Eddy current flaw detection method

Abstract

To provide an eddy current flaw detection method that can prevent the influence of irregularities on an outer face of a tube body, and can maintain the accuracy of detecting cracks.SOLUTION: Provided is an eddy current flaw detection method for detecting cracks in an inner face of a tube body by bringing a probe of an eddy current flaw detection probe closer to an outer face of the tube body, and the method includes: a step of forming a membrane layer on the outer face; a step of setting an eddy current penetration distance on the basis of the depth of the cracks from the inner face and the thickness of the tube body, and calculating a current frequency on the basis of the set eddy current penetration distance; and a step of executing flaw detection of the tube body by using the probe from a top face of the membrane layer on the basis of the calculated current frequency.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an eddy current flaw detection method for detecting cracks on the inner surface of a pipe by bringing a probe of an eddy current flaw detection probe close to the outer surface of the pipe. [Background technology]

[0002] In the blowing process of molten steel, molten steel is charged into a converter and subjected to treatment such as deoxidation. The high-temperature exhaust gas generated by the treatment is recovered in a gas recovery device (boiler) installed above the converter and sent to an external exhaust device connected to the outside.

[0003] The gas recovery device has a piping system for transporting the recovered exhaust gas. The piping system has a membrane-structured wall made of multiple pipes arranged in a circular ring and assembled by welding these pipes. This configuration allows heat exchange between the recovered exhaust gas and the water flowing through the circularly arranged pipes, making it possible to recover heat from the exhaust gas.

[0004] Here, due to the operation of the converter and the recovery of exhaust gas in the gas recovery device, a temperature difference occurs between the inner surface of the annularly arranged tube, which is in contact with the water flow, and the outer surface, which is in contact with the exhaust gas. If air bubbles remain in the water flow on the inner surface of the tube, the air bubbles will affect the heat exchange with the exhaust gas, leading to thermal fatigue.

[0005] Furthermore, if thermal fatigue continues for a long period of time, cracks (heat cracks) will occur in the tube. These cracks will extend from the inner surface of the tube to the outer surface, and if they reach the outer surface, water leakage will occur.

[0006] For this reason, flaw detection probes are used to inspect whether or not cracks have occurred on the inner surface of the pipe from the outer surface. In particular, it is necessary to accurately detect the depth of cracks that have occurred on the inner surface when they reach a certain depth, so that repairs and pipe replacement work can be carried out before water leaks occur.

[0007] As a flaw detection technique, ultrasonic flaw detection is often used to detect cracks inside an object (a pipe body). Patent Document 1 discloses a method of eddy current flaw detection using an eddy current flaw detection probe for inspecting internal defects (cracks) in a pipe body of a heat recovery boiler. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-173144 Summary of the Invention [Problem to be solved by the invention]

[0009] However, ultrasonic and eddy current testing, which rely on ultrasonic and eddy current techniques to perform inspections based on ultrasonic and eddy current techniques with the probe of the flaw detection probe positioned close to the target's outer surface, are significantly affected by irregularities on the target's outer surface. Furthermore, the outer surface of the tube may be corroded by contact with exhaust gases generated in the converter, causing the tube to deform into an irregular shape. Therefore, applying ultrasonic and eddy current testing without prior treatment (removal of irregularities) to the tube's outer surface can result in problems with crack detection accuracy. Even if prior treatment is performed on the outer surface to maintain detection accuracy, there are cases where the material is difficult to treat or the area to be inspected is large, requiring significant effort before the inspection.

[0010] The present invention has been made in consideration of the above circumstances, and its purpose is to provide an eddy current flaw detection method that can suppress the influence of unevenness on the outer surface of a pipe body while maintaining the accuracy of crack detection. [Means for solving the problem]

[0011] [1] An eddy current flaw detection method for detecting cracks on the inner surface of a tubular body by bringing a probe of an eddy current flaw detection probe close to the outer surface of the tubular body, the eddy current flaw detection method comprising a step of forming a film layer on the outer surface. [2] An eddy current flaw detection method for detecting cracks on the inner surface of a tube by bringing a probe of an eddy current flaw detection probe close to the outer surface of the tube, the eddy current flaw detection method comprising the steps of: forming a film layer on the outer surface; setting an eddy current penetration distance based on the depth of the crack from the inner surface and the thickness of the tube, and calculating a current frequency based on the set eddy current penetration distance; and inspecting the tube from the top surface of the film layer using the probe based on the calculated current frequency. [3] The eddy current flaw detection method according to [2], wherein the current frequency is calculated based on equation (1).

