JP2010261836A - Method for inspection of magnetic material pipe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a defect inspection method of a magnetic material pipe for accurately detecting a defect of the magnetic material pipe, and the defect of the magnetic material pipe in a baffle portion provided outside the magnetic material pipe. <P>SOLUTION: The defect inspection method of the magnetic material pipe uses a probe. In the probe, permanent magnets are mounted around a central part of a columnar yoke so as to have the magnetization direction in the axial direction of the yoke. The permanent magnets are mounted around the yoke on both sides so as to have the magnetization direction in the radial direction of the yoke and cause a magnetic pole on the yoke side to be different. A detection coil is disposed on the permanent magnet in the central part. A flaw is inspected within the magnetic material pipe by an eddy current. Additionally, the flaw is inspected by a multiple frequency method. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for accurately inspecting a defect of a magnetic tube.

One of the inspection methods for metal materials is the eddy current flaw detection method. In the inspection of non-magnetic pipes such as austenitic stainless steel, titanium, and copper alloy, eddy current flaw detection using an interpolated eddy current flaw detection probe is widely performed.
In the inspection of magnetic tubes such as carbon steel, ferritic stainless steel, and dual-phase stainless steel consisting of two phases of ferrite phase and austenite phase, eddy currents should flow only on the surface with eddy current flaw detection probes for non-magnetic tubes. Since the noise caused by local fluctuations in the magnetic permeability adversely affects the detectability, it is impossible to detect defects with high accuracy.

As a probe for eddy current flaw detection of a duplex stainless steel heat transfer tube, a detection coil is arranged around the central part of a cylindrical yoke, the magnetization direction is the yoke radial direction around the yokes on both sides thereof, and the magnetic pole on the yoke side There are known probes equipped with permanent magnets so that they are different from each other (for example, see Non-Patent Document 1).
By using this probe, eddy current flaw detection of weak magnetic pipes such as duplex stainless steel pipes is possible, but it is not sufficient for flaw detection of small defects in ferromagnetic pipes such as carbon steel. Therefore, there is a demand for a method capable of inspecting even a small defect with higher accuracy.

Nondestructive Inspection Vol.42, No.9, 520-526, 1993

  The present invention provides a method for accurately inspecting a defect in a magnetic tube, and further, a magnetic tube in a portion of a baffle plate, a tube plate, a support, etc. (hereinafter simply referred to as a baffle) provided outside the magnetic tube. It is an object of the present invention to provide a method for accurately inspecting defects of the above.

  As a result of intensive investigations on the eddy current flaw detection method for the magnetic tube, the present inventor has arranged a detection coil around the central portion of the cylindrical yoke, and the magnetization direction around the yokes on both sides is the radial direction of the yoke. Using a probe with a permanent magnet attached at the center of the probe with a permanent magnet attached so that the magnetic poles on the yoke side are different, the eddy current flaw detection is performed inside the magnetic tube. It is found that the defect of the magnetic pipe can be inspected with high accuracy, and further, the defect of the magnetic pipe in the baffle portion provided outside can be inspected with high accuracy by conducting the eddy current flaw detection by the multi-frequency method. Invented.

  That is, the present invention is (1) mounting a permanent magnet around the central part of a cylindrical yoke so that the magnetization direction is the axial direction of the yoke, and placing the permanent magnet around the yokes on both sides, Eddy current flaw detection in a magnetic tube using a probe that is mounted so that the yoke side magnetic poles are different and the detection coil is arranged on the permanent magnet in the center. A feature of the defect inspection method for a magnetic tube, and (2) a defect inspection method for a magnetic tube according to (1) above, wherein eddy current flaw detection is performed by a multi-frequency method.

  According to the method of the present invention, defects in the magnetic tube can be inspected with high accuracy, and further defects in the magnetic tube in the baffle portion provided outside the magnetic tube can be inspected with high accuracy.

It is a cross-sectional schematic diagram which shows one embodiment of the probe of this invention. It is a schematic diagram of the permanent magnet attachment method of the probe of this invention. It is a schematic diagram of the permanent magnet attachment method of the probe of this invention. It is a schematic diagram of the permanent magnet attachment method of the probe of this invention. It is a circuit diagram of the probe of the present invention. It is a figure for demonstrating the multi-frequency method. It is a figure which shows the result of Example 1. It is a figure which shows the other result of Example 1. It is a figure which shows the result of Example 3. It is a figure which shows the other result of Example 3. It is a figure which shows the other result of Example 3. It is a figure which shows the other result of Example 3.

Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic sectional view showing an embodiment of the probe of the present invention.
A permanent magnet 2 is mounted around the central portion of the columnar yoke 1 so that the magnetization direction is the axial direction of the yoke. In the figure, it is mounted so that the N pole is on the left side and the S pole is on the right side.
Permanent magnets 3 and 4 are mounted around the yokes on both sides of the permanent magnet 2 such that the magnetization direction is the radial direction of the yoke and the magnetic poles on the yoke side are different between the permanent magnet 3 and the permanent magnet 4. Yes. In the figure, the permanent magnet 3 is mounted so that the yoke side is the south pole and the outside is the north pole, and the permanent magnet 4 is mounted so that the yoke side is the north pole and the outside is the south pole.
A detection coil 5 is disposed on the permanent magnet 2 in the center. Inner excitation coils 6 are arranged on both sides thereof.
Guides 7 and 8 are provided at both ends of the probe. An air introduction hole 9 is provided at a substantially central portion of the yoke 1, and a plurality of air injection holes 10 extending in the radial direction from the air introduction hole are provided at both ends.
Note that the coil conductors and their extraction holes are not shown.

As the yoke, a high permeability metal such as carbon steel or low alloy steel is used.
As the permanent magnet, for example, a high-performance permanent magnet such as a neodymium magnet is used. As the permanent magnet 2 attached to the center portion, a ring-shaped one having a length in the axial direction of the yoke of about 5 to 10 mm is used. As the permanent magnet 3 and the permanent magnet 4 mounted on both sides of the permanent magnet 2, a ring-shaped one having an axial length of about 5 to 30 mm, preferably about 10 to 30 mm is used. The longer the permanent magnet 3 and the permanent magnet 4, the better the flaw detection accuracy, but even if it exceeds about 30 mm, an effect commensurate with it cannot be obtained. The size and thickness of the permanent magnet 3 and the permanent magnet 4 in the radial direction of the yoke are changed in accordance with the size of the magnetic substance tube to be detected.

  Since a ring-shaped permanent magnet having a magnetization in the radial direction is expensive to manufacture, it is usually used by being mounted around a cylindrical yoke in a divided shape such as a quadrant as shown in FIG. When the quarter-turn divided permanent magnet is mounted on the cylindrical yoke, as shown in FIG. 2-B, a permanent magnet having a magnetization direction opposite to that of the dummy iron piece or the mounted permanent magnet is sandwiched. Since the repulsive force of the permanent magnet is eliminated, it can be easily mounted. A method for attaching the permanent magnet to the columnar yoke is not particularly limited, but the permanent magnet is attached using an adhesive, for example, an acrylic adhesive. After mounting the pair of permanent magnets, remove the permanent magnets whose magnetization direction is opposite to that of the dummy iron pieces or the mounted permanent magnets, and apply another adhesive to the contact surface with the cylindrical yoke at this position. The permanent magnet can be easily attached to the cylindrical yoke by inserting the magnet and holding and holding the facing permanent magnet with a vise or the like until the adhesive exhibits an adhesive effect. Alternatively, as shown in FIG. 2C, permanent magnets having magnetization in the radial direction are arranged around the cylindrical yoke via an adhesive, and the magnetization directions are reversed so as to cover the permanent magnets. When the dummy permanent magnets are arranged across adjacent permanent magnets, the repulsive force is reduced and the permanent magnets can be easily bonded to the cylindrical yoke without being fixed using a vise or the like. . The dummy permanent magnet may be removed after the adhesive exhibits an effect.

As the two detection coils 5 and the two inner excitation coils 6, for example, a copper wire having a wire diameter of about 0.05 to 0.1 mmφ, a width of about 0.8 to 1.2 mm, and a depth is used. Is about 0.8 to 1.2 mm and is formed by winding about 60 to 80 times.
The inner excitation coil is preferably provided because it suppresses the eddy current conduction range only in the vicinity of the defect, improves the S / N ratio of a minute defect, and reduces the influence in the vicinity of the tube end.

  The guides 7 and 8 at both ends of the probe are formed of acetal resin, stainless steel or the like, and are attached to the yoke by a screw structure.

