JP2017111023A - Evaluation method in magnetic flux leakage method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an evaluation method for solving a problem in a conventional magnetic flux leakage method.SOLUTION: An evaluation method in a magnetic flux leakage method includes steps of: magnetizing multiple test pieces that are composed of the same prescribed material and differ in board thickness; measuring a value of magnetization current and a value of leakage magnetic flux density signal, with respect to each of the test pieces; creating a waveform of magnetization current-leakage magnetic flux density, with respect to each of the test pieces, on the basis of the value of magnetization current and the value of leakage magnetic flux density signal; creating a master curve on the basis of the waveform of magnetization current-leakage magnetic flux density; and using the master curve to evaluate a sample composed of the prescribed material.SELECTED DRAWING: Figure 8

Description

  The present invention relates to an improved evaluation method in the leakage magnetic flux method, and more particularly to an improved damage evaluation method in the low magnetization strength leakage magnetic flux method. Here, the low magnetization intensity is defined as “when a magnetic substance is put in a magnetic field, it is weakly magnetized and the magnetization intensity is small near the origin of the magnetization curve in the magnetization curve” (hereinafter the same).

  The conventional magnetic flux leakage method in the magnetic flaw detection test for damage (including defects (including cracks, thinning, etc.), material deterioration, etc., the same applies hereinafter) that occurred in a ferromagnetic steel structure is as follows. . That is, when a ferromagnetic specimen is magnetized, if there is damage on the surface or directly under the surface, the flow of magnetic flux lines is disturbed, and a leakage magnetic flux appears on the surface (see FIG. 1). By detecting this leakage magnetic flux, damage on the surface and directly under the surface is detected. The leakage magnetic flux depends on the degree of magnetization of the subject (for example, see Non-Patent Documents 1 and 2).

Japanese Patent No. 3355322

New Nondestructive Inspection Handbook p6-p7, p516-p533, edited by Japan Nondestructive Inspection Association, published by Nikkan Kogyo Shimbun Non-destructive Testing Handbook 2ndedition, Vol. 4, Electromagnetic Testing, p608-p630, published by The American Society For Nondestructive Testing

The conventional magnetic flux leakage method is intended for defects on the front surface or surface layer, and there is a problem that it is difficult to apply to defects on the back surface and deep portions.
In the conventional leakage magnetic flux method, a magnetic field of about several hundred to several thousand A / m is applied according to the magnetization characteristics of the subject, and the magnetic flux density inside the subject is set to about 0.7-0 of the saturation magnetic flux density of the corresponding material. .Times.8 (see FIG. 2), ordinary steel materials need to be magnetized to about 1.0 to 1.4 tesla. In particular, stronger magnetization is required to detect damage to the inside and back of the subject.

  In order to magnetize a subject to such a high magnetic field / high magnetic flux density level, a magnetizing device (magnetizer) and a power source that can provide a large electromotive force are required. For example, when a subject is magnetized by the inter-electrode method using an electromagnet, a coil having a large number of turns and a large current and a large-capacity power supply device are required. Therefore, there are problems such as an increase in size and weight of the magnetic head, a decrease in spatial resolution of detection, and heat generation of the coil, and a problem that the apparatus such as a magnetizer is expensive. In addition, there is a problem that the burden of field operation increases due to an increase in the size of the apparatus, and there is a problem that the place of use is limited. Furthermore, the subject is magnetized by the high-intensity magnetization at the time of inspection, and a process of degaussing after the inspection is required, resulting in a problem that inspection efficiency is lowered. The conventional leakage flux method evaluates damage from the leakage flux signal itself, but a method for evaluating damage easily and quantitatively from the leakage flux detection signal has not yet been established.

  Moreover, when evaluating the material (deterioration) of a ferromagnetic steel material using the conventional leakage magnetic flux method, there are the following problems. That is, it is well known that magnetic characteristics change with deterioration. When using the AC magnetization method (for example, refer to Patent Document 1) to magnetize the subject, since the high frequency is used, the magnetic flux concentrates on the surface of the subject due to the skin effect, and the internal material change may not be grasped. There is a problem of having sex. Moreover, since the influence of both conductivity and magnetic permeability appears in the eddy current, there is a problem that the evaluation becomes more complicated. Furthermore, in the method of strongly magnetizing the material and detecting the change in magnetic properties, attention is paid to parameters that are insensitive to 'tissue change' such as saturation, and there is a possibility of overlooking permeability change that is sensitive to 'tissue change'. There is a problem that there is.

