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WO2012086845A1 - Surface property magnetic evaluation device and method - Google Patents

Surface property magnetic evaluation device and method Download PDF

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Publication number
WO2012086845A1
WO2012086845A1 PCT/JP2011/080544 JP2011080544W WO2012086845A1 WO 2012086845 A1 WO2012086845 A1 WO 2012086845A1 JP 2011080544 W JP2011080544 W JP 2011080544W WO 2012086845 A1 WO2012086845 A1 WO 2012086845A1
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WO
WIPO (PCT)
Prior art keywords
under inspection
magnetic sensor
magnetic
surface property
object under
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/JP2011/080544
Other languages
French (fr)
Inventor
Yoshiyasu MAKINO
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Sintokogio Ltd
Original Assignee
Sintokogio Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Sintokogio Ltd filed Critical Sintokogio Ltd
Priority to JP2013526026A priority Critical patent/JP6104161B2/en
Priority to CN201180045278.5A priority patent/CN103119432B/en
Publication of WO2012086845A1 publication Critical patent/WO2012086845A1/en
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/72—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables
    • G01N27/80—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating mechanical hardness, e.g. by investigating saturation or remanence of ferromagnetic material
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B7/00—Measuring arrangements characterised by the use of electric or magnetic techniques
    • G01B7/02—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness
    • G01B7/06—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness for measuring thickness
    • G01B7/10—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness for measuring thickness using magnetic means, e.g. by measuring change of reluctance
    • G01B7/105—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness for measuring thickness using magnetic means, e.g. by measuring change of reluctance for measuring thickness of coating
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00—Measuring force or stress, in general
    • G01L1/12—Measuring force or stress, in general by measuring variations in the magnetic properties of materials resulting from the application of stress
    • G01L1/125—Measuring force or stress, in general by measuring variations in the magnetic properties of materials resulting from the application of stress by using magnetostrictive means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00—Measuring force or stress, in general
    • G01L1/12—Measuring force or stress, in general by measuring variations in the magnetic properties of materials resulting from the application of stress
    • G01L1/127—Measuring force or stress, in general by measuring variations in the magnetic properties of materials resulting from the application of stress by using inductive means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/72—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables
    • G01N27/82—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws
    • G01N27/90—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws using eddy currents
    • G01N27/9046—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws using eddy currents by analysing electrical signals

Definitions

  • the present invention relates to a surface property evaluation device and surface property evaluation method, and more particularly to a surface property evaluation device and surface property evaluation method for evaluating surface properties such as residual stress and hardness in an object under inspection.
  • JP-A-2008-2973 discloses a non-destructive inspection device for shot-peened treatment surfaces, wherein an AC signal is input as frequency is varied to an inspection circuit furnished with a coil, disposed above a shot-peening treatment surface, and the frequency response characteristics of the impedance in that test circuit are used to inspect the state of residual stress in an object under inspection.
  • a surface property evaluation device for evaluating the surface properties of an object under inspection, comprising: a magnetic sensor for detecting a magnetic properties of the surface of the object under inspection, the magnetic sensor including a core having a magnetic body and a coil wound around the core; power supply means for supplying AC power to the coil of the magnetic sensor; signal detection means for detecting a surface property signal in response to the magnetic properties of the surface of the object under inspection detected by a magnetic sensor; memory means for storing predetermined values showing the relationship between the surface property signal and the surface properties of the object under inspection; and surface property calculation means for calculating the surface properties of the object under inspection based on the values stored in the memory means and the surface property signals detected by the signal detection means; wherein the power supply means supplies the AC power at a predetermined frequency to the coil in the magnetic sensor so as to excite the core of the magnetic sensor and to form a closed magnetic path with the surface of the object under inspection.
  • the magnetic properties of the surface of the object under inspection are detected by the magnetic sensor supplied with AC power from the power supply means; the surface property signal corresponding to the magnetic properties of the surface of the object under inspection detected by the magnetic sensor in the signal detection means is detected; and, using the surface property calculation means, the surface properties of the object under inspection can be calculated based on the predetermined value stored in the memory means indicating the relationship between the surface property signal and the surface properties of the object under inspection, and on a surface property signal.
  • the core forms a closed magnetic path with the object under inspection, therefore magnetic attenuation and leakage between the object under inspection and the magnetic sensor can be prevented.
  • the strength of the surface property signal can be increased and the magnetic property detection sensitivity of the magnetic sensor can be improved, therefore the surface properties of the object under inspection can be non-destructively and accurately evaluated.
  • the surface properties of the object under inspection are evaluated by supplying AC power at the predetermined frequency to the coil, the magnetic penetration depth into the object under inspection can be held fixed, therefore surface properties having a distribution in the depth direction can be accuately discerned.
  • Calculating surface properties here refers to evaluating surface properties not only by calculating absolute values such as residual stress or hardness, but also by calculating whether the surface property signal is within a predetermined range relative to a reference value.
  • the predetermined values indicating the relationship between the predetermined surface property signal and the surface properties of the object under inspection are preferably represented by a calibration curve indicating the correlation between a surface property signal and an object under inspection.
  • the surface properties of the object under inspection e.g., the thickness of a nitrided layer when performing nitriding treatment, or the depth of compressive residual stress when performing shot-peening treatment, etc.
  • the surface properties of the object under inspection e.g., the thickness of a nitrided layer when performing nitriding treatment, or the depth of compressive residual stress when performing shot-peening treatment, etc.
  • the predetermined values indicating the relationship between the surface property signal and the surface properties of the object under inspection are preferably reference values indicating a surface property signal in a reference sample having predetermined surface properties.
  • a quality determination of the object under inspection e.g., whether there is sufficient thickness of a nitrided layer when performing nitriding treatment, or sufficient depth of compressive residual stress when performing shot-peening treatment
  • a quality determination of the object under inspection can be performed by calculating the difference between the detected surface property signal and the reference values.
  • the core of the magnetic sensor is preferably capable of making a contact with an object under inspection along the surface shape thereof.
  • the magnetic sensor core is capable of making contact with the object under inspection along the surface shape thereof, magnetic attenuation and leakage between the object under inspection and the magnetic sensor can be prevented by bringing the core into contact with the surface of the object under inspection.
  • the strength of the surface property signal can be increased and the magnetic property detection sensitivity of the magnetic sensor can be improved.
  • the core of the magnetic sensor is preferably capable of being disposed so that the distance between the core and the surface of the object under inspection is equal to or less than 3.0 mm.
  • the fact that the core can be disposed so that the distance between the core and the object under inspection surface is equal to or less than 3.0 mm means that a closed magnetic path is formed between the magnetic sensor and the surface of the object under inspection so that a sufficiently strong magnetic detection signal is obtained, thus enabling the surface properties of the object under inspection to be non-destructively and accurately evaluated.
  • the core of the magnetic sensor is preferably capable of being disposed so that the distance between the core and the surface of the object under inspection is equal to or less than 0.3 mm.
  • the core can be disposed so that the distance between the core and the surface of the object under inspection is equal to or less than 0.3 mm, therefore surface properties of the object under inspection can be non-destructively and accurately evaluated even when measuring an object under inspection with a weak magnetic property detection signal.
  • the core of the magnetic sensor is preferably formed of ferromagnetic material.
