JP3705327B2 - Metal pipe internal damage inspection method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for inspecting internal damage in a ferromagnetic metal tube having a nonmagnetic metal layer formed on the inner surface, and particularly in a carbon steel tube having a molten aluminum plating layer formed on the inner surface.
[0002]
[Prior art]
Copper alloy pipes are generally used for heat exchangers that use seawater for cooling water. However, when copper alloy pipes cannot be used for process reasons, carbon steel pipes with molten aluminum plating (hereinafter melted) are used. Aluminium-plated carbon steel pipe).
[0003]
The aluminum plating layer in the hot-dip aluminum-plated carbon steel tube functions as a coating material for carbon steel, and also functions as a corrosion-proof sacrificial material that utilizes the difference in corrosion potential between aluminum and carbon steel. However, once the aluminum plating layer disappears due to seawater, due to the difference in corrosion potential between the alloy layer and the carbon steel, the local corrosion of the carbon steel rapidly proceeds, starting from fine defects that exist in the alloy layer. However, there is a high possibility of causing a serious leakage accident by reaching the tube opening.
[0004]
Therefore, when using a hot-dip aluminized carbon steel pipe, it is important to quickly and reliably find the location where the aluminum plating layer has disappeared or the location where the carbon steel has been locally corroded, and various techniques have been implemented and devised for that purpose. I came.
[0005]
First, the most commonly performed method includes a visual inspection using a fiberscope and a water immersion ultrasonic flaw detection test. However, these methods have a low inspection speed, and a great deal of labor is required to inspect a unit having several hundreds of metal tubes such as a heat exchanger. Further, depending on the pretreatment state and the damage form, not all defects can be detected, and there is a problem in terms of reliability.
[0006]
Next, there is a method using an insertion probe type eddy current flaw detector.
In this method, a probe is generally inserted into a metal tube to receive a signal from a damaged portion. At this time, a self-comparison probe in which a pair of test coils are arranged in parallel is used. There are many. The self-comparison probe detects a difference between two adjacent portions of the specimen. Therefore, it responds well to local changes such as drill holes, conversely, gentle changes due to subtle changes in composition, shape, dimensions, relative position changes due to carrier vibration, ambient temperature changes, etc. The influence of factors other than the inspection purpose is offset by the two coils, and the inspection accuracy is excellent.
[0007]
However, for example, when a metal pipe base material including a hot-dip aluminized carbon steel pipe considered as an inspection target is a ferromagnetic material, noise due to a change in magnetic permeability is generated, and so-called S / N ratio is remarkably lowered. It becomes difficult to detect a defect signal. Therefore, even if the eddy current flaw inspection is general as a maintenance inspection inspection technique for a non-magnetic metal tube, it cannot be applied to a ferromagnetic metal tube as it is.
[0008]
Therefore, by applying high-frequency alternating current of multiple frequencies to the test coil, adding a magnetic field having a fundamental frequency component and a magnetic field having a higher-frequency component to the metal tube, noise of signals obtained from two types of magnetic fields For example, “Non-destructive inspection Vol. 42, No. 2, PP 86-93” discloses a technique for synthesizing detection signals so that components are canceled out. It is possible to eliminate the influence of.
[0009]
By the way, when an alternating current is applied to the test coil, a direct magnetic field and an indirect magnetic field are generated around the excitation coil. The general eddy current flaw detection method is a method of detecting a change in eddy current as a change in an indirect magnetic field, but at this time, it is also directly influenced by a magnetic field. Therefore, there is no problem when inspecting a non-magnetic metal tube having a uniform direct magnetic field as described above, but a large noise is generated in the inspection of a ferromagnetic metal tube whose direct magnetic field fluctuates. Will cause trouble. However, at a position 2 to 3 times the tube diameter, there is a region called a remote field where the direct magnetic field is reduced and almost only an indirect magnetic field exists. Here, there is almost no noise due to magnetic field inhomogeneity. A method for measuring an indirect magnetic field (remote field inspection, abbreviated as RFT) is known as an effective method particularly for inspection of a ferromagnetic metal tube.
