# US 10,768,129 B2 · Surface characteristics evaluation method for steel material

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- Number: US 10,768,129 B2
- Document ID: US10768129B2
- Status: Granted
- Kind: B2
- Application number: 15/737,981
- Filed: 2016-06-07
- Published: 2018-10-18
- Granted: 2020-09-08
- Earliest claimed: 2015-11-09
- Assignees: Sintokogio, Ltd.
- Applicants: Sintokogio, Ltd.
- Inventors: Yoshiyasu Makino
- Web page: https://patentgrep.com/patent/US10768129B2

## CPC

- G01N 27/023: {where the material is placed in the field of a coil}
- G01N 27/02: by investigating impedance
- G01N 27/025: {a current being generated within the material by induction}
- G01N 27/72: by investigating magnetic variables
- G01N 27/80: for investigating mechanical hardness, e.g. by investigating saturation or remanence of ferromagnetic material
- G01N 27/9046: {by analysing electrical signals}
- C21D 7/06: by shot-peening or the like
- G01N 27/82: for investigating the presence of flaws
- G01N 27/90: using eddy currents
- G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q)
- C21D: MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING (surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by this subclass C23F17/00; unidirectional solidification of eutectic materials or unidirectional demixing of eutectoid materials C30B)

## Abstract

To provide a surface characteristics evaluation method that evaluates the residual stress in a subject made of steel material subjected to a surface modification treatment. A surface characteristics evaluation method includes a step of preparing a surface characteristics evaluation apparatus, a step of placing the subject in such a manner that an alternating magnetic field induced by a coil of the surface characteristics evaluation apparatus permeates into the steel material, a step of generating an eddy current in the subject, a step of successively changing the alternating magnetic field applied to the coil, a step of calculating the impedance Z1 for each of different frequencies from the potential difference across the coil and the value of the current flowing through the coil, a step of performing computation based on the impedance Z1, and a step of evaluating the residual stress in the steel material based on the computation result.

## Claims

### Claim 1

A surface characteristics evaluation method of evaluating a residual stress in a subject made of a steel material subjected to a surface modification treatment, comprising:
a preparation step of preparing a surface characteristics evaluation apparatus, wherein the surface characteristics evaluation apparatus comprises
an oscillator including an AC power supply and a frequency changing circuit,
a detector including a coil connected to the oscillator, an amplifier amplifying a potential difference between opposite ends of the coil, a first absolute value circuit connected to the amplifier, an I/V conversion circuit connected to one end of the coil and a second absolute value circuit connected to the I/V conversion circuit, and
a measuring instrument connected to the frequency changing circuit and the first and second absolute value circuits of the detector;
a subject placement step of placing the subject in such a manner that an alternating magnetic field induced by the coil permeates into the subject;
an eddy current generation step of generating an eddy current in the subject by activating the AC power supply to make the coil induce the alternating magnetic field and allowing the alternating magnetic field to permeate into the subject;
a frequency change step of successively changing the frequency of the alternating current by the frequency changing circuit, thereby successively changing a depth of permeation of the alternating magnetic field into the subject;
an impedance calculation step of calculating a magnitude of impedance Z1 for each of different frequencies by detecting a signal representing a potential difference outputted from the first absolute value circuit and a signal representing a value of a current flowing through the coil from the second absolute value circuit;
a reference impedance measurement step of preliminary measuring a magnitude of reference impedance Z0 at each of different frequencies for a subject yet to be subjected to the surface modification treatment; and
an evaluation step of evaluating the residual stress in the subject based on the magnitude of the impedance Z1,
wherein in the evaluation step, impedance ratios between the magnitude of the reference impedance Z0 and the magnitude of the impedance Z1 are computed for each of the different frequencies, and the residual stress in the subject subjected to the surface modification treatment is evaluated based on the impedance ratios.

### Claim 2 (depends on 1)

The surface characteristics evaluation method according to claim 1, wherein in the evaluation step, the frequency of the alternating current is converted into the depth of permeation of the alternating magnetic field according to the following equation, and the distribution of the residual stress in the subject subjected to the surface modification treatment in the depth direction is evaluated based on the converted depth of permeation:
y
=
κ
⁢
1
πχμσ
where
y: depth of permeation of alternating magnetic field (μm),
k: correction coefficient,
x: frequency of alternating current (Hz),
μ: magnetic permeability of subject (H/m), and
σ: electric conductivity of subject (S/m).

### Claim 3 (depends on 1)

The surface characteristics evaluation method according to claim 1, wherein in the evaluation step, a set of the impedance ratios is plotted on a coordinate system whose coordinate axes indicate the impedance ratios between the reference magnitude of the impedance Z0 and the magnitude of the impedance Z1 and the frequency, respectively.

### Claim 4 (depends on 3)

The surface characteristics evaluation method according to claim 3, wherein in the evaluation step, the residual stress in the subject subjected to the surface modification treatment is evaluated based on a trajectory of a graph of the set of the impedance ratios plotted on the coordinate system and one or more of the impedance ratios including an extreme of the trajectory.

### Claim 5 (depends on 4)

The surface characteristics evaluation method according to claim 4, wherein in the evaluation step, the quality of the surface modification treatment is determined by comparing one or more of the impedance ratios including the extreme with one or more preset thresholds.