  • the magnetic sensor core is formed of ferromagnetic material, enabling a high magnetic flux density inside the core and a high S/N ratio (S: magnetism penetrating into the object under inspection; N: leaked magnetism), thereby making it possible to improve magnetic property detection sensitivity by the magnetic sensor.
  • the core of the magnetic sensor is preferably an E-shaped core in which a coil is wound around a leg portion at the center.
  • the coil is sandwiched by the core, therefore magnetic leakage can be effectively suppressed, and a closed magnetic path easily formed.
  • the magnetic sensor core preferably has a cylindrical portion around which the coil is wound, and a round pipe portion surrounding the cylindrical portion, closed off at one end by a base portion, the cylindrical portion is disposed at the axial center of the round pipe portion, and one end of the cylindrical portion is connected to the base portion of the round pipe portion.
  • the coil is surrounded by the core, therefore magnetic leakage can be effectively suppressed, and a closed magnetic path easily formed.
  • the core is easy to manufacture and low in cost.
  • a surface property evaluation method for evaluating the surface properties of an object under inspection comprising the steps of: preparing a magnetic sensor for detecting magnetic properties of the surface of an object under inspection, the magnetic sensor including a core having a magnetic body and a coil wound around the core; supplying AC power at a predetermined frequency to the coil of the magnetic sensor so as to excite the core of the magnetic sensor and to form a closed magnetic path with the surface of the object under inspection; detecting a surface property signal in response to the magnetic properties of the surface of the object under inspection detected by the magnetic sensor; storing predetermined values showing the relationship between the surface property signal and the surface properties of the object under inspection; and calculating the surface properties of the object under inspection based on the stored value and the detected surface property signals.
  • Fig. 1 is a block diagram showing a surface property evaluation device according to an embodiment of the present invention
  • Fig 2 is an explanatory diagram showing the magnetic sensor in a surface property evaluation device according to an embodiment of the present invention
  • Fig. 3 is a perspective views showing multiple respective variant examples of the magnetic sensor in a surface property evaluation device according to an embodiment of the present invention
  • Fig. 4 is a line diagram showing the distribution in the depth direction of residual stress in a steel material subjected to shot-peening treatment in a first embodiment of the present invention
  • Fig. 5 is a line diagram showing the distribution in the depth direction of the amount of retained austenite in a steel material subjected to shot-peening treatment in a first example of the present invention
  • Fig. 6 is a line diagram showing the relationship between hardness and the voltage value of the surface property signal in a second example of the present invention.
  • Fig. 7 is a line diagram showing the relationship between the thickness of a nitrided layer and the voltage value of a surface property signal in a fourth example of the present invention.
  • the surface property evaluation device 1 comprises: a magnetic sensor 10 for detecting magnetic properties such as changes in magnetic permeability or inverse magnetostriction in the surface of an object under inspection and outputting a magnetic detection signal; a power supply means 20 for supplying AC power to the magnetic sensor 10; a signal detection means 21 for extracting and detecting from the magnetic detection signal detected by the magnetic sensor 10 a surface property signal responsive to the magnetic properties of the surface of the object under inspection; a surface property calculation means 22 for calculating surface properties such as residual stress and hardness of the object under inspection based on surface property signals obtained from this signal detection means 21 ; and a memory means 23 for storing predetermined values indicating the relationship between a surface property signal and the surface properties detected by the signal detection means 21 , or more specifically calibration curve indicating the relationship between the surface property signal and surface properties and/or surface property signals (reference values) obtained in advance using a reference sample with known surface properties such as hardness, and residual stress. It also comprises a
  • the surface property evaluation device 1 may, for example, be furnished with other components such as amplifiers or the like.
  • “Surface properties” here refers to the close vicinity of the surface of the object under inspection, and are properties down to a predetermined depth to which surface treatments impart an effect; “surface magnetic properties,” indicates magnetic properties in a region down to a predetermined depth of the object under inspection where magnetism excited by the magnetic sensor 10 penetrates and is detected.
  • the signal detection means 21 comprises a synchronous detector 21a for synchronously detecting a magnetic detection signal output from the magnetic sensor 10, and a low pass filter 21b for extracting from the detection output of the synchronous detector 21a a surface property signal in response to the magnetic properties of the surface of the object under inspection.
  • the magnetic sensor 10 has a shape capable of forming a closed magnetic path using the magnetic sensor 10 and the surface of the object under inspection.
  • a magnetic sensor furnished with an E-shaped core is explained.
  • An E-shaped core is easy to manufacture and low in cost.
  • the magnetic sensor 10 comprises an E-shaped core 11 made of a magnetic body, and a coil 12.
  • the core 11 comprises a leg portion 11a, leg portions 11b and 11c disposed on both side of the leg portion 11a, and a base portion 11d disposed in opposition to the surface 30a of the object 30 under inspection.
  • the one ends of the leg portions 11a, 11 b and 11c are respectively connected to the base portion 11d.
  • the core 11 is elected so as to form an E-shape from the base portion 11d toward the surface 30a.
  • the coil 12 is wound around the leg portion 11a.
  • the core 11 is here preferably formed of ferromagnetic material such as ferrite; the magnetic flux density inside the core can be raised and the S/N ratio (S: magnetism penetrating into the steel material; N: leaked magnetism) can be increased, thereby improving the magnetic property detection sensitivity of the magnetic sensor 10.
  • ferromagnetic material include iron, super permalloy, permalloy, silicon steel, ferrite (Mn-Zn based and Ni-Zn based), carbonyl iron dust, molybdenum permalloy, sendust, and the like.
  • the magnetic sensor 10 is formed so that the respective tip portions of the leg portions 11a, 11b, and 11c are able to contact the surface 30a of the object 30 under inspection.
  • the magnetic sensor 10 is formed so that the tips of the leg portions 11a, 11 b, and 11 c lie on the same plane.
  • the magnetic sensor 10 is disposed so that the leg portions 11a, 11 b, and 11c contact the surface 30a of the object 30 under inspection.
  • contact in the present embodiment includes cases in which at least one portion of the leg portions 11a, 11b, and 11c contacts the surface 30a of the object 30 under inspection (e.g., it includes cases in which not all of said leg portions 11a, 11b, and 11c are adhered, due to the shape of the surface 30a or to manufacturing tolerances, etc. in the manufacture of the magnetic sensor 10).
  • Magnetic attenuation and leakage between the object 30 under inspection and the magnetic sensor 10 can be prevented by disposing the magnetic sensor 10 so that it contacts the surface 30a of the object 30 under inspection.
  • the strength of the surface property signal can be increased and the magnetic property detection sensitivity of the magnetic sensor 10 can be improved.
  • the magnetic sensor 10 does not have to be brought into contact with the surface 30a of the object 30 under inspection if a closed magnetic path can be formed by the magnetic sensor 10 and the surface (the compound layer 30b) of the object 30 under inspection such that a sufficiently strong magnetic detection signal can be obtained.
  • a distance between the magnetic sensor 10 and the object 30 under inspection of 3.0 mm or less is desirable, and a distance of 0.3 mm or less is more desirable.
  • the object 30 under inspection is a material with a strong magnetic detection signal such as ferromagnetic material, for example, sufficient magnetic detection signal strength can be obtained using the magnetic sensor 10, therefore the distance can be 3.0 mm or less.