[0010]
[Problems to be solved by the invention]
However, in the method of eliminating magnetic non-uniformity by mixing, a skilled engineer having sufficient technology and experience is required for performing the inspection. That is, a high level of skill is required for optimum test frequency combinations, mixing condition settings, and the like, and it has not been a simple inspection method.
[0011]
Further, in RFT, there is a limit to the size of a defect that can be detected for local thinning, and there is a region that cannot be inspected, that is, a dead zone in the vicinity of the support plate of the metal tube. In order to prevent leakage accidents due to local corrosion as described above in hot-dip aluminum-plated carbon steel pipes, full length inspection is required, so it is necessary to use other inspection methods in combination with this part, which is inefficient. It was.
[0012]
Therefore, the problem of the present invention is that it is possible to remove the influence of magnetic non-uniformity on a ferromagnetic metal tube having a nonmagnetic metal layer formed on its inner surface, particularly a hot-dip aluminized carbon steel tube, In addition, an internal damage inspection method capable of inspecting the entire region simply and efficiently is provided.
[0013]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, a metal tube internal damage inspection method according to the present invention includes a metal tube in a plurality of different damage stages and an unused metal tube in a ferromagnetic metal tube having a nonmagnetic metal layer formed on the inner surface. A flaw detection test is conducted with an interpolated probe type eddy current flaw detector, and a reference figure is established by graphicizing the information obtained by this test. In the metal pipe internal damage inspection method for identifying a damage stage of a metal pipe to be inspected by comparing a figure obtained by performing a flaw detection test and the reference figure, the interpolated probe type eddy current flaw detection apparatus has a test coil of a probe. It has magnetic saturation means with weak magnetic permanent magnets on both sides, so that the penetration depth of eddy currents is within the magnetic saturation range by the weak magnetic permanent magnets. Line the scratch test Flaw detection of ferromagnetic metal tubes and non-magnetic metal layers on the inner surface of metal tubes It is characterized by performing.
As a result, the metal pipe internal damage inspection method of the present invention can only compare figures. Ferromagnetic metal tube and non-magnetic metal layer inside the metal tube The damage stage can be identified, which is convenient. In the metal tube internal damage inspection method according to the present invention, an eddy current flaw detection device in which weak magnetic permanent magnets are arranged on both sides of the test coil of the probe is used, and the weak magnetic permanent magnet has a penetration depth of eddy current in the weak magnetic permanent. Since the magnetic force is such that it is within the range of being magnetically saturated by the magnet, the magnetic non-uniformity due to the ferromagnetic material is eliminated, and the inspection can be performed without being affected by it. In addition, the scanning property is excellent and the shape is compact.
[0014]
The metal tube internal damage inspection method of the present invention is characterized by using an eddy current flaw detector having a self-comparing probe. Thereby, the difference of two adjacent parts in a test object can be detected, and the thinning by the local corrosion of a metal tube can be detected reliably.
According to the metal tube internal damage inspection method of the present invention, the frequency of the alternating current applied to the test coil is determined according to the depth in the tube thickness direction that requires inspection of the metal tube to be inspected. . Thereby, a desired depth can be inspected by the penetration depth of the magnetic field determined by the type and frequency of the constituent material.
[0016]
Moreover, the metal pipe internal damage inspection method of this invention is characterized by determining the frequency of the alternating current applied to a test coil according to the constituent material of a test object metal pipe. Thereby, a desired depth can be inspected by the penetration depth of the magnetic field determined by the type and frequency of the constituent material.
[0017]
Further, the metal pipe internal damage inspection method according to the present invention is an interpolation probe having a magnetic saturation means for an unused pipe and a plurality of used pipes in different damage stages in a carbon steel pipe having a molten aluminum plating layer formed on the inner surface. A flaw detection test is performed using a eddy current flaw detector, a reference graphic is established by converting the information obtained by the test into a graphic, and the inspection target pipe is compared with the reference graphic similarly obtained for the inspection target pipe. It is characterized by specifying the damage stage. Thereby, the magnetic nonuniformity by the carbon steel which is a ferromagnetic body is eliminated, and it can test | inspect, without receiving the influence. In addition, the damage stage of the hot-dip aluminized carbon steel pipe can be identified only by comparing the figures, which is convenient.
