JP2011185623A - Device for evaluation of surface treatment - Google Patents

Abstract

An object of the present invention is to provide a surface treatment evaluation apparatus capable of evaluating a surface treatment without being affected by the arrangement of inspection objects and without requiring a calibration curve.
A surface treatment evaluation apparatus 20 includes a sensor unit 30 including an excitation coil 32 that applies an alternating magnetic field to an inspection object 10, a detection coil 34 that detects an eddy current generated by the alternating magnetic field, and a plurality of excitation coils 32. An excitation setting unit that applies an alternating current while switching the inspection frequency in a range of frequencies, and an eddy current signal of the inspection object 10 is obtained by the detection coil 34 for each of the inspection frequencies, and the same kind in which surface treatment is not performed. A signal ratio that is a ratio of the unprocessed object to the eddy current signal is calculated for each inspection frequency, and a peak extraction unit that extracts a peak frequency at which the signal ratio reaches a peak, and based on the extracted peak frequency An evaluation value calculation unit that calculates an evaluation value of the surface treatment of the inspection object 10.
[Selection] Figure 1

Description

  The present invention relates to a surface treatment evaluation apparatus, and more particularly, to a surface treatment evaluation apparatus that performs evaluation based on an eddy current signal related to an eddy current generated by applying an alternating magnetic field to an object to be inspected.

  For example, when quenching is performed as a surface treatment for steel materials, the depth of hardness can be obtained by calculating the depth distribution of hardness and quenching the depth at which the hardness changes as the evaluation of surface treatment. Depth is done. Although this method is a destructive inspection, in order to evaluate the quenching depth in a nondestructive manner, it is utilized that the electrical and magnetic properties of the steel material change depending on the quenching depth. As one of them, the quenching depth is evaluated from the magnitude of the eddy current generated by applying an alternating magnetic field to the steel material.

  For example, in Patent Document 1, as a quenching depth measurement method, a steel material to be inspected is magnetized in a direction along the surface by a low-frequency AC magnetic field generated by an exciting coil, and induced by an eddy current generated thereby. It is described that the magnetic field is detected by a detection coil, and the output voltage of the detection coil is compared with the correlation data of the depth of quenching calibrated in advance to measure the quenching depth.

  Patent Document 2 discloses, as a hardened layer depth inspection method, a vortex including a probe having a configuration in which two detection coils arranged concentrically or coaxially and having different physical structures are inserted concentrically with an excitation coil. Using a current sensor, an alternating current is passed through the exciting coil to generate a magnetic field. The eddy current generated in the subject by the magnetic field is detected by the two detection coils, and a value based on the difference between the detection signals of the two detection coils is calculated. It is described that the quality of the hardened layer is determined as compared with a predetermined quality determination threshold value. As described above, since the difference between detection values of two detection coils having different physical structures is used, it is stated that 100% inspection can be performed without performing temperature correction even in a production line in which environmental temperature is likely to change. Yes.

  Non-Patent Document 1 describes a quenching depth estimation method that does not use calibration data. Here, a method is used in which an alternating current is applied to both ends of the steel rod and the impedance between two points along the axial direction of the steel rod is measured. Here, an axisymmetric model is used to obtain the relationship between the frequency of the alternating current and the real part and imaginary part of the impedance, and the actual part and imaginary part of the impedance are changed by actually changing the frequency of the alternating current. It is shown that the measurement formula is calculated and a calculation formula adapted thereto is obtained, and the conductivity σ and the permeability μ are obtained from the result.

  Then, in the case of a steel rod with a homogeneous structure without a hardened layer and the case of a steel rod with a two-layered structure with a hardened layer, the measured values and the calculation formulas corresponding thereto are obtained, respectively. The permeability and permeability in the case of a two-layer structure and the quenching depth that is the boundary between the two-layer structure are obtained. And it was shown that the quenching depth obtained in this way matches well with the quenching depth found from the change in Rockwell hardness. It is stated that it can be sought.

Here, SAE / AISI 1056 steel rod is used in the experiment in the US standard, and it is shown that the electric conductivity is 4.77 MSm ⁻¹ and the relative magnetic permeability is 66 in the case of a homogeneous structure without a hardened layer. Yes. Further, it is shown that the two-layer structure having a quenching layer is an average value for three quenching depths, and has an electrical conductivity of 3.84 MSm ⁻¹ and a relative magnetic permeability of 49.4.

JP 2002-14081 A JP 2009-36682 A

John R. Bowler et.al., Alternating current potential-drop measurement of the depth of case hardening steel rods, Measurement Science and Technology, 19 (2008), 075204, p1-8

  According to the methods of Patent Document 1 and Patent Document 2, an inspection object is inserted into each annular hole of the excitation coil and the detection coil, an alternating magnetic field is applied by the excitation coil, and the magnitude of the eddy current is increased by the detection coil. Since the evaluation is performed, the detected value of the eddy current is influenced by the arrangement relationship of the inspection object with respect to each coil.

  For example, the detected value of the eddy current may be different between the case where the inspection object is arranged coaxially with respect to the annular hole of the exciting coil or the annular hole of the detection coil and the case where the inspection object is arranged eccentrically. Further, when the inspection object is inserted obliquely with respect to the central axis of the annular hole of the exciting coil and the annular hole of the detection coil, the detected value of the eddy current may change depending on the degree of inclination.

