JPH0517503B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is capable of accurately detecting surface flaws such as cracks and pit flaws existing on the surface of inspected materials such as steel pipes and slabs, regardless of their properties. This paper proposes a possible flaw detection method and the equipment used for its implementation.

[Prior art]

Various destructive inspection methods have been put into practical use as methods for detecting surface flaws in metal materials, and one or more methods are applied depending on the defects expected to exist.

For example, magnetic flaw detection, which detects leakage magnetic flux from the surface of the material to be inspected, is mainly applied to detect cracks where the expected direction of the flaw is determined to some extent.
Eddy current flaw detection is applied to detect bit flaws that only extend in the thickness direction.

The former magnetic flaw detection method is excellent for detecting surface defects in ferromagnetic materials such as steel materials. Even defects such as ground scratches without open cracks can be detected. It has the advantage that it is possible to detect the position of the defect and its length on the surface.

Furthermore, in the latter eddy current flaw detection method, the flaw detection results are directly obtained as electrical output. Since it is non-contact, testing speed is fast. Suitable for detecting surface defects. It can also track defects, material changes, dimensional changes, etc., and has a wide range of applications. It has advantages such as a substantially proportional relationship between the signal and the defect volume.

In addition, the magnetic flaw detection method described above is effective when magnetized in the direction perpendicular to the flaw, but if it is magnetized in the same direction as the flaw, no magnetic pole is generated at the flaw, so there is no magnetic pole from the surface of the material to be inspected. The leakage magnetic flux was so small that flaw detection was impossible. However, as shown below, it has now become possible to detect flaws regardless of their direction by using a method that uses multiple magnetic fields.

For example, as shown in FIG. 1, a round steel bar 1 is directly energized in the axial direction to magnetize it in the circumferential direction, and a coil surrounding the round bar 1 is energized to be magnetized in the axial direction.
A method is known in which the former detects surface flaws 1a in the circumferential direction, and the latter detects surface flaws 1b in the axial direction.

In addition, as shown in FIG.
A pair of coils 2, 2 and magnets 3 with magnetic poles facing each other on both sides of the tube 1' in the diametrical direction are arranged in tandem, and the former magnetizes the tube 1' in the axial direction, and the magnetic field generates magnetization in the circumferential direction. The surface flaw 1'b is detected by the magnetic field detector 2a, and the latter magnetizes the tube material 1' in the circumferential direction, and the magnetic field detects the surface flaw 1'a in the axial direction by the magnetic field detector 3a.
Detection methods are also known.

However, in addition to cracking defects, there are also defects called pit defects that occur on the surface of metal materials, and it is difficult to detect pit defects by the magnetic flaw detection described above. Therefore, if it is necessary to detect pit flaws, it is necessary to rely on eddy current flaw detection. For this reason, depending on the material to be inspected and the nature of the flaw, it is necessary to use a plurality of flaw detection methods, which is cumbersome.

When multiple flaw detection methods are applied, it is necessary to install dedicated flaw detection equipment for cracks and pit flaws on the pass line of the inspected material, which increases the size of the equipment and increases the cost. However, since each flaw detection device performs the inspection independently, there was a drawback that the inspection cost was high. Further, as mentioned above, in the conventional magnetic flaw detection method, the means for magnetizing the material to be inspected is large, and the detection part has a poor ability to follow the material to be inspected, making it difficult to perform accurate detection.

〔the purpose〕

The present invention was made in view of the above circumstances, and consists of two coils whose axes are located on the same line along the surface of the material to be inspected, and the maximum value (or minimum value) of their waveforms can be measured. By applying excitation currents of different frequencies so that the phases match each other, and so that the phase of one local maximum value (or minimum value) and the phase of the other local minimum value (or maximum value) match. , a magnetic field oriented along this and a magnetic field oriented perpendicular to this are periodically formed on the surface of the material to be inspected facing both coils, and magnetic flux changes of these magnetic fields are provided between both coils. Magnetic flaw detection is a method of synchronous detection using a magnetic field detector.
It is an object of the present invention to provide a flaw detection method that performs eddy current flaw detection intermittently and can accurately detect flaws such as cracks and pit flaws regardless of the type thereof, and an apparatus used for carrying out the flaw detection method.