[0018]
Moreover, the metal pipe internal damage inspection method of this invention is characterized by applying an alternating current of 100 to 400 KHz to the test coil. Thereby, the penetration depth of the eddy current is limited to a range where the magnetic saturation is caused by the weak magnetic permanent magnet, and magnetic noise generation by the lower carbon steel is suppressed, and the inspection can be surely performed.
[0019]
In the present invention, in a hot dip aluminized carbon steel pipe, an eddy current flaw detection test was conducted using an unused pipe and a used pipe at a plurality of different damage stages as model pipes, and the information obtained by this test was figured out and established. The reference figure is used and compared with the figure obtained in the same manner for the inspection target tube. The damage stage may be appropriately determined according to the maintenance management purpose, and the same applies to the number of damage stages. For example, in the hot-dip aluminized carbon steel pipe 100 shown in FIG.
(A) Stage in which aluminum plating layer 103 remains thick on the entire surface
(B) The aluminum plating layer 103 remains on the almost entire surface, but is partially disappearing.
(C) The stage where the disappearance of the aluminum plating layer 103 progresses and the alloy layer 102 is partially exposed.
(D) The stage in which the aluminum plating layer 103 has almost disappeared and most of the alloy layer 102 is exposed.
(E) The stage in which the aluminum plating layer 103 has completely disappeared and the entire surface of the alloy layer 102 is exposed.
(F) What is necessary is just to set the stage etc. which the local thinning of the alloy layer 102 and the carbon steel 101 has generate | occur | produced in the position which makes the starting minute defect a starting point.
[0020]
Moreover, in this invention, an insertion probe type eddy current flaw detector is used. At this time, it is desirable to provide a magnetic saturation means in order to improve the S / N ratio by suppressing magnetic nonuniformity due to the ferromagnetic material and to perform a highly accurate inspection. In the conventional magnetic saturation means, it is common to use a strong magnetic permanent magnet or a magnetic saturation coil. However, in the present invention, a very shallow surface of the inner surface of a hot-dip aluminized carbon steel pipe Since it is sufficient that only the layer can be magnetically saturated, the probe 200 in which the weak magnetic permanent magnets 202 are arranged on both sides of the test coil 201 is used as shown in FIG.
Since the weak magnetic permanent magnet 202 is thinner and smaller than conventional strong magnetic permanent magnets and coils, the probe can be reduced in weight and size, and the magnetic attraction is small, so the scanning performance is good.
[0021]
In addition, the probe used at this time is a self-comparison probe for the purpose of reliably detecting thinning due to local corrosion and unevenness of the aluminum plating layer.
[0022]
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 alternating current applied and the penetration depth of the magnetic field is specific to the material. Accordingly, the frequency is selected according to the constituent material of the metal pipe to be inspected, the inner surface magnetic saturation depth, and the like.
[0023]
Regarding the hot-dip aluminized carbon steel pipe, the frequency is determined by the thickness and uneven height of the aluminum plating layer and the inner surface magnetic saturation depth depending on the strength of the permanent magnet. The relationship between the frequency and the penetration depth in carbon steel and aluminum is as shown in FIG. The penetration depth of the alloy layer is unknown, but is estimated to be close to 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. However, when an alternating current of 100 KHz is applied, the penetration depth of the aluminum plating layer is several hundred μm according to FIG. In general, the unevenness 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 using the above-described eddy current flaw detection probe provided with the weak magnetic permanent magnet, an alternating current of 100 KHz is applied to a carbon steel pipe not subjected to hot dip aluminum plating. As a result of testing, almost no noise due to magnetic nonuniformity of carbon steel was detected, and it was found that the inner surface thickness of several tens of μm in the alloy layer and carbon steel was sufficiently magnetically saturated. From the above, except when the thickness of the aluminum plating layer or the height of the unevenness is particularly large, an accurate inspection can be performed by applying an alternating current having a frequency of 100 KHz or more.
