JP2000356624A - Eddy current flaw detection method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an eddy current flaw detection method capable of expanding a phase difference between a flaw signal and a vibration noise even in the case of application to a non-magnetic material or a magnetic material heated to a temperature above its Curie point, to enhance the improving effect of an S/N ratio by phase detection. SOLUTION: In this eddy current flaw detection method, an eddy current is induced in a specimen 16 by flowing an alternating current through a solenoid- like test coil 10 with passing the specimen 16 made of a non-magnetic material or a magnetic material heated to a temperature above its Curie point through the test coil 10, and a change of the eddy current is detected. The frequency of the alternating current flowing through the test coil 10 is controlled to 200-1,000 kHz, preferably, 300-600 kHz.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an eddy current flaw detection method, and more particularly, to barges, cracks, and the like generated on a rod or wire made of a nonmagnetic material or a magnetic material heated to a temperature higher than the Curie point. The present invention relates to an eddy current flaw detection method suitable for non-destructively inspecting defects such as linear flaws and surface irregularities.

[0002]

2. Description of the Related Art When a conductive test object is placed in a magnetic field formed by flowing an alternating current through a coil, an eddy current is induced in the test object by an electromagnetic induction phenomenon. This eddy current is disturbed if there is a defect or the like in the test body.
If this is detected as a change in coil impedance,
The presence or absence of a defect or the like in the test body can be inspected nondestructively. A nondestructive inspection method for defects using such a change in eddy current is known as an eddy current flaw detection test.

In the eddy current test, a relatively high resolution can be obtained without directly contacting a test coil with a test piece.
Since it has the advantage that it is not affected even if a dielectric such as an oil film exists between the test coil and the test piece,
It is widely used as a non-destructive inspection method for detecting cracks, scabs, linear scratches, etc. generated on metal wires, bars, etc., or defects such as irregularities formed on the metal plate surface. is there. In the case where the shape of the test body is a rod or a wire, a hot eddy current flaw detector of a penetration coil type is generally used. The frequency of the alternating current flowing through the coil of the above device is generally 30 to 125 kHz.

In the eddy current testing, eddy currents induced in a test body are affected by not only defects present in the test body but also changes in the relative positions of the test coil and the test body. For example, penetrate a metal wire inside the test coil,
When performing flaw detection while moving the metal wire, if the metal wire is eccentric or moves up and down in the test coil, the eddy current changes and appears as a change in the impedance of the test coil. The same applies to the case where flaw detection is performed while scanning the test coil on the surface of the metal plate. If the distance between the test coil and the metal plate changes or the test coil tilts, the impedance of the test coil changes.

[0005] The impedance change caused by the relative position change between the test coil and the test specimen is actively used as a method for measuring the change in the shape and dimensions of a product, the change in the thickness of a film formed on a substrate, and the like. However, in the flaw detection test, it appears as a large vibration noise, and the S / N
It causes the ratio to decrease. Therefore, in the eddy current flaw detection test, it is necessary to minimize vibration noise in order to increase the flaw detection accuracy.

As a method of suppressing such vibration noise, for example, a pair of test coils (a so-called self-comparison type test coil) is juxtaposed to a test body, and two adjacent portions of the test body are connected. There are known a method of detecting a difference, a method of removing vibration noise using a phase detector, and a method of extracting only a frequency having a specific frequency component using a bandpass filter.

[0007]

As shown in FIG. 4, the suppression of vibration noise by phase detection is such that a flaw detection signal is displayed two-dimensionally on a voltage plane, and the phase of vibration noise is detected via a phase shifter. A control signal (straight line OT) having a phase difference of 90 ° with respect to the straight line OB) is applied to the phase detection circuit, and the defect signal (straight line O
This is performed by generating an output proportional to the projection length (straight line OC) of the straight line OT in A). Therefore, when the phase difference between the flaw signal and the vibration noise is 90 °, the effect of improving the S / N ratio by the phase detection is maximized.

However, in the case of non-magnetic material, when a flaw is detected in a frequency range of 30 to 125 kHz which is generally used in the past, the phase difference between the flaw signal and the vibration noise is only 7 to 8 °, and the obtained flaw detection is performed. Even if phase detection is performed on the signal, the flaw signal is also suppressed at the same time, so that there has been a problem that highly accurate flaw detection cannot be performed.

On the other hand, when the specimen is a magnetic material, the phase difference between the flaw signal and the vibration noise is large (several tens of degrees) even if the frequency range used for flaw detection is 30 to 125 kHz.
Therefore, even if the specimen is a magnetic material, for example, if it is a wire, it is difficult to perform a flaw detection test after the wire is wound into a coil shape. It is common to perform a flaw detection test before the is wound into a coil. Therefore, in the case of hot rolling, the flaw detection test is performed in a state where the temperature of the material exceeds the Curie point, so that the phase difference between the flaw signal and the vibration noise is small as in the case of the non-magnetic material. Therefore, there is a problem that high-precision flaw detection cannot be performed.

An object of the present invention is to increase the phase difference between a flaw signal and vibration noise even when applied to a non-magnetic material or a magnetic material heated to a temperature higher than the Curie point. Accordingly, it is an object of the present invention to provide an eddy current flaw detection method capable of improving the effect of improving the S / N ratio by phase detection.

[0011]

In order to solve the above-mentioned problems, the present invention provides a method in which a specimen is passed through a solenoid-shaped magnetic coil, and an eddy current is generated in the specimen by passing an alternating current through an exciting coil. In the eddy current flaw detection method for inducing and detecting a change in the eddy current, the test body is made of a non-magnetic material or a magnetic material heated to a temperature equal to or higher than the Curie point, and the frequency of an alternating current flowing through the exciting coil is 200 to 1000k
The gist is that the frequency is controlled to Hz.

According to the eddy current flaw detection method of the present invention having the above configuration, when the test body is a non-magnetic material or a magnetic material heated to a temperature equal to or higher than the Curie point, the frequency is controlled to 200 to 1000 kHz. As a result, the phase difference between the flaw signal and the vibration noise increases. Therefore, if the phase detection is performed on the flaw detection signal obtained by such a method, the S / N ratio is improved, and the flaw detection accuracy can be improved.

[0013]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows an example of the test coil. In FIG. 1, the test coil 10
Is a so-called self-comparison mutual induction type penetration coil in which the detection coil 14 is arranged inside the excitation coil 12.
A linear test body 16 is inserted inside the test coil 10.

The exciting coil 12 is a solenoid-shaped through coil. A transmitter 30 is connected to both ends thereof, so that a high-frequency current having a predetermined frequency can flow through the exciting coil 12.

The detection coil 14 is a so-called self-comparison type coil composed of two solenoid-shaped through coils, that is, a first coil 14a and a second coil 14b. The two coils are connected differentially. I have. The output terminals 32 and 34 at both ends of the detection coil 14 are connected to an amplifier (not shown) so as to output the amplified flaw detection signal to a phase detector (not shown). Thus, the detection coil 14 in which the two coils are differentially connected
The use of the method suppresses gentle changes such as the material and the shape and dimensions of the test article, vibration noise, and the like, so that there is an advantage that a minute defect can be stably detected without using a bridge resistor.

Here, the test coil 1 is transmitted via the transmitter 30.
The frequency to flow to 0 needs to be 200 to 1000 kHz. If the frequency is less than 200 kHz, the phase difference between the flaw signal and the vibration noise becomes several degrees or less, and the effect of improving the S / N ratio is not recognized. Also, as the frequency increases, the phase difference between the flaw signal and the vibration noise tends to increase, but when the frequency exceeds 1000 kHz,
On the contrary, the phase difference decreases, and the S / N ratio decreases, which is not preferable. The high frequency is more preferably 300 to
It is 600 kHz.

In the present invention, the test piece 16 includes:
A non-magnetic material or a magnetic material heated to a temperature equal to or higher than the Curie point is used. When the present invention is applied to such a material exhibiting non-magnetism, there is an advantage that separation of a flaw signal and vibration noise, which has been difficult with the conventional method, is facilitated.

The test body 16 may have a linear shape, a tubular shape, or a rod shape. In addition, test coil 1
The characteristic of 0 is not particularly limited. Even when coils having different resonance frequencies are used, the phase difference between the flaw signal and the vibration noise can be enlarged by the method of the present invention.

Next, a flaw detection method using the test coil 10 shown in FIG. 1 will be described. First, a high-frequency current having a predetermined frequency is caused to flow through the exciting coil 12 using the transmitter 30 in a state where the test body 16 is inserted into the test coil 10.
When a high-frequency current flows through the exciting coil 12, an eddy current is induced in the test body 16, and a magnetic flux is generated in the exciting coil 12.

When a magnetic flux is generated in the exciting coil 12, a part of the magnetic flux is linked to the detecting coil 14. At this time, at both ends of the first coil 14a and the second coil 14b, an electromotive force corresponding to the magnitude of the magnetic flux linking the inside of the detection coil 14 is generated, but the first coil 14a and the second coil 14b are generated. Are connected differentially, so that if there is no defect in the test body 16, only vibration noise is output as a flaw detection signal between the output terminals 32 and.

