JPH01206246A - Method of checking defect of metal pipe - Google Patents

Abstract

PURPOSE:To detect a defect accurately by disposing first and second coil elements within a sphere wherein a change in the quality of a metal pipe and a joint have the same effect substantially on the phases of reception signals of the two coil element, and by taking a difference signal between the two reception signals as a reception signal. CONSTITUTION:While a transmission coil 12 and reception coils 13-15 are moved in the direction of a pipe axis, an electromagnetic wave is generated in the direction of an arrow B from the transmission coil 12. This electromagnetic wave is received by the reception coils 13-15, a change in the phase of this reception signal accompanying a defect 11a of a metal pipe is monitored, and thereby the presence and position of the defect 11a of the metal pipe 11 are detected. First and second coil elements 13a and 14a are disposed within a sphere wherein a change in the quality of the metal pipe 11 and a joint have the same effect substantially on the phases of the reception signals of the first and second coil elements 13a and 13b, and a difference signal between the reception signals of the two coils 13a and 13b is taken as a reception signal of the reception coil 13.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention is applicable to corrosion of metal pipes such as steel pipes buried underground such as gas pipes and water pipes, or metal pipes such as steel pipes of boilers and heat exchangers. The present invention relates to a metal pipe defect inspection method for non-destructively detecting defects such as the above.

[Conventional technology]

The conventional defect inspection method for metal pipes is shown in Figs. 17 and 18.
As shown in the figure, a transmitting coil 2 and a plurality of receiving coils 3, 4, 5, etc. can be moved within a metal tube 1 made of steel while maintaining a fixed interval (for example, around twice the tube diameter). The transmitter coil 2 and the receiver coil 3, 4.5. ...
While moving in the tube axis direction, an electromagnetic wave is generated from the transmitting coil 2 as shown by arrow A, and this electromagnetic wave is transmitted to the receiving coil 3°4.5. ..., receiving coil 3.
4. 5. Defect 1a of metal tube l of received signal due to...
The metal tube 1 is inspected for defects based on the phase change caused by the change in phase.

In this case, a plurality of receiving coils 3, 4, 5 . ... are arranged close to the inner surface of the metal tube 1 and arranged in a line in the circumferential direction of the metal tube 1.

Now, as shown in FIG. 17, when there is a defect 1a in some part of the metal tube 1, the transmitter coil 2 and the receiver coils 3, 4, 5, . . . are kept at a constant distance. While moving the metal tube 1 in the tube axis direction, the radio waves generated from the transmitting coil 2 are transmitted to the receiving coils 3, 4, 5, .・・・
・When the signal is received at
(As shown in C1, there is almost no change. On the other hand, the phase of the received signal by the receiving coil 4 installed near the defect 1a will be affected by the defect 1a,
As shown in FIG. 19 (bl), it changes depending on the position where the defect 1a exists.

In this way, if the defect 1a exists in the metal tube 1, the defect 1
Since the phase of the signal received by the receiving coil 4, for example, installed near the transmitting coil 2 and receiving coils 3, 4, 5, .a, will change in accordance with the position of the defect 1a.・
While moving the... in the tube axis direction, each receiving coil 3, 4,
5. By monitoring the phase of the received signal by..., the presence and position of the defect 1a in the metal tube 1 can be detected.

[Problem to be solved by the invention]

However, with this metal tube defect inspection method, changes in the tube material (magnetic permeability, etc.) occur (resulting in noise when detecting defects) due to residual stress caused by rolling during manufacturing of the metal tube. Even in the case where a joint (which causes noise when detecting a defect) is provided in the middle of the metal tube, multiple receiving coils 3° 4.5. ... will occur, and the phase change due to the defect 1a will be buried, resulting in a problem of low defect detection accuracy.

This point will be explained in detail below with reference to FIGS. 20 and 21. Now, as shown in FIG. 20, there is a part 1b in which the material of the pipe changes along the entire circumference in the middle of the metal pipe 1.
Let us also consider a case where a defect 1c exists in a part of the pipe material changed portion lb. In this case, the pipe material change portion 1b exists over a considerably wider range than the defect lc, and the material change does not occur suddenly but gradually.

In the above case, the transmitting coil 2 and the receiving coil 3. 4. 5. While moving . 4. 5°..., the phase of the received signal by the receiving coils 3 and 5 installed at a position away from the defective IC is
The phase is hardly affected by the change in pipe material, and the phase changes only by the influence of the change in pipe material 1b, and as shown in FIG. do.