[0024]
On the other hand, when a too high frequency is applied, the penetration depth becomes shallow, and when the unevenness of the aluminum plating layer is large, the detection sensitivity is lowered by lift-off. Moreover, when the penetration depth into the alloy layer becomes shallower, the detection sensitivity of thinning due to local corrosion also decreases. Therefore, it is desirable to apply a frequency of 400 KHz or less.
[0025]
The latest eddy current flaw detector has a function of recording while simultaneously applying a plurality of frequencies to the test coil. If such an apparatus is used, a flaw detection test can be performed by applying a frequency of 100 to 400 KHz and selecting a frequency that is most easily identified depending on the situation.
[0026]
Now, various model metal pipes are inspected by the above procedure, and the obtained information is made into a graphic. As the figure, an output to a conventionally used XY chart and a vector display expressed by a two-dimensional plane are used. By comparing the reference graphic established in this inspection with the graphic similarly obtained for the inspection target pipe, the damage stage of the inspection target pipe is specified. The reference graphic at each stage of damage has a unique pattern that can be distinguished at a glance. Therefore, it is easy to specify which reference graphic corresponds to the figure of the inspection target pipe, and no special skill is required for the inspection.
[0027]
In addition, since the indirect magnetic field is not measured as in RFT, the inspection method is based on the direct magnetic field measurement, and unlike the normal eddy current flaw detection inspection, only the shallow layer on the inner surface is inspected. The entire length can be inspected in the vicinity and directly below without causing any trouble.
[0028]
【Example】
One embodiment of the present invention will be described below. In this example, for hot-dip aluminized carbon steel pipes used in heat exchangers that use seawater for cooling water, model pipe selection at each stage of damage, establishment of reference figures, and flaw detection tests for the pipes used And the stage of damage.
[0029]
The configuration of the interpolating probe type eddy current flaw detector used in this example is shown in FIG. As shown in FIG. 2, the probe 200 was used in which a self-comparison type test coil 201 was disposed and a weak magnetic permanent magnet 202 was disposed on both sides thereof. The probe 200 is scanned over the entire area of the hot-dip aluminized carbon steel pipe 100 while applying high-frequency alternating current from the eddy current flaw detector 410. 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.
[0030]
Before conducting a flaw detection test on a hot-dip aluminized carbon steel pipe that is actually used in a heat exchanger, first, as shown in FIG.
(A) Stage in which aluminum plating layer 103 remains thick on the entire surface
(B) The aluminum plating layer 103 remains on the almost entire surface, but is partially disappearing.
(C) The stage where the disappearance of the aluminum plating layer 103 progresses and the alloy layer 102 is partially exposed.
(D) The stage in which the aluminum plating layer 103 has almost disappeared and most of the alloy layer 102 is exposed.
(E) The stage in which the aluminum plating layer 103 has completely disappeared and the entire surface of the alloy layer 102 is exposed.
(F) Prepared are hot-dip aluminized carbon steel pipes 100 in a stage where thinning due to local corrosion of the alloy layer 102 and the carbon steel 101 occurs at a position starting from a latent fine defect. To establish a reference figure.
[0031]
As described above, the frequency of the alternating current applied to the test coil 201 is determined according to the thickness and uneven height of the aluminum plating layer and the inner surface magnetic saturation depth caused by the magnetic force of the permanent magnet. Generally, 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. 3, the frequency is 100 to 400 KHz. It becomes.
Here, the frequency of 4 channels is used so that the most advantageous frequency for identification can be selected according to the situation. However, since the frequency can be sufficiently identified by 100 KHz alone, it is expressed only by the result of 100 KHz.