Air is introduced from an air introduction hole 9 provided in the yoke 1, and air is ejected from an air ejection hole 10. In the flaw detection of a magnetic tube, it is difficult to scan and center the probe by sticking to the inner surface of the tube with a strong permanent magnet attached to the probe, but when air is injected vertically from the air injection hole, Can be reduced, and the probe can be easily scanned.
For example, the air injection hole 10 has a hole diameter of about 2 mmφ and is provided in the circumferential direction from the air introduction hole 9 with about 6 to 10 holes.

The coil is connected to the eddy current flaw detector by a conducting wire, and the time-voltage characteristics are determined to inspect the defect.
FIG. 3 shows a circuit diagram of the probe. Two detection coils L1, L2 and two inner excitation coils L3, L4 and four variable resistors R1, R2, R3, R4 are connected in parallel to the lock-in amplifier, and the detection coils L1, L2 The variable resistors R1 and R2 are connected to the input signal terminals of the lock-in amplifier so as to form a Whiston bridge circuit.

The flaw detection is performed as follows.
The impedance of the detection coil and the inner excitation coil is measured at a predetermined test frequency, for example, 100 kHz, which has a high flaw detection sensitivity in actual flaw detection, and an applied voltage of 5 v, and the resistance values of the variable resistors R1 and R2 are measured by the measured resistance. Adjust to the value. In addition, the combined impedance of the detection coil and the variable resistor at this time is measured, and the resistance values of the variable resistors R3 and R4 connected to the inner excitation coil are changed before and after the measured resistance value to detect flaws. In addition, flaw detection is performed under conditions with good detection sensitivity.
The flaw detection speed by the probe is about 2 to 50 mm / second, and about 2 to 10 mm / second is preferable in order to detect a smaller defect with high accuracy.

By the above method, it is possible to accurately detect defects in the magnetic tube where the baffle is not provided outside the magnetic tube, but the baffle signal interferes with the portion where the baffle is provided. Thus, defects in the magnetic tube cannot be detected with high accuracy.
As a method for removing such disturbing signals and improving the accuracy of eddy current flaw detection, a multi-frequency method is known in which eddy current flaw detection is performed by adding two or more frequencies to a detection coil. The multi-frequency method is described in, for example, “Non-Destructive Inspection Series Eddy Current Test II” (October 5, 2002, 6th printing, published by Japan Nondestructive Inspection Association).

Hereinafter, the inspection of the defect of the magnetic tube in the portion where the baffle is provided by the multi-frequency method using two frequencies will be described. FIG. 4 is a diagram for explaining this.
When flaw detection is performed on a magnetic tube having no defect in a portion where the baffle is provided at a frequency of f ₁ , a baffle signal (Lissajous waveform) as shown in (A) is obtained (f ₁ processing). When flaw detection by the frequency f _2, the baffle signal as shown in (B) is obtained (f ₂ processing). Rotate the amplitude X, Y and phase θ of the baffle signal obtained at the frequency f ₂ and adjust it so that the baffle signal obtained at the frequency f ₁ has the same amplitude and inclination as possible. (C). Since the signal obtained by adjusting the signal obtained at the frequency f ₂ is obtained, this processing is defined as f ₂ ′.
f ₁ process _'process taking the difference of the processing of the signal (f ₁ -f _2' signal and f ₂ of when the) signals baffle for that is a similar Lissajous waveform disappears is offset (D) .

When flaw detection is performed on a magnetic tube having a defect in a portion where the baffle is provided at a frequency of f _1, a signal in which the baffle and the defect are combined as shown in (E) is obtained (f ₁ processing).
Here, the processing (f ₁ −f ₂ ′) is performed to remove the baffle signal in the same manner as described above, and only the defect signal from which the baffle signal has been removed is taken out (F) and compared with (D). Inspect.

If the frequencies f ₁ and f ₂ are in the same band, the baffle signal and the defect signal are similar to each other. Therefore, if the frequency is calculated, most of the defect signal may be lost by the calculation. On the other hand, if the frequencies f ₁ and f ₂ are set to completely different bands, the basic waveform shape changes, so that the baffle signals of the f ₁ process and the f ₂ ′ process have similar shapes no matter how much adjustment is performed. As a result, the baffle signal cannot be erased. Frequency _{f 2} is typically about 1 / 2-1 / 8 of the frequency _{f 1.} The frequency varies depending on the material of the magnetic tube, and an optimum frequency is selected by scanning.