[Structure of the invention]

In the flaw detection method according to the present invention, two
An excitation current of an appropriate frequency is applied to one of the two coils, and an excitation current of an integral multiple of the frequency is applied to the other coil, so that the phase of the maximum value (or minimum value) of the waveform of one excitation current is the maximum value of the waveform of the other excitation current. and energizing while adjusting the phase so that there is a period that substantially coincides with the minimum value,
A magnetic field oriented along the surface of the material to be inspected facing both coils and a magnetic field oriented perpendicular thereto are periodically formed, respectively, and a magnetic field detector provided at this portion is used to detect each of the coils. It is characterized by detecting changes in magnetic flux when a magnetic field is formed.

〔principle〕

First, the principle of the present invention will be explained. As shown in FIG. 3, excitation coils 31a and 31b are respectively arranged at positions spaced apart from each other by the same distance in the left-right direction from directly above the center of the steel pipe 11, which is a material to be inspected, which is phased in the axial direction. The axes of both excitation coils 31a and 31b face the tangential direction of the steel pipe 11, and share the same axis. A magnetic field detector 32 made of a magnetosensitive diode is provided at the center of the excitation coils 31a, 31b at a position slightly upwardly separated from the uppermost side surface of the steel pipe 11.

In this configuration, the coil 31a is supplied with an exciting current having a frequency f as shown in FIG. 4a from the first oscillator 41a. Further, as an example, the coil 31b is supplied with an excitation current of double frequency 2f from the second oscillator 41b as shown in FIG. There is a period in which the phase of the maximum value (or minimum value) matches the phase of the maximum value (or minimum value), and the phase of the maximum value (or minimum value) matches the phase of the minimum value (or maximum value) of the excitation current of frequency f. Turn on electricity. That is, the fourth
As shown in the figure, the current is applied so that the phase of the excitation current of frequency 2f applied to the excitation coil 31b lags the phase of the excitation current of frequency f applied to the excitation coil 31a by 45 degrees (or 135 degrees). That's what I do. Assuming that the winding directions of the coils 31a and 31b are the same, the maximum value (or minimum value) of both currents
During the period in which the two currents match, the directions of the magnetic fields due to both currents on the surface of the steel pipe 11 become the same. In this case, a magnetic field along the surface of the steel pipe 11 (hereinafter referred to as a co-directional magnetic field) is formed in the surface layer of the steel pipe 11 located below both excitation coils 31a and 31b, as shown in FIG. 5a. On the other hand, during the period in which the phase of the minimum value (or maximum value) of the current with frequency f and the phase of the maximum value (or minimum value) of the current with frequency 2f match, the directions of the magnetic fields due to both currents on the surface of the steel pipe 11 are opposite. become.
Therefore, in this case, both excitation coils 31a, 31b
As shown in FIG. 5b, a magnetic field perpendicular to the surface of the steel pipe 11 (hereinafter referred to as a different direction magnetic field) is formed between the two excitation coils 31.
An eddy current centered on the magnetic field is generated on the surface of the steel pipe 11 located below the magnetic field a and 31b. Such magnetic fields in the same direction and magnetic fields in different directions appear periodically in accordance with the waveform of the excitation current of frequency f, as shown in FIG.

Therefore, if a crack C [see Fig. 5a] exists on the surface of the steel pipe 11 when a codirectional magnetic field is formed, magnetic flux leaks from the codirectional magnetic field at the crack as shown in Fig. 5a. . This leakage magnetic flux is detected by a magnetic field detector 32 that detects a magnetic field in a direction perpendicular to the surface of the steel pipe 11.