[0032]
The information obtained by performing the flaw detection test according to the above procedure, that is, the received signal is graphed, and prior to that, the phase is set first. It is known that when the surface of aluminum has irregularities, its signal phase changes according to the height of the irregularities as shown in FIG. 5 and rotates clockwise as the height increases. It is also known that the signal phase of the thinned portion in the alloy layer which is a ferromagnetic material and carbon steel is constant regardless of the depth.
Therefore, if a flaw detection test is performed with such a constant phase signal set to 90 degrees, the relative change in the signal phase accompanying the change in the unevenness of the aluminum plating layer becomes clear, and the reference graphic, particularly the vector display, is displayed. Features at each stage of damage can be revealed.
[0033]
Therefore, a test piece in which a 3, 4, 6, 8 mmφ through-drilled hole was drilled 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. Is set so that the phase of the signal obtained by the above is 90 degrees. The XY chart and vector display of the received signal at this time are shown in FIG. The signal from each through hole shows 90 degrees regardless of the opening diameter, although there is some variation.
[0034]
Next, under the above settings, a flaw detection test was performed on the unused hot-dip aluminized carbon steel pipe. The result is shown in FIG. The XY chart shows continuous forest-like signals, and individual signal separation is difficult, but when the signal phase is read in vector display, it is roughly divided into two groups of 50 to 70 degrees and 220 to 250 degrees. . This is considered to be a signal due to surface irregularities in the aluminum plating layer. That is, it is considered that 50 to 70 degrees corresponds to a signal by a concave portion and 220 to 250 degrees corresponds to a signal by a convex portion. As shown in FIG. 6, this is also the case of 50 to 70 degrees and the phase 220 to 250 degrees separately from the signal from the through drill hole (phase 90 degrees) in the previous inspection of the through drill hole specimen. This is supported by the signal appearing in
[0035]
When performing a flaw detection test on an unused aluminum-plated 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 signal (the signal due to the unevenness of the aluminum plating layer) is adjusted so as to be about 30% of the full scale. However, the adjustment is not limited to this, and may be appropriately adjusted according to the situation. Note that it is preferable to use a recording medium in which the sensitivity setting can be freely changed after recording because adjustment is easy in all situations.
[0036]
Next, a flaw detection test was performed on each model tube at each damage stage under the above-described conditions. The results are shown in FIGS. Note that the model tube in FIG. 1 (a) that is not damaged is the same as in the unused state in FIG. 7, and is omitted to avoid duplication.
[0037]
FIG. 8 shows the flaw detection test results for “the stage in which the aluminum plating layer remains almost entirely but is partially disappearing” in FIG. Looking at the vector display, the phase of the signal was 50 to 70 degrees and 230 to 250 degrees, which almost coincided with the signal phase in the unused pipe described above, but was slightly rotated counterclockwise (standing vertically) ). When the phase signal rotates counterclockwise, it indicates that the unevenness height of the aluminum plating layer is reduced as described above. That is, the vector display of FIG. 8 shows that the original unevenness height of the aluminum plating layer was reduced by corrosion, and it was found that the actual situation at this damage stage was well expressed characteristically.
[0038]
Next, FIG. 9 shows the flaw detection test results for “the stage in which the disappearance of the aluminum plating layer has progressed and the alloy layer is partially exposed” in FIG. Looking at the vector display, the amplitude of the signal decreases and the phases show 60-80 degrees and 240-260 degrees, which are clearly counterclockwise compared to the previous stage. This indicates that the unevenness height of the aluminum plating layer is reduced, and it was found that the state in which corrosion further progressed characteristically well.
[0039]
Next, FIG. 10 shows the flaw detection test results for the “stage where the aluminum plating layer is almost disappeared and most of the alloy layer is exposed” in FIG. Looking at the vector display, the signal amplitude is significantly reduced and the phases are 85-90 degrees and 265-270 degrees, almost vertical. Moreover, when the XY enlarged chart is seen, it turns out that the main signal shows about 270 degree | times. This indicates that a slight amount of the aluminum plating layer remains and that the remaining portion has a convex shape, which clearly shows the actual state.
