A multi-field coupling thermal fatigue test device and method for expansion welding joint

Abstract

The present application relates to a kind of expansion welding joint multi-field coupling thermal fatigue test device and test method, containing sample clamping system, heating furnace, independent temperature control system in pipe, independent pressure sealing system, mechanical loading system and with on-line leakage monitoring function measurement control system;Heating furnace and temperature control system in pipe are independent of each other, can be decoupled and controlled to control the temperature difference of tube plate side, heat exchange pipe internal temperature and form, pressure sealing system and temperature control structure are separated, mechanical loading system can apply axial alternating tension-compression load, central controller cooperates and controls multiple systems synchronous operation.This application realizes pipe internal and external temperature free control by internal and external double independent temperature control structure, can be matched with internal pressure and alternating load to complete temperature, pressure, stress three-field coupling loading, relies on on-line leakage monitoring to determine joint failure in real time, accurately measure thermal fatigue life, completely restores the real service condition of heat exchanger, test precision is high, working condition covers widely, can be used for heat exchanger expansion welding joint material screening and expansion welding process optimization.

Description

Technical Field

[0001] This invention relates to the field of material performance testing technology, and more specifically to a multi-field coupled thermal fatigue testing device and method for expansion welded joints, used to simulate the service behavior of expansion welded joints of heat exchanger tube sheets under the combined effects of temperature difference between inside and outside the tube, internal pressure and mechanical load. Background Technology

[0002] Heat exchangers are critical equipment widely used in petrochemical, nuclear power, and marine propulsion industries. The connection between the tube sheet and heat exchange tubes is the weakest link in the equipment. There are three main connection methods between the tube sheet and heat exchange tubes: expansion joint, welding, and a combination of expansion and welding. During actual service, due to the temperature difference between the tube-side and shell-side media, as well as equipment start-up, shutdown, and fluctuations in operating conditions, the expanded weld joint is subjected to the coupled effects of the temperature difference load between the inside and outside of the tube, the pressure of the medium inside the tube, and the axial stress caused by the obstruction of thermal expansion. This can easily lead to joint sealing failure or structural damage. Therefore, accurately simulating the actual service load conditions of the expanded weld joint and quantitatively evaluating its thermal fatigue performance is of critical engineering value for heat exchanger material selection, expansion welding process optimization, and equipment life prediction.

[0003] Currently, the testing equipment and methods used in the industry for testing the fatigue performance of expanded welded joints have many inherent defects:

[0004] First, existing testing equipment cannot independently establish and precisely control the temperature field on both sides of the heat exchanger tubes and tube sheet. Conventional creep testing machines and static material tensile testing machines can only provide a single constant temperature environment and can only complete unidirectional mechanical tensile tests under constant temperature conditions. They cannot form a controllable temperature difference at the interface between the tube sheet and the heat exchanger tubes. The few dedicated testing devices that can realize temperature difference conditions mostly adopt a passive temperature difference construction scheme with single-sided heating and fluid flow into the opposite pipe. The fluid simultaneously undertakes the dual functions of cooling and establishing internal pressure. Temperature control and pressure control are deeply coupled and cannot be decoupled. The size of the temperature difference is limited by the fluid temperature and flow rate, making it difficult to freely set the internal and external temperatures and temperature difference values. It cannot flexibly simulate various actual operating conditions such as hot tube side and cold shell side, or cold tube side and hot shell side. At the same time, the test objects of this type of temperature difference testing equipment are mostly heat exchanger plugs. The test objective is only to verify the reliability of the plugging and sealing process, and it is impossible to conduct research on the thermal fatigue failure mechanism of the expansion weld interface.

[0005] Secondly, existing equipment struggles to simultaneously apply axial alternating mechanical loads based on the coupling of temperature difference and internal pressure fields, making it impossible to fully replicate the real coupled stress environment. Most current tube sheet joint testing equipment can only achieve coupled loading of temperature and pressure fields, lacking an independent mechanical loading module capable of applying reciprocating tensile and compressive loads. This makes it impossible to simulate the alternating axial thermal stress generated by limited thermal expansion during heat exchanger start-up and shutdown. A few existing devices that can achieve coupling of temperature, pressure, and load fields use bellows expansion joints as the test carrier. The sample structure and force transmission method are fundamentally different from those of tube sheet expansion joints. Furthermore, these devices employ an overall sealed heating structure, failing to construct independent temperature fields inside and outside the heat exchange tubes, thus failing to meet the testing requirements for temperature difference conditions in expansion joints.

[0006] Third, the lack of online real-time joint failure determination methods during testing leads to large errors in thermal fatigue life test results. Current mainstream testing procedures involve removing the sample from the equipment after completing a preset number of hot and cold cycles, and then using offline detection methods to determine whether the joint leaks or develops fatigue cracks. This approach cannot capture the critical moment of joint seal failure in real time during cyclic loading, making it difficult to accurately determine the precise number of thermal fatigue failure cycles for the joint. If the sample leaks and fails midway, the equipment will continue to complete the remaining preset cycles, resulting in a waste of test consumables, energy, and time. Summary of the Invention

[0007] To address the problems of existing technologies, such as the inability to decouple and control the internal and external temperatures of heat exchange tubes, the inability to synchronously achieve temperature-pressure-stress three-field coupled loading, and the inability to determine joint failure in real time during the test, this invention aims to provide a multi-field coupled thermal fatigue test device and method for expansion welded joints.