On the other hand, the phase of the received signal by the receiving coil 4 installed in the vicinity of the defective IC is affected by both the tube material change section 1b and the defective IC.
As shown in FIG. 1 (bl), the pipe material changes depending on the location of the changed portion 1b and the defective IC.

Note that the same problem as above occurs also when there is a defect in the vicinity of the pipe joint.

Therefore, it is an object of the present invention to provide a metal tube defect inspection method that can accurately detect the presence or absence and location of defects in metal tubes.

[Means for Solving the Problems] The metal tube defect inspection method of the present invention has considerably smaller defects than the parts of the metal pipe where the pipe material changes and the joints. This was done with the focus on the fact that it is possible to set the size to be sufficiently smaller than the joint and approximately equal to or larger than the defect.

In other words, this metal tube defect inspection method involves movably disposing a transmitting coil and a receiving coil within a metal tube at a fixed interval, and transmitting electromagnetic waves from the transmitting coil while moving the transmitting coil and receiving coil in the tube axis direction. In a metal tube defect inspection method, the existence and location of a defect in the metal tube is detected by emitting electromagnetic waves and receiving this electromagnetic wave by a receiving coil, and monitoring the phase change of the signal received by the receiving coil due to a defect in the metal tube. The coil is composed of a first and a second coil part, and the first and second coil parts are arranged so that the tube material of the metal tube and the joint are substantially the same in phase with the re-received signals of the first and second coil parts. , and the difference signal between the reception signals of the first and second coil sections is used as the reception signal of the reception coil.

[For production]

According to the configuration of the present invention, the defects are considerably smaller than the parts where the pipe material of the metal tube changes and the joints, and the receiving coil is made sufficiently smaller than the parts where the pipe material of the metal pipes changes and the joints, and is approximately equal to or larger than the defects. Focusing on the fact that it is possible to set the size to is arranged within a range that has substantially the same effect on the phase of the re-received signal of the first and second coil parts, and a difference signal between the reception signals of the first and second coil parts is used as the reception signal of the reception coil, The existence and location of defects in the metal tube is detected by monitoring the phase change of the received signal of this receiving coil, so phase changes due to changes in the material of the metal tube and the presence of joints are canceled out. Therefore, the presence or absence and position of defects in the metal tube can be detected with high accuracy regardless of changes in the tube material of the metal tube and the presence of joints.

In addition, since the first and second coil parts are arranged side by side in the circumferential direction of the metal tube to prevent positional deviation in the tube axis direction, even when the tube material changes suddenly in the axial direction of the metal tube, Even when the first and second coil sections cross a sudden change part or when the first and second coil sections cross a joint between a metal tube and a joint, the re-received signals of the first and second coil sections are have the same waveform, and the above part shows no phase change in the difference signal between the received signals of the first and second coil parts at the above position, allowing highly accurate detection of the presence and location of defects in the metal tube. can do.

〔Example〕

A first embodiment of the present invention will be described based on FIGS. 1 to 13. This metal pipe defect inspection method detects defects that are considerably smaller than the parts where the pipe material of the metal pipe changes and the joints, and detects the receiving coil which is sufficiently smaller than the parts where the pipe material of the metal pipe changes and the joints and is almost equivalent to the defects. This was done by focusing on the fact that it is possible to set the size to a size larger than that.

That is, in this metal tube defect inspection method, as shown in FIGS. 1 and 2, a transmitting coil 12 and a plurality of receiving coils 13, 14, 15, . The transmitter coil 12 and the receiver coil 13 . l While moving 4.15°... in the tube axis direction, transmit electromagnetic waves from the transmitting coil 12 in the direction of arrow B.
This electromagnetic wave is generated in the direction of receiving coil 1.
3. ．． 14.15. ..., receiving coil 13.1
4,15. Defect 1 in metal tube 1) of received signal due to...
) The existence and position of the defect Za in the metal tube 1) is detected by monitoring the phase change associated with a.

In this case, a plurality of receiving coils 13, 14, 15°...
are arranged close to the inner surface of the metal tube 1 and arranged in a line in the circumferential direction of the metal tube 1).

Further, the receiving coil 13 includes the first and second coil portions 1
3a and 13b, and the first and second coil parts 13a and 13b are arranged so that the change in the tube material of the metal tube 1) and the joint are approximately in phase with the re-received signals of the first and second coil parts 13a and 13b. placed within the same range of influence,
That is, they are arranged side by side in the circumferential direction of the metal tube 1), and the difference signal between the reception signals of the first and second coil parts 13a and 13b is used as the reception signal of the reception coil 13. Further, the receiving coils 14 and 15 have the same configuration as the receiving coil 13, and each has a first coil portion 14a.

15a and second coil portions 14b and 15b.

In addition, in the above example, every other coil part 13a.

13b; 14a, 14b; 15a, 15b are paired, but this combination can be set arbitrarily.

In this way, for example, the reception coil 13 is configured with the first and second coil parts 13a and 13b, and the difference signal between the reception signals of the first and second coil parts 13a and 13b is used as the reception signal of the reception coil 13. When the phase is monitored, even if a phase change due to a change in the pipe material of the metal tube 1) and the presence of a joint appears in the received signal of each of the first and second coil parts 13a and 13b, the phase change of the metal tube 1) Since the changes in the pipe material and the joints are arranged within a range that has approximately the same effect on the phase of the re-received signals of the first and second coil parts 13a and 13b, the difference signal includes the metal pipe 1). There are no phase changes due to changes in pipe material or the presence of joints.

Here, as shown in Fig. 1, there is a defect 1) a in some places in the middle of the metal pipe 1) where there is no change in the pipe material. A case will be described in which the changed portion 1)b exists over the entire circumference and the defect 1)C exists at some locations within the changed portion 1)b of the pipe material.

Transmitting coil 12 and receiving coil l 3. 14.15
While moving the metal tube 1) at a constant interval in the tube axis direction, the radiated radio waves from the transmitting coil 12 are transmitted to the receiving coils 13, 14, 15, . ..., the phase of the received signal due to defect 1)a is due to the receiving coil 13.15 installed at a position far from the IIC, and the change due to the change in tube material 1)b is Canceled and defective 1)
8.1) No influence from C, Figure 3+a), (C1
As shown, there is almost no change.

On the other hand, the phase of the received signal by the receiving coil 14 installed in the vicinity of defects 1) a, 1) c is canceled due to the change in tube material change part 1) b, and defects 1) a, 1)
) c remains, and as shown in FIG. 3 Tbl, it only changes in response to defects 1) a and llc.

In this way, if defects 1)a, IIC exist in the metal tube 1), the phase of the received signal by the receiving coil, for example 14, installed near the defects 1)a, llc will change due to the defect 1)a, llc.
c will change depending on the position, so the transmitting coil 12 and the receiving coils 13, 14, 15 . Each receiving coil 13, 14, 15 is moved while moving ... in the tube axis direction.
．． By monitoring the phase of the received signal by...
The presence and position of defects 1) a, llc in the metal tube 1) can be detected.

Here, the configuration of a metal tube defect inspection apparatus used in the metal tube defect inspection method will be explained based on FIGS. 4 to 6. Figure 4 shows an example of a metal pipe defect inspection device, and Figure 5 shows an example of a metal pipe defect inspection device.
The figures and FIG. 6 show other examples.

First, in Fig. 4, the metal pipe defect inspection device
A flexible connecting rod 21 made of a non-magnetic material such as plastic is covered with a resin coil bobbin 22 and fixed with screws 23 . A transmitting coil 24 is wound around the coil bobbin 22 so that the winding axis is parallel to the tube axis of the metal tube, and an insulating resin 25 is applied to the outer periphery of the coil bobbin 22 . Also,
A centering cushion 26 made of sponge or the like is adhered entirely or partially to the outer periphery of both flanges 22a of the coil bobbin 22. 27 is a screw for fixing the end of the transmitting coil 24.

Further, receiving coil holders 28 are externally mounted on the connecting rod 21 at intervals of about twice the diameter of the metal tube and are fixed with screws 29. The receiving coil holder 28 has, for example, twelve receiving coil holding recesses 28a on its outer circumferential surface, and the receiving coil 31 wound around the coil bobbin 30 is held in the receiving coil holding recesses 28a, and the winding axis thereof is a metal tube. They are fitted perpendicularly and sealed and fixed with resin (not shown). Also,
A centering cushion 33 made of sponge or the like is adhered to the outer peripheral surface of the maximum diameter portion of the receiving coil holder 28 all around or partially.

Note that 34.35 indicates the transmitting coil 24 and the receiving coil 3.
These are the lead wires connected to 1 respectively.

