JPH0412266A - Method for non-destructive inspection of heating tube - Google Patents

Abstract

PURPOSE:To execute flaw detection with high accuracy without disassembling a heating tube by forming a winding shape of a probe coil to a closed shape having a linear part, and allowing the linear part to abut on the outside of the heating tube or to approach it. CONSTITUTION:A probe 10 is constituted by winding a pair of probe coils 12, 13 to a core 11 of bakelite whose cross section is semicircular, and forming its winding shape to a closed shape having a linear part. In such a state, the linear parts of the coils 12, 13 are allowed to abut on the outside of a heating tube 21 being a measuring object or to approach it, and by moving them gradually in the lengthwise direction, a flaw detection is executed. In this case, a magnetic flux generated from the coils 12, 13 passes more through the inside of the heating tube 21 centering around linear parts 22, 23, respectively, and the heating tube 21 is allowed to generate an eddy current being orthogonal to the magnetic flux. If there is a flaw on the heating tube 21, a pair of coils 12 13 are not balanced well, therefore, with respect to a flaw such as a crack, etc., generated from the inside thereby, a vortex flaw detection can be executed efficiently from the outside of the heating tube 21.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method of non-destructively testing heating tubes (referred to as heating radiation tubes and heat exchanger tubes) using eddy currents.

[Conventional technology] Heating radiant tubes (also called radiant tubes) used in the annealing process at steel mills tend to crack from the inside because high-temperature gas flows inside them, and inspection for these cracks is currently done by visual inspection from the outside. It was only done by.

On the other hand, the eddy current flaw detection method has been used for heat exchangers and the like for some time. To give an overview of this method, a probe coil is inserted into the metal tube to be detected, and an alternating magnetic field is generated in this coil to cause eddy current to flow through the tube. A current is induced, but if there are defects such as thinning or cracks in the pipe, the eddy current will be disturbed.
This changes the impedance of the probe coil, and the tube is tested for flaws by analyzing this change in impedance as a flaw detection signal.

In this case, flaw detection of the tube is also carried out by inserting the tube into the probe coil.

[Problem to be solved by the invention]

As mentioned above, visual inspection of heating radiant tubes can only detect through-hole defects where cracks from the inner surface are exposed to the outside, so replacement of heating radiant tubes is limited to tubes with the above-mentioned through-hole defects and tubes that have been significantly deformed. was done, and therefore the defects were overlooked and the tubes that did not need to be replaced were replaced.

Therefore, it is most preferable to non-destructively inspect internally generated flaws from the outside, and the inventors of the present invention have thought that it would be appropriate to use the eddy current flaw detection method as described above, and have conducted extensive research on this method.

However, since the heating radiant tube is meandering, it is necessary to partially disassemble the heating radiant tube to perform the inspection by inserting the probe coil inside the heating radiant tube as described above, which is difficult to implement. The problem is that it is time consuming.

Therefore, it is theoretically possible to perform flaw detection by forming the probe coil so that the heating radiant tube is located inside the probe coil, but the heating radiant tube is subject to considerable deformation due to heat and cannot be smoothly circular. There was a problem in that the probe coil could not be brought into contact with the surrounding area.

There is also a method of inspecting the surface of the heating radiant tube using a probe coil that uses a magnetic core, but if you use a magnetic core, it becomes a pencil-type probe, and you will be able to detect flaws near the point. When inspecting a heating radiant tube with a relatively large diameter, there was the problem of extremely low efficiency.Furthermore, a probe coil with a circular cross section was used, and the probe coil was placed parallel to the axis of the heating radiant tube. Although it is possible to perform flaw detection on the heating radiant tube, the magnetic field generated by the coil does not effectively penetrate the heating radiant tube.
There was a problem with poor measurement accuracy.

The present invention was made in view of the above circumstances, and it is an object of the present invention to provide a non-destructive testing method for heating tubes that has relatively good measurement accuracy and can be carried out without requiring the trouble of disassembling the heating tube. purpose.

[Means to solve the problem]

A method for non-destructive testing of a heating tube according to claim 1, which meets the above object, is an island flow flaw detection method in which a probe coil is placed outside the heating tube to detect shear cracks that occur in the heating tube.
The probe coil is formed by a pair of coils, and the winding shape of the coil is a closed shape with a straight part, and the straight part of the pair of coils is brought into contact with or close to the heating tube from the outside to perform flaw detection. It is configured to do this.

Here, the closed shape having a straight line portion refers to a shape obtained by dividing a closed shape such as a circle, an ellipse, a polygon, or an egg shape by straight lines.

The method for non-destructive testing of heating tubes according to claim 2 is characterized in that in the method according to claim 1, the alternating current flowing through the probe coil is a mixture of alternating currents of two different frequencies.
It consists of multiple frequency alternating current.

[Effect]

In the method for non-destructive testing of a heating tube according to claim 1, the probe coil is constituted by a pair of coils, and the winding shape of the coil is constituted by a closed shape having a straight part. If it touches or comes close to a heating tube,
More magnetic flux generated from the probe coil can pass through the inside of the heating tube, thereby generating an eddy current that is perpendicular to the magnetic flux, and if there is a crack etc. that intersects with the eddy current, the paired coil The balance is disrupted, and cracks, etc. can be detected by this.

In the method for non-destructive testing of heating tubes according to claim 2, since alternating currents of different frequencies are mixed and applied to the probe coil, the magnetic flux generated by the high-frequency alternating currents flows near the surface of the heating tube, and Since the lower alternating current flows from the surface of the heating tube to the middle layer, this allows the detection depth to be expanded, and by calculating each detected signal value, noise can be removed and only the signal can be output. I can do it.