On the other hand, if a pit flaw P (see Fig. 5b) exists on the surface of the steel pipe 11 when a magnetic field is formed in a different direction, the direction of the eddy current is disturbed at the pit flaw, and the magnetic field due to the eddy current is accordingly disturbed. This is magnetic field detector 3
Detected at 2. And these magnetic field detectors 3
The outputs when the two magnetic fields are generated in the same direction and when the magnetic fields are generated in different directions are synchronously detected by synchronous detection circuits 43a and 43b connected to the magnetic field detector 32, respectively. This makes it possible to detect and differentiate cracks C and pit defects P. In addition, the phases of the local maximum values (or local minimum values) of both excitation currents or the phase of one local maximum value (or local minimum value) and the other local minimum value (or local maximum value)
It is not necessary for the phases of the two to match completely, but it is sufficient that the phases are within a range that can form a magnetic field in the same direction and a magnetic field in different directions.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on drawings showing embodiments thereof. Fig. 6 is a schematic front view showing the structure around the detection part of the device used to implement the flaw detection method according to the present invention, Fig. 7 is a left side view thereof, and Fig. 8 is a schematic front view showing the structure around the detector holder of the detection part. An enlarged front sectional view,
FIG. 9 is a left half sectional view thereof.

In the figure, reference numeral 11 denotes a steel pipe, which is rotated about its axis and transported in the axial direction by a transport device (not shown).
Above the transfer area of the steel pipe 11, a pair of arms 12, 12 which are spaced apart by an appropriate length in the transfer direction of the steel pipe 11 are pivotally supported by a drive means (not shown) so as to be rotatable in a vertical plane. A detection unit A of the device of the present invention is pivotally supported at the tips of the arms 12, 12 so as to be rotatable in a vertical plane. The detection part A is moved downward when the steel pipe 11 is transferred, and when the tip thereof passes, the arms 12 and 12 are rotated downward to lower the detection part A and come into contact with the upper surface of the steel pipe 11. flaw detection is carried out. If the steel pipe 11 is not below the detection part A, the arms 12 and 12 are rotated upward to be raised to a predetermined retracted position. The vertical rotation of the arms 12, 12 is performed by a tube end detection signal from a photo sensor (not shown) installed in the transfer area of the steel tube 11.

Next, the detection section A will be explained. Arm 12,
A rectangular detection unit mounting plate 13 is pivotally connected to the distal end of the detection unit 12 by horizontal connecting pins 14, 14 so as to be swingable in the left-right direction. Cylindrical follow-up roller support members 15, 15 are vertically disposed on the left side of the lower surface of the mounting plate 13, and follow-up roller support members 15a, 15a each having an inverted U-shape in side view are provided at their tips. On the lower right side of the mounting plate 13, a similar follow-up roller support member 16 (only the front side is visible in the drawing) is connected via a support member 13a vertically provided on the lower surface of the mounting plate 13, with its longitudinal direction being the horizontal direction. installed. A follow-up roller support member 16a similar to the follow-up roller support member 15a is provided at its tip facing leftward. Each support member 15a, 15
a, 16a, follow rollers 17, 17... are attached to the steel pipe 1.
The following rollers 17, 17, . There is. Each follower roller 17,1
7... are in rolling contact with the upper surface of the steel pipe 11 rotating on its axis, allowing the detection part A to follow the steel pipe 11.