Furthermore, in the above description, the transmitting coil 24 and the receiving coil 3
1 is a coreless type, but a type having a core may also be used.

Then, the metal tube defect inspection device shown in FIG. 4 is inserted into the metal tube and moved inside the metal tube in the tube axis direction.

5 and 6 (al, (bl) are sectional views of main parts of another example of a metal tube defect inspection device. In this metal tube defect inspection device, the reception coil holder 28 is A receiving coil 37 wound around a U-shaped coil bobbin 36 is fitted into the receiving coil holding recess 28b provided on the outer circumferential surface so that the winding axis is parallel to the tube axis of the metal tube, and is sealed and fixed with resin (not shown). ing.

38 is a lead wire drawn out from the receiving coil 37;
Other details are the same as in FIG. 4.

Next, for the purpose of clarifying the effect of the configuration of the embodiment, experiments were conducted on defect inspection of metal tubes using the embodiment and the conventional example.

First, as an example, as shown in FIG.
the transmitting coil 42 and the first and second coil parts 43a,
As shown in FIG. 8, as a conventional example, as shown in FIG. A receiving coil holding block 48 having a coil 47 is used, and as shown in FIG. In the case where the defect 53 was present, the phase change of the received signal of the receiving coils 43 and 47 was examined while moving inside the metal tube 51 in the tube axis direction (direction of arrow C).

The measurement conditions were that the distance between the transmitting coil 42 and the receiving coil 43 and the distance between the transmitting coil 46 and the receiving coil 47 were both 240 ml, the excitation frequency for the transmitting coils 42 and 46 was 30 Hz, and the excitation voltage was 15 Vr. -r. Further, the metal tube defect inspection device was moved so that one of the receiving coils 43 and 47 passed directly above the defect 53.

FIG. 10 fat shows the phase change of the received signal in the case of the conventional example, and FIG. 10 fbl shows the phase change of the received signal in the case of the embodiment. As is clear from FIG. 1O (al), in the case of the conventional example, a phase change P1 due to the rolling portion 52 appears in the received signal, and a phase change P2 due to the defect 53 appears superimposed on this phase change P1. Therefore, the phase change P2 due to the defect 53 is the rolling part 5.
2, it is difficult to detect the phase change P2 due to the defect 53, and the detection accuracy of the defect 53 is low.

On the other hand, as is clear from FIG. This clearly appears without being buried, and the phase change Q2 due to the defect 53 can be easily detected, and the presence or absence and position of the defect 53 can be detected with high accuracy.

In addition, in the waveform of FIG. 10 fat, both ends are high because a phase change due to the tube ends appears, and this also disappears in the case of the embodiment.

Next, using the metal tube defect inspection apparatus shown in FIGS. 7 and 8, as shown in FIG. In the case where the joint 63 is provided, the metal pipe 61. 6
2 was moved in the tube axis direction (in the direction of the arrow), and the phase change of the received signal of the receiving coil 43° 47 was examined.

The measurement conditions are that the distance between the transmitting coil 42 and the receiving coil 43 and the distance between the transmitting coil 46 and the receiving coil 47 are both 300cm, the excitation frequency for the transmitting coil 42° and 46 is 301)z, and the excitation is the same. The voltage is 15VP-P. Further, the metal tube defect inspection device was moved so that one of the receiving coils 43 and 47 passed directly above the defect 64.

FIG. 12 fat shows the phase change of the received signal before the formation of the defect 64 in the conventional example, FIG. 12 tb+ shows the phase change of the received signal after the formation of the defect 64 in the conventional example, and FIG. Figure +81 shows the phase change of the received signal before the formation of the defect 64 in the case of the example, and Fig. 13 +81 shows the phase change of the received signal after the formation of the defect 64 in the case of the example.

As is clear from FIG. 12+a+ and tb+, in the case of the conventional example, before the defect 64 is formed, a phase change R1 due to the joint 63 appears in the received signal, and after the defect 64 is formed, the received signal has a phase change R1 due to the joint 63. A phase change R1 appears, and a phase change R2 due to the defect 64 is superimposed on this phase change R1.
appears, the phase change R2 due to the defect 64 is buried in the phase change R1 due to the joint 63, and the defect 64
It is difficult to detect the phase change R2 due to this, and the detection accuracy of the defect 64 is low.