  As described above, in the method of inserting the inspection object into the annular holes of the plurality of coils, the detected value of the eddy current varies depending on the arrangement relationship, and accurate evaluation may be difficult. In addition, in order to obtain the quenching depth, it is necessary to obtain a calibration curve for the relationship between the quenching depth and the magnitude of the eddy current in advance, and the quenching depth is directly determined from the detected value of the eddy current. Cannot be asked.

  The objective of this invention is providing the surface treatment evaluation apparatus which can evaluate surface treatment, without being influenced by arrangement | positioning of a test target object. Another object is to provide a surface treatment evaluation apparatus that can evaluate surface treatment without requiring a calibration curve. The following means contribute to at least one of the above objects.

  A surface treatment evaluation apparatus according to the present invention includes a sensor unit including an excitation coil that applies an alternating magnetic field to an object to be inspected and a detection coil that detects an eddy current signal related to an eddy current generated by the alternating magnetic field. An excitation setting unit that applies an alternating current while switching the inspection frequency in a range of a plurality of frequencies set in advance in the excitation coil of the sensor unit, and an eddy current signal of the inspection object is detected by the detection coil for each of the inspection frequencies. A peak extraction unit that calculates a signal ratio, which is a ratio to an eddy current signal for an unprocessed object of the same type that has not been subjected to surface treatment, for each inspection frequency, and extracts a peak frequency at which the signal ratio reaches a peak And an evaluation value calculation unit that calculates an evaluation value of the surface treatment of the inspection object based on the extracted peak frequency.

  Further, in the surface treatment evaluation apparatus according to the present invention, the evaluation value calculation unit evaluates the surface treatment of the inspection object based on the physical property value related to the eddy current of the inspection object and the peak frequency obtained in advance. It is preferable to calculate the value.

  Further, in the surface treatment evaluation apparatus according to the present invention, the evaluation value calculation unit uses the conductivity and permeability as physical property values related to eddy current when the surface treatment is a quenching treatment, and uses the equation for calculating the skin depth. Based on this, it is preferable to calculate the evaluation value of the surface treatment of the inspection object.

  Moreover, in the surface treatment evaluation apparatus according to the present invention, the sensor unit includes two excitation coils that are spaced apart from each other so as to apply an alternating magnetic field in a vertical direction to the surface on which the surface treatment of the inspection object is performed. And at least one detection coil arranged between the two exciting coils and arranged to detect an eddy current signal in a direction parallel to the surface of the object to be inspected.

  With the above configuration, the surface treatment evaluation apparatus includes an excitation coil and a detection coil, and applies an alternating current while switching the inspection frequency in a range of a plurality of frequencies set in advance in the excitation coil. An eddy current signal of the inspection object is obtained by the detection coil, and a signal ratio, which is a ratio to the eddy current signal of the same kind of untreated object that has not been surface-treated, is calculated for each inspection frequency. The peak frequency that becomes the peak is extracted. Then, an evaluation value of the surface treatment of the inspection object is calculated based on the extracted peak frequency.

The eddy current has a skin effect and flows in a portion having a skin depth δ. Skin depth [delta] is expressed by _{δ = {1 / (πμ 0} μ r σf)} 1/2. Here, μ ₀ is the vacuum permeability, μ _r is the relative permeability, σ is the conductivity, and f is the inspection frequency. Thus, since the skin depth has frequency dependency and permeability dependency, the depth at which the current flows varies depending on the change in the inspection frequency, and the depth at which the current flows also varies depending on the change in the permeability. . Assuming that the magnetic permeability changes due to surface treatment, the eddy current changes when the frequency is changed for the untreated object and the eddy current when the frequency is changed for the inspection object. The state of change will be different.

  In particular, when the inspection frequency is low, the inspection object has a two-layer structure having a characteristic in which eddy currents are generated differently in the surface portion where the surface treatment is performed and the central portion where the surface treatment is not performed. However, when the skin depth is shallower than the depth at which the surface treatment is performed at a higher frequency, eddy currents are generated only in the layer on which the surface treatment has been performed. That is, the method of generating eddy currents differs greatly depending on whether the skin depth determined by the inspection frequency is shallower or deeper than the surface treatment depth. In that sense, the frequency characteristics of the eddy current in the inspection object vary greatly from the inspection frequency corresponding to the skin depth of the same depth as the surface treatment depth.

  On the other hand, the unprocessed object still has the effect that eddy currents are generated in the layer that has not been surface-treated even when the skin depth is reduced by changing the inspection frequency, and the change in the inspection frequency. The state of eddy current generation does not change significantly depending on Here, taking the ratio between the eddy current characteristics of the inspection object and the eddy current characteristics of the untreated object, and taking the frequency characteristics of the ratio, the surface treatment depth is the same as the skin depth. It has been experimentally found that the characteristic has a peak at the inspection frequency.

  If the inspection frequency at the peak is called a peak frequency, the peak frequency is a frequency at which the skin depth matches the surface treatment depth. For example, the arrangement relationship of the inspection object with respect to the excitation coil and the detection coil Little affected by variation. As described above, the surface treatment evaluation such as the surface treatment depth is performed based on the peak frequency, so that the surface treatment can be evaluated without being influenced by the arrangement of the inspection object.