[0040]
Next, FIG. 11 shows the flaw detection test results for “the stage in which the aluminum plating layer has completely disappeared and the entire surface of the alloy layer is exposed” in FIG. There are almost no discernible signals on the XY chart and the vector display. In microscopic observation with a microscope, considerable irregularities are observed on the surface of the alloy layer and the boundary between the alloy layer and the carbon steel pipe, but these irregularities are macroscopically averaged as flat surfaces and boundaries as signals. It is thought that it was not detected.
[0041]
Next, FIG. 12 shows the flaw detection test results for FIG. 1 (f), “the stage in which thinning due to local corrosion of the alloy layer and carbon steel occurs at a position starting from a potential fine defect”. . At first glance, the XY chart does not seem to be different from that of FIG. 10, but when the vector display is viewed, the signal phases all show 90 degrees, which is opposite to that of FIG. 10 showing 270 degrees. Yes. Therefore, when the XY chart was enlarged, it was found that clear single signals were scattered or swept, and the phase of the Y chart was also opposite to that of FIG. That is, the signal in FIG. 10 is due to the convex remaining portion of the aluminum plating layer, whereas the signal in FIG. 12 is a signal indicating a concave portion, and the alloy layer is destroyed and the alloy layer and the carbon steel pipe It was found that the thinning due to local corrosion was well represented.
[0042]
Based on the above results, enumerating the features of the reference figures at each damage stage,
(A) Stage: The XY chart shows a vector signal in which forest-like signals appear on the entire surface, and the phase display is 50 to 70 degrees and 220 to 250 degrees.
(B) Stage: A forest-like signal appears on the entire surface of the XY chart, and apparently indistinguishable from the stage (a), the vector display shows phases of 50 to 70 degrees and 230 to 250 degrees, and rotates slightly counterclockwise.
(C) Stage: The amplitude of the forest signal decreases in the XY chart, and especially the vector display in which the amplitude of the XY chart decreases shows a phase of 60 to 80 degrees and 240 to 260 degrees, which is quite counterclockwise. Rotate to and close to vertical
(D) Stage: XY chart The amplitude of the forest signal is very small, and in particular, the vector display in which the X chart is almost a straight line shows a phase of 265 to 270 degrees and is almost vertical.
(E) Stage: No signal appears in XY chart and vector display
Step (f): The vector display in which the amplitude is small on the XY chart and clear single signals are scattered or swarmed is 90 degrees in phase and vertical.
[0043]
Here, a flaw detection test was conducted on the total number of hot-dip aluminized carbon steel tubes actually used in one heat exchanger, and the damage stage was determined. Moreover, about the actual damage stage, the cross section was visually observed, and some typical things were observed with the microscope. The use tube shown in FIG. 13 has a portion where a clear single signal having a phase of 90 degrees appears in order from the bottom (step (f)), a portion where no signal appears (step (e)), a phase of 230 to 250 degrees, and a phase of 50 to 70. There was a portion (stage (b)) where a forest-like signal with a large amplitude appeared, and the damage stage was determined to be in stages (b) to (f) over the entire length. Further, in the use tube shown in FIG. 14, a signal corresponding to the stage (b) was not detected, and a part showing the stage (c) and a part showing the stage (f) were seen. In particular, in the portion showing the lower stage (f), the signal amplitude is increased, so it was determined that the thinning due to local corrosion of the carbon steel was progressing.
[0044]
About the said use pipe | tube, the result of having observed the inner surface under the microscope was combined with FIG.13 and FIG.14. The damage stage estimated by the figure selected from the flaw detection test and the damage stage by microscopic observation almost coincided, and the effectiveness of the metal pipe internal damage inspection method according to the present invention was proved.
[0045]
Further, for all the hot-dip aluminized carbon steel pipes other than the above, a clear correlation was found between the determination result of the damage stage by the flaw detection test and the damage stage confirmed by visual observation of the cross section.
[0046]
At this time, the portion where the support plate is present was particularly paid attention to, but the above correlation was also observed for this portion.