On the other hand, as is clear from Fig. 13+a+, (bl),
In the case of the embodiment, before the defect 64 is formed, the phase change S1 caused by the joint 63 in the received signal is extremely small;
The phase change S2 due to the defect 64 appears clearly without being buried in the phase change S1 due to the joint 63, making it easy to detect the phase change S2 due to the defect 64, and the presence or absence and position of the defect 64 is detected with high accuracy.

Also, first and second coil portions 13a, 13b.

14a, 14b, 15a, 15b, etc. are arranged side by side in the circumferential direction of the metal tube 1) so that there is no displacement in the tube axis direction, so that the tube material of the metal tube 1) in the axial direction is When the change is sudden, the first and second coil parts 1
3a, 13b, 14a, 14b.

15a, 15b, . . . when crossing or cutting a sudden change part or at the joint between the metal tube 1) and the joint.

15b, . . . also cross the first and second coil portions 13a, 13b, 14a, 14b.

Both received signals 15a, 15b, . . . have the same waveform, and the above portions correspond to the first and second coil portions 13a.

No phase change appears in the difference signal between the received signals 13b, 14a, 14b, 15a, 15b, -- at the above positions, making it possible to detect the existence and location of defects in the metal tube 1) with extremely high accuracy. can.

A second embodiment of the invention will be explained based on FIGS. 14 to 16. In this embodiment of the metal tube defect inspection method, the receiving coils 13, 14 . ．． The first and second coil portions 13a constitute 15°...

13b, 14a, 14b, 15a, 15b, . . . are arranged close to each other in the tube axis direction of the metal tube 1), and the other configurations are the same as in the first embodiment.

Even in this configuration, it is possible to remove the influence of the changed part 1)b of the tube material of the metal tube 1) and detect the phase change only due to the defect 1)C, The presence or absence and position of the defect 1)c can be detected with high accuracy without being affected by the presence of the material-changed portion 1)b. This is because the material of the metal tube 1) changes slowly in the axial direction.

Next, for the purpose of clarifying the effect of the configuration of the embodiment, experiments were conducted on defect inspection of metal tubes using the embodiment and the conventional example.

First, as an example, as shown in FIG.
1 includes a transmitting coil 42 and first and second coil parts 43a.
, 43b and a receiving coil holding block 44 having a receiving coil 43 arranged close to each other in the axial direction.As a conventional example, as shown in FIG.
A receiving coil holding block 48 having a transmitting coil 46 and a receiving coil 47 is used, and as shown in FIG. In the case where a defect 53 existed in the (residual stress) portion 52, the phase change of the received signal of the receiving coils 43 and 47 was examined while moving inside the metal tube 51 in the tube axis direction (direction of arrow C).

The measurement conditions are that the distance between the transmitting coil 42 and the receiving coil 43 and the distance between the transmitting coil 46 and the receiving coil 47 are both 240 m, and the distance between the centers of the coil parts 43a and 43b that constitute the receiving coil 43 is 1). 5 mm, the spacing between the coil parts 43a and 43b is l mm, and the transmitting coil 4
The excitation frequency for 2.46 is 40Hz, and the excitation voltage is 15V F-P. Further, the metal tube defect inspection device was moved so that the receiving coils 43 and 47 passed directly above the defect 53.

FIG. 16 fat shows the phase change of the received signal in the case of the conventional example, and FIG. 16 (bl shows the phase change of the received signal in the case of the embodiment.) In the case of the example, a phase change T1 due to the rolling portion 52 appears in the received signal, and a phase change T2 due to the defect 53 appears superimposed on this phase change T1.
It is difficult to detect the phase change T2 due to the defect 53, and the detection accuracy of the defect 53 is low.

On the other hand, as is clear from FIG. This clearly appears without being buried, and the phase change V2 due to the defect 53 can be easily detected, and the presence or absence and position of the defect 53 can be detected with high accuracy.

In the waveforms of fat and (bl in Figure 16), the reason why both ends are high is that a phase change due to the tube ends appears.In this example, the material is When there is a sudden change, the phase changes.

In addition to iron, the metal pipes subject to defect inspection include copper,
It may be made of any material such as aluminum.

