JP2012189326A - Interpolation probe for eddy current flaw detection for ferromagnetic steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an interpolation probe for eddy current flaw detection which detects a flaw of a steel pipe caused by an eddy current by magnetizing a ferromagnetic steel pipe in a circumferential direction, and in which noise variation is reduced.SOLUTION: An interpolation probe 1 for eddy current flaw detection comprises: a rectangular parallelepiped permanent magnet 10 of which the cross section provided in the center is rectangular and which is magnetized in a counter side direction; a yoke 20 of which the cross section is arc-shaped coupled to both magnetic pole planes of the rectangular parallelepiped permanent magnet; and a coil holder of which the cross section is arc-shaped coupled to different planes from both the magnetic pole planes. In the interpolation probe for eddy current flaw detection, a void 30 is provided in a portion in contact with each of both the magnetic pole planes of the permanent magnet, in the yoke near the center in the length direction of the interpolation probe. Two inspection coils 40a and 40b are wound around linearly in the void and along the arc shape in the coil holder, thereby obtaining differential output of the two inspection coils.

Description

The present invention relates to an insertion probe for measuring the degree of damage inside a ferromagnetic heat transfer tube used in a chemical plant or the like by eddy current flaw detection.

Since heat transfer tubes used in chemical plants and the like are difficult to detect flaws from the outside, they usually detect internal and external flaws using an insertion probe. As an insertion probe, there are an ultrasonic flaw detection method and an eddy current flaw detection method. However, the ultrasonic flaw detection method needs to fill a flaw detection portion with water or oil as a contact medium, and has high accuracy but takes extra man-hours. The eddy current flaw detection method does not require a contact medium and can perform flaw detection efficiently.

On the other hand, the eddy current flaw detection method is affected by the magnetic permeability when the test material is a ferromagnetic material. There is no problem when the heat transfer tube is made of a non-magnetic material. However, when the heat transfer tube is made of a ferromagnetic material, if there is a partial variation in the magnetic permeability, noise is generated and the accuracy of eddy current flaws is lowered. Conventionally, an eddy current flaw detection probe 5 shown in FIG. 5 has been used as a solution to this problem.

The eddy current flaw detection probe 5 has cylindrical permanent magnets 10 fitted to both ends of a columnar yoke 20, and an inspection coil 45 via an insulating material 35 at the center of the yoke 20. Is attached. The direction of magnetization of the cylindrical permanent magnet 10 is perpendicular to the axis of the yoke 20. The permanent magnet 10 at one end has an N pole on the outer periphery, an S pole on the inner periphery, and the permanent magnet 10 on the other end. The outer circumference is S pole and the inner circumference is N pole, and a magnetic path as shown by a one-dot chain line in the figure is formed, and the ferromagnetic steel tube T is saturated and magnetized in the tube axis direction. Thereby, the change of the magnetic permeability by the residual distortion of the ferromagnetic steel tube T and the local dispersion | variation in material is suppressed, and the influence which it has on the test | inspection coil 45 is eliminated.

However, even with the above method, there is a problem that when the thickness of the steel pipe is increased, the ferromagnetic steel pipe T cannot be saturated and magnetized, the magnetic permeability varies, and the accuracy of eddy current flaw detection is lowered. As a countermeasure against this, in Patent Document 1, as shown in FIG. 6, a permanent magnet 10 having a fan-shaped cross section is joined to the outer periphery of a columnar yoke 20 to saturate and magnetize the ferromagnetic steel tube T in the circumferential direction. Further, an invention is disclosed in which a cylindrical inspection coil 48 is disposed in a portion of the yoke 20 that is not coupled to the permanent magnet 10 in order to further increase the circumferential resolution.

JP-A-63-24152

In the invention of Patent Document 1, the magnetic field strength in the circumferential direction is stronger than that in the conventional method. However, a cylindrical inspection coil 48 is arranged through an insulating material 35 on the portion of the yoke 20 that is not coupled to the permanent magnet 10. Therefore, when the distance between the inspection coil 48 and the inner surface of the ferromagnetic steel tube T is changed (lifted off), the detection signal is changed and the accuracy of eddy current flaws is lowered.