  • the direction in which the magnetic sensor 10 is disposed relative to the object 30 under inspection may also be changed to fit the shape of the object 30 under inspection. Specifically, if the object 30 under inspection has a curved surface, for example if the object 30 under inspection is cylindrical as shown in Fig. 3(D), the magnetic sensor 10 may be disposed along the longitudinal direction of the cylindrical shape.
  • Fluctuation errors in the surface property signal caused by liftoff can be eliminated by setting the distance between the magnetic sensor 10 and the surface of the object 30 under inspection to be the same as the distance at the time the calibration curve or reference value is obtained.
  • Non-contact evaluation allows the object 30 under inspection to be measured as it is being transported, without stopping, therefore the time required for inspection can be shortened.
  • the AC magnetic field H which interlinks with the coil 12 varies in response to the magnetic properties of the compound layer 30b into which magnetism has penetrated, therefore magnetic properties can be detected by the coil 12 in response to properties (surface properties) of the compound layer 30b.
  • the variation in the amount of magnetism arising based on magnetic permeability and the inverse magnetostriction effect, which changes in response to surface properties, is output from the coil 12 to the signal detection means 21 as a magnetic detection signal.
  • magnetic permeability is reduced, for example, by hardening of the surface or formation of a compound layer. Magnetic permeability drops due to the inverse magnetostriction effect when a compressive residual stress has been imparted by shot-peening treatment or the like. When magnetic permeability drops, the amount of magnetism in magnetic circuits is reduced, therefore the strength of the magnetic detection signal drops.
  • the AC power frequency is appropriately set according to factors such as the material of the object 30 under inspection, the properties being evaluated, and the depth being evaluated. For example, magnetism can be set to penetrate in a concentrated manner relative to a depth of 100-200 urn from the outermost surface of the steel material.
  • the signal detection means 21 detects a surface property signal as a voltage signal in response to the magnetic properties of the surface 30a of an object 30 under inspection (the magnetic properties of the compound layer 30b) using a magnetic detection signal input from the magnetic sensor 10.
  • Magnetic detection signals input from the magnetic sensor 10 are input to a synchronous detector 21a; in the synchronous detector 21a these are then detected using a carrier wave with the same frequency as the AC power supplied to the coil 12 by the power supply means 20.
  • the detection output of the synchronous detector 21a is output to a low pass filter 21b; in the low pass filter 21b a surface property signal responsive to the magnetic properties of the surface 30a of the object 30 under inspection is extracted as a voltage signal from the detection output, then output to the surface property calculation means 22.
  • the surface property calculation means 22 calculates surface properties such as residual stress and hardness of the object 30 under inspection based on a signal obtained by the signal detection means
  • the surface property calculation means 22 is capable of calculating hardness, residual stress, and the like based on a calibration curve stored in the memory means 23 as a relationship between voltage and surface properties. In cases where the voltage value of the surface property signal is sufficient as a value for managing the surface properties of the object 30 under inspection, it is not necessary to calculate surface properties using a calibration curve.
  • the surface property calculation means 22 may also be arranged to be capable of making a quality determination based on whether calculated surface properties are within a predetermined range.
  • a quality determination based on the differential value between a surface property signal (reference value) for a reference sample indicating predetermined surface properties, and a measured surface property signal is also acceptable.
  • a reference sample indicating predetermined surface properties is first prepared; the surface property signal is measured in advance and stored as a reference value in the memory means 23.
  • the surface property calculation means 22 calculates the difference between this reference value and the measured surface property signal, then makes a quality determination based on whether the surface property signal is within a predetermined range.
  • a reference value is set using a sample with a hardness H as the reference sample; the differential value of the surface property signal relative to a is set as a threshold, and it can be determined as "poor" when the calculated differential value exceeds the threshold.
  • the surface properties and determination results calculated by the surface property calculation means 22 are output to the display means 24, and the display means 24 displays the surface properties and determination results using a screen, a voice output, or the like. For example, values for surface properties such as hardness, residual stress, and the like can be displayed. It is also possible to display only the voltage value of the surface property signal.
  • the surface property calculation means 22 may also implement a poor quality warning display using a warning sound or warning light.
  • a closed magnetic path is formed by the magnetic sensor 10 and a region up to a predetermined depth from the surface 30a of the object 30 under inspection, therefore magnetic attenuation and leakage between the object 30 under inspection and the magnetic sensor 10 can be prevented.
  • the strength of the surface property signal can be increased and the magnetic property detection sensitivity of the magnetic sensor 10 can be improved, therefore the surface properties of the object 30 under inspection can be non-destructively and accurately evaluated.
  • the magnetic penetration depth into the object 30 under inspection can be held fixed, therefore surface properties having a distribution in the depth direction can be accurately discerned. Evaluation and the like of the nitrided layer thickness can thus be accomplished.
  • the depth of magnetic penetration can be changed by varying the frequency of the AC power supplied to the coil 12 by the power supply means 20. Since the depth of magnetic penetration becomes shallower as frequency increases, surface properties in a region of shallow depth from the surface can be evaluated. Changing the frequency of the AC power in this way enables evaluation of the distribution of residual stress in the depth direction, compound layer thickness, and so forth.
  • a magnetic sensor provided with an E-shaped core 11 was used, but the invention is not limited thereto, and magnetic sensors with various core shapes can be used so long as a closed magnetic path is formed by the magnetic sensor and the region from the surface of the object under inspection to a predetermined depth thereof.
  • a magnetic sensor 40 furnished with a cylindrical portion 41a around which a coil 42 is wound, and with a round pipe portion 41b disposed to surround this cylindrical portion 41a and closed off by a base portion 41c at one end thereof, wherein the cylindrical portion 41a is disposed on the axial center of the round pipe portion 41 b, with one end of the cylindrical portion 41a connected to the base portion of the round pipe portion 41b.
  • a magnetic sensor 50 in which a coil 52 is wound onto a U-shaped core 51 , as shown in Fig. 3(B), or a magnetic sensor 60 in which a coil 62 is wound onto an L-shaped core 61 , as shown in Fig. 3(C), may also be used.
  • a magnetic sensor 10 using a core 11 in which the shapes of the leg portions 11a, 11b, and 11 c are constituted to be capable of making contact along the shape of an object 30 under inspection.
  • a closed magnetic path is formed by the magnetic sensor 10 and the region up to a predetermined depth from the surface 30a of the object 30 under inspection, therefore magnetic attenuation and leakage between the object 30 under inspection and the magnetic sensor 10 can be prevented.
  • the strength of the surface property signal can be increased and the magnetic property detection sensitivity of the magnetic sensor 10 can be improved, therefore the surface properties of the object 30 under inspection can be non-destructively and accurately evaluated. Since the surface properties of the object 30 under inspection are evaluated by supplying AC power at a certain frequency to the coil 12, the magnetic penetration depth into the object 30 under inspection can be held fixed, therefore surface properties having a distribution in the depth direction can be accurately discerned.