In the eddy current flaw detection and calculation processing, the frequencies f ₁ and f ₂ are simultaneously applied to the detection coil, a circuit is formed so as to perform the calculation processing, and the defect can be inspected simultaneously with scanning the probe.
Alternatively, the frequencies f ₁ and f ₂ can be separately applied to the detection coil for scanning, and then the obtained signal can be processed to detect defects.

In addition, in order to eliminate a signal other than the baffle, such as a backlash signal of the probe (a signal generated by changing the distance between the tube and the detection coil due to rattling of the probe in the tube) is large. In some cases, the third frequency is used.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated, this invention is not limited to a following example.

Example 1
A probe similar to that shown in FIG. 1 was prepared. The materials and shapes used are shown below.
Yoke 1: Carbon steel S15C annealing material Permanent magnet 2: Neodymium magnet (manufactured by Asahi Corporation)
Ring-shaped permanent magnets 3 and 4: outer diameter φ25.5 mm × inner diameter φ21 mm × length 6.4 mm: Neodymium magnet (manufactured by Asahi Corporation)
Ring shape with outer diameter φ28mm x inner diameter φ21mm x length 30mm divided into four parts.
Detection coil 5, inner excitation coil 6:
Each coil uses a copper wire with a wire diameter of φ0.08mm, and the dimensions are 1.0mm width x
The depth was 1.0 mm, the number of turns was 70, and the coil interval was 0.8 mm.
Guides 7 and 8: Polyacetal (copolymer) Duracon (registered trademark) (manufactured by Polyplastic Co., Ltd.), outer diameter φ28.4 mm
Adhesive for bonding the permanent magnet to the yoke:
Acrylic adhesive hard rock (registered trademark) (manufactured by Electrochemical Industry Co., Ltd.)
The lock-in amplifier is LI5640 (manufactured by NF Circuit Design Block Co., Ltd.), the oscilloscope is TDS3104B (manufactured by Nippon Tektronix Co., Ltd.), and the stage controller that scans the probe is CAT-E (Chuo Seiki Co., Ltd.). Made).

  A circuit diagram of the probe was produced as shown in FIG. Since the impedance of the detection coil and the inner excitation coil when the test frequency was 100 kHz and the applied voltage was 5 v was measured to be about 93Ω, the resistance values of the variable resistors R1 and R2 were adjusted to about 93Ω. Further, when the combined impedance of the detection coil and the variable resistor at this time was measured, it was about 172Ω. Under these conditions, flaw detection was performed by changing the resistance values of the variable resistors R3 and R4 connected to the inner excitation coil to 150 to 500Ω.

For a ferromagnetic tube (carbon steel STB340, outer diameter φ34 mm × thickness 2.3 mm × length 900 mm) provided with through holes of φ2.0 mm, φ1.5 mm, φ1.0 mm, φ0.5 mm at 100 mm intervals, Scanning was performed by scanning at a speed of 30 mm / second and 4 mm / second.
FIG. 5 shows the results when the flaw detection speed is 30 mm / second, and FIG. 6 shows the results when flaw detection is performed on through holes with φ1.0 mm and φ0.5 mm at the flaw detection speed of 4 mm / second.
In the figure, “without inner excitation (2 coils)” indicates that the flaw detection was performed only with the detection coil without using the inner excitation coil, and 150... 500Ω represents the resistance value of the variable resistors R3 and R4.

  For a minute defect of φ2 mm or less, by using a probe provided with an inner excitation coil, the S / N ratio is clearly improved as compared with the case of using a probe having no inner excitation coil and only a detection coil. Also, when the flaw detection speed is about 4 mm / sec, the S / N ratio is poor without the inner excitation coil, and both φ1 mm and φ0.5 mm through-holes cannot be detected. Detection is possible with sufficient sensitivity.