[0047]
As described above, the probe of the eddy current flaw detector is scanned about the inside of the hot-dip aluminized carbon steel pipe, the information obtained from the internal damage location is figured out, and the shape with the reference figure is simply compared and efficiently and It was possible to accurately identify the damage stage for all areas. Therefore, in the damage stage (e), the “aluminum plating layer has completely disappeared and the entire surface of the alloy layer is exposed”, or in the damage stage (f), the alloy is positioned at a position starting from a potential fine defect. To prevent serious accidents such as tube opening and leakage, etc. by sequentially performing replacement work after considering the open cycle for those that have reached the stage where thinning due to local corrosion of the layer and carbon steel has occurred. Is possible.
[0048]
【The invention's effect】
As described above, according to the present invention, in a ferromagnetic metal tube having a nonmagnetic metal layer formed on the inner surface, particularly in a hot-dip aluminized carbon steel tube, the influence of magnetic nonuniformity due to the ferromagnetic material is removed. In addition, the internal damage inspection can be performed on the entire area simply and efficiently.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining a damage stage of a hot-dip aluminized steel pipe.
FIG. 2 is a schematic configuration diagram of a probe of an 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 block 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 a flaw detection test result in a drill hole test piece.
FIG. 7 is a diagram showing a flaw detection test result in an unused pipe.
FIG. 8 is a diagram showing a flaw detection test result in a damage stage (b) model tube.
FIG. 9 is a diagram showing a flaw detection test result in a damage stage (c) model tube.
FIG. 10 is a diagram showing a flaw detection test result in a damage stage (d) model tube;
FIG. 11 is a diagram showing a flaw detection test result in a damage stage (e) model tube;
FIG. 12 is a diagram showing a flaw detection test result in a damage stage (f) model tube;
FIG. 13 is a diagram showing a test result of a pipe used in a heat exchanger.
FIG. 14 is a diagram showing a test result of a pipe used in a heat exchanger.
[Explanation of symbols]
100 Hot-dip aluminized carbon steel pipe
101 carbon steel
102 Alloy layer
103 Aluminum plating layer
200 probes
201 test coil
202 Weak magnetic permanent magnet
210 Probe cable
400 heat exchanger
410 Eddy current flaw detector
420 Output recording device

Claims (6)

A flaw detection test was conducted on a metal tube at different stages of damage of a ferromagnetic metal tube with a nonmagnetic metal layer formed on the inner surface and an unused metal tube using an interpolated probe type eddy current flaw detector. The reference figure is established by making the information obtained into a figure, and the inspection is performed by comparing the reference figure with the figure obtained by conducting the flaw detection test on the metal pipe to be inspected by the interpolated probe type eddy current flaw detector. In the metal pipe internal damage inspection method for identifying the damage stage of the target metal pipe, the interferometric probe type eddy current flaw detector can weakly magnetize only a very shallow surface layer inside the metal pipe on both sides of the test coil of the probe. has a magnetic saturation means arranged, performs flaw detection test to the penetration depth of the eddy current by the weak force permanent magnet is in the range that is magnetically saturated, Metal pipe internal damage inspection method characterized by performing flaw detection of the magnetic metal tube and the metal layer of the nonmagnetic metal pipe surface.  The eddy current flaw detection apparatus having a self-comparison probe is used.  The method for inspecting a metal pipe internal damage according to claim 1 or 2, wherein the frequency of the alternating current applied to the test coil is determined according to a depth in the tube thickness direction that requires an inspection of the metal pipe to be inspected.  The method for inspecting a metal pipe internal damage according to any one of claims 1 to 3, wherein an AC frequency applied to the test coil is determined according to a constituent material of the metal pipe to be inspected.  The metal pipe internal damage inspection method according to any one of claims 1 to 4, wherein the nonmagnetic metal layer is a molten aluminum plating layer.  6. The method for inspecting a metal pipe internal damage according to claim 5, wherein an alternating current of 100 to 400 KHz is applied to the test coil.

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