Nondestructive testing method for austenite steel pipe cutting decision

Abstract

The invention discloses a nondestructive testing method for pipe cutting decision of an austenitic steel pipe. The nondestructive testing method comprises the following steps: pretreating the surface of a to-be-tested 18Cr-8Ni series austenitic steel pipe to specified roughness; calibrating and detecting relative permeability values of a plurality of positions of the pretreated steel pipe under set parameters by using a permeability meter; comparing each measured relative permeability value with a pre-established corresponding steel grade aging rating threshold database; and according to the proportion of the detection points exceeding the threshold value, whether the 18Cr-8Ni series austenite steel pipes need to be subjected to pipe cutting sampling or not is decided. According to the method, the quantitative relation between the relative permeability and the aging degree of the material is established through nondestructive testing of the relative permeability of the 18Cr-8Ni series austenite steel pipe in service, so that the aging level of the metallographic structure is evaluated, and a basis is provided for on-site pipe cutting decision.

Description

Technical Field

[0001] This invention belongs to the field of non-destructive testing technology for 18Cr-8Ni series austenitic steel pipes, specifically relating to a non-destructive testing method for making decisions on cutting austenitic steel pipes. Background Technology

[0002] 18Cr-8Ni series austenitic steel pipes, with their excellent resistance to intergranular corrosion, good weldability, thermal stability, and cold working properties, are widely used in high-temperature components such as high-temperature superheaters and high-temperature reheaters in power plant boilers, with a maximum operating temperature of approximately 700℃. However, long-term exposure of 18Cr-8Ni series austenitic steel pipes to high-temperature flue gas can lead to pipe rupture due to flue gas erosion, high-temperature corrosion, and overheating, which is one of the main causes of unscheduled outages in thermal power units. Relevant standards have strict requirements for the testing of 18Cr-8Ni series austenitic steel pipes during maintenance. For example, DL / T 438-2023 "Technical Supervision Regulations for Metals in Thermal Power Plants" stipulates that after 50,000 hours of boiler operation, the tubes of the high-temperature superheater and reheater must be cut and sampled during each A-level overhaul. If the microstructure aging of 18Cr-8Ni series austenitic steel pipes reaches level 4, material evaluation and life assessment are required. However, this also brings the following drawbacks: first, destructive testing of the tube samples is required; second, due to the lack of professional skills of construction workers, on-site tube cutting is blind and disorderly, and the tube cutting and sampling are not representative.

[0003] Related studies have shown that 18Cr-8Ni series austenitic steel pipes in service exhibit weak magnetism. This magnetism primarily originates from the combined effects of deformation-induced martensite (α′ phase) and ferromagnetic precipitates (M23C6 and σ phase). In high-temperature creep and stress concentration regions, especially on the fire-facing side of burst pipes, around grain boundaries and pores, the austenitic matrix undergoes a γ→α′ phase transformation. The α′ phase is a ferromagnetic phase, and its content and distribution directly determine the magnetic response intensity. Furthermore, the higher the aging level of the microstructure, the more M23C6 and σ phase precipitates are present, and the more α′-M precipitates are induced by Cr depletion at grain boundaries, resulting in stronger weak magnetism in the corresponding pipe sample. Based on the correlation between microstructure aging level and weak magnetism, this invention proposes a non-destructive testing method for deciding whether to cut 18Cr-8Ni series austenitic steel pipes. This method aims to determine whether to cut and sample the pipe by measuring the relative permeability of the 18Cr-8Ni series austenitic steel pipe in the field. Summary of the Invention

[0004] The purpose of this invention is to provide a non-destructive testing method for making decisions on cutting austenitic steel pipes, which solves the problem of blind cutting of 18Cr-8Ni series austenitic steel pipes on site.

[0005] The technical solution adopted in this invention is: a non-destructive testing method for decision-making regarding the cutting of austenitic steel pipes, comprising the following steps:

[0006] Step 1: Pre-treat the surface of the 18Cr-8Ni series austenitic steel pipe to be tested to the specified roughness; Step 2: Using a permeability meter, calibrate and test the relative permeability values ​​at multiple locations on the pretreated steel pipe under the set parameters; Step 3: Compare the measured relative permeability values ​​with the pre-established aging rating threshold database for the corresponding steel grades; Step 4: Based on the proportion of detection points exceeding the threshold, decide whether it is necessary to cut and sample the 18Cr-8Ni series austenitic steel pipe.

[0007] The invention is further characterized in that, Step 1 specifically involves: using 80-mesh louvers to perform initial grinding on the outer surface of the 18Cr-8Ni series austenitic steel pipe, followed by secondary grinding using 280-mesh louvers.

