Heat transfer tube expansion transition area stress corrosion prediction and evaluation method and application
By preparing damaged specimens in the heat transfer tube expansion transition zone and conducting stress corrosion tests, combined with model building and eddy current signal conversion, the problem of predicting stress corrosion in the heat transfer tube expansion transition zone was solved, achieving high-precision evaluation and safety assessment, and reducing test costs.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-15
- Publication Date
- 2026-04-03
AI Technical Summary
Existing technologies are insufficient to effectively predict and assess the initiation and propagation of stress corrosion cracks in the heat transfer tube expansion transition zone, especially in the presence of surface mechanical damage defects, which threatens the safe operation of nuclear power plants.
By preparing heat transfer tube samples with different types and depths of damage, and then expanding the tubes, stress corrosion tests were conducted in a simulated water chemistry environment. Combining the three-dimensional geometric morphology and the crack initiation law after corrosion, a stress corrosion prediction model was constructed and converted into eddy current signal parameters for evaluation.
It enables accurate prediction of stress corrosion in the expansion transition zone of heat transfer tubes, reduces testing cycle and cost, and provides important assessment basis for the safe operation of nuclear power plants.
Smart Images
Figure CN121783749A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of power equipment commissioning technology, and in particular to a method and application for predicting and evaluating stress corrosion in the transition zone of heat transfer tube expansion. Background Technology
[0002] In nuclear power plants, the steam generator is a key component connecting the primary and secondary loops. Its heat transfer tubes act as a critical barrier between the reactor pressure boundary and the primary and secondary loops, and their integrity is paramount to the safety of the nuclear power plant. Currently, 690 nickel-based alloy is widely used as the heat transfer tube material due to its excellent corrosion resistance. The heat transfer tubes are fixed to the tube sheet using an expansion joint process, forming an expansion transition zone between the expanded and non-expanded areas. See the schematic diagram below. Figure 1 As shown, the framed area is the tube expansion transition zone, located on the secondary side, while the back of the tube sheet is on the primary side. The expansion process of the heat transfer tubes involves plastic deformation of the tubes and elastic deformation of the tube sheet under pressure, resulting in a tight bond that achieves sealing and tensile strength. In the tube expansion zone, the tube body first undergoes elastic deformation under pressure until it yields, and the gap between the drilled holes in the tube body and tube sheet gradually decreases. Once the tube body and tube sheet are in contact, the tube sheet also undergoes elastic deformation. When the expansion pressure is released, recovery deformation begins. Because the elastic recovery of the tube sheet is greater than that of the tube body, adhesion pressure is generated between the tube body and tube sheet, thus fixing the tube body in the drilled holes in the tube sheet. The tube expansion transition zone between the tube body and tube sheet experiences significant residual stress due to the expansion. Additionally, during service, there may be sediment deposition, localized high-concentration alkaline or lead-containing water environments, or minor defects (such as scratches or dents) caused by foreign object impacts. The coupled effects of stress, defects, and corrosive environment make the expansion tube transition zone a frequent area for stress corrosion cracking (SCC), posing a potential threat to the long-term safe operation of nuclear power plants.
[0003] Due to the complex structure, confined space, and harsh service water environment of the tube sheet expansion transition zone in steam generators, conducting related stress corrosion performance tests and identifying corresponding defects presents significant technical challenges. Furthermore, predicting and assessing the stress corrosion sensitivity and stress corrosion crack propagation of typical components in the expansion transition zone under specific service conditions is extremely difficult. To date, the industry still lacks predictive assessment models for the initiation and cracking of stress corrosion cracks in typical tube sheet expansion transition zones with surface mechanical damage defects.
[0004] Therefore, it is urgent to conduct systematic research on the tube expansion transition zone of defective tube bodies and tube sheets, scientifically evaluate the impact of surface defects of different scales on the microstructure and corrosion performance of thin-walled tube surfaces, and establish corresponding prediction models. This is an urgent need for nuclear power and other industries. Summary of the Invention
[0005] The technical problem to be solved by the present invention is to provide a method and application for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion.