A support member 18 having a U-shape in side view is attached to the center of the lower surface of the mounting plate 13 . A cylindrical bearing housing 19 is fitted into the center of the bottom surface of the support member 18 with its longitudinal direction facing up and down, and a thrust bearing 20 is fitted concentrically into the bearing housing 19. A sliding shaft 21 is fitted inside the thrust bearing 20, and a disc-shaped stopper 22 is fixed to the upper end of the thrust bearing 20 to prevent the sliding shaft 21 from coming off from the thrust bearing 20. The lower end of the sliding shaft 21 has a flange 21a with a slightly larger diameter, and a coil spring is attached between the flange 21a and the bearing housing 19 and surrounded by the sliding shaft 21 in order to bias the sliding shaft 21 downward. 2
8 has been inserted. A rectangular spring receiving plate 23 is fixed to the lower end of the flange 21a, and a holder support member 24 having a gate shape in front view is fixed to the lower surface of the spring receiving plate 23. Support member 24
Bearings 25, 25 are fitted into both side walls, respectively, with the axial direction being the left and right direction. Support member 24
A rectangular parallelepiped-shaped holder cover 26 with an open lower surface, which is slightly shorter in the left-right direction and longer in the front-rear direction than the support member 24, is loosely fitted between the supporting members 24, and a shaft provided at the center of the left and right side walls of the holder cover 26 is loosely fitted therebetween. The parts 26a, 26a are fitted into the inner rings of the bearings 25, 25. As a result, the holder cover 26 is attached to the bearings 25, 25.
It can freely rotate around its axis. Springs 27, 27 are engaged at predetermined positions between the spring receiving plate 23 and the holder cover 26 to urge the holder cover 26 downward. The springs 27, 27 and the coil spring 28 are bent into the steel pipe 11,
Bearing 2 of detection part A when there is deformation etc.
This is to absorb vibrations around the axes 5 and 25 and in the vertical direction, thereby improving the ability of the detection section A to follow the steel pipe 11.

A detector holder 29, which has the same dimensions as the holder cover 26 in plan view, is detachably attached to the lower edge of the holder cover 26. Coil bobbins 30a and 30b, which have a frame-like shape when viewed from the side and are long in the front-rear direction, are arranged at symmetrical positions on the detector holder 29. The coil bobbins 30a and 30b are arranged such that their axial center directions are in the left-right direction, and excitation coils 31a and 31b are wound in grooves on their peripheries. Excitation coil 3
The longitudinal lengths of 1a and 31b are selected to be appropriate lengths in consideration of the flaw detection area determined by the number of magnetic field detectors 32, 32, etc. or the feed rate of the steel pipe 11, which will be described below.

Eight (only four are shown in the drawing) magnetic field detectors 3 are located in the center between the excitation coils 31a and 31b.
2, 32... are arranged in series according to the length of the coils 31a, 31b in the front-rear direction, thereby expanding the flaw detection area. The magnetic field detectors 32, 32... use magnetically sensitive diodes as detection elements, and the excitation coil 3
The magnetic field detectors 32, 32...
A predetermined electrical signal is input to a signal processing circuit B (see FIG. 10), which will be described later, through connectors 33, 33, . . . A U-shaped guide shoe 34 with an open top when viewed from the front is attached to the lower surface of the detector holder 29. The guide shoe 34 is made of a highly wear-resistant material, such as ceramics, and is used to prevent the detector holder 29 from being worn.

Next, the signal processing circuit B will be explained based on FIG. FIG. 10 is a block diagram of the signal processing circuit B. The coil 31a is supplied with a high frequency current of 2.5 kHz as shown in FIG. 11a from the first oscillator 41a, and the coil 31b is supplied with a high frequency current of 2.5 kHz as shown in FIG. 11a from the second oscillator 41b as shown in FIG.
A high frequency current of 5 kHz as shown in is applied. Note that the frequency of the high-frequency current applied to the coil 31b is not limited to a frequency that is twice the frequency applied to the coil 31a, and as described above, the frequency of the high-frequency current applied to the coil 31b is not limited to a frequency that is a multiple of the frequency applied to the coil 31a. (or local minimum value) phase and the other local minimum value (or local maximum value)
It is sufficient if the frequency is an integer multiple and satisfies the condition that a period in which the phase of

A phase shifter 42 is used to adjust the phase of the output of the second oscillator 41b using the output of the first oscillator 41a. That is, so that there is a period in which the phase of the maximum value (or minimum value) of the current waveform shown in FIG. 11a matches the phase of the maximum value (or minimum value) of the current waveform shown in FIG. 11b, In addition, the phase is adjusted so that there is a period in which the phase of the minimum value (or maximum value) of the current waveform in FIG. 11a matches the phase of the maximum value (or minimum value) of the current waveform in FIG. 11b. In this way, in order to match the phases of the maximum values (or minimum values) or the phase of one maximum value (or minimum value) and the phase of the other minimum value (or maximum value) using frequency doublers, , the output of the second oscillator 41b is
It is delayed by 45 degrees with respect to the output of the oscillator 41a.