Example 2
A φ4 mm air introduction hole is provided inside the yoke of the probe used in Example 1 in the axial direction, and eight φ2 mm air injection holes each extending radially from the air introduction hole are provided on both sides (guide portions) of the permanent magnet. It was.
A spring balance is attached to the end of the probe, inserted into the same ferromagnetic tube as in Example 1, and air whose pressure is adjusted by a regulator is supplied to the air introduction hole so as to be constant while being ejected from the air injection hole. The tensile force immediately before the probe started to move was measured. Measurement was performed 5 times under the same conditions, and an average value was obtained.
The results are shown in Table 1. By increasing the air pressure and ejecting more air, the tensile force is reduced and scanning is facilitated.

Example 3
A ferromagnetic tube (carbon steel STB340, outer diameter φ34 mm × thickness 2.3 mm × length 900 mm), a through hole having a diameter of 1 mmφ as a simulated defect, an outer surface having a width of 5 mm × length of 12.5 mm and a depth of thickness A rectangular groove having a width of 25%, a rectangular groove having a width of 5 mm × length of 17.5 mm and a depth of 50% of the thickness, an inner circumferential groove having a width of 1.5 mm and a depth of 20% of the thickness, Inner circumferential groove with a width of 1.5 mm and a depth of 70% of the thickness, outer circumferential groove with a width of 1.5 mm and a depth of 50%, a width of 1.5 mm and a depth of 80 % Outer circumferential groove.
A ferromagnetic tube was inserted into a hole of a simulated baffle (carbon steel SS400, length 100 mm × width 100 mm × thickness 15 mm, diameter 34.4 mmφ hole in the center), and the same probe and eddy current flaw detector as in Example 1 were used for simulation. The position of the baffle was changed to detect eddy currents, and the defect was inspected.

The flaw detection speed was about 4 mm / second, flaw detection was first performed at a frequency f ₁ of 20 kHz (f ₁ treatment), and then flaw detection was performed at a frequency f ₂ of 10 kHz (f ₂ treatment).
The obtained data was processed. That is, the amplitudes X and Y and the phase θ of the baffle signal obtained by the f ₂ process are rotated, and the baffle signal obtained by the f ₁ process is processed so as to have a signal having the same amplitude and inclination as possible ( f ₂ 'treatment).
A process (f ₁ −f ₂ ′) that takes the difference between the signal of the f ₁ process and the signal of the f ₂ ′ process was performed, and the defect was inspected. The obtained Lissajous waveforms are shown in FIGS. (1) in FIG. 7 is a defect-free region, (2) is a through hole having a diameter of 1 mmφ, (3) in FIG. 8 is a rectangular groove having a width of 5 mm × length of 12.5 mm and a depth of 25% of the thickness, (4) is a square groove having a width of 5 mm × length of 17.5 mm and a depth of 50% of the thickness. FIG. 9 (5) is an inner circumferential groove having a width of 1.5 mm and a depth of 20% of the thickness. (6) is an inner circumferential groove having a width of 1.5 mm and a depth of 70%, and FIG. 10 (7) is an outer circumferential groove having a width of 1.5 mm and a depth of 50%. (8) shows an outer circumferential groove having a width of 1.5 mm and a depth of 80% of the thickness.
From the comparison of the Lissajous waveforms of (1) and (2) to (8), it can be seen that a defect in the magnetic tube in the baffle portion provided outside is detected.

DESCRIPTION OF SYMBOLS 1 Yoke 2 Permanent magnet 3 Permanent magnet 4 Permanent magnet 5 Detection coil 6 Inner excitation coil 7 Guide 8 Guide 9 Air introduction hole 10 Air ejection hole

Claims (4)

  A permanent magnet is mounted around the center of the cylindrical yoke so that the magnetization direction is the axial direction of the yoke, the permanent magnet is mounted around the yokes on both sides, and the magnetization direction is the radial direction of the yoke. A defect in a magnetic tube characterized by vortex flaw detection in a magnetic tube using a probe that is mounted so that the magnetic poles on the yoke side are different, and a detection coil is arranged on the permanent magnet in the center. Inspection method.   2. The defect inspection method for a magnetic tube according to claim 1, wherein a probe is used in which inner excitation coils are arranged on both sides of the detection coil.   2. A probe comprising an air introduction hole in the axial direction of the yoke and a plurality of air injection holes extending radially from the air introduction hole on both sides of the permanent magnets on both sides. Inspection method for defects in magnetic tube.   The defect inspection method for a magnetic tube according to any one of claims 1 to 3, wherein eddy current testing is performed by a multi-frequency method.

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