  The present invention aims to solve these problems.

An embodiment of the evaluation method in the leakage magnetic flux method according to the present invention is as follows, for example.
[Embodiment 1]
In the evaluation method in the leakage magnetic flux method, a step of magnetizing a plurality of test pieces made of the same predetermined material and having different plate thicknesses, and a value of a magnetizing current and a value of a leakage magnetic flux density signal for each of the plurality of test pieces , A step of creating a magnetization current-leakage flux density waveform for each of the plurality of test pieces based on the magnetization current value and the leakage flux density signal value, and the magnetization current-leakage flux An evaluation method comprising: creating a master curve based on a waveform of density; and evaluating a subject made of the predetermined material using the master curve.

[Embodiment 2]
The evaluation method according to the first embodiment, wherein the content of the evaluation relates to the thickness of the subject made of the predetermined material.

[Embodiment 3]
In the step of creating the master curve, the waveform of the magnetization current-leakage magnetic flux density is normalized, and each of the initial leakage resistances is normalized using the waveform of the magnetization current-leakage magnetic flux density normalized for each of the plurality of test pieces. The evaluation method according to the second embodiment, in which the master curve is created by calculating the rate and indicating the relationship between the initial leakage resistivity and the thickness of the test piece.

[Embodiment 4]
In the step of creating the master curve, the waveform of the magnetization current-leakage magnetic flux density is normalized, and each of the initial leakage resistances is normalized using the waveform of the magnetization current-leakage magnetic flux density normalized for each of the plurality of test pieces. A test having a plate thickness other than the first test piece from the initial leakage resistivity of the first test piece having the maximum first plate thickness among the plate thicknesses of the plurality of test pieces. By subtracting each initial leakage resistivity of each piece, the difference in initial leakage resistivity is obtained, and by subtracting each value of the other plate thickness from the first plate thickness, the difference in plate thickness is obtained, The evaluation method according to the second embodiment, in which the master curve indicating a relationship between a difference in initial leakage resistivity and a difference in plate thickness of the test piece is created.

[Embodiment 5]
The evaluation method according to Embodiment 3 or 4, wherein the master curve is a linear function.
[Embodiment 6]
In the evaluation method in the leakage flux method, a step of magnetizing a plurality of test pieces having different characteristics, a step of measuring a value of a magnetization current and a value of a leakage flux density signal for each of the plurality of test pieces, and the magnetization Based on the value of the current and the value of the leakage magnetic flux density signal, a step of creating a magnetization current-leakage magnetic flux density waveform for each of the plurality of test pieces, and a step of normalizing the magnetization current-leakage magnetic flux density waveform Determining each initial leakage resistivity using a normalized magnetization current-leakage magnetic flux density waveform for each of the plurality of test pieces; and for each of the plurality of test pieces, the magnetization current-leakage A step of obtaining an area surrounded by an upper portion and a lower portion in a waveform of magnetic flux density, and a waveform of the initial leakage resistivity and the magnetization current-leakage magnetic flux density waveform And creating a master curve showing the relationship between serial areas, using the master curve, and a step of evaluating the characteristics of the subject evaluation method.

[Embodiment 7]
The evaluation method according to the sixth embodiment, wherein the characteristic of the subject is a PWHT process.
[Embodiment 8]
The evaluation method according to Embodiment 6, wherein the characteristic of the subject is a magnetic characteristic.

[Embodiment 9]
In the step of creating the magnetization current-leakage magnetic flux density waveform, a predetermined averaging process is performed on the measured value of the magnetization current and the leakage magnetic flux density signal to reduce noise. The evaluation method according to any one of the above items.

[Embodiment 10]
10. The evaluation method according to any one of Embodiments 1 to 9, wherein the test piece and the subject are made of a ferromagnetic material.

The evaluation method in the leakage magnetic flux method according to the present invention can provide the following advantageous effects, for example.
That is, the present evaluation method can be applied not only to the surface of the subject but also to the back surface (see FIG. 3) and deep defects (not shown). Since this evaluation method can be implemented at low excitation and low magnetization, it can solve the above-described various problems of the conventional leakage magnetic flux method that requires strong magnetization. In addition, a simple defect evaluation method can be established by this evaluation method.

It is a schematic diagram for demonstrating the principle of the conventional magnetic flux leakage method. An example of an initial magnetization curve of a ferromagnetic material and an operating point (H ₀ , B ₀ ) in a conventional leakage magnetic flux method is shown. In the figure, B represents magnetic flux density (unit: Tesla), and H represents magnetic field strength (unit: A / m). It is a schematic diagram for showing the test object which has a defect in a back surface. In the figure, H ₁ is the strength of the magnetic field (leaked) in the vicinity of the surface of the test piece (hereinafter the same). _HTP is the strength of the magnetic field in the subject (the same applies hereinafter). T and t are the thickness and the remaining thickness of the subject (the same applies hereinafter). In FIG. 2, it is a figure for showing the low magnetization area | region which this invention pays attention. In the figure, the right axis shows the differential permeability. It is a schematic diagram for demonstrating the basic theory of this invention. In the figure, “core” and “gap” indicate predetermined gaps between the magnetic core, the electromagnet, and the subject, respectively. It is a figure which shows the "magnetization current-leakage magnetic flux density" waveform for demonstrating the basic theory of this invention. In the figure, the horizontal axis represents the magnetization current (i), and the vertical axis represents the leakage magnetic flux density (B _l ) (hereinafter, the same applies to the “magnetization current-leakage magnetic flux density” waveform). It is a figure for demonstrating the method of normalizing the "magnetization current-leakage magnetic flux density" waveform in FIG. In the _figure, it shows B _la, respectively _{B ld} is decomposed on (: ascend) portion of the waveform, the same lower (descend) moiety. B _ln is the normalized leakage magnetic flux density. It is a flowchart which shows the outline of the procedure of the evaluation method in the magnetic flux leakage method which concerns on this invention. It is a figure which shows the "magnetization current-leakage magnetic flux density" waveform calculated | required in Example 1-1. In the figure, the horizontal axis indicates the magnetization current (i), and the vertical axis indicates the leakage magnetic flux density (B _l ) (hereinafter the same applies to the “regular magnetization current−leakage magnetic flux density” diagram). It is a figure which shows the "regular magnetization current-leakage magnetic flux density" figure calculated | required in Example 1-1. “Magnetic current—leakage magnetic flux density” waveform (same as in the above graph, FIGS. 12-16 below) and “normal magnetization current—leakage magnetic flux density” of the test piece having a thickness of 3 mm obtained in Example 1-1 FIG. 17 is a diagram showing a lower graph (the same applies to FIGS. 12-16 below). It is a figure which shows the "magnetization current-leakage magnetic flux density" waveform and the "regular magnetization current-leakage magnetic flux density" figure of the test piece of 4 mm in thickness calculated | required in Example 1-1. It is a figure which shows the "magnetization current-leakage magnetic flux density" waveform and the "regular magnetization current-leakage magnetic flux density" figure of the test piece of thickness 6mm calculated | required in Example 1-1. It is a figure which shows the "magnetization current-leakage magnetic flux density" waveform and the "regular magnetization current-leakage magnetic flux density" figure of the test piece of thickness 8mm calculated | required in Example 1-1. It is a figure which shows the "magnetization current-leakage magnetic flux density" waveform and the "regular magnetization current-leakage magnetic flux density" figure of the 10-mm-thick test piece calculated | required in Example 1-1. It is a figure which shows the "magnetization current-leakage magnetic flux density" waveform and the "regular magnetization current-leakage magnetic flux density" figure of the 12-mm-thick test piece calculated | required in Example 1-1. It is a master curve (it shows with a broken line in the figure) which shows the relationship between the initial stage leakage resistivity calculated | required in Example 1-1, and the plate | board thickness of a test piece. In the figure, the horizontal axis represents the thickness of the test piece, and the vertical axis represents the initial leakage resistivity. It is a figure which shows the "magnetization current-leakage magnetic flux density" waveform calculated | required in Example 1-2. It is a figure which shows the "normal magnetization current-leakage magnetic flux density" figure calculated | required in Example 1-2. It is a master curve (it shows with a broken line in the figure) which shows the relationship between the initial leakage resistivity calculated | required in Example 1-2, and the plate | board thickness of a test piece. In the figure, the horizontal axis represents the thickness of the test piece, and the vertical axis represents the initial leakage resistivity. 2 is a master curve (indicated by a broken line in the figure) showing the relationship between the initial leakage resistivity and the thinning amount of the test piece obtained in [Utilization of Experimental Results] of Example 1. In the figure, the horizontal axis indicates the amount of thinning of the test piece, and the vertical axis indicates the amount of change in the initial leakage resistivity. It is a figure which shows the "magnetization current-leakage magnetic flux density" waveform calculated | required in Example 2-1. It is a figure which shows the "normal magnetization current-leakage magnetic flux density" figure calculated | required in Example 2-1. The “magnetization current—leakage magnetic flux density” waveform (the same as in the above graph, FIGS. 25-28 below) and the “normal magnetization current—leakage magnetic flux density” diagram of the As received test piece obtained in Example 2-1. It is a figure which shows the lower graph and the following FIGS. 25-28). It is a figure which shows the "magnetization current-leakage magnetic flux density" waveform and the "regular magnetization current-leakage magnetic flux density" figure of the test piece of PWHT temperature 682 degreeC calculated | required in Example 2-1. It is a figure which shows the "magnetization current-leakage magnetic flux density" waveform and the "regular magnetization current-leakage magnetic flux density" figure of the test piece of PWHT temperature 720 degreeC calculated | required in Example 2-1. It is a figure which shows the "magnetization current-leakage magnetic flux density" waveform and the "regular magnetization current-leakage magnetic flux density" figure of the test piece of PWHT temperature 740 degreeC calculated | required in Example 2-1. It is a figure which shows the "magnetization current-leakage magnetic flux density" waveform and the "regular magnetization current-leakage magnetic flux density" figure of the test piece of PWHT temperature 762 degreeC calculated | required in Example 2-1. It is a figure which shows the graph of the area of the initial stage leakage resistivity calculated | required in Example 2-1, and a "magnetization current-leakage magnetic flux density" waveform. It is a figure which shows the "magnetization current-leakage magnetic flux density" waveform calculated | required in Example 2-2. It is a figure which shows the "regular magnetization current-leakage magnetic flux density" figure calculated | required in Example 2-2. It is a figure which shows the graph of the area of the initial stage leakage resistivity calculated | required in Example 2-2, and the "magnetization current-leakage magnetic flux density" waveform.

