Low-defect welding method for large-diameter HDPE (high-density polyethylene)

Abstract

The invention provides a low-defect welding method for large-diameter HDPE (high-density polyethylene), and relates to the field of large-diameter pipeline welding, an initial mounting gap not greater than the maximum thermal expansion elongation of to-be-welded pipelines is obtained, and marking lines are arranged on the two pipelines; the electric smelting sleeve is installed outside the pipe ends, and it is ensured that the distance between the pipe ends is the initial installation gap and the distances between the two marking lines and the corresponding ends of the sleeve are equal; and periodically monitoring the distance between the marking lines, and starting welding after the pipeline is naturally jacked tightly. According to the method, through precise initial gap design, mechanical locking is achieved through pipeline thermal expansion, axial displacement is thoroughly eliminated, the defects of pipe end disengagement, electric heating wire displacement and the like are eradicated, and the first-pass yield is increased. Complex equipment is not needed, displacement return-to-zero is achieved only through simple measurement and opportunity control, cost is low, reliability is high, and the method is suitable for complex construction scenes such as outdoor sunlight and air temperature changes.

Description

Technical Field

[0001] This application relates to the field of pipeline welding technology, specifically to a low-defect welding method for large-diameter HDPE. Background Technology

[0002] High-density polyethylene (HDPE) pipes, especially large-diameter pipes (such as DN1000 and above), are widely used in municipal, water conservancy, and industrial transportation fields. In the electrofusion welding of large-diameter HDPE pipes, axial expansion and contraction displacement caused by ambient temperature (especially sunlight) is a major risk factor affecting weld quality. This invention completely eliminates this risk through a systematic timing control and monitoring method.

[0003] Taking a DN1400 pipeline in a southern project as an example, under strong sunlight variations, the measured daily axial displacement of its exposed section can exceed 10cm. This displacement is equivalent to the width of the sleeve's molten zone itself (typically about 10-15cm). During the welding and cooling process, which can last for several hours (4-7 hours for large-diameter sleeves under different process parameters), axial displacement of the pipeline will result in: the pipe end detaching from the molten zone, causing complete lack of fusion; displacement of the resistance wire, causing localized wire aggregation and separation, resulting in uneven temperature distribution in the molten zone; displacement or breakage of the partially molten resistance wire, damaging the welded structure; and the generation of enormous internal stress during cooling. All of these factors will directly lead to weld joint failure.

[0004] Currently, there is a lack of simple and reliable methods to eliminate the axial degree of freedom of pipelines during the critical welding period. Only complex external constraint tools can be used, which are ineffective and costly. Summary of the Invention

[0005] In view of the above-mentioned defects or deficiencies in the prior art, it is desirable to provide a low-defect welding method for large-diameter HDPE, comprising the following steps: Obtain the initial installation gap, cut the pipe to be welded, and make the distance between the ends of the two opposite pipes to be welded the initial installation gap; An electrofusion sleeve is fitted over the ends of two pipes to be welded that are close to each other. Each of the two pipes to be welded has a marking line. The distance between the two marking lines and the pipe ends of the corresponding pipes to be welded is equal. The distance from each marking line to the pipe end of the adjacent electrofusion sleeve is defined as the marking line distance. The two marking lines are equal in distance. Periodically monitor the distance between the two marked lines; Welding begins when the distance between the two marked lines indicates that the two pipes to be welded are in a naturally tightened state.

[0006] According to the technical solution provided in the embodiments of this application, the method for determining whether two pipes to be welded are in a naturally tightened state is as follows: Set a stable duration; When, within a stable time period, the sum of the lengths of the two pipes to be welded within the electrofusion sleeve, calculated by the distance between the two marked lines, equals the length of the electrofusion sleeve, it is determined that the two pipes to be welded are in a naturally tightened state.

[0007] According to the technical solution provided in the embodiments of this application, obtaining the initial installation gap includes the following steps: The pipeline temperature change database is retrieved to obtain the pipeline temperature change amount, wherein the pipeline temperature change database includes at least environmental conditions and pipeline temperature change amounts corresponding to the environmental conditions; The temperature change of the pipeline is input into the pipeline expansion model to obtain the maximum elongation of the pipeline to be welded. The initial installation gap is less than the maximum elongation of the pipeline to be welded.

[0008] According to the technical solution provided in the embodiments of this application, the pipe expansion and contraction model is as follows: Formula (1) in, ΔT This represents the change in pipe temperature (°C). L 0 represents the length of the free section of the pipe (mm). β The coefficient of linear expansion of HDPE material (°C) -1 The length of the free section is the length of the pipe to be welded that is not buried and can be freely expanded and contracted. ΔL This represents the maximum elongation of the pipe to be welded.

[0009] According to the technical solution provided in the embodiments of this application, two welding zones corresponding to the pipe to be welded are provided axially on the inner wall of the electrofusion sleeve. Each welding zone is wound with an electric heating wire. The welding zone is provided with a first side zone, a central zone and a second side zone from the inside to the outside. The winding pitch of the electric heating wire placed in the central zone is greater than the winding pitch of the electric heating wire placed in the first side zone and the second side zone.

