A method for treating corrosion defects of a bolt

Abstract

The present application relates to the technical field of nuclear power plant main pump maintenance, and discloses a bolt corrosion defect processing method, comprising the following steps: S1, detecting the bolt corrosion condition to obtain the measured value of the bolt defect parameter; S2, defect grading of the bolt based on the measured value and the critical value of the bolt defect parameter to obtain the defect grading result; S3, replacing or grading repairing the bolt according to the defect grading result. The bolt corrosion defect processing method scientifically processes and repairs defects, reduces the replacement frequency and operation and maintenance cost of the bolt, provides technical support for the full-cycle safe operation and maintenance of the nuclear main pump seal chamber bolt, and meets the safe operation requirement of the nuclear main pump seal chamber bolt in the whole life cycle.

Description

Technical Field

[0001] This invention relates to the field of nuclear power plant main pump maintenance technology, and in particular to a method for treating bolt corrosion defects. Background Technology

[0002] The bolts of the No. 1 sealing chamber of the main pump in a nuclear power plant are critical fastening components at the pressure boundary of the primary circuit. They operate under high temperature, high pressure, and high humidity conditions (150℃, 173 bar) for extended periods. Affected by backflow from the No. 3 sealing chamber, they are prone to corrosion defects caused by the coupling of chemical and electrochemical corrosion, as well as surface damage from impacts. Corrosion is concentrated on the bottom end face, the root of the polished shank, and the first thread, forming corrosion pits and cracks. The thread-polished shank transition area and the first thread are stress concentration zones. When the pit depth exceeds 0.6 mm or the crack depth exceeds 0.15 mm, the connection strength is significantly reduced, potentially leading to sealing chamber leakage and even main pump shutdown, threatening the safety of nuclear power plant operations.

[0003] Currently, corrosion control of bolts in the sealing chamber of nuclear main pumps primarily employs conventional coating protection or periodic replacement strategies. Conventional coating protection is ill-suited to the stress corrosion environment under high temperature and pressure, easily leading to coating failure. Periodic replacement strategies lack scientific replacement standards based on critical defect parameters, resulting in wasted spare parts or overlooked failure risks. Both lack precise control plans for different corrosion types and defect parameters of bolts, as well as on-site repair of corrosion defects. For bolts that do not meet replacement standards but have minor defects, their connection performance cannot be effectively restored. This results in poor targeted protection, inaccurate defect handling, and high maintenance costs, failing to meet the safe operation requirements of nuclear main pump sealing chamber bolts throughout their entire lifecycle. Summary of the Invention

[0004] The technical problem to be solved by this invention is that bolts have poor corrosion protection, unscientific defect handling and repair, high replacement frequency and maintenance costs, and provides a method for handling bolt corrosion defects.

[0005] A method for treating bolt corrosion defects includes the following steps: S1. Inspect the corrosion of bolts and obtain the measured values ​​of bolt defect parameters; S2. Based on the measured values ​​and critical values ​​of bolt defect parameters, the bolts are classified into defects to obtain the defect classification results; S3. Based on the defect classification results, replace or classify and repair the bolts.

[0006] In one embodiment, step S1, which involves detecting bolt corrosion and obtaining measured values ​​of bolt defect parameters, includes: The bolt corrosion pits are detected and their size and distribution parameters are obtained. The size of the bolt corrosion pits includes length, width and depth. Based on the distribution parameters, the bolt corrosion pits are divided into transition area pits and thread pits. Detect cracks in bolts and obtain the length, depth, and location parameters of the cracks.

[0007] In one embodiment, step S2, the defect classification of the bolt based on the measured values ​​and critical values ​​of bolt defect parameters, to obtain the defect classification result, includes: If the depth of the pit in the transition region is less than or equal to the depth of the pit in the first transition region, the depth of the thread pit is less than or equal to the depth of the first thread pit, the depth of the crack is less than or equal to the depth of the first crack, the length is less than or equal to the length of the first crack, and the crack is located in the low stress region of the bolt, then the crack is determined to be a micro crack, and the bolt is a first-level defect. When the depth of the pit in the transition region is between the depth of the pit in the first transition region and the depth of the pit in the second transition region, the depth of the thread pit is between the depth of the thread pit in the first thread and the depth of the thread pit in the second thread, the depth of the crack is between the depth of the first crack and the depth of the second crack, the length is between the first length and the second length, and the crack is located at the edge of the stress concentration zone of the bolt, the crack is determined to be a microcrack, and the bolt is a secondary defect. If the depth of the pit in the transition region is greater than the depth of the pit in the second transition region, the depth of the threaded pit is greater than the depth of the threaded pit in the second threaded pit, the depth of the crack is greater than the depth of the second crack, and the length is greater than the second length; or, if the size of the pit in the transition region or / and the threaded pit exceeds a preset value, the bolt is determined to be a level three defect.

[0008] In one embodiment, the method further includes the following step before step S3: The new bolts are subjected to primary protection, which includes applying a TiAlCrN coating to the high-risk corrosion areas of the new bolts and adjusting the phosphating process parameters on the bolt surface. Secondary protection is applied to in-service bolts. The secondary protection includes: removing corrosion products from in-service bolts that are free of defects or have primary defects, and installing a seal on the bolt-flange mating surface to prevent water from adjacent areas from entering the bolt connection area.