  • a closed magnetic path is formed between the magnetic sensor 10 and the surface of the object 30 under inspection when the core 11 of the magnetic sensor 10 is disposed to be non-contacting with the surface 30a of the object 30 under inspection and the distance between the magnetic sensor 10 and the surface 30a of the object 30 under inspection is 3.0 mm or less; a sufficiently strong magnetic detection signal can thus be obtained, so that the surface properties of the object 30 under inspection can be evaluated in a nondestructive and accurate manner. Even when measuring an object under inspection in which the obtained magnetic detection signal is weak, the surface properties of the object under inspection can be evaluated in a nondestructive and accurate manner by disposing the device so that the distance between the core and the object under inspection is 0.3 mm or less. Also, non-contact evaluation allows the object 30 under inspection to be measured as it is being transported, without stopping, therefore the time required for inspection can be shortened.
  • the magnetic flux density inside the core can be raised and the S/N ratio (S: magnetism penetrating into the steel material; N: leaked magnetism) can be increased, thereby improving the magnetic property detection sensitivity of the magnetic sensor 10.
  • SCH420H gas carburized material
  • An untreated material to which no surface treatment was applied was used as a comparison material.
  • shot-peening treatment a shot material with a grain diameter of 100um and a hardness of Hv 900-950 was used; shot pressure was 0. 2 MPa.
  • Evaluation of residual stress was effectuated by bringing a magnetic sensor with an E-shaped core into contact with a surface.
  • the core was composed of ferromagnetic material of Mn-Zn based ferrite, formed so that 4mm x 4mm x 3mm square-columnar leg portions were disposed thereon at 3 mm intervals.
  • the AC current supplied to the coil was set at 20kHz, 3.5mA.
  • the voltage of the surface property signal detected by the surface property evaluation device dropped down to 0.71V with the surface treated material vs. 0.79V for the untreated material.
  • Fig. 4 shows the distribution in the depth direction of residual stress in a surface treated material. Residual stress was measured by an X-ray stress measurement method as the surface treated material was ground down a few microns at a time by electrolytic polishing. As shown in Fig. 5, it is clear that compared to untreated material, compressive residual stress is imparted to the treated material at a peak depth of 15 urn.
  • Fig. 5 shows the distribution of the retained austenite quantity in the depth direction. It is clear, as shown in Fig. 5, that compared to the untreated material there is a large reduction in the amount of retained austenite in the surface treated material.
  • the change in voltage of the surface property signal corresponds to a change in magnetic permeability, and it was confirmed that an accurate evaluation of residual stress can be performed using the surface property evaluation device of the present invention.
  • Example 2 evaluation of a surface property signal relative to steel material hardness was performed using steel material (SKS3) in which hardness was varied by heat treatment.
  • the evaluation of hardness was performed by bringing a magnetic sensor with an E-shaped core into contact with a surface.
  • the AC current supplied to the coil was set at 20kHz, 3.5mA.
  • Fig. 6 The relationship between Vickers hardness and the surface property signal voltage (device output) is shown in Fig. 6. A trend is observed whereby the voltage of the surface property signal decreases as the Vickers hardness increases, and it was confirmed that hardness can be accurately evaluated using the surface property evaluation device of the present invention.
  • the surface property signal was sufficiently strong at 0. 2V, but when the core was brought into contact, the surface property signal increased significantly to 0. 8V. It was thus confirmed that the strength of the surface property signal is increased by causing the magnetic sensor core to contact the object under inspection, thereby enabling an improvement in detection sensitivity.
  • an evaluation of surface property signal relative to thickness of nitrided layer was performed using steel provided with a nitrided layer by nitriding treatment.
  • a nitrided layer of 1.5um to 10.0um thickness was created by heating a steel material (SKD61) to 500-60CTC in an NH3 atmosphere.
  • the evaluation was performed by bringing a magnetic sensor with an E- shaped core into contact with a surface.
  • the AC current supplied to the coil was set at 20kHz, 3. 5mA.
  • the voltage of the surface property signal detected by the surface property evaluation device increased as the nitrided layer thickness increased, as shown in Fig. 7. It was thus confirmed that an accurate evaluation of nitrided layer thickness can be performed.

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Abstract

A surface property evaluation device capable of non-destructively and accurately evaluating the surface properties of an object under inspection subjected to heat treatment, nitriding treatment, shot-peening treatment, or the like is provided. The surface property evaluation device 1 of the present invention comprises a magnetic sensor 10 for detecting the magnetic properties of the surface of an object under inspection and outputting a surface property signal, a power supply means 20 for supplying AC power at a predetermined frequency to the magnetic sensor 10; a signal detection means 21 for extracting from a magnetic detection signal a surface property signal in response to the magnetic properties of the surface of an object under inspection; a surface property calculation means 22 for calculating surface properties based on a surface property signal; and a memory means 23 for storing the calibration curve showing the relationship between surface property signals and surface properties and/or reference values obtained in advance using a reference sample of which surface properties are already known. The magnetic sensor 10 comprises a core 11 having a magnetic body, and a coil 12; a closed magnetic path is formed by the magnetic sensor 10 and the surface of the object under inspection.

Description

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SURFACE PROPERTY MAGNETIC EVALUATION DEVICE AND METHOD Technical Field
The present invention relates to a surface property evaluation device and surface property evaluation method, and more particularly to a surface property evaluation device and surface property evaluation method for evaluating surface properties such as residual stress and hardness in an object under inspection.
Background Art
In steel components such as gears or shafts used in automobile parts and the like, surface hardening by heat-treatment or nitriding treatment and surface treatments such as shot-peening are used to improve wear- resistance, fatigue strength, and the like.
Conventionally, evaluation of post-surface treatment surface properties such as residual stress and hardness in such products was done by sample destructive testing. This led to the inability to test 100% of products, and because testing was destructive, tested products were rendered unusable.
There is therefore an increasing need to develop a device capable of non-destructive inspection of product surface properties. JP-A-2008-2973, for example, discloses a non-destructive inspection device for shot-peened treatment surfaces, wherein an AC signal is input as frequency is varied to an inspection circuit furnished with a coil, disposed above a shot-peening treatment surface, and the frequency response characteristics of the impedance in that test circuit are used to inspect the state of residual stress in an object under inspection.
Summary of the Invention
Technical Problems
However, in an inspection device in which, as in the conventional technology described above, a coil with an open magnetic path structure is disposed above a shot-peentng treatment surface, there is a large attenuation and leakage of magnetism between the object under inspection and the sensor, leading to the problem of reduced detection sensitivity and measurement value reproducibility. Using this method, in which an AC signal is input as frequency is varied to obtain impedance frequency response characteristics, the problem arose that the depth of magnetic penetration into an object under inspection varied, thereby preventing the accurate discernment of surface properties which were distributed in the depth direction.
It is therefore an object of the present invention to provide a surface property evaluation device and surface property evaluation method capable of non-destructively and accurately evaluating the surface properties of an object under inspection subjected to heat treatment, nitriding treatment, shot- peening treatment, or the like.
Solution to Problems
The above object is achieved according to the present invention by providing a surface property evaluation device for evaluating the surface properties of an object under inspection, comprising: a magnetic sensor for detecting a magnetic properties of the surface of the object under inspection, the magnetic sensor including a core having a magnetic body and a coil wound around the core; power supply means for supplying AC power to the coil of the magnetic sensor; signal detection means for detecting a surface property signal in response to the magnetic properties of the surface of the object under inspection detected by a magnetic sensor; memory means for storing predetermined values showing the relationship between the surface property signal and the surface properties of the object under inspection; and surface property calculation means for calculating the surface properties of the object under inspection based on the values stored in the memory means and the surface property signals detected by the signal detection means; wherein the power supply means supplies the AC power at a predetermined frequency to the coil in the magnetic sensor so as to excite the core of the magnetic sensor and to form a closed magnetic path with the surface of the object under inspection.