[Basic theory]
The basic theory of the present invention is as follows. That is, the magnetic permeability of air is not zero, the magnetic flux always 'leaks' into the air, the permeability of the low magnetization region does not change depending on the magnetic field strength, is almost constant, and the tangential component of the magnetic field is continuous. Based on this fact, attention is focused on a low magnetic field region having magnetization characteristics as shown in FIG. In the low magnetic field region, the following equation is established by Rayleigh's law. Here, the low magnetic field region is “a region where the value of the magnetic field (H) near the origin (magnetic flux density B = 0, magnetic field strength H = 0) is small in the BH curve representing the magnetic characteristics of the ferromagnetic material”. (Hereinafter the same). H is the magnetic field strength, μ (H) is the magnetic permeability at the magnetic field strength H, and μ _i0 is the initial magnetic permeability when the magnetic field strength H is zero.

  In the present invention, a weak magnetic flux density that leaks into the air, preferably using a highly sensitive magnetic sensor, by passing a direct current or low frequency alternating current that changes near zero to the exciting coil of the magnetizer to magnetize the test piece with low strength. Detect and evaluate damage by detecting signals.

  An example will be described in which a subject, which is a plate material having predetermined vertical and horizontal dimensions and plate thickness T shown in FIG. A thinned portion, that is, a concave portion (remaining thickness t in the thinned portion) is provided on the back surface of the subject. In the same figure, a predetermined alternating current is passed through a magnetizing coil of a small-yoke type electromagnet (magnetizer) wound around a magnetic core denoted by “core” to magnetize the subject. Note that a predetermined gap (denoted as “gap” in the figure) is provided between the electromagnet and the subject. The cross-sectional area and permeability of each part of the magnetic circuit are uniform. Of course, a magnetization method other than the inter-pole method may be employed. Each indicator is defined as follows.

  Magnetoresistance R of the entire magnetic circuit consisting of an electromagnet, an air gap and a subject:

Here, R _core , R _gap , and R _TP are the respective magnetoresistances of the core of the electromagnet as the magnetic circuit, the predetermined gap, and the test piece (hereinafter the same). μ _core , μ ₀ , and μ _TP are the magnetic permeability of the electromagnet core, air (that is, in the predetermined gap) as a magnetic circuit, and the test piece, respectively (hereinafter the same). A _core , A _gap and A _TP are respectively the core of the electromagnet as the magnetic circuit, the predetermined gap, and the cross-sectional area of the test piece (the same applies hereinafter). l _core , l _gap , and l _TP are the magnetic path lengths of the core of the electromagnet as the magnetic circuit, the predetermined gap, and the test piece, respectively (the same applies hereinafter).