As a result, a magnetic field in the same direction is formed on the surface of the steel pipe 11 during the period in which the phases of the maximum values (or minimum values) of the current waveforms shown in FIGS.
Magnetic fields in different directions are formed during a period in which the phase of the minimum value (or maximum value) and the phase of the maximum value (or minimum value) of the current waveforms shown in FIGS. 1a and 1b match. Each of the magnetic field detectors 32, 32, . . . detects changes in magnetic flux of these magnetic fields, and outputs electric signals corresponding to the changes in magnetic flux to synchronous detection circuits 43a and 43b, respectively. The synchronous detection circuit 43a receives signals from the first oscillator 41a to the first
As shown in Figure 1c, a synchronization pulse of an appropriate width that appears during the period when the maximum values of the current waveforms in Figure 11a and b match is input, and each time the synchronization pulse is input, the magnetic field detector The input signals from 32, 32, . The detection result is obtained as a magnetic flaw detection type signal waveform having maximum and minimum values as shown in FIG. 12a. This detection result is recorded by the recorder 44a and is also input to the comparator 45a. A threshold value is set in the comparator 45a as a criterion for determining whether the crack C is harmful or harmless, as shown by the two-dot chain line in FIG. If it is large, it is inputted to a marking device (not shown) as a harmful flaw detection signal. As a result, the steel pipe 11 is marked.

On the other hand, the second oscillator 41 is connected to the synchronous detection circuit 43b.
As shown in FIG. 11b to FIG. 11d, a synchronization pulse of an appropriate width is input that appears during the period in which the phase of the minimum value of the current waveform in FIG. 11a and the phase of the maximum value of the current waveform in FIG. 11b match. Each time the synchronization pulse is input, input signals from the magnetic field detectors 32, 32, . . . are synchronously detected, and changes in magnetic flux of magnetic fields in different directions are detected. The detection result is obtained as an eddy current flaw detection type signal waveform having only the minimum value as shown in FIG. 12b. This detection result is recorded by the recorder 44b and is also input to the comparator 45b. A threshold value is set in the comparator 45b as a criterion for determining whether the pit flaw P is harmful or harmless, as shown by the two-dot chain line in FIG. If it is smaller, a harmful flaw detection signal is input to the marking device. As a result, the steel pipe 11 is marked.

Next, the effects of the present invention will be explained based on examples. FIG. 13 is a graph showing the detection results when artificial defects corresponding to cracks C and pit defects P are detected by the present invention. Figure 13a shows the output level of the magnetic field detector for detecting changes in magnetic flux in the same direction magnetic field and different direction magnetic field when an artificial defect corresponding to the crack C is detected, and the depth of the crack C as the vertical axis. This is a graph showing the horizontal axis. As is clear from the graph, in the case of crack C, the output level of the magnetic field detector for the magnetic field in the same direction appears to be sufficiently higher than that for the magnetic field in the different direction, so that the two can be clearly distinguished.

Fig. 13b shows the output level of the magnetic field detector for detecting magnetic flux changes in the same direction magnetic field and the different direction magnetic field when an artificial defect corresponding to the pit flaw P is detected, and the depth of the pit flaw P is plotted on the vertical axis. This is a graph showing the horizontal axis. As is clear from the graph, in the case of pit flaws P, the detection level of the magnetic field in the different direction appears sufficiently higher than that of the magnetic field in the same direction, so that the two can be clearly distinguished.

FIG. 14 shows cracks with different depths according to the present invention.
This is a chart showing the results of pit flaw detection.
FIG. 14a shows the magnetic flux change of the same direction magnetic field, and b shows the magnetic flux change of the different direction magnetic field, and the vertical axis shows the detection level of each. As is clear from the graph, it is possible to distinguish between the two. Furthermore, since the flaw depth and the detection level are in a proportional relationship, the flaw depth can be determined quantitatively. In addition, although the above-mentioned embodiment described the case where the present invention is applied to a steel pipe, it goes without saying that it can also be applied to steel materials such as slabs, and furthermore, it can be applied to other metal materials other than steel materials.