[0008] The multi-field coupled thermal fatigue testing device for expansion joints according to the present invention includes: a main frame and a sample clamping system for clamping a tube-plate joint sample comprising a tube sheet and at least one heat exchange tube; a thermal circulation system including a heating furnace covering the tube-plate joint sample, wherein the heating furnace is provided with multiple independently temperature-controlled heating zones for constructing a first temperature field on the tube sheet side; an independent temperature control system inside the tube for constructing a second temperature field independent of the first temperature field inside the heat exchange tube, wherein the independent temperature control system inside the tube is independent of the temperature control system of the heating furnace, and can decouple the control of the tube sheet side temperature T1 and the internal temperature T2 of the heat exchange tube, forming a controllable temperature difference ΔT at the expansion joint interface; and a pressure sealing system, which is connected to the inner cavity of the heat exchange tube and is set independently of the independent temperature control system inside the tube, for use in heat exchange... A constant pressure is established and maintained inside the tube; a mechanical loading system, including a servo actuator and a tie rod, is used to apply axial alternating tensile and compressive loads to the heat exchange tube via a dynamic sealing joint; a measurement and control system includes a temperature sensing component, a displacement sensing component, an online leak monitoring device, and a central controller; the online leak monitoring device is a helium mass spectrometer leak detector, which is connected to the inner cavity of the heat exchange tube or the side cavity of the tube sheet, and is used to continuously collect the joint leakage rate throughout the process and output a failure signal when the leakage rate exceeds a threshold; the central controller is electrically connected to the heating furnace, the independent temperature control system inside the tube, the pressure sealing system, and the mechanical loading system, respectively, and is used to coordinate and regulate the operating sequence and amplitude of temperature, pressure, and load, and to collect various test data in real time. This invention, by setting up independent pipe-side and pipe-inside dual temperature control structures, combined with an independent pressure sealing circuit and a mechanical loading mechanism capable of applying alternating loads, and equipped with a collaborative control central controller integrating online leakage monitoring function, can simultaneously construct a controllable temperature difference between inside and outside the pipe, and apply two types of service loads: high pressure inside the pipe and axial alternating tension and compression, thus completely replicating the multi-field coupled service environment of the heat exchanger expansion weld joint.

[0009] In a preferred embodiment, the independent temperature control system inside the tube includes a miniature heating rod and / or a fluid circulation loop. The miniature heating rod extends into the inner cavity of the heat exchange tube, and the fluid circulation loop connects to the inner cavity of the heat exchange tube and delivers a medium to regulate the temperature inside the tube. This invention, through the combination of two optional in-tube temperature control structures—a miniature heating rod and a fluid circulation loop—can flexibly adapt to different heating and cooling conditions, accurately constructing an in-tube temperature field that does not interfere with the tube sheet side. Both temperature control schemes can independently achieve decoupled control of the in-tube temperature and the tube sheet side temperature.

[0010] In a preferred embodiment, the dynamic sealing joint includes a metal bellows compensator and a water-cooled sealing seat; one end of the metal bellows compensator is sealed to the end of the heat exchange tube, and the other end is sealed to the water-cooled sealing seat. The water-cooled sealing seat is connected to a tie rod to compensate for deformation displacement and continuously maintain the pipeline seal. This invention, through the composite dynamic sealing structure of the metal bellows compensator and the water-cooled sealing seat, can continuously maintain a high-pressure seal inside the heat exchange tube under varying temperature and axial displacement conditions, avoiding interference from media leakage during loading that could affect the accuracy of leakage monitoring data.

[0011] In a preferred embodiment, the online leak monitoring device is a helium mass spectrometer leak detector. The helium mass spectrometer leak detector is connected to the inner cavity of the heat exchange tube or the side cavity of the tube sheet, continuously collecting the joint leakage rate throughout the process and outputting a failure signal when the leakage rate exceeds a threshold. This invention, through continuous online monitoring of the joint leakage rate using a helium mass spectrometer leak detector, can instantly identify the critical point of seal failure, accurately obtain the precise thermal fatigue cycle life of the joint, and avoid the loss of failure points in traditional offline detection.

[0012] In a preferred embodiment, the heating furnace is provided with an observation window, and the displacement sensing component includes a non-contact video extensometer. The non-contact video extensometer measures the axial displacement of the heat exchange tube relative to the tube sheet through the observation window and aligned with the expansion joint area. This invention, by combining a furnace observation window with a non-contact video extensometer, enables real-time, non-contact measurement of the deformation at the expansion joint interface under a closed, temperature-controlled loading environment. This avoids the limitation of traditional contact displacement sensors, which cannot be placed in a closed, high-temperature furnace, and does not interfere with the service conditions of the sample.

[0013] In a preferred embodiment, the heating furnace has a split, openable / closable structure, which, when closed, completely covers the tube sheet and expansion joint interface area of ​​the tube sheet joint sample. This invention, through its openable / closable furnace design, facilitates rapid sample loading and unloading and the arrangement of observation windows, thereby improving experimental efficiency.

[0014] In a preferred embodiment, the central controller has a built-in safety interlock program that automatically cuts off the heating, pressurizing, and loading mechanisms when leakage exceeds limits or temperature or pressure exceeds set ranges. This invention, through its safety interlock protection mechanism, covers abnormal operating conditions such as over-temperature, over-pressure, and leakage, ensuring the safety of equipment and personnel.