As a result of earnest research to solve the above problems, the ferromagnetic steel pipe is saturated and magnetized in the circumferential direction with a permanent magnet and a yoke, and the two test coils are separated from the ferromagnetic steel pipe at the part where the yoke is located. In the part without the yoke, it is wound in the circumferential direction so as to approach the ferromagnetic steel pipe, and furthermore, by taking the differential output of the two inspection coils, it is possible to cope with fluctuations in lift-off and to provide an insertion probe with high detection accuracy. Heading The present invention has been completed.

In order to solve the above-mentioned problem, an eddy current flaw detection insertion probe according to claim 1 of the present invention is a rectangular parallelepiped permanent magnet having a rectangular cross section provided in the center and magnetized in the opposite direction, and the rectangular parallelepiped shape. An eddy current flaw detection insertion probe in which a yoke having a bow shape in cross section on both magnetic pole surfaces of a permanent magnet and a coil holder having a bow shape in cross section on a different surface from both magnetic pole surfaces, A gap is provided in a portion of the yoke in contact with the both magnetic pole surfaces of the yoke near the center in the length direction of the insertion probe, and two inspection coils are formed in a straight line in the gap and in a bow shape in the coil holder. An eddy current flaw detection probe characterized in that it is wound along and a differential output of the two inspection coils is obtained.

According to the present invention, the eddy current flaw detection insertion probe has a rectangular cross section provided at the center thereof, and a cross section is provided on both magnetic pole surfaces of the rectangular parallelepiped permanent magnet magnetized in the opposite direction and the rectangular parallelepiped permanent magnet. Because the shape of the yoke is combined with a coil holder having a bow-shaped cross section on a surface different from both magnetic pole faces, a magnetic path with high magnetic flux density in the circumferential direction of the ferromagnetic steel pipe when inserted into the ferromagnetic steel pipe Is formed. In addition, a through-hole is provided in the portion in contact with both magnetic pole faces of the yoke permanent magnet near the center in the length direction of the eddy current flaw detection insertion probe, and the two inspection coils are linear so that they are separated from the ferromagnetic steel tube in the gap. Since the coil holder is wound so as to approach the ferromagnetic steel pipe along the bow shape, it is possible to detect eddy current from a portion of the ferromagnetic steel pipe having a high magnetic flux density. For this reason, detection of eddy current is mainly performed from a portion where variation in magnetic permeability is reduced, and noise is reduced. Further, by obtaining the differential output of the two inspection coils, the problem of signal fluctuation due to lift-off can be solved even when the center of the probe is deviated from the center of the ferromagnetic steel tube.

The eddy current flaw detection probe according to claim 2, wherein the coil holder is nonmagnetic and nonconductive.

According to such an invention, since the coil holder is non-magnetic and non-conductive, there is no eddy current loss from the AC voltage of the coil.

The invention according to claim 3 is characterized in that a cross section of the eddy current flaw detection insertion probe in which the permanent magnet, the yoke, and the coil holder are combined is substantially circular. It is an insertion probe for eddy current flaws described.

According to such an invention, since the cross section of the eddy current flaw detection insertion probe in which the permanent magnet, the yoke, and the coil holder are combined is substantially circular, the inner surface of the ferromagnetic steel tube when inserted into the ferromagnetic steel tube The part where the distance is constant increases and the output improves.

According to a fourth aspect of the present invention, a groove is formed in the coil holder, and the two inspection coils are wound around the groove. This is an eddy current flaw detection probe.

According to such an invention, since the two inspection coils are wound along the groove of the coil holder, the coil does not directly contact the inner surface of the ferromagnetic steel tube, and the coil is deformed by cutting or contact. Decreases, durability is improved, and noise due to contact decreases.