[0008] After grinding, the surface roughness of the steel pipe is Ra≤0.5μm.

[0009] In step 2, a FerroPro compact permeability meter with an FPC-5 probe is used as the detection device.

[0010] The parameters of the permeability meter were set as follows: excitation field strength 30 kA / m, measurement mode relative permeability μ. r The detection error is (μ r -1)×5%; then place the probe vertically in the air, adjust the instrument to make the reading 0, and then use a standard test block with a relative permeability of 1.380 to calibrate the permeability meter.

[0011] In step 2, when detecting the relative permeability of the pretreated steel pipe, three radial sections are selected in the pretreatment area. The three sections are evenly distributed along the axial direction of the steel pipe. Four detection points are evenly selected on each section. The four detection points are evenly distributed along the circumference of the section. The relative permeability is measured three times for each detection point. The three measurement values ​​are recorded and the arithmetic mean is calculated as the final measurement value of a single detection point.

[0012] The relative permeability thresholds for different materials in the pre-established aging rating threshold database for corresponding steel grades in step 3 are as follows: the relative permeability threshold for TP347H material is >1.032; the relative permeability threshold for TP347HFG material is >1.013; the relative permeability threshold for TP304H material is >1.017; the relative permeability threshold for TP316H material is >1.014; and the relative permeability threshold for 1Cr18Ni9Ti material is >1.016.

[0013] Step 4 is as follows: Statistically analyze the test results obtained in Step 2. If 50% of the test results are greater than the relative permeability threshold of the corresponding material in Step 3, then the aging degree of the 18Cr-8Ni series austenitic steel pipe is determined to exceed level 3.5, and the 18Cr-8Ni series austenitic steel pipe needs to be cut and sampled. If 50% of the test results are less than the relative permeability threshold of the corresponding material in Step 3, then the aging degree of the 18Cr-8Ni series austenitic steel pipe is determined to not exceed level 3.5, and the 18Cr-8Ni series austenitic steel pipe does not need to be cut and sampled.

[0014] The beneficial effects of this invention are as follows: This invention provides a non-destructive testing method for austenitic steel pipe cutting decisions. By non-destructively testing the relative magnetic permeability of in-service 18Cr-8Ni series austenitic steel pipes, a quantitative relationship between relative magnetic permeability and the degree of material aging is established, thereby assessing the metallographic aging level and providing a basis for on-site pipe cutting decisions. This eliminates the need for destructive sampling tests on in-service pipes, preserving their integrity. Furthermore, it features targeted pipe cutting, low cost, and high testing accuracy, significantly improving testing efficiency. Attached Figure Description

[0015] Figure 1 This is a scatter plot of the relative magnetic permeability and metallographic structure rating of TP347H austenitic steel pipe. Figure 2 This is a scatter plot of the relative magnetic permeability and metallographic structure rating of TP347HFG austenitic steel pipe. Figure 3 This is a scatter plot of the relative magnetic permeability and metallographic structure rating of TP304H austenitic steel pipe. Figure 4 This is a scatter plot of the relative magnetic permeability and metallographic structure rating of TP316H austenitic steel pipe. Figure 5 This is a scatter plot of the relative magnetic permeability and metallographic structure rating of 1Cr18Ni9Ti austenitic steel pipe. Figure 6 It is the metallographic structure of TP304H austenitic high-temperature superheater tube with 25,000 hours of service. Detailed Implementation

[0016] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0017] This invention provides a non-destructive testing method for deciding on the cutting of austenitic steel pipes. By non-destructively testing the relative magnetic permeability of the sample surface, a quantitative relationship between relative magnetic permeability and the degree of material aging is established, thereby assessing the aging level of the metallographic structure and providing a basis for on-site pipe cutting decisions. The specific principle is as follows: During service, the M23C6 and σ phases precipitated in austenitic heat-resistant steel cause chromium depletion at grain boundaries by consuming stabilizing elements, and stress concentration due to lattice mismatch and creep voids, jointly inducing martensitic phase transformation. As the microstructure ages more rapidly, the amount of magnetic martensite and precipitated phases increases, enhancing the material's magnetism. Based on the above principles and in accordance with the "Microstructure Aging Rating Standard for 18Cr-8Ni Series Austenitic Stainless Steel Boiler Tubes" (DL / T1422-2015), a database and related relationship diagrams of metallographic aging rating and relative magnetic permeability for five types of 18Cr-8Ni series austenitic steel tubes (TP347H, TP347HFG, TP304H, TP316H, and 1Cr18Ni9Ti) were established. The results are shown in […]. Figures 1-5 As shown, the results indicate that there is a clear correlation between relative magnetic permeability and the degree of aging. Table 1 shows the cutting thresholds for different 18Cr-8Ni series austenitic steel pipes, with aging level 3.5 as the cutting threshold.