[0015] The multi-field coupled thermal fatigue test method for expansion welded joints based on the above-mentioned device according to the present invention includes the following steps: S1, assembling the tube sheet joint sample into the sample clamping system, connecting the pressure sealing system and the independent temperature control system inside the tube, and installing temperature sensing components; S2, starting the heating furnace, heating the tube sheet side to a first set temperature T1 according to a set heating rate and holding it at that temperature to form a uniform first temperature field; S3, starting the independent temperature control system inside the tube, adjusting the heat exchange tube to a second set temperature T2, forming a preset temperature difference ΔT=T1-T2 at the expansion joint interface, where T1 and T2 can be adjusted independently, and the magnitude of the temperature difference and the direction of the temperature gradient can be freely switched; S4, through pressure sealing... The system fills the inner cavity of the heat exchange tube with a high-pressure medium and maintains a constant pressure inside the tube; S5, the mechanical loading system is activated, and an axial alternating load is applied to the heat exchange tube according to a preset load spectrum. The phase relationship between the temperature cycle and the load cycle includes two modes: in-phase loading and out-of-phase loading; S6, the leakage rate at the expansion joint interface is collected in real time through an online leakage monitoring device, and displacement and load data are collected simultaneously. When the leakage rate exceeds the threshold or the displacement suddenly increases, the joint is judged to be in failure and the number of failure cycles Nf is recorded; S7, if the sample completes the preset total number of cycles without failure, a pull-out test is carried out on the sample. The pull-out test can be performed under normal temperature or high temperature conditions according to the simulated working conditions to determine the remaining strength of the joint. This invention, through a complete process of constructing independent internal and external temperature fields, pressure stabilization loading, synchronous coupling of temperature and alternating load, real-time monitoring and judgment of failure, and residual strength detection, can comprehensively quantify the sealing durability and remaining load-bearing capacity of the expansion welded joint under different working conditions.

[0016] In a preferred embodiment, the phase relationship between the temperature cycle and the load cycle in step S5 includes two modes: in-phase loading and out-of-phase loading. In-phase loading means that the tensile load increases simultaneously with the temperature increase, while out-of-phase loading means that the compressive load increases simultaneously with the temperature increase. By setting two temperature-load matching phase modes, this invention can simulate the alternating thermal stress borne by the welded joint under different equipment start-up and shutdown conditions, thus broadening the scope of test conditions.

[0017] In a preferred embodiment, the method includes a thermal shock test mode; the thermal shock test mode includes: heating the sample to a target temperature and holding it at that temperature, rapidly introducing a cooling medium to quickly cool the sample, and repeating the rapid heating and cooling cycle. The cooling medium can be compressed air or cooling water. This invention, by adding a rapid heating and cooling thermal shock test procedure, can simulate extreme temperature fluctuation conditions and evaluate the damage to the joint caused by instantaneous and drastic temperature differences.

[0018] In a preferred embodiment, after each fixed number of rapid heating and cooling cycles, the sample is removed for metallographic analysis and pull-out strength testing. This invention, through a segmented, offline metallographic and strength testing mechanism after cycling, can quantitatively analyze the correlation between the number of thermal shock cycles and the propagation of interface cracks and the decay of residual joint strength.

[0019] In a preferred embodiment, in step S5, the central controller synchronously operates the heating furnace, the independent temperature control system inside the tube, the pressure sealing system, and the mechanical loading system, ensuring that the temperature field, pressure field, and stress field continuously and synergistically couple and act on the tube sheet joint sample. This invention, through the unified and synchronous control of four sets of control and loading systems by the central controller, can ensure that temperature, pressure, and alternating loads act synchronously on the sample throughout the entire process, highly replicating the actual synchronous service load state of the equipment.

[0020] This invention addresses the shortcomings of existing technologies that cannot decouple and control the internal and external temperatures of tubes through an independent internal and external dual temperature control structure. It achieves synchronous coupling loading of temperature, pressure, and stress fields using an independent pressure sealing circuit and a servo-driven alternating mechanical loading mechanism. This overcomes the limitation of existing equipment that can only couple two fields and cannot simulate real alternating axial stress. Combined with a fully online helium mass spectrometry leak monitoring method, it can accurately capture the critical point of joint seal failure in real time, solving the problem of traditional offline testing's inability to accurately determine the failure moment. The invention is equipped with a central controller that can synchronously link multiple systems and a complete test process with phase-separated and expandable thermal shock modes. This not only comprehensively replicates various real service load conditions of heat exchanger expansion weld joints, significantly improving the testing accuracy of thermal fatigue life and joint performance, but also allows for material selection and expansion welding process optimization without requiring whole-machine testing, effectively reducing test consumables, time, and R&D costs. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the overall structure of a multi-field coupled thermal fatigue testing device for expansion welded joints according to a preferred embodiment of the present invention. Detailed Implementation

[0022] The preferred embodiments of the present invention are given below with reference to the accompanying drawings and described in detail.

[0023] like Figure 1 As shown, the multi-field coupled thermal fatigue testing device for expansion welded joints according to the present invention includes a main frame 1, a sample clamping system, a thermal circulation system, an independent temperature control system inside the tube, a pressure sealing system, a mechanical loading system, and a measurement and control system.

[0024] The main frame 1 serves as the foundation for the entire device. The sample clamping system is fixedly assembled at the bottom of the main frame 1, including the lower clamping fixture 3. The thermal circulation system completely covers the area to be tested, including the heating furnace 8. The independent temperature control system inside the tube extends into the sample, including a miniature internal heating rod 4. The pressure sealing system is connected to the inner cavity of the heat exchange tube via pipelines, including a high-pressure medium source 15, a pressure holding valve 16, and a pressure sensor 17. The mechanical loading system is mounted on the upper part of the main frame 1 and sealed downwards to the top of the sample, including a servo actuator 11, a pull rod 10, and a dynamic sealing joint 9. The measurement and control system includes thermocouples 6 on the outside of the tube sheet, thermocouples 7 on the inside of the tube, a force sensor, a displacement sensor, a non-contact video extensometer, an online helium mass spectrometer leak detector, and a central controller. Various sensing and monitoring elements are respectively arranged in the temperature control, loading, and pressure loops. All controlled elements are electrically connected to the central controller to achieve multi-parameter coordinated timing control.

