Low-frequency eddy current-based thermal power plant drain pipe seat inner wall defect detection method

Abstract

The present invention relates to a low-frequency eddy current-based thermal power plant drain pipe seat inner wall defect detection method. The method comprises: organizing and collecting data to determine the distribution and characteristics of crack defects in inner walls of drain pipe seats of pressure parts, so as to provide a theoretical basis for the next step of specimen manufacturing; determining a scanning mode and designing reference specimens having cracks, so as to configure the detection sensitivity by means of the reference specimens; by means of an eddy current penetration depth formula and the reference specimens, configuring a detection frequency, and performing parallel-line or grid-line scanning along a scanning path at fillet weld parts of pipe seats and at parts from fusion lines below fillet welds to fusion lines above butt welds; and introducing an equivalent method and a formula calculation algorithm, so as to perform defect quantification and position determination on the basis of the amplitude of trajectory curve signals. The present invention solves the problem of nondestructive detection of crack defects in inner walls of drain pipe seats of pressure parts in in-service thermal power units, and can effectively avoid pipeline leakage accidents caused by expansion of crack defects. The method is simple, safe, reliable and widely applicable.

Description

A method for detecting inner wall defects of drain pipe sockets in thermal power plants based on low-frequency eddy current Technical Field

[0001] The present invention relates to a method for detecting defects in a drain pipe seat of a high-temperature pressure-bearing component in a thermal power plant. Specifically, a low-frequency eddy current detection method is used to detect defects such as cracks on the inner wall of the drain pipe seat of the high-temperature pressure-bearing component, belonging to the field of non-destructive testing. Background Art

[0002] As the difference between peak and valley loads in the power grid increases year by year, installed capacity is growing rapidly, but electricity consumption is increasing more slowly. Thermal power utilization hours and load factors are declining significantly, placing higher demands on energy conservation. Thermal power units operating on the grid must implement deep peak load regulation, exceeding the minimum steady-fire load of power plant boilers. Peak load regulation requires units to dive deeper, with faster startup and load change rates, significantly impacting the service life of metal components.

[0003] The drain from the high-temperature pressure-bearing components of the boiler in service power station is the condensed water generated by the steam in the pipeline due to the pressure and temperature drop. The drain should be discharged in time, otherwise it will not only absorb the heat of the steam in the pipe and affect the steam flow, but also seriously cause water hammer, resulting in serious consequences. Therefore, welding the drain pipe seat on the pipeline to connect the small pipe outside the furnace can realize the normal discharge of the drain.

[0004] However, due to factors such as deep peak shaving, when the temperature around the drain pipe orifice fluctuates repeatedly, a temperature differential develops between the orifice and the pipe wall, causing the pipe wall to expand and contract, generating thermal stress cycles. This can lead to fatigue damage in the material and easily induce thermal fatigue, resulting in internal surface cracks. Thermal fatigue cracks often originate from multiple fatigue sources, often initiating several cracks or a network of cracks. One of these cracks develops into the primary crack, while the others grow slowly or cease to expand due to thermal stress relaxation. Thermal fatigue expands irregularly and in a jumpy manner, with the cracks filled with loose oxides. If cracks in the inner wall of the drain pipe seat are not promptly addressed, leaks can seriously threaten the safe and stable operation of the thermal power unit and cause significant economic losses to the power plant.

[0005] The diameter of the drain pipe seat, a high-temperature, pressure-bearing component in a thermal power plant, is small (32mm to 89mm) and thin (4.0mm to 12.0mm). Currently, magnetic powder, penetrant, and ultrasonic methods are generally used for inspection of the drain pipe seat. Magnetic powder and penetrant methods can only detect surface or near-surface defects, and have high requirements for the surface finish of the pipe wall, requiring polishing. However, due to the thin wall of the drain pipe seat, excessive polishing is not dared, and the surface finish of the weld generally cannot meet the requirements, which easily leads to missed detection. Conventional ultrasonic detection requires the pipe wall to be polished to produce a metallic luster, which easily leads to thinning of the pipe wall. In addition, ultrasonic waves can only emit an axial sound beam with a fixed angle, and can only detect defects perpendicular to the sound beam direction, namely circumferential defects, but cannot detect parallel defects, namely axial defects. Ultrasonic waves also have the disadvantages of sound velocity diffusion, low reflectivity, and low defect detection rate, resulting in missed defects. In recent years, as the units are operating in high temperature, high pressure environments and deep peak regulation, there have been many unit non-shutdown accidents caused by defects in the drain pipe seat, especially crack defects that expand and leak, which have seriously affected the unit's power generation and grid stability. There is an urgent need for a new detection method to detect axial and circumferential crack defects inside the drain pipe seat, to prevent them in advance and ensure the safety of the unit.