[0009] In one embodiment, step S3, replacing or classifying the bolt according to the defect classification result, includes: Bolts with Level 1 defects were repaired on-site. Bolts with secondary defects were repaired offline. Replace any bolts with level three defects.

[0010] In one embodiment, the on-site repair includes: Remove the loose corrosion products from the surface of the bolt corrosion pit and apply a repair agent to the surface of the bolt corrosion pit; The microcracks are clad and a cladding layer is formed at the microcracks.

[0011] In one embodiment, the offline repair includes: Remove the bolts, use machining to remove the defect layer in the area of ​​bolt corrosion pits, and apply a TiAlCrN coating. The microcracks were closed by ultrasonic impact.

[0012] In one embodiment, step S3 is followed by: S4, inputting the current service time of the bolt into the trained corrosion trend prediction model to obtain the predicted value of the bolt defect parameter output by the corrosion trend prediction model. The corrosion trend prediction model is used to predict the measured value of the bolt defect parameter within a future preset time period based on the mapping relationship between the service time of the bolt and the measured value of the bolt defect parameter. When the predicted value of the bolt defect parameter approaches the critical value of the bolt defect parameter, preventive repair measures are initiated.

[0013] In one embodiment, step S4 is followed by: S5, obtaining the measured values ​​of bolt defect parameters and the corresponding service time within the target time period, wherein the start time of the target time period is the time when the corrosion trend prediction model was previously fine-tuned, and the end time is the current time. The corrosion trend prediction model is fine-tuned based on the measured values ​​of bolt defect parameters within the target time period and the corresponding service duration. Correspondingly, the step of inputting the current service life of the bolt into the trained corrosion trend prediction model to obtain the predicted bolt defect parameters output by the corrosion trend prediction model includes: Input the current service life of the bolt into the latest corrosion trend prediction model to obtain the predicted bolt defect parameters output by the corrosion trend prediction model.

[0014] In one embodiment, step S5 is followed by: S6. Analyze the corrosion products to obtain the microscopic morphology and structural changes of the bolts and the phase changes of the crystal composition, and adjust the parameters of the protective coating and the repair parameters.

[0015] The bolt corrosion defect treatment method of this invention targets the corrosion mechanism of coupled chemical and electrochemical corrosion in bolts. Combining the critical parameter characteristics of bolt defects, it constructs a defect detection-graded repair system to achieve source control of bolt corrosion, accurate identification of defects, and graded treatment. This improves the bolt corrosion protection effect, scientifically treats and repairs defects, reduces bolt replacement frequency and maintenance costs, and provides technical support for the full-cycle safe operation and maintenance of nuclear main pump sealing chamber bolts, thereby meeting the safe operation requirements of nuclear main pump sealing chamber bolts throughout their entire life cycle. Attached Figure Description

[0016] To more clearly illustrate the technical solution of the present invention, the present invention will be further described below in conjunction with the accompanying drawings and embodiments. It should be understood that the following drawings only show some embodiments of the present invention and should not be considered as a limitation of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort. In the drawings: Figure 1 This is a schematic diagram of the structure of the bolt in the No. 1 sealing chamber of the nuclear main pump in one embodiment of the present invention; Figure 2 This is a flowchart of a bolt corrosion defect treatment method in one embodiment of the present invention. Detailed Implementation

[0017] To provide a clearer understanding of the technical features, objectives, and effects of this invention, specific embodiments are now described in detail with reference to the accompanying drawings. In the following description, it should be understood that the orientations or positional relationships indicated by terms such as "front," "rear," "upper," "lower," "left," "right," "longitudinal," "horizontal," "vertical," "horizontal," "top," "bottom," "inner," "outer," "head," and "tail" are based on the orientations or positional relationships shown in the accompanying drawings, and are constructed and operated in a specific orientation. They are only for the convenience of describing this technical solution and do not indicate that the device or element referred to must have a specific orientation; therefore, they should not be construed as limitations on this invention.

[0018] It should also be noted that, unless otherwise explicitly specified and limited, terms such as "installation," "connection," "linking," "fixing," and "setting" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. When an component is referred to as being "on" or "below" another component, the component can be located "directly" or "indirectly" on the other component, or there may be one or more intermediary components. The terms "first," "second," "third," etc., are only for the convenience of describing this technical solution and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, features defined with "first," "second," "third," etc., may explicitly or implicitly include one or more of that feature. For those skilled in the art, the specific meaning of the above terms in this invention can be understood according to the specific circumstances.

[0019] In the following description, specific details such as particular system architectures and techniques are set forth for illustrative purposes and not for limitation, in order to provide a thorough understanding of the embodiments of the invention. However, those skilled in the art will understand that the invention can be implemented in other embodiments without these specific details. In other instances, detailed descriptions of well-known systems, apparatuses, circuits, and methods are omitted so as not to obscure the description of the invention with unnecessary detail.