Also, the skin depth [delta], as described above is expressed by _{δ = {1 / (πμ 0} μ r σf)} 1/2. Thus, it puts the test frequency f and the peak frequency f _C, by using the physical property values that are known to σ and mu ₀ mu _r, it is possible to obtain the skin depth δ at the peak frequency f _C. The skin depth δ at the peak frequency f _C is the surface treatment depth as described above. Thus, once the peak frequency f _C is known, the surface treatment depth can be directly obtained using known physical property values, and no special calibration curve or the like is required.

In particular, when the surface treatment is a quenching treatment, the σ and mu ₀ mu _r by using the conductivity and permeability of hardened steel, as described above, based on the formula for skin depth, baked The depth of penetration can be determined directly.

  Further, in the surface treatment evaluation apparatus, as the sensor unit, two excitation coils arranged so as to be separated from each other so as to apply an alternating magnetic field in a direction perpendicular to the surface on which the surface treatment of the inspection object is performed, and two At least one detection coil is used which is arranged between the excitation coils and is arranged to detect an eddy current signal in a direction parallel to the surface of the inspection object. As described above, since the inspection object is not inserted into the annular hole of the coil, there is no problem of the arrangement relationship such as the degree of coaxiality between the annular hole of the coil and the inspection object.

It is a figure explaining the structure of the surface treatment evaluation apparatus of embodiment which concerns on this invention. It is a figure explaining the arrangement | positioning relationship of the exciting coil of a sensor part and detection coil in embodiment which concerns on this invention. In embodiment which concerns on this invention, it is a top view which shows the relationship between the electric current which flows into an exciting coil, and the eddy current which flows into a test object. FIG. 4 is a diagram for explaining the state of an eddy current flowing through an inspection object, a magnetic field due to the eddy current, a leakage magnetic field from an excitation coil, and a magnetic field detected by a detection coil corresponding to FIG. 3. In the embodiment according to the present invention, the magnitude of the magnetic field due to the eddy current for each of the untreated object, the object that has been completely quenched, and the inspection object that has been quenched from the surface to the quenching depth. It is a figure explaining the mode of the frequency characteristic. FIG. 6 is a diagram for explaining the frequency characteristics of the ratio of the eddy current signal of the inspection object to the eddy current signal of the unprocessed object with respect to the eddy current signal detected by the detection coil, corresponding to FIG. 5. In embodiment which concerns on this invention, it is a flowchart explaining the procedure of the surface treatment evaluation method. It is a figure explaining a mode that the peak frequency when a quenching depth is 1.00 mm is calculated | required according to the method which concerns on this invention. It is a figure explaining a mode that a peak frequency when a quenching depth is 2.00 mm is calculated | required with respect to FIG. It is a figure explaining a mode that the peak frequency when a hardening depth is 3.00 mm is calculated | required with respect to FIG. 8, FIG. It is the figure which put together the comparison with the calculated value of the surface treatment depth based on the result of FIGS. 8-10, and actual quenching depth.

  Embodiments according to the present invention will be described below in detail with reference to the drawings. Hereinafter, as the surface treatment, a quenching process from the surface of the steel material to a predetermined depth will be described. However, other surface treatments may be used as long as the surface treatment changes the skin depth. For example, it may be a strong working process that generates a residual stress layer such as peening, a carburizing process from the surface of the steel material to a predetermined depth, a plating process, a coating process of a conductive material, and the like.

  In the following description, the number of excitation coils is 2 and the number of detection coils is 2. However, the number of excitation coils may be 3 or more, and the number of detection coils is 1 or 3 or more. May be.

  The materials, shapes, dimensions, frequencies, and the like described below are illustrative examples, and can be appropriately changed according to the contents of the inspection object or the specifications of the surface treatment inspection apparatus. In the following, the quenching depth is obtained from the calculation result without using the calibration curve, but of course, an appropriate calibration curve may be used. As a calibration curve in that case, a curve obtained by obtaining a relationship between the quenching depth and the peak frequency in advance can be used.

  Below, the same code | symbol is attached | subjected to the same element in all the drawings, and the overlapping description is abbreviate | omitted. In the description in the text, the symbols described before are used as necessary.

  FIG. 1 is a diagram for explaining the configuration of a surface treatment evaluation apparatus 20 used for the evaluation of the quenching depth. Here, although not a constituent element of the surface treatment evaluation apparatus 20, as the inspection object 10, the base material portion 12 that has not been subjected to the quenching process and the quenching process that has been quenched from the surface to the quenching depth d. A steel material having a treated layer 14 is shown.

  The surface treatment evaluation apparatus 20 includes a sensor unit 30 including an excitation coil 32 and a detection coil 34, an oscillator 40 for supplying an excitation signal to the excitation coil 32, a current source 42, and an eddy current signal from the detection coil 34. Connected to the eddy current signal measurement unit 50 to measure, the evaluation value calculation unit 60 that processes the signal from the eddy current signal measurement unit 50 and outputs the surface treatment evaluation value, and the evaluation value calculation unit 60, and is necessary for processing And a storage unit 70 that stores various physical property values.

