JP4087312B2 - Inspection method and apparatus for deterioration of metal using high sensitivity magnetic sensor - Google Patents

Description

The present invention relates to a method and apparatus for inspecting deterioration such as fatigue and damage of metals constituting equipment and structures such as thermal power / nuclear power generation facilities, petrochemical plants, and railway vehicles. In particular, it allows non-destructive inspections such as fatigue and deterioration of metals in the initial stage based on the addition of stress below the elastic limit without repeated metallographic changes due to aging of the structure. .

  Fatigue fractures and cracks in equipment and structures are not limited to minor accidents and can lead to major accidents, which are often caused by unexpected material aging. For this reason, establishment of a non-destructive diagnosis technique and remaining life evaluation technique for facilities such as plants is strongly desired.

  For example, many thermal power generation facilities and petrochemical plants were built in the 1960s, and these high-temperature equipment deteriorates over time due to long-term use of high temperatures, so the existing facilities are aging. is doing. Recently, the problems of cracks in the core bulkhead (shroud) of nuclear power plants and cracks in the bogies of subway vehicles have been highlighted. These cracks and cracks are considered to be caused by internal stress remaining in the structure.

  In particular, large-scale facilities that constitute social infrastructures such as energy and transportation have become difficult to secure new locations and sites in recent years, and there is a need for efficient operation and life extension of existing facilities. There is a growing need for non-destructive inspection of materials and structures.

  Conventional techniques for nondestructive inspection of fatigue, damage, cracks, etc. in metals constituting equipment and structures such as thermal and nuclear power generation facilities, petrochemical plants, railway vehicles, etc. are visual (VT), hammering, metal Examples include tissue observation, magnetic particle testing (MT), penetration testing (PT), eddy current testing (ET), and ultrasonic testing (UT). Among them, the UT is widely adopted as a method for nondestructively inspecting deterioration of crack generation due to fatigue and damage in equipment and structures such as various plants and railway vehicles.

  However, the UT is an inspection method that requires skill in the operation method. According to the announcement, the environment for fostering UT engineers is established in the United States, but the response is delayed in Japan. Moreover, UT has the fault that it cannot test | inspect, when a test object is a multilayered structure, For example, it cannot apply to the test | inspection of the heat resistant metal in which the coating was given to the surface. Furthermore, it goes without saying that UT is a technique for inspecting the presence or length of cracks and cracks, and the like, and at the initial stage of deterioration such as fatigue and damage in equipment and structures, that is, before the stage of cracks and cracks. Non-destructive inspection cannot be performed. This is not limited to the UT, and the above-described prior art is a method for finding cracks that have already occurred without exception. If inspections are performed at intervals of one year, the occurrence of cracks may be overlooked for up to one year. Become.

  Based on the above, (1) simple inspection method and inspection device that do not require skill in operation methods, (2) inspection is possible even for metal with a multilayer structure, (3) deterioration of metal fatigue, damage, etc. There is a need for a non-destructive inspection method for the initial stage.

  In recent years, the environment in which equipment and structures are used tends to be harsh, and therefore there are increasing opportunities for stainless steel having excellent corrosion resistance to be used. Among them, austenitic stainless steel including SUS304 is frequently used. These non-magnetic austenitic stainless steels have long been known to produce weak magnetism when mechanical force is applied. One of the causes of this phenomenon is a transformation to a ferromagnetic martensite phase due to plastic deformation. It is clear that martensitic transformation (for example, the crystal structure changes from fcc to bcc) is not limited to stainless steel, but exists in many metals and alloys, regardless of whether it is iron or non-ferrous. Detailed studies on magnetism and plasticity of ordered alloys have been made (for example, Non-Patent Document 1). In addition, coercivity (the force that keeps the magnetic moment of an atom tightly bound in a specific direction in the crystal, that is, the force that antagonizes the force to eliminate magnetism) is qualitatively pinned by the grain boundary. Therefore, if the defect or crystal structure changes due to external factors such as stress, the magnetism of the metal changes.

  Thus, the crystal structure and microstructure of metal are closely related to magnetism, and are physical properties that are extremely sensitive to structural changes. Therefore, in many metals such as austenitic stainless steel, if their magnetism (leakage magnetic flux) is measured, the inspection method can generally detect with high sensitivity whether or not mechanical force has been applied to the metal. It is expected that

  From this point of view, Yuji Nagae et al. Clarified the correlation between weak magnetic properties generated in specimens that have been subjected to tensile or creep damage to steel materials such as austenitic stainless steel and material mechanical strain. An attempt to apply the test results to non-destructive inspection is reported (Non-Patent Document 2).