On the other hand, when the test object 16 has the defect 16a, the test object 16 is moved, and when the defect 16a reaches the first coil 14a, the eddy current flowing through the test object 16 is changed by the defect 16a. And the first coil 14
Only the impedance of a changes. As a result, a flaw signal corresponding to the vibration noise and the defect 16a is output as a flaw detection signal between the output terminals 32 and 34.

When the defect 16a completely passes through the first coil 14a, only vibration noise is output as a flaw detection signal between the output terminals 32 and 34.
Is moved further, and when the defect 16a reaches the second coil 14b, only the impedance of the second coil 14b changes contrary to the above. Therefore, the output terminals 32 and 3
Between the four, the vibration noise and the flaw signal are output again as the flaw detection signal.

When the flaw detection signal is output from the output terminals 32 and 34, the flaw detection signal amplified by an amplifier (not shown) is output to a phase detector (not shown), and the phase detector performs phase detection. However, in the case where the test body 16 is a material showing non-magnetism and the frequency of the test coil 10 via the transmitter 30 is 30 to 125 kHz, the phase difference between the flaw signal and the vibration noise is small.
Even if the phase detection is performed, no improvement in the S / N ratio is observed.

On the other hand, when the frequency of the high frequency supplied to the test coil 10 is set to 200 to 1000 kHz, more preferably 300 to 600 kHz, the phase difference between the flaw signal and the vibration noise is increased to 30 to 60 degrees. Therefore, if phase detection is performed in such a state, only vibration noise is preferentially suppressed, and the S / N ratio can be significantly improved.

[0025]

EXAMPLE A test coil 10 having the shape shown in FIG. 1 was used to measure the phase difference between a flaw signal and vibration noise. The test coil 10 has a resonance frequency of 500 kHz and an inner diameter of 13 mm.
Self-comparison method, a mutual induction type penetration coil (hereinafter referred to as “coil A”), and a resonance frequency of 1.6 MH
Two kinds of coils, z, a self-comparison method with an inner diameter of 13 mm, and a mutual induction type penetration coil (hereinafter referred to as "coil B") were used.

As the test body 16, a wire made of austenitic stainless steel (SUS316) having a diameter of 8 mm was used. Also, an artificial defect for flaw detection is provided on the surface of the wire, and its shape is a flat bottom hole having a diameter of 3 mm and a depth of 0.1 mm when flaw detection is performed using the coil A, and a test coil B is used. Was a flat bottom hole having a diameter of 3 mm and a depth of 0.2 mm. Further, a frequency (hereinafter, referred to as a frequency) flowing through the test coil
This is called “flaw detection frequency”. ) Is 30-700kHz
z and the test temperature was room temperature.

Under these measurement conditions, the specimen 1
6, the test piece 16 was moved inside the test coil 10 without vibration, the test piece 16 was hit with a hammer to intentionally move up and down, and the test piece 16 was intentionally tilted. The signal was output, and the phase difference between the flaw signal and the vibration noise was measured.

FIG. 3 shows an example of an oscilloscope waveform of the flaw detection signal. In the case where the test coil A is used and the flaw detection frequency is 90 kHz, as shown in FIG. 3A, the phase difference between the flaw signal and the vibration noise caused by the inclination is 2
5 degrees. However, both the flaw signal and the vibration noise caused by the vertical movement appeared on the oscilloscope in the horizontal direction, and the phase difference was 0 degree.

On the other hand, assuming that the flaw detection frequency is 400 kHz, as shown in FIG. 3B, the flaw signal appears almost horizontally on the oscilloscope. The vibration noise caused by the inclination of 16 appeared on the oscilloscope in an oblique direction, and the phase difference between the flaw signal and the vibration noise was increased to 30 degrees.

When the test coil B was used and the flaw detection frequency was 100 kHz, the flaw signal appeared horizontally on the oscilloscope as shown in FIG. 3C. On the other hand, the phase difference between the vibration noise and the flaw signal caused by the vertical movement was 65 degrees, while the phase difference between the vibration noise and the flaw signal caused by the inclination was 20 degrees.

On the other hand, assuming that the flaw detection frequency is 400 kHz, as shown in FIG. 3D, the flaw signal appears almost horizontally on the oscilloscope, while the vibration noise caused by the vertical movement and the tilt is generated. All appeared obliquely on the oscilloscope, and the phase difference between the flaw signal and the vibration noise was increased to 55 degrees.