In the present invention thus constituted, the magnetic properties of the surface of the object under inspection are detected by the magnetic sensor supplied with AC power from the power supply means; the surface property signal corresponding to the magnetic properties of the surface of the object under inspection detected by the magnetic sensor in the signal detection means is detected; and, using the surface property calculation means, the surface properties of the object under inspection can be calculated based on the predetermined value stored in the memory means indicating the relationship between the surface property signal and the surface properties of the object under inspection, and on a surface property signal.
In the magnetic sensor, the core forms a closed magnetic path with the object under inspection, therefore magnetic attenuation and leakage between the object under inspection and the magnetic sensor can be prevented. Thus the strength of the surface property signal can be increased and the magnetic property detection sensitivity of the magnetic sensor can be improved, therefore the surface properties of the object under inspection can be non-destructively and accurately evaluated.
Since the surface properties of the object under inspection are evaluated by supplying AC power at the predetermined frequency to the coil, the magnetic penetration depth into the object under inspection can be held fixed, therefore surface properties having a distribution in the depth direction can be accuately discerned.
"Calculating surface properties" here refers to evaluating surface properties not only by calculating absolute values such as residual stress or hardness, but also by calculating whether the surface property signal is within a predetermined range relative to a reference value.
In the present invention, the predetermined values indicating the relationship between the predetermined surface property signal and the surface properties of the object under inspection are preferably represented by a calibration curve indicating the correlation between a surface property signal and an object under inspection.
In the present invention thus constituted, the surface properties of the object under inspection (e.g., the thickness of a nitrided layer when performing nitriding treatment, or the depth of compressive residual stress when performing shot-peening treatment, etc.) can be calculated.
In the present invention, the predetermined values indicating the relationship between the surface property signal and the surface properties of the object under inspection are preferably reference values indicating a surface property signal in a reference sample having predetermined surface properties.
In the present invention thus constituted, a quality determination of the object under inspection (e.g., whether there is sufficient thickness of a nitrided layer when performing nitriding treatment, or sufficient depth of compressive residual stress when performing shot-peening treatment) can be performed by calculating the difference between the detected surface property signal and the reference values.
In the present invention, the core of the magnetic sensor is preferably capable of making a contact with an object under inspection along the surface shape thereof.
In the present invention thus constituted, because the magnetic sensor core is capable of making contact with the object under inspection along the surface shape thereof, magnetic attenuation and leakage between the object under inspection and the magnetic sensor can be prevented by bringing the core into contact with the surface of the object under inspection. Thus the strength of the surface property signal can be increased and the magnetic property detection sensitivity of the magnetic sensor can be improved.
In the present invention, the core of the magnetic sensor is preferably capable of being disposed so that the distance between the core and the surface of the object under inspection is equal to or less than 3.0 mm. In the present invention thus constituted, in cases where the magnetic property detection signal detected by the magnetic sensor is strong, such as when the object under inspection is a ferromagnetic body, the fact that the core can be disposed so that the distance between the core and the object under inspection surface is equal to or less than 3.0 mm means that a closed magnetic path is formed between the magnetic sensor and the surface of the object under inspection so that a sufficiently strong magnetic detection signal is obtained, thus enabling the surface properties of the object under inspection to be non-destructively and accurately evaluated.
In the present invention, the core of the magnetic sensor is preferably capable of being disposed so that the distance between the core and the surface of the object under inspection is equal to or less than 0.3 mm.
In the present invention thus constituted, the core can be disposed so that the distance between the core and the surface of the object under inspection is equal to or less than 0.3 mm, therefore surface properties of the object under inspection can be non-destructively and accurately evaluated even when measuring an object under inspection with a weak magnetic property detection signal.
In the present invention, the core of the magnetic sensor is preferably formed of ferromagnetic material.
In the present invention thus constituted, the magnetic sensor core is formed of ferromagnetic material, enabling a high magnetic flux density inside the core and a high S/N ratio (S: magnetism penetrating into the object under inspection; N: leaked magnetism), thereby making it possible to improve magnetic property detection sensitivity by the magnetic sensor. In the present invention, the core of the magnetic sensor is preferably an E-shaped core in which a coil is wound around a leg portion at the center.
In the present invention thus constituted, the coil is sandwiched by the core, therefore magnetic leakage can be effectively suppressed, and a closed magnetic path easily formed.
In the present invention, the magnetic sensor core preferably has a cylindrical portion around which the coil is wound, and a round pipe portion surrounding the cylindrical portion, closed off at one end by a base portion, the cylindrical portion is disposed at the axial center of the round pipe portion, and one end of the cylindrical portion is connected to the base portion of the round pipe portion.
In the present invention thus constituted, the coil is surrounded by the core, therefore magnetic leakage can be effectively suppressed, and a closed magnetic path easily formed. The core is easy to manufacture and low in cost.
The above object is achieved according to the present invention by providing a surface property evaluation method for evaluating the surface properties of an object under inspection, comprising the steps of: preparing a magnetic sensor for detecting magnetic properties of the surface of an object under inspection, the magnetic sensor including a core having a magnetic body and a coil wound around the core; supplying AC power at a predetermined frequency to the coil of the magnetic sensor so as to excite the core of the magnetic sensor and to form a closed magnetic path with the surface of the object under inspection; detecting a surface property signal in response to the magnetic properties of the surface of the object under inspection detected by the magnetic sensor; storing predetermined values showing the relationship between the surface property signal and the surface properties of the object under inspection; and calculating the surface properties of the object under inspection based on the stored value and the detected surface property signals.
Brief Description of Drawings
Fig. 1 is a block diagram showing a surface property evaluation device according to an embodiment of the present invention;
Fig 2 is an explanatory diagram showing the magnetic sensor in a surface property evaluation device according to an embodiment of the present invention;
Fig. 3 is a perspective views showing multiple respective variant examples of the magnetic sensor in a surface property evaluation device according to an embodiment of the present invention;
Fig. 4 is a line diagram showing the distribution in the depth direction of residual stress in a steel material subjected to shot-peening treatment in a first embodiment of the present invention;
Fig. 5 is a line diagram showing the distribution in the depth direction of the amount of retained austenite in a steel material subjected to shot-peening treatment in a first example of the present invention;
Fig. 6 is a line diagram showing the relationship between hardness and the voltage value of the surface property signal in a second example of the present invention; and
Fig. 7 is a line diagram showing the relationship between the thickness of a nitrided layer and the voltage value of a surface property signal in a fourth example of the present invention.
Descriptions of Embodiments
Referring to the attached drawings, a surface property evaluation device according to an embodiment of the present invention is explained.
