JPH0447263A - Eddy-current flaw detection method for nonmagnetic steel - Google Patents

Abstract

PURPOSE:To enable highly-precise and sure detection of a minute flaw of the surface of a nonmagnetic steel material by making a high-frequency current of a specific frequency approach the surface of the steel material and by detecting the amount of a change in an eddy current generated in the surface of the steel material. CONSTITUTION:When a surface flaw of a peeling material 9 is inspected, an alternating current of an oscillator 1 is made to accord with a frequency of a prescribed value within a range of 1 to 3 MHz and to approach the surface of peeling material 9 to be inspected, so that an eddy current be made to flow. A signal from a probