TECHNICAL FIELD
[0001] The present disclosure relates to a method for detecting a softened phase existing in a metal member. 5 The present application claims priority based on Japanese Patent Application No. 2020-072227 filed on April 14, 2020, the entire content of which is incorporated herein by reference.
BACKGROUND ART 10
[0002] In recent years, there have been reports of cases where softened phases are generated in welds and heat affected zones (HAZs) of base materials of high-temperature steam pipes (high-chromium steel pipes) used in thermal power plants, for example (see Non-Patent Document 1, for example). Such a softened phase generated in a metal member used under high temperature and high 15 pressure conditions has significantly lower hardness than the normal region (base material and weld metal), and the crystal grains are coarsened. Therefore, there is a possibility that the presence of the softened phase reduces the strength of the metal member, and the need for non-destructive detection of the softened phase is expected to increase in the future. 20
[0003] As the method for non-destructively inspecting a metal member, ultrasonic testing (UT) has been conventionally known (see Patent Document 1). In ultrasonic testing, an ultrasonic short pulse signal is sent (transmitted) from a flaw detector to an inspection target, such as a weld of a high-temperature steam pipe, and its reflection (echo signal) is received and analyzed. Thereby, it is 25 possible to estimate the presence or absence of defects and the position thereof in
2
the inspection target.
Citation List
Patent Literature
[0004] Patent Document 1: JP2018-205033A 5
Non-Patent Literature
[0005] Non-Patent Document 1: S. Zhang, et. al., “Effect of softening structural on creep strength of long-term used 9Cr steel welded joint”, Proceedings of 57th Symposium on Strength of Materials at High Temperature, 10 pp.46-50, December 5-6 (2019)
SUMMARY
Problems to be Solved
[0006] However, at present, there is no report of a technique for non-15 destructively detecting a softened phase precipitated within a metal structure. There are many research cases for detecting defects such as microcracks and creep voids (cavities) (Patent Document 1, etc.), but there are no research cases aimed at detecting non-cavity material structures such as softened phases.
[0007] In this regard, the present inventors have intensively studied and found 20 that in order to able to detect a softened phase by ultrasonic testing, it is necessary to use ultrasonic waves with a wavelength corresponding to the grain size of crystal grains forming the softened phase, otherwise the signal values of reflected waves (time course of echo signal) of the emitted ultrasonic waves from the softened phase are too small to detect the softened phase (see FIGs. 5A and 5B 25 described later). In other words, we have found a strong correlation between the 3
wavelength (frequency) of ultrasonic waves and the grain size of crystal grains forming the softened phase that can be detected when using ultrasonic testing to detect the softened phase.
[0008] In view of the above, an object of at least one embodiment of the present invention is to provide a softened phase detection method for detecting a 5 softened phase within metal.
Solution to the Problems
[0009] A softened phase detection method according to at least one embodiment of the present invention is for detecting a softened phase within an 10 inspection target of a metal member by ultrasonic testing, comprising: a setting step of setting an assumed grain size of a crystal grain forming the softened phase; a decision step of deciding a set frequency which is a frequency of an ultrasonic wave used in the ultrasonic testing so as to satisfy an ultrasonic condition that defines a relationship between the assumed grain size and the set frequency; a 15 testing step of executing the ultrasonic testing on the inspection target by the ultrasonic wave having the set frequency; and a determination step of determining presence or absence of the softened phase within the inspection target, on the basis of an execution result of the ultrasonic testing. 20
Advantageous Effects
[0010] At least one embodiment of the present invention provides a softened phase detection method for detecting a softened phase within metal.
BRIEF DESCRIPTION OF DRAWINGS 25
[0011] FIG. 1 is a schematic configuration diagram of an ultrasonic test
4
system according to an embodiment of the present invention. FIG. 2 is a diagram showing the case where ultrasonic testing is performed on softened phases that are intermittent in the thickness direction according to an embodiment of the present invention. FIG. 3 is a flowchart of a softened phase detection method according to an 5 embodiment of the present invention. FIG. 4 is an image diagram showing a softened phase composed of multiple crystal grains according to an embodiment of the present invention. FIG. 5A is a diagram showing a relationship between the wavelength of ultrasonic waves and the echo signal height from a softened phase according to an 10 embodiment of the present invention. FIG. 5B is a diagram showing a relationship between the set frequency of ultrasonic waves and the echo signal height from a softened phase according to an embodiment of the present invention. FIG. 6 is a diagram showing the frequency response of reflected waves of 15 ultrasonic waves according to an embodiment, where the solid line indicates a softened phase and the dashed line indicates a flaw. FIG. 7 is a flowchart of the determination step according to an embodiment of the present invention, based on the presence or absence of high frequency shift. FIG. 8 is a flowchart of the determination step according to an embodiment 20 of the present invention, based on the shape of the frequency response. FIG. 9 is a flowchart of the determination step according to an embodiment of the present invention, based on the propagation velocity of shear waves. FIG. 10 is a flowchart of the determination step according to an embodiment of the present invention, based on the responsiveness according to used frequency 25 in ultrasonic testing.
5
FIG. 11A is a diagram illustrating the reflection direction of an echo signal when the reflection source is a softened phase according to an embodiment of the present invention, without directivity. FIG. 11B is a diagram illustrating the reflection direction of an echo signal when the reflection source is a flaw according to an embodiment of the present 5 invention, with directivity. FIG. 12 is a flowchart of the determination step according to an embodiment of the present invention, based on the presence or absence of directivity of an echo signal. 10
DETAILED DESCRIPTION
[0012] Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly identified, dimensions, materials, shapes, relative positions, and the like of components described in the embodiments shall be interpreted as 15 illustrative only and not intended to limit the scope of the present invention. For instance, an expression of relative or absolute arrangement such as “in a direction”, “along a direction”, “parallel”, “orthogonal”, “centered”, “concentric” and “coaxial” shall not be construed as indicating only the arrangement in a strict literal sense, but also includes a state where the arrangement is relatively 20 displaced by a tolerance, or by an angle or a distance whereby it is possible to achieve the same function. For instance, an expression of an equal state such as “same” “equal” and “uniform” shall not be construed as indicating only the state in which the feature is strictly equal, but also includes a state in which there is a tolerance or a 25 difference that can still achieve the same function.
6
Further, for instance, an expression of a shape such as a rectangular shape or a cylindrical shape shall not be construed as only the geometrically strict shape, but also includes a shape with unevenness or chamfered corners within the range in which the same effect can be achieved. On the other hand, an expression such as “comprise”, “include”, “have”, 5 “contain” and “constitute” are not intended to be exclusive of other components.
[0013] FIG. 1 is a schematic configuration diagram of an ultrasonic test system 1 according to an embodiment of the present invention. FIG. 2 is a diagram showing the case where ultrasonic testing is performed on softened phases that are intermittent in the thickness direction according to an embodiment 10 of the present invention. FIG. 3 is a flowchart of a softened phase detection method according to an embodiment of the present invention. FIG. 4 is an image diagram showing a softened phase T composed of multiple crystal grains G according to an embodiment of the present invention. FIG. 5A is a diagram showing a relationship between the wavelength λ of ultrasonic waves and the echo 15 signal height from a softened phase T according to an embodiment of the present invention. FIG. 5B is a diagram showing a relationship between the set frequency F of ultrasonic waves and the echo signal height from a softened phase T according to an embodiment of the present invention.
[0014] The softened phase detection method is a method for detecting a 20 softened phase T existing in a metal material. More specifically, the softened phase detection method is a method for detecting a softened phase T within a portion of an inspection target 9 (hereinafter, simply inspection target 9) of a metal member, for example, by ultrasonic testing (UT). The metal member may be a pipe, such as a high chromium steel pipe. The inspection target 9 may 25 include at least one of a weld (weld metal) of the pipe or a heat affected zone 7
(HAZ) of a base material thereof.
[0015] First, the ultrasonic test system 1 and measurement conditions will be described. As shown in FIG. 1, the ultrasonic test system 1 includes a flaw detector 2 having a transmitter 2s for transmitting ultrasonic waves and a receiver 2r for receiving an echo signal (reflected waves) of the ultrasonic waves, a 5 measurement device 3 for generating reflected wave signal data sm by measuring (observing) the signal level of the echo signal (hereinafter referred to as echo signal level) using the receiver 2r after the ultrasonic waves (ultrasonic pulse) are transmitted from the transmitter 2s, and a processing device 4 for generating an acoustic image I based on a well-known flaw inspection principle, for instance, by 10 performing aperture synthesis processing, on the basis of multiple signal data sm measured at multiple measurement positions pm of the inspection target 9.
[0016] More specifically, probes (sensors) for generating or receiving ultrasonic waves are incorporated in the transmitter 2s and the receiver 2r. After ultrasonic waves (ultrasonic pulse) are transmitted from the transmitter 2s, the 15 measurement device 3 measures the signal level of reflected waves of the ultrasonic waves using the receiver 2r to generate signal data sm(t) (time-series data; hereinafter sm as appropriate) which is the time course of the echo signal level observed during a predetermined period from the transmission of ultrasonic waves from the transmitter 2s. Such measurement is performed at each of the 20 multiple (M) measurement positions pm (m = 1, 2, ..., M; the same shall apply hereinafter) of the inspection target 9 by placing (moving) the flaw detector 2 to each measurement position pm to obtain M number of signal data sm, as shown in FIG. 1. The processing device 4 outputs the generated acoustic image I to a display device 12 such as a display (FIG. 1) or a printer that can visualize the 25 image, so that the acoustic image I is displayed.
8
[0017] The example of FIG. 1 shows the case where a softened phase T exists in the inspection target 9. The softened phase T has physical properties different from those of the base material of the inspection target 9, and serves as a reflection source of ultrasonic waves. The reflection source reflects the ultrasonic waves transmitted from the transmitter 2s, and the reflected waves are received by 5 the receiver 2r. Thus, there is a portion where the signal level changes greatly (reflection source echo). Further, since each signal data sm is measured at different measurement positions, there is a variation in the position of the reflection source echo in each signal data sm. The same is true when the reflection source is a flaw h. The above-described “t” is, for example, the elapsed time from 10 the transmission of ultrasonic waves.
[0018] In such an ultrasonic test system 1 for detecting the softened phase T, the probes (sensors) incorporated in the transmitter 2s and the receiver 2r may be well-known probes. The incorporated probe may be a single probe, or an array probe combining multiple probes. However, in order to detect the softened phase 15 T in a small area, it is preferable to use a focused beam B with a strong sound pressure (high sensitivity), and a focusing probe or an array probe may be used. The principle of testing may be based on general pulse echo method, pitch catch method, phased array method, aperture synthesis method, FMC/TFM (full matrix capture/total focusing method), PWI (plain wave imaging) or the like. As 20 described above, it is preferable to use a focusing probe in the pulse-echo method and pitch catch method. Further, it is preferable to use an array probe in the phased array method, synthetic aperture method, FMC/TFM, and PWI method.
[0019] Since the focusing probe has the problem that it is difficult to inspect the entire area within a metal material with high sensitivity, the focusing probe 25 may be designed or applied according to the inspection range in the thickness
9
direction, or a variable-refractive-index delay material may be used to transmit a focused beam B to any position for flaw inspection. Here, in order to increase the detection sensitivity, the probe may have a curvature in the longitudinal direction to focus the beam. The focal depth may be freely changed by using the refractive-index-variable delay material. 5
[0020] In recent years, it has been reported that softened phases T may be detected intermittently or continuously in the thickness direction (z direction in FIG. 1; the same shall apply hereinafter) in a weld metal or a groove surface (see FIG. 2). However, such softened phases T are difficult to detect by angle beam ultrasonic testing, which is generally used for weld inspection. In this angle beam 10 ultrasonic testing, ultrasonic waves are obliquely incident on the inspection target 9 to inspect a weld. Then it is better to inspect the softened phases T by transmitting ultrasonic waves vertically from the top surface (outer surface) of the inspection target 9, which can be the interior of the weld metal or the groove surface, or if the metal member is, for example, a pipe, by emitting ultrasonic 15 waves in the circumferential direction of the pipe. When transmitting ultrasonic waves from above the weld, the excess weld metal may be ground and flattened before flaw inspection, or adaptive ultrasonic testing may be used. The adaptive ultrasonic testing is a flaw inspection method in which ultrasonic waves are emitted to the excess weld metal as it is, and the refraction effect of the excess 20 weld metal is corrected by combining waveform processing.
[0021] In order to detect the softened phases T (see FIG. 2) that are intermittent (continuous) in the thickness direction over the entire area from the surface to deep portions, it is necessary to emit a strong ultrasonic beam into the deep portion for inspection. To this end, in some embodiments, the flaw detector 25 2 for ultrasonic testing is configured to emit ultrasonic waves along the depth
10
direction (z direction in FIG. 1) from the surface of the inspection target 9. More specifically, a focused beam B is emitted from above the weld with a point-focusing vertical probe. In this case, the beam focal depth is matched with the depth to be inspected. In order to optionally change the inspection depth for testing over the entire area, focusing probes with different focal depths may be 5 used, or a refractive-index-variable delay material may be used to apply a focused beam B to any position for flaw inspection.
[0022] PWI is preferable for imaging using an array probe. The array probe is composed of a plurality of micro-oscillators, and PWI is a method in which simultaneously excited waveforms are transmitted from all the micro-oscillators, 10 and the waveforms are reconstructed to image the inside of the inspection target 9. Here, PWI imaging with the excess weld metal as it is by combining the adaptive ultrasonic testing logic eliminates the need for grinding the excess weld metal. This makes it possible to accurately detect softened phases T even when the softened phases T are generated layer by layer along the depth direction. 15
[0023] The softened phase detection method performed with the ultrasonic test system 1 under measurement conditions as described above includes a setting step S1, a decision step S2, a testing step S3, and a determination step S4, which are executed in this order, as shown in FIG. 3. The softened phase detection method will be described in order of steps of 20 FIG. 3.
[0024] The setting step S1 is a step of, assuming that a softened phase T exists within the inspection target 9, setting an assumed value (hereinafter, assumed grain size d) of the grain size of crystal grains G forming the softened phase T. As shown in FIG. 4, the softened phase T is usually composed of multiple crystal 25 grains G. The grain size of the crystal grains G (hereinafter, grain size) is not 11
certain at this point, but the grain size of the softened phase T that can occur in the inspection target 9 placed in a certain environment can be estimated from past cases of the same type of inspection target placed in the same environment, or by simulation (numerical analysis), or the like. Then, on the basis of such past cases or the like, the crystal grain size assumed when the softened phase T exists within 5 the inspection target 9 is set as the assumed grain size d. In an embodiment, the assumed grain size d may be 100 μm or more.
[0025] In the example of FIG. 4, the region (portion) where relatively large crystal grains G are concentrated is the softened phase T, and the region where relatively small crystal grains are concentrated adjacent to the softened phase T 10 (lower part of FIG. 4) is the base material portion (hereinafter, normal phase Tm) of the inspection target 9 where the softened phase T does not exist. The crystal grain size of the crystal grains G forming the softened phase T is about 100 μm to 200 μm (about 150 μm on average). Therefore, if the metal microstructure shown in FIG. 4 is a past case or a simulation result of a portion of the same type as the 15 inspection target 9, the assumed grain size d can be set to any value between 100 μm and 200 μm, or it may be set to a value outside but close to this numerical range.
