JP2000065801A - Inspecting mehtod for damage inside metal tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inspect a whole area conveniently and efficiently eliminating the effect of magnetic inhomogeneity on a ferromagnetic metal tube with a non- magnetic metal layer formed therein, particularly on a hot-dip aluminum-plated carbon steel tube. SOLUTION: In this inspecting method, an unused tube and a plurality of tubes at different damage stages are tested for flaws by an insertion probe type eddy current flaw detector, and on the basis of the information therefrom a reference graphic is formed. By likewise forming a graphic about a metal tube being inspected and by comparing it with the reference graphic, a damage stage of the metal tube being inspected is determined. Also, a permanent magnet 202 with a weak magnetic force is arranged on both sides of a test coil 201 in a probe 200.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for inspecting internal damage in a ferromagnetic metal tube having a non-magnetic metal layer formed on its inner surface, particularly a carbon steel tube having a molten aluminum plating layer formed on its inner surface. About.

[0002]

2. Description of the Related Art Heat exchangers utilizing seawater for cooling water include:
Copper alloy pipes are generally used, but when copper alloy pipes cannot be used for process reasons, carbon steel pipes with hot-dip aluminum plating (hereinafter referred to as hot-dip aluminum-coated carbon steel pipes) can be used. is there.

The aluminum plating layer in the hot-dip aluminum-plated carbon steel pipe functions not only as a coating material for carbon steel, but also as a sacrificial material for corrosion prevention utilizing a corrosion potential difference between aluminum and carbon steel. However, once the aluminum plating layer has disappeared due to seawater,
Due to the corrosion potential difference between the alloy layer and the carbon steel, local corrosion of the carbon steel can rapidly progress from the fine defects etc. existing in the alloy layer, which may lead to a tube opening and cause a serious leakage accident. high.

Therefore, when using a hot-dip aluminum-coated carbon steel pipe, it is important to quickly and surely find a place where the aluminum plating layer has disappeared or a place where the carbon steel has locally corroded. And has been devised.

First, the most commonly used methods include a visual inspection using a fiberscope and a water immersion ultrasonic inspection test. However, these methods have a low inspection speed, and require a great deal of labor to inspect a unit having hundreds of metal tubes per unit, such as a heat exchanger. Further, depending on the pretreatment state and the damage form, not all defects can be always detected, and there is a problem in reliability.

[0006] Next, there is a method using an eddy current flaw detection device of an interpolation probe type. Generally, this method inserts a probe into a metal tube and receives signals from the damaged part.In this case, use a self-comparison type probe with a pair of test coils arranged in parallel. There are many. The self-comparison probe detects a difference between two adjacent portions of the test sample. Therefore, it responds well to local changes such as drill holes, and conversely, gently changes due to subtle changes in composition, shape, dimensions, changes in relative position due to transport vibration, changes in ambient temperature, etc. The effect of factors other than the purpose of the inspection is offset by the two coils, and the high inspection accuracy is excellent.

However, when the metal pipe base material including the hot-dip aluminum-plated carbon steel pipe to be inspected here is a ferromagnetic material, noise is generated due to a change in magnetic permeability, and the so-called S / N ratio is remarkably increased. This makes it difficult to detect a defect signal. Therefore, the above-mentioned eddy current inspection is not applicable to ferromagnetic metal tubes as it is, even though it is a general technique for maintenance inspection of non-magnetic metal tubes.

Therefore, a high-frequency alternating current of a plurality of frequencies is applied to the test coil, and a magnetic field having a fundamental frequency component and a magnetic field having a higher frequency component are added to a metal tube.
The method of synthesizing (mixing) detection signals so that noise components of signals obtained from different types of magnetic fields are offset,
For example, it is disclosed in “Non-Destructive Inspection Vol. 42, No. 2, PP 86-93”, and it is possible to remove the influence of magnetic non-uniformity due to a ferromagnetic material by this method.