[0006] Summary of the Invention

[0007] The purpose of the present invention is to overcome the above-mentioned shortcomings and thus provide a method for detecting defects on the inner wall of the drain pipe seat of a thermal power plant based on low-frequency eddy currents. The method can effectively solve the problem of non-destructive detection of internal defects of the drain pipe seat of the high-temperature pressure-bearing component of an in-service thermal power unit, so as to prevent the occurrence of unit non-shutdown accidents caused by the expansion and leakage of internal defects of the drain pipe seat, thereby improving work efficiency, being safe and reliable, and having a wide range of applications.

[0008] The technical solution provided by the present invention is: a method for detecting inner wall defects of a drain pipe seat of a thermal power plant based on low-frequency eddy current, comprising the following steps:

[0009] Step 1: Determine the scanning path and scanning method

[0010] The outer wall surface of the drain pipe seat and the outer surface of the drain pipe seat fillet weld are used as the detection surface, and the circumference of the detection surface along the outer surface area of ​​the drain pipe seat fillet weld is the scanning path 1; the scanning method of the eddy current probe along the scanning path 1 is parallel line scanning;

[0011] The inspection surface of the outer surface area from the lower fusion line of the fillet weld of the drain pipe seat to the upper fusion line of the butt weld is the scanning area of ​​scanning path 2. In the scanning area of ​​scanning path 2, the eddy current probe scans along the outer wall surface of the pipe seat in the axial and circumferential directions in a grid line manner as scanning path 2;

[0012] Step 2: Instrument and probe selection

[0013] The instrument selected is a digital low-frequency eddy current scanner, and the probe is a point eddy current probe;

[0014] Step 3: Selection of comparison samples

[0015] Two comparison test blocks were used, namely test block DBSY-1 and test block DBSY-2, with the following parameters:

[0016] DBSY-1: 100mm in length, t1 in wall thickness, and d1 in diameter. Seven 0.5mm circular holes are machined on the inner wall of the comparison block at depths of 2%, 5%, 10%, 20%, 40%, 60%, and 80% of the wall thickness, respectively.

[0017] DBSY-2: 100 mm in length, t2 in wall thickness, and d2 in diameter. Seven circumferential grooves and seven axial grooves are engraved on the inner wall of the comparison block. The length × width of the circumferential and axial grooves are 5 mm × 0.5 mm, and the depths are 2%, 5%, 10%, 20%, 40%, 60%, and 80% of the wall thickness, respectively. There are 14 grooves in total.

[0018] Step 4: Setting the detection frequency and sensitivity

[0019] Formula for penetration depth of eddy current using placed coil on semi-infinite plane conductor Approximately estimate the initial detection frequency when the tube seat thickness is t, and then determine it using the response of the artificial defects on the reference sample at this initial detection frequency (δ-standard penetration depth, unit is m; f-operating frequency, unit is Hz; σ-conductivity, S / m);

[0020] When the probe scans along path 1 using the parallel line method, calibration is performed using test block DBSY-1. The probe is placed on the outer surface of the pipe wall corresponding to seven Φ0.5mm circular holes with depths of 2%, 5%, 10%, 20%, 40%, 60%, and 80% of the wall thickness. Seven trajectory curve signals with peak values ​​are displayed on the instrument's screen, and the trajectory curve signal amplitudes vary. The closer the circular hole is to the outer surface of the pipe wall, the higher the trajectory curve signal amplitude. The trajectory curve signal amplitude F1 corresponding to a depth of 2% of the wall thickness is adjusted to 40% of the full scale of the instrument's screen as the detection sensitivity. The percentage of the trajectory curve signal amplitude F1 at different groove depths relative to the full scale of the instrument's screen is recorded.