  However, the above report uses a fluxgate (FG) type sensor having a resolution of 0.001 Gauss as a magnetic sensor, so that a higher sensitivity measurement than a conventional type magnetic sensor (for example, a Hall element) is expected. Although it is possible, the measurement sensitivity is not always sufficient. Also, the spatial resolution is not sufficient due to the size problem of the magnetic sensor itself. Furthermore, a strain of 3% is applied to the test piece at the minimum, and stress is applied up to the plastic deformation region, and there is no mention of stress application in the elastic region.

  Yoshida has proposed an example of a non-destructive inspection apparatus using a SQUID for a SUS304 tensile specimen (Non-Patent Document 3). There are only three types of test pieces: in front of the tensile test piece, in the elastic deformation region, and 15% residual strain. It has already been reported that the austenite phase, which is a paramagnetic material, has been transformed into a martensitic phase of a ferromagnetic material due to distortion, although a magnetic change has been taken in the elastic deformation region. In other words, the relationship between tensile strength and SQUID signal is not quantitative.

  In addition, Tsuyoshi Uchiyama et al. Have reported that a magnetic impedance (MI) sensor can detect the magnetism due to local crystal strain associated with plastic deformation due to steel tension in one type of hot rolled steel (Non-Patent Document 4). ). Although steel materials that have undergone plastic deformation and steel materials that have not undergone plastic deformation can be distinguished, the correlation between the magnitude of tension and the magnetic signal is not clarified.

As described above, various changes in the magnetic properties when stress is applied to the metal have been considered, but the specific content is that of the change in the magnetic properties when stress is applied to the plastic deformation region. However, the change in magnetic properties when the stress in the elastic region is applied is not clarified.
Devices and structures are designed with a safety factor in mind, and it is usually difficult to imagine that stress is applied to a region where the metal of the structure is distorted.

  From the above, an inspection method and an inspection apparatus capable of non-destructively examining the damage level (index) associated with the addition of stress in the so-called elastic region before that, not only in the region with distortion as in the prior art. Development is required.

  On the other hand, Japanese Patent Application Laid-Open No. 2002-350403 discloses a prior art for predicting the occurrence timing of a crack before a crack is generated due to the cumulative fatigue of a metal material. An evaluation method and an evaluation apparatus for predicting the degree of cumulative fatigue or the remaining life of crack generation by creating a database of relationships are disclosed (Patent Document 1). In this method, a magnetic field is applied to the metal material from the outside, the magnetic anisotropy is measured by the magnetic anisotropy detection sensor, and the cumulative fatigue of the metal material is evaluated from the magnitude of the anisotropy. Ancillary equipment for adding is necessary, measurement is complicated, and it is generally difficult to apply a constant magnetic field non-destructively to large equipment.

Japanese Patent Laid-Open No. 2001-21538 discloses a deterioration detection method based on the relationship between a magnetic susceptibility coefficient c calculated based on measurement of a magnetic susceptibility at a predetermined magnetic field strength of a ferromagnetic structure and tensile stress ( Patent Document 2).
The magnetic susceptibility coefficient of the ferromagnetic structure to be inspected is measured and calculated, and the tensile stress σ is derived from the universal curve (or calibration curve) obtained from the experimental data showing the above relationship in advance for the ferromagnetic body to be inspected. The deterioration state at the stage before the crack is generated is inspected by comparing the stress value σ with the tensile stress value σ0 in the initial state of the ferromagnetic structure to be inspected.
However, this disclosure also shows an example only when a strain of about several percent is included, and there is no description regarding an elastic region that is normally regarded as having no change in material mechanics.
JP 2002-350403 A Japanese Patent Laid-Open No. 2001-21538 Takahasi et al., Phys. Stat. Solid B69 (1979) 27. Nagae Yuji et al., Cycle Mechanism Technical Report, No. 14 (2002.2.3), p. p. 125-135 [Development of damage nondestructive detection technology for austenitic stainless steels] Yoshida Yoshikazu, Inspection Technology '99 .5 (1999), p. p. 12-15 [Practical application of nondestructive inspection equipment using SQUID] Takeshi Uchiyama et al., Journal of Japan Society of Applied Magnetics, Vol23, Vo. 4-2 (1999), p. p. 1465 to 1468 [Evaluation of plastic deformation of structural steel by MI magnetic field difference sensor]