[0026] The decision step S2 is a step of deciding a set frequency F, which is a frequency of ultrasonic waves used in ultrasonic testing, according to the assumed 20 grain size d so as to satisfy a predetermined ultrasonic condition C that defines a relationship between the assumed grain size d and the set frequency F. This step is based on findings by the present inventors as a result of intensive studies that in order to able to detect a softened phase T by ultrasonic testing, it is necessary to use ultrasonic waves with a wavelength according to the grain size of crystal 25 grains G forming the softened phase T, otherwise the signal level of an echo signal 12
observed (received) as a result of reflection of the emitted ultrasonic waves from a reflection source within the inspection target 9 are too small to detect the softened phase T even if the time course of the echo signal (hereinafter, reflected waves) includes an echo signal from the softened phase T (see FIGs. 5A and 5B). In other words, there is a strong correlation between the wavelength (frequency) of 5 ultrasonic waves and the grain size of crystal grains forming the softened phase T that can be detected when using ultrasonic testing to detect the softened phase T.
[0027] Specifically, in some embodiments, the ultrasonic condition C is that the wavelength λ (λ = V/F) (V: sound velocity of ultrasonic waves) of ultrasonic waves corresponding to the set frequency F is twice or less the assumed grain size 10 d (λ ≤ 2d). Therefore, in the decision step S2, the set frequency F is decided on the basis of the assumed grain size d so as to satisfy this ultrasonic condition C. For example, the wavelength λ of ultrasonic waves may be calculated from a calculation equation λ = 2d-α, where α is a constant not less than 0 that can be arbitrarily determined, and the set frequency F may be calculated. 15
[0028] For example, in FIGs. 5A and 5B, the horizontal axis represents the wavelength λ of ultrasonic waves, and the vertical axis represents the echo signal height, which is a ratio of a certain echo signal level to the maximum echo signal level. In the example of FIG. 5A, when the wavelength λ (horizontal axis) of ultrasonic waves used in ultrasonic testing is more than 2d (λ > 2d) approximately, 20 the echo signal height of the echo signal represented by the vertical axis is near 0, but when the wavelength λ is equal to or less than 2d approximately, the echo signal height rises to the extent that it can be distinguished from the base echo (noise). Further, the shorter the wavelength λ, the larger the echo signal height tends to be. 25
[0029] The example of FIG. 5B shows the wavelength λ in FIG. 5A converted
13
to frequency (frequency = 1/λ), and as in FIG. 5A, the echo signal height is 0% on the low frequency side and rises to the extent that it can be distinguished from the base echo (noise) on the high frequency side after 1/2d, which is a frequency corresponding to 2d approximately. Further, the higher the frequency, the larger the echo signal height tends to be. In FIGs. 5A and 5B, 2d = 300 to 400 μm, and 5 when the observed echo signal level is equal to or lower than the base echo (noise), it is assumed to be 0% as unevaluable.
[0030] Thus, when the wavelength λ corresponding to the set frequency F satisfies the ultrasonic condition C, the softened phase T within the inspection target 9 can be detected more reliably. In the example of FIG. 4, by setting the 10 smaller grain size (specifically, e.g., 100 μm) of the multiple crystal grains G as the assumed grain size d, the entire softened phase T can be detected more accurately.
[0031] The testing step S3 is a step of executing ultrasonic testing on the inspection target 9 by ultrasonic waves having the set frequency F decided in the 15 decision step S2. Specifically, with the ultrasonic test system 1 as shown in FIGs. 1 and 2, ultrasonic waves having the set frequency F (wavelength λ) are emitted (transmitted) from the flaw detector 2 in multiple positions of the inspection target 9, and the presence of the interface between the softened phase T or a flaw h (internal defect) such as a crack and a creep void (cavity) within the inspection 20 target 9 and the base material is detected on the basis of the signal data sm indicating the time course (reflected waves) of a reflected signal (echo signal) of the emitted ultrasonic waves from the reflection source.
[0032] The center frequency of a probe may be set so as to satisfy the ultrasonic condition C, or the entire band of a wideband frequency sensor may 25 satisfy the ultrasonic condition C. Conversion to wavelength takes into 14
consideration the difference between longitudinal waves and transverse waves used in ultrasonic testing.
[0033] The determination step S4 is a step of determining the presence or absence of the softened phase within the inspection target 9, on the basis of an execution result of ultrasonic testing executed in the testing step S3. By executing 5 ultrasonic testing with ultrasonic waves having the set frequency F, if the softened phase T exists within the inspection target 9, as already described, the echo signal level from the softened phase T, which has a heterogeneous interface with the normal phase Tm, rises enough to be detected by the flaw detector 2, so the presence thereof can be detected. 10
[0034] In the embodiment shown in FIG. 3, the softened phase detection method further includes an evaluation step S5 of calculating an evaluation index including at least one of the size of the softened phase T or the position of the softened phase T within the inspection target 9. When the acoustic image I is displayed on the display device 12, the evaluation index calculated in the 15 evaluation step S5 is also displayed.