[Examples] Next, examples embodying the present invention will be described with reference to the same accompanying drawings to provide an understanding of the present invention.

Here, FIG. 1 is a cross-sectional view showing a state in which a method for non-destructive testing of heating tubes according to an embodiment of the method of the present invention is applied, and FIG.
The figure is a perspective view of the probe used in the above embodiment, and FIG. 3 is a schematic configuration diagram of the flaw detection apparatus used in the above embodiment.

As shown in FIG. 2, first, a probe 10 used in a method for non-destructive testing of heating tubes according to an embodiment of the present invention is shown.
As shown in the figure, a groove having a depth of 1 to 3 mm (preferably 2 mm) is provided around a heckleite core 11 having a semicircular cross section, and probe coils 12 and 13 are wound around this groove.

Each of the probe coils 12 and 13 has about 200 turns in this embodiment, and the interval between them is a=5 mm.
b=about 4 mm, and the dimension of the core 11 is c=50
mm, d=50 mm.

As shown in FIG. 3, this probe 10 is connected to an eddy current flaw detection device 14 having a well-known structure.
The transmitter 15 transmits three frequencies of 10kHz and 40kHz.
16, mixed by mixer 17, and added to bridge circuit 18. This bridge circuit 1
8 is connected to the probe coils 12 and 13, the voltage source is the three-frequency mixed voltage, the output of the bridge circuit 18 is detected, amplified appropriately with an amplifier, and then compared with the reference frequency to determine its phase and After detecting the absolute value and adding the necessary phase rotation, vector outputs (x+-yz) and (Xy 2) corresponding to 10 kHz and 40 kHz are output.

Add this to the next cathode ray tube display device 19, and
The RT 20 is configured to draw the above vector output and xy small output rissage figures obtained by appropriately adding and subtracting the vector output.

Therefore, when inspecting the heating radiant tube 21 (SCH22) as shown in FIG. A test pipe is prepared, the probe 10 is brought into contact with it so that the center of the straight portion 22.23 of the probe coil 12.13 exactly coincides with the side line of the test pipe, and the above device 14.19 is used to detect noise. Adjust the whole thing so that it doesn't happen. This adjustment is performed while displaying on the CRT 20 two frequency signals of 10 kHz and 40 kHz, a signal obtained by rotating the phase of each signal, and a signal obtained by adjusting the phase of each signal.

Next, the probe 10 is brought into contact with the heating radiation tube 21 to be measured in the direction shown in FIG. 1, and this is gradually moved in the lengthwise direction (in some cases circumferential direction) .

In this case, the magnetic flux generated by the 10-build coils I2 and I3 passes through the inside of the heating radiation tube 21 centering on the straight portions 22 and 23, respectively, and passes through the heating radiation tube 21, which is a conductor.
1 generates eddy currents, which act as effective resistance and change impedance.

If there is a flaw in the heating radiation tube 21, the eddy current will change, and the balance of the bridge circuit 18 of the eddy current flaw detector 14 will be lost depending on the position with respect to the probe coil 12, 13, causing an output to be generated.

When eddy current flaw detection was performed using the above method, the experimental results shown in Table 1 were obtained.

Table 1 Therefore, from this table, by using the above method,
Scratches, cracks, etc. located 3 mm from the outer surface can be reliably detected.

In the above embodiment, a coil with a semicircular winding shape was used, but it is also possible to use a coil with a rectangular or square winding shape if it has a straight part. It is also possible to change the number of turns depending on the frequency or detection depth.

In the above embodiment, dual-frequency alternating current is passed through the probe coils 12 and 13 to remove noise and expand the measurement depth, but depending on the conditions, single-frequency alternating current corresponding to the detection depth may be used. It is also possible to do so.

[Effects of the Invention] As is clear from the above explanation, the method for non-destructive testing of a heating tube according to claim 1 involves bringing a probe coil having a linear portion into contact with or very close to the outside of the heating tube. By passing a large amount of magnetic flux through the heating tube, changes in the generated eddy current made it possible to detect cracks occurring from the inside of the heating tube from the outside, which was difficult to measure using conventional cylindrical coils. .

In this case, the above-mentioned probe coil has a straight section, and inspection can be performed while simultaneously contacting or very close to the surface of the heating tube over a long distance, compared to when using a conventional pencil-type probe. This made it possible to perform eddy current flaw detection efficiently.

In the method for non-destructive testing of heating tubes according to claim 2, alternating current with different frequencies is mixed and applied to the probe coil, so flaw detection can be performed from shallow to deep ranges, and furthermore, each of the detected Since the noise can be removed by calculating the signal value,
Defects can now be detected more accurately.

[Brief explanation of drawings]

Fig. 1 is a sectional view showing a state in which a non-destructive inspection method for heating tubes according to an embodiment of the method of the present invention is applied, Fig. 2 is a perspective view of a probe used in the above embodiment, and Fig. 3 is a It is a schematic block diagram of the flaw detection apparatus used in the said Example. [Explanation of symbols]

Claims (2)

[Claims] (1) In an eddy current flaw detection method in which a probe coil is placed outside a heating tube to detect cracks occurring in the heating tube, the probe coil is formed by a pair of coils, and the winding shape of the coil is changed to a straight part. A closed shape having
A method for non-destructive testing of a heating tube, characterized in that flaw detection is performed by bringing the straight portions of the pair of coils into contact with or close to the heating tube from the outside. (2) Claim 1, wherein the alternating current flowing through the probe coil is a dual frequency alternating current that is a mixture of alternating currents of two different frequencies.
Non-destructive testing method for heating tubes as described in section.