  As described above, the sensor unit 30 includes the excitation coil 32 and the detection coil 34, and these are integrated together in a predetermined arrangement relationship. The lower surface of the excitation coil 32 and the lower surface of the detection coil 34 are contact lower surfaces that contact the surface of the inspection object 10.

  In FIG. 1, a gap between the contact lower surface and the surface of the inspection object 10 is shown as a lift-off amount 16. This lift-off amount 16 is obtained when the lower surfaces of the excitation coil 32 and the detection coil 34 are not necessarily one plane when the sensor unit 30 is integrated, or when there is a material thickness for integration. This occurs when the surface of the inspection object 10 is not necessarily a flat surface. This lift-off amount 16 becomes an error in the arrangement relationship in the case of the surface treatment evaluation apparatus 20. The fact that the lift-off amount 16 hardly affects the surface treatment evaluation will be described later.

  FIG. 2 is a perspective view showing a specific arrangement relationship of the sensor unit 30. Here, two excitation coils 32 and 33 are used, and two detection coils 34 and 35 are also used.

  The exciting coils 32 and 33 are annular winding coils for applying an alternating magnetic field to the inspection object 10 to generate eddy currents. The two exciting coils 32 and 33 are used as described above, and the two exciting coils 32 and 33 apply an alternating magnetic field in the vertical direction to the surface of the inspection object 10 on which the surface treatment is performed. So as to be spaced apart from each other. That is, the exciting coils 32 and 33 are winding coils wound in an annular shape, and are arranged so that the center axis of the annular shape is perpendicular to the surface of the inspection object 10.

  The two exciting coils 32 and 33 have the same basic structure and the same dimensions. The arrangement of the two exciting coils 32 and 33 can be appropriately determined as long as the intervals are sufficient for the detection coils 34 and 35 to be arranged.

  The detection coils 34 and 35 are annular winding coils for detecting a magnetic field due to an eddy current generated in the inspection object 10 by the excitation coils 32 and 33. Two detection coils 34 and 35 are used as described above, and the two detection coils 34 and 35 are disposed between the two excitation coils 32 and 33 and are parallel to the surface of the inspection object 10. It is arranged to detect eddy current signals in various directions. That is, the detection coils 34 and 35 are winding coils wound in an annular shape, and are arranged so that the central axis of the annular shape is parallel to the surface of the inspection object 10.

  The two detection coils 34 and 35 have the same basic structure and the same dimensions. The heights of the two detection coils 34 and 35 are set lower than the heights of the excitation coils 32 and 33. That is, there is a deviation between the (1/2) height axis that is the center line in the height direction of the excitation coils 32 and 33 and the center axis of the annular shape of the detection coils 34 and 35, and the former is a higher position. It is in. As a result, even when the inspection object 10 is not present, the detection coils 34 and 35 detect the leakage magnetic field flowing between the excitation coils 32 and 33 when an excitation current is passed through the excitation coils 32 and 33. The excitation current corresponds to an excitation signal for generating an eddy current in the inspection object 10.

  Since the two detection coils 34 and 35 are used in order to increase the detection sensitivity of the magnetic field due to the eddy current, it is preferable that the two detection coils 34 and 35 are arranged closely.

  Returning to FIG. 1 again, the oscillator 40 and the current source 42 correspond to an excitation current supply unit that supplies a plurality of AC excitation currents to the excitation coils 32 and 33.

  The oscillator 40 is an AC signal generation device that can generate a plurality of AC signals. The oscillator 40 generates AC signals having a plurality of frequencies at preset frequency intervals and supplies the AC signals to the current source 42. The frequency switching timing is performed based on a signal transmitted from the eddy current signal measuring unit 50. As the plurality of frequencies, for example, in the range of several tens of Hz to several kHz, the frequency can be set to be 10 to several tens of types. Since the frequency range is several digits, the frequency interval is preferably set so as to have an even width with respect to the logarithmic scale.

  The current source 42 is a driver circuit for outputting a current having a predetermined current amplitude with respect to the AC signal supplied from the oscillator 40. The current amplitude can be set to a suitable value between several mApp and several App, for example.

  The eddy current signal measuring unit 50 extracts only the detection signal having the same frequency as the AC excitation signal supplied to the excitation coils 32 and 33 from the signals detected by the detection coils 34 and 35 as described above. The signal processing circuit has a function of appropriately amplifying the signal and outputting the signal to the evaluation value calculation unit 60 as an eddy current signal. The eddy current signal measuring unit 50 includes a phase detector 52, an amplifier 54, a filter 56 and a display 58.

  As described above, since excitation signals of a plurality of frequencies are supplied to the excitation coils 32 and 33, the eddy current signal measurement unit 50 transmits a signal to that effect to the oscillator 40 when measurement for one frequency is completed. To do. When the signal is received, the oscillator 40 outputs an AC signal having the next frequency to the current source 42. Therefore, this signal corresponds to a frequency switching signal.