[0035] More specifically, in the evaluation step S5, UT imaging is performed to obtain a cross-sectional mapping image of the softened phase T which is a reflection source exceeding a certain echo signal height. Then, the size (height, width, depth) of the softened phase T is obtained based on this image. For 20 example, the size and volume can be obtained by extracting the softened phase T from the image or by binarizing. When a focusing probe is used for the flaw detector 2, the size of the softened phase T can be specified by scanning in the front, back, left, and right directions, performing beam scanning with a refractive-index-variable wedge, and recording only echo signal heights satisfying a criterion. 25
[0036] The evaluation index may include a proportion of the softened phase T
15
in the inspection target 9 (proportion of the softened phase T to the thickness of the inspection target 9). Specifically, the proportion can be obtained by recording the range of the softened phase T by the above-described imaging method using an array probe. More specifically, waveforms in the thickness direction can be acquired by ultrasonic testing vertically to the inspection target 9, and by 5 determining the proportion of a waveform specific to the softened phase T on the basis of this waveform information, the proportion of the generated softened phase T to the thickness can be obtained. Further, as will be described later, the proportion can also be obtained by observing the change in the propagation velocity of shear waves caused by the softened phase T. Specifically, the 10 proportion of the generated softened phase T can be obtained by measuring the amount of change in the propagation velocity in the thickness direction and dividing it by the thickness.
[0037] According to the above configuration, the frequency (set frequency F) of ultrasonic waves used in ultrasonic testing is decided so as to satisfy the 15 ultrasonic condition C, on the basis of the grain size (assumed grain size d) of crystal grains forming the softened phase T assumed when the softened phase T is generated within the inspection target 9 of the metal member. Further, the inspection target 9 is measured by ultrasonic testing using the decided set frequency F, and the presence or absence of the softened phase T is determined on 20 the basis of the result thereof. Thus, the softened phase T within the inspection target 9 can be detected in a non-destructive manner.
[0038] Next, some embodiments related to the determination step S4 will be described. Although the ultrasonic testing using the set frequency F can detect the 25 softened phase T (non-cavity structure), this method can also detect a flaw h
16
(internal defect) in addition to the softened phase T. In other words, the reflection source of ultrasonic waves detected by ultrasonic testing may actually be the softened phase T or the flaw h. Therefore, an identification method for identifying whether the detected reflection source is the softened phase T or not is required.
[0039] Hereinafter, five embodiments relating to the identification method 5 will be described sequentially with reference to FIGs. 6 to 11B. The following five embodiments may be used alone or in combination; for example, some of them may be implemented, and when the results of all the implemented identification methods are the same, the identification result may be adopted, or a quality engineering method (e.g., MT method) may be used to obtain a final 10 identification result.
[0040] (1. Identification method through analysis of frequency response of reflected waves of ultrasonic waves) FIG. 6 is a diagram showing the frequency response of reflected waves of ultrasonic waves according to an embodiment, where the solid line indicates a 15 softened phase T and the dashed line indicates a flaw. FIG. 7 is a flowchart of the determination step S4 according to an embodiment of the present invention, based on the presence or absence of high frequency shift.
[0041] The present inventors have intensively studied and found that there is a difference in the frequency response (frequency spectrum) of reflected waves of 20 ultrasonic waves depending on whether the reflection source in the inspection target 9 that reflects the ultrasonic waves is the softened phase T or not (see FIG. 6). Specifically, as shown in FIG. 6, when the softened phase T exists, the peak (intensity peak) in the frequency response shifts to a higher frequency as a whole compared to when the softened phase T does not exist (hereinafter, referred to as 25 “high frequency shift”). Further, since the softened phase T is usually composed 17
of multiple crystal grains G, as shown in FIG. 6, the shape of the frequency response due to the softened phase T is forest-like (grass-like) and has multiple peaks, while the shape of the frequency response due to a flaw is mountain-like and has approximately one peak.
[0042] For this reason, in some embodiments, the identification may be 5 performed by determining whether there is a high frequency shift of the frequency response (see FIG. 6) as described above. Specifically, as shown in FIG. 7, the determination step S4 includes: a target characteristic acquisition step S71 of acquiring a frequency response (hereinafter, target frequency response Dt) of reflected waves of ultrasonic wave emitted to the inspection target 9; a first 10 analysis step S72 of obtaining an evaluation value (hereinafter, target peak position Vp) of position (peak position) of frequency where a peak exists in the target frequency response Dt; and a first identification step S73 (S73a to S73c) of acquiring an evaluation value (hereinafter, reference peak position Vr) of peak position of a frequency response (hereinafter, reference frequency response Dr, 15 see FIG. 6) of reflected waves of ultrasonic waves obtained when a reflection source causing the target frequency response Dt is a flaw h, and identifying the reflection source as the softened phase T if the target peak position Vp is equal to or larger than a first threshold La that has a larger value than the reference peak position Vr (Vp > La > Vr). 20
[0043] The target frequency response Dt is obtained by performing Fourier transform such as fast Fourier transform (FFT) on the reflected waves. The target peak position Vp is an index for determining whether there is a high frequency shift. The peak position of each of the target peak position Vp and the reference peak position Vr may be, for example, the center frequency or the peak position 25 having the maximum intensity. The first threshold La may be determined by 18
multiplying or adding a predetermined coefficient β (β > 1) to the reference peak position Vr, for example.
[0044] The reference peak position Vr can be obtained by simulation or ultrasonic testing on a standard test piece having an internal defect. Specifically, regarding the simulation, since the position and size of the reflection source in the 5 inspection target 9 can be obtained by the testing step S3, in a numerical model of the inspection target 9 that can simulate ultrasonic testing, the measurement conditions of ultrasonic testing that has been actually performed may be reflected (set), and the flaw h may be set at the position (actual position) of the reflection source detected by the actually performed ultrasonic testing to perform numerical 10 analysis. Regarding the standard test piece, ultrasonic testing may be performed on a test piece such that the relative positional relationship between the flaw detector 2 and the actual position is the same. Alternatively, the reference peak position Vr may be estimated on the basis of the execution result of ultrasonic testing on a standard test piece in which the position of internal defect is known, 15 and the estimated value may be used as the reference peak position Vr.