[0020] This invention discloses a method for treating bolt corrosion defects. This method is applied to the bolts of the No. 1 sealing chamber of a nuclear main pump to achieve corrosion protection and defect repair of the bolts. It can also be extended to the protection and defect treatment of high-strength alloy steel bolts under similar high-temperature and high-pressure environments. Please refer to... Figure 1 and Figure 2 The bolt corrosion defect treatment method in this embodiment includes the following steps: S1. Inspect the corrosion of the bolts and obtain the measured values ​​of the bolt defect parameters.

[0021] In this embodiment, a combination of non-destructive testing and parameter monitoring is required to assess the condition of the bolts, comprehensively detecting corrosion defects while preventing further damage. Step S1 involves measuring bolt corrosion and obtaining the measured values ​​of bolt defect parameters, including: S11. Detect bolt corrosion pits and obtain the size and distribution parameters of the bolt corrosion pits. The dimensions of the bolt corrosion pits include length, width, and depth. Based on the distribution parameters, the bolt corrosion pits are divided into transition area pits and thread pits. Here, bolt corrosion pits refer to bolt corrosion pits caused by bolt corrosion, rather than depressions on the bolt surface caused by processing.

[0022] S12. Inspect the bolt for cracks and obtain the length, depth, and location parameters of the cracks.

[0023] It should be noted that in this embodiment, the implementation of steps S11 and S12 is not in any particular order. For example, steps S11 and S12 can be implemented sequentially, or steps S12 can be implemented first and then steps S11.

[0024] In step S11, when detecting the size and distribution of bolt corrosion pits, ultrasonic testing can be used to obtain the size (including the length, width, and depth) and distribution of the bolt corrosion pits. This allows for non-destructive testing of bolt corrosion without disassembling the equipment or damaging the bolts. Furthermore, it enables the detection of bolt corrosion pits inside the bolt and in concealed locations at the base of the bolt, achieving precise positioning and dimensional quantification of the corrosion pits.

[0025] In other embodiments, bolt corrosion pits can also be detected and their size and distribution parameters obtained through laser profilometry or optical microscopy. Laser profilometry, through three-dimensional laser scanning, can acquire the three-dimensional morphology of the bolt surface, directly detect bolt corrosion pits, and output accurate data such as the size and distribution parameters of the bolt corrosion pits; optical microscopy, through a combination of high-magnification optical imaging and electronic rulers, can directly observe bolt corrosion pits and accurately measure their size and distribution parameters.

[0026] The structure of the bolt is as follows Figure 1 As shown, it includes a threaded portion 1, a smooth shank portion 2, and a threaded head portion 3 connected in sequence. The threaded portion 1 is located at the bottom of the bolt, and the threaded head portion 3 is located at the top of the bolt. When the bolt is corroded, the corrosion is concentrated on the bottom end face (i.e., the end face adjacent to the threaded portion 1), the root of the smooth shank (i.e., the part where the smooth shank portion 2 connects to the threaded head portion 3), and the first thread (i.e., the part of the threaded portion 1 near the bottom end face). When corrosion occurs, bolt corrosion pits and cracks will appear in the above-mentioned parts. The transition area between the threaded portion 1 and the smooth shank portion 2 (hereinafter referred to as the transition area) and the first thread are stress concentration areas, while the rest of the bolt is a low-stress area.

[0027] In this embodiment, a crack refers to a tiny, newly formed crack in the bolt under stress concentration and corrosion, which has not yet expanded into a large crack. These cracks are often on the micrometer scale and are difficult to detect with the naked eye. They will rapidly propagate under alternating loads, directly leading to bolt fracture. In step S12, magnetic particle testing is used to detect the length, depth, and location of the crack. Magnetic particle testing is particularly sensitive to initial microcracks on and near the surface of ferromagnetic bolts. It can detect nascent cracks that cannot be detected by visual inspection or ordinary checks, and can accurately determine the location, length, and relative depth of the crack. It has the advantages of fast detection speed and no damage or disassembly of the bolt body. In this embodiment, magnetic particle testing is performed on all bolts to be tested. When the detected crack length and depth are 0, it is determined that there is no crack on the bolt; when the detected crack length and depth are greater than 0, it is determined that there is a crack on the bolt. In other embodiments, AC electromagnetic field detection, ultrasonic detection, or phased array ultrasonic detection can be used to detect cracks on the bolt. Alternating current electromagnetic field testing is based on electromagnetic induction. It identifies cracks through magnetic field distortion and can directly obtain parameters such as the location, length, and relative depth of the crack. Ultrasonic testing uses ultrasonic waves to penetrate bolts and determines crack defects based on the reflected echoes, and measures parameters such as the location, length, and relative depth of the crack. Phased array ultrasonic testing uses a controllable acoustic beam fan-shaped scan to image and display parameters such as the location, length, and relative depth of the crack, achieving accurate measurement of the crack.

[0028] S2. Based on the measured values ​​and critical values ​​of bolt defect parameters, the bolts are classified into defects to obtain the defect classification results.

[0029] In step S2, the bolt is classified into defects based on the measured values ​​and critical values ​​of the bolt defect parameters, and the defect classification results include: If the depth of the pit in the transition region is less than or equal to the depth of the pit in the first transition region, the depth of the threaded pit is less than or equal to the depth of the first threaded pit, the depth of the crack is less than or equal to the depth of the first crack, the length is less than or equal to the length of the first crack, and the crack is located in the low stress region of the bolt, the crack is determined to be a micro crack and the bolt is a first-level defect.