[0010] According to the technical solution provided in the embodiments of this application, the welding process includes the following steps: Obtain the target welding temperature range; Obtain the effective initial resistance and the effective temperature coefficient of resistance; The heating wire inside the electrofusion sleeve is energized to begin welding; The average temperature of each of the welding zones is obtained in real time based on the effective initial resistance and the effective resistance temperature coefficient. Determine whether the average temperature is within the target welding temperature range. If not, adjust the welding parameters until the average temperature is within the target welding temperature range.

[0011] According to the technical solution provided in the embodiments of this application, obtaining the effective initial resistance and the effective temperature coefficient of resistance includes the following steps: Obtain a calibration temperature range, and set multiple calibration temperatures within the calibration temperature range; The calibration coil, made of the heating wire, is heated by an electric current. The calibration coil includes two sub-coils, and the heating wires of the two sub-coils are wound in opposite directions. Multiple calibration sets are obtained, each calibration set including the true average temperature of the calibration coil and the real-time resistance value of the calibration coil at each calibration temperature; By fitting multiple calibration combinations, the effective initial resistance and the effective resistance temperature coefficient are obtained.

[0012] According to the technical solution provided in the embodiments of this application, multiple temperature values ​​measured by multiple temperature measuring components set on the calibration coil are obtained, and the true average temperature of the calibration coil is obtained by averaging the multiple temperature values.

[0013] According to the technical solution provided in the embodiments of this application, obtaining the target welding temperature range includes the following steps: Obtain welding process strategies, including standard mode, fast mode, and ultra-low defect mode; The target welding temperature range is obtained by retrieving the welding rule database, wherein the welding rule database includes at least welding process strategies and the corresponding target welding temperature range.

[0014] According to the technical solution provided in the embodiments of this application, the welding process further includes the following steps: Continuously monitor the distance between the marked lines; If the distance between the marking lines increases, the two pipes to be welded are moved so that their ends are pressed together.

[0015] Compared with the prior art, the beneficial effects of this application are as follows: Achieving fundamental mechanical locking: Through precise initial gap design, the pipeline system is actively transformed from a "free expansion and contraction" state to a "pressure-locked" state by utilizing the pipeline's own thermal expansion. In this state, the axial relative displacement between the pipeline and the sleeve is completely eliminated, creating a stable geometric basis for welding.

[0016] Eliminate all welding defects caused by displacement: Since the entire welding process is carried out in a locked position, quality problems directly caused by displacement, such as pipe end detachment, heating wire displacement or breakage, are fundamentally eliminated, significantly improving the first-pass welding qualification rate.

[0017] The method is ingenious, extremely low-cost, and highly reliable: It requires no electricity, no complex sensors or actuators, and achieves the "displacement to zero" control effect that traditionally requires a complex closed-loop control system through simple length measurement and timing. It is highly practical and economical in engineering.

[0018] Especially suitable for on-site construction in complex environments: It perfectly solves the problem of pipeline displacement caused by changes in sunlight and temperature during outdoor construction, and provides a simple and effective guarantee for high-reliability welding of large-diameter pipelines in critical positions such as closure sections and crossing sections. Attached Figure Description

[0019] Other features, objects, and advantages of this application will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a schematic diagram of the structure of the electrofusion sleeve provided in the embodiments of this application; Figure 2 for Figure 1 A magnified view of part A in the image; Figure 3 A flowchart of a low-defect welding method for large-diameter HDPE provided in an embodiment of this application; The text labels in the image represent: 1. Electrofusion sleeve; 2. Welding zone; 21. First side zone; 22. Second side zone; 23. Central zone; 3. Terminal block. Detailed Implementation

[0020] The present application will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, only the parts relevant to the invention are shown in the accompanying drawings.

[0021] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0022] Example 1 This application proposes a large-diameter HDPE electrofusion sleeve. The electrofusion sleeve 1 is sleeved on the outside of the pipe openings of two pipes to be welded distributed along a first direction. The inner wall of the electrofusion sleeve is provided with two welding areas 2 corresponding to the pipes to be welded along the first direction. Each welding area 2 is wound with a heating wire. The welding area 2 is provided with a first side area 21, a central area 23 and a second side area 22 from the inside to the outside. The winding pitch of the heating wire placed in the central area 23 is greater than the winding pitch of the heating wire placed in the first side area 21 and the second side area 22. like Figure 1 and Figure 2 As shown, the electrofusion sleeve 1 is a hollow cylindrical plastic body, usually injection molded from the same high-density polyethylene (HDPE) material as the pipes to be welded. It is used to sleeve and permanently connect two pipes to be welded. The first direction is the axial direction of the electrofusion sleeve 1. The heating wire is embedded in the inner wall of the electrofusion sleeve 1, and its material is usually copper wire coated with insulating varnish. The two ends of the heating wire in each welding zone 2 are led out to the terminals 3 on the outer wall of the sleeve for connecting the welding power source.