[0021] When the probe scans along path 2 using a grid line method, DBSY-2 is used for calibration. The probe is placed on the outer surface of the pipe wall corresponding to 14 circumferential grooves and axial grooves with depths of 2%, 5%, 10%, 20%, 40%, 60%, and 80% of the wall thickness. 14 trajectory curve signals with peak values ​​are displayed on the instrument's fluorescent screen, and the trajectory curve signal amplitudes vary. The closer the circular hole is to the outer surface of the pipe wall, the higher the trajectory curve signal amplitude. The trajectory curve signal amplitude F2 corresponding to a depth of 5% of the wall thickness is adjusted to 40% of the full scale of the instrument's fluorescent screen as the detection sensitivity. The percentage of the trajectory curve signal amplitude F2 of different groove depths on the full scale of the instrument's fluorescent screen is recorded at this time.

[0022] Step 5: Scan

[0023] The eddy current probe is placed on the outer surface of the area between the lower fusion line of the fillet weld of the pipe seat and the upper fusion line of the butt weld. The probe is in stable contact with the outer surface and kept perpendicular to the surface of the sample. Scan along scanning path 1 and scanning path 2 using the parallel line method or the grid line method.

[0024] Step 6: Result evaluation

[0025] After scanning, the defects are quantified and located:

[0026] Defect quantification:

[0027] (1) Pipe socket fillet weld: When the trajectory curve signal amplitude F1 exceeds 20% of the full scale of the instrument's fluorescent screen, i.e., F1>20%, it is judged as a dangerous defect, unqualified, and directly scrapped; when the trajectory curve signal amplitude is 10%≤F1≤20%, the defect should be recorded and inspected regularly; when the trajectory curve signal amplitude F1<10%, no record is required;

[0028] (2) From the lower fusion line of the fillet weld to the upper fusion line of the butt weld: When the trajectory curve signal amplitude F2 exceeds 30% of the full scale of the instrument's fluorescent screen, i.e., F2>30%, it is determined to be a dangerous defect, unqualified, and directly scrapped; when the trajectory curve signal amplitude is 15%≤F2≤30%, the defect should be recorded and inspected regularly; when the trajectory curve signal amplitude F2<15%, no record is required;

[0029] Defect location determination:

[0030] (1) Pipe seat fillet weld: When the trajectory curve signal amplitude F1 is greater than 20%, the defect position is determined by comparing the trajectory defect signal with the percentage of the full scale of the instrument's fluorescent screen. That is, first determine that the instrument's fluorescent screen trajectory curve signal amplitude is within the specific range of the comparison sample's trajectory curve signal amplitude. After determining the range, calculate based on the specific value. If the amplitude of the defect trajectory curve signal of the test block DBSY-1 is Fx and Fy, the defect depth corresponding to the amplitude is Hx and Hy, and the amplitude of the workpiece defect trajectory curve signal is F, then the depth is:

[0031] (2) From the lower fusion line of the fillet weld to the upper fusion line of the butt weld: When the trajectory curve signal amplitude F2 is greater than 30%, the defect position is determined by comparing the trajectory defect signal with the percentage of the defect trajectory curve signal amplitude of the test block DBSY-2 on the full scale of the instrument screen. That is, first determine that the amplitude of the instrument screen trajectory curve signal is within the specific range of the comparison sample trajectory curve signal amplitude. After determining the range, calculate based on the specific value. If the DBSY-2 defect trajectory curve signal amplitude is Fx, Fy, the defect depth corresponding to the amplitude is Hx, Hy, and the workpiece defect trajectory curve signal amplitude is F, then the depth is:

[0032] Compared with the prior art, the present invention has the following beneficial technical effects:

[0033] 1. Through data collation and collection, the distribution and characteristics of crack defects on the inner wall of the drain pipe seat of the pressure-bearing component were determined, providing a theoretical basis for the next step of sample preparation.

[0034] 2. The scanning method was determined, and a comparison specimen with cracks was designed. The detection sensitivity was set based on the comparison specimen. The detection frequency was set based on the eddy current penetration depth formula and the comparison specimen. Parallel line or grid line scanning was performed along the scanning path at the pipe seat fillet weld and from the lower fusion line of the fillet weld to the upper fusion line of the butt weld. The equivalent method and formula calculation method were introduced, and the defect quantification and location were determined based on the amplitude of the trajectory curve signal.