The evaluation value calculation unit 60 has a function of calculating and outputting the surface treatment evaluation value of the inspection object 10 based on the eddy current signal output from the eddy current signal measurement unit 50 as described above. It is. The evaluation value calculation unit 60 includes an A / D converter 62, a data processing unit 64, a memory 66, and a display 68. The memory 66 is a device that temporarily stores an eddy current signal value for each frequency output from the eddy current signal measurement unit 50. The data processing unit 64 is the core of the evaluation value calculation unit 60. The data processing unit 64 obtains a peak frequency f _C described later from eddy current signal values for a plurality of frequencies stored in the memory 66, and sets the peak frequency f _C to the peak frequency f _C. Therefore, it has a function of calculating the surface treatment evaluation value. In this case, the surface treatment evaluation value is the quenching depth. The details of the calculation of the surface treatment evaluation value will be described later.

The storage unit 70 is a storage device for storing in advance physical property values and the like necessary for data processing by the data processing unit 64. The storage unit 70 and the memory 66 may be combined to form one storage device. The storage unit 70 stores unquenched material data 72 that is an eddy current signal value for each of a plurality of frequencies for an unquenched material that has not been subjected to a quenching process using the same steel material as that of the inspection object 10. . Since the unquenched material is the same material as the inspection object 10 and has not been subjected to surface treatment, it can be called an untreated object. Further, the storage unit 70 stores physical property values of the quenching material that is the inspection object 10. Here, the electrical conductivity σ and the magnetic permeability μ for the quenched material are stored. The permeability mu, but relative permeability mu _r is stored, may store mu ₀ mu _r is a value obtained by multiplying the permeability mu ₀ of vacuum relative permeability mu _r.

  Next, the operation and the like of the surface treatment evaluation apparatus 20 having the above configuration will be described. 3 and 4 are schematic diagrams for explaining how the eddy current of the inspection object 10 is measured. FIG. 3 corresponds to a plan view of the surface of the inspection object 10 and FIG. 4 corresponds to a front view of a cross section of the inspection object 10.

  In FIG. 3, the excitation coils 32 and 33 and the directions of the excitation currents 80 and 81 flowing through them are shown. The excitation currents 80 and 81 are alternating currents as described above. As shown in FIG. 3, the direction of the excitation current 80 flowing through the excitation coil 32 and the direction of the excitation current 81 flowing through the excitation coil 33 are as follows. When viewed in a plan view, the directions are opposite to each other. In the example of FIG. 3, the direction of the excitation current 80 is clockwise on the paper surface, and the direction of the excitation current 81 is counterclockwise on the paper surface. Therefore, in this example, the exciting coil 32 generates a magnetic field from the upper side to the lower side of the paper, and the exciting coil 33 generates a magnetic field from the lower side to the upper side of the paper.

  In the inspection object 10, eddy currents 82 and 83 are generated so as to cancel the excitation currents 80 and 81. That is, eddy current 82 flows on the opposite side to the direction in which exciting current 80 flows, and eddy current 83 flows on the opposite side to the direction in which exciting current 81 flows. The eddy currents 82 and 83 flow in opposite directions on the paper surface. As a result, in the portion where the detection coil between the two excitation coils 32 and 33 is arranged, the eddy currents 82 and 83 are in the same direction. It flows all together. 3, eddy currents 82 and 83 flow from the lower side to the upper side of the page.

FIG. 4 shows a magnetic field 86 generated by the eddy currents 82 and 83. In the example of FIG. 4, the magnetic field 86 flows in the clockwise direction on the paper surface, and becomes the magnetic field _BE flowing from the left side to the right side of the paper in the detection coils 34 and 35. On the other hand, the leakage magnetic field 84 between the two excitation coils 32 and 33 becomes a magnetic field B _C that flows from the right side to the left side of the paper on the detection coils 34 and 35. That is, the magnetic field detected by the detection coils 34 and 35 is (leakage magnetic field B _C of the excitation coils 32 and 33) − (magnetic field B _{E of the} eddy currents 82 and 83).

The detection coils 34 and 35 detect a magnetic field and output a current signal. If this signal is called an eddy current signal S, then S = (signal S _C due to leakage magnetic field B _C of excitation coils 32 and 33) − (signal S _E due to magnetic field B _E of eddy currents 82 and 83). Thus, the detection coil 34 and 35, the excitation current 80, 81 is passed through the exciting coil 32, even in the absence of a diagnosis object 10, as an eddy current signal S, and outputs a S _C, inspected If there is an object 10, a value obtained by subtracting S _E from the S _C is output.

  Next, the relationship between the eddy current signal S and the frequency f will be described. The frequency f is the frequency of the excitation signal, and as described with reference to FIG. 1, the signal detected by the detection coils 34 and 35 in the eddy current signal measuring unit 50 is extracted from the signal corresponding to the frequency of the excitation signal. Therefore, it is also the frequency of the detection signal.

Eddy current signal S (signal S _C due to the leakage magnetic field B _C of the exciting coil 32, 33) as described above - is a (signal S _E by the magnetic field B _E of eddy currents 82 and 83), S _E epidermal Although it is affected by the frequency f due to the effect, S _C is due to the leakage magnetic field between the exciting coils 32 and 33, and therefore may be considered as a constant value having no frequency characteristics.

The value of S _E varies depending on which depth the eddy currents 82 and 83 flow in the inspection object 10. Even when the eddy currents 82 and 83 flow near the surface of the inspection object 10 and when they flow deeper, the former S _{E is} greater than the latter S _E even if the eddy currents 82 and 83 have the same magnitude. Also grows. The depth through which the eddy currents 82 and 83 flow can be expressed by the skin depth δ.