[0023] From the inside out, along the first direction, from the center of the electrofusion sleeve 1 to the two ports of the electrofusion sleeve 1, the second side region 22 is near the port, while the first side region 21 is near the other welding area 2. Since the second side region 22 is close to the port of the electrofusion sleeve 1, it experiences strong heat radiation and convection. The first side region 21, being adjacent to the other welding area 2 and located inside, has a relatively enclosed thermal environment and different heat dissipation paths. The central region 23 is surrounded by the first and second side regions 21, hindering axial heat transfer and causing heat to easily accumulate. Traditional equal-pitch designs ignore this difference, leading to inherent axial temperature unevenness. This application uses a smaller pitch in the second side region 22 (with better heat dissipation) and the first side region 21 (with a special thermal environment) to compensate for heat loss; and a larger pitch in the central region 23 (where heat easily accumulates) to actively suppress overheating. This effectively solves the overheating defects (such as holes and material coking) that are easily caused by heat accumulation in the middle of the welding area 2, which is key to improving the reliability of large-diameter electrofusion joints. At the same time, it ensures that both sides have sufficient melting capacity. A weld zone 2 with a more uniform initial heat distribution makes the melting, flow and crystallization behavior in the subsequent welding process more consistent. This provides an ideal controlled object for implementing precise closed-loop temperature control (as described in the claims of the method of this invention), and is a structural guarantee for obtaining welded joints with uniform performance and long-term stability.

[0024] In a preferred embodiment, the ratio of the pitch of the central region 23 to the pitch of the first side region 21 or the second side region 22 is between 1.05 and 1.8. The outer wall region corresponding to the welding area 2 of the electrofusion sleeve 1 has a surface roughness of less than or equal to 6.3 μm and an axial flatness error of less than or equal to 0.1 mm.

[0025] The sparser windings reduce the overall resistance of solder area 2. According to the power formula P=U... 2 Given the same welding power P, reducing the resistance R allows for the use of a lower voltage U. This naturally leads to "low-voltage design." For example, a conventional close-wound design might require a welding voltage of 55V or higher, while the optimized design of this invention might only require 39.5V or 44V. Lower voltage reduces electrical safety risks and the requirements for welder insulation levels, making it possible to weld large-diameter pipes using more widely available and economical "low-power welders" (which typically have limited output current capability but can meet power requirements at low voltage).

[0026] After welding, PAUT (Probe-Inspected Ultrasonic Test) is required to detect welding defects. During PAUT, the probe needs to be in close contact with the surface of the object being tested, and a coupling agent is applied between them to eliminate air. The efficiency of ultrasonic wave propagation between the probe wafer and the workpiece surface depends heavily on the quality of the contact surface. Excessive roughness will generate a large number of messy scattered signals, drowning out weak defect echoes; excessive flatness error will cause local gaps between the probe and the surface (even with coupling agent), resulting in sound energy loss and beam distortion. By controlling the roughness and flatness within a certain range, the incident energy of the ultrasonic waves can be maximized, obtaining clear, stable, and repeatable detection signals. Therefore, this application greatly improves the accuracy, reliability, and sensitivity of PAUT nondestructive testing by limiting the values ​​of surface roughness and axial flatness errors. It provides the necessary technical conditions for the detection and quantitative assessment of internal micro-defects (such as millimeter-level pores).

[0027] Example 2 Based on Example 1, this application provides a low-defect welding method for large-diameter HDPE. This method is based on a scenario where the sleeve and the pipe to be welded are installed in the early morning when the temperature is low, and welding is performed at a later time after the temperature rises. The two pipes to be welded are coaxially arranged and fixed at opposite ends. Before installing the sleeve, in order to ensure the best matching between the electrofusion sleeve 1 and the pipe to be welded and to create reliable initial conditions for subsequent precision monitoring, the following checks and preparations are preferably performed: S001. Pipe measurement, specifically including pipe diameter measurement and out-of-roundness measurement; For example, for a welded pipe with DN=1400mm, use a steel ruler to measure the circumference of the pipe end at 5cm, 10cm, and 25cm, accurate to the mm, and calculate the pipe diameter at different positions based on the circumference (a special π ruler can also be used for direct measurement); the calculated diameter at 5cm from the pipe end should not be less than 1400mm. If the pipe end is significantly constricted and the diameter is less than 1400mm, the constricted section of 10-20cm can be cut off; the pipe end out-of-roundness should be less than 3cm. S002. Measurement of the electrofusion sleeve 1, specifically including measuring the total length of the electrofusion sleeve 1 and the position of the welding zone 2, as well as checking whether the electric heating wire inside the electrofusion sleeve 1 is conductive or not; Among them, four points are selected in the circumferential direction according to the "meter" shape for measurement and estimation of the inner diameter of the electrofusion sleeve 1, accurate to millimeters, and compared with the calculated diameter at 25 cm from the pipe end. If there is an interference fit, the scraping depth of the oxide layer can be appropriately increased. After scraping the oxide layer, measure and calculate the pipe end diameter again, and it is necessary to ensure that the calculated pipe end diameter ≥ 1400 mm. The nominal resistance value of the DN1400 electrofusion sleeve 1 is 0.7 Ω, and the actual resistance value measured by the multimeter is about 0.6 - 0.8 Ω. If the measured resistance value is too large or unstable, it may be that the copper terminal 3 is oxidized and rusted. The surface of the terminal can be polished with a file or the oxide layer of the terminal can be removed with a special copper cleaner, and electrofusion welding can be carried out only after the resistance value is restored to normal.