When an alternating current is applied to the test coil, a direct magnetic field and an indirect magnetic field are generated around the exciting coil.
The general eddy current inspection method described above is a method of detecting a change in an eddy current as a change in an indirect magnetic field. At this time, the method is also affected by a direct magnetic field. Therefore, there is no problem when directly inspecting a non-magnetic metal tube in which the magnetic field is uniform as described above. Will cause trouble. However, at a position two to three times the diameter of the tube, there is a region called a remote field where the direct magnetic field is reduced and almost only an indirect magnetic field is present. Here, since there is almost no noise due to non-uniform magnetic field, this region is used. Method for measuring indirect magnetic fields in the field (remote field inspection, abbreviated as R
FT) is known as a particularly effective method for inspecting ferromagnetic metal tubes.

[0010]

However, in the method of eliminating magnetic non-uniformity by mixing, a skilled technician having sufficient skills and experience is required for performing the inspection. That is, a high level of skill is required for optimal combinations of test frequencies and setting of mixing conditions, and thus cannot be said to be a simple inspection method.

Further, in the RFT, there is a limit to a defect size that can be detected with respect to local thinning, and there is an untestable area, that is, a dead zone near the support plate of the metal tube. In order to prevent leakage accidents due to local corrosion as described above in hot-dip aluminum-coated carbon steel pipes,
Since full-length inspection is required, it is necessary to use another inspection method in combination with this part, which is inefficient.

An object of the present invention is to eliminate the effects of magnetic non-uniformity on a ferromagnetic metal tube having a non-magnetic metal layer formed on the inner surface, particularly a hot-dip aluminum carbon steel tube. An object of the present invention is to provide an internal damage inspection method that is possible, and that can easily and efficiently inspect all areas.

[0013]

According to the present invention, there is provided a method for inspecting the inside of a metal tube for damage to solve the above-mentioned problem. A metal tube and a metal tube at different stages of damage are subjected to a flaw detection test using an interpolated probe type eddy current flaw detector having magnetic saturation means, a reference pattern is established from the information obtained at this time, and the same applies to the metal tube to be inspected. The damage stage of the metal tube to be inspected is specified by comparing the figure obtained in (1) with the reference figure. Thereby, the magnetic non-uniformity due to the ferromagnetic material is eliminated, and the inspection can be performed without being affected by the non-uniformity. Further, the damage stage of the metal tube can be determined only by comparing the figures, which is simple.

Further, the metal pipe internal damage inspection method of the present invention is characterized in that an eddy current flaw detector having a self-comparison type probe is used. This makes it possible to detect a difference between two adjacent portions of the test body, and reliably detect a thinning due to local corrosion of the metal tube.

Further, the metal tube internal damage inspection method according to the present invention is characterized by using an eddy current flaw detector in which weak magnetic permanent magnets are arranged on both sides of a test coil of a probe. Thereby, the magnetic non-uniformity due to the ferromagnetic material is eliminated, and the inspection can be performed without being affected by the non-uniformity. In addition, it is excellent in scannability and has a compact shape.

Further, in the metal pipe internal damage inspection method according to the present invention, the frequency of the alternating current applied to the test coil is determined in accordance with the depth in the thickness direction of the metal pipe requiring inspection. . Thus, it is possible to inspect a desired depth based on the penetration depth of the magnetic field determined by the type and frequency of the constituent material.

Further, the metal tube internal damage inspection method according to the present invention is characterized in that the frequency of the alternating current applied to the test coil is determined according to the constituent material of the metal tube to be inspected. Thus, it is possible to inspect a desired depth based on the penetration depth of the magnetic field determined by the type and frequency of the constituent material.

Further, the metal pipe internal damage inspection method of the present invention has a magnetic saturation means for an unused pipe and a used pipe at a plurality of different damage stages in a carbon steel pipe having a molten aluminum plating layer formed on an inner surface thereof. By performing a flaw detection test with an interpolated probe type eddy current flaw detector, establishing a reference figure by graphicizing the information obtained by this test, and comparing the figure obtained similarly with the reference figure with respect to the inspection target pipe by It is characterized in that the damage stage of the tube to be inspected is specified. Thereby, the magnetic non-uniformity due to the ferromagnetic carbon steel is eliminated, and the inspection can be performed without being affected by the magnetic non-uniformity. Further, the damage stage of the hot-dip aluminum-coated carbon steel pipe can be determined only by comparing the figures, which is simple.