According to the eddy current flaw detection probe of the present invention, a ferromagnetic steel pipe is magnetized in the circumferential direction by a permanent magnet and a yoke, and two inspection coils are placed on the ferromagnetic steel pipe side at a portion where the magnetic flux density of the ferromagnetic steel pipe is high. The coil is wound close to the part where the magnetic flux density is low, away from the ferromagnetic steel pipe side, and magnetically shielded by the yoke existing between the ferromagnetic steel pipe and the coil. The current can be detected. For this reason, detection of eddy current is mainly performed from a portion where variation in magnetic permeability is reduced, and noise is reduced. Further, since the differential outputs of the two inspection coils are obtained, the problem of signal fluctuation due to lift-off can be solved even when the center of the probe is deviated from the center of the ferromagnetic steel tube.

It is a figure which shows the cross section of the insertion probe for eddy current flaw detection of this invention. (a) Cross section in the tube axis direction (b) AA cross section (c) BB cross section It is an example of the block diagram of the signal detection circuit of this invention. It is a figure which shows the calculation result of a pipe axial direction of a pipe | tube internal magnetic flux density, and a pipe peripheral direction. It is a figure which shows the influence of the lift-off of the eddy current test insertion probe using the cylindrical coil of the conventional method, and the eddy current test insertion probe of this invention. It is a figure of the conventional insertion probe for eddy current flaw detection. It is a figure which shows the conventional insertion probe for eddy current flaws using a cylindrical test | inspection coil. (a) Schematic diagram of eddy current flaw detection probe (b) CC cross section

Embodiments of the present invention will be described below.

FIG. 1 shows an embodiment of an insertion probe for eddy current flaw detection according to the present invention. 1A is a sectional view in the tube axis direction, FIG. 1B is an AA sectional view, and FIG. 1C is a BB sectional view.

The eddy current flaw detection probe 1 has a rectangular cross section provided at the center, and has a rectangular parallelepiped shape on both magnetic pole surfaces of a rectangular parallelepiped permanent magnet 10 magnetized in the opposite direction and a rectangular parallelepiped permanent magnet 10. A coil holder 50 having a bow-shaped cross section is coupled to the iron 20 and a surface different from both magnetic pole surfaces. The magnetic lines of force exit from the N pole toward the S pole inside the permanent magnet 10, but outside the permanent magnet 10, exit from the N pole so that the magnetic path length is shortened and pass through the circumferential direction of the ferromagnetic steel tube T. In order to return to the pole, a part of the circumferential direction of the ferromagnetic steel tube T is magnetized as shown in FIG. As the permanent magnet 10, an NdFeB-based or SmCo-based rare earth magnet is desirable because of its high saturation magnetization and large coercive force.

Further, as the cross-sectional shape of the permanent magnet 10, if the side in the magnetization direction is the length and the side perpendicular to the magnetization direction is the width, the ratio of the length to the width is 0.5 (ratio 1: 2) to 2.0. A ratio of about 2: 1 is desirable. When the length-to-width ratio is less than 0.5, the magnetic pole area increases, but the demagnetizing factor increases, the operating magnetic field decreases, the overall magnetic flux decreases, and the length-to-width ratio exceeds 2.0. This is because the demagnetizing factor is reduced and the operating magnetic field is increased, but the magnetic pole area is reduced and the overall magnetic flux is reduced. In this embodiment, an NdFeB magnet having a length to width ratio of 0.8 is used.

Further, a gap 30 is provided in a portion of the yoke 20 in contact with both magnetic pole surfaces of the yoke 20 near the center in the length direction of the eddy current flaw detection probe 1. Two inspection coils 40a and 40b are linearly formed in the gap 30. The coil holder 50 is wound along a bow shape so as to obtain a differential output of the two inspection coils 40a and 40b. The inspection coils 40a and 40b are obtained by winding an 80 μm coated copper wire 60 times.