An object of the present invention relates to a technique for nondestructively inspecting signs and the like leading to the occurrence of cracks, such as fatigue and damage of equipment and structures such as thermal power / nuclear power generation facilities, petrochemical plants, and railway vehicles. More specifically, it is an object of the present invention to provide an inspection method and apparatus for nondestructively detecting the progress of fatigue and deterioration of a metal at an extremely early stage due to repeated stress application in an elastic region without a strain applied by daily use.

The present invention uses a superconducting quantum interference device (SQUID) to detect the magnetism of a sample to be inspected, and compares the detected value with a predetermined reference value, thereby repeatedly applying stress in an elastic region without any metallic structural change. Detects the presence or absence of initial deterioration due to.

The first technical means of the present invention is a non-destructive inspection method for inspecting the progress of metal degradation based on the addition of stress below the elastic limit not accompanied by a metallographic change, and comprising a superconducting quantum interference device (SQUID) Using a plurality of samples made of the same material as the specimen metal, after applying different stresses below the elastic limit, each leakage flux is detected, and two-dimensional mapping data indicating the distribution of the leakage flux is created. By calculating the area of the local leakage magnetic flux increase region or the decrease region and obtaining reference data indicating the correspondence between the stress application and the increase region or the decrease region, the superconducting quantum interference element ( (SQUID) is used to detect the leakage flux of the subject metal, perform two-dimensional mapping of the leakage flux, and increase the local leakage flux obtained based on the mapping, or Calculating the area of the reduction area, characterized by non-destructive inspection method for a metal to inspect the progress of deterioration by comparing the detection result with the reference data.

The second technical means is a magnetic sensor composed of a superconducting quantum interference element (SQUID), and detects the leakage magnetic flux of the subject metal using the sensor, and the detection output is compared with a predetermined threshold to increase or decrease the leakage magnetic flux. A data processing unit for creating two-dimensional mapping data indicating the distribution of the magnetic field, and an analysis unit for calculating an area of a local leakage magnetic flux increase region or a decrease region based on the mapping data. It features a nondestructive inspection device for metals used in nondestructive inspection methods.

A third technical means is the second technical means, wherein the increase area and the decrease area of the leakage magnetic flux are indicated as color data.

According to a fourth technical means, in the second or third technical means, a monitoring device for displaying two-dimensional mapping data and a recording device for the data are provided.

According to the present invention, fatigue at an early stage before a metallographic change occurs due to the addition of stress within the elastic limit in a metal that is a material of equipment or a structure such as a thermal power / nuclear power generation facility, a petrochemical plant, and a railway vehicle. Etc. can be inspected nondestructively.

  Below, SUS304L and SUS304 were selected from austenitic stainless steel, which is a typical metal used in various devices and structures, and test pieces were artificially subjected to tensile damage (damage). An example is shown.

  In FIG. 1, 10 is a subject, 20 is an inspection apparatus, and the inspection apparatus 20 includes a magnetic measurement unit 21 using a high-sensitivity magnetic sensor such as a SQUID, a magnetic impedance sensor, or a fluxgate sensor, and a detected detection. A storage unit 22 for storing magnetic signals of values, an image processing unit 23 for mapping the recorded magnetic data into a required two-dimensional distribution to create a magnetic image, an average value, integral value, and local magnetism in the magnetic image Data processing such as calculating the signal intensity of the difference between the increase / decrease peaks and the area (surface distribution) of the peak area of the magnetic increase / decrease, and the analysis unit 24 that compares the detection data or the processed data with the reference value And a monitoring / recording device unit 25.

  As a high-sensitivity magnetic sensor used for the magnetic measurement unit 21, an MI sensor can be used for an object that can be detected with a magnetic field sensitivity of a minimum of about 1 nanotesla. On the other hand, when it is necessary to detect a weaker magnetic field than this, the SQUID can be used. When the magnetic field emitted from the subject is extremely small, a highly sensitive magnetic sensor is used to cancel the geomagnetism and magnetic noise. It is necessary to inspect it by installing it inside the magnetic shield. On the other hand, the latest fluxgate type sensor can be used if the magnetic field is about 10 nanotesla. Even the latest Hall element cannot be used due to insufficient magnetic field sensitivity.

