander to extend into the heat exchange tube at a uniform speed to the preset expansion depth, dividing the rolling expansion process into four stages, and performing graded expansion of the pressure vessel tube sheet and the heat exchange tube; and simultaneously activating the helium cooling system during the entire rolling expansion process, filling the air inlet of the rolling expander with high-purity, high-pressure helium, so that the helium in the central air channel is continuously injected and circulated back, and the temperature of the expansion area is stably controlled within the normal temperature range.

[0036] Specifically, the ball-driven tube expander extends into the heat exchanger tube at a uniform speed to the preset expansion depth. It employs a four-stage incremental, low-speed, and shock-free graded expansion process, releasing the stored deformation energy step by step to expand the pressure vessel tube sheet and heat exchanger tubes, avoiding stress accumulation from a single large deformation. The four-stage progressive expansion structure is as follows: Figure 2 As shown, the heat exchange tube 4 is located inside the tube hole of the tube sheet 3. The tube expander 1 is inserted into the tube hole of the heat exchange tube 4, and the heat exchange tube 4 and the tube sheet 3 are expanded together with the helium gas filled in through the helium gas inlet 2.

[0037] The graded rolling expansion process includes four rolling expansion stages: The micro-deformation pre-fitting stage involves low-speed, small-feed light rolling to eliminate assembly gaps, correct micro-deformation at the tube end, and hold the rolling position for 2-4 seconds to eliminate initial assembly stress.

[0038] The low-speed incremental plastic forming stage involves a small gradient increase in rolling speed and feed rate, which allows the tube wall metal to expand slowly and steadily, achieving uniform metal rheology and eliminating instantaneous high stress.

[0039] The constant torque stabilization and shaping stage involves maintaining a constant torque for 5–12 seconds after the preset expansion joint dimensions are reached, balancing the circumferential and axial stress distribution of the pipe wall and offsetting local stress peaks.

[0040] The micro-speed repair average stress stage involves reducing the rotation speed and low load during rolling to smooth out the deformation texture of the pipe wall, further homogenize the circumferential stress field, and completely eliminate stress concentration points.

[0041] The helium cooling system is activated simultaneously throughout the entire expansion process to prevent the accumulation of thermal stress. For example... Figure 3 As shown, the tube expander equipped with helium cooling includes an expansion shell 5, an expansion rod 6, an expansion ball 7, a cap nut 8, an air inlet connector 9, a thrust bearing 10, and a square connector 11.

[0042] During the expansion process, a helium circulation cooling system is used. Utilizing the high thermal conductivity, good fluidity, inertness, and non-oxidizing properties of helium, high-purity, high-pressure helium is injected from the air inlet of the ball bearing expander throughout the entire expansion process. This continuously sprays and circulates the gas into the central air passage of the expander, carrying away the heat from the rolling friction. This keeps the temperature of the expansion area stably controlled within the normal temperature range, preventing defects such as pipe strength fluctuations, localized thermal stress accumulation, and pipe wall thermal rebound caused by temperature rise. This also suppresses the problems of thermal stress and uneven metal rheology caused by frictional heat generation.

[0043] Step S6: Real-time collection of rolling torque fluctuation, pipe wall deformation and expansion uniformity; calculation of estimated residual stress; when the estimated residual stress exceeds the standard or the deformation is uneven, adjustment of the rolling process parameters of the next expansion section.

[0044] Specifically, step S6 includes: using a high-precision torque sensor and a laser deformation sensor to collect data in real time on the rolling torque fluctuation, pipe wall deformation, and expansion uniformity, and to calculate the estimated residual stress in real time; when the estimated residual stress exceeds the standard or the deformation is uneven, the rolling process parameters of the next expansion section are adjusted.

[0045] Specifically, by using high-precision torque sensors and laser deformation sensors, data such as rolling torque fluctuations, pipe wall deformation, and expansion uniformity are collected in real time, and the estimated residual stress is calculated in real time. Since the current expansion is staged, if the estimated residual stress peak exceeds the standard or the deformation is uneven in the current expansion section, the system can automatically fine-tune the rolling process parameters of the next expansion section. The rolling process parameters include radial feed, static pressure, rolling speed, etc., to achieve precise control of shape and stress through a dual closed loop, and also ensure the stress consistency of batch processing.

[0046] Step S7: After the pressure vessel expansion is completed, the pressure vessel is subjected to low-frequency micro-vibration aging treatment at a set frequency, and residual stress is relieved by low-temperature constant temperature insulation treatment.