As shown in Fig. 1 , the surface property evaluation device 1 according to an embodiment of the present invention comprises: a magnetic sensor 10 for detecting magnetic properties such as changes in magnetic permeability or inverse magnetostriction in the surface of an object under inspection and outputting a magnetic detection signal; a power supply means 20 for supplying AC power to the magnetic sensor 10; a signal detection means 21 for extracting and detecting from the magnetic detection signal detected by the magnetic sensor 10 a surface property signal responsive to the magnetic properties of the surface of the object under inspection; a surface property calculation means 22 for calculating surface properties such as residual stress and hardness of the object under inspection based on surface property signals obtained from this signal detection means 21 ; and a memory means 23 for storing predetermined values indicating the relationship between a surface property signal and the surface properties detected by the signal detection means 21 , or more specifically calibration curve indicating the relationship between the surface property signal and surface properties and/or surface property signals (reference values) obtained in advance using a reference sample with known surface properties such as hardness, and residual stress. It also comprises a display means 24 such as a display screen or audio output device for displaying surface properties calculated by the surface property calculation means 22.
Note that the surface property evaluation device 1 may, for example, be furnished with other components such as amplifiers or the like.
"Surface properties" here refers to the close vicinity of the surface of the object under inspection, and are properties down to a predetermined depth to which surface treatments impart an effect; "surface magnetic properties," indicates magnetic properties in a region down to a predetermined depth of the object under inspection where magnetism excited by the magnetic sensor 10 penetrates and is detected.
The signal detection means 21 comprises a synchronous detector 21a for synchronously detecting a magnetic detection signal output from the magnetic sensor 10, and a low pass filter 21b for extracting from the detection output of the synchronous detector 21a a surface property signal in response to the magnetic properties of the surface of the object under inspection.
The magnetic sensor 10 has a shape capable of forming a closed magnetic path using the magnetic sensor 10 and the surface of the object under inspection. Here, as an example of such a magnetic sensor, a magnetic sensor furnished with an E-shaped core is explained. An E-shaped core is easy to manufacture and low in cost.
As shown in Fig. 2, the magnetic sensor 10 comprises an E-shaped core 11 made of a magnetic body, and a coil 12. The core 11 comprises a leg portion 11a, leg portions 11b and 11c disposed on both side of the leg portion 11a, and a base portion 11d disposed in opposition to the surface 30a of the object 30 under inspection. The one ends of the leg portions 11a, 11 b and 11c are respectively connected to the base portion 11d. The core 11 is elected so as to form an E-shape from the base portion 11d toward the surface 30a. The coil 12 is wound around the leg portion 11a.
The core 11 is here preferably formed of ferromagnetic material such as ferrite; the magnetic flux density inside the core can be raised and the S/N ratio (S: magnetism penetrating into the steel material; N: leaked magnetism) can be increased, thereby improving the magnetic property detection sensitivity of the magnetic sensor 10. Examples of ferromagnetic material include iron, super permalloy, permalloy, silicon steel, ferrite (Mn-Zn based and Ni-Zn based), carbonyl iron dust, molybdenum permalloy, sendust, and the like.
The magnetic sensor 10 is formed so that the respective tip portions of the leg portions 11a, 11b, and 11c are able to contact the surface 30a of the object 30 under inspection. For example, when the object 30 under inspection is flat, the magnetic sensor 10 is formed so that the tips of the leg portions 11a, 11 b, and 11 c lie on the same plane.
Next a method for evaluating the surface properties of an object under inspection using the surface property evaluation device 1 is explained. Here, as an example of an object 30 under inspection, steel material in which a compound layer 30b is formed in the vicinity of the surface 30a by nitriding treatment is used for the explanation.
First, the magnetic sensor 10 is disposed so that the leg portions 11a, 11 b, and 11c contact the surface 30a of the object 30 under inspection. Note that "contact" in the present embodiment includes cases in which at least one portion of the leg portions 11a, 11b, and 11c contacts the surface 30a of the object 30 under inspection (e.g., it includes cases in which not all of said leg portions 11a, 11b, and 11c are adhered, due to the shape of the surface 30a or to manufacturing tolerances, etc. in the manufacture of the magnetic sensor 10).
Magnetic attenuation and leakage between the object 30 under inspection and the magnetic sensor 10 can be prevented by disposing the magnetic sensor 10 so that it contacts the surface 30a of the object 30 under inspection. Thus the strength of the surface property signal can be increased and the magnetic property detection sensitivity of the magnetic sensor 10 can be improved.
The magnetic sensor 10 does not have to be brought into contact with the surface 30a of the object 30 under inspection if a closed magnetic path can be formed by the magnetic sensor 10 and the surface (the compound layer 30b) of the object 30 under inspection such that a sufficiently strong magnetic detection signal can be obtained. Here, a distance between the magnetic sensor 10 and the object 30 under inspection of 3.0 mm or less is desirable, and a distance of 0.3 mm or less is more desirable. For materials having a weak magnetic detection signal, it is desirable that said distance be 0.3 mm or less in order to obtain sufficient magnetic detection signal strength. When the object 30 under inspection is a material with a strong magnetic detection signal such as ferromagnetic material, for example, sufficient magnetic detection signal strength can be obtained using the magnetic sensor 10, therefore the distance can be 3.0 mm or less.
Note that the direction in which the magnetic sensor 10 is disposed relative to the object 30 under inspection may also be changed to fit the shape of the object 30 under inspection. Specifically, if the object 30 under inspection has a curved surface, for example if the object 30 under inspection is cylindrical as shown in Fig. 3(D), the magnetic sensor 10 may be disposed along the longitudinal direction of the cylindrical shape.
Fluctuation errors in the surface property signal caused by liftoff can be eliminated by setting the distance between the magnetic sensor 10 and the surface of the object 30 under inspection to be the same as the distance at the time the calibration curve or reference value is obtained.
Non-contact evaluation allows the object 30 under inspection to be measured as it is being transported, without stopping, therefore the time required for inspection can be shortened.
Next, when AC power of a predetermined frequency is supplied to the coil 12 by the power supply means 20, an AC magnetic field H arises on the core 11 ; magnetism penetrates to a predetermined depth in the compound layer 30b of the object 30 under inspection in response to frequency, and a closed magnetic path is formed by a region up to a predetermined depth of the leg portions 11a and 11c and the compound layer 30b of the object 30 under inspection.
The AC magnetic field H which interlinks with the coil 12 varies in response to the magnetic properties of the compound layer 30b into which magnetism has penetrated, therefore magnetic properties can be detected by the coil 12 in response to properties (surface properties) of the compound layer 30b. The variation in the amount of magnetism arising based on magnetic permeability and the inverse magnetostriction effect, which changes in response to surface properties, is output from the coil 12 to the signal detection means 21 as a magnetic detection signal.
In the relationship between surface properties and magnetic properties, magnetic permeability is reduced, for example, by hardening of the surface or formation of a compound layer. Magnetic permeability drops due to the inverse magnetostriction effect when a compressive residual stress has been imparted by shot-peening treatment or the like. When magnetic permeability drops, the amount of magnetism in magnetic circuits is reduced, therefore the strength of the magnetic detection signal drops.
The AC power frequency is appropriately set according to factors such as the material of the object 30 under inspection, the properties being evaluated, and the depth being evaluated. For example, magnetism can be set to penetrate in a concentrated manner relative to a depth of 100-200 urn from the outermost surface of the steel material.
The signal detection means 21 detects a surface property signal as a voltage signal in response to the magnetic properties of the surface 30a of an object 30 under inspection (the magnetic properties of the compound layer 30b) using a magnetic detection signal input from the magnetic sensor 10.