In addition, in the case of SQUID, magnetism can be detected with a resolution of about 10 microns at the maximum, which is suitable for local magnetic analysis. A magneto-impedance sensor or the like can be detected with a resolution on the order of millimeters, so that it can be applied to inspections that do not require high resolution. As the performance of each sensor improves, the resolution further improves.
The high-sensitivity magnetic sensor is driven by a sensor control system configured by electronic equipment.

  The storage unit 22 corresponds to a measurement pitch in which the magnitude of the magnetic signal output via the AD converter is set in advance (for example, if a SQUID is used, a scan area of 1 mm square is measured at 10 μm intervals at 10,000 points). Record.

  The image processing unit 23 is devised so that detection values can be colored and mapped in color or black and white according to the magnetic intensity, and the degree of deterioration can be detected very easily. Does not require special skills. Of course, magnetic images can also be displayed as contour lines or histograms.

  The analysis unit 24 compares the data in the storage unit 22 and the image processing unit 23 with a calibration curve (or database), and uses the threshold value set based on data measured in advance for each application target as a reference for the health of the subject. Make a decision.

  The monitoring / recording device unit 25 displays the result analyzed by the analysis unit 24 on a display or prints it on recording paper.

  When SQUID is used for a sensor, the detected value of the sensor becomes a relative value, so that the intensity between the peaks of the local detected value (difference in increase / decrease in the detected value of local magnetism), the average value of the detected values, The magnetic strength is grasped by the integral value.

  FIG. 2 shows the shape of a tensile test piece produced for SUS304L, and the test piece was cut out in the same shape as FIG. 2 by electric discharge machining (wire cut) from a material manufactured in large quantities on a mass production line. Thereafter, in order to remove the residual stress generated when the material was cut out, the distortion was removed by electrolytic polishing of the hatched portion in FIG. 2 by about 0.3 mm. Although a method for removing strain by heat treatment is inexpensive and general, electrolytic polishing is used to suppress subtle structural changes due to thermal history. The presence of no excessive strain in each test piece was examined by measuring residual stress on the γ-Fe (311) diffraction surface with a microscopic x-ray stress measurement device (Rigaku Electric).

Fig. 3 is a stress-strain diagram when this specimen is pulled with a tensile tester until it breaks.
Circled number 1 is the reference (non-tensioned) load, none Circled number 2 is the tensile load up to 20% of 0.2% proof stress (or yield stress),
The number 3 with a circle is the tensile load up to 50% of the 0.2% proof stress,
The number 4 with a circle is the tensile load up to 70% of the 0.2% proof stress,
The number 5 with a circle is the tensile load up to 90% of 0.2% proof stress,
The circled number 6 is the maximum load,
The rounded number 7 is the intermediate load between the break and the maximum load,
The number 8 with a circle indicates a time when a breaking load is applied.
As apparent from FIG. 3, when a load of about 20,000 N was applied, the test piece broke. From the elastic region to the plastic deformation region and fracture, eight types of stress indicated by arrows in FIG. 3 were applied only once to prepare eight types of test pieces.

  FIG. 4 shows a magnetic image inspected with respect to the above-mentioned eight types of SUS304L test pieces (circled numbers 1 to 8) using a SQUID as a magnetic sensor. The range of the magnetic image diagram shown in FIG. 4 is 1 mm square, and 10,000 points of data are displayed at intervals of 10 μm. Qualitatively, it is magnetized by an environmental magnetic field (such as geomagnetism), and qualitatively, as the stress intensity increases, the relative magnetic field increases (eg, displayed in red) and the relative magnetic field decreases (eg, displayed in blue). As shown, an increase in the area of the vortex clustered in the magnetization region is shown. The strength of the magnetic field corresponding to each color is as per the color scale criteria shown in FIG. In color display, two colors are interpolated, and in FIG. 4 (color) shown as an example, white: 190 μT red: 145 μT orange: 100 μT yellow: 55 μT green: 10 μT light blue: −35 μT blue: −80 μT pink: −120 μT black : -165 μT and a reference value were determined and displayed. Of course, it is also possible to display with contour lines or histograms instead of distinguishing colors.