[0028] The low-defect welding method includes the following steps, as Figure 3 shown: S100. Obtain the initial installation gap, cut the pipes to be welded, so that the distance between the two opposite pipe ends to be welded is the initial installation gap; it includes the following steps: S110. Retrieve the pipeline temperature change database to obtain the pipeline temperature change amount. Among them, the pipeline temperature change database at least includes environmental conditions and the corresponding pipeline temperature change amount; Among them, the pipeline temperature change database refers to a pre-constructed structured data set that stores the corresponding relationship between different environmental conditions and the pipeline temperature change amount of HDPE pipelines. The data sources include laboratory simulation tests, on-site measured data, and industry standards and specifications (such as CJJ63 - 2018), and support quick retrieval and matching of data according to environmental conditions. For example, before installation, first collect the environmental parameters at the construction site, measure the environmental conditions (real-time environmental temperature, sunlight intensity, wind force level, and atmospheric humidity, etc.) through an on-site weather station, and record the pipeline temperature change amount of HDPE pipelines. Call the pre-constructed pipeline temperature change database and retrieve according to the combined conditions of "environmental temperature + sunlight intensity" to obtain the matching pipeline temperature change amount ΔT . For example, for an open-air laid DN1400 HDPE pipeline, the environmental temperature is 25 °C, and the sunlight intensity is 800 W / m², and the retrieved pipeline temperature change amount Δ T = 15 °C.

[0029] S120. Input the pipeline temperature change amount into the pipeline expansion model to obtain the maximum elongation amount of the pipes to be welded, and the initial installation gap is less than the maximum elongation amount of a single pipe to be welded; among them, the pipeline expansion model is shown as the following formula: Formula (1) Among them, ΔT This represents the change in pipe temperature (°C). L 0 represents the length of the free section of the pipe (mm). β The coefficient of linear expansion of HDPE material (°C) -1 The length of the free section is the length of the pipe to be welded that is not buried and can be freely expanded and contracted. ΔL This represents the maximum elongation of the pipe to be welded. For example, if the maximum elongation is 7cm, the initial installation gap is set to 3cm.

[0030] It should be noted here that if the initial installation gap is not obtained by retrieving the database, the maximum elongation of the pipe to be welded from early morning (expected cutting time) to noon (expected welding time) can be obtained by measurement the day before cutting the pipe to be welded.

[0031] S130. Cut the pipes to be welded so that the distance between the ends of the two pipes is the initial installation distance. The cutting work is usually completed in the early morning.

[0032] S200. An electrofusion sleeve is fitted over the pipe ends of two pipes to be welded that are close to each other. Each of the two pipes to be welded has a marking line. The distance between the two marking lines and the pipe ends of the corresponding pipes to be welded is equal. The distance from each marking line to the pipe end of the adjacent electrofusion sleeve is set as the marking line distance. The distance between the two marking lines is equal. The length of the electrofusion sleeve is less than the distance between the two marking lines. Therefore, the condition of the pipe to be welded inside the electrofusion sleeve can be determined by monitoring the movement of the marking lines.

[0033] Before installing the sleeve, perform the following preparations: S003. Before installing the electrofusion sleeve 1, mark a line along the circumference at a distance of 30cm or 50cm from the end of the pipe to be welded (Note: It must be confirmed that the distance of the mark line from the end of the two pipes to be welded is equal, that is, the distance of the mark line is equal, so that when the sleeve cannot be directly observed after installation, the sleeve position can be measured and calculated by the mark line to determine whether the sleeve position is centered and to determine the specific size of the gap at the pipe end after thermal expansion and contraction). S004. Use an electric planer to chamfer the ends of the pipes to be welded at a 45° angle to facilitate the installation of the electrofusion sleeve 1. The chamfer width is generally 1.5~2cm. It is crucial that the sum of the chamfer widths of the two pipes to be welded is less than the distance between the two welding zones 2 of the electrofusion sleeve 1. In other words, when the ends of the two pipes are tightly joined, the chamfer will not fall within the welding zone 2. If the chamfer extends into the welding zone 2 of the electrofusion sleeve 1, it will lead to two serious problems: first, the effective welding contact area will be significantly reduced, weakening the joint's mechanical strength and sealing performance; second, the sharp edge formed by the chamfer will cause current and heat concentration, easily leading to localized overheating, carbonization, or even burn-through of the PE material, forming voids and defects. This directly contradicts the fundamental purpose of this invention: achieving "low defect rate welding."

[0034] S005. Scraping the oxide layer on the outer wall of the pipe to be welded in the fusion zone: The scraping length should not be less than 1 / 2 of the sleeve length. An electric planer or a dedicated scraper can be used for oxide layer scraping; angle grinders are prohibited. When using an electric planer to scrape the oxide layer, be careful not to adjust the planing depth too deeply to ensure good planing smoothness. If necessary, a trial scraping can be performed on the surface of a scrapped pipe section first. After scraping, the calculated diameter of the pipe end is recommended to be controlled between 1400 and 1402 mm. Ensure that weld zone 2 is completely covered by clean, active PE material to prevent incomplete welds caused by oxide layer obstruction.

[0035] The installation of the electrofusion sleeve 1 includes the following steps: S210. By utilizing the inherent flexibility of the HPDE pipe and setting an appropriate length for the free section (unfilled section), sufficient lateral displacement space can be generated at one end of the pipe to be welded, thereby enabling the installation of the electrofusion sleeve 1.