Further, in the metal tube internal damage inspection method of the present invention, an alternating current of 100 to 400 KHz is applied to the test coil. As a result, the penetration depth of the eddy current is limited to a range in which the eddy current is magnetically saturated by the weak magnetic permanent magnet, the generation of magnetic noise by the lower carbon steel is suppressed, and the inspection can be performed reliably.

In the present invention, in a hot-dip aluminum-plated carbon steel pipe, an eddy current flaw detection test is performed using an unused pipe and a used pipe in a plurality of different damage stages as model pipes, and information obtained by this test is graphically drawn. Using the established reference figure, a comparison is made with the figure obtained in the same manner for the pipe to be inspected. The damage stage may be appropriately determined according to the maintenance management purpose, and the same applies to the number of damage stage divisions. For example, in the hot-dip aluminum-plated carbon steel pipe 100 shown in FIG. 1, (a) a stage in which the aluminum-plated layer 103 remains thick over the entire surface; and (b) the aluminum-plated layer 103 remains almost entirely but partially disappears. (C) The stage where the disappearance of the aluminum plating layer 103 progresses and the alloy layer 102 is partially exposed. (D) The stage where the aluminum plating layer 103 almost disappears and most of the alloy layer 102 is exposed. (E) A stage in which the aluminum plating layer 103 has completely disappeared and the entire surface of the alloy layer 102 is exposed. (F) An alloy layer 1 is located at a position starting from a latent minute defect.
02 and the stage at which local thinning of the carbon steel 101 occurs.

In the present invention, an interpolation probe type eddy current testing device is used. At this time, it is desirable to provide a magnetic saturation means in order to suppress the magnetic non-uniformity due to the ferromagnetic material, improve the S / N ratio, and perform a highly accurate inspection. In the conventional magnetic saturation means, it is common to use a permanent magnet having a strong magnetic force or use a coil for magnetic saturation. In the present invention, however, a very shallow surface of the inner surface of a hot-dip aluminized carbon steel pipe is used. Since it is sufficient that only the layer can be magnetically saturated, as shown in FIG.
02 arranged on both sides of the test coil 201
0 was used. The low-magnetism permanent magnet 202 is thinner and smaller than conventional high-magnetism permanent magnets and coils, so that the probe can be reduced in weight and size, and scanning performance is good because the magnet is less attracted.

The probe used at this time is a self-comparison type probe for the purpose of reliably detecting wall thinning due to local corrosion and unevenness of the aluminum plating layer.

The probe is inserted into the tube and scanned while applying an alternating current to the test coil. The relationship between the frequency of the applied alternating current and the penetration depth of the magnetic field is specific to the material. Therefore, selection of the frequency is performed according to the constituent material of the metal tube to be inspected, the inner surface magnetic saturation depth, and the like.

[0024] For the hot-dip aluminum-coated carbon steel pipe, the frequency is determined by the thickness of the aluminum-plated layer and the height of the irregularities and the inner surface magnetic saturation depth depending on the strength of the permanent magnet. The relationship between frequency and penetration depth in carbon steel and aluminum is as shown in FIG. The depth of penetration of the alloy layer is unknown, but is presumed to be close to that of steel. Generally, the thickness of the aluminum plating layer is about 100 μm, and the thickness of the alloy layer is also about 100 μm. When an alternating current of 100 KHz is applied, according to FIG. 3, the penetration depth of the aluminum plating layer is several hundred μm. In general, the irregularities of the aluminum plating layer can be sufficiently detected at this frequency. On the other hand, the penetration depth of carbon steel is several tens of μm, while applying an alternating current of 100 KHz to a carbon steel pipe that has not been subjected to hot-dip aluminum plating by using the eddy current flaw detection probe provided with the above-described weak magnetic permanent magnet. As a result of the test, almost no noise due to the magnetic non-uniformity of the carbon steel was detected, and it was found that the inner layer thickness of several tens of μm in the alloy layer and the carbon steel was sufficiently magnetically saturated. From the above, unless the thickness of the aluminum plating layer or the height of the irregularities is particularly large, 100K
If an alternating current having a frequency of not less than Hz is applied, accurate inspection can be performed.