  Magnetic flux Φ flowing in a magnetic circuit composed of an electromagnet core and a subject:

Here, Φ is a magnetic flux flowing in a magnetic circuit composed of an electromagnet core, an air gap, and a subject, n is the number of turns of the magnetization coil, and i is a current flowing in the magnetization coil (the same applies hereinafter).
Further, since the magnetic flux is considered to be continuous in the circuit, the following equation holds.

Here, Φ _core , Φ _gap , and Φ _TP are the magnetic core of the electromagnet as the magnetic circuit, the predetermined gap, and the magnetic flux of the test piece (hereinafter the same).
Magnetic flux density B _TP in the subject:

Magnetic field strength H _TP in the subject:

Since R _gap and R _core do not change depending on the subject, the sum is described as R _0, and the following equation holds.

Further, considering the continuity of the tangential component of the magnetic field, the following equation holds. H ₁ is the strength of the magnetic field (leaked) in the vicinity of the surface of the test piece (hereinafter the same).

Using a predetermined measuring instrument such as a high-sensitivity magnetic sensor, the magnetization current (i) and the leakage magnetic flux density (B _l ) are recorded. The magnetization current and leakage flux density signals are plotted on the horizontal and vertical axes, respectively, to create a “magnetization current-leakage flux density” waveform (FIG. 6). At this time, as shown in the figure, a predetermined averaging process (averaging process) may be performed on the raw signal directly measured by the measuring instrument to perform noise reduction.

Next, the “magnetization current—leakage magnetic flux density” waveform is normalized to create “normal magnetization current—leakage magnetic flux density”. FIG. 7 shows a “magnetization current-leakage magnetic flux density” waveform. In the figure, the vertical axis represents the measured magnetic flux density ( _Bl signal), and the horizontal axis represents the measured current (Current signal). In the “magnetization current-leakage magnetic flux density” waveform, the decomposed upper part (ascend) (in the figure, the value of the magnetic flux density at the upper part is indicated as “B _la ”) and the lower part (descend) In the figure, the value of the leakage magnetic flux density in the lower part is indicated as “B _ld ”). A curve obtained by tracing the average value “B _ln ” of each value of the leakage magnetic flux density with respect to the same current value is expressed as “normal magnetization current−leakage”. Magnetic flux density "waveform. Corresponding to the slope of the “normal magnetization current-leakage magnetic flux density” waveform, “differential magnetic leakage reluctivity γ” expressed by the following equation is obtained.

Next, “Initial magnetic leakage reluctivity γ | _{i → 0} ” expressed by the following equation when the current becomes 0, that is, when the applied magnetic field becomes almost 0, is extracted.

Since B ₁ = μ ₀ H ₁ , the following equations are obtained from the equations [Equation 9], [Equation 10], and [Equation 12].

[Detection and evaluation of remaining thickness]
In the above equation [13], when μ _i0 R ₀ / nμ ₀ is constant and thinning occurs in a sufficiently large region, the remaining thickness t is proportional to the cross-sectional area,

  Therefore, the following equation is established.

The procedure described above is performed for each specimen having a different remaining thickness (or reduced thickness) or a thickness of a healthy part, and an “initial leakage resistance ratio−remaining thickness” relationship diagram is obtained. By referring to the “initial leakage resistivity—remaining thickness” relationship diagram (master curve) obtained in advance in this way, the remaining thickness (or reduced thickness) of the object or the thickness of the healthy part Can be evaluated.
[Detection and evaluation of magnetic properties of materials]
Further, the following equation is obtained in the same manner as the equation [Equation 13].

When no thinned portion is provided on the back surface of the subject and the thickness is constant, that is, A _TP , R ₀ , and n are constant, γ | _{i → 0} changes μ _i0 as shown in the following equation. reflect.

In this way, by using the correlation (linear relationship) between γ | _{i → 0} and μ _i0 , the analysis of the area surrounded by the initial leakage resistivity and the “magnetization current-leakage magnetic flux density” waveform, etc. By evaluating the magnetic characteristics, it is possible to evaluate the change of the material from the previously obtained relationship of “magnetic characteristic change—structure or mechanical characteristic change”.

FIG. 8 shows an outline of the procedure of the evaluation method in the above-described leakage magnetic flux method according to the present invention.
[Example 1]
Example 1 is an experiment related to detection / evaluation of backside thinning. Two types of experiments were performed by changing the material of the subject made of a ferromagnetic material. Here, ferromagnetism is defined as “a magnetic material that is strongly magnetized and attracts a magnet strongly when placed in a magnetic field, such as iron” (the same applies hereinafter).