[0035] 3. Solve the problem of non-destructive detection of crack defects on the inner wall of the drain pipe seat of the pressure-bearing component of the in-service thermal power unit, which can effectively solve the occurrence of pipeline leakage accidents caused by the expansion of crack defects. This method is simple, safe and reliable and has a wide range of applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 is a structural diagram of the drain pipe seat.

[0037] Figure 2 is a sectional front view of the drain pipe seat.

[0038] Figure 3 is a position diagram of scanning path 1.

[0039] FIG4 is a position diagram of scanning path 2. FIG4 is a position diagram of scanning path 2.

[0040] Figure 5 is a schematic diagram of the probe.

[0041] Figure 6 is a schematic diagram of the structure of the comparative sample DBSY-1.

[0042] Figure 7 is a schematic diagram of the structure of the comparative sample DBSY-2.

[0043] FIG8 is a schematic diagram of the probe placement position.

[0044] Figure 9 is a physical picture of the drain pipe seat.

[0045] Figure 10 is a defect detection diagram. DETAILED DESCRIPTION

[0046] The specific implementation of the present invention is further described in detail below with reference to the accompanying drawings and examples.

[0047] The stress characteristics and failure analysis results of the high-temperature, pressure-bearing drain pipe socket structure were collected and collated. It was determined that the primary defect in the drain pipe socket is cracks. In the initial stage, cracks are distributed along the axial direction of the inner wall of the socket. In the mid-stage, cracks form a network along the axial and circumferential directions. In the final stage, cracks on the inner wall of the socket develop to the outer surface, leading to leakage. A diagram of the drain pipe socket structure is shown in Figure 1.

[0048] Based on this, the present invention provides a method for detecting inner wall defects of a drain pipe seat of a thermal power plant based on low-frequency eddy current, comprising the following steps:

[0049] Step 1: Determine the scanning path and scanning method

[0050] As shown in Figure 2, the pipe seat and pressure-bearing components are cut open along the steam flow direction to show the front view. The outer wall surface of the drain pipe seat and the outer surface of the drain pipe seat fillet weld are used as the inspection surface. The circumference of the inspection surface along the outer surface area of ​​the drain pipe seat fillet weld is the scanning path 1. The eddy current probe scans along the scanning path 1 in a parallel line manner, and the probe scanning coverage rate is 20% to prevent missed detection, as shown in Figure 3.

[0051] Scanning Path 2 covers the outer surface area from the lower fusion line of the fillet weld to the upper fusion line of the butt weld on the drain pipe socket. Within this area, the eddy current probe scans the outer wall of the pipe socket in a grid pattern along the axial and circumferential directions. This is scan path 2, with a probe coverage rate of 5% to prevent missed inspections, as shown in Figure 4. (Scanning coverage rate is the percentage of the width of the last probe scan that covers the width of the previous probe scan.)

[0052] Step 2: Instrument and probe selection

[0053] The instrument selected is a digital low-frequency eddy current scanner, and the probe is a point-type eddy current probe; the contact point diameter of the point-type probe is 2 mm; the actual picture of the probe is shown in FIG5 .

[0054] Step 3: Selection of comparison samples

[0055] Two comparison test blocks were used, namely test block DBSY-1 and test block DBSY-2, with the following parameters:

[0056] DBSY-1: 100mm in length, t1 in wall thickness, and d1 in diameter. Seven 0.5mm circular holes are machined on the inner wall of the comparison block at depths of 2%, 5%, 10%, 20%, 40%, 60%, and 80% of the wall thickness, respectively.

[0057] DBSY-2: 100 mm in length, t2 in wall thickness, and d2 in diameter. Seven circumferential grooves and seven axial grooves are engraved on the inner wall of the comparison block. The length × width of the circumferential and axial grooves are 5 mm × 0.5 mm, and the depths are 2%, 5%, 10%, 20%, 40%, 60%, and 80% of the wall thickness, respectively. There are 14 grooves in total.