[0036] S220. Use a lever hoist and sledgehammer to fully fit the electrofusion sleeve 1 onto one end of the pipe. During installation, be sure to clean any dust adhering to the inside of the electrofusion sleeve 1 regularly. Because the inner diameter of the electrofusion sleeve 1 is very close to the outer diameter of the pipe, installing the electrofusion sleeve 1 fitting can be somewhat difficult. It is recommended to use four lever hoists arranged in a cross pattern to facilitate adjusting the direction of force on the electrofusion sleeve 1 during installation.

[0037] S230. Close and align the two ends of the pipe, and precisely adjust the initial installation gap with the axis of the pipe to be welded; Slowly close the pipe to be welded on the side where the electrofusion sleeve 1 is located with the pipe to be welded on the other side (not in a tight-fitting state). First, visually adjust to align the axes of the two pipes (avoiding significant offset angles). Then, using a feeler gauge with an accuracy of 0.1mm, select four measuring points evenly in a star pattern around the circumference of the two pipe openings to measure the current gap value between the two pipes in real time. By pushing the free end of the pipe to be welded or finely adjusting the force direction of the lever hoist, slowly adjust the position of the pipe along the pipe axis to accurately adjust the pipe end gap to the preset initial installation gap (error ≤ ±0.5mm), and ensure the uniformity of the gap at the four measuring points (maximum difference ≤ 0.5mm). Finally, confirm that the coaxiality error of the two pipes is ≤1°, with no tilting or misalignment, laying the foundation for the subsequent centering and welding of the electrofusion sleeve 1. S240. Pull the electrofusion sleeve 1 back to the center position of the pipe end gap (confirm whether the sleeve is centered by measuring the marking lines on both ends of the pipe body).

[0038] S300. Periodically monitor the distance between the two marked lines; After the electrofusion sleeve 1 is installed, monitor the distance between the two marked lines in real time and record the data every 5 minutes. Avoid external disturbance to the pipeline during monitoring (such as prohibiting personnel from bumping into it or equipment from running over it). If there are interferences such as strong winds or direct sunlight, a temporary protective shed can be erected for shielding.

[0039] S400. When the distance between the two marked lines indicates that the two pipes to be welded are in a naturally tightened state, welding work begins. The method for determining that the two pipes to be welded are in a naturally tightened state is as follows: S410. Set the stable duration; Among them, the stabilization time refers to the "observation window" for judging whether the pipeline is in a natural tightening state. Its value needs to balance the stability of ambient temperature and construction efficiency, and is usually 30 to 60 minutes. S420. When, within a stable time period, the sum of the lengths of the two pipes to be welded within the electrofusion sleeve, calculated by the distance between the two marked lines, is equal to the length of the electrofusion sleeve, it is determined that the two pipes to be welded are in a naturally tightened state.

[0040] During construction, the pipes to be welded are cut in the cooler morning to create an initial installation gap. As the temperature rises, the two pipes expand and elongate. Let the length of the two pipes inside the electrofusion sleeve be L, the distance between the two marking lines be L1 and L2, the length of the electrofusion sleeve be H, and the distance between the two marking lines and the corresponding pipe ends be L0. When (L0-L1)+(L0-L2)=H, the two sleeves are in a naturally tightened state. Once the pipe ends are tightened, a mechanically stable structure is formed. Even if the temperature continues to rise, the pipes will not displace relative to the sleeves due to axial compression rather than free expansion. Since the initial installation gap is less than the maximum elongation of a single pipe, when the two pipes are naturally tightened, there is an axial force on the pipes. Because the two ends of the pipes are fixed, a "bulge" will appear on the pipe outside the electrofusion sleeve.

[0041] It should be noted that the timing of the mechanical locking of the pipe end requires welding to begin at noon. During the afternoon period (when the sunlight is strong and the pipe to be welded continues to elongate and tighten), the pipe end remains in a continuously tightened state during the cooling process of the welding machine for several hours.

[0042] In a preferred embodiment, initiating the welding process includes the following steps: S500. Obtain the target welding temperature range; including the following steps: S510. Obtain welding process strategies, including standard mode, fast mode, and ultra-low defect mode. Experiments show that HDPE materials can be welded well at 180~230℃. The target welding temperature range of the constant temperature welding method is not fixed, but rather the most suitable welding temperature is selected under the premise of ensuring welding quality. The standard mode is for general engineering projects and construction conditions, and the target welding temperature range can be set to 210℃-220℃. The fast mode is for construction environments with significant changes in air temperature and sunlight, where pipelines are prone to axial temperature deformation and displacement, and welding needs to be completed as soon as possible. In this mode, high-power welding parameters are selected and the target welding temperature range is set to 230℃-240℃ to complete the welding as quickly as possible. The ultra-low defect mode is for engineering projects with strict requirements on the defect rate of electrofusion fittings, requiring non-destructive testing (such as nuclear power projects, which exceed general standards). Low-power welding parameters can be used, and the target welding temperature range is set to 180-190℃ for long-term welding, which can achieve electrofusion welding construction with extremely low defect rates.

[0043] S520. Retrieve the welding rule database to obtain the target welding temperature range, wherein the welding rule database includes at least welding process strategies and the corresponding target welding temperature range.

[0044] The welding rules database can be a spreadsheet, a relational database table, or a data structure embedded in the welding machine / controller software. It stores at least one core mapping relationship: "welding process strategy" and its corresponding "target welding temperature range." The "welding process strategy" is explicitly designed to include at least three modes: standard mode, fast mode, and ultra-low defect mode. Each mode is associated with a preset target welding temperature range.