[0045] In the embodiment shown in FIG. 7, the target frequency response Dt is acquired in step S71, and the center frequency is calculated as the target peak position Vp of the acquired target frequency response Dt in step S72. Then, in step S73, the first identification step S73 is performed. Specifically, in step S73a, 20 it is determined whether the target peak position Vp is equal to or more than the first threshold La. In this embodiment, the coefficient β is 1.2 (β = 1.2), and the first threshold La is set to 1.2×β (La = β×Vr). Then, if Vp < TLa in step S73a, the reflection source of ultrasonic waves is identified as the flaw h in step S73b. Conversely, if Vp ≥ TLa in step S73a, the reflection source of ultrasonic waves is 25 identified as the softened phase T in step S73c.
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[0046] That is, in the embodiment shown in FIG. 7, a shift from the reference peak position Vr (in this case, center frequency) to the high frequency side by a percentage corresponding to β (20% in FIG. 7) or more is regarded as a significant difference, and if this condition is satisfied, the reflection source is identified as the softened phase T. The inventors have confirmed that the reflection source can 5 be properly identified by the first threshold La determined with the coefficient β being 1.2.
[0047] According to the above configuration, the target peak position Vp, which may be, for example, the center frequency, of the frequency response of reflected waves (reflected wave signal) of the ultrasonic waves is calculated. 10 Further, the first threshold La to be compared with the target peak position Vp is determined on the basis of the reference peak position Vr that would be obtained when the reflection source is not the softened phase T but the flaw h, and if the target peak position Vp is equal to or more than the first threshold La (Vp ≥ La > Vr), the reflection source detected by ultrasonic testing is identified as the 15 softened phase T. Thus, it is possible to appropriately identify whether the reflection source is the non-cavity softened phase T or the flaw h, and it is possible to appropriately detect the softened phase T within the inspection target 9.
[0048] In some embodiments, the identification may be performed by determining whether the frequency response of the reflected waves has the 20 appearance of grass echo. FIG. 8 is a flowchart of the determination step S4 according to an embodiment of the present invention, based on the shape of the frequency response.
[0049] Specifically, as shown in FIG. 8, the determination step S4 includes: a 25 target characteristic acquisition step S81 (the same as S71) of acquiring a target
20
frequency response Dt of reflected waves of ultrasonic wave emitted to the inspection target 9; a second analysis step S82 of evaluating the shape of the target frequency response Dt; and a second identification step S83 (S83a to S83c) of identifying whether a reflection source causing the target frequency response Dt is the softened phase T, on the basis of a result of the shape evaluation. Specifically, 5 in the second identification step S83, as a result of the shape evaluation, if the shape of the target frequency response is grass-like, the reflection source is identified as the softened phase T, while if the shape is not grass-like such as mountain-like, the reflection source is identified as not being the softened phase T but, for example, the flaw h. 10
[0050] Various methods for the shape evaluation are possible. For example, in some embodiments, the number of peaks in the target frequency response Dt may be tallied. For tallying the number of peaks, for example, the number of peaks having an intensity equal to or higher than a predetermined percentage (for example, 50%) of the maximum intensity may be counted. Further, it may be 15 determined that the shape of the target frequency response is grass-like if the number of peaks is equal to or more than a second threshold which is a predetermined number of 2 or more. For example, since the number of peaks in the frequency response of reflected waves due to a flaw is approximately one, the second threshold may be 3, for example, considering the possibility of two peaks. 20 Further, for example, when the inspection target 9 includes a weld, since a noise echo due to recrystallization is generated depending on the welding conditions, the reflected waves also include this noise echo. Considering such a case, the second threshold may be set to more than 3 according to the noise echo state (welding conditions) in the inspection target 9. 25
[0051] In some embodiments, a smoothed distribution (smoothed frequency 21
response) may be generated by smoothing the target frequency response Dt, and the points where the intensity of the smoothed frequency response is equal to or more than a third threshold may be tallied to determine whether the target frequency response Dt is grass-like. Also in this case, the third threshold may be set according to the noise echo state (welding conditions) in the inspection target 9. 5 For the smoothing process, a well-known technique such as moving average may be used.
[0052] In some embodiments, when the reflected waves include an echo signal originated from the softened phase T and an echo signal originated from the flaw, after extracting only the former echo signal, the shape may be evaluated by 10 the evaluation method as described above. Specifically, in some embodiments, shapes approximating the target frequency response Dt may be created by using multiple functions in bell shape (hereinafter, bell-shaped functions) such as Gaussian functions, Lorentz functions, and Voigt functions, and the created shapes may be used to extract the echo signal due to the softened phase T from the target 15 frequency response Dt.