[0058] The wall thickness t of the test block DBSY-1 ranges from 4.0 mm to 12.0 mm, and the diameter d ranges from 32 mm to 89 mm; the wall thickness t of the test block DBSY-2 ranges from 4.0 mm to 12.0 mm, and the diameter d ranges from 32 mm to 89 mm.

[0059] Step 4: Setting the detection frequency and sensitivity

[0060] Formula for penetration depth of eddy current using placed coil on semi-infinite plane conductor (δ-standard penetration depth, in m; f-working frequency, in Hz; σ-conductivity, S / m) Approximately estimate the initial detection frequency when the tube seat thickness is t, and then determine the final detection frequency by using the response of the artificial defects on the comparison sample at the initial detection frequency: ① Determination of detection frequency of scanning path 1: Input the initial detection frequency into the low-frequency eddy current scanner, and use the point eddy current probe to scan the circular hole of the test block DBSY-1 with a depth of 2% of the wall thickness. If the peak of the trajectory curve signal displayed on the instrument screen is sharp and clearly visible, the frequency is the final detection frequency; if the peak of the trajectory curve signal displayed on the instrument screen is not If there is a sharp or unclear peak in the trajectory curve signal, adjust the initial detection frequency until the peak of the trajectory curve signal is sharp and clearly visible. This frequency is the final detection frequency. ② Determine the detection frequency of scanning path 2: Input the initial detection frequency into the low-frequency eddy current scanner, and use a point eddy current probe to scan the circumferential groove of the test block DBSY-2 with a depth of 2% of the wall thickness. If the peak of the trajectory curve signal displayed on the instrument screen is sharp and clearly visible, this frequency is the final detection frequency. If there is no or unclear peak in the trajectory curve signal displayed on the instrument screen, adjust the initial detection frequency until the peak of the trajectory curve signal is sharp and clearly visible. This frequency is the final detection frequency.

[0061] When the probe scans along path 1 using the parallel line method, calibration is performed using test block DBSY-1. The probe is placed on the outer surface of the pipe wall corresponding to seven Φ0.5mm circular holes with depths of 2%, 5%, 10%, 20%, 40%, 60%, and 80% of the wall thickness. Seven trajectory curve signals with peak values ​​are displayed on the instrument's screen, and the trajectory curve signal amplitudes vary. The closer the circular hole is to the outer surface of the pipe wall, the higher the trajectory curve signal amplitude. The trajectory curve signal amplitude F1 corresponding to a depth of 2% of the wall thickness is adjusted to 40% of the full scale of the instrument's screen as the detection sensitivity. The percentage of the trajectory curve signal amplitude F1 at different groove depths relative to the full scale of the instrument's screen is recorded.

[0062] When the probe scans along path 2 using a grid line method, DBSY-2 is used for calibration. The probe is placed on the outer surface of the pipe wall corresponding to 14 circumferential grooves and axial grooves with depths of 2%, 5%, 10%, 20%, 40%, 60%, and 80% of the wall thickness. 14 trajectory curve signals with peak values ​​are displayed on the instrument's fluorescent screen, and the trajectory curve signal amplitudes vary. The closer the circular hole is to the outer surface of the pipe wall, the higher the trajectory curve signal amplitude. The trajectory curve signal amplitude F2 corresponding to a depth of 5% of the wall thickness is adjusted to 40% of the full scale of the instrument's fluorescent screen as the detection sensitivity. The percentage of the trajectory curve signal amplitude F2 of different groove depths on the full scale of the instrument's fluorescent screen is recorded at this time.

[0063] Schematic diagrams of test blocks DBSY-1 and DBSY-2 are shown in Figures 6 and 7 , with wall thickness t ranging from 4.0 mm to 12.0 mm and diameter d ranging from 32 mm to 89 mm.

[0064] Step 5: Scan

[0065] The eddy current probe is placed on the outer surface of the pipe seat, from the lower fusion line of the fillet weld to the upper fusion line of the butt weld. The probe is in stable contact with the outer surface and maintained perpendicular to the specimen surface. Scan along scanning path 1 and scanning path 2 using a parallel line method or a grid line method. The probe should maintain a constant speed of 2.5 mm / s during the test, and the speed fluctuation should not exceed ±5% of the average speed. The probe placement is shown in Figure 8.