On the other hand, when an excessively high frequency is applied, the penetration depth becomes shallow, and when the unevenness height of the aluminum plating layer is large, the detection sensitivity is lowered by lift-off. Further, when the penetration depth into the alloy layer becomes shallow, the detection sensitivity of the wall thinning due to local corrosion also decreases. Therefore, it is desirable to apply a frequency of 400 KHz or less.

The latest eddy current flaw detector has a function of performing recording while simultaneously applying a plurality of frequencies to the test coil. With such a device, 100-400 KH
The flaw detection test can be performed by applying the frequency of z and selecting the frequency that is most easily identified according to the situation.

According to the above-described procedure, various model metal tubes are inspected, and the obtained information is made into a graphic. The shape is
The conventional output to the XY chart and the vector display expressed in a two-dimensional plane are used. By comparing the reference graphic established in this inspection with a graphic obtained similarly for the inspection target tube, the damage stage of the inspection target tube is specified. The reference graphic at each damage stage has a unique pattern that can be distinguished at a glance. Therefore, it is easy to specify which reference figure corresponds to the figure of the pipe to be inspected, and no special skill is required for the inspection.

In addition, the inspection method is to measure the direct magnetic field instead of measuring the indirect magnetic field as in RFT,
In addition, unlike the normal eddy current inspection, only the shallow layer on the inner surface is to be inspected, so that the entire length can be inspected in the vicinity of and just below the support plate without any trouble.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below. In this example, for a hot-dip aluminum-plated carbon steel pipe used in a heat exchanger using seawater for cooling water, a model pipe at each damage stage was selected, a reference pattern was established, and a flaw detection test of the pipe used was performed. And the stage of damage.

FIG. 4 shows the configuration of the interpolated probe type eddy current flaw detector used in this embodiment. Probe 200
As shown in FIG. 2, a self-comparison type test coil 201 is arranged, and the weak magnetic permanent magnets 2 are provided on both sides thereof.
02 was used. This probe 200
The entire area inside the hot-dip aluminum-plated carbon steel pipe 100 is scanned while applying a high-frequency alternating current from the eddy current flaw detector 410 to the. The signal obtained at this time is recorded as an XY chart and a vector display by an output recording device 420 having an electronic recording medium.

Before conducting a flaw detection test on a hot-dip aluminized carbon steel pipe actually used in a heat exchanger,
First, as shown in FIG. 1 as a model brass at each damage stage, (a) a stage in which the aluminum plating layer 103 remains thick over the entire surface; and (b) a stage in which the aluminum plating layer 103 remains almost entirely. (C) A stage where the disappearance of the aluminum plating layer 103 progresses and a part of the alloy layer 102 is exposed. (D) A stage where the aluminum plating layer 103 almost disappears, and most of the alloy layer 102. (E) The aluminum plating layer 103 has completely disappeared and the entire surface of the alloy layer 102 has been exposed. (F) The alloy layer 1 is located at a position starting from a latent minute defect.
02 and a hot-dip aluminized carbon steel pipe 100 at the stage where the thinning of the carbon steel 101 due to local corrosion has occurred are prepared, and a flaw detection test is performed on these pipes to establish a reference figure.

As described above, the frequency of the alternating current applied to the test coil 201 is determined according to the thickness of the aluminum plating layer, the height of the unevenness, and the inner surface magnetic saturation depth caused by the magnetic force of the permanent magnet. In general, the thickness of the aluminum plating layer is 200 μm or less, and it is required to obtain a penetration depth of 200 to 400 μm in terms of aluminum including the alloy layer. Therefore, referring to FIG. 400 KHz. Here, four channel frequencies are used so that the most advantageous frequency for identification can be selected depending on the situation. However, since 100 KHz alone can sufficiently be identified, only the result of 100 KHz is expressed.

The information obtained by performing the flaw detection test according to the above procedure, that is, the received signal is formed into a graphic.
First, the phase is set. It is known that, when the surface of the aluminum has irregularities, the signal phase changes according to the height of the irregularities as shown in FIG. 5, and as the height increases, the signal rotates clockwise. It is also known that the signal phase of the thinned portion in the ferromagnetic alloy layer and carbon steel is constant regardless of the depth. Therefore, if a flaw detection test is performed by setting such a constant phase signal to 90 degrees, the relative change of the signal phase due to the change in the height of the unevenness of the aluminum plating layer becomes clear, and the reference figure, particularly, the vector display. Features at each damage stage can be revealed.