Magnetic detection signals input from the magnetic sensor 10 are input to a synchronous detector 21a; in the synchronous detector 21a these are then detected using a carrier wave with the same frequency as the AC power supplied to the coil 12 by the power supply means 20.
The detection output of the synchronous detector 21a is output to a low pass filter 21b; in the low pass filter 21b a surface property signal responsive to the magnetic properties of the surface 30a of the object 30 under inspection is extracted as a voltage signal from the detection output, then output to the surface property calculation means 22.
The surface property calculation means 22 calculates surface properties such as residual stress and hardness of the object 30 under inspection based on a signal obtained by the signal detection means The surface property calculation means 22 is capable of calculating hardness, residual stress, and the like based on a calibration curve stored in the memory means 23 as a relationship between voltage and surface properties. In cases where the voltage value of the surface property signal is sufficient as a value for managing the surface properties of the object 30 under inspection, it is not necessary to calculate surface properties using a calibration curve.
The surface property calculation means 22 may also be arranged to be capable of making a quality determination based on whether calculated surface properties are within a predetermined range.
It is also acceptable to make a quality determination based on the differential value between a surface property signal (reference value) for a reference sample indicating predetermined surface properties, and a measured surface property signal. In such cases, a reference sample indicating predetermined surface properties is first prepared; the surface property signal is measured in advance and stored as a reference value in the memory means 23. The surface property calculation means 22 calculates the difference between this reference value and the measured surface property signal, then makes a quality determination based on whether the surface property signal is within a predetermined range.
For example, when the hardness of the object 30 under inspection should be controlled to H ± a, a reference value is set using a sample with a hardness H as the reference sample; the differential value of the surface property signal relative to a is set as a threshold, and it can be determined as "poor" when the calculated differential value exceeds the threshold.
The surface properties and determination results calculated by the surface property calculation means 22 are output to the display means 24, and the display means 24 displays the surface properties and determination results using a screen, a voice output, or the like. For example, values for surface properties such as hardness, residual stress, and the like can be displayed. It is also possible to display only the voltage value of the surface property signal.
When performing a quality determination of the object under inspection, the surface property calculation means 22 may also implement a poor quality warning display using a warning sound or warning light.
Using the surface property evaluation device 1 , a closed magnetic path is formed by the magnetic sensor 10 and a region up to a predetermined depth from the surface 30a of the object 30 under inspection, therefore magnetic attenuation and leakage between the object 30 under inspection and the magnetic sensor 10 can be prevented. Thus the strength of the surface property signal can be increased and the magnetic property detection sensitivity of the magnetic sensor 10 can be improved, therefore the surface properties of the object 30 under inspection can be non-destructively and accurately evaluated.
Also, since the surface properties of the object 30 under inspection are evaluated by supplying AC power at a certain frequency to a coil 12, the magnetic penetration depth into the object 30 under inspection can be held fixed, therefore surface properties having a distribution in the depth direction can be accurately discerned. Evaluation and the like of the nitrided layer thickness can thus be accomplished.
In the present embodiment, the evaluation of surface properties 30 formed of a steel material on which a compound layer 30b is formed by nitriding treatment is explained, but surface properties associated with changes in magnetic properties of the surface such as hardness and residual stress can also be accurately evaluated with this method.
The depth of magnetic penetration can be changed by varying the frequency of the AC power supplied to the coil 12 by the power supply means 20. Since the depth of magnetic penetration becomes shallower as frequency increases, surface properties in a region of shallow depth from the surface can be evaluated. Changing the frequency of the AC power in this way enables evaluation of the distribution of residual stress in the depth direction, compound layer thickness, and so forth.
Next, referring to Fig. 3, variant examples of a magnetic sensor are explained. In the embodiment explained above, a magnetic sensor provided with an E-shaped core 11 was used, but the invention is not limited thereto, and magnetic sensors with various core shapes can be used so long as a closed magnetic path is formed by the magnetic sensor and the region from the surface of the object under inspection to a predetermined depth thereof.
As shown in Fig. 3(A), it is possible to utilize a magnetic sensor 40 furnished with a cylindrical portion 41a around which a coil 42 is wound, and with a round pipe portion 41b disposed to surround this cylindrical portion 41a and closed off by a base portion 41c at one end thereof, wherein the cylindrical portion 41a is disposed on the axial center of the round pipe portion 41 b, with one end of the cylindrical portion 41a connected to the base portion of the round pipe portion 41b.
With this magnetic sensor 40, because the entire perimeter of the coil
42 is surrounded by the round pipe portion 41b, magnetic leakage can be effectively suppressed and a closed magnetic path easily formed.
A magnetic sensor 50 in which a coil 52 is wound onto a U-shaped core 51 , as shown in Fig. 3(B), or a magnetic sensor 60 in which a coil 62 is wound onto an L-shaped core 61 , as shown in Fig. 3(C), may also be used.
Furthermore, as shown in Fig. 3(D), it is also possible to utilize a magnetic sensor 10 using a core 11 in which the shapes of the leg portions 11a, 11b, and 11 c are constituted to be capable of making contact along the shape of an object 30 under inspection.
Next the effects of a surface property evaluation device according to the above-described embodiment of the present invention are explained.
(1) With the surface property evaluation device 1 according to the present embodiment, a closed magnetic path is formed by the magnetic sensor 10 and the region up to a predetermined depth from the surface 30a of the object 30 under inspection, therefore magnetic attenuation and leakage between the object 30 under inspection and the magnetic sensor 10 can be prevented. Thus the strength of the surface property signal can be increased and the magnetic property detection sensitivity of the magnetic sensor 10 can be improved, therefore the surface properties of the object 30 under inspection can be non-destructively and accurately evaluated. Since the surface properties of the object 30 under inspection are evaluated by supplying AC power at a certain frequency to the coil 12, the magnetic penetration depth into the object 30 under inspection can be held fixed, therefore surface properties having a distribution in the depth direction can be accurately discerned.
(2) By disposing the core 10 of the magnetic sensor 10 core 11 so that it contacts the surface 30a of the object 30 under inspection, magnetic attenuation and leakage between the object 30 under inspection and the magnetic sensor 10 can be prevented. Thus the strength of the surface property signal can be increased and the magnetic property detection sensitivity of the magnetic sensor 10 can be improved. Also, fluctuation errors in the surface property signal caused by liftoff can be eliminated.
(3) If the object under inspection is ferromagnetic material, a closed magnetic path is formed between the magnetic sensor 10 and the surface of the object 30 under inspection when the core 11 of the magnetic sensor 10 is disposed to be non-contacting with the surface 30a of the object 30 under inspection and the distance between the magnetic sensor 10 and the surface 30a of the object 30 under inspection is 3.0 mm or less; a sufficiently strong magnetic detection signal can thus be obtained, so that the surface properties of the object 30 under inspection can be evaluated in a nondestructive and accurate manner. Even when measuring an object under inspection in which the obtained magnetic detection signal is weak, the surface properties of the object under inspection can be evaluated in a nondestructive and accurate manner by disposing the device so that the distance between the core and the object under inspection is 0.3 mm or less. Also, non-contact evaluation allows the object 30 under inspection to be measured as it is being transported, without stopping, therefore the time required for inspection can be shortened.