[0066] Step 6: Result evaluation

[0067] After scanning, the defects are quantified and located:

[0068] Defect quantification:

[0069] (1) Pipe socket fillet weld: When the trajectory curve signal amplitude F1 exceeds 20% of the full scale of the instrument's fluorescent screen, i.e., F1>20%, it is judged as a dangerous defect, unqualified, and directly scrapped; when the trajectory curve signal amplitude is 10%≤F1≤20%, the defect should be recorded and inspected regularly; when the trajectory curve signal amplitude F1<10%, no record is required;

[0070] (2) From the lower fusion line of the fillet weld to the upper fusion line of the butt weld: When the trajectory curve signal amplitude F2 exceeds 30% of the full scale of the instrument's fluorescent screen, i.e., F2>30%, it is determined to be a dangerous defect, unqualified, and directly scrapped; when the trajectory curve signal amplitude is 15%≤F2≤30%, the defect should be recorded and inspected regularly; when the trajectory curve signal amplitude F2<15%, no record is required;

[0071] Defect location determination:

[0072] (1) Pipe seat fillet weld: When the trajectory curve signal amplitude F1 is greater than 20%, the defect position is determined by comparing the trajectory defect signal with the percentage of the full scale of the instrument's fluorescent screen. That is, first determine that the instrument's fluorescent screen trajectory curve signal amplitude is within the specific range of the comparison sample's trajectory curve signal amplitude. After determining the range, calculate based on the specific value. If the amplitude of the defect trajectory curve signal of the test block DBSY-1 is Fx and Fy, the defect depth corresponding to the amplitude is Hx and Hy, and the amplitude of the workpiece defect trajectory curve signal is F, then the depth is:

[0073] (2) From the lower fusion line of the fillet weld to the upper fusion line of the butt weld: When the trajectory curve signal amplitude F2 is greater than 30%, the defect position is determined by comparing the trajectory defect signal with the percentage of the defect trajectory curve signal amplitude of the test block DBSY-2 on the full scale of the instrument screen. That is, first determine that the amplitude of the instrument screen trajectory curve signal is within the specific range of the comparison sample trajectory curve signal amplitude. After determining the range, calculate based on the specific value. If the DBSY-2 defect trajectory curve signal amplitude is Fx, Fy, the defect depth corresponding to the amplitude is Hx, Hy, and the workpiece defect trajectory curve signal amplitude is F, then the depth is:

[0074] Example:

[0075] This embodiment takes the drain pipe seat in front of the plugging valve of the reheating hot section steam pipeline as an example for testing, with a specification of Φ38mm×7mm and a material of P91.

[0076] 1. Preparation of comparative samples:

[0077] The comparison sample DBSY-1 has a length of 100mm, a wall thickness of 7mm, and a diameter of 38mm. A total of 7 Φ0.5mm circular hole defects with depths of 0.14mm, 0.35mm, 0.7mm, 1.4mm, 2.8mm, 4.2mm, and 5.6mm are machined in the comparison test block. The comparison sample DBSY-2 has a length of 100mm, a wall thickness of 7mm, and a diameter of 38mm. Seven circumferential grooves and 7 axial grooves are machined in the comparison test block. The circumferential grooves are: 5mm long × 0.5mm wide × 0.14mm deep, 5mm long × 0.5mm wide × 0.35mm deep, 5mm long × 0.5mm wide × 0.7mm deep, 5mm long × 0.5mm wide × 1.4mm deep, 5mm long × 0.5mm wide × 2.8mm deep, and 5mm long × 0.5mm wide. m×depth 4.2mm, length 5mm×width 0.5mm×depth 5.6mm; axial groove: length 5mm×width 0.5mm×depth 0.14mm, length 5mm×width 0.5mm×depth 0.35mm, length 5mm×width 0.5mm×depth 0.7mm, length 5mm×width 0.5mm×depth 1.4mm, length 5mm×width 0.5mm×depth 2.8mm, length 5mm×width 0.5mm×depth 4.2mm, length 5mm×width 0.5mm×depth 5.6mm.