[0045] Furthermore, within the welding rule database, under the same welding process strategy, in addition to a fixed target temperature range, multiple sets of different initial voltage / time parameters can be associated. For example, for the same DN1400 sleeve, parameter group A (low voltage 39.5V, long time T1) and parameter group B (higher voltage 44V, short time T2) can be designed. Both sets of parameters, after calculation and verification, can achieve the same target welding temperature range and welding effect under closed-loop control. The construction party can flexibly select the starting parameter group based on the power of the welding machine available on site, greatly increasing construction convenience. This application achieves "one-click" high-quality welding, lowering the operational threshold. Skilled workers do not need to memorize or search complex parameter tables; they only need to select the mode according to the working conditions, simplifying the operation process and reducing human error. It also greatly expands the adaptability and economy of the welding system. The three modes cover all scenarios from conventional engineering to extremely harsh environments (rapid repair) to top-level quality requirements (nuclear power). The "multi-parameter design" allows the same pipe fitting to be adapted to welding machines of different power levels, saving users equipment investment and improving the versatility of the pipe fitting.

[0046] S600. Obtain the effective initial resistance and effective temperature coefficient of resistance; including the following steps: S610. Obtain a calibration temperature range and set multiple calibration temperatures within the calibration temperature range; The calibration temperature range should cover the actual temperature range that welding may reach, typically set to 130℃ to 270℃. Within this range, a series of discrete, stable temperature points are selected as calibration points, for example, one point can be set every 20℃ or 30℃, such as 150℃, 180℃, 210℃, 240℃, and 270℃.

[0047] S620. The calibration coil made of the heating wire is energized and heated, wherein the calibration coil includes two sub-coils, and the heating wires of the two sub-coils are wound in opposite directions; The calibration coil is manufactured as follows: A heating wire of the same batch and specification as the one embedded in soldering area 2 is folded in half. The first half is wound clockwise to form a sub-coil, and the second half is wound counterclockwise to form another sub-coil, with the two sub-coils placed closely side-by-side. In the AC circuit, the impedance of the calibration coil includes resistance (R) and inductive reactance (ωL). The two sub-coils wound in opposite directions generate magnetic flux in opposite directions, and their inductive reactances cancel each other out, making the measured total impedance almost a pure resistance, thus ensuring the accuracy of the calibration coil's resistance. S630. Obtain multiple calibration sets, each calibration set including the true average temperature of the calibration coil and the real-time resistance value of the calibration coil at each calibration temperature; in a preferred embodiment, obtain multi-point temperature values ​​measured by multiple temperature measurement components disposed on the calibration coil, and obtain the true average temperature of the calibration coil by averaging the multi-point temperature values.

[0048] Optionally, the temperature measuring component is a type K or type T thermocouple. The number of thermocouples is usually no less than four, for example, eight. The arrangement strategy is to install the measuring ends of the thermocouples at representative positions such as the inner ring, outer ring, upper part, lower part, and the middle and two ends of the axial direction of the calibration coil, and fix them with high-temperature resistant adhesive to ensure good contact with the heating wire.

[0049] The calibration coil is placed in a well-insulated environment (such as being wrapped in thick insulation cotton) and connected to a welding power source and a monitoring recorder. Power is applied for heating, and by finely adjusting the power output, the coil temperature is slowly and steadily raised to each of the previously set "calibration temperatures," maintaining this temperature at each point for a sufficient time to achieve thermal equilibrium. On one hand, real-time voltage and current values ​​are acquired to calculate the real-time resistance of the calibration coil. On the other hand, multiple high-precision thermocouples independently arranged inside and on the surface of the coil are used to measure and calculate the true average temperature of the calibration coil at that moment. Thus, a set of data pairs is obtained at each calibration temperature point, and all the data pairs from all calibration points constitute "multiple calibration sets."

[0050] S640. Fit multiple calibration combinations to obtain the effective initial resistance and the effective resistance temperature coefficient.

[0051] Among these methods, multiple sets of collected data are used to fit the most accurate "effective initial resistance" and "effective resistance temperature coefficient" through mathematical methods such as linear regression, or to directly determine the optimal parameters of the temperature calculation formula. The temperature calculation formula is as follows: T_calculated = (R_calculated - R 0) / ( R 0* α ) + T refFormula (2) Where T_calculated is the calculated temperature, and R_calculated is the real-time resistance value. R 0 represents the effective initial resistance. α The effective temperature coefficient of resistance, T ref To and R The reference temperature corresponding to 0.

[0052] This invention aims to stably control the effective initial resistance error within 0.005 Ω using the aforementioned calibration method, thereby ensuring the accuracy of temperature calculations from the outset. In contrast, existing technologies such as initial pulse heating measurement methods have limited measurement accuracy and may lack practicality due to insufficient consideration of complex on-site conditions, posing a risk of significant temperature calculation deviations. Analogously from a control theory perspective, the feedback calibration based on true average temperature employed in this invention represents a significant improvement in accuracy and reliability compared to feedforward estimations that rely on instantaneous, single-point measurements.