[0018] Table 1 shows the sampling thresholds for magnetic guide cutting tubes of different austenitic heat-resistant steels.

[0019] Based on this, by measuring the relative permeability of the in-service tube, its aging state can be quickly and accurately determined. An established mathematical model can then be used to assess the tissue aging and determine whether tube cutting and sampling are necessary. This method is a non-destructive testing technique that maintains sample integrity and offers advantages such as ease of operation, low cost, high sensitivity, and suitability for on-site batch testing. The assessment results can directly guide whether tube cutting and sampling are necessary, and whether further material evaluation and lifespan assessment are required, thereby enabling targeted maintenance and safety management.

[0020] This invention provides an efficient, economical, and reliable non-destructive testing method for the microstructure aging rating of 18Cr-8Ni series austenitic steel pipes, which has significant engineering application value. The specific technical solution is as follows: S1: The surface of 18Cr-8Ni series austenitic steel pipes is polished using 80-mesh and 280-mesh louvered blades; S2: Set the parameters of the relative permeability detector. The specific parameters are: excitation field strength of 30kA / m, measurement mode of relative permeability μr, detection error of (μr-1)×5%, and calibration is performed on a standard test block with a relative permeability of 1.038 and in air. S3: Detect the relative magnetic permeability of 18Cr-8Ni series austenitic steel pipes; S4: Compare the test results with the relative permeability thresholds in the database; S5: Determine whether it is necessary to cut the tube for sampling based on the comparison results.

[0021] The method of this invention has the following technical features: 1) Based on the principle of weak magnetic detection: Utilizing the magnetic enhancement effect caused by strain-induced martensitic transformation, aging-induced martensitic transformation and precipitation formation in 18Cr-8Ni series austenitic steel pipes during service.

[0022] 2) Advantages of non-destructive testing: The testing process does not damage the sample and maintains the integrity of the pipe sample on site. It is suitable for direct testing of 18Cr-8Ni series austenitic steel pipes used in thermal power plants during maintenance.

[0023] 3) Simple and efficient operation: The FerroPro compact permeability meter is used with the FPC-5 probe, which supports multi-point detection, automatic recording and curve generation, significantly improving the efficiency of on-site detection.

[0024] 4) High sensitivity and accuracy: It is highly sensitive to the aging state of materials and can accurately determine whether tube cutting and sampling are necessary by using the characteristics of relative magnetic permeability distribution.

[0025] 5) Wide range of applications: It complies with the relevant provisions of ASME BPCV I-2015 and DL / T 438-2023 for TP347H austenitic heat-resistant steel, has good engineering applicability, and can be extended to other austenitic heat-resistant steel materials.

[0026] 6) Standardized testing process: The standardized operation covers the entire process from sample pretreatment and equipment calibration to data comparison, ensuring the reliability and consistency of test results.

[0027] To further deepen the understanding of the non-destructive testing method for cutting decisions of 18Cr-8Ni series austenitic steel pipes proposed in this invention, relative magnetic permeability tests were performed on TP347H austenitic steel pipes, TP347HFG austenitic steel pipes, TP304H austenitic steel pipes, TP316H austenitic steel pipes, and 1Cr18Ni9Ti austenitic steel pipes used in thermal power plants to provide a detailed discussion of the method proposed in this invention. Example 1 This case study focuses on the high-temperature superheater tubes of a 660MW thermal power unit that have been in service for 45,000 hours. The tubes are made of TP347H material.

[0028] Step 1: Use 80-mesh and 280-mesh louvers to grind the TP347H austenitic steel pipe in sequence. The grinding range is a whole circle with a height of about 100mm, ensuring that the surface roughness Ra≤0.5μm.

[0029] Step 2: Use a FerroPro compact permeability meter with an FPC-5 probe to detect the weak magnetism of the TP347H austenitic steel tube in service. Set the excitation field strength to 30kA / m, the measurement mode to relative permeability (μr), and the detection error to (μr-1)×5%. After installing the probe, calibrate the instrument to ensure that the relative permeability of the probe is 0 in the vertical state in air. Then, use a standard test block with a relative permeability of 1.380 for calibration.