[0053] For example, N+1 bell-shaped functions may be prepared, where N is the number of peaks of the target frequency response Dt. Among them, the line widths (FWHM: Full Width at Half Maximum) of N bell-shaped functions are made the same, and only the line width of the remaining bell-shaped function is 20 made different from the others. Further, the parameters (line width, peak height) of the N+1 bell-shaped functions are adjusted so that the difference from the target frequency response Dt is equal to or less than a specified value. As a result, it is determined that the synthetic waveform of the N bell-shaped functions having the same line width when the difference is equal to or less than the specified value is 25 originated from the softened phase T, and the remaining one bell-shaped function 22
is originated from the flaw h. In some embodiments, the shape of the target frequency response Dt may be approximated by a synthetic waveform of only bell-shaped functions with narrow line width.
[0054] In the embodiment shown in FIG. 8, the target frequency response Dt is acquired in step S81, and the shape of the acquired target frequency response Dt 5 is evaluated by at least one of the above-described embodiments in step S82. Then, in step S83, the second identification step S83 is performed. Specifically, in step S83a, it is determined whether the shape of the target frequency response Dt is grass-like, and if the shape is not grass-like (if it is mountain-like), the reflection source of ultrasonic waves is identified as the flaw h in step S83b. 10 Conversely, if the shape is grass-like in step S83a, the reflection source of ultrasonic waves is identified as the softened phase T in step S83c.
[0055] According to the above configuration, the shape of the target frequency response Dt is evaluated, and it is identified whether the reflection source detected by the ultrasonic testing is the softened phase T on the basis of the 15 evaluation result. Thus, the softened phase T within the inspection target 9 can be detected appropriately on the basis of the shape of the target frequency response Dt.
[0056] In some other embodiments, the identification method based on the high frequency shift and the identification method based on the shape of the 20 frequency response may be combined, and if the same result is obtained in either method, the result may be adopted. Further, the present method may be applied not only for echoes directly reflected from the softened phase T, but also for reflected echoes (e.g., bottom echoes) due to a shape after the softened phase T have been passed. Additionally, the set parameters during ultrasonic testing, such 25 as pulse voltage and pulse width, may be swept to check reproducibility using 23
multiple waveforms or to perform discrimination based on differences in responsiveness. Thereby, it is possible to improve the determination precision by the determination step S4.
[0057] (2. Identification method based on propagation velocity v (propagation time) of ultrasonic waves in softened phase T) 5 FIG. 9 is a flowchart of the determination step S4 according to an embodiment of the present invention, based on the propagation velocity v of shear waves.
[0058] The mechanical properties of the softened phase T differ from those of the surrounding base material (metal structure) in physical properties such as 10 stiffness (Young's modulus), elastic modulus, and density ρ, resulting in different acoustic impedance. In the medical field, which is another technical field, a method of measuring the stiffness of biological tissue by generating shear waves (elastic waves) in the tissue and measuring the propagation velocity v of the shear waves is in practical use. Then, the present inventors have considered generating 15 shear waves from a reflection source of ultrasonic waves detected by ultrasonic testing by emitting ultrasonic pulses to the reflection source, and identifying whether the reflection source is the softened phase T on the basis of the measurement result of the propagation velocity v of the shear waves.
[0059] Specifically, the identification is performed by a comparison of the 20 measured velocity of shear waves generated from the reflection source, based on the fact that, qualitatively, with the same density ρ, the harder the structure, the higher the propagation velocity v, and the softer the structure, the lower the propagation velocity v. Alternatively, Young’s modulus or modulus of rigidity, or density ρ may be calculated from the theoretical relationship between the 25 propagation velocity v of shear waves, density ρ, and Young’s modulus or 24
modulus of rigidity, and the identification may be performed on the basis of the agreement between them and the physical properties of the base material or the softened phase T.
[0060] For this reason, in some embodiments, as shown in FIG. 9, the determination step S4 includes: a propagation velocity acquisition step S91 of 5 acquiring the propagation velocity v of shear waves generated by a reflection source of the ultrasonic waves detected by ultrasonic testing; and a third identification step S92 of identifying whether the reflection source is the softened phase T, on the basis of the acquired propagation velocity v. More specifically, the propagation velocity acquisition step S91 includes a shear wave generation 10 step S91a of emitting focused ultrasonic waves (focused beam B; the same shall apply hereinafter) to a reflection source detected by ultrasonic testing, and a velocity measurement step 91b of measuring the propagation velocity v of shear waves generated from the reflection source by step S91a.
[0061] As a specific method for applying the focused ultrasonic waves, any 15 known method may be used. For example, the phased array method or PWI may be used, and a focusing probe having a curvature may be used. The focused beam (focus beam) may be applied to the reflection source, or the focused beam B whose focus position is intermittently changed in the depth direction by delay time correction or by a refractive-index variable-delay material or the like may be 20 applied.