[0025] This invention achieves a controllable temperature difference between the inside and outside of the tube by independently configuring the thermal circulation system and the independent temperature control system inside the tube, decoupling the pressure sealing system from the two temperature control systems, and coordinating the synchronous coupling of the temperature difference and the internal pressure load in the mechanical loading system. It can also apply two types of service loads, namely high pressure inside the tube and axial alternating tension and compression, to completely restore the multi-field coupled service environment of the heat exchanger expansion weld joint.

[0026] 1) Host Framework

[0027] The main frame 1 is used to bear all the reaction forces generated by the output load of the servo actuator 11 during the mechanical loading process, ensuring that the overall structure has no offset or deformation during the axial tensile and compressive loading process, and providing a stable installation reference for all other systems.

[0028] The main frame 1 consists of uprights 13, a fixed lower crossbeam 2, and a top crossbeam 14, forming a rigid closed-loop frame. The uprights 13 are vertically fixed to both sides of the base. The fixed lower crossbeam 2 is horizontally supported at the lower part of the two uprights 13, and the top crossbeam 14 is horizontally supported at the top of the two uprights 13. A movable upper crossbeam 12 is slidably mounted on the uprights 13, and its height can be adjusted vertically along the uprights 13. The maximum upward travel of the movable upper crossbeam 12 is limited by the top crossbeam 14.

[0029] 2) Sample clamping system

[0030] The lower clamping fixture 3 is assembled on the top surface of the fixed lower crossbeam 2 to clamp and fix the tube sheet section of the tube sheet joint sample 5, and to restrict all horizontal and vertical displacement of the tube sheet joint sample 5.

[0031] The tube sheet joint specimen 5 is an expansion welded composite specimen simulating the structure of a real heat exchanger. It includes a tube sheet substrate and at least one heat exchange tube, which extends upward to connect the mechanical loading system and the pressure sealing pipeline.

[0032] The present invention can quickly assemble and disassemble tube sheet joint samples 5 of different specifications through the lower clamping fixture 3, and is suitable for testing samples of various pipe diameters and tube sheet sizes, making it more versatile.

[0033] 3) Thermal circulation system

[0034] The heating furnace 8 has a split, openable structure that, when closed, completely covers the tube sheet and expansion joint interface area of ​​the tube sheet joint sample 5. The furnace cavity of the heating furnace 8 is divided into multiple independently controlled heating zones, each with adjustable heating power. This counteracts the temperature difference on the tube sheet surface caused by faster heat dissipation at the edges and slower heat dissipation at the center, resulting in a more uniform overall temperature for the tube sheet. Furthermore, the heating furnace 8 only covers and heats the outer area of ​​the tube sheet, without heating the heat exchange tubes, thus creating a uniform and stable primary temperature field on the tube sheet side.

[0035] Thermocouple 6 is attached to the outer wall of the tube sheet joint sample 5 to collect the temperature value of the first temperature field in real time.

[0036] A transparent observation window 81 is provided on the side wall of the heating furnace 8. The observation window 81 is used to provide an observation channel for a non-contact video extensometer, enabling real-time observation of the deformation of the expansion joint area without disassembling the furnace.

[0037] 4) Independent temperature control system inside the pipe

[0038] The micro-in-tube heating rod 4 serves as an internal heating element, extending from the bottom of the heat exchange tube in the tube sheet joint sample 5 into the inner cavity of the heat exchange tube. It is used to establish a second temperature field independent of the first temperature field inside the heat exchange tube. The micro-in-tube heating rod 4 can be replaced by a fluid circulation loop, which is connected to the inner cavity of the heat exchange tube. Heating or cooling media are introduced to achieve heating or cooling inside the tube.

[0039] Thermocouple 7 is attached and fixed to the inner wall of the heat exchange tube. The signal line of thermocouple 7 is led out to the central controller for real-time acquisition of the temperature value of the second temperature field.

[0040] The tube has an independent temperature control system with dedicated temperature control equipment, which is completely decoupled from the temperature control system of the heating furnace 8. It can independently set and maintain the tube temperature T2, and form any preset temperature difference ΔT=T1-T2 with the tube sheet side T1. The magnitude of the temperature difference and the direction of hot and cold can be freely switched, which can simulate two typical working conditions: hot tube side and cold shell side, and cold tube side and hot shell side.

[0041] This invention achieves decoupled temperature control between the tube sheet side and the heat exchange tube inside by using two completely independent temperature control structures, namely the heating furnace 8 and the micro tube heating rod 4. The pressure sealing system does not change the temperature inside and outside the tube during the pressurization process, and the temperature adjustment does not interfere with the pressure value inside the tube, thus completely solving the technical defects of mutual restraint between temperature and pressure parameters in traditional equipment.

[0042] 5) Pressure sealing system

[0043] The high-pressure medium source 15 provides high-pressure gas (such as helium) or liquid, which is connected to the inner cavity of the heat exchange tube through a pressure-resistant pipeline for delivering the high-pressure medium into the heat exchange tube. A pressure sensor 17 and a pressure holding valve 16 are connected in series on the pipeline.

[0044] Pressure sensor 17 collects the pressure of the medium inside the pipe in real time and transmits it to the central controller.