[0047] Specifically, step S7 includes: after the pressure vessel is expanded, it is subjected to low-frequency micro-vibration aging treatment and low-temperature constant temperature insulation in conjunction with the set temperature; the residual stress of mechanical rolling deformation is gradually released through the composite process, so that the residual stress dissipation rate is greater than or equal to the dissipation rate threshold, and the residual stress of the joint is controlled within the set value of the material yield strength.

[0048] The low-temperature constant temperature insulation is achieved using a trolley-type gas-fired heat treatment furnace, which heats up at a set heating rate and maintains the temperature according to a set holding time.

[0049] Specifically, after the entire pressure vessel is expanded, it is subjected to low-frequency micro-vibration aging treatment of 25-35Hz for 15-25 minutes, while being kept at a low temperature of 160-200℃ (temperature fluctuation ≤ ±3℃). Through a composite process, the residual tensile stress of mechanical rolling deformation is gradually released, the residual stress relief rate is ≥60%, and the residual stress of the joint is finally stably controlled within 30% of the material yield strength.

[0050] Low-temperature constant temperature heat treatment can be achieved using a trolley-type gas-fired heat treatment furnace, with a heating rate of 50℃ / h and a heat treatment time of 30-60min.

[0051] Step S8: After the expansion and shaping are completed, control the servo mechanism to slowly retract the tool at a constant speed within the set speed range of the feed rate.

[0052] Specifically, step S8 includes: after the expansion joint is shaped, the servo mechanism is controlled to slowly retract the tool at a constant low speed within the feed speed setting range. The retraction speed is controlled within the setting speed range to stabilize the expansion joint size and stress state.

[0053] Specifically, after the expansion joint is shaped, the servo mechanism is controlled to slowly retract the tool at a constant low speed of 30% to 50% lower than the feed speed. The retraction speed is controlled at 0.2 to 0.5 mm / s to completely avoid secondary deformation and additional residual stress caused by rapid retraction, and to ensure the stability of the expansion joint size and stress state.

[0054] Step S9: Perform performance and stress compliance tests on the finished product after processing. If the product is qualified, it will be put into storage; if it is not qualified, it will be marked for rework.

[0055] Among them, performance and stress compliance testing includes residual stress testing, dimensional re-inspection, and airtightness and water pressure resistance testing.

[0056] Specifically, the finished products after the tumbling process undergo residual stress testing, dimensional re-inspection, and airtightness and water pressure resistance testing to ensure that the expansion joint dimensions, connection strength, sealing performance, and stress indicators are all compliant. Qualified products are put into storage, and unqualified products are automatically marked for rework.

[0057] In this embodiment, qualified pressure vessel tube sheets and heat exchange tubes are obtained through refined preprocessing and compatibility testing. Sealant is applied to the mating surfaces of the pressure vessel tube sheets to meet the requirements of low-stress mechanical expansion molding. Pressure vessel tube sheet drawings and heat exchange tube dimensions are obtained for initial tube hole positioning. Three-dimensional scanning modeling is performed on all tube holes in the pressure vessel tube sheet to correct the spatial position of the tube expansion mechanism, achieving high-precision coaxial alignment between the ball-bearing tube expander and the heat exchange tube. A staged rolling expansion process drives the rolling tube expander to uniformly extend into the heat exchange tube to the preset expansion depth, and helium is used for circulating cooling throughout the rolling expansion process to avoid stress accumulation and thermal stress superposition during single large deformation. Real-time data collection of rolling torque fluctuations, tube wall deformation, and expansion uniformity is used to calculate the estimated residual stress. If the estimated residual stress exceeds... When the deformation is uneven, adjust the rolling expansion process parameters for the next batch to ensure stress consistency in batch processing. After expansion, perform low-frequency micro-vibration aging treatment on the pressure vessel using a set frequency, and combine it with low-temperature constant temperature insulation treatment to release residual stress, thereby homogenizing and eliminating residual stress. After expansion and shaping, control the servo mechanism to slowly retract the tool at a constant speed below the set feed speed to ensure the stability of the expansion joint size and stress state. Finally, perform performance and stress compliance testing on the finished product. If the product is qualified, it is put into storage; if it is unqualified, it is marked for rework and awaits rework. This achieves dual precise control of shape and stress throughout the process, ensuring low residual stress and uniform stress field, and is suitable for precision machining of pressure vessels of multiple materials and pressure levels.