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[0012] According to the present invention, it is possible to suppress the influence of irregularities on the outer surface of the pipe body and maintain the accuracy of crack detection. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a diagram showing the configuration of a converter, a gas recovery device, a tube body, and fins according to an embodiment of the present invention. FIG. [Figure 2] 1 is a simplified schematic diagram showing a state in which water flows through the inside of a pipe body and a state in which exhaust gas flows through the inside of a pipe. FIG. [Figure 3] 1A and 1B are diagrams showing a state in which a crack occurs on the inner surface of a pipe body and a configuration related to an eddy current flaw detection method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. Fig. 1 shows the configuration of a converter 1, a gas recovery device 2, a tubular body 3a, and fins 3b of this embodiment. Fig. 1(a) is a diagram showing the arrangement of the converter 1 and the gas recovery device 2. Fig. 1(b) shows a cross-sectional view of the piping 3 based on the cross section seen from the arrow AA in Fig. 1(a), and also shows an enlarged view of the arrangement of the tubular body 3a and fins 3b that constitute the piping 3.

[0015] As shown in Fig. 1(a), the gas recovery device 2 is installed above the converter 1. Therefore, high-temperature exhaust gas generated in the converter 1 during the molten steel blowing process rises and is recovered in the gas recovery device 2, and then sent to an external exhaust device connected to the outside.

[0016] 1(b), the gas recovery device 2 also has a pipe 3 through which the exhaust gas flows. The pipe 3 has a membrane structure in which multiple pipes 3a are arranged in a circular ring shape and the pipes 3a are welded together via fins 3b. This configuration allows heat exchange between the water flowing inside the circularly arranged pipes 3a and the recovered exhaust gas, making it possible to recover heat from the exhaust gas.

[0017] Next, the state in which water flows inside the pipe body 3a and the state in which exhaust gas flows inside the pipe 3 will be described with reference to Fig. 2. Fig. 2 is a schematic diagram simply showing the state in which water flows inside the pipe body 3a and the state in which exhaust gas flows inside the pipe 3. Fig. 2 shows an enlarged view of the configuration (piping 3 and pipe body 3a) relating to range B in Fig. 1.

[0018] As shown in Figure 2, exhaust gas G flows through the pipe 3 of the gas recovery device 2 in a flow direction E. At the same time, water flow W flows through the pipe body 3a in a flow direction F. Therefore, as described above, heat exchange occurs between the water flow W and the exhaust gas G, and the heat of the high-temperature exhaust gas G is transferred to the water flow W. In other words, the heat is recovered by the water flow W.

[0019] Here, in the annularly arranged pipe body 3a, a temperature difference occurs between the inner surface in contact with the water flow W and the outer surface in contact with the exhaust gas G. Furthermore, when air bubbles Z are retained inside the water flow W on the inner surface of the pipe body 3a, the air bubbles Z affect the heat exchange with the exhaust gas G, leading to thermal fatigue.

[0020] Furthermore, if thermal fatigue continues for a long period of time, cracks (heat cracks) will occur in the tube 3a. The cracks that occur in the tube 3a will extend from the inner surface to the outer surface of the tube 3a, and if the cracks subsequently reach the outer surface, water leakage will occur.

[0021] Next, a state in which a crack occurs on the inner surface of the tube 3a and the configuration of the eddy current flaw detection method of the present invention will be described with reference to Figure 3. Figure 3 is a diagram showing a state in which a crack occurs on the inner surface of the tube 3a and the configuration of the eddy current flaw detection method of the present invention. Note that the thickness C of the tube 3a and the thickness D of the film layer R shown in Figure 3 are merely examples shown schematically for the sake of convenience, and in actuality, the layer thickness D will be infinitesimally small compared to the thickness C.

[0022] 3, the eddy current flaw detection method according to the present invention is characterized in that, in order to detect a crack K extending from the inner surface M of the pipe body 3a to the outer surface N, a film layer R is formed on the outer surface N of the pipe body 3a, and the probe P of the eddy current flaw detection probe is brought into contact with the upper surface S of the film layer R to detect the crack K. The eddy current flaw detection method according to the present invention will be described in detail below.