[0045] After pressurization is completed, the pressure-holding valve 16 closes the pipeline to maintain long-term stable pressure inside the pipeline.

[0046] The entire pressure sealing system piping is laid out independently, without sharing a loop with the independent temperature control system piping inside the pipe, thus achieving decoupling between pressure control and internal temperature control.

[0047] 6) Mechanical loading system

[0048] The servo actuator 11 is the power source of the mechanical loading system, which can precisely control the waveform, amplitude, and frequency of the output axial tensile and compressive load. The servo actuator 11 is fixedly installed at the center of the bottom surface of the movable upper crossbeam 12. The vertical movement of the movable upper crossbeam 12 can accommodate tube sheet joint samples 5 of different lengths.

[0049] The pull rod 10 is fixedly connected to the output end of the servo actuator 11. The pull rod 10 is made of high-temperature resistant alloy material, suitable for long-term variable temperature operation. The bottom end of the pull rod 10 is connected to the upper end of the dynamic sealing joint 9.

[0050] The lower end of the dynamic sealing joint 9 seals the top of the heat exchange tube of the sample 5 in the pipe plate joint, and simultaneously connects to the high-pressure pipeline of the pressure sealing system. The dynamic sealing joint 9 integrates a metal bellows compensator and a water-cooled sealing seat. One end of the metal bellows compensator is sealed to the end of the heat exchange tube, and the other end connects to the water-cooled sealing seat. The water-cooled sealing seat is externally connected to a circulating cooling water circuit to continuously remove heat from the sealing location. A tie rod 10 is fixed to the upper end of the water-cooled sealing seat. Under variable temperature conditions, the metal bellows compensator absorbs the thermal expansion deformation of the sample and tie rod, as well as the axial displacement output by the servo actuator. The water-cooled sealing seat continuously cools to ensure the stability of the sealing structure, guaranteeing no leakage of the high-pressure medium inside the heat exchange tube throughout the process. Thus, the dynamic sealing joint 9 simultaneously achieves the dual functions of variable temperature reciprocating displacement compensation and high-pressure sealing, preventing medium leakage during axial alternating tension and compression loading, and ensuring the accuracy and validity of online leak detection data.

[0051] 7) Measurement and Control Systems

[0052] Thermocouple 6 on the outside of the tube sheet and thermocouple 7 on the inside of the tube collect real-time temperatures on the tube sheet side and inside the tube, respectively. A force sensor is integrated into the output of the servo actuator 11 to collect real-time axial load. A displacement sensor is used in conjunction with a non-contact video extensometer. The non-contact video extensometer is aligned with the expansion joint interface through the observation window 81 of the heating furnace 8, measuring the axial deformation displacement of the heat exchange tube relative to the tube sheet without contact. The online helium mass spectrometer leak detector can be connected to the inner cavity of the heat exchange tube or switched to connect to the outer cavity of the tube sheet. Both monitoring methods can achieve continuous acquisition of joint leakage rate throughout the process. The online helium mass spectrometer leak detector can compare the measured leakage rate with the preset failure threshold in real time. The leakage threshold, number of cycles, and cooling rate can be flexibly set according to the actual operating parameters of the heat exchanger. Once the leakage exceeds the standard, a failure signal is immediately transmitted to the central controller, and the current number of thermal cycles is recorded simultaneously. The central controller is electrically connected to the heating furnace 8, the independent temperature control system inside the pipe, the high-pressure medium source 15, the servo actuator 11, and all sensing and monitoring elements. It coordinates the operating sequence, amplitude, and phase of temperature cycling, pressure maintenance, and alternating loads, and synchronously collects and stores all temperature, pressure, load, leakage, and displacement test data. It can also export the original test curves to an external terminal. The central controller has a built-in safety interlock program that automatically cuts off the heating, pressurizing, and loading mechanisms when leakage exceeds the limit or temperature or pressure exceeds the set range, preventing damage to the sample or equipment and covering abnormal operating conditions such as over-temperature, over-pressure, and leakage.

[0053] The central controller's collaborative control logic is as follows: before the test begins, the temperature cycle curve (the timing and rate of heating-holding-cooling), pressure setpoint, load spectrum (waveform, amplitude, frequency), and phase relationship (in-phase / out-of-phase) are set. After startup, the controller synchronously outputs control signals to the heating furnace, the pipe temperature control system, the pressure holding valve, and the servo actuator according to the time axis. At the same time, it collects all sensor data at a sampling rate of not less than 10Hz to achieve closed-loop regulation.

[0054] The test method for the above-mentioned device is briefly described below.

[0055] S1: Sample assembly and system connection: Place the processed tube sheet joint sample 5 above the fixed lower crossbeam 2 and lock it in place using the lower clamping fixture 3; insert the micro-tube heating rod 4 into the inner cavity of the heat exchange tube and connect it to the independent temperature control system circuit inside the tube; if a fluid circulation loop scheme is selected, connect the hot and cold medium pipelines to the inner cavity of the heat exchange tube; connect the high pressure medium source 15, pressure holding valve 16, pressure sensor 17 and the dynamic sealing joint 9 at the top of the heat exchange tube; install the thermocouples 6 on the outside of the tube sheet and the thermocouples 7 on the inner wall of the tube, and connect all the sensing circuits to the central controller.

[0056] S2: Construct the first temperature field on the tube sheet side: Close the heating furnace 8, start the heating furnace temperature control program, heat the tube sheet side to the first preset temperature T1 according to the set heating rate, keep the temperature constant until the temperature value collected by the thermocouple 6 on the outside of the tube sheet is stable, and a uniform first temperature field is formed on the tube sheet side.