[0058] In one embodiment, the above-mentioned method for mechanical expansion jointing of a pressure vessel with low residual stress is further illustrated with examples: For 316L stainless steel heat exchange tubes with a diameter of Φ19mm and a wall thickness of 2.0mm, a tube sheet thickness of 35mm, and a container design pressure of 2.5MPa, the pre-treatment assembly gap is 0.18mm, high-precision coaxial alignment calibration is performed, with a pre-bonding speed of 280r / min, a shaping speed of 430r / min, a finishing rolling speed of 310r / min, a feed rate of 0.45mm / s, a rolling torque of 19N·m, and a retraction speed of 0.3mm / s; the entire process is under dynamic helium cooling at a supply pressure of 0.16MPa, with four-stage low-speed progressive rolling expansion, a 10s rolling hold to stabilize the stress field, and a temperature control of ≤38℃ throughout; stress relief is achieved through 32Hz micro-vibration aging for 22min followed by a 50min stress-relief holding at 170℃.

[0059] Finished product inspection: The residual stress is 28% of the material's yield strength, there is no thermal stress or local stress peaks, and the fatigue resistance, pressure resistance, and corrosion resistance meet the standards for high-pressure vessels.

[0060] In summary, the present invention has the following advantages over the prior art: First, it abandons the traditional one-time large deformation impact expansion joint and adopts a four-stage graded rolling expansion of pre-bonding, low-speed shaping, constant torque shaping and micro-speed smoothing. Combined with gradient speed change, smooth feed and impact-free start and stop process, the tube wall metal slowly and uniformly rheomorphizes and releases the deformation energy in stages, eliminating the high residual stress and local stress concentration caused by instantaneous large deformation. Second, a special tube expander with a central air channel is used to circulate and spray high-purity helium gas throughout the four-stage rolling expansion process to remove the rolling friction heat in real time and keep the temperature of the expansion joint area constant below 40°C. This completely eliminates thermal coupling defects such as thermal stress, thermal rebound, and local softening of the tube caused by mechanical rolling friction heat generation from the source. Third, the gradient speed change and low-speed non-impact retraction process is adopted, and there is no secondary processing stress throughout the process, which ensures that the expansion size, interference fit and stress field are stable and do not deform. Fourth, the stress relief process combining micro-vibration and low-temperature isothermal control further homogenizes the stress field, significantly reduces residual stress values, and achieves integrated intelligent manufacturing that controls shape, temperature, and stress.

[0061] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0062] The above descriptions are merely embodiments of the present invention. Commonly known structures and characteristics are not described in detail here. Those skilled in the art are aware of all common technical knowledge in the field prior to the application date or priority date, are aware of all existing technologies in that field, and have the ability to apply conventional experimental methods prior to that date. Those skilled in the art can, under the guidance of this application, improve and implement this solution in combination with their own capabilities. Some typical known structures or methods should not be obstacles for those skilled in the art to implement this application. It should be noted that those skilled in the art can make several modifications and improvements without departing from the structure of the present invention. These should also be considered within the scope of protection of the present invention, and will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for mechanical expansion jointing of a pressure vessel with low residual stress, characterized in that, Includes the following steps: S1. Perform fine pretreatment and compatibility testing on the workpieces to obtain qualified workpieces, including pressure vessel tube sheets and heat exchange tubes. S2. Apply sealant to the mating surfaces of the pressure vessel tube sheet; S3. Obtain the tube sheet drawings and heat exchange tube dimensions of the pressure vessel and perform initial positioning of the tube holes; S4. Perform three-dimensional scanning modeling of all tube holes in the pressure vessel tube sheet, and correct the spatial position of the tube expansion mechanism based on the scanning results to achieve coaxial alignment between the ball tube expander and the heat exchange tube. S5. Drive the ball expander to extend into the heat exchange tube at a uniform speed to the preset expansion depth. Use a staged expansion process to expand the pressure vessel tube sheet and heat exchange tube in sections, and use helium for circulating cooling. S6. Real-time acquisition of rolling torque fluctuation, pipe wall deformation and expansion uniformity, calculation of estimated residual stress, and adjustment of rolling process parameters for the next expansion section when the estimated residual stress exceeds the standard or the deformation is uneven. S7. After the pressure vessel is expanded, the pressure vessel is subjected to low-frequency micro-vibration aging treatment at a set frequency, and residual stress is relieved by low-temperature constant temperature insulation treatment. S8. After the expansion joint is shaped, control the servo mechanism to slowly retract the tool at a constant speed within the feed speed range. S9. Perform performance and stress compliance tests on the finished products after processing. If the products are qualified, they will be put into storage; if they are not qualified, they will be marked for rework.