[0006] The technical solution adopted by this invention to solve its technical problem is: to provide a method for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion, comprising the following steps: S1. Provide several heat transfer tube samples, perform damage preparation on the heat transfer tube samples, and analyze and characterize the three-dimensional morphology of the damage. S2. The heat transfer tube sample is loaded into the tube sheet and expanded to form a sample assembly; the damage on the heat transfer tube sample is located in the tube expansion transition zone of the sample assembly. S3. Place the sample assembly in an aqueous chemical environment for stress corrosion testing; S4. According to the set time interval, the sample assembly is taken out from the water chemical environment in sequence and analyzed to obtain the relationship between microstructure changes and material stress corrosion, so as to obtain the characteristic mechanism and propagation law of stress corrosion crack initiation in the tube expansion transition zone of the sample assembly under multi-factor coupling. S5. Combining the three-dimensional geometric morphology of the damage on the sample assembly and the law of crack initiation and propagation after corrosion, a stress corrosion prediction model is constructed. S6. The stress corrosion prediction model parameters are converted into eddy current signal parameters.
[0007] Preferably, in step S1, the damage includes axial scratches, circumferential scratches, and dents; After the damage preparation is completed, the heat transfer tube samples include at least one of the following: heat transfer tube samples with axial scratches, heat transfer tube samples with circumferential scratches, and heat transfer tube samples with dents.
[0008] Preferably, the length of the axial scratch is 10mm to 20mm and the depth is 30μm to 90μm; the length of the circumferential scratch is 90mm to 120mm and the depth is 30μm to 90μm; and the depth of the indentation is 0.04mm to 0.6mm.
[0009] Preferably, in step S3, the water chemical environment includes a simulated secondary loop water chemical environment, an alkaline water chemical environment, and an alkaline lead-added water chemical environment. Several of the sample assemblies were grouped and placed into simulated secondary loop water chemical environment, alkaline water chemical environment and alkaline lead-added water chemical environment respectively for stress corrosion testing.
[0010] Preferably, in step S1, the heat transfer tube samples are divided into at least three groups, the damage of the first group of heat transfer tube samples is axial scratches, the damage of the second group of heat transfer tube samples is circumferential scratches, and the damage of the third group of heat transfer tube samples is dents. In step S3, the sample assemblies formed by the first group of heat transfer tube samples are grouped and placed in simulated secondary loop water chemical environment, alkaline water chemical environment and alkaline lead-added water chemical environment respectively for stress corrosion test. The sample assemblies formed from the second group of heat transfer tube samples were grouped and placed in simulated secondary loop water chemical environment, alkaline water chemical environment and alkaline lead-added water chemical environment for stress corrosion test. The sample assemblies formed from the third group of heat transfer tube samples were grouped and placed in simulated secondary loop water chemical environment, alkaline water chemical environment, and alkaline lead-added water chemical environment for stress corrosion testing.
[0011] Preferably, the conditions for simulating the secondary loop water chemical environment are: temperature 325℃, pH 25℃ = 9.7~9.9, dissolved oxygen content ≤ 5μg / kg, and pressure 13MPa.
[0012] Preferably, the alkaline aqueous chemical environment conditions are: temperature 325 ℃, aqueous solution prepared with 10 wt.% NaOH, dissolved oxygen content ≤ 5 μg / kg, and pressure 13 MPa.
[0013] Preferably, the alkaline lead-containing water chemical environment conditions are: a temperature of 325 °C, an aqueous solution prepared with 60 ppm Pb and 10 wt.% NaOH, a dissolved oxygen content of ≤5 μg / kg, and a pressure of 13 MPa.
[0014] Preferably, in step S5, the three-dimensional geometry of the damage includes depth, aspect ratio, and root tip radius.
[0015] Preferably, step S5 includes: S5.1 Data Acquisition and Geometric Parameterization: The damage is characterized in three dimensions and the input parameters of the model are extracted. The input parameters of the model include damage depth, damage width and length, damage aspect ratio, damage apical radius of curvature and crack initiation time. S5.2 Physical field simulation based on geometric shape: Based on the three-dimensional geometric shape data of the damage obtained in the above steps, a high-fidelity finite element model is constructed. The local stress-strain field distribution of the damage under service load and residual stress is obtained by calculation. The fracture mechanics parameters that play a decisive role in crack initiation and propagation are extracted from the stress field results. S5.3, Model Building.
[0016] Preferably, the method for predicting and evaluating stress corrosion in the heat transfer tube expansion transition zone further includes the following steps: S1.1 Prepare several damage calibration tubes, perform eddy current detection on the damage on the damage calibration tubes, record the eddy current signal impedance diagram, and obtain the correspondence between the depth, length and location information of the damage on the damage calibration tubes and the eddy current signal.
[0017] Preferably, the damage calibration tube is formed by damaging the heat transfer tube sample; the damage includes axial scratches, circumferential scratches, and dents; The aforementioned damage calibration tubes include at least one of the following: calibration tubes with axial scratches, calibration tubes with circumferential scratches, and calibration tubes with dents.