FIG. 5 summarizes the signal intensity between the peaks of the partial increase and decrease of the magnetic field, the average value of the detected magnetic field, and the integral value thereof. The signal intensity between the peaks of the magnetic field is the difference between the maximum intensity and the minimum intensity from the measurement result of each sample, and its unit is Gauss (G). One micro tesla (μT) corresponds to 10 milligauss. Next, the average value of the 10,000 detected signal intensities is simply the average value of the detected magnetic field, and its unit is Gauss. Furthermore, the volume of the strength of each measurement point, that is, a value obtained by integrating and dividing by 1 mm ² of the scanning range is displayed as an integrated value. The integral value of each point is expressed by a unit weber (Wb) of magnetic flux from a simple calculation. This is divided by the scanning area of 10 ⁻⁶ m ² and converted into magnetic flux density, which is shown in FIG. Here, 1T (Tesla) = 1 Wb / m ² .

  When stress is applied up to the elastic region, these three parameters hardly increase, but a rapid increase is observed where a stress in the plastic deformation region exceeding the maximum stress in the elastic region is applied.

  FIG. 6 shows the result of observing the surface metallographic structure at 400 times using an optical microscope for samples cut out from the same eight types of SUS304L test pieces inspected in FIG. Such structure observation corresponds to the replica method as a conventional method. In the sample (circled number 1 to circled number 5) to which stress up to the elastic region is applied, no significant difference is observed in terms of metal structure. On the other hand, in the sample (circled number 6 to circled number 8) to which the stress in the plastic deformation region was applied, a systematic change appeared and deformation stripes were observed. Consistent with this result, also in FIG. 5 shown in the example, a rapid increase was observed for the above three parameters for the sample to which stress was applied to the plastic deformation region. Furthermore, in the structure observation, it is not possible to distinguish a clear difference at first glance with respect to the deformation stripes of the three samples in which stress is applied up to the plastic deformation region, whereas in this example, all three parameters increase systematically in proportion to the strength of the stress. Therefore, this method can be said to be a detection method with high accuracy.

  As described above, by focusing on the detected magnetism grasped by these three parameters (the signal intensity between the peaks of the local increase / decrease of the magnetic field, the average value of the detected magnetic field, and its integral value) By detecting the increased region, it is possible to inspect with high accuracy whether or not the sample has been subjected to stress greater than the elastic region, that is, permanent strain.

7 quantifies the clustered vortex area or surface distribution (corresponding to the area where the magnetic change has occurred) of the magnetic field of the magnetic image shown in FIG. 4 (color display) with pixels by the image processing unit. Results are shown. The inspected 1 mm square corresponds to 300 × 300 pixels. The standard deviation (σ ₁ , σ ₂ ) is plotted with the relative magnetic field decrease (blue) vortex area in blue and the relative magnetic field increase (red) vortex area in red. That is, in quantifying the area, calculation was performed based on RGB (R: red, G: green, B: blue) of the image. Cluster color vortex areas were calculated by setting red as R: 200, G: 0, B: 0, blue as R: 0, G: 0, and B: 50. In the example shown here, the red area counts 40 μT or more, and the blue area counts −40 μT or less. Of course, the calculation of the area may be based on the color or the intensity of the magnetic signal. In FIG. 7, σ ₁ indicates the standard deviation of the area of the blue vortex, and σ ₂ indicates the standard deviation of the area of the red vortex.

  In FIG. 5, no change was detected in the addition of stress in the elastic region, but in FIG. 7, the elastic region before the plastic deformation region also tends to increase in proportion to the applied stress. I understand that. This is because even in a region that is reversible in terms of material mechanics, changes in the atomic level metal structure left in the specimen due to stress applied to the specimen are caused by local residual magnetization (in a broad sense, spontaneous magnetization). ) Is considered to be measured. In the metal structure observation shown in FIG. 6, no structural differences were observed for the five samples up to the elastic region, and events associated with changes in the metal structure at the atomic level that could not be detected by conventional observation techniques such as the replica method were detected. can do. In the X-ray diffraction measurement of the test piece, the structural change in the elastic region could not be detected, and it can be seen that the accuracy of the present invention is high.

  Further, in the plastic deformation region, a rapid change is observed, which coincides with the tendency shown in FIG. 6, and it is possible to inspect whether or not a stress greater than the elastic region is applied to the test piece. It is one of the features of the present invention that the history of stress applied in the elastic region can be found by inspecting the area (surface distribution) of the region showing the magnetic change in this way.