[0057] S3: Construct a second temperature field inside the tube and form a preset temperature difference: Start the independent temperature control system inside the tube, raise the temperature by the micro-in-tube heating rod 4 or cool down by the introduction of cooling medium, adjust the inside of the heat exchange tube to the second preset temperature T2, and form a stable temperature difference ΔT=T1-T2 at the expansion joint interface; because the heating furnace 8 is decoupled from the independent temperature control system inside the tube, T1 and T2 can be adjusted independently, and the tube side hot and shell side cold and tube side cold and shell side hot working conditions can be flexibly switched, that is, the magnitude of the temperature difference and the direction of the temperature gradient can be freely switched.

[0058] S4: Intra-tube pressurization and stabilization: Turn on the high-pressure medium source 15 to introduce high-pressure medium into the inner cavity of the heat exchange tube. After the pressure sensor 17 detects that the pressure reaches the set pressure P, turn off the high-pressure medium source 15 and lock the pressure holding valve 16 to maintain a constant pressure inside the tube throughout the process.

[0059] S5: Synchronous Coupling of Temperature Cycling and Axial Alternating Load: The temperature cycling program and mechanical loading program are started by the central controller. The servo actuator 11 outputs axial alternating tensile and compressive loads to the heat exchange tubes according to the preset load spectrum. The central controller can be configured with two load phase modes: in-phase loading and out-of-phase loading. In-phase loading increases the tensile load synchronously during the temperature rise process, while out-of-phase loading increases the compressive load synchronously during the temperature rise process, which fully simulates the alternating thermal stress generated by the start-up and shutdown of the heat exchanger. The central controller synchronously links the four sets of control loading systems to operate synchronously. The three fields of temperature cycling, pressure, and axial load continuously coordinate and couple, realizing the coupling of the temperature field, pressure field, and stress field.

[0060] S6: Online Real-time Monitoring and Failure Judgment: The online helium mass spectrometer leak detector, non-contact video extensometer, and various sensors are activated throughout the test to continuously collect data on leakage rate, temperature, pressure, load, and displacement. When the leakage rate detected by the online helium mass spectrometer exceeds the preset threshold, or when the displacement of the expansion joint interface detected by the video extensometer suddenly increases, the central controller immediately determines that the joint has failed and automatically records the number of cycles Nf corresponding to the failure.

[0061] S7: Residual strength test of unfailed specimens: If the specimen does not trigger the failure judgment after completing the preset total number of cycles, stop the temperature, pressure and mechanical loading, take out the tube sheet joint specimen 5, carry out the room temperature or high temperature pull-out test according to the simulated working conditions, measure the remaining pull-out strength of the expansion weld joint, and quantitatively evaluate the degree of attenuation of the joint bearing performance by thermal cycling.

[0062] This device can switch between thermal shock test modes to simulate extreme rapid heating and cooling conditions. The operation procedure is as follows: heat the tube sheet joint sample 5 to the target temperature and keep it at a uniform temperature; quickly cut off the heat source of the heating furnace 8, start the matching rapid cooling system to spray compressed air or cooling water into the expansion joint area, so that the sample cools down rapidly within a specified time to complete a single thermal shock cycle; repeat the rapid heating and cooling cycle, and after each set number of cycles, stop the machine and take out the sample to carry out metallographic analysis to observe microcracks at the expansion joint interface, and at the same time perform a pull-out test to obtain the remaining strength data and evaluate the degree of damage to the expansion joint caused by thermal shock.

[0063] During the test, if leakage exceeds the standard or displacement suddenly increases, the central controller will immediately record the number of failure cycles and automatically shut down to avoid subsequent invalid cycles and ensure the accuracy of life data.

[0064] Example 1: Thermal fatigue test of nickel-based alloy expansion welded joint at 600℃

[0065] This embodiment uses an Inconel 625 nickel-based alloy expansion welded joint as the test object to simulate its thermal fatigue behavior under a reference temperature of 600℃ and a temperature difference of 100℃.

[0066] A tube sheet joint sample made of Inconel 625 material was selected. The tube sheet dimensions were 100mm × 100mm × 30mm, the heat exchange tube outer diameter was 19mm, the wall thickness was 2mm, and the expansion joint length was 25mm. The sample was installed on the main frame, and the tube sheet was fixed to the lower crossbeam with a special clamp. The upper end of the heat exchange tube was connected to the high-temperature tie rod through a high-temperature dynamic sealing joint.

[0067] The high-temperature heating furnace was started, and the tube sheet side was heated to 550°C at a heating rate of 10°C / min and held for 30 minutes. Subsequently, the miniature heating rods in the independent temperature control system inside the tubes were activated to independently raise the inner wall temperature of the heat exchange tubes to 650°C, creating a 100°C temperature difference at the expansion joint interface (hot tube side, cold shell side). Multi-point thermocouple monitoring confirmed that the temperature field on both the tube sheet side and inside the tubes was stable, and that the temperature control on both sides was independent, with a control accuracy of ±3°C.

[0068] High-purity helium gas at 5 MPa was introduced into the heat exchange tubes through a pressure sealing system, the pressure holding valve was closed, and pressure stability was monitored. An online helium mass spectrometer leak detector was set up to continuously monitor the leakage rate, with a sensitivity set to 1×10⁻⁶. -9 Pa·m 3 / s.