[0018] Preferably, in the damage calibration sample tube: the length of the axial scratch is 10mm to 20mm and the depth is 30μm to 90μm; the length of the circumferential scratch is 90mm to 120mm and the depth is 30μm to 90μm; the depth of the indentation is 0.04mm to 0.6mm.
[0019] This invention also provides an application of a method for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion. When performing eddy current testing on the heat transfer tubes of a nuclear power plant steam generator, the method performs parameter equivalent transformation on the stress corrosion prediction model based on the eddy current detection signal obtained from the test, obtains the three-dimensional geometric morphology of the corresponding damage, quantitatively evaluates the possible stress corrosion cross-sectional crack length of the damaged heat transfer tube, and determines whether the damaged heat transfer tube needs to be plugged.
[0020] The beneficial effects of this invention are as follows: by quantitatively pre-inducing damage in the expansion transition zone of the heat transfer tube sample, conducting stress corrosion tests in an aqueous chemical environment, and comprehensively analyzing relevant data to construct a stress corrosion prediction model, which is used to predict and evaluate the stress corrosion failure of the expansion transition zone of the heat transfer tube in a nuclear power plant steam generator.
[0021] This invention offers high prediction accuracy, reduces testing cycles, and lowers testing costs, providing an optimized solution for stress corrosion prediction and assessment of typical components in related industries. Attached Figure Description
[0022] The present invention will be further described below with reference to the accompanying drawings and embodiments. In the accompanying drawings: Figure 1 This is a schematic diagram of the structure of an assembly formed by fixing heat transfer tubes to a tube sheet through an expansion joint process. Figure 2 This is a schematic diagram of the structure of the sample assembly with axial scratches in this invention; Figure 3 This is a schematic diagram of the structure of the sample assembly with circumferential scratches in this invention; Figure 4 , Figure 5 These are schematic diagrams of the sample assembly with indentations in this invention. Figure 6 This is a schematic diagram of the structure of the sample assembly after it is installed in the tube sheet and expanded. Detailed Implementation
[0023] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.
[0024] refer to Figures 2-6 The method for predicting and evaluating stress corrosion in the transition zone of heat transfer tube expansion according to the present invention includes the following steps: S1. Provide several heat transfer tube samples 10, prepare damage to the heat transfer tube samples 10, and analyze and characterize the three-dimensional morphology of the damage.
[0025] Several heat transfer tube samples 10, numbering in the dozens or even hundreds. Heat transfer tube samples 10 include, but are not limited to, 690 alloy tubes.
[0026] The damage includes two main categories: scratches and dents 13. Scratches further include axial scratches 11 and circumferential scratches 12. Therefore, the damage can include axial scratches 11, circumferential scratches 12 and dents 13.
[0027] The length of the axial scratch 11 is 10mm to 20mm, and the depth is 30μm to 90μm.
[0028] The length of the circumferential scratch 12 is 90mm to 120mm, and the depth is 30μm to 90μm.
[0029] The depth of dent 13 is 0.04mm to 0.6mm.
[0030] The preparation of axial scratch 11 can be performed as follows: use a special tooling to fix the heat transfer tube sample on the lathe fixture, lock the lathe to keep it stationary and not rotate, fix the tool on the lathe tool post, and perform axial scratch according to the position of the scratch mark.
[0031] The preparation of circumferential scratch 12 can refer to the preparation of axial scratch 11.
[0032] To simulate the actual working condition of high-speed fluid carrying foreign objects and causing dent damage to heat transfer tubes in nuclear power plants, a free-fall method was used. A device with a punch was dropped from a certain height to impact the surface of the heat transfer tube sample to create dents. During the process, a dedicated fixing device was used to secure the heat transfer tube and prevent deformation. The material, shape, and size of the punch were optimized through simulation experiments based on actual working conditions and research requirements to ensure that the dents realistically simulate actual damage. During operation, the drop height of the punch was precisely controlled in conjunction with its mass to ensure controllable dent depth and shape, guaranteeing the repeatability and accuracy of the dent damage preparation method. Different dent depths were achieved by adjusting the drop height of the punch. Through multiple experiments and precise measurements using measuring tools (such as microscopes and 3D profilometers), the dent depth was ensured to be within the set range of 0.04 mm to 0.6 mm.
[0033] After the damage preparation is completed, the heat transfer tube samples 10 include at least one of the following: a heat transfer tube sample 10 with axial scratches 11, a heat transfer tube sample 10 with circumferential scratches 12, and a heat transfer tube sample 10 with dents 13. The heat transfer tube sample 10 with axial scratches 11 can be as follows: Figure 2 As shown, the heat transfer tube sample 10 with circumferential scratches 12 can be used as follows: Figure 3 As shown, the heat transfer tube sample 10 with the dent 13 can be as follows: Figure 4 and Figure 5 As shown. By Figure 4 and Figure 5 It can be seen that, based on the depth and shape of the dent 13, the dent 13 may include strip-shaped dents and hole-shaped dents.