[0030] When the depth of the pit in the transition region is between the depth of the pit in the first transition region and the depth of the pit in the second transition region, the depth of the thread pit is between the depth of the thread pit in the first thread and the depth of the thread pit in the second thread, the depth of the crack is between the depth of the first crack and the depth of the second crack, the length is between the first length and the second length, and the crack is located at the edge of the stress concentration zone of the bolt, the crack is determined to be a microcrack, and the bolt is a secondary defect.

[0031] If the depth of the pit in the transition area is greater than the depth of the pit in the second transition area, the depth of the threaded pit is greater than the depth of the threaded pit in the second threaded pit, the depth of the crack is greater than the depth of the second crack, and the length is greater than the second length; or, if the size of the pit in the transition area or / and the threaded pit exceeds a preset value, the bolt is determined to be a level three defect.

[0032] In some embodiments, the bolts being tested can be bolts installed in the sealing chamber of a nuclear power plant, bolts installed on the flanges of pressure vessels, or equipment connection bolts or flange fastening bolts under high temperature, high pressure, and corrosive media conditions, such as flange bolts of steam turbines, condensers, heaters, etc. in thermal power or heating systems, or bolts of circulating water pipes or cooling water pipes; they can also be stainless steel bolts and high-strength alloy bolts arranged in salt spray, water vapor, acid and alkali media environments, such as bolts on offshore platforms or fastening bolts of underwater equipment. In this embodiment, the bolts being tested are the bolts of the No. 1 sealing chamber of the nuclear main pump, specifically M52×3 (metric thread nominal diameter of 52mm, pitch of 3mm) rolled thread 40NCD7.03 alloy steel bolts of the No. 1 sealing chamber of the nuclear main pump of a pressurized water reactor nuclear power plant. The critical parameters for bolt defects are: pit depth in the transition area ≤ 0.6mm, crack depth ≤ 0.15mm. For this type of bolt, the pit depth in the first transition area is 0.3mm, the pit depth in the first thread is 0.4mm, the crack depth is 0.08mm, and the length is 0.1mm; the pit depth in the second transition area is 0.6mm, the pit depth in the second thread is 0.8mm, the crack depth is 0.15mm, and the length is 0.2mm. For example, the specific defect classification is as follows: If the depth of the pit in the transition area is less than or equal to 0.3 mm, the depth of the thread pit is less than or equal to 0.4 mm, the depth of the crack is less than or equal to 0.08 mm, the length is less than or equal to 0.1 mm, and the crack is located in the low stress area of ​​the bolt, the crack is judged to be a micro crack, and the bolt is a first-level defect. In this case, the bolt has a minor defect.

[0033] When the depth of the pit in the transition area is between 0.3-0.6 mm, the depth of the thread pit is between 0.4-0.8 mm, the depth of the crack is between 0.08-0.15 mm, the length is between 0.1-0.2 mm, and the crack is located at the edge of the stress concentration zone, the crack is judged to be a microcrack, and the bolt is a secondary defect. At this time, the bolt has a moderate defect.

[0034] If the depth of the pit in the transition area is greater than 0.6 mm, the depth of the threaded pit is greater than 0.8 mm, and the depth of the crack is greater than 0.15 mm and the length is greater than 0.2 mm; or, if the size of the pit in the transition area and / or the threaded pit exceeds 3 mm in length, 0.75 mm in width, and 0.6 mm in depth, that is, if at least one of the pits in the transition area and the threaded pit exceeds 3 mm in length, 0.75 mm in width, and 0.6 mm in depth, the bolt is judged to have a level three defect, in which case the bolt has a serious defect.

[0035] S3. Based on the defect classification results, replace or classify and repair the bolts. That is, classify and repair the bolts according to different defect conditions in order to scientifically and specifically address different bolt defects.

[0036] In step S3, replacing or repairing the bolts according to the defect classification results includes: Bolts with Level 1 defects were repaired on-site. Bolts with secondary defects were repaired offline. Replace any bolts with level three defects.

[0037] The above-mentioned on-site repair, offline repair, and replacement methods constitute the three-level treatment. When treating a bolt, based on its defect classification results, only one of the above three-level treatment methods needs to be performed.

[0038] When repairing bolts with Level 1 defects on-site, the bolts can be repaired directly without disassembling them. On-site repair of bolts with Level 1 defects includes: removing loose corrosion products from the surface of the bolt corrosion pits and applying a repair agent to the surface of the corrosion pits; cladding treatment of micro-cracks and forming a cladding layer at the micro-cracks.

[0039] For example, for bolt corrosion pits, a precision grinding tool is used to remove loose corrosion products from the surface of the pits. A corrosion-resistant repair agent is then applied, and the agent is cured at a curing temperature of 80°C / 1h (i.e., continuous heating at 80°C for 1 hour) to fill and protect the bolt corrosion pits. In this embodiment, grinding tools such as thread grinding ring gauges / plug gauges, centerless grinding wheels, diamond grinding strips, and abrasive belts can be used to remove corrosion products from the surface of the pits. An epoxy repair agent can be used as the corrosion-resistant repair agent. For microcracks (i.e., cracks detected by magnetic particle testing on the surface of bolts with first-level defects), laser cladding technology is used for micro-area repair. The cladding material is matched with the composition of 40NCD7.03 alloy steel (i.e., the cladding material is the same as the bolt material), and the cladding layer thickness is 0.3-0.5mm. After repair, stress relief treatment is performed. In this way, through laser cladding, the cladding material fills the microcrack and fuses with the inner wall of the microcrack, eliminating the microcrack while ensuring the structural strength of the bolt.