[0030] Step 3: Select 3 sections in the grinding area of ​​TP347H austenitic steel pipe, select 4 test points evenly in each section, perform 3 measurements at each test point, and take the average value of the 3 measurements. During the test, ensure that the probe is perpendicular to the outer surface of the steel pipe.

[0031] Step 4: The test results of the upper section in clockwise direction are: 1.035, 1.037, 1.037, 1.040; the test results of the middle section in clockwise direction are: 1.038, 1.036, 1.037, 1.039; the test results of the lower section in clockwise direction are: 1.042, 1.039, 1.039, 1.038.

[0032] The decision on whether to cut the TP347H austenitic steel pipe in service for testing was made: 50% of the test results were greater than the threshold of TP347H austenitic steel pipe, i.e., 1.032, indicating that the aging degree of the TP347H austenitic steel pipe in service exceeded level 3.5, and the pipe needed to be cut for sampling.

[0033] Example 2 This case study focuses on the superheater tubes of a 350MW thermal power unit that have been in service for 96,000 hours. The tubes are made of TP347HFG material.

[0034] The operation methods for steps 1 to 3 are the same as those in Example 1.

[0035] Step 4: The test results of the upper section in clockwise direction are: 1.024, 1.026, 1.024, 1.027; the test results of the middle section in clockwise direction are: 1.025, 1.027, 1.026, 1.027; the test results of the lower section in clockwise direction are: 1.030, 1.028, 1.028, 1.029.

[0036] A decision was made regarding whether to cut the TP347H austenitic steel pipe in service for testing: 50% of the test results were greater than the threshold of TP347H austenitic steel pipe, i.e., 1.013, indicating that the aging degree of the TP347H austenitic steel pipe in service exceeded level 3.5, and pipe cutting and sampling were required.

[0037] Example 3 This case study focuses on the high-temperature superheater tubes of a 600MW thermal power unit that have been in service for 25,000 hours. The tubes are made of TP304H material.

[0038] The operation methods for steps 1 to 3 are the same as those in Example 1.

[0039] Step 4: The test results of the upper section in clockwise direction are: 2.061, 2.056, 2.053, 2.055; the test results of the middle section in clockwise direction are: 2.058, 2.056, 2.053, 2.054; the test results of the lower section in clockwise direction are: 2.057, 2.060, 2.057, 2.058.

[0040] A decision was made regarding whether to perform pipe cutting tests on the in-service TP304H austenitic steel pipe: 50% of the test results were greater than the threshold value of 1.017 for TP304H austenitic steel pipe, indicating that the aging degree of the in-service TP304H austenitic steel pipe exceeded level 3.5, requiring pipe cutting and sampling. Actual pipe cutting and metallographic analysis is shown in the attached image. Figure 6 The tissue showed a aging level of 5, indicating a fully aged state, which also verified the accuracy of the method of the present invention.

[0041] Example 4 This case study focuses on the high-temperature superheater tubes of a 660MW thermal power unit that have been in service for 103,000 hours. The tubes are made of TP316H material.

[0042] The operation methods for steps 1 to 3 are the same as those in Example 1.

[0043] Step 4: The test results of the upper section in clockwise direction are: 1.022, 1.023, 1.022, 1.025; the test results of the middle section in clockwise direction are: 1.026, 1.026, 1.023, 1.022; the test results of the lower section in clockwise direction are: 1.027, 1.023, 1.025, 1.025.

[0044] The decision on whether to cut the TP316H austenitic steel pipe in service for testing was made: 50% of the test results were greater than the threshold of TP316H austenitic steel pipe, i.e., 1.014, indicating that the aging degree of the TP316H austenitic steel pipe in service exceeded level 3.5, and the pipe needed to be cut for sampling.

[0045] Example 5 This case study focuses on the screen-type superheater tubes of a 330MW thermal power unit that have been in service for 120,000 hours. The material used in this case study is 1Cr18Ni9Ti.

[0046] The operation methods for steps 1 to 3 are the same as those in Example 1.

[0047] Step 4: The test results of the upper section in clockwise direction are: 1.010, 1.009, 1.011, 1.008; the test results of the middle section in clockwise direction are: 1.008, 1.010, 1.010, 1.009; the test results of the lower section in clockwise direction are: 1.007, 1.006, 1.010, 1.009.

[0048] The decision on whether to cut the 1Cr18Ni9Ti austenitic steel pipe in service was made: 50% of the test results were less than the threshold of 1Cr18Ni9Ti austenitic steel pipe, i.e., 1.016, indicating that the aging degree of the 1Cr18Ni9Ti austenitic steel pipe in service did not exceed level 3.5, and it was not necessary to cut the pipe for sampling.