  The numbers 1 to 8 with circles in the arrows in FIG. 3 correspond to the numbers 1 to 8 with circles in FIGS. 4, 5, 6, and 7, respectively. The circled numbers 5 are samples in the elastic region, and the circled numbers 6 to 8 are plastic deformation regions.

  FIG. 8 shows a test result using a magnetic impedance sensor (MI sensor) as a high-sensitivity magnetic sensor, replacing the steel type with SUS304. Eight types of specimens with the shape shown in FIG. 2 as in the SUS304L system, which are artificially pulled and stressed once from the elastic region to the plastic deformation region under the same conditions as those shown in FIG. Of the samples were examined. In this inspection, the image processing unit was not operated. Although the SQUID is not completely the same because it represents a relative value, the magnetic field strength of the MI sensor shown in FIG. 8A corresponds to the integral value of FIG. 5 measured by the SQUID. Similar to SUS304L in FIG. 5, a large magnetic signal is clearly recorded in the sample in which stress is applied to the plastic deformation region.

  On the other hand, FIG. 8 (B) shows that the same steel type SUS304 is subjected to a stress of 90% of the elastic limit (90% of proof stress of 0.2%) at a speed of 20 times / second with a fatigue tester once, 10,000 times. The magnetic signal strength (leakage magnetic flux density) of the sample subjected to 100,000 times and 1,000,000 times of repeated stress is shown. In terms of metal structure, as is apparent from FIG. 9, no structural change is observed at all even with a test piece subjected to 1 million cycles of fatigue. On the other hand, in FIG. 8 (B), a significant increase in magnetic signal is observed for three samples subjected to 10,000 times or more of repeated stress, and in response to increasing the number of repeated stress, The intensity of the magnetic signal is increasing.

  A result consistent with the inspection result by the MI sensor shown in FIG. 8 is confirmed even when the SQUID is used, and the area of the peak region of the local magnetic increase / decrease similar to FIG. 7 tends to increase by 10,000. It was clearly shown in the sample to which repeated stress was applied. As described above, in SUS304L, it has been shown that the local residual strain in the elastic region can be detected by the SQUID, which is the most sensitive magnetic sensor, in the test piece applied with stress only once. When N is increased to N times, local strain is accumulated and the remaining spontaneous magnetization increases. As a result, it can be seen that even a magnetic sensor at the MI sensor level can be detected.

  For this reason, data on the relationship between the application of stress within the elastic limit, the magnetic signal strength, and the area of the local increase / decrease area of the magnetic field is accumulated for the material of the structure to be inspected. It is possible to inspect the progress of deterioration of the initial stage with high accuracy.

In SUS304, according to “Metallic Material Fatigue Reliability Design Data Collection (Japan) Society of Materials Science (1998)”, fatigue limit = tensile strength (σ _u ) × 0.283, and mill sheet of specimen From σ _u = 618 MPa, if the stress amplitude is 175 MPa and the fatigue limit is the total stress, 0 to 350 MPa is calculated. In the example of FIG. 8B, as an example, an inspection example is shown in which stress is set to 50%, which is half of the fatigue limit, and cyclic stress of 0 to 175 MPa is applied 10,000, 100,000, and 1 million times. . The degree of this value compared to the level allowed by SUS304 was estimated by checking the allowable design stress strength of the American Society of Mechanical Engineers (ASME) standard. For austenitic steel pipes, the value obtained by multiplying the sum of 0.2% proof stress (σ _y ) (or yield stress) and tensile strength (σ _u ) by 0.5 is the flow stress (σ _f ). “Fracture Mechanics (Baifukan) Yagawa Motoki 1988”, which is almost equal to 3 times the allowable design stress strength.
From the mill sheet, σ _y = 269 MPa, σ _u = 618 MPa, and σ _f = 443 MPa, so 175 MPa added to the test piece is close to the allowable design stress strength (148 MPa) and is a test example reflecting the actual situation to some extent. .

Furthermore, according to “Handbook of Iron and Steel Materials (Maruzen) The Japan Institute of Metals, Japan Iron and Steel Institute”, generally, under the low stress amplitude corresponding to the life of 10 ⁵ to 10 ⁷ repetitions as shown in this example. In this case, when the life is about 90%, the crack finally grows to about 0.1 cm. However, no cracks are observed even in the specimen subjected to stress applied 1 million times shown in FIG. 9, and no change in the metal structure can be observed. Therefore, it is considered that there is still a sufficient remaining life.