[0040] Offline repair of bolts with secondary defects involves disassembling the bolts and performing the repair at a different location. Offline repair of bolts with secondary defects includes: removing the bolt, machining to remove the defect layer in the corrosion pit area, and applying a TiAlCrN coating; and using ultrasonic impact to close microcracks (i.e., cracks detected on the surface of the bolt with secondary defects by magnetic particle testing).

[0041] For example, during offline processing of disassembled bolts, for bolt corrosion pits, the defect layer in the pit area is removed by machining to ensure that the remaining material thickness meets strength requirements. Then, a TiAlCrN coating is reapplied. The defect layer removal can be performed using laser cutting, tool cutting, or other methods. After the defect layer is removed, a removal zone is formed, and the TiAlCrN coating can be applied to this zone using methods such as plasma spraying or magnetron sputtering. In this way, by removing the defect layer, the impact of stress changes caused by corrosion on this area can be reduced. Simultaneously, the TiAlCrN coating provides corrosion protection to this area and ensures its structural strength. For microcrack areas, ultrasonic impact technology is used to close the microcracks, achieving the purpose of eliminating microcracks. At the same time, the stress concentration factor is reduced, and the fillet radius of the transition area after impact is ≥5mm. Ultrasonic impact testing (UIT / UP) is a highly efficient surface strengthening and defect repair process that uses high-frequency, high-energy mechanical impact to induce plastic flow and compressive stress closure in the metal surface layer. This results in the physical healing of surface / near-surface microcracks (typically <0.5mm), tip passivation, and stress field reconstruction. Through high-frequency impact plastic extrusion, microcracks are physically closed, and the high-stress concentration area at the microcrack tip undergoes localized plastic deformation, increasing the radius of curvature and significantly reducing the stress concentration factor, thus preventing crack propagation. Furthermore, residual stress can be introduced to offset the working tensile stress and inhibit crack reopening and propagation.

[0042] Replacement of bolts with level 3 defects includes: For bolts that meet the criteria for severe defects (i.e., level 3 defect bolts), directly replace the bolts with new bolts. When replacing, use a torque wrench to control the preload and ensure that the preload is 2.5 times the working load to avoid excessive installation stress that could lead to new corrosion.

[0043] It should be noted that after defect classification and before step S3, the following steps are also included: classifying and protecting bolts at different service stages in order to inhibit corrosion from the source.

[0044] Specifically, graded protection includes primary protection for new bolts and secondary protection for in-service bolts. New bolts refer to bolts that have been manufactured but not yet put into use, while in-service bolts refer to bolts that have been put into use and installed. In this embodiment, in-service bolts refer to bolts that have been put into use and installed in the No. 1 sealing chamber of the nuclear main pump. During graded protection, both primary and secondary protection employ coating protection and environmental isolation to achieve precise protection.

[0045] Level 1 protection includes applying a TiAlCrN coating to high-risk corrosion areas of the new bolt and adjusting the phosphating process parameters on the bolt surface. For example, the TiAlCrN coating thickness is 50-80 μm, and the adhesion strength between the TiAlCrN coating and the bolt substrate is greater than or equal to 50 N. Specifically, high-risk corrosion areas of the new bolt include the transition area between the threaded portion 1 and the smooth shank portion 2, and the bottom end face of the bolt. The TiAlCrN coating is applied to these high-risk corrosion areas using plasma spraying. Plasma spraying ensures thorough powder melting and high particle kinetic energy, improving the coating's bonding strength. It also results in good density and low porosity, reducing oxygen permeability. The TiAlCrN coating has a hardness of 2600-3500 HV and an oxidation resistance temperature of up to 1000℃. It is a hard protective coating that combines the high-temperature hardness of TiAlN with the oxidation resistance, corrosion resistance, and toughness of AlCrN, reducing oxygen migration to the bolt surface.

[0046] In addition, after spraying the TiAlCrN coating, the new bolts undergo surface phosphating treatment. By optimizing the bolt surface phosphating process, the density of the phosphating layer on the bolt surface is improved, achieving a pretreatment of the bolts. After pretreatment, high-temperature curing at 200℃ / 2h (i.e., continuous heating at 200℃ for 2 hours) is used to enhance the corrosion resistance of the TiAlCrN coating and the phosphating layer. During the surface phosphating treatment of the bolts, the bolts are placed in a phosphoric acid-based chemical solution. Through chemical reaction, a water-insoluble phosphate conversion film (i.e., the phosphating layer) is formed on the surface. This phosphate conversion film has a passivation and anti-corrosion effect. After oil immersion, it can isolate air and moisture, resist atmospheric corrosion and slight salt spray. In this embodiment, by strengthening surface degreasing and rust removal and activation pretreatment, optimizing the acidity ratio and accelerator content of the phosphoric acid-based chemical solution, controlling the phosphating temperature, time and bath circulation, and combining post-phosphating water washing and sealing treatment, the phosphating grains are refined, porosity defects are reduced, and the density and uniformity of the phosphating layer are significantly improved.