(4) When the core 11 is formed of ferromagnetic material such as ferrite, the magnetic flux density inside the core can be raised and the S/N ratio (S: magnetism penetrating into the steel material; N: leaked magnetism) can be increased, thereby improving the magnetic property detection sensitivity of the magnetic sensor 10. Examples
Examples of surface property measurement using the surface property evaluation device of the present invention are shown below.
(Example 1)
In this example, residual stress in a steel material surface treated by shot-peening was evaluated.
SCH420H, gas carburized material, is shot-peened on its surface and used as surface-treated material for evaluation sample. An untreated material to which no surface treatment was applied was used as a comparison material. For shot-peening treatment, a shot material with a grain diameter of 100um and a hardness of Hv 900-950 was used; shot pressure was 0. 2 MPa.
Evaluation of residual stress was effectuated by bringing a magnetic sensor with an E-shaped core into contact with a surface. The core was composed of ferromagnetic material of Mn-Zn based ferrite, formed so that 4mm x 4mm x 3mm square-columnar leg portions were disposed thereon at 3 mm intervals. The AC current supplied to the coil was set at 20kHz, 3.5mA. The voltage of the surface property signal detected by the surface property evaluation device dropped down to 0.71V with the surface treated material vs. 0.79V for the untreated material.
Fig. 4 shows the distribution in the depth direction of residual stress in a surface treated material. Residual stress was measured by an X-ray stress measurement method as the surface treated material was ground down a few microns at a time by electrolytic polishing. As shown in Fig. 5, it is clear that compared to untreated material, compressive residual stress is imparted to the treated material at a peak depth of 15 urn.
Retained austenite is present in the SCH420H, gas carburized material; the shot-peening treatment causes a martensite metamorphosis, such that the amount of retained austenite declines. Fig. 5 shows the distribution of the retained austenite quantity in the depth direction. It is clear, as shown in Fig. 5, that compared to the untreated material there is a large reduction in the amount of retained austenite in the surface treated material.
When compressive residual stress is imparted, changes in magnetic properties caused by the inverse magnetostriction effect are dominant compared to the change in magnetic permeability caused by the reduction in the amount of retained austenite, and that change in the magnetic permeability of the surface-treated material as a whole is reduced.
The change in voltage of the surface property signal corresponds to a change in magnetic permeability, and it was confirmed that an accurate evaluation of residual stress can be performed using the surface property evaluation device of the present invention.
(Example 2) In the present example, evaluation of a surface property signal relative to steel material hardness was performed using steel material (SKS3) in which hardness was varied by heat treatment.
The evaluation of hardness was performed by bringing a magnetic sensor with an E-shaped core into contact with a surface. The AC current supplied to the coil was set at 20kHz, 3.5mA.
The relationship between Vickers hardness and the surface property signal voltage (device output) is shown in Fig. 6. A trend is observed whereby the voltage of the surface property signal decreases as the Vickers hardness increases, and it was confirmed that hardness can be accurately evaluated using the surface property evaluation device of the present invention.
(Example 3)
In the present example the effect of bringing a magnetic sensor into contact with an object under inspection was evaluated.
The evaluation sample and conditions are the same as in Example 1.
When the device was disposed so the core was not brought into contact, and the distance from the evaluation sample surface was 1mm, the surface property signal was sufficiently strong at 0. 2V, but when the core was brought into contact, the surface property signal increased significantly to 0. 8V. It was thus confirmed that the strength of the surface property signal is increased by causing the magnetic sensor core to contact the object under inspection, thereby enabling an improvement in detection sensitivity.
(Example 4)
In the present example an evaluation of surface property signal relative to thickness of nitrided layer was performed using steel provided with a nitrided layer by nitriding treatment. A nitrided layer of 1.5um to 10.0um thickness was created by heating a steel material (SKD61) to 500-60CTC in an NH3 atmosphere.
The evaluation was performed by bringing a magnetic sensor with an E- shaped core into contact with a surface. The AC current supplied to the coil was set at 20kHz, 3. 5mA.
The voltage of the surface property signal detected by the surface property evaluation device increased as the nitrided layer thickness increased, as shown in Fig. 7. It was thus confirmed that an accurate evaluation of nitrided layer thickness can be performed.

Claims

1. A surface property evaluation device for evaluating the surface properties of an object under inspection, comprising:
a magnetic sensor for detecting a magnetic properties of the surface of the object under inspection, the magnetic sensor including a core having a magnetic body and a coil wound around the core;
power supply means for supplying AC power to the coil of the magnetic sensor;
signal detection means for detecting a surface property signal in response to the magnetic properties of the surface of the object under inspection detected by a magnetic sensor;
memory means for storing predetermined values showing the relationship between the surface property signal and the surface properties of the object under inspection; and
surface property calculation means for calculating the surface properties of the object under inspection based on the values stored in the memory means and the surface property signals detected by the signal detection means;
wherein the power supply means supplies the AC power at a predetermined frequency to the coil in the magnetic sensor so as to excite the core of the magnetic sensor and to form a closed magnetic path with the surface of the object under inspection.
The surface property evaluation device according to claim 1 , wherein predetermined values indicating the relationship between the predetermined surface property signal and the surface properties of the object under inspection are represented by a calibration curve indicating the correlation between a surface property signal and an object under inspection.
3. The surface property evaluation device according to claim 1 , wherein the predetermined values indicating the relationship between the surface property signal and the surface properties of the object under inspection are reference values indicating a surface property signal in a reference sample having predetermined surface properties.
4. The surface property evaluation device according to any one of claims 1 to 3, wherein the core of the magnetic sensor is capable of making a contact with an object under inspection along the surface shape thereof.
5. The surface property evaluation device according to any one of claims 1 to 3, wherein the core of the magnetic sensor is capable of being disposed so that the distance between the core and the surface of the object under inspection is equal to or less than 3.0 mm.
6. The surface property evaluation device according to any one of claims 1 to 3, wherein the core of the magnetic sensor is capable of being disposed so that the distance between the core and the surface of the object under inspection is equal to or less than 0.3 mm.
7. The surface property evaluation device according to any one of claims 1 to 3, wherein the core of the magnetic sensor is formed of ferromagnetic material.
8. The surface property evaluation device according to any one of claims 1 to 3, wherein the core of the magnetic sensor is an E-shaped core in which a coil is wound around a leg portion at the center.
9. The surface property evaluation device according to any one of claims 1 to 3, wherein the magnetic sensor core has a cylindrical portion around which the coil is wound, and a round pipe portion surrounding the cylindrical portion, closed off at one end by a base portion, the cylindrical portion is disposed at the axial center of the round pipe portion, and one end of the cylindrical portion is connected to the base portion of the round pipe portion.
10. A surface property evaluation method for evaluating the surface properties of an object under inspection, comprising the steps of:
preparing a magnetic sensor for detecting magnetic properties of the surface of an object under inspection, the magnetic sensor including a core having a magnetic body and a coil wound around the core;
supplying AC power at a predetermined frequency to the coil of the magnetic sensor so as to excite the core of the magnetic sensor and to form a closed magnetic path with the surface of the object under inspection;
detecting a surface property signal in response to the magnetic properties of the surface of the object under inspection detected by the magnetic sensor; storing predetermined values showing the relationship between the surface property signal and the surface properties of the object under inspection; and
calculating the surface properties of the object under inspection based on the stored value and the detected surface property signals.
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