Skin depth [delta] is expressed by _{δ = {1 / (πμ 0} μ r σf)} 1/2. Here, μ ₀ is the vacuum permeability, μ _r is the relative permeability, σ is the conductivity, and f is the frequency. In this way, the skin depth δ is inversely proportional to f ^1/2 , so that the skin depth δ decreases as the frequency f increases. Further, the magnetic permeability μ ^1/2 = (μ ₀ μ _r ) ^1/2 is inversely proportional. Since the magnetic permeability of the material subjected to the quenching treatment is smaller than the magnetic permeability of the untreated material, the former skin depth is larger than the latter skin depth.

Thus, in the case of a steel material having the base material portion 12 not subjected to the quenching treatment and the quenching treatment layer 14 subjected to the quenching treatment from the surface to the quenching depth d as the inspection object 10, the S The relationship between _E and frequency f is that the frequency dependence of the skin depth δ, the difference in permeability between the base material portion 12 and the quenching treatment layer 14, the difference in the skin depth δ due to the difference, the quenching depth, It is affected by the magnitude relationship between the thickness d and the skin depth δ.

Although S _E is related to the magnitude of the magnetic field B _E due to eddy currents, FIG. 5, the frequency characteristic of the B _E, the magnetic field B _E0 for the untreated object quenching treatment is not performed, the entire steel The experimentally verified results for the magnetic field B _EQ for the entire quenching object being quenched and the magnetic field B _Eq for the inspection object being quenched from the surface to the quenching depth d. It is a schematic diagram shown. The frequency characteristics of S _E of the inspection object 10 will be described with reference to FIG.

When the frequency is a low frequency, the eddy currents 82 and 83 of the inspection object 10 flow through two layers of the quenching treatment layer 14 and the base material portion 12. As the frequency increases, the eddy currents 82 and 83 are concentrated in the direction of the surface because the skin depth δ is inversely proportional to f ^1/2 as described above. Only the incoming processing layer 14 is concentrated.

At this time, the skin depth δ coincides with the quenching depth d, but until that frequency f _C , eddy currents 82 and 83 flow across the two layers. Up to the frequency f _C, the flow depth of the eddy currents 82 and 83 moves in the direction of the surface as the frequency f increases. Therefore, the magnetic field B _Eq due to the eddy current increases as the frequency f increases. On the other hand, since the untreated object is a homogeneous material, the depth through which the eddy currents 82 and 83 flow is moved in the direction of the surface as the frequency f increases. Therefore, also in this case, the magnetic field B _E0 due to the eddy current increases as the frequency f increases.

When these two cases are compared, B _Eq of the inspection object 10 flows into two layers, the quenching treatment layer 14 and the base material part 12, and B _{E0 of the} unprocessed object is homogeneous with the same material as the base material part 12. Therefore, the rate of increase of the magnetic field _BE with respect to the frequency f is greater in the latter case.

Next, when the skin depth δ is in the vicinity of the frequency f _C that matches the quenching depth d, most eddy currents 82 and 83 flow in the quenching treatment layer 14 in the inspection object 10. Since the magnetic permeability of the quench treatment layer 14 is smaller than the magnetic permeability of the base material portion 12, the increase in the eddy currents 82 and 83 generated in the quench treatment layer 14 is the smallest, and is caused by the eddy current near the detection coils 34 and 35. The increase in the magnetic field B _Eq is the smallest. On the other hand, since the untreated object is a homogeneous material, the depth through which the eddy currents 82 and 83 flow is moved in the direction of the surface as the frequency increases. Therefore, in this case as well, the magnetic field B _E0 due to the eddy current increases as the frequency f increases.

Next, when the frequency becomes higher than the frequency f _C, the eddy currents 82 and 83 of the inspection object 10 approach the surface in the quenching treatment layer 14. Therefore, the magnetic field B _Eq due to the eddy current in the vicinity of the detection coils 34 and 35 increases as the frequency f increases. The degree of this increase is less than when the eddy currents 82 and 83 are divided into two layers. On the other hand, since the untreated object is a homogeneous material, the depth through which the eddy currents 82 and 83 flow is moved in the direction of the surface as the frequency increases. Therefore, in this case as well, the magnetic field B _E0 due to the eddy current increases as the frequency f increases.

Summarizing the above, as shown in FIG. 5, B _E0 in the case of an unprocessed object monotonously increases as the frequency f increases. This increase characteristic indicates the frequency dependence of the skin depth δ. On the other hand, B _{Eq of the} object to be inspected 10 increases monotonously as the frequency f increases on the lower frequency side than the frequency f _{C at} which the skin depth δ is the same as the quenching depth d, but the frequency f _C. At higher frequencies, the increase in B _Eq is much less than before. As shown in FIG. 5, B _Eq becomes an inflection point at the frequency f _C. As described above, the frequency characteristics of B _Eq of the inspection object 10 are greatly different from each other at the frequency f _C.