[0047] Secondary protection includes: removing corrosion products from in-service bolts without defects or with primary defects, and installing seals at the bolt-flange mating surfaces to prevent water from adjacent areas from entering the bolted connection area. When the bolts are for the No. 1 sealing chamber bolts of the nuclear main pump, the seals are used to prevent backflow from the No. 3 sealing chamber from entering the bolted connection area, thereby creating a dry protective environment to slow down the bolt corrosion process.

[0048] For example, for bolts in service without defects or with only level one defects, the bolt surface can be periodically rinsed with a concentrated boron solution of 2000 ppm to remove corrosion products, followed by hot air drying to remove moisture. This removes residual corrosive media from the bolt surface, preventing further corrosion. Alternatively, corrosion products can be removed by grinding. The seal at the bolt-flange mating surface can be a fluororubber gasket, or a gasket or sealing ring made of other corrosion-resistant materials can be installed as a seal at the bolt-flange mating surface.

[0049] It should be noted that, in practical applications, the above steps can be used individually or in combination to achieve bolt corrosion prevention and defect repair. For example, when bolt cracks are visible to the naked eye, there is no need for defect classification and graded protection; the bolt can be replaced directly. When the bolt has a level II or III defect in service, the graded protection step can be skipped.

[0050] In one embodiment, step S3 is followed by S4: inputting the current service life of the bolt into a trained corrosion trend prediction model to obtain the bolt defect parameter prediction value output by the corrosion trend prediction model. The corrosion trend prediction model is used to predict the measured value of the bolt defect parameter within a preset time period based on the mapping relationship between the bolt's service life and the measured value of the bolt defect parameter. When the predicted value of the bolt defect parameter approaches the critical value of the bolt defect parameter, preventive repair measures are initiated. Preferably, preventive repair measures are initiated when the predicted value of the bolt defect parameter approaches 80% of the critical value of the bolt defect parameter. In this embodiment, when using the bolt of the No. 1 sealing chamber of the nuclear main pump, the critical value of the bolt defect parameter is: the pit depth in the transition area ≤ 0.6 mm, and the crack depth ≤ 0.15 mm. When the predicted value of the bolt defect parameter approaches 80% of the critical value, that is, based on the aforementioned bolt defect classification conditions, the bolt is classified according to the predicted value of the bolt defect parameter, and the bolt is further processed according to the classification result. Specifically, it can participate in the aforementioned defect classification and processing process, which will not be elaborated here.

[0051] It should be noted that after step S4, the method further includes: S5, obtaining the measured values ​​of bolt defect parameters and their corresponding service durations within the target time period. The start time of the target time period is the time of the previous fine-tuning of the corrosion trend prediction model, and the end time is the current time. Based on the measured values ​​of bolt defect parameters and their corresponding service durations within the target time period, the corrosion trend prediction model is fine-tuned. Correspondingly, the step of inputting the current service duration of the bolt into the trained corrosion trend prediction model to obtain the bolt defect parameter prediction values ​​output by the corrosion trend prediction model includes: inputting the current service duration of the bolt into the latest corrosion trend prediction model to obtain the bolt defect parameter prediction values ​​output by the corrosion trend prediction model. In other words, in this embodiment, the actual measured values ​​of bolt defect parameters obtained by detecting bolts within a specified target time period, as well as the service duration of the bolts during the detection, can be obtained. This allows the obtained measured values ​​of bolt defect parameters and their corresponding service durations to be input into the corrosion trend prediction model, and the corrosion trend prediction model to be fine-tuned (i.e., updated) through data training. By periodically updating the corrosion trend prediction model, the bolt defect parameter prediction values ​​output by the corrosion trend prediction model become closer to the actual bolt conditions.

[0052] Step S5 is followed by: S6. Analyze the corrosion products to obtain the microscopic morphology and structural changes of the bolts and the phase changes of the crystal composition, and adjust the parameters of the protective coating and the repair parameters.