[0062] When the focused ultrasonic waves are emitted to the reflection source, the reflection source is excited by the ultrasonic waves to generate shear waves. The shear waves are detected at two or more positions on the propagation path of the shear waves, and the shear wave propagation velocity v is measured based on 25 the difference in detection time of the shear waves detected at these positions and
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the difference in propagation distance. For measuring the shear wave displacement, the autocorrelation method, which is used in the Doppler method or the like, can be used. Further, for measuring a local shear wave at each position, the sampling interval may be narrowed, or time shift tracking may be performed to measure at fast cycles. 5
[0063] For example, the third identification step S92 may include calculating the stiffness of the reflection source on the basis of the acquired propagation velocity v, and determining whether the reflection source is the softened phase T on the basis of the calculated stiffness of the reflection source. It is known that the shear wave propagation velocity v is equal to the square root of the shear 10 modulus (modulus of rigidity) divided by the density ρ. Also, it is known that Young’s modulus can be approximated by 3×ρ×propagation velocity v. Alternatively, the third identification step S92 may include, if the acquired propagation velocity v is slower than the reference propagation velocity that would be obtained when the reflection source is not the softened phase T but the 15 flaw h, identifying the reflection source detected by ultrasonic testing as the softened phase T. The reference propagation velocity may be obtained by emitting focused ultrasonic waves to a position where no reflection source has been detected by ultrasonic testing, and measuring the propagation velocity v of shear waves thus generated as described above. 20
[0064] In the embodiment shown in FIG. 9, in step S91, the propagation velocity acquisition step S91 is performed. Specifically, in step S91a, focused ultrasonic waves are emitted to the reflection source. In step S91b, the propagation velocity v of the shear waves generated from the reflection source by step S91a is measured. Thus, since the propagation velocity v of the measurement 25 target is acquired, thereafter, in step S92, the reflection source is identified on the 26
basis of the acquired propagation velocity v.
[0065] According to the above configuration, the propagation velocity v of shear waves generated by emitting ultrasonic waves to a reflection source of the ultrasonic waves detected by ultrasonic testing is measured, and it is identified whether the reflection source is the softened phase T, on the basis of the measured 5 propagation velocity v. Thus, the softened phase T within the inspection target 9 can be detected appropriately.
[0066] (3. Identification method using multiple frequencies with different detection sensitivities to softened phase T) FIG. 10 is a flowchart of the determination step S4 according to an 10 embodiment of the present invention, based on the responsiveness according to used frequency in ultrasonic testing.

I/We Claim:
1. A softened phase detection method for detecting a softened phase within an inspection target of a metal member by ultrasonic testing, comprising: a setting step of setting an assumed grain size of a crystal grain forming the 5 softened phase; a decision step of deciding a set frequency which is a frequency of an ultrasonic wave used in the ultrasonic testing so as to satisfy an ultrasonic condition that defines a relationship between the assumed grain size and the set frequency; 10 a testing step of executing the ultrasonic testing on the inspection target by the ultrasonic wave having the set frequency; and a determination step of determining presence or absence of the softened phase within the inspection target, on the basis of an execution result of the ultrasonic testing. 15
2. The softened phase detection method according to claim 1, wherein the ultrasonic condition is λ ≤ 2×d, where λ is a wavelength of the ultrasonic wave corresponding to the set frequency, and d is the assumed grain size. 20
3. The softened phase detection method according to claim 1 or 2, wherein the determination step includes: a target distribution acquisition step of acquiring a target frequency response which is a frequency response of a reflected wave of the ultrasonic wave; 25 a first analysis step of obtaining a target peak position which is an
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evaluation value of frequency where a peak exists in the target frequency response; and a first identification step of acquiring a reference peak position which is the evaluation value when a reflection source causing the target frequency response is a flaw, and identifying the reflection source as the softened phase if 5 the target peak position is equal to or larger than a first threshold that has a larger value than the reference peak position.
4. The softened phase detection method according to any one of claims 1 to 3, wherein the determination step includes: 10 a target distribution acquisition step of acquiring a target frequency response which is a frequency response of a reflected wave of the ultrasonic wave; a second analysis step of evaluating a shape of the target frequency response; and a second identification step of identifying whether a reflection source 15 causing the target frequency response is the softened phase, on the basis of a result of the shape evaluation.
5. The softened phase detection method according to any one of claims 1 to 4, wherein the determination step includes: 20 a propagation velocity acquisition step of acquiring a propagation velocity of a shear wave generated by a reflection source of the ultrasonic wave detected by the ultrasonic testing; and a third identification step of identifying whether the reflection source is the softened phase, on the basis of the acquired propagation velocity. 25
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6. The softened phase detection method according to any one of claims 1 to 5, wherein the determination step includes: a reference testing step of executing the ultrasonic testing on the inspection target by a reference ultrasonic wave having a frequency that does not satisfy the ultrasonic condition if a reflection source of the ultrasonic wave is 5 detected in the testing step; and a fourth identification step of identifying the reflection source as the softened phase if the reflection source detected in the testing step is not detected in the reference testing step. 10
7. The softened phase detection method according to any one of claims 1 to 6, wherein the determination step includes: a directivity determination step of determining presence or absence of directivity of a reflected wave of the ultrasonic wave from a reflection source detected by the ultrasonic testing; and 15 a fifth identification step of identifying the reflection source as the softened phase if it is determined that directivity of the reflected wave is present.
8. The softened phase detection method according to any one of claims 1 to 7, wherein the determination step includes 20 a sixth identification step of, with a learning model capable of outputting an identification result whether a reflection source detected by the ultrasonic testing is the softened phase, identifying whether the reflection source is the softened phase from an execution result of the ultrasonic testing on the inspection target. 25
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9. The softened phase detection method according to any one of claims 1 to 8, further comprising an evaluation step of calculating an evaluation index including at least one of a size of the softened phase or a position of the softened phase detected within the inspection target. 5
10. The softened phase detection method according to any one of claims 1 to 9, wherein a flaw detector for the ultrasonic testing is configured to emit the ultrasonic wave along a depth direction of the inspection target from a surface of the inspection target.