[Example 1-1]
Experiments were conducted using SB410 steel plates having dimensions of 100 mm long × 100 mm wide × predetermined plate thickness as test pieces. The thickness of each test piece is 3 mm, 4 mm, 6 mm, 8 mm, 10 mm, and 12 mm, respectively. In this experiment, 0.6 Ampere pp (peak-to-peak), 1Hz frequency alternating current was applied to the magnetizing coil of a small yoke-type electromagnet (magnetizer) to magnetize each test piece, and the surface of the test piece A signal of magnetic flux density leaking into the air is measured using a predetermined measuring instrument at a predetermined location corresponding to the thinned portion. Assuming that a thinned portion (concave portion) exists on the back surface of the subject to be used in the actual site of the method according to the present invention, the distance between the poles of the magnetizing coil is along the direction of the distance between the poles. When the distance between both edges of the thinned portion is sufficiently large, the plate thickness of each test piece used in this experiment can be regarded as the remaining thickness t of the thinned portion of the subject.

  As shown in FIG. 9, a “magnetization current-leakage magnetic flux density” waveform (horizontal axis: magnetization current, vertical axis; leakage magnetic flux density, the same applies to the same waveform) corresponding to the test pieces of each plate thickness is obtained. Next, the “magnetization current-leakage magnetic flux density” waveform is normalized, and a “normal magnetization current-leakage magnetic flux density” diagram (horizontal axis: magnetization current, vertical axis: normal) as shown in FIG. The obtained leakage magnetic flux density, the same applies to FIG. As the normalization method, the above-described method is employed, but other methods can be appropriately employed. Each of FIGS. 11-16 is the “magnetization current-leakage magnetic flux density” waveform of each test piece having a thickness of 3 mm, 4 mm, 6 mm, 8 mm, 10 mm, and 12 mm and “ FIG. 4 is a diagram of “normal magnetization current−leakage magnetic flux density”. In each of the above drawings, the “magnetization current-leakage magnetic flux density” waveform is shown on the upper side of the drawing, and the “normal magnetization current-leakage magnetic flux density” diagram is shown on the lower side of the drawing.

  Subsequently, as shown in FIG. 17, after plotting the initial leakage resistivity of each test piece obtained based on the above “regular magnetization current-leakage magnetic flux density” diagram, the relationship between the initial leakage resistivity and the plate thickness indicated by a broken line is plotted. Is obtained as a master curve (in this experiment, a linear function). By using this master curve, the remaining thickness or the plate thickness of the thinned portion can be estimated from the measurement signal under the same test conditions.

[Example 1-2]
A similar experiment was carried out by changing the material, that is, using an SM490 steel plate having a predetermined plate thickness dimension of 150 mm long × 150 mm wide as a test piece. In this experiment, test pieces having thicknesses of 6 mm, 8 mm, 9 mm, and 10 mm were used. Similar to [Example 1-1] above, each test piece is magnetized by passing an alternating current of 0.6 Ampere pp (peak-to-peak) and a frequency of 1 Hz to correspond to a thinned portion on the surface of the test piece. At a location, a signal of magnetic flux density leaking into the air is measured using a predetermined measuring instrument.

  Similar to [Example 1-1] above, “magnetization current-leakage magnetic flux density” waveform (FIG. 18), “regular magnetization current-leakage magnetic flux density” diagram (FIG. 19), plot of initial leakage resistivity and plate thickness. A master curve (linear function in this experiment) representing the relationship between the initial leakage resistivity and the plate thickness can be obtained by creating a diagram (FIG. 20). By using this master curve, it is possible to estimate the plate thickness or the remaining thickness from the measurement signal under the same test conditions.

[Conclusion of Example 1]
From the above, it was proved that the plate thickness or the remaining thickness of the subject can be estimated from the measurement signal by applying the present invention to various ferromagnetic subjects.

[Application Example of Example 1]
[theory]
The experiment in [Example 1-1] above proves that the initial magnetic leakage resistivity is a linear function of the remaining thickness t. Use this result to find a master curve with a different viewpoint. The rationale for this is as follows. The following formula is established for a healthy part with no thinning.

Here, _AT is a cross-sectional area at the plate thickness T. This is substituted into the above [Formula 13] to obtain the following equation.

  On the other hand, the following formula is established for the thinned portion where there is thinning.

Here, A _t is the cross-sectional area of the remaining wall thickness t at the reduced thickness portion. This is substituted into the above [Formula 13] to obtain the following equation.