[0053] In this embodiment, during the unit overhaul, all bolts are thoroughly disassembled and inspected to analyze their corrosion products. This allows for the acquisition of the changes in the bolt's microstructure and crystalline composition, enabling the adjustment and optimization of the protective coating and repair process parameters. Specifically, EDS (energy dispersive spectroscopy), XRD (X-ray diffraction), and SEM (scanning electron microscopy) are used to analyze the corrosion products. EDS can analyze the elemental types and relative content of the corrosion products, XRD can analyze the crystalline phases and compound composition of the corrosion products, and SEM can observe the microstructure of the corrosion products, track morphological evolution, and locate micro-area corrosion. By combining EDS and XRD to analyze the corrosion product composition, the types of compounds composed of these elements can be further confirmed after elemental analysis, thus determining the type of corrosive substance. The complementary nature of elemental and phase detection improves the accuracy of the detection results and allows for precise identification of the corrosion type. For example, the detection of Cl and chloride phases indicates pitting corrosion or crevice corrosion; the detection of multiple iron oxides and ferric hydroxides indicates atmospheric oxidation corrosion or electrochemical corrosion. Thus, by analyzing the composition of corrosion products, the corrosive environment can be deduced, providing a basis for crack and failure mechanisms. This facilitates the optimization of anti-corrosion measures based on the corrosion products, such as improving the environment, reducing its impact on corrosion, or improving bolt materials, selecting corrosion-resistant materials for bolt production. Analyzing the evolution of corrosion products using XRD and SEM—SEM showing changes in microstructure and XRD showing changes in crystal composition and phases—allows for a complete understanding of the corrosion product evolution process from macroscopic morphology to microscopic composition. It clarifies the stages of corrosion development, identifying the morphological characteristics and phase composition of each stage. Combining the principles of porous / dense morphology and phase transformation, it analyzes the intrinsic mechanisms of pitting corrosion, stress corrosion, and the initiation and propagation of corrosion cracks, revealing the corrosion mechanism. Thus, by understanding the evolution of corrosion products (i.e., the changes in the microstructure and crystal composition of bolts), processes such as phosphating, coating, ultrasonic impact, and repair agent protection can be optimized. Phosphating process parameters (such as adjusting the phosphating temperature or the type of phosphoric acid chemical solution), coating parameters (coating type, thickness, surface roughness, etc.), ultrasonic impact parameters (ultrasonic frequency), and repair agent protection parameters (repair agent type and dosage) can be adjusted to delay bolt corrosion and the formation of microcracks.

[0054] In addition, bolt corrosion defect treatment methods also include full-cycle monitoring and maintenance of bolts to detect bolt corrosion and cracks as early as possible, prevent sudden fracture accidents, extend service life, achieve precise maintenance, reduce operation and maintenance costs, and ensure the long-term safe and stable operation of equipment. Full-cycle monitoring and maintenance of bolts includes: Establish a full life cycle file for bolts, recording protection time, defect detection data, and repair records.

[0055] Regular monitoring of the condition of in-service bolts is essential to understand their defects and enable timely detection and intervention. This monitoring includes detecting changes in the size of corrosion pits and crack propagation, as well as monitoring the temperature and humidity of the sealed chamber. Specifically, a portable ultrasonic detector is used to detect changes in pit size, and a magnetic particle detector is used to detect crack propagation, achieving non-destructive and accurate inspection of the bolt condition. Monitoring the temperature and humidity of the sealed chamber allows for adjustments to be made when these values ​​exceed preset limits, thus slowing down bolt corrosion.

[0056] The above-mentioned bolt corrosion defect treatment method targets the corrosion mechanism of coupled chemical and electrochemical corrosion in bolts. Combining the critical parameter characteristics of bolt defects, it constructs a defect detection-graded repair system to achieve source control of bolt corrosion, accurate identification of defects, and graded treatment. This improves the bolt corrosion protection effect, scientifically treats and repairs defects, reduces bolt replacement frequency and maintenance costs, and provides technical support for the full-cycle safe operation and maintenance of the sealing chamber bolts of the nuclear main pump, so as to meet the safe operation requirements of the sealing chamber bolts of the nuclear main pump throughout their entire life cycle.

[0057] Compared with existing technologies, the bolt corrosion defect treatment method employing the aforementioned three-level treatment system of precision protection, defect detection, and graded repair has the following significant advantages: 1) Strong protection and significant corrosion inhibition effect: For different corrosion types and high-risk areas of bolts, a combination of TiAlCrN coating protection and environmental isolation is adopted. The corrosion resistance of the coating is 15 times that of the stainless steel substrate, and the oxidation temperature reaches 1000℃. It can effectively resist corrosion in high temperature, high pressure and humid environment, reduce the frequency of bolt corrosion defects by more than 90%, and significantly extend the service life of bolts.

[0058] 2) Precise defect detection and scientific grading: Grading standards are formulated based on the critical parameters of bolt defects (depth of pit in transition area ≤ 0.6mm, crack depth ≤ 0.15mm). Defects are accurately identified through non-destructive testing, avoiding waste of spare parts due to excessive replacement, and preventing safety risks caused by missed defects, reducing operation and maintenance costs by 40%.

[0059] 3) Practical repair technology and reliable connection performance: Corresponding repair processes are adopted for different levels of defects. Minor defects are repaired on-site and moderate defects are strengthened offline. After repair, the tensile strength of the bolt is ≥1500MPa and the stress distribution is uniform. It can effectively restore the bolt connection performance and reduce maintenance costs by 60% compared with direct replacement.

[0060] 4) Full-cycle monitoring and early warning with high safety redundancy: Establish a full life cycle monitoring system and corrosion trend prediction model to achieve early warning and preventive treatment of defects, avoid the risk of leakage in the sealing chamber caused by sudden bolt failure, improve the operational reliability of the nuclear main pump sealing system, and provide important protection for the safe and stable operation of the primary loop equipment of the nuclear power plant.

[0061] It is understood that the above embodiments only illustrate preferred embodiments of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can freely combine the above technical features without departing from the concept of the present invention, and can also make several modifications and improvements, all of which fall within the protection scope of the present invention. Therefore, all equivalent transformations and modifications made with respect to the scope of the claims of the present invention should fall within the scope of the claims of the present invention.