FIG. 6 is a diagram illustrating a state in which the ratio between the eddy current signal S _q of the inspection object 10 and the eddy current signal S ₀ of the unprocessed object is calculated and the frequency characteristics thereof are examined. As described above, S = (signal S _C due to the leakage magnetic field B _C of the exciting coil 32, 33) - are the (signal due to the magnetic field B _E of eddy currents _{82,83 S E), S 0 =} ( exciting coil 32 , signal S _C due to the leakage magnetic field B _C 33) - (denoted as signal S _E0) by the magnetic field B _E0 of eddy currents 82 and 83, S _q = (signal S _C due to the leakage magnetic field B _C of the exciting coils 32, 33 )-(Signal S _Eq due to magnetic field B _Eq of eddy currents 82 and 83).

Therefore, S _q / S ₀ = (S _C −S _Eq ) / (S _C −S _E0 ) = (S _Eq −S _C ) / (S _E0 −S _C ), and S _C is constant with respect to the frequency f. Therefore, S _q / S ₀ has characteristics that are substantially close to the ratio of B _Eq and B _{E0 in} FIG. Therefore, since described in FIG. 5, the frequency characteristics of S _q / S ₀ is a frequency lower frequency than f _C is S _q / S ₀ increases with increasing frequency f, the frequency is higher than f _C On the high frequency side, since the increasing tendency of B _Eq is smaller than the increasing tendency of B _E0 , S _q / S ₀ decreases as the frequency f increases. From this, S _q / S ₀ shows a peak at the frequency f _C. This was confirmed by experiments as described later.

Thus, since the frequency f _C is a frequency at which the ratio S _q / S ₀ of the eddy current signal shows a peak, this can be called a peak frequency. As described above, this peak frequency is also a frequency at which the skin depth δ matches the quenching depth d. For example, the arrangement of the inspection object 10 with respect to the excitation coils 32 and 33 and the detection coils 34 and 35 is performed. It is expected that it will be largely unaffected by variations in relationships. As will be described later, even if the lift-off amount 16 described in FIG. 1 is actually changed, the peak frequency hardly changes. In this way, by performing the surface treatment evaluation such as the quenching depth d based on the peak frequency, it is possible to evaluate the surface treatment without being affected by the arrangement of the inspection object 10. .

  FIG. 7 is a flowchart showing a procedure for performing surface treatment evaluation based on the above description. First, measurement conditions are input (S10). The measurement conditions include the frequency range of the inspection frequency in the oscillator 40, the frequency interval, the current amplitude value in the current source 42, and the like. Next, the measurement start frequency is set in the oscillator 40 based on the measurement conditions. For example, by arranging the inspection frequencies in order at predetermined frequency intervals and giving a measurement start signal, the oscillator 40 can generate an AC signal of the measurement start frequency with the initial frequency as the measurement start frequency.

Next, an excitation current corresponding to the AC signal of the measurement start frequency is applied to the excitation coils 32 and 33 (S14). Specifically, the AC signal from the oscillator 40 is supplied to the exciting coils 32 and 33 as an exciting current having a predetermined current amplitude in the current source 42. Then, the signals from the detection coils 34 and 35 are measured, stored and displayed (S16). Specifically, a signal of the detection coil 34, 35 to extract the same frequency components as the excitation signal in the eddy current signal measurement section 50, and appropriately amplify, its value as an eddy current signal S _q in the measurement start frequency It memorize | stores in the memory 66, and displays on the indicator 58,68 as needed.

Then, the next frequency is set in the oscillator 40 (S18), an excitation current is applied under the conditions (S20), and signals from the detection coils 34 and 35 are measured, stored, and displayed (S22). That is, the procedure performed for the measurement start frequency is repeated while changing the frequency. In this way, the eddy current signal S _{q at} the next frequency is stored in the memory 66.

Next, it is determined whether or not the frequency is a stop frequency (S24). The stop frequency is the last frequency of the plurality of inspection frequencies arranged in the oscillator 40 according to the frequency range of the inspection frequency of the measurement condition and the frequency interval. If the determination in S24 is negative, the process returns to S18, the measurement is continued for the remaining frequency, and the obtained eddy current signal _Sq is stored in the memory 66.

Thus, when the eddy current signal _Sq is measured and stored in the memory 66 for all the inspection frequencies determined by the measurement conditions, the frequency f _C of the maximum signal ratio is extracted (S28). . The signal ratio is S _q / S ₀ described above, and the maximum signal ratio frequency f _C is the peak frequency described in FIG. The calculation of the peak frequency is executed in the data processing unit 64 by reading the unquenched material data 72 in the storage unit 70 (S26). The unquenched material data 72 is an eddy current signal S ₀ of an unprocessed object for all inspection frequencies determined by the frequency range and frequency interval of the measurement conditions.

Using this eddy current signal S ₀ of the unprocessed object and the eddy current signal S _q of the inspection object 10 in the memory 66, the signal ratio S _q / S ₀ is calculated for every inspection frequency. Then, a frequency at which the signal ratio S _q / S ₀ has a peak is extracted, and the frequency is set as a peak frequency f _C.

When the peak frequency f _C is obtained, the quenching depth d is calculated, stored, and displayed (S32). Here, using the fact that the skin depth δ of the peak frequency f _C coincides with the quenching depth d, the relationship is _expressed as skin depth δ = {1 / (πμ ₀ μr σf)} ^1/2 . Fit f _C to f in the equation. Then, the baked Irizai physical properties 74 of the storage unit 70, the conductivity sigma, reads the magnetic permeability μ ₀ μ _r (S30), applying the values to equation skin depth [delta]. When the skin depth δ at the peak frequency f _C is thus obtained by calculation, this is output as the quenching depth d and displayed on the display 68, for example.