Since T = d + t, A _T −A _t = LXd (where d is the amount of thinning in the thinned portion, and L is the width in the cross section). Using the above [Equation 19] and [Equation 21] and taking the difference between the initial magnetic leakage resistivity of the healthy part and the initial magnetic leakage resistance of the thinned part, the following equation is obtained.

From the above, it was proved that Δγ _t | _{i → 0} is proportional to the thinning amount d.
[Use of experimental results]
The plate thickness of 12 mm obtained in the above [Example 1-1] is the plate thickness of the healthy portion, and the plate thicknesses of 10 mm, 8 mm, 6 mm, 4 mm, and 3 mm are the remaining thickness t of the thinned portion (that is, the reduced thickness d respectively. = 2mm, 4mm, 6mm, 8mm, 9mm), and FIG. 21 is obtained in accordance with the theory described above. Using the master curve obtained in advance as described above, the thinning amount d of the subject can be obtained on site.
[Example 2]
Example 2 is an experiment relating to inspection / evaluation of material change (deterioration) of a ferromagnetic material. Two types of experiments were performed with different specimen materials.

[Example 2-1]
This experiment relates to the evaluation of the presence / absence / PWHT temperature of PWHT (abbreviation of Post Weld Heat Treatment, a so-called “post-weld heat treatment”) of 9Cr—Mo steel welds.
In this experiment, a 9Cr-Mo welding material having the same shape and dimensions (length 60 mm × width 50 mm × thickness 20 mm) was used as a test piece.

  The magnetizer used in Example 1 described above is a 9Cr-Mo welded As-received specimen (in short, no PWHT treatment), and at 682 ° C, 720 ° C, 740 ° C, and 762 ° C for 2.25 hours, respectively. It is placed on the surface of each test piece that has been subjected to PWHT treatment, and the magnetic property change of each test piece is detected and evaluated. The magnetization conditions are the same as in the first embodiment.

  Similarly to Example 1, the “magnetization current-leakage magnetic flux density” waveform (FIG. 22) and the “regular magnetization current-leakage magnetic flux density” diagram (FIG. 23) corresponding to each test piece are created. Each of FIGS. 24-28 shows “magnetization current-leakage magnetic flux density” waveforms and “normal magnetization current-leakage magnetic flux density” diagrams of As-received, 682 ° C, 720 ° C, 740 ° C, and 762 ° C test pieces. It is shown. In each of the above drawings, the “magnetization current-leakage magnetic flux density” waveform is shown on the upper side of the drawing, and the “normal magnetization current-leakage magnetic flux density” diagram is shown on the lower side of the drawing. Next, as in Example 1, the initial leakage resistivity is obtained based on the “normal magnetization current−leakage magnetic flux density”.

  Subsequently, as shown in FIG. 29, a graph of “initial leakage resistivity and“ area of “magnetization current-leakage magnetic flux density” waveform ”as a master curve is obtained. Here, the same area is an area surrounded by an upper part and a lower part in the “magnetization current-leakage magnetic flux density” waveform.

  In the figure, the value of the initial leakage resistivity of the As-received test piece is about -0.21, whereas the value of the initial leakage resistivity of each test piece subjected to the PWHT treatment is around -0.12 to -0.004. In a certain point, the value of the initial leakage resistivity of the As-received test piece is significantly different from the value of the initial leakage resistivity of each test piece subjected to the PWHT treatment. The value of the area of the “magnetization current-leakage magnetic flux density” waveform is remarkably different between the As-received test piece and each test piece subjected to the PWHT process, as is apparent from FIG. Further, as the temperature of the PWHT process is higher, the value of the area of the same waveform becomes larger.

  From the above, it was proved that by preparing such a master curve in advance, it is possible to distinguish between the As-received test piece and the test piece subjected to PWHT treatment. It has also been proved that the temperature of the PWHT process can be estimated.

[Example 2-2]
This experiment relates to the characterization of SM400A and SM490YB steels with different magnetic properties. In this experiment, a test piece of SM400A steel and a test piece of SM490YB steel having the same shape and dimensions (length 60 mm x width 50 mm x thickness 20 mm) were used.

  The magnetizer used in Example 1 described above is placed on the surface of each test piece of SM400A steel and SM490YB steel, and the magnetic property change of each test piece is detected and evaluated. The magnetization conditions are the same as in the first embodiment. In the same manner as in Example 2-1, a “magnetization current-leakage magnetic flux density” waveform (FIG. 30) and “normal magnetization current-leakage magnetic flux density” (FIG. 31) corresponding to each test piece were created. The initial leakage resistivity is obtained based on “magnetization current-leakage magnetic flux density”. Subsequently, a graph of “initial leakage resistivity and“ magnetization current-leakage magnetic flux density ”waveform area” as shown in FIG. 32 is obtained.