[0023] The eddy current flaw detection method according to the present invention includes a step of forming a film layer R on the outer surface N of the tube 3a before flaw detection is performed. The film layer R preferably contains a material that has excellent corrosion resistance against high-temperature exhaust gas G. By forming the film layer R on the outer surface N of the tube 3a, oxidation and corrosion caused by contact with high-temperature exhaust gas G on the outer surface N of the tube 3a can be suppressed, and the occurrence of uneven shapes due to corrosion, etc. can be suppressed. In other words, by forming the film layer R before flaw detection is performed, the influence of unevenness on the outer surface N of the tube 3a during flaw detection can be suppressed.

[0024] Next, the eddy current flaw detection method according to the present invention includes a step of setting an eddy current penetration distance V based on the depth L from the inner surface M of the crack K to be detected and the thickness C of the pipe body 3a, and calculating a current frequency f based on the set eddy current penetration distance V.

[0025] Specifically, first, the depth L from the inner surface M of the crack K that needs to be detected is determined. At this time, when detecting a deep crack ("crack K1" in FIG. 3), it is preferable to set the depth to "L1." Furthermore, when detecting many cracks occurring on the inner surface M, it is preferable to set the depth to "L2" in order to detect a shallow crack ("crack K2" in FIG. 3). In this embodiment, the purpose is to detect the deep crack K1, and the following description will be given taking as an example a case where the depth is determined to be "L1."

[0026] It is preferable to set the eddy current penetration distance V based on the determined depth L1 of the crack K1 and the thickness C of the tubular body 3a. As mentioned above, in reality, the layer thickness D of the membrane layer R is infinitesimally small compared to the thickness C. For this reason, it is preferable to set the eddy current penetration distance V as a value obtained by subtracting the determined depth L1 of the crack K1 from the thickness C of the tubular body 3a.

[0027] Thereafter, it is preferable to calculate the current frequency f using the following formula (1) based on the set eddy current penetration distance V. In formula (1), y is the eddy current penetration distance V (m), π is the circular constant (rad), f is the current frequency f (kHz), μ is the magnetic permeability (H / m) of the tube body 3a, and σ is the conductivity (S / m) of the tube body 3a.

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[0028] Next, the eddy current flaw detection method according to the present invention includes a step of detecting flaws in the tube 3a using the probe P from the upper surface S of the coating layer R based on the calculated current frequency f. Specifically, as shown in Fig. 3, it is preferable to perform flaw detection on the tube 3a while moving the probe P on the upper surface S of the coating layer R. At this time, the flaw detection is performed in the probe P based on the calculated current frequency f and the eddy current penetration distance V, and therefore, as shown in Fig. 3, the eddy current U generated in the probe P makes it possible to detect a crack K1 with a depth L1.

[0029] In the present invention, since the coating layer R is formed before eddy current testing, testing is possible with the probe P in contact with the upper surface S of the coating layer R. This brings the probe P close to the outer surface N of the tube 3a, and the distance between the probe P and the outer surface N of the tube 3a is maintained constant. In other words, since the film thickness D of the coating layer R is formed to be constant, the distance between the probe P and the outer surface N of the tube 3a can be maintained constant (ensuring lift-off). Therefore, the eddy current testing method according to the present invention stabilizes the penetration distance V of the eddy current penetrating into the tube 3a, suppressing the influence of irregularities on the outer surface N of the tube 3a and maintaining crack detection accuracy.

[0030] Furthermore, since the current frequency f can be calculated using equation (1) when the eddy current penetration distance V is changed, even if various cracks K with different depths L occur on the inner surface of the tube body 3a, it is possible to detect the cracks K corresponding to each depth L by adjusting the current frequency f and the penetration depth of the magnetic flux (eddy current penetration distance V).

[0031] In the eddy current flaw detection method according to the present invention, the step of forming a film layer R on the outer surface N of the tubular body 3a is preferably carried out when the tubular body 3a is replaced with a new one in the piping 3 of the gas recovery device 2. In this case, the progression of corrosion due to contact with the exhaust gas G is constantly suppressed, and eddy current flaw detection can be performed on the tubular body 3a at a desired timing that coincides with the operation of the converter 1 and the gas recovery device 2. In other words, there is no restriction on the timing at which eddy current flaw detection is performed, and when eddy current flaw detection is performed, the distance between the probe P and the outer surface N of the tubular body 3a is always maintained constant.