[0053] S700. The heating wire inside the electrofusion sleeve 1 is energized to begin welding; S800. Based on the effective initial resistance and the effective resistance temperature coefficient, the average temperature of each of the solder zones 2 is obtained in real time; including the following steps: S810. Obtain the real-time resistance of the heating wire in each welding zone 2; while welding is in progress, calculate the real-time resistance using Ohm's law based on the real-time voltage and current. S820. Input the resistor into the temperature calculation model to obtain the average temperature of each of the solder zones 2, wherein the temperature calculation model is shown in the following formula: Formula (3) in, T The average temperature. R t For the resistor, R 0 represents the effective initial resistance; α The effective temperature coefficient of resistance, T ref To and R The reference temperature corresponding to 0.

[0054] S900. Determine whether each of the average temperatures is within the target welding temperature range. If not, adjust the welding parameters until the average temperature is within the target welding temperature range. Adjusting the welding parameters includes the following methods: S910. When the average temperature is lower than the lower limit of the target welding temperature, increase the output voltage or current; S920. When the average temperature is higher than the upper limit of the target welding temperature, reduce the output voltage or current.

[0055] In a preferred embodiment, the welding process further includes the following steps: Continuously monitor the distance between the marked lines; If the distance between the marking lines increases, the two pipes to be welded are moved so that their ends are pressed together.

[0056] In certain special scenarios, when the temperature drops suddenly during welding or the worker's work time is delayed, causing the temperature to decrease, the two pipes to be welded in the naturally tightened state will experience cold shrinkage. By flattening the above-mentioned "bulge" and taking advantage of the flexibility of HDPE material, the pipes to be welded can be moved axially to the end for tightening before or during welding.

[0057] In a preferred embodiment, the following steps are further included after welding is completed: S1001. Acquire welding inspection images; In this process, after the welded joint has completely cooled (usually after 24 hours) to room temperature, a phased array ultrasonic (PAUT) testing device is used to scan the outer wall of the electrofusion sleeve 1. The probe moves circumferentially and axially, and the internal computer of the device processes the received ultrasonic echo signals to generate a two-dimensional or three-dimensional color image, i.e., a "weld inspection image." Different colors or brightness in the image represent differences in acoustic impedance within the material, thereby revealing defects such as fusion surface quality, porosity, and inclusions.

[0058] S1002. Obtain the image feature vector of the welding inspection image, which includes multiple feature values. The system extracts a series of quantifiable "feature values" from the region in the image corresponding to the suspected defect. These feature values ​​may include, but are not limited to: the maximum amplitude of the defect echo (related to the defect size), the position coordinates of the defect in the image (depth, axial, and circumferential position), the length and width of the defect indication (pixel size), the shape factor of the defect region (such as roundness), and the spectral characteristics of the echo signal. Arrange these feature values ​​in a certain order to form a mathematical "image feature vector," such as an array containing 5-10 dimensions.

[0059] S1003. Compare with the feature value database to obtain the welding defect type. The feature value database includes multiple image feature vectors and the welding defect type corresponding to the image feature vectors.

[0060] The process involves creating comparative test blocks containing a series of known types, sizes, and locations of artificial defects (such as flat-bottomed holes, transverse holes, and grooves of different diameters). These test blocks are scanned using the same PAUT system. For each known defect, its image feature vector is extracted and stored in a database corresponding to the "defect type" (e.g., "φ2mm flat-bottomed hole," "5mm long, 0.5mm wide groove"). The image feature vector is then automatically compared with the feature vectors of a large number of known defects stored in the database. Finally, the system outputs the type (and equivalent size) of the known defect with the highest similarity to the extracted image feature vector, serving as the "quantitative assessment" result for the suspected defect. For example, the system might report: "A defect was found at location (X,Y,Z), whose features best match the record 'φ3.2mm pore' in the database."

[0061] This application represents a leap from qualitative to quantitative assessment of welding defects. Reports no longer simply state "defective" or "no defect," but rather "how large and what type of defect exists," providing extremely precise data support for project acceptance, life assessment, and rework decisions, far exceeding existing standards. It also significantly improves the objectivity, efficiency, and consistency of non-destructive testing. It reduces subjective judgment differences among testing personnel, enabling the digital and automated analysis of test results, facilitating big data accumulation and intelligent diagnosis. Furthermore, combined with process data, it forms an irrefutable quality traceability system. By linking quantitative defect reports with the "time-temperature curve" generated by closed-loop control, the process causes of defects can be precisely traced (e.g., temperature exceeding limits at a certain moment), enabling rapid location of quality problem causes and continuous process optimization, forming a truly intelligent closed-loop quality management system.

[0062] This application adopts a variable pitch heating wire design to balance the temperature of the large-diameter welding zone 2 and avoid overheating of the central hole and cold welding on both sides; the outer wall of the welding zone 2 is precision machined to improve the PAUT inspection accuracy and accurately identify minute defects, thereby reducing risks from both "defect reduction" and "defect identification".

[0063] By precisely designing the initial gap (temperature database + expansion model), we ensure that the pipeline is tightly sealed due to thermal expansion, thus eliminating welding displacement defects; by building a closed-loop temperature control system (reverse winding calibration + real-time parameter adjustment), we stabilize the temperature of weld zone 2 and avoid over-welding / cold welding; and by monitoring welding displacement (tracking marker line), we adjust the displacement in time to prevent pipeline misalignment during welding from damaging weld zone 2.