The results of the experiment conducted according to the above procedure are indicated by black circles in each of FIGS. 8 to 10, the horizontal axis represents the inspection frequency f, the vertical axis represents the eddy current signal ratio S _q / S ₀ , FIG. 8 shows the quenching depth d = 1.0 mm, and FIG. 9 shows the quenching. FIG. 10 shows the result of the inspection object 10 having the depth d = 2.0 mm and the quenching depth d = 3.0 mm.

In FIG. 8, the result for the inspection object 10 with the quenching depth d = 1.0 mm shows a peak characteristic with a clean frequency characteristic of the eddy current ratio, and the peak frequency f _C is obtained as 500 Hz. Similarly, in FIG. 9, the result for the inspection object 10 with the quenching depth d = 2.0 mm is the peak frequency f _C = 251 Hz, and the inspection object with the quenching depth d = 3.0 mm in FIG. The result for 10 is a peak frequency f _C = 126 Hz.

8 to 10 are data when the lift-off amount 16 is 3 mm. In the data of the black circles described above, the lift-off amount 16 is 2 mm. When the lift-off amount 16 was 2 mm, experiments were performed three times each for confirming reproducibility. In all cases, the data values almost overlapped with the black circles. Even when the lift-off amount 16 is 3 mm, the calculated value of the peak frequency f _C is hardly changed from that when the lift-off amount 16 is 2 mm. From these experiments, it can be seen that according to the evaluation method of the above configuration, the value of the peak frequency f _C is hardly influenced by the lift-off amount 16.

Figure 11 is based on the peak frequency f _C obtained in FIGS. 8-10, the skin depth _{δ = {1 / (πμ 0} μ r σf)} 1/2, are described in Non-Patent Document 1 Applying conductivity σ = 3.84 MSm ⁻¹ and relative permeability μ _r = 49.4, skin depth δ = quenching depth d was determined, and the results compared with measured quenching depth d were summarized. It is a thing. From the result of FIG. 11, it can be seen that the quenching depth obtained from the calculation without using any calibration curve and the measured quenching depth are in good agreement. In particular, considering that the practical quenching depth is often 2 mm or more, the surface treatment evaluation method is considered to have sufficient practical accuracy.

  The surface treatment evaluation apparatus according to the present invention can be used for surface treatment evaluations such as quenching, strong processing such as peening, carburization of steel, plating, and coating of conductive material.

  DESCRIPTION OF SYMBOLS 10 Inspection object, 12 Base material part, 14 Hardening process layer, 16 Lift-off amount, 20 Surface treatment evaluation apparatus, 30 Sensor part, 32, 33 Excitation coil, 34, 35 Detection coil, 40 Oscillator, 42 Current source, 50 Eddy current signal measurement unit, 52 phase detector, 54 amplifier, 56 filter, 58, 68 display, 60 evaluation value calculation unit, 62 A / D converter, 64 data processing unit, 66 memory, 70 storage unit, 72 not yet Hardened material data, 74 Hardened material property values, 80, 81 Excitation current, 82, 83 Eddy current, 84 (leakage) magnetic field, 86 (generated by eddy current).

Claims (4)

A sensor unit including an excitation coil that applies an alternating magnetic field to an object to be inspected and a detection coil that detects an eddy current signal related to an eddy current generated by the alternating magnetic field;
An excitation setting unit that applies an alternating current while switching the inspection frequency in a range of a plurality of frequencies set in advance in the excitation coil of the sensor unit;
For each inspection frequency, an eddy current signal of the inspection object is obtained by the detection coil, and a signal ratio that is a ratio to the eddy current signal of the same kind of untreated object that is not subjected to surface treatment is determined for each inspection frequency. And a peak extraction unit that extracts a peak frequency at which the signal ratio becomes a peak,
An evaluation value calculation unit that calculates an evaluation value of the surface treatment of the inspection object based on the extracted peak frequency;
A surface treatment evaluation apparatus comprising: In the surface treatment evaluation apparatus according to claim 1,
The evaluation value calculation unit
A surface treatment evaluation apparatus that calculates an evaluation value of a surface treatment of an inspection object based on a physical property value related to an eddy current of the inspection object and a peak frequency obtained in advance. In the surface treatment evaluation apparatus according to claim 2,
The evaluation value calculation unit
When the surface treatment is a quenching treatment, the evaluation value of the surface treatment of the object to be inspected is calculated based on the skin depth calculation formula using the electrical conductivity and the magnetic permeability as the physical property values related to the eddy current. Surface treatment evaluation device. In the surface treatment evaluation apparatus according to claim 1,
The sensor part
Two excitation coils arranged apart from each other so as to apply an alternating magnetic field in a direction perpendicular to the surface on which the surface treatment of the inspection object is performed;
At least one detection coil arranged between the two excitation coils spaced apart and arranged to detect an eddy current signal in a direction parallel to the surface of the object to be examined;
A surface treatment evaluation apparatus comprising:

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