A cross-sectional view of the inspection coil 40a is shown in FIG. 1 (c). In the portion where the ferromagnetic steel tube T is saturated and magnetized, the inspection coil 40a projects in a bow shape along the coil holder 50, and the ferromagnetic steel tube T However, in the portion where the magnetic field is weak and insufficient for saturation magnetization, the inspection coil 40a is the portion of the portion in contact with both the magnetic pole surfaces of the yoke 20 and the permanent magnet 10. Since the gap 30 is linearly passed, the distance from the ferromagnetic steel pipe T is long, and the shield 20 is magnetically shielded. Therefore, the signal intensity contributing from this portion is weak.

In the eddy current flaw detection according to the present invention, the flaw detection is performed by the signals of the inspection coils 40a and 40b from the portion projecting in a bow shape along the coil holder 50, and therefore the flaw detection of the entire surface of the ferromagnetic steel tube T in the circumferential direction is performed. This can be dealt with by rotating the eddy current flaw detection probe 1 at a predetermined speed or connecting a plurality of eddy current flaw detection probes 1 at a predetermined angle and connecting them in series.

In order to obtain the differential outputs of the two inspection coils 40a and 40b, for example, the signals of the inspection coils 40a and 40b are taken out from the cable 60 that also serves as the suspension cable of the eddy current flaw detection insertion probe 1, and a bridge circuit is obtained. A differential output can be obtained. Furthermore, it is possible to incorporate a substrate on which a bridge circuit or the like is mounted in the eddy current flaw detection insertion probe 1.

The coil holder 50 is made of, for example, a nonmagnetic and nonconductive material such as bakelite, nylon, or polyacetal. Thereby, the influence of the residual magnetization from the coil holder 50 and the generation of eddy current can be suppressed, and the noise to the signal of the ferromagnetic steel tube T can be reduced.

Moreover, the cross section of the eddy current flaw detection insertion probe 1 in which the permanent magnet 10, the yoke 20, and the coil holder 50 are combined is substantially circular. Thereby, when inserted into the ferromagnetic steel tube T, the distance between the inner surface of the ferromagnetic steel tube T and the inspection coils 40a and 40b projecting in a bow shape contributing to signal detection increases, and the output is improved. .

Two concave grooves having a depth of 1 mm and a width of 2 mm are formed in the coil holder 50, and each of the two inspection coils 40a and 40b is wound around the concave groove so that the inspection coils 40a and 40b are deformed by cutting or contact. I am trying not to.

FIG. 2 shows a block diagram of the signal detection circuit of the present invention. The high-frequency signal from the oscillator is amplified by the power amplifier and transmitted to the inspection coils 40a and 40b. The differential output of the inspection coils 40a and 40b is detected by the bridge circuit and amplified by the amplifier. The amplified signal and the signal from the oscillator are transmitted to the synchronous detector 1 to detect the X component. A signal obtained by phase-shifting the amplified signal and the oscillator signal by 90 degrees with a phase shifter is transmitted to the synchronous detector 2 to detect the Y component.

FIG. 3 shows the calculation results of the tube axis direction magnetization and tube circumferential direction magnetization of the tube internal magnetic flux density. The calculation results are obtained by three-dimensional finite element analysis of the magnetic flux density at the center of the steel pipe thickness when the pipe outer diameter is 25.4 mm as a function of the steel pipe thickness. In the case where the outer diameter of the probe is 1 mm smaller than the inner diameter of the steel pipe, the permanent magnet 10 is calculated using the magnetic characteristics of the NdFeB magnet, and the yoke 20 is calculated using the magnetic characteristics of pure iron. From this result, it can be said that the direction of magnetization in the circumferential direction of the steel pipe can magnetize the steel pipe more strongly than the direction of magnetization in the axial direction of the conventional steel pipe.

A flaw detection test was conducted to investigate the effect of lift-off between the eddy current flaw detection probe using a conventional cylindrical coil and the eddy current flaw detection probe of the present invention. The ferromagnetic steel pipe T used for the flaw detection test is a carbon steel pipe having an outer diameter of 19 mm and a wall thickness of 2.3 mm, and has an outer diameter and a depth as shown in Table 1 artificially created on the outer surface of the steel pipe. It is.