[0069] A temperature cycling curve was set: using 600℃ as the intermediate value, the temperature was cycled between 550℃ and 650℃, with a cycle time of 2 hours / cycle (30 minutes of heating, 60 minutes of holding, and 30 minutes of cooling). Simultaneously, a mechanical loading system was activated to apply an axial tensile load in phase with the temperature: when the temperature rose to 650℃, the tensile load increased to 2kN; when the temperature dropped to 550℃, the tensile load decreased to 0.5kN, simulating the thermal stress caused by impeded thermal expansion.

[0070] Temperature, load, displacement, and leakage rate data were recorded in real time. During the 1572nd cycle of the test, the online helium mass spectrometer leak detector detected a leakage rate increasing from 1.2 × 10⁻⁶. -9 Pa·m 3 / s suddenly increased to 5.6×10 -6 Pa·m 3 / s, exceeding the set threshold, immediately determine the joint seal failure, and record the failure cycle number Nf=1572.

[0071] After the test, the sample was taken out for metallographic analysis, which revealed intergranular microcracks at the expansion joint interface. The cracks originated from the edge of the sealing groove and propagated inward, confirming that thermo-mechanical coupling fatigue was the main cause of failure.

[0072] Example 2: Thermal Shock Test

[0073] The same set of equipment was used to carry out thermal shock mode testing, and the sample specifications were consistent with those in Example 1.

[0074] Heat the sample to 600℃ and hold it at that temperature to ensure uniform temperature.

[0075] Immediately shut off the power to the heating furnace and simultaneously activate the rapid cooling system. Inject compressed air into the expansion joint area through nozzles to reduce the surface temperature of the sample from 600°C to below 150°C within 20 seconds, with a cooling rate of approximately 22.5°C / s.

[0076] The above rapid heating and cooling process was repeated 100 times. After every 20 cycles, the sample was removed for metallographic examination to observe whether microcracks appeared at the expansion joint interface. The results showed that microcracks began to appear at the expansion joint interface after the 60th cycle; after the 100th cycle, the cracks significantly expanded.

[0077] After 100 thermal shock cycles, the sample was subjected to a room temperature pull-out test, and the pull-out force was measured to be 28.5 kN, which was 19% lower than that of the control sample that had not undergone thermal shock (pull-out force 35.2 kN), indicating that thermal shock caused significant degradation of the joint performance.

[0078] Example 3: Comparative Test of Different Materials

[0079] A comparative test was performed on dissimilar material expanded welded joints (S30408 stainless steel pipe + Q345R tube sheet) using the same device, with the same test conditions as above. The results show that under the same thermal cycle conditions, the dissimilar material joint experienced leakage failure after only 856 cycles, which is significantly lower than 1572 cycles of the same nickel-based alloy, verifying the important influence of the matching of material thermal expansion coefficients on thermal fatigue life.

[0080] The beneficial effects of the present invention are briefly described below:

[0081] 1) Decoupling control of temperature inside and outside the pipe, which truly simulates the temperature difference between media inside and outside the pipe. Different from the passive temperature difference establishment method in the prior art that adopts unilateral heating combined with fluid flow on the opposite side (the fluid simultaneously undertakes the dual functions of cooling and pressure building, so temperature and pressure cannot be decoupled), the present invention realizes, for the first time, independent establishment and precise control of the tube sheet side temperature T1 and the inner tube temperature T2 on a single tube-sheet joint specimen through the mutual independent control of the independent temperature control system inside the pipe and the external high-temperature heating furnace, can flexibly simulate various working conditions such as hot tube side and cold shell side (T1<T2), cold tube side and hot shell side (T1>T2), and can precisely control the magnitude and direction of the temperature difference ΔT.

[0082] 2) Synergistic coupling of three fields, which is closer to real service conditions. The synergistic control and real-time monitoring of temperature field-pressure field-stress field are realized, and the synergistic effect of temperature difference load inside and outside the pipe, medium pressure in the pipe and axial alternating thermal stress generated by blocked thermal expansion during the operation of the heat exchanger can be simulated. Different from the existing tube-sheet joint testing devices (which only have two-field coupling), the present invention realizes three-field coupling through an independent mechanical loading system, and more realistically reproduces the actual service conditions of the expanded welded joint.

[0083] 3) Online real-time failure determination, which accurately captures the failure critical point. Different from the off-line detection method after the end of variable temperature cycling in the prior art, the present invention continuously monitors the leakage rate through an online helium mass spectrometer leak detector, can capture the critical point of joint sealing failure in real time during the test, and accurately measure the thermal fatigue life of the expanded welded joint, avoiding the problem that the failure moment may be missed in off-line detection.

[0084] 4) Low cost and high efficiency. Compared with the whole machine bench test, the device of the present invention has compact structure, simple sample processing, short test period and flexible parameter adjustment, which significantly reduces the R & D cost.

[0085] The above description is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Various changes can be made to the above embodiments of the present invention. That is, all simple equivalent changes and modifications made based on the claims and the description of the present invention application shall fall within the protection scope of the claims of the present invention. What is not described in detail in the present invention is conventional technical content.