[0049] Example 6 This case study focuses on the high-temperature superheater tubes of a 600MW thermal power unit that have been in service for 67,000 hours. The tubes are made of TP304H material.

[0050] The operation methods for steps 1 to 3 are the same as those in Example 1.

[0051] Step 4: The test results of the upper section in clockwise direction are: 1.012, 1.009, 1.013, 1.011; the test results of the middle section in clockwise direction are: 1.013, 1.009, 1.012, 1.012; the test results of the lower section in clockwise direction are: 1.014, 1.012, 1.010, 1.012.

[0052] The decision on whether to cut the TP304H austenitic steel pipe in service for testing was made as follows: 50% of the test results were less than the threshold of TP304H austenitic steel pipe, i.e., 1.017, indicating that the aging degree of the TP304H austenitic steel pipe in service did not exceed level 3.5, and it was not necessary to cut the pipe for sampling.

Claims

1. A non-destructive testing method for deciding on the cutting of austenitic steel pipes, characterized in that, Includes the following steps: Step 1: Pre-treat the surface of the 18Cr-8Ni series austenitic steel pipe to be tested to the specified roughness; Step 2: Using a permeability meter, calibrate and test the relative permeability values ​​at multiple locations on the pretreated steel pipe under the set parameters; Step 3: Compare the measured relative permeability values ​​with the pre-established aging rating threshold database for the corresponding steel grades; Step 4: Based on the proportion of detection points exceeding the threshold, decide whether it is necessary to cut and sample the 18Cr-8Ni series austenitic steel pipe.

2. The non-destructive testing method for austenitic steel pipe cutting decision-making as described in claim 1, characterized in that, Step 1 specifically involves: using 80-mesh louvers to perform initial grinding on the outer surface of the 18Cr-8Ni series austenitic steel pipe, followed by secondary grinding using 280-mesh louvers.

3. The non-destructive testing method for austenitic steel pipe cutting decision-making as described in claim 2, characterized in that, After the grinding is completed, the surface roughness of the steel pipe reaches Ra≤0.5μm.

4. The non-destructive testing method for austenitic steel pipe cutting decision-making as described in claim 1, characterized in that, In step 2, a FerroPro compact permeability meter with an FPC-5 probe is used as the detection device.

5. The non-destructive testing method for austenitic steel pipe cutting decision-making as described in claim 4, characterized in that, The parameters of the permeability meter are set as follows: excitation field strength 30 kA / m, measurement mode relative permeability μ. r The detection error is (μ r -1)×5%; then place the probe vertically in the air, adjust the instrument to make the reading 0, and then use a standard test block with a relative permeability of 1.380 to calibrate the permeability meter.

6. The non-destructive testing method for austenitic steel pipe cutting decision-making as described in claim 1, characterized in that, In step 2, when detecting the relative permeability of the pretreated steel pipe, three radial sections are selected within the pretreatment area. The three sections are evenly distributed along the axial direction of the steel pipe. Four detection points are evenly selected on each section. The four detection points are evenly distributed along the circumference of the section. Three relative permeability measurements are performed on each detection point. The three measurement values ​​are recorded and the arithmetic mean is calculated as the final measurement value of a single detection point.

7. The non-destructive testing method for austenitic steel pipe cutting decision-making as described in claim 1, characterized in that, The relative permeability thresholds for different materials in the pre-established aging rating threshold database for corresponding steel grades in step 3 are as follows: the relative permeability threshold for TP347H material is >1.032; the relative permeability threshold for TP347HFG material is >1.013; the relative permeability threshold for TP304H material is >1.017; the relative permeability threshold for TP316H material is >1.014; and the relative permeability threshold for 1Cr18Ni9Ti material is >1.

016.

8. The non-destructive testing method for austenitic steel pipe cutting decision-making as described in claim 1, characterized in that, Step 4 specifically involves: statistically analyzing the test results obtained in step 2. If 50% of the test results are greater than the relative permeability threshold of the corresponding material in step 3, then the aging degree of the 18Cr-8Ni series austenitic steel pipe is determined to exceed level 3.5, and the 18Cr-8Ni series austenitic steel pipe needs to be cut and sampled. If 50% of the test results are less than the relative permeability threshold of the corresponding material in step 3, then the aging degree of the 18Cr-8Ni series austenitic steel pipe is determined to not exceed level 3.5, and the 18Cr-8Ni series austenitic steel pipe does not need to be cut and sampled.

Images