Therefore, a test piece having a drilled hole of 3, 4, 6, and 8 mmΦ was prepared for an unused hot-dip aluminized carbon steel pipe, and a flaw detection test was performed in the same manner as in the above procedure. The setting was made so that the phase of the signal obtained from the drill hole portion was 90 degrees. FIG. 6 shows an XY chart of the received signal and a vector display at this time. The signal from each through hole shows 90 degrees regardless of the opening diameter, though there is some variation.

Next, a flaw detection test was performed on an unused hot-dip aluminum-plated carbon steel pipe under the above-mentioned settings. FIG. 7 shows the result. The XY chart shows a continuous forest-like signal, and it is difficult to discriminate individual signals.
It is roughly divided into two groups of 0 degree and 220 to 250 degrees. This is considered to be a signal caused by surface irregularities in the aluminum plating layer. That is, 50 to 70 degrees are concave,
It is considered that 220 to 250 degrees corresponds to the signal due to the convex portion. This means that, as shown in FIG. 6, even in the inspection of the through-drilled hole test piece performed earlier, apart from the signal (phase 90 °) from the through-drilled hole, the phase is 50 to 70 degrees and the phase is 220 to 250 degrees. This is supported by the appearance of the signal in the section.

In conducting a flaw detection test on an unused hot-dip aluminized carbon steel pipe, it is preferable to adjust the flaw detection sensitivity at the same time. That is, the sensitivity is adjusted so that the received signal is drawn within an appropriate range with respect to the Y scale of the XY chart. Here, the adjustment is performed so that the signal (the signal due to the unevenness of the aluminum plating layer) is about 30% of the full scale. However, the adjustment is not limited thereto, and may be appropriately adjusted depending on the situation. If you use a recording medium whose sensitivity can be changed freely after recording,
Adjustment is easy and preferable in all situations.

Next, flaw detection tests were performed on the model tubes in each damage stage under the above conditions.
The results are shown in FIGS. It should be noted that the model tube in the stage where there is almost no damage in FIG. 1A is the same as in the unused state in FIG. 7, and is omitted to avoid duplication.

FIG. 8 shows the results of a flaw detection test for “a stage where the aluminum plating layer is almost completely left but partially disappears” in FIG. 1B. Looking at the vector representation, the phase of the signal is 50-70 degrees and 230-
Although it was 250 degrees and almost coincided with the signal phase in the above-mentioned unused tube, it was slightly rotated counterclockwise (standing vertically). When the phase signal rotates counterclockwise, it indicates that the height of the unevenness of the aluminum plating layer is reduced as described above. That is, the vector display in FIG. 8 indicates that the original unevenness height of the aluminum plating layer was reduced by the corrosion, and it was found that the actual situation at this damage stage was characteristically well represented.

Next, FIG. 9 shows the results of a flaw detection test of FIG. 1 (c) "a stage where the disappearance of the aluminum plating layer has progressed and the alloy layer has been partially exposed". Looking at the vector representation, as the amplitude of the signal decreases, the phase becomes 60
-80 degrees and 240-260 degrees, clearly rotating counterclockwise as compared to the previous step. This indicates that the height of the concavities and convexities of the aluminum plating layer has decreased, and it has been found that the state in which the corrosion has further progressed is characteristically well represented.

Next, FIG. 10 shows the results of a flaw detection test in FIG. 1D for the “stage where the aluminum plating layer has almost disappeared and most of the alloy layer is exposed”. Looking at the vector representation, the signal amplitude is significantly reduced and the phases are between 85 and 90 degrees and between 265 and 270 degrees and are almost vertical. Also, when looking at the XY enlarged chart, it can be seen that the main signal shows approximately 270 degrees. This indicates that the aluminum plating layer slightly remains, and that the remaining portion has a convex shape, which clearly shows the actual state.