  However, as shown in FIG. 8B, the detected value of magnetism is consistent with the number of repeated fatigue, the magnetic field strength is increasing, and the initial stage deterioration due to repeated application of stress below the elastic limit is detected. It shows what you can do. This is fatigue at an initial stage that cannot be detected by the conventional replica method, and shows that the technique of the present application is highly accurate.

  Based on the above, the analysis unit compares the data of the recording unit and the image processing unit with a calibration curve (or database), thereby determining the soundness of the subject with reference to a certain threshold set in advance by the operator. It can be done automatically. As a threshold value, the value of the slightly increased magnetic field due to repeated stress application in the elastic region is set as the first reference value, and the value of the greatly increased magnetic field due to stress applied in the plastic deformation region is set as the second reference value. By comparing with the detection value of the high-sensitivity magnetic sensor, the degree of initial deterioration due to repeated application of stress in the elastic region and the damage due to stress addition in the plastic deformation region can be identified and nondestructively inspected.

  When the SQUID is used for the magnetic sensor, as shown in FIG. 7, the stress in the elastic region can be applied only once by increasing the area (surface distribution) of the local magnetic increase / decrease region in the magnetic field mapping data. It is possible to detect the accompanying deterioration of the metal, and it is possible to diagnose and detect the progress of the very initial deterioration due to repeated stress application. In this manner, the combined use of the inspections shown in FIGS. 7 and 8 can diagnose the progress of deterioration from the very early stage of aging deterioration to the limit stage of life, although no metal structural change occurs.

  In the application of the present invention, in the case of a large plant, a structure, or a large apparatus, an MI sensor (or a flux gate type sensor) can be applied as a high-sensitivity magnetic sensor in consideration of resolution and workability. Conceivable. In other words, a relatively inexpensive (<10,000 yen) MI sensor is attached to a specific part of a structure or plant, and the change in the strength of the magnetic signal is monitored over time to collate with the database of the analysis unit. The remaining life and soundness can be determined at low cost. Hall elements are produced at a level of 100 million units per year mainly for mobile phones, so they are easily available and inexpensive, but the magnetic field sensitivity is not sufficient and unfortunately they cannot be applied to the present invention at present. .

  On the other hand, when the subject is expensive and can be removed (for example, a gas turbine blade, etc.), the SQUID is applied as a high-sensitivity magnetic sensor. By using such an index, it is possible to inspect the progress of deterioration even at a very early stage. In addition, SQUID is expensive including peripheral equipment, and since cooling is required, maintenance cost must be high.

  As described above, by using the inspection method and inspection apparatus of the present invention, the initial stage of deterioration such as fatigue and damage of metal can be detected nondestructively. Of course, even if it is cracked or damaged, it can be detected.

  When SQUID is used, the stress in the plastic deformation region is detected by the signal strength between peaks, the average value of the detected magnetic field, and its integrated value. In this case, it is necessary to consider all these three parameters. The parameters to be extracted can be arbitrarily selected according to the type of subject and the measurement object.

  Typical high-sensitivity magnetic sensors that can be applied to the present invention and their noise levels (lowest detection magnetic field when the performance can be maximized) are 5 picotesla (pT) for SQUID and 1 nanotesla for MI sensor. (NT), 10 nanotesla (nT) etc. are reference values in the latest FG type sensor, and the Hall element is 50 microtesla (μT) (Magnetic sensor science and engineering, Yoshitoshi Mohri (Corona) )). In addition, since an unbalanced voltage (offset voltage) exists in the Hall element, the relationship between the Hall voltage and the detected magnetic field is near zero, that is, in a weak magnetic field, the linear relationship is broken and the effect as a magnetic sensor cannot be exhibited. . Although it has been devised to reduce the unbalanced voltage, even an InSb high-sensitivity Hall element has an offset of 10 G (1 mT) or more when converted to a magnetic field (Shibazaki et al., Applied Physics Vol. 67 (1998)). For this reason, the Hall element can measure the Hall voltage without any problem, but at present, the value of the magnetic field of 10 G or less is meaningless and requires attention. In the present invention, for example, as shown in FIG. 8, it is necessary to measure a magnetic field of 10 gauss or less with high accuracy, and a Hall element cannot be used. On the other hand, the MI sensor and the latest FG sensor can be stably used as a high-sensitivity magnetic sensor by recent technological development, and have finally reached the high-sensitivity level used in the present invention.