[0034] Each type of damage has multiple different depths and / or different morphologies, which can ensure the quantitative geometric accuracy of damage preparation. The depth and morphology of the prepared indentation are controllable and highly accurate.
[0035] After damage preparation and before stress corrosion testing, laser confocal microscopy and SEM were used to characterize and analyze the damage (dents and scratches) preparation and the surface geometry of the specimens. Specifically, this included three-dimensional geometric parameters such as dent depth, dent contour, dent aspect ratio, scratch depth, and radius of curvature at the root of the scratch. Images of the damaged specimens were observed and recorded in advance for comparative analysis after the test.
[0036] In some embodiments, a plurality of heat transfer tube samples 10 are divided into at least three groups. The damage to the first group of heat transfer tube samples 10 is axial scratches 11, the damage to the second group of heat transfer tube samples 10 is circumferential scratches 12, and the damage to the third group of heat transfer tube samples 10 is dents 13. Correspondingly, subsequent stress corrosion tests are performed on the heat transfer tube samples 10 with the three types of damage, respectively.
[0037] S2. Install all heat transfer tube samples 10 into the tube sheet 20 and expand them to form a sample assembly; the damage on the heat transfer tube sample 10 is located in the tube expansion transition zone, such as... Figures 2-5 As shown, various types of damage, such as axial scratches 11, circumferential scratches 12, and dents 13, are all located in the tube expansion transition zone. Among them, circumferential scratches 12 and dents 13 are located close to the tube sheet 20 in the tube expansion transition zone.
[0038] Specifically, after preparing the surface damage of the heat transfer tube sample 10, a hydraulic expansion process is used to expand the heat transfer tube sample 10 to the tube sheet 20. Before expansion, the end of the heat transfer tube sample 10 is flush with the surface of the primary side weld overlay of the tube sheet 20, ensuring that the damaged area of the heat transfer tube sample 10 is located in the expected expansion transition zone on the secondary side of the tube sheet 20. During expansion, the end core of the high-pressure water pipe used to provide pressure and the expansion gun should protrude 3mm~4mm. During the connection process, hold the end nut of the water pipe and manually rotate the expansion gun until it cannot be rotated, then tighten the end nut of the water pipe with a wrench. The tube-tube sheet positioning expansion pressure is 125MPa~140MPa, and the pressure holding time is 5~6 seconds. After expansion, samples are taken and cut into sample assemblies of the final size, such as... Figure 6 As shown.
[0039] S3. Place the sample components in an aqueous chemical environment for stress corrosion testing.
[0040] The water chemistry environment includes a simulated secondary loop water chemistry environment, an alkaline water chemistry environment, and an alkaline lead-added water chemistry environment.
[0041] The conditions for simulating the secondary loop water chemistry environment are: temperature 325℃, pH 25℃ = 9.7~9.9, dissolved oxygen content ≤5μg / kg, and pressure 13MPa. For simulating the secondary loop water chemistry environment, high-purity water is used to ensure water quality requirements are met. An automatic control system is used to ensure that the dissolved oxygen (DO) concentration (controlled using nitrogen and oxygen) is ≤5μg / kg, and ammonia is used to adjust the pH.
[0042] The conditions for an alkaline aquatic chemical environment are: temperature 325℃, aqueous solution prepared with 10wt.% NaOH, dissolved oxygen content ≤5μg / kg, and pressure 13MPa. For this alkaline aquatic chemical environment, high-purity water is used to ensure water quality requirements are met. An automatic control system is used to ensure the dissolved oxygen (DO) concentration (controlled using nitrogen and oxygen) is ≤5μg / kg, and a high-precision balance with a precision of 1 / 100,000 is used to ensure solution concentration.
[0043] The conditions for an alkaline lead-added water chemical environment are: a temperature of 325 ℃, an aqueous solution prepared with 60 ppm Pb and 10 wt.% NaOH, a dissolved oxygen content ≤5 μg / kg, and a pressure of 13 MPa. For this alkaline lead-added water chemical environment, high-purity water is used to ensure water quality requirements are met. An automatic control system is used to ensure that the dissolved oxygen (DO) concentration (controlled using nitrogen and oxygen) is ≤5 μg / kg, and a high-precision balance with a precision of 1 / 100,000 is used to ensure solution concentration.