〔Effect of the invention〕

According to the metal pipe defect inspection method of the present invention, the defects are considerably smaller than those of the pipe material change parts and joints of metal pipes.
Focusing on the fact that it is possible to set the receiving coil to a size that is sufficiently smaller than the changing part of the pipe material of the metal pipe and the joint, and approximately the same size or larger than the defect, we set the receiving coil to the first and second parts. It consists of a coil part and a first and second
The coil portion of the first and second coil portions is arranged within a range where changes in the tube material of the metal tube and the joint have a negative effect on the phase of both the received signals of the first and second coil portions, and The difference signal between the received signals is used as the received signal of the receiving coil, and by monitoring the phase change of the received signal of the receiving coil, the existence and location of defects in the metal tube can be detected. The phase change caused by the presence of the changing portion and the joint is canceled out, and the presence or absence and position of a defect in the metal pipe can be detected with high accuracy regardless of the change in the tube material of the metal pipe and the presence of the joint.

In addition, since the first and second coil parts are arranged side by side in the circumferential direction of the metal tube to prevent positional deviation in the tube axis direction, even when the tube material changes suddenly in the axial direction of the metal tube, Even when the first and second coil sections cross a sudden change part or when the first and second coil sections cross a joint between a metal tube and a joint, both received signals of the first and second coil sections are have the same waveform, and the above part shows no phase change in the difference signal between the received signals of the first and second coil parts at the above position, allowing highly accurate detection of the presence and location of defects in the metal tube. can do.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIGS. 1 and 2 are schematic sectional views of a metal tube defect inspection device used in the metal tube defect inspection method of the first embodiment of the present invention, and FIG. 3 is a receiving coil FIG. 4 is a sectional view showing a specific example of a metal tube defect inspection device; FIG. 5 is a sectional view of a main part of another specific example of a metal tube defect inspection device; Figure 6 (al, (bl)
FIG. 7 is a front view and a side view showing the configuration of a receiving coil, respectively. Figures 8 and 9 are schematic diagrams for explaining the experimental conditions, respectively, and Figures 10 (a) and tbl are phase changes of the received signal in the conventional example and the example when there is a material change in the metal tube, respectively. Figure 1) is a schematic diagram for explaining the experimental conditions, Figure 12 (al, (bl)
are waveform diagrams showing the phase change of the receiver signal 3 in the conventional example before and after the formation of a defect near the joint of a metal pipe, respectively, and (bl is a waveform diagram showing the phase change of the receiver signal 3 in the conventional example before and after the formation of a defect near the joint of a metal pipe, respectively. FIG. 14 is a waveform diagram showing the phase change of the received signal in the embodiment after formation.
A schematic cross-sectional view of the metal tube defect inspection apparatus used in the metal tube defect inspection method of Example 1, 15th is a schematic diagram for explaining the experimental conditions, FIG. 16 (F), (bl is a metal tube, respectively) Waveform diagram showing the phase change of the received signal in the conventional example and the example when there is a material change in the 17th waveform diagram
FIG. 18 is a schematic cross-sectional view of a metal tube defect inspection device used in a conventional metal tube defect inspection method, respectively;
Figure 19 is a waveform diagram of reception No. 13 by the reception coil,
FIG. 0 is a sectional view for explaining the drawbacks of the conventional example, and FIG. 21 is a waveform diagram of the received signal, also for explaining the drawbacks. 1)...Metal tube, 12...Transmission coil, 13, 14
゜15--Receiving coil, 13a, 14a, 15a--
First coil section, 13b, 14b, 15b... Second coil section Patent applicant Osaka Gas Co., Ltd. Tokyo Gas Co., Ltd. b Figure 1 Figure 2 B ↑ Figure 3 Figure 5 (a) (b ) Figure 6 Figure 7 Figure 8 Figure 9 Figure 1) Figure 12 Figure 13 Figure 14 Figure 15 Figure 17 Figure 18 Figure 16

Claims (2)

[Claims] (1) A transmitting coil and a receiving coil are movably disposed within a metal tube at a constant interval, and while the transmitting coil and receiving coil are moved in the tube axis direction, an electromagnetic wave is generated from the transmitting coil, and the electromagnetic wave is generated. is received by the receiving coil, and the presence or absence and position of a defect in the metal tube is detected by monitoring a phase change of the reception signal caused by the defect in the metal tube by the receiving coil, the receiving The coil is composed of first and second coil parts, and the first and second coil parts are arranged so that the tube material of the metal tube changes and the joint changes the phase of both received signals of the first and second coil parts. A method for inspecting a metal tube for defects, characterized in that the first and second coil parts are arranged within a range that has substantially the same influence on the coil parts, and a difference signal between reception signals of the first and second coil parts is used as a reception signal of the reception coil. (2) The metal tube defect inspection method according to claim (1), wherein the first and second coil portions are arranged side by side in the circumferential direction of the metal tube.