2. The method for mechanical expansion joint of a pressure vessel with low residual stress according to claim 1, characterized in that, Step S1 specifically includes: The inner wall of the tube hole of the pressure vessel tube sheet and the outer wall of the heat exchange tube are polished and ground to make the roughness of the inner wall of the tube hole and the roughness of the outer wall of the heat exchange tube within the corresponding roughness range, and then wiped and cleaned with anhydrous ethanol. The roundness of the tube holes, the diameter and wall thickness of the heat exchange tubes are checked, qualified workpieces are screened, and the single-sided assembly gap between the tube holes and the heat exchange tubes is calculated. Qualified workpieces within the gap range are selected for pre-assembly and positioning.

3. The method for mechanical expansion jointing of a pressure vessel with low residual stress according to claim 1, characterized in that, Step S2 specifically includes: A micro-volume uniform coating process is adopted to uniformly coat an ultra-thin sealant layer on the mating area of ​​the inner wall of the tube sheet and the mating area of ​​the outer wall of the heat exchange tube. The coating thickness is controlled within a set thickness range. The sealant is a modified polytetrafluoroethylene high-temperature resistant sealant adapted to pressure vessel conditions. After coating, it is pre-cured at room temperature and allowed to stand to ensure uniform film coverage.

4. The method for mechanical expansion joint of a pressure vessel with low residual stress according to claim 1, characterized in that, Step S4 specifically includes: Using machine vision combined with a laser ranging module, three-dimensional scanning modeling is performed on all tube holes in the tube sheet to identify tube hole coordinates, perpendicularity and depth parameters. Based on the scanning results, the spatial position of the tube expansion mechanism is corrected so that the coaxiality deviation between the ball tube expander and the heat exchange tube axis is less than or equal to the deviation threshold.

5. The method for mechanical expansion joint of a pressure vessel with low residual stress according to claim 1, characterized in that, Step S5 specifically includes: The rolling expansion device is driven to extend into the heat exchange tube at a uniform speed to the preset expansion depth. The rolling expansion process is divided into four stages to carry out the graded expansion of the pressure vessel tube sheet and the heat exchange tube. During the entire rolling expansion process, the helium cooling system is activated simultaneously, and high-purity, high-pressure helium is introduced from the air inlet of the ball expander, so that the helium in the central air channel is continuously injected and circulated back, and the temperature of the expansion joint area is stably controlled within the normal temperature range.

6. The method for mechanical expansion joint of a pressure vessel with low residual stress according to claim 1, characterized in that, Step S6 specifically includes: By using high-precision torque sensors and laser deformation sensors, the rolling torque fluctuation, pipe wall deformation and expansion uniformity are collected in real time, and the residual stress is calculated and estimated in real time. If the estimated residual stress exceeds the limit or the deformation is uneven, adjust the rolling expansion process parameters of the next expansion section.

7. The method for mechanical expansion joint of a pressure vessel with low residual stress according to claim 1, characterized in that, Step S7 specifically includes: After the pressure vessel is expanded, it undergoes low-frequency micro-vibration aging treatment and is kept at a low temperature and constant temperature in conjunction with a set temperature. By gradually releasing the residual stress from mechanical rolling deformation through a composite process, the residual stress relief rate is made greater than or equal to the relief rate threshold, and the residual stress in the joint is controlled within the set value of the material yield strength.

8. The method for mechanical expansion joint of a pressure vessel with low residual stress according to claim 7, characterized in that, The low-temperature constant temperature insulation is achieved using a trolley-type gas-fired heat treatment furnace, which heats up at a set heating rate and maintains the temperature for a set holding time.

9. A method for mechanical expansion jointing of a pressure vessel with low residual stress according to claim 1, characterized in that, Step S8 specifically includes: After the expansion joint is shaped, the control servo mechanism slowly retracts the tool at a constant low speed below the feed speed set range. The retraction speed is controlled within the set speed range to stabilize the expansion joint size and stress state.

10. A method for mechanical expansion jointing of a pressure vessel with low residual stress according to claim 1, characterized in that, The performance and stress compliance testing includes residual stress testing, dimensional re-inspection, and airtightness and water pressure resistance testing.

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