[0044] Specifically, several sample components were grouped and placed into simulated secondary loop water chemical environment, alkaline water chemical environment and alkaline lead-added water chemical environment respectively for stress corrosion testing, so that each type of damage was subjected to stress corrosion testing in simulated secondary loop water chemical environment, alkaline water chemical environment and alkaline lead-added water chemical environment respectively.
[0045] In the embodiment where the heat transfer tube samples include three sets, the sample assemblies formed with the tube sheet also correspond to three sets. The sample assemblies formed from the first set of heat transfer tube samples are grouped and placed in simulated secondary loop water chemistry environment, alkaline water chemistry environment, and alkaline lead-added water chemistry environment respectively for stress corrosion testing; the sample assemblies formed from the second set of heat transfer tube samples are grouped and placed in simulated secondary loop water chemistry environment, alkaline water chemistry environment, and alkaline lead-added water chemistry environment respectively for stress corrosion testing; the sample assemblies formed from the third set of heat transfer tube samples are grouped and placed in simulated secondary loop water chemistry environment, alkaline water chemistry environment, and alkaline lead-added water chemistry environment respectively for stress corrosion testing.
[0046] S4. According to the set time interval, take out the sample assembly from the water chemical environment in sequence and analyze it to obtain the relationship between the microstructure changes of the tube expansion transition zone on the sample assembly and the material stress corrosion, so as to obtain the characteristic mechanism and propagation law of stress corrosion crack initiation in the tube expansion transition zone on the sample assembly under multi-factor coupling.
[0047] In the post-experiment, SEM and other characterization methods were used to analyze the stress corrosion cracking, corrosion product morphology, oxide film thickness, and oxide composition at the crack tip of the sample assembly. The stress corrosion sensitivity of the sample assembly material was analyzed, and the relationship between the surface scratch (dent) parameters, microstructure changes, and material stress corrosion was obtained. The initiation mechanism and propagation law of stress corrosion cracks in the heat transfer tube expansion transition section under multi-factor coupling were obtained, the research method of heat transfer tube expansion transition section was mastered, and stress corrosion data of heat transfer tube under the multi-factor coupling of scratch / dent, stress, and environment were obtained. This provides important reference and data support for the application performance evaluation and long-term service safety of heat transfer tubes.
[0048] The set time intervals vary depending on the specific water chemistry environment. For example, in a simulated secondary loop water chemistry environment, samples are taken every 200 hours for analysis; in an alkaline water chemistry environment, samples are taken every 100 hours for analysis; and in an alkaline lead-added water chemistry environment, samples are taken every 50 hours for analysis. These sampling intervals are just examples; the specific intervals can be adjusted based on actual conditions and are not limited to the 200, 100, and 50 hours mentioned above.
[0049] The analytical structures sampled at different times are recorded and accumulated. For example, for a sample assembly with axial scratches, after multiple samplings and analyses at different times, the relationship between the parameter changes, microstructure changes, and material stress corrosion of the axial scratches in a water chemical environment (e.g., a simulated secondary loop water chemical environment) is obtained. The characteristic mechanism and propagation law of stress corrosion crack initiation in the heat transfer tube expansion transition section under multi-factor coupling in a simulated secondary loop water chemical environment are obtained.
[0050] In addition, during stress corrosion testing, the aqueous chemical environment is changed to avoid the influence of corrosive substances on the test. For example, the aqueous chemical environment is changed after 50 to 100 hours of testing.
[0051] S5. Combining the three-dimensional geometric morphology of damage on the sample assembly with the crack initiation and propagation law after corrosion, a stress corrosion prediction model is constructed.
[0052] This step primarily uses the three-dimensional geometry (depth, aspect ratio, root tip radius) of scratches and dents as direct or indirect input to the model, achieving a leap from qualitative assessment of damage presence to quantitative evaluation of its geometric impact. The physics-based damage mechanics model transforms the complex stress corrosion problem into a continuously evolving field variable (damage variable D) problem solvable through finite element analysis. It couples the geometry of the damage, the mechanical behavior of the material, and the chemical effects of the environment into a unified framework.
[0053] The three-dimensional geometry of the damage includes depth, aspect ratio, and root tip radius.
[0054] Specifically, step S5 may include: S5.1 Data Acquisition and Geometric Parametricization: The damage was characterized in three dimensions, and the input parameters of the model were extracted. These parameters included damage depth, damage width and length, damage aspect ratio, damage apical radius of curvature, and crack initiation time. Crack initiation time was the time from the start of the experiment to the formation of microcracks.