It is an Example block diagram of the test | inspection apparatus of this invention which performs the test | inspection of a metal. It is a figure which shows the shape of the tensile test piece of SUS304L and SUS304 which showed the test result by the apparatus of this invention as an Example. It is the figure which showed the relationship of stress and distortion as a typical example about the case where the SUS304L test piece which showed the test result by the apparatus of this invention as an Example has the same shape as FIG. It is the figure which showed the inspection result by the apparatus of this invention about 8 types of tensile test pieces of SUS304L, using SQUID as a high-sensitivity magnetic sensor, and inspecting a 1 square mm area | region at a 10 micron space | interval as a magnetic image (magnetic) The image can also be displayed in color, contours, histograms, etc.). FIG. 5 is a diagram summarizing the test results of the apparatus of the present invention for eight types of tensile test pieces of SUS304L with respect to the signal intensity between peaks, average value, and integral value from the magnetic image diagram shown in FIG. 4. It is the figure which observed the metal structure of the tensile test piece (eight types) of SUS304L cut out from the same test piece as FIG. 4 with a 400 times magnification with the optical microscope. The results of the inspection by the apparatus of the present invention are calculated from the magnetic image diagram shown in FIG. 4, the cluster area of the subduction is calculated by the number of pixels, and the standard deviation is summarized for 8 types of tensile test pieces of SUS304L. It is a figure. Σ ₁ represents the standard deviation of the area of the blue vortex, σ ₂ represents the standard deviation of the area of the red vortex, blue is −40 μT or less, and red is 40 μT or more. The result of the inspection by the apparatus of the present invention was measured for eight types of tensile test pieces of SUS304 using an MI element as a high-sensitivity magnetic sensor, and a diagram summarizing the strength of the magnetic signal and the inspection by the apparatus of the present invention The strength of the magnetic signal of the SUS304 test piece to which 0-90% cyclic stress was applied N times in the elastic region was measured using an MI element as a high-sensitivity magnetic sensor. It is the figure which summarized the relationship. It is the figure which observed the metal structure by the magnification of 400 times using the optical microscope the sample cut out from the same test piece as the fatigue test piece shown in FIG.8 (B).

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Subject, 20 ... Inspection apparatus, 21 ... Magnetic measurement part, 22 ... Memory | storage part, 23 ... Image processing part, 24 ... Analysis part, 25 ... Monitoring and recording apparatus part.

Claims (4)

A non-destructive inspection method for inspecting the progress of metal degradation based on the addition of stress below the elastic limit without metallographic change,
  Two-dimensional mapping showing the distribution of leakage flux after detecting leakage flux after applying different stresses below the elastic limit for multiple samples of the same material as the subject metal using a superconducting quantum interference device (SQUID) Create data, calculate the area of the increase area or decrease area of the local leakage magnetic flux from the mapping data, and provide reference data indicating the correspondence between the stress application and the area of the increase area or the decrease area Acquired,
  The leakage flux of the subject metal is detected using a superconducting quantum interference device (SQUID), the leakage flux is two-dimensionally mapped, and the increase area of the local leakage flux obtained based on the mapping or the decrease area A nondestructive inspection method for a metal, characterized in that the progress of deterioration is inspected by calculating an area and comparing the detection result with the reference data. A two-dimensional mapping showing the distribution of increase and decrease of leakage flux by detecting the leakage flux of the subject metal using the magnetic sensor composed of a superconducting quantum interference element (SQUID) and comparing the detection output with a predetermined threshold. The non-destructive metal according to claim 1, further comprising: a data processing unit that generates data; and an analysis unit that calculates an area of an increase area or a decrease area of the local leakage magnetic flux based on the mapping data. Metal nondestructive inspection equipment used for inspection methods. 3. The nondestructive inspection apparatus for metal according to claim 2, wherein an increase area and a decrease area of leakage magnetic flux are indicated as color data. 4. The metal nondestructive inspection apparatus according to claim 2, comprising a monitor device for displaying two-dimensional mapping data and a recording device for the data.

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