FIG. 2 shows the relationship between the flaw detection frequency and the phase difference. In FIG. 2, the “phase difference” on the horizontal axis is the smaller of the phase difference between the flaw signal and the vibration noise caused by the vertical movement and the phase difference between the flaw signal and the vibration noise caused by the inclination. Indicates phase difference. In the case of coil B, the detection frequency is 200k
When the frequency was set to Hz or more, the phase difference expanded to 30 degrees or more. On the other hand, in the case of the coil A, when the flaw detection frequency was 300 kHz or more, the phase difference between the flaw signal and the vibration noise was increased to 20 degrees or more.

In addition, both the coil A and the coil B
Up to the flaw detection frequency of 600 kHz, the phase difference increased almost monotonously with the increase of the flaw detection frequency.
At 0 kHz, the phase difference was slightly reduced.

From the above results, when performing an eddy current flaw detection test on a non-magnetic material, the flaw detection frequency was set to 300 kHz.
It was found that when the value was z or more, the phase difference between the flaw signal and the vibration noise increased regardless of the characteristics of the test coil 10. In addition, it was found that when flaw detection was actually performed under such conditions, the S / N ratio was improved and the flaw detection accuracy was improved even when the test piece was made of a nonmagnetic material.

Although the embodiments of the present invention have been described in detail, the present invention is not limited to the above-described embodiments, and various modifications are possible without departing from the gist of the present invention. is there.

For example, in the above embodiment, an example in which the present invention is applied to an austenitic stainless steel at room temperature has been described. However, other non-magnetic materials or defects of a magnetic material heated to a temperature equal to or higher than the Curie point can be used. The present invention can be applied to inspection. In addition, the present invention can be applied not only to the case of detecting a defect on the surface of a wire using a penetrating coil but also to the case of detecting a defect on the inner surface of a pipe using an insertion coil.

Further, in the above-described embodiment, an example in which the present invention is used for flaw detection of a defect has been described. The present invention can be applied to the case. At this time, if the frequency of the high frequency flowing through the coil is within a predetermined range, the vibration noise can be effectively removed, and the measurement accuracy of the shape, dimensions, and the like can be improved.

[0038]

According to the present invention, an eddy current is induced in a test body by passing the test body through a solenoid-shaped magnetic coil and passing an alternating current through an exciting coil to detect a change in the eddy current. In the flaw detection method, since a high frequency having a frequency of 200 to 1000 kHz is used as an alternating current flowing through the excitation coil, even if the test specimen is a non-magnetic material or a magnetic material heated to a temperature equal to or higher than the Curie point, The phase difference between the signal and the vibration noise increases, and S in phase detection
There is an effect that the / N ratio improvement effect is improved.

When the frequency of the alternating current flowing through the exciting coil is controlled to 300 to 600 kHz, the phase difference between the flaw signal and the vibration noise becomes maximum, and the effect of improving the S / N ratio in the phase detection is further improved. is there.

As described above, according to the eddy current flaw detection method according to the present invention, it is possible to detect minute defects generated in non-magnetic materials, which have been difficult in the past, with high accuracy. If applied to wire rod defect inspection, seamless steel pipe defect inspection, ERW pipe welding defect inspection, heat exchanger piping corrosion inspection, etc., it will contribute to the improvement of product quality, This is a very large invention.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a test coil used in the present invention.

FIG. 2 is a diagram showing the influence of a flaw detection frequency on a phase difference between a flaw signal and vibration noise.

FIG. 3 is a diagram illustrating an example of an oscilloscope waveform when an eddy current flaw detection test is performed using test coils having different resonance frequencies and high frequencies having different flaw detection frequencies.

FIG. 4 is a conceptual diagram for explaining a phase detection method.

[Explanation of symbols]

 10 Test coil 16 Specimen 16a Defect

Claims (2)

[Claims] An eddy current flaw detection method for causing a test body to pass through a solenoid-shaped magnetic coil and causing an alternating current to flow through an exciting coil to induce an eddy current in the test body and detect a change in the eddy current. The test body is made of a non-magnetic material or a magnetic material heated to a temperature equal to or higher than the Curie point. The frequency of an alternating current flowing through the excitation coil is 200 to 1000.
An eddy current flaw detection method characterized in that the frequency is controlled to kHz. 2. The frequency of an alternating current flowing through the exciting coil is 3
2. The eddy current flaw detection method according to claim 1, wherein the eddy current flaw detection is controlled at 00 to 600 kHz.