[0032] Furthermore, the step of forming the film layer R on the outer surface N of the pipe 3a may be performed immediately before performing eddy current testing on the pipe 3a. In this case, even if the outer surface N of the pipe 3a becomes uneven due to the progression of corrosion caused by contact with the exhaust gas G, by forming the film layer R immediately before performing eddy current testing, the distance between the probe P and the outer surface N of the pipe 3a is maintained constant when performing eddy current testing.

[0033] Although the film layer R in this embodiment has been described as being configured to maintain a constant distance between the probe P and the outer surface N of the tubular body 3a, it is preferable to use a material that improves the dissociation of dust particles and the like to prevent the adhesion and accumulation of dust particles and the like on the upper surface S of the film layer R. In this case, it is preferable to use an antistatic material that provides an antistatic effect to prevent the adhesion of dust particles and the like. When forming the film layer R for the purpose of antistatic purposes, it is preferable to apply a coating agent such as "Ceracoat 22." Specifically, it is preferable to form the film layer R by sandblasting or sanding the outer surface N of the tubular body 3a, and then applying the coating agent using a roller or brush. By improving the dissociation of dust particles and the like on the upper surface S of the film layer R, the distance between the probe P and the outer surface N of the tubular body 3a is maintained more constant during eddy current testing, thereby improving the accuracy of crack detection. Note that the coating agent "Ceracoat 22" not only functions as an antistatic material that improves the dissociation of dust particles and the like, but also has the effect of suppressing oxidation and corrosion caused by contact with high-temperature exhaust gas G.

[0034] Furthermore, it is preferable to use a material containing a non-magnetic material for the film layer R. This is because including a non-magnetic material in the film layer R reduces the rate at which the magnetic flux of the eddy current U emitted from the probe P is attenuated as it penetrates the film layer R. Furthermore, from the viewpoint of maintaining the detection accuracy of cracks K by eddy current flaw detection, it is preferable that the film layer R be made of a material whose expansion coefficient and thermal conductivity are similar to those of iron. The method for forming the film layer R is not limited, as long as it is a method that can form a layer, such as spraying or painting as a coating agent on the outer surface N of the tube body 3a.

[0035] Furthermore, in this embodiment, as described above, it is preferable that the eddy current penetration distance V is determined based on the depth L of the crack K and the thickness C of the tubular body 3a. However, to further improve the detection accuracy of the crack K, the layer thickness D of the membrane layer R may also be taken into consideration. In this case, for example, it is preferable that the eddy current penetration distance V is set as a value obtained by subtracting the depth L of the crack K from the sum of the thickness C of the tubular body 3a and the layer thickness D (e.g., 0.25 mm) of the membrane layer R.

[0036] In this way, by determining the eddy current penetration distance V taking into consideration not only the depth L of the crack K and the thickness C of the pipe body 3a but also the thickness D of the membrane layer R, it is possible to more accurately set the eddy current penetration distance V and calculate the current frequency f. Then, by performing eddy current testing of the pipe body 3a based on the calculated current frequency f, it is possible to further improve the detection accuracy of the crack K. [Example]

[0037] The results of implementing the eddy current flaw detection method according to the present invention will be described. First, a first tubular body was prepared with cracks of 0.5 to 3.0 mm at multiple locations on the inner surface, and eddy current flaw detection was carried out by bringing the probe of an eddy current flaw detection probe into contact with the outer surface of the first tubular body. The first tubular body was a test specimen prepared using an unused steel pipe product (STB410) as a gas recovery device, and had a structure without irregularities on the outer surface. The first tubular body had a thickness of 5 mm and a magnetic permeability of 2.2 x 10 -4 H / m, conductivity 5.9×10 6 The same was true for the second and third tubular bodies used in the following examples.

[0038] In the eddy current testing of the first tube, the eddy current penetration distance was set and the current frequency was calculated assuming a crack depth of 1.0 mm, and the eddy current testing was performed by bringing the probe of the eddy current testing probe into contact with the outer surface of the tube based on the calculated current frequency. In this case, the eddy current penetration distance was set to the value obtained by subtracting the crack depth (1.0 mm) from the thickness of the tube.

[0039] As a result of eddy current testing of the first tube, among the multiple cracks on the inner surface of the tube, cracks with a depth of 1.0 to 3.0 mm were detected. This can be said to be the result of determining the depth of the cracks to be detected as 1.0 mm, and being able to detect cracks with a depth of 1.0 mm or more without forming a film layer on the outer surface of the tube, because the outer surface of the tube does not have an uneven shape.