[0078] 2. Detection frequency setting:

[0079] δ=t=7mm,σ=9.93×106S / m,using the formula The test frequency was calculated to be f = 5 Hz. This frequency was then used to conduct preliminary tests on comparison samples DBSY-1 and DBSY-2, accurately detecting a defect at 0.14 mm. Finally, f = 4 Hz was determined for testing.

[0080] 3. Scan the comparison samples DBSY-1 and DBSY-2.

[0081] The low-frequency eddy current scanner was adjusted to 5Hz for testing. Using a point-type eddy current probe, the detection sensitivity of a 0.16mm deep defect on the comparison sample DBSY-1 was adjusted to 20% of the full scale. The detection sensitivity of a 0.16mm deep defect on the comparison sample DBSY-2 was adjusted to 30% of the full scale. The test results are shown in the table below:

[0082] 4. Scan the workpiece with a size of Φ38mm×7mm

[0083] Using the frequency and scanning sensitivity set on the comparison sample, the pipe seat fillet weld of the workpiece under test was scanned in a parallel line manner, and the area from the lower fusion line of the fillet weld to the upper fusion line of the butt weld was scanned in a grid line manner. The actual image and defect detection diagram are detailed in Figures 9 and 10. Defect location of the pipe seat fillet weld: Axial defects from the lower fusion line of the fillet weld to the upper fusion line of the butt weld:

[0084] The test results are shown in the table below:

[0085] The test results show that the low-frequency eddy current testing method can effectively detect workpiece defects, and accurately calculate the defect depth with small error through the introduced equivalent method and formula method.