Next, FIG. 11 shows the results of a flaw detection test of FIG. 1 (e) "the stage where the aluminum plating layer has completely disappeared and the entire surface of the alloy layer has been exposed". Almost no discernable signal is seen on the XY chart and the vector display. In microscopic observation with a microscope, considerable irregularities are recognized at the alloy layer surface and at the boundary between the alloy layer and the carbon steel pipe, but these irregularities are macroscopically averaged as flat surfaces and boundaries, and the signal It is probable that it was not detected.

Next, FIG. 12 shows the results of a flaw detection test of FIG. 1 (f) at the “stage where the thinning due to local corrosion of the alloy layer and carbon steel occurs at the position starting from the latent fine defect”. showed that. At first glance, the XY chart does not seem to be different from the one shown in FIG. 10, but when looking at the vector display, all the signal phases show 90 degrees, and FIG. 1 showing 270 degrees.
The phase is opposite to 0. Then, when the XY chart was enlarged, it was found that clear single signals were sporadic or clustered, and the phase of the Y chart was also opposite to that of FIG. That is, while the signal in FIG. 10 was caused by the convex residual portion of the aluminum plating layer, the signal in FIG. 12 was a signal indicating the concave portion, and the alloy layer was broken and the alloy layer and the carbon steel pipe were damaged. It was found that the state where the wall thinning due to local corrosion occurred was well represented.

Based on the above results, the features of the reference figure at each damage stage are listed as follows: (a) Stage: The XY chart shows a forest-like signal on the entire surface. (B) stage: a forest-like signal appears on the entire surface of the XY chart, and it is apparently indistinguishable from the (a) stage. The vector display shows a phase of 50 to 70 degrees and a phase of 230 to 250 degrees. Rotate clockwise (step (c)): The amplitude of the forest-like signal decreases in the XY chart, and the amplitude of the XY chart in particular decreases. Vector display has a phase of 60 to 80 degrees and 240 to 260.
(D) Stage: XY chart The amplitude of the forest-like signal becomes very small, especially the X chart becomes almost straight. Vector display has a phase of 265. (E): no signal appears in both the XY chart and vector display (f): the amplitude is small in the XY chart, and a clear single signal is sporadic or clustered Vector display is vertical with all 90 degrees phase

Here, a flaw detection test was performed on all the hot-dip aluminized carbon steel pipes actually used in one heat exchanger, and the damage stage was determined. The cross sections of the actual damage stages were visually observed, and some representative ones were observed with a microscope. The use pipe shown in FIG. 13 has a portion where a clear single signal having a phase of 90 degrees appears in order from the bottom ((f) stage), a portion where no signal appears (stage (e)), phases 230 to 250 degrees and 50 phases.
Part where large forest-like signal with large amplitude of ~ 70 degrees appears ((b)
Stage), and the damage stage is (b)-
(F) It was determined that it was in the stage. In addition, the use pipe shown in FIG. 14 does not detect a signal corresponding to the stage (b),
A portion indicating the stage (c) and a portion indicating the stage (f) were observed. In particular, in the portion showing the lower stage (f), since the signal amplitude was increased, it was determined that the wall thinning due to the local corrosion of the carbon steel was progressing.

The results of microscopic observation of the inner surface of the above used tube are shown in FIGS. 13 and 14. The damage stage estimated by the figure selected from the flaw detection test and the damage stage by microscopic observation were almost the same, demonstrating the effectiveness of the metal tube internal damage inspection method according to the present invention.

In addition, with respect to the total number of hot-dip aluminized carbon steel pipes other than the above, a clear correlation was found between the result of the judgment of the damage stage by the flaw detection test and the damage stage confirmed by visual observation of the cross section. .

At this time, attention was paid particularly to the portion where the support plate is located, and the above correlation was found also for this portion.

As described above, the probe of the eddy current flaw detector scans the inside of the hot-dip aluminized carbon steel pipe, converts the information obtained from the damaged part into a figure, and compares the shape with the reference figure. It was possible to efficiently and accurately identify the damage stage for all areas. Therefore, in the damage stage (e), “the stage in which the aluminum plating layer is completely lost and the entire surface of the alloy layer is exposed”, or in the damage stage (f), “the alloy is located at a position starting from a latent minute defect. To prevent serious accidents such as tube opening and leaks, by sequentially replacing those that have reached the stage where the wall loss due to local corrosion of the layer and carbon steel has occurred. Becomes possible.