[0040] Next, a pipe was collected from a used pipe in the gas recovery device and prepared as the second pipe. It was confirmed that cracks of 0.5 to 3.0 mm had occurred in multiple locations on the inner surface of the second pipe. Then, eddy current testing was performed by contacting the probe of an eddy current testing probe with the outer surface of the second pipe. Since the second pipe had been used as a component of the piping of the gas recovery device, erosion had progressed due to contact with exhaust gas, forming an uneven shape on the outer surface.

[0041] In the eddy current testing of the second tube, the eddy current penetration distance was set and the current frequency was calculated with the crack depth to be detected set to 1.0 mm, and the eddy current testing was performed by bringing the probe of the eddy current testing probe into contact with the outer surface of the tube based on the calculated current frequency. The eddy current penetration distance was set as the value obtained by subtracting the crack depth (1.0 mm) from the thickness of the tube.

[0042] As a result of eddy current testing of the second pipe, among the multiple cracks formed on the inner surface of the pipe, cracks with a depth of 3.0 mm or more could be detected, but cracks with a depth of less than 3.0 mm could not be detected. This can be attributed to the fact that the outer surface of the pipe had an uneven shape, which caused the penetration distance of the eddy current into the inside of the pipe to fluctuate irregularly, resulting in a deterioration in crack detection accuracy.

[0043] Next, a pipe was collected from a used pipe in the gas recovery device, and a film layer was formed on the outer surface of the pipe, which had an uneven shape, to prepare a test specimen as the third pipe. It was confirmed that cracks of 0.5 to 3.0 mm had occurred in multiple locations on the inner surface of the third pipe. Eddy current testing was then performed by contacting the probe of an eddy current testing probe with the film layer on the outer surface of the third pipe. The thickness of the film layer formed on the outer surface of the third pipe was 0.25 to 0.30 mm.

[0044] In the eddy current testing of the third tube, the eddy current penetration distance was set and the current frequency was calculated assuming a crack depth of 1.0 mm, and the eddy current testing was performed by bringing the probe of the eddy current testing probe into contact with the film layer based on the calculated current frequency. In this case, the eddy current penetration distance was set to the value obtained by subtracting the crack depth (1.0 mm) from the thickness of the tube.

[0045] As a result of eddy current testing of the third tube, among the multiple cracks formed on the inner surface of the tube, cracks with depths of 1.0 to 3.0 mm were detected. This can be said to be the result of determining the depth of the cracks to be detected as 1.0 mm and forming a film layer on the outer surface of the tube, which stabilized the penetration distance of the eddy currents penetrating into the inside of the tube, thereby maintaining the accuracy of crack detection. [Explanation of symbols]

[0046] 1 Converter 2 Gas recovery device 3 Piping 3a Body 3b fins C (body thickness) D (film layer) thickness E Transmission direction F Distribution direction G. Exhaust gas K crack L (crack) depth M Inner surface N External surface P probe R membrane layer S (membrane layer) top surface U Eddy current Z bubble

Claims

1. An eddy current flaw detection method for detecting cracks on an inner surface of a pipe by bringing a probe of an eddy current flaw detection probe close to the outer surface of the pipe, comprising: An eddy current flaw detection method comprising the step of forming a film layer on the outer surface.

2. An eddy current flaw detection method for detecting cracks on an inner surface of a pipe by bringing a probe of an eddy current flaw detection probe close to the outer surface of the pipe, comprising: forming a membrane layer on the outer surface; setting an eddy current penetration distance based on the depth of the crack from the inner surface and the thickness of the pipe body, and calculating a current frequency based on the set eddy current penetration distance; a step of detecting flaws in the tubular body from the upper surface of the film layer using the probe based on the calculated current frequency; An eddy current flaw detection method comprising:

3. The eddy current flaw detection method according to claim 2 , wherein the current frequency is calculated based on equation (1). [Equation 1] Here, y is the eddy current penetration distance (m), π is the circumference constant (rad), f is the current frequency (kHz), μ is the magnetic permeability of the tube body (H / m), and σ is the conductivity of the tube body (S / m).

4. The eddy current flaw detection method according to any one of claims 1 to 3, wherein the film layer includes a non-magnetic material.

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