  As is apparent from the figure, the initial magnetic leakage resistivity values of the test pieces of SM400A steel and SM490YB steel are significantly different. Moreover, the values of the areas of the “magnetization current-leakage magnetic flux density” waveforms of the test pieces of SM400A steel and SM490YB steel are significantly different. Thus, it has been demonstrated that these two types of steel can be characterized using this method.

[Conclusion of Example 2]
From the above, it was proved that the subject can be characterized from the measurement signal by applying the present invention to the subject of various materials.

H ₁ Magnetic field strength in the vicinity of the surface of the specimen (leaked) H Magnetic field intensity in the _TP specimen B ₁ Leakage magnetic flux density T Thickness of the specimen / specimen t Remaining thickness d Reduction in thickness γ | _{i → 0} initial magnetic leakage resistivity μ _i0 initial magnetic permeability Δγ _t | _{i → 0} change in initial magnetic leakage resistivity

Claims (10)

In the evaluation method in the leakage magnetic flux method,
Magnetizing a plurality of test pieces made of the same predetermined material and having different plate thicknesses;
Measuring the value of the magnetizing current and the value of the leakage magnetic flux density signal for each of the plurality of test pieces;
Creating a magnetization current-leakage magnetic flux density waveform for each of the plurality of test pieces based on the value of the magnetization current and the value of the leakage magnetic flux density signal;
Creating a master curve based on the magnetization current-leakage magnetic flux density waveform;
Using the master curve to evaluate a subject made of the predetermined material.   The evaluation method according to claim 1, wherein the content of the evaluation relates to a plate thickness of the subject made of the predetermined material. In the step of creating the master curve,
Normalizing the magnetization current-leakage flux density waveform;
The master showing the relationship between the initial leakage resistivity and the plate thickness of the test piece by obtaining each initial leakage resistivity using a normalized magnetization current-leakage magnetic flux density waveform for each of the plurality of test pieces. The evaluation method according to claim 2, wherein a curve is created. In the step of creating the master curve,
Normalizing the magnetization current-leakage flux density waveform;
Determine each initial leakage resistivity using a normalized magnetization current-leakage flux density waveform for each of the plurality of test specimens,
From the initial leakage resistivity of the first test piece having the maximum first plate thickness among the plate thicknesses of the plurality of test pieces, each initial of the test pieces having other plate thicknesses other than the first test piece By reducing the leakage resistivity respectively, the difference in the initial leakage resistivity is obtained,
By subtracting each value of the other plate thickness from the first plate thickness, respectively, to obtain the difference in plate thickness,
The evaluation method according to claim 2, wherein the master curve indicating the relationship between the difference in the initial leakage resistivity and the difference in plate thickness of the test piece is created.   The evaluation method according to claim 3 or 4, wherein the master curve is a linear function. In the evaluation method in the leakage magnetic flux method,
Magnetizing a plurality of specimens with different properties;
Measuring the value of the magnetizing current and the value of the leakage magnetic flux density signal for each of the plurality of test pieces;
Creating a magnetization current-leakage magnetic flux density waveform for each of the plurality of test pieces based on the value of the magnetization current and the value of the leakage magnetic flux density signal;
Normalizing the magnetization current-leakage flux density waveform;
Determining each initial leakage resistivity using a magnetized current-leakage flux density waveform normalized for each of the plurality of test specimens;
For each of the plurality of test pieces, obtaining an area surrounded by an upper part and a lower part in the waveform of the magnetizing current-leakage magnetic flux density;
Creating a master curve indicating the relationship between the initial leakage resistivity and the area of the waveform of the magnetizing current-leakage magnetic flux density;
Evaluating the characteristics of the subject using the master curve.   The evaluation method according to claim 6, wherein the characteristic of the subject is PWHT processing.   The evaluation method according to claim 6, wherein the characteristic of the subject is a magnetic characteristic. In the step of creating the magnetization current-leakage magnetic flux density waveform,
The evaluation method according to any one of claims 1 to 8, wherein a noise is reduced by performing a predetermined averaging process on the measured value of the magnetizing current and the leakage magnetic flux density signal.   The evaluation method according to claim 1, wherein the test piece and the subject are made of a ferromagnetic material.