〔effect〕

As detailed above, in the case of the present invention, an excitation current of an appropriate frequency is applied to one of the two coils arranged in parallel on the same line along the surface of the material to be inspected, and an excitation current of an integral multiple of the frequency is applied to the other coil. , the phase is adjusted so that there is a period in which the phase of the maximum value (or minimum value) of the waveform of one excitation current substantially matches the phase of the maximum value and the phase of the minimum value of the waveform of the other excitation current. Since it was decided to energize, a magnetic field in the same direction and a magnetic field in different directions are periodically formed on the surface of the material to be inspected, and a magnetic field detector detects changes in the magnetic flux of the magnetic fields in the same direction and different directions when a flaw exists. Therefore, accurate detection can be performed regardless of the nature of the flaw, that is, regardless of the type of flaw such as cracks or pit flaws.
Unlike conventional methods, there is no need to apply multiple flaw detection methods depending on the type of flaw, and equipment costs and inspection costs can be reduced. Furthermore, since the detection unit can be made smaller and lighter, the ability to follow the surface of the material to be inspected can be improved, and highly accurate detection becomes possible, and the present invention has excellent effects.

[Brief explanation of the drawing]

Figures 1 and 2 are schematic diagrams showing the implementation state of the conventional method, Figures 3 to 5 are explanatory diagrams of the principle of the present invention, Figure 6 is a schematic front view showing the structure around the detection section of the device of the present invention, Fig. 7 is a left side view, Fig. 8 is a front sectional view showing an enlarged view of the detector holder, Fig. 9 is a left half sectional view thereof, Fig. 10 is a block diagram of the signal processing circuit, and Fig. 11 is a front sectional view showing the area around the detector holder. A signal waveform diagram for explaining the operation, FIG. 12 is a graph showing the detection results of the synchronous detection circuit, and FIG. 13 is a graph showing the results when artificial defects corresponding to cracks and pit defects are detected by the present invention. , FIG. 14 is a chart showing the results of detecting cracks and pit defects of different depths according to the present invention. 11... Steel pipe, 31a, 31b... Excitation coil, 32, 32... Magnetic field detector, 41a... First
Oscillator, 41b...second oscillator, 42...phase shifter, 43a, 43b...synchronous detection circuit, A...detection section, B...signal processing circuit.

Claims (1)

[Scope of Claims] 1. Two coils are arranged in parallel so that their axes are on the same line along the surface of the material to be inspected, one of which is excited with an appropriate frequency, and the other coil is excited with an integral multiple of the frequency. The phase of the current is adjusted so that there is a period in which the phase of the maximum value (or minimum value) of the waveform of one excitation current substantially matches the phase of the maximum value and the phase of the minimum value of the waveform of the other excitation current. energized to periodically form a magnetic field along the surface of the material to be inspected, and a magnetic field perpendicular to the coils, respectively, on the surface of the material to be inspected, facing the coils. A flaw detection method characterized by detecting changes in magnetic flux when each magnetic field is formed using a magnetic field detector. 2. first and second coils arranged in parallel so that their axes are on the same line along the surface of the material to be inspected; and a first oscillator that supplies a first excitation current of an appropriate frequency to the first coil. , A second excitation current having a frequency that is an integral multiple of the frequency is applied to the second coil, and the phase of the maximum value (or minimum value) of the waveform thereof is substantially the same as the phase of the maximum value and minimum value of the waveform of the first excitation current. a second oscillator that is energized so that the periods coincide with each other; a magnetic field detector that is provided between the first and second coils and faces the surface of the material to be inspected; a first detection circuit that synchronously detects the output of the magnetic field detector during a period in which the phases of the maximum value or the minimum value of the waveform substantially coincide; 2. A flaw detection apparatus comprising: a second detection circuit that synchronously detects the output of the magnetic field detector during a period that substantially coincides with the phase of the minimum value (or maximum value) of the waveform of the excitation current.