[0064] The design incorporates multiple process modes (including an ultra-low defect mode at 180-190℃), with a gentle thermal process to reduce fusion defects. The same pipe fittings can be adapted to different welding machines with multiple parameters. PAUT quantitative detection accurately determines the type and size of defects, facilitating rework and reverse optimization of the process, forming a quality closed loop.

[0065] The above description is merely a preferred embodiment of this application and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in this application is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the inventive concept. For example, technical solutions formed by substituting the above features with (but not limited to) technical features with similar functions disclosed in this application.

Claims

1. A low-defect welding method for large-diameter HDPE, characterized in that, Includes the following steps: Obtain the initial installation gap, cut the pipe to be welded, and make the distance between the ends of the two opposite pipes to be welded the initial installation gap; An electrofusion sleeve (1) is fitted over the pipe ends of two pipes to be welded that are close to each other. Marking lines are provided on the two pipes to be welded. The distance between the two marking lines and the pipe ends of the corresponding pipes to be welded is equal. The distance between each marking line and the pipe end of the adjacent electrofusion sleeve (1) is set as the marking line distance. The distance between the two marking lines is equal. Periodically monitor the distance between the two marked lines; Welding begins when the distance between the two marked lines indicates that the two pipes to be welded are in a naturally tightened state.

2. The low-defect welding method for large-diameter HDPE according to claim 1, characterized in that, The method for determining whether two pipes to be welded are in a naturally tightened state is as follows: Set a stable duration; When the sum of the lengths of the two pipes to be welded within the electrofusion sleeve (1), calculated by the distance between the two marked lines, is equal to the length of the electrofusion sleeve (1) within a stable period, it is determined that the two pipes to be welded are in a naturally tightened state.

3. The low-defect welding method for large-diameter HDPE according to claim 1, characterized in that, The process of obtaining the initial installation gap includes the following steps: The pipeline temperature change database is retrieved to obtain the pipeline temperature change amount, wherein the pipeline temperature change database includes at least environmental conditions and pipeline temperature change amounts corresponding to the environmental conditions; The temperature change of the pipeline is input into the pipeline expansion model to obtain the maximum elongation of the pipeline to be welded. The initial installation gap is less than the maximum elongation of a single pipeline to be welded.

4. The low-defect welding method for large-diameter HDPE according to claim 3, characterized in that, The pipe expansion / contraction model is shown in the following equation: Official (1) in, ΔT This represents the change in pipe temperature (°C). L 0 represents the length of the free section of the pipe (mm). β The coefficient of linear expansion of HDPE material (°C) -1 The length of the free section is the length of the pipe to be welded that is not buried and can be freely expanded and contracted. ΔL This represents the maximum elongation of the pipe to be welded.

5. The low-defect welding method for large-diameter HDPE according to claim 1, characterized in that, The inner wall of the electrofusion sleeve (1) is provided with two welding areas (2) corresponding to the pipe to be welded along the axial direction. Each welding area (2) is wound with an electric heating wire. The welding area (2) is provided with a first side area (21), a central area (23) and a second side area (22) from the inside to the outside. The winding pitch of the electric heating wire placed in the central area (23) is greater than the winding pitch of the electric heating wire placed in the first side area (21) and the second side area (22).

6. The low-defect welding method for large-diameter HDPE according to claim 5, characterized in that, The welding process includes the following steps: Obtain the target welding temperature range; Obtain the effective initial resistance and the effective temperature coefficient of resistance; The heating wire inside the electrofusion sleeve (1) is energized to begin welding; The average temperature of each of the welding zones (2) is obtained in real time based on the effective initial resistance and the effective resistance temperature coefficient. Determine whether the average temperature is within the target welding temperature range. If not, adjust the welding parameters until the average temperature is within the target welding temperature range.

7. The low-defect welding method for large-diameter HDPE according to claim 6, characterized in that, The process of obtaining the effective initial resistance and the effective temperature coefficient of resistance includes the following steps: Obtain a calibration temperature range, and set multiple calibration temperatures within the calibration temperature range; The calibration coil, made of the heating wire, is heated by an electric current. The calibration coil includes two sub-coils, and the heating wires of the two sub-coils are wound in opposite directions. Multiple calibration sets are obtained, each calibration set including the true average temperature of the calibration coil and the real-time resistance value of the calibration coil at each calibration temperature; By fitting multiple calibration combinations, the effective initial resistance and the effective resistance temperature coefficient are obtained.

8. The low-defect welding method for large-diameter HDPE according to claim 7, characterized in that, The true average temperature of the calibration coil is obtained by averaging the multiple temperature measurement components set on the calibration coil.

9. The low-defect welding method for large-diameter HDPE according to claim 7, characterized in that, The process of obtaining the target welding temperature range includes the following steps: Obtain welding process strategies, including standard mode, fast mode, and ultra-low defect mode; The target welding temperature range is obtained by retrieving the welding rule database, wherein the welding rule database includes at least the welding process strategy and the corresponding target welding temperature range.

10. The low-defect welding method for large-diameter HDPE according to claim 6, characterized in that, The welding process also includes the following steps: Continuously monitor the distance between the marked lines; If the distance between the marking lines increases, the two pipes to be welded are moved so that their ends are pressed together.

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