[0055] S5.1 Physical field simulation based on geometric morphology: Based on the three-dimensional geometric morphology data of the damage obtained in the above steps, a high-fidelity finite element model is constructed, and the local stress-strain field distribution of the damage under service loads and residual stresses is obtained through calculation. Fracture mechanics parameters that play a decisive role in crack initiation and propagation are extracted from the stress field results. Geometric morphology is the key to affecting the local stress state. This step uses numerical simulation methods to build a bridge between geometric parameters and fracture mechanics parameters. Based on the accurate three-dimensional geometric morphology data obtained in S5.1, a high-fidelity finite element model is constructed. By calculating the local stress-strain field distribution at the tip of the scratch or dent under service loads and residual stresses. Fracture mechanics parameters that play a decisive role in crack initiation and propagation are extracted from the stress field results, and the most important one is the stress intensity factor (K). This makes the input of the model change from macroscopic geometry to micro-mechanics driven.
[0056] S5.1 Model construction.
[0057] The crack initiation process of stress corrosion is regarded as a continuous process in which internal micro-damage accumulates continuously and finally reaches a critical state under the combined action of the service environment (stress, temperature, chemical medium) of the material. The crack initiation process of stress corrosion does not directly predict "cracks", but quantifies an internal state variable - the damage variable (D).
[0058] The damage variable (Damage Variable, D) is a dimensionless scalar, and its value range is [0, 1].
[0059] D = 0: It means that the material is in an initial, undamaged ideal state.
[0060] 0 < D < 1: It means that internal micro-damage (such as micro-voids, micro-cracks, slip band oxidation, etc.) has occurred in the material, but no macroscopic crack has been formed. The larger the value of D, the higher the degree of damage.
[0061] D = 1 (or a critical value Dc close to 1): It means that the bearing capacity of the material at this point is completely lost, and a macroscopic crack begins to form (i.e., crack initiation).
[0062] a) Construction of damage evolution function Define the relationship of the damage evolution function, which describes the rate of increase of the damage variable D with time, and its mathematical form is usually expressed as a function of stress, strain, temperature and environmental chemical parameters. A conceptual expression is as follows: [[ID=Here, T represents temperature, which accelerates the damage process by affecting the diffusion rate and chemical reaction kinetics of the material. C represents the chemical environment: parameters of the secondary loop water chemical environment (such as pH, caustic alkali concentration, and lead concentration) significantly affect the corrosion rate, and these parameters are introduced as variables into the function f.
[0063] b) Coupled Constitutive Law The accumulation of damage can, in turn, weaken the mechanical properties of a material; this is known as "damage coupling." The model addresses this by introducing the concept of effective stress σ_eff.
[0064] σ_eff = σ / (1 - D) σ_eff is the actual stress borne by the undamaged part of the material.
[0065] As the damage D increases, (1-D) decreases. Even if the macroscopic stress σ remains unchanged, the effective stress σ_eff will increase sharply. This will further accelerate the accumulation of damage, forming a positive feedback process until eventual failure.
[0066] c) Crack Initiation Criterion The model uses damage variables to determine whether a crack has started. The criterion is clear: when the accumulated damage variable D(t) at a certain point in the material reaches its critical value Dc, it is determined that a macroscopic crack has started at that point.
[0067] D(t) = ∫[f(σ, ε, T, C, ...)] dt ≥ Dc Dc is a material constant that needs to be calibrated through specialized experiments (such as creep or low-cycle fatigue tests under specific conditions). Crack initiation time (ti) is the time t taken for this integral to reach Dc.
[0068] S6. Convert the parameters of the stress corrosion prediction model into equivalent eddy current signal parameters.
[0069] Based on the established relationship between the eddy current signal and the depth and length of damage on the surface of the heat transfer tube, the parameters of the stress corrosion prediction model constructed above are further converted into eddy current signal parameters. This enables the quantitative assessment of the possible stress corrosion cross-sectional crack length of the damaged heat transfer tube by directly using the eddy current signal from the damage detection during eddy current detection. Furthermore, through further integrity assessment, it can be determined whether the heat transfer tube with damage such as dents needs to be plugged.
[0070] Furthermore, the method for predicting and evaluating stress corrosion in the transition zone of heat transfer tube expansion according to the present invention also includes the following steps: S1.1 Prepare several damage calibration tubes, perform eddy current detection on the damage on the damage calibration tubes, record the eddy current signal impedance diagram, and obtain the correspondence between the depth, length and location of the damage on the damage calibration tubes and the eddy current signal.