FIG. 4A shows the effect of lift-off of the eddy current flaw detection probe using a conventional cylindrical coil, and FIG. 4B shows the effect of lift-off with the eddy current flaw detection probe of the present invention. Show. The outer diameter of the eddy current flaw detection probe was 12 mm, the permanent magnet 10 was made of NdFeB magnet, the yoke 20 was made of soft iron, and the frequency of the oscillator was 50 kHz. In the eddy current flaw detection probe 8 in which the cylindrical inspection coil 48 is disposed in the portion of the conventional yoke 20 that is not coupled to the permanent magnet 10, when the lift-off of the cylindrical inspection coil 48 fluctuates, the steel pipe is induced. The eddy current generated varies greatly. That is, in FIG. 6B, when the cylindrical inspection coil 48 at 3 o'clock approaches the steel pipe, the cylindrical inspection coil 48 at 9 o'clock moves away from the steel pipe, and the detected signal varies greatly. Furthermore, even if the differential of two inspection coils arranged adjacent to each other is taken, it is not possible to suppress lift-off fluctuations accompanying yaw angle fluctuation. For this reason, it is impossible to detect minute defects (for example, scratches D and F) having a large variation in the zero point level.

On the other hand, in the eddy current flaw detection probe 1 according to the present invention, the inspection coils 40a and 40b are arranged in a straight line so as to be separated from the steel pipe in a portion where the yoke 20 is present, and in a portion so as to approach the steel pipe in a portion where the yoke 20 is not present. Since it is wound in the direction, the influence of the lift-off fluctuation accompanying the probe swing is small. In other words, when the bow shape at 3 o'clock approaches the steel pipe in Fig. 1 (c), the bow shape at 9 o'clock moves away from the steel pipe and the sum of eddy currents induced in the steel pipe does not change so much, and the lift off to the inspection coil itself It has a fluctuation compensation function. Furthermore, since the differential outputs of the two inspection coils 40a and 40b are obtained, there is little fluctuation in the zero point even when there is a yaw angle fluctuation when the steel pipe is approached, away from the steel pipe, and fine defects are good. Can be detected.

As described above, the eddy current flaw detection probe according to the present invention saturation-magnetizes the ferromagnetic steel pipe in the circumferential direction with the permanent magnet and the yoke, and separates the two inspection coils from the ferromagnetic steel pipe in the portion with the yoke. As described above, since the coil is wound in the circumferential direction so as to approach the ferromagnetic steel pipe in the portion without the yoke, and the differential of the two inspection coils is taken, there is no signal fluctuation due to lift-off, and the detection accuracy is improved. A high eddy current flaw detection probe can be provided.

DESCRIPTION OF SYMBOLS 1 Insertion probe for eddy current flaw detection 10 Permanent magnet 20 yoke 30 Air gap 40a, 40b Inspection coil 50 Coil holder 60 Cable T Ferromagnetic steel pipe

Claims (4)

A rectangular permanent magnet magnetized in the opposite direction in the central portion, a yoke with a bow cross section on both magnetic pole surfaces of the rectangular permanent magnet, and a surface different from the two magnetic pole surfaces An eddy current flaw detection probe having a bow-shaped cross section coupled to a coil holder, which is in contact with both magnetic pole surfaces of the permanent magnet of the yoke near the longitudinal center of the eddy current flaw detection probe. A gap is provided in the portion, and two inspection coils are wound along a straight line in the gap and along a bow shape in the coil holder, so that a differential output of the two inspection coils is obtained. Interpolation probe for eddy current testing. The eddy current flaw detection insertion probe according to claim 1, wherein the coil holder is nonmagnetic and nonconductive. The eddy current flaw detection insertion probe according to claim 1 or 2, wherein a cross section of the eddy current flaw detection insertion probe in which the permanent magnet, the yoke and the coil holder are combined is substantially circular. The eddy current flaw detection insertion probe according to any one of claims 1 to 3, wherein a groove is formed in the coil holding body, and the two inspection coils are wound around the groove.

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