[0049]

As described above, according to the present invention, in a ferromagnetic metal tube having a non-magnetic metal layer formed on the inner surface, particularly in a hot-dip aluminum-plated carbon steel tube, the magnetic non-uniformity due to the ferromagnetic material is reduced. The internal damage inspection can be easily and efficiently performed on the entire area while removing the influence.

[Brief description of the drawings]

FIG. 1 is a view illustrating a damage stage of a hot-dip aluminum-plated steel pipe.

FIG. 2 is a schematic diagram of a configuration of a probe of the eddy current flaw detector.

FIG. 3 is a diagram showing a relationship between an AC frequency and a magnetic field penetration depth in carbon steel and aluminum.

FIG. 4 is a configuration diagram of an apparatus according to an embodiment of the present invention.

FIG. 5 is a diagram showing the relationship between the unevenness of the aluminum surface and the signal phase.

FIG. 6 is a diagram showing the results of a flaw detection test on a drill hole test piece.

FIG. 7 is a diagram showing the results of a flaw detection test on an unused pipe.

FIG. 8 is a diagram showing the results of a flaw detection test performed on a model tube at the damage stage (b).

FIG. 9 is a diagram showing the results of a flaw detection test on a model tube at the damage stage (c).

FIG. 10 is a diagram showing the results of a flaw detection test performed on a model tube at the damage stage (d).

FIG. 11 is a diagram showing the results of a flaw detection test performed on a model tube at the damage stage (e).

FIG. 12 is a diagram showing the results of a flaw detection test performed on a model tube at a damage stage (f).

FIG. 13 is a diagram showing an inspection result of a used pipe in the heat exchanger.

FIG. 14 is a diagram showing an inspection result of a used pipe in the heat exchanger.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 molten aluminum-plated carbon steel pipe 101 carbon steel 102 alloy layer 103 aluminum-plated layer 200 probe 201 test coil 202 weak magnetic permanent magnet 210 probe cable 400 heat exchanger 410 eddy current flaw detector 420 output recorder

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takeshi Takeshi 1-1, Mikage-cho, Tokuyama-shi, Yamaguchi Pref. Tokuyama Co., Ltd. (72) Inventor Hiroshige Seki 4135, Nibancho, Tokuyama-shi, Yamaguchi F-term (reference) 2G053 AA11 AB21 BA12 BA23 BA26 BB12 BC02 BC07 BC20 CA03 CB02 CB29 CB30 CC04 DA03 DA06 DB20 DB27

Claims (7)

[Claims] A ferromagnetic metal tube having a nonmagnetic metal layer formed on an inner surface thereof, wherein an unused metal tube and a metal tube in a plurality of different damage stages are provided with a magnetic saturation means. A flaw detection test is performed by an eddy current flaw detector, a reference figure is established by graphically drawing information obtained by this test, and a test is performed by comparing a figure obtained similarly for the inspection target metal pipe with the reference figure. A method for inspecting internal damage of a metal tube, wherein a damage stage of the target metal tube is specified. 2. The method according to claim 1, wherein an eddy current flaw detector having a self-comparison type probe is used. 3. The method according to claim 1, wherein an eddy current flaw detector is used in which weak magnetic permanent magnets are arranged on both sides of the test coil of the probe. 4. The frequency of the alternating current applied to the test coil is determined according to the depth of the metal tube to be inspected in the thickness direction in which the inspection is required. Of metal tube internal damage inspection method. 5. The metal tube internal damage inspection method according to claim 1, wherein an AC frequency applied to the test coil is determined according to a constituent material of the metal tube to be inspected. . 6. In a carbon steel pipe having a molten aluminum plating layer formed on an inner surface, an unused pipe and a used pipe in a plurality of different damage stages are subjected to a flaw detection test by an insertion probe type eddy current flaw detector having magnetic saturation means. Then, a reference figure is established by graphicizing the information obtained by this test, and a damage stage of the inspection pipe is specified by comparing a figure obtained similarly with respect to the inspection pipe with the reference graphic. The metal pipe internal damage inspection method according to any one of claims 1 to 4, wherein: 7. The method according to claim 6, wherein an alternating current of 100 to 400 KHz is applied to the test coil.