[0071] Step S1.1 is applied to step S6 to achieve the equivalent conversion between the input parameters of the constructed stress corrosion prediction model and the eddy current signal parameters.
[0072] Based on the damage distribution characteristics (outline and depth) of the heat transfer tube sample in step S1, and combined with common non-destructive testing methods used in the manufacturing, installation, and operation of nuclear power heat transfer tubes, a damage calibration sample tube is prepared. The depth range must cover the damage of all samples used in the stress corrosion test. In step S1.1, the damage calibration sample tube is formed by preparing damage on the heat transfer tube sample; the damage includes axial scratches, circumferential scratches, and dents.
[0073] The damage calibration tubes include at least one of the following: calibration tubes with axial scratches, calibration tubes with circumferential scratches, and calibration tubes with dents. The length of the axial scratches is 10 mm to 20 mm, and the depth is 30 μm to 90 μm; the length of the circumferential scratches is 90 mm to 120 mm, and the depth is 30 μm to 90 μm; the depth of the dents is 0.04 mm to 0.6 mm.
[0074] For a number of calibration tubes with axial scratches, the scratches have different depths, which can be set in increments of 0.05 μm. For a number of calibration tubes with circumferential scratches, the scratches have different depths, which can be set in increments of 0.05 μm. For a number of calibration tubes with dents, the dents have different depths, which can be set in increments of 0.05 μm.
[0075] After the above steps S1.1, a database corresponding to eddy current signals and damage is constructed. In actual testing, the eddy current signals of the detected heat transfer tube damage can be matched with the corresponding damage parameters and development trends in the database, thereby obtaining the possible stress corrosion cross-sectional crack length for evaluation.
[0076] The application of the stress corrosion prediction and assessment method in the heat transfer tube expansion transition zone according to an embodiment of the present invention is as follows: When performing eddy current testing on the heat transfer tubes of a nuclear power plant steam generator, the parameters of the stress corrosion prediction model are equivalently transformed based on the eddy current detection signal obtained by the test to obtain the three-dimensional geometric morphology of the corresponding damage, quantitatively assess the possible stress corrosion cross-sectional crack length of the damaged heat transfer tube, and then determine whether the damaged heat transfer tube needs to be plugged through further integrity assessment.
[0077] This invention addresses the stress corrosion aging failure of the tube sheet expansion transition zone in nuclear power steam generators, which is influenced by multiple factors including materials, structural form, stress state, and water chemistry conditions. It provides a systematic solution for predicting and assessing stress corrosion in the tube sheet expansion transition zone of nuclear power steam generators with surface damage, under typical locally concentrated water chemistry conditions. The stress corrosion prediction and assessment method of this invention is simple in process, has high prediction accuracy, reduces experimental cycle time, and lowers experimental costs, providing an optimized solution for stress corrosion prediction and assessment of typical components in related industries.
[0078] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.
Claims
1. A method for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion, characterized in that, Includes the following steps: S1. Provide several heat transfer tube samples, perform damage preparation on the heat transfer tube samples, and analyze and characterize the three-dimensional morphology of the damage. S2. The heat transfer tube sample is loaded into the tube sheet and expanded to form a sample assembly; the damage on the heat transfer tube sample is located in the tube expansion transition zone of the sample assembly. S3. Place the sample assembly in an aqueous chemical environment for stress corrosion testing. S4. According to the set time interval, the sample assembly is taken out from the water chemical environment in sequence and analyzed to obtain the relationship between microstructure changes and material stress corrosion, so as to obtain the characteristic mechanism and propagation law of stress corrosion crack initiation in the tube expansion transition zone of the sample assembly under multi-factor coupling. S5. Combining the three-dimensional geometric morphology of the damage on the sample assembly and the law of crack initiation and propagation after corrosion, a stress corrosion prediction model is constructed. S6. The parameters of the stress corrosion prediction model are converted into equivalent eddy current signal parameters.
2. The method for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion as described in claim 1, characterized in that, In step S1, the damage includes axial scratches, circumferential scratches, and dents; After the damage preparation is completed, the heat transfer tube samples include at least one of the following: heat transfer tube samples with axial scratches, heat transfer tube samples with circumferential scratches, and heat transfer tube samples with dents.
3. The method for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion as described in claim 2, characterized in that, The axial scratch has a length of 10mm to 20mm and a depth of 30μm to 90μm; the circumferential scratch has a length of 90mm to 120mm and a depth of 30μm to 90μm; the indentation has a depth of 0.04mm to 0.6mm.