Claims

1. A method for detecting inner wall defects of drain pipe sockets in thermal power plants based on low-frequency eddy currents, characterized in that: The following steps are involved: Step 1: Determine the scanning path and scanning method The outer wall surface of the drain pipe seat and the outer surface of the drain pipe seat fillet weld are used as the detection surface, and the circumference of the detection surface along the outer surface area of ​​the drain pipe seat fillet weld is the scanning path 1; the scanning method of the eddy current probe along the scanning path 1 is parallel line scanning; The inspection surface of the outer surface area from the lower fusion line of the fillet weld of the drain pipe seat to the upper fusion line of the butt weld is the scanning area of ​​scanning path 2. In the scanning area of ​​scanning path 2, the eddy current probe scans along the outer wall surface of the pipe seat in the axial and circumferential directions in a grid line manner as scanning path 2; Step 2: Instrument and probe selection The instrument selected is a digital low-frequency eddy current scanner, and the probe is a point eddy current probe; Step 3: Selection of comparison samples Two comparison test blocks were used, namely test block DBSY-1 and test block DBSY-2, with the following parameters: DBSY-1: 100mm in length, t1 in wall thickness, and d1 in diameter. Seven 0.5mm circular holes are machined on the inner wall of the comparison block at depths of 2%, 5%, 10%, 20%, 40%, 60%, and 80% of the wall thickness, respectively. DBSY-2: 100 mm in length, t2 in wall thickness, and d2 in diameter. Seven circumferential grooves and seven axial grooves are engraved on the inner wall of the comparison block. The length × width of the circumferential and axial grooves are 5 mm × 0.5 mm, and the depths are 2%, 5%, 10%, 20%, 40%, 60%, and 80% of the wall thickness, respectively. There are 14 grooves in total. Step 4: Setting the detection frequency and sensitivity Formula for penetration depth of eddy current using placed coil on semi-infinite plane conductor The initial inspection frequency when the tube seat thickness is t is approximately estimated, and then the final inspection frequency is determined by using the response of the artificial defects on the comparison sample at the initial inspection frequency; When the probe scans along path 1 using the parallel line method, calibration is performed using test block DBSY-1. The probe is placed on the outer surface of the pipe wall corresponding to seven Φ0.5mm circular holes with depths of 2%, 5%, 10%, 20%, 40%, 60%, and 80% of the wall thickness. Seven trajectory curve signals with peak values ​​are displayed on the instrument's screen, and the trajectory curve signal amplitudes vary. The closer the circular hole is to the outer surface of the pipe wall, the higher the trajectory curve signal amplitude. The trajectory curve signal amplitude F1 corresponding to a depth of 2% of the wall thickness is adjusted to 40% of the full scale of the instrument's screen as the detection sensitivity. The percentage of the trajectory curve signal amplitude F1 at different groove depths relative to the full scale of the instrument's screen is recorded. When the probe scans along path 2 using a grid pattern, DBSY-2 is used for calibration. The probe is placed on the outer surface of the pipe wall corresponding to 14 circumferential and axial grooves with depths of 2%, 5%, 10%, 20%, 40%, 60%, and 80% of the wall thickness. Fourteen trajectory curve signals with peak values ​​are displayed on the instrument's screen. The amplitudes of the trajectory curve signals vary, with the closer the circular hole is to the outer surface of the pipe wall, the higher the amplitude. The amplitude F2 of the trajectory curve signal corresponding to a depth of 5% of the wall thickness is adjusted to 40% of the full scale of the instrument's screen as the detection sensitivity. The percentage of the amplitude F2 of the trajectory curve signal at different groove depths relative to the full scale of the instrument's screen is recorded. Step 5: Scan The eddy current probe is placed on the outer surface of the area between the lower fusion line of the fillet weld of the pipe seat and the upper fusion line of the butt weld. The probe is in stable contact with the outer surface and kept perpendicular to the surface of the sample. Scan along scanning path 1 and scanning path 2 using the parallel line method or the grid line method. Step 6: Result evaluation After scanning, the defects are quantified and located: Defect quantification: (1) Pipe socket fillet weld: When the trajectory curve signal amplitude F1 exceeds 20% of the full scale of the instrument's fluorescent screen, i.e., F1>20%, it is judged as a dangerous defect, unqualified, and directly scrapped; when the trajectory curve signal amplitude is 10%≤F1≤20%, the defect should be recorded and inspected regularly; when the trajectory curve signal amplitude F1<10%, no record is required; (2) From the lower fusion line of the fillet weld to the upper fusion line of the butt weld: When the trajectory curve signal amplitude F2 exceeds 30% of the full scale of the instrument's fluorescent screen, i.e., F2>30%, it is determined to be a dangerous defect, unqualified, and directly scrapped; when the trajectory curve signal amplitude is 15%≤F2≤30%, the defect should be recorded and inspected regularly; when the trajectory curve signal amplitude F2<15%, no record is required; Defect location determination: (1) Pipe seat fillet weld: When the trajectory curve signal amplitude F1 is greater than 20%, the defect position is determined by comparing the trajectory defect signal with the percentage of the full scale of the instrument's fluorescent screen. That is, first determine that the instrument's fluorescent screen trajectory curve signal amplitude is within the specific range of the comparison sample's trajectory curve signal amplitude. After determining the range, calculate based on the specific value. If the amplitude of the defect trajectory curve signal of the test block DBSY-1 is Fx and Fy, the defect depth corresponding to the amplitude is Hx and Hy, and the amplitude of the workpiece defect trajectory curve signal is F, then the depth is: (2) From the lower fusion line of the fillet weld to the upper fusion line of the butt weld: When the trajectory curve signal amplitude F2 is greater than 30%, the defect position is determined by comparing the trajectory defect signal with the percentage of the defect trajectory curve signal amplitude of the test block DBSY-2 on the full scale of the instrument screen. That is, first determine that the amplitude of the instrument screen trajectory curve signal is within the specific range of the comparison sample trajectory curve signal amplitude. After determining the range, calculate based on the specific value. If the DBSY-2 defect trajectory curve signal amplitude is Fx, Fy, the defect depth corresponding to the amplitude is Hx, Hy, and the workpiece defect trajectory curve signal amplitude is F, then the depth is:

2. The method for detecting inner wall defects of drain pipe sockets in thermal power plants based on low-frequency eddy current according to claim 1, characterized in that: The contact point diameter of the point probe is 2 mm; the actual picture of the probe is shown in FIG5 .

3. The method for detecting inner wall defects of drain pipe sockets in thermal power plants based on low-frequency eddy current according to claim 1, characterized in that: The wall thickness t of the test block DBSY-1 ranges from 4.0 mm to 12.0 mm, and the diameter d ranges from 32 mm to 89 mm; the wall thickness t of the test block DBSY-2 ranges from 4.0 mm to 12.0 mm, and the diameter d ranges from 32 mm to 89 mm.

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