Claims

1. A method for treating bolt corrosion defects, characterized in that, Includes the following steps: S1. Inspect the corrosion of bolts and obtain the measured values ​​of bolt defect parameters; S2. Based on the measured values ​​and critical values ​​of bolt defect parameters, the bolts are classified into defects to obtain the defect classification results; S3. Based on the defect classification results, replace or classify and repair the bolts.

2. The bolt corrosion defect treatment method according to claim 1, characterized in that, In step S1, the measurement values ​​of bolt defect parameters obtained by detecting bolt corrosion include: The bolt corrosion pits are detected and their size and distribution parameters are obtained. The size of the bolt corrosion pits includes length, width and depth. Based on the distribution parameters, the bolt corrosion pits are divided into transition area pits and thread pits. Detect cracks in bolts and obtain the length, depth, and location parameters of the cracks.

3. The bolt corrosion defect treatment method according to claim 2, characterized in that, In step S2, the defect classification of the bolt based on the measured values ​​and critical values ​​of the bolt defect parameters, and the resulting defect classification results, include: If the depth of the pit in the transition region is less than or equal to the depth of the pit in the first transition region, the depth of the thread pit is less than or equal to the depth of the first thread pit, the depth of the crack is less than or equal to the depth of the first crack, the length is less than or equal to the length of the first crack, and the crack is located in the low stress region of the bolt, then the crack is determined to be a micro crack, and the bolt is a first-level defect. When the depth of the pit in the transition region is between the depth of the pit in the first transition region and the depth of the pit in the second transition region, the depth of the thread pit is between the depth of the thread pit in the first thread and the depth of the thread pit in the second thread, the depth of the crack is between the depth of the first crack and the depth of the second crack, the length is between the first length and the second length, and the crack is located at the edge of the stress concentration zone of the bolt, the crack is determined to be a microcrack, and the bolt is a secondary defect. If the depth of the pit in the transition region is greater than the depth of the pit in the second transition region, the depth of the threaded pit is greater than the depth of the threaded pit in the second threaded pit, the depth of the crack is greater than the depth of the second crack, and the length is greater than the second length; or, if the size of the pit in the transition region or / and the threaded pit exceeds a preset value, the bolt is determined to be a level three defect.

4. The bolt corrosion defect treatment method according to claim 3, characterized in that, The steps preceding step S3 also include: The new bolts are subjected to primary protection, which includes applying a TiAlCrN coating to the high-risk corrosion areas of the new bolts and adjusting the phosphating process parameters on the bolt surface. Secondary protection is applied to in-service bolts. The secondary protection includes: removing corrosion products from in-service bolts that are free of defects or have primary defects, and installing a seal on the bolt-flange mating surface to prevent water from adjacent areas from entering the bolt connection area.

5. The bolt corrosion defect treatment method according to claim 4, characterized in that, In step S3, replacing or repairing the bolt according to the defect classification results includes: Bolts with Level 1 defects were repaired on-site. Bolts with secondary defects were repaired offline. Replace any bolts with level three defects.

6. The bolt corrosion defect treatment method according to claim 5, characterized in that, The on-site repair includes: Remove the loose corrosion products from the surface of the bolt corrosion pit and apply a repair agent to the surface of the bolt corrosion pit; The microcracks are clad and a cladding layer is formed at the microcracks.

7. The bolt corrosion defect treatment method according to claim 5, characterized in that, The offline repair includes: Remove the bolts, use machining to remove the defect layer in the area of ​​bolt corrosion pits, and apply a TiAlCrN coating. The microcracks were closed by ultrasonic impact.

8. The bolt corrosion defect treatment method according to claim 1, characterized in that, Step S3 is followed by: S4, inputting the current service time of the bolt into the trained corrosion trend prediction model to obtain the bolt defect parameter prediction value output by the corrosion trend prediction model. The corrosion trend prediction model is used to predict the measured value of the bolt defect parameter within a future preset time period based on the mapping relationship between the service time of the bolt and the measured value of the bolt defect parameter. When the predicted value of the bolt defect parameter approaches the critical value of the bolt defect parameter, preventive repair measures are initiated.

9. The bolt corrosion defect treatment method according to claim 8, characterized in that, Step S4 is followed by: S5, obtaining the measured values ​​of bolt defect parameters and the corresponding service time within the target time period, wherein the start time of the target time period is the time when the corrosion trend prediction model was previously fine-tuned, and the end time is the current time. The corrosion trend prediction model is fine-tuned based on the measured values ​​of bolt defect parameters within the target time period and the corresponding service duration. Correspondingly, the step of inputting the current service life of the bolt into the trained corrosion trend prediction model to obtain the predicted bolt defect parameters output by the corrosion trend prediction model includes: Input the current service life of the bolt into the latest corrosion trend prediction model to obtain the predicted bolt defect parameters output by the corrosion trend prediction model.

10. The bolt corrosion defect treatment method according to claim 9, characterized in that, Step S5 is followed by: S6. Analyze the corrosion products to obtain the microscopic morphology and structural changes of the bolts and the phase changes of the crystal composition, and adjust the parameters of the protective coating and the repair parameters.

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