4. The method for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion as described in claim 2, characterized in that, In step S3, the water chemical environment includes a simulated secondary loop water chemical environment, an alkaline water chemical environment, and an alkaline lead-added water chemical environment. Several of the sample assemblies were grouped and placed into simulated secondary loop water chemical environment, alkaline water chemical environment and alkaline lead-added water chemical environment respectively for stress corrosion testing.
5. The method for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion as described in claim 4, characterized in that, In step S1, several heat transfer tube samples are divided into at least three groups. The damage to the first group of heat transfer tube samples is axial scratches, the damage to the second group of heat transfer tube samples is circumferential scratches, and the damage to the third group of heat transfer tube samples is dents. In step S3, the sample assemblies formed by the first group of heat transfer tube samples are grouped and placed in simulated secondary loop water chemical environment, alkaline water chemical environment and alkaline lead-added water chemical environment respectively for stress corrosion test. The sample assemblies formed from the second group of heat transfer tube samples were grouped and placed in simulated secondary loop water chemical environment, alkaline water chemical environment and alkaline lead-added water chemical environment for stress corrosion test. The sample assemblies formed from the third group of heat transfer tube samples were grouped and placed in simulated secondary loop water chemical environment, alkaline water chemical environment, and alkaline lead-added water chemical environment for stress corrosion testing.
6. The method for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion as described in claim 4, characterized in that, The conditions for simulating the secondary loop water chemistry environment are: temperature 325℃, pH 25℃ = 9.7~9.9, dissolved oxygen content ≤5μg / kg, and pressure 13MPa.
7. The method for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion as described in claim 4, characterized in that, The alkaline aqueous chemical environment conditions are: temperature 325 ℃, aqueous solution prepared with 10 wt.% NaOH, dissolved oxygen content ≤5 μg / kg, and pressure 13 MPa.
8. The method for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion as described in claim 4, characterized in that, The conditions for the alkaline lead-containing water chemical environment are: a temperature of 325 ℃, an aqueous solution prepared with 60 ppm Pb and 10 wt.% NaOH, a dissolved oxygen content of ≤5 μg / kg, and a pressure of 13 MPa.
9. The method for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion as described in claim 1, characterized in that, In step S5, the three-dimensional geometry of the damage includes depth, aspect ratio, and root tip radius.
10. The method for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion as described in claim 1, characterized in that, Step S5 includes: S5.1 Data Acquisition and Geometric Parameterization: The damage is characterized in three dimensions and the input parameters of the model are extracted. The input parameters of the model include damage depth, damage width and length, damage aspect ratio, damage apical radius of curvature and crack initiation time. S5.2 Physical field simulation based on geometric shape: Based on the three-dimensional geometric shape data of the damage obtained in the above steps, a high-fidelity finite element model is constructed. The local stress-strain field distribution of the damage under service load and residual stress is obtained by calculation. The fracture mechanics parameters that play a decisive role in crack initiation and propagation are extracted from the stress field results. S5.3, Model Building.
11. The method for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion as described in any one of claims 1-10, characterized in that, The method for predicting and evaluating stress corrosion in the transition zone of heat transfer tube expansion also includes the following steps: S1.1 Prepare several damage calibration tubes, perform eddy current detection on the damage on the damage calibration tubes, record the eddy current signal impedance diagram, and obtain the correspondence between the depth, length and location information of the damage on the damage calibration tubes and the eddy current signal.
12. The method for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion as described in claim 11, characterized in that, The damage calibration tube is formed by damaging the heat transfer tube sample; the damage includes axial scratches, circumferential scratches, and dents. The aforementioned damage calibration tubes include at least one of the following: calibration tubes with axial scratches, calibration tubes with circumferential scratches, and calibration tubes with dents.
13. The method for predicting and evaluating stress corrosion in the transition zone of a heat transfer tube expansion as described in claim 12, characterized in that, In the damage calibration sample tube: the length of the axial scratch is 10mm to 20mm and the depth is 30μm to 90μm; the length of the circumferential scratch is 90mm to 120mm and the depth is 30μm to 90μm; the depth of the dent is 0.04mm to 0.6mm.
14. An application of the stress corrosion prediction and evaluation method for the heat transfer tube expansion transition zone according to any one of claims 1-13, characterized in that, When performing eddy current testing on the heat transfer tubes of a nuclear power plant steam generator, the parameters of the stress corrosion prediction model are equivalently transformed based on the eddy current detection signal obtained from the test to obtain the three-dimensional geometric morphology of the corresponding damage, quantitatively assess the possible stress corrosion cross-sectional crack length of the damaged heat transfer tube, and determine whether the damaged heat transfer tube needs to be plugged.