Claims

1. A multi-field coupled thermal fatigue testing device for expanded welded joints, characterized in that, include: The main frame and sample clamping system are used to clamp the tube sheet joint sample containing the tube sheet and at least one heat exchange tube (5). The thermal circulation system includes a heating furnace (8) covering the tube sheet joint sample (5), wherein the heating furnace (8) is provided with multiple independent temperature-controlled heating zones to construct a first temperature field on the tube sheet side; The independent temperature control system (4) inside the tube is used to construct a second temperature field that is independent of the first temperature field inside the heat exchange tube. The independent temperature control system (4) inside the tube is independent of the temperature control system of the heating furnace (8). It can decouple the control of the tube sheet side temperature T1 and the internal temperature T2 of the heat exchange tube, and form a controllable temperature difference ΔT at the expansion joint interface. The pressure sealing system is connected to the inner cavity of the heat exchange tube and is set independently of the independent temperature control system inside the tube (4), and is used to establish and maintain a constant pressure inside the heat exchange tube; The mechanical loading system includes a servo actuator (11) and a tie rod (10), the tie rod (10) being connected to the end of the heat exchange tube via a dynamic sealing joint (9) for applying axial alternating tensile and compressive loads to the heat exchange tube; The measurement and control system includes a temperature sensing component, a displacement sensing component, an online leak monitoring device, and a central controller; the online leak monitoring device is a helium mass spectrometer leak detector, which is connected to the inner cavity of the heat exchange tube or the side cavity of the tube sheet, and is used to continuously collect the joint leakage rate throughout the process and output a failure signal when the leakage rate exceeds the threshold; the central controller is electrically connected to the heating furnace (8), the independent temperature control system (4) inside the tube, the pressure sealing system, and the mechanical loading system, respectively, and is used to coordinate and regulate the running sequence and amplitude of temperature, pressure, and load, and to collect various test data in real time.

2. The multi-field coupled thermal fatigue testing device for expanded welded joints according to claim 1, characterized in that, The independent temperature control system (4) inside the tube includes a miniature heating rod and / or a fluid circulation loop. The miniature heating rod extends into the inner cavity of the heat exchange tube, and the fluid circulation loop connects to the inner cavity of the heat exchange tube and delivers a medium to regulate the temperature inside the tube.

3. The multi-field coupled thermal fatigue testing device for expanded welded joints according to claim 1, characterized in that, The dynamic sealing joint (9) includes a metal bellows compensator and a water-cooled sealing seat; one end of the metal bellows compensator is sealed to the end of the heat exchange tube, and the other end is sealed to the water-cooled sealing seat. The water-cooled sealing seat is connected to the tie rod (10) to compensate for deformation displacement and continuously maintain pipeline sealing.

4. The multi-field coupled thermal fatigue testing device for expanded welded joints according to claim 1, characterized in that, The online leak monitoring device is a helium mass spectrometer leak detector. The heating furnace (8) is a left-right split structure that can be opened and closed. When closed, it completely covers the tube sheet and expansion joint interface area of ​​the tube sheet joint sample (5).

5. The multi-field coupled thermal fatigue testing device for expanded welded joints according to claim 1, characterized in that, The heating furnace (8) is provided with an observation window. The displacement sensing component includes a non-contact video extensometer. The non-contact video extensometer measures the axial displacement of the heat exchange tube relative to the tube sheet by aligning it with the expansion joint area through the observation window.

6. A method for multi-field coupled thermal fatigue testing of expanded welded joints based on the device described in any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Assemble the tube sheet joint sample (5) into the sample clamping system, connect the pressure sealing system with the independent temperature control system (4) inside the tube, and install the temperature sensing components. S2. Start the heating furnace (8), heat the tube sheet side to the first set temperature T1 according to the set heating rate and keep it at the set temperature to form a uniform first temperature field; S3. Start the independent temperature control system inside the tube (4) and adjust the heat exchange tube to the second set temperature T2. A preset temperature difference ΔT=T1-T2 is formed at the expansion joint interface. T1 and T2 can be adjusted independently, and the magnitude of the temperature difference and the direction of the temperature gradient can be switched freely. S4. High-pressure medium is injected into the inner cavity of the heat exchange tube through a pressure sealing system and the pressure inside the tube is kept constant. S5. Start the mechanical loading system and apply axial alternating load to the heat exchange tube according to the preset load spectrum. The phase relationship between the temperature cycle and the load cycle includes two modes: in-phase loading and out-of-phase loading. S6. Real-time collection of leakage rate at the expansion joint interface through online leakage monitoring device, and simultaneous collection of displacement and load data. When the leakage rate exceeds the threshold or the displacement suddenly increases, the joint is determined to be in failure and the number of failure cycles Nf is recorded. S7. If the sample does not fail after completing the preset total number of cycles, conduct a pull-out test on the sample to determine the remaining strength of the joint.

7. The multi-field coupled thermal fatigue test method for expanded welded joints according to claim 6, characterized in that, In step S5, the phase relationship between temperature cycling and load cycling includes two modes: in-phase loading and out-of-phase loading. In-phase loading means that the tensile load increases synchronously with the temperature increase, while out-of-phase loading means that the compressive load increases synchronously with the temperature increase.

8. The multi-field coupled thermal fatigue test method for expanded welded joints according to claim 6, characterized in that, The method includes a thermal shock test mode; the thermal shock test mode includes: heating the sample (5) to the target temperature and keeping it at that temperature, rapidly introducing a cooling medium to rapidly cool the sample (5), and repeating the rapid heating and cooling cycle.

9. The multi-field coupled thermal fatigue test method for expanded welded joints according to claim 8, characterized in that, After each fixed number of rapid heating and cooling cycles, the sample (5) is taken out and metallographic analysis and pull-out strength test are carried out respectively.

10. The multi-field coupled thermal fatigue test method for expanded welded joints according to claim 6, characterized in that, In step S5, the central controller synchronously links the heating furnace (8), the independent temperature control system inside the tube (4), the pressure sealing system, and the mechanical loading system to operate synchronously, so that the temperature field, pressure field, and stress field continuously and synergistically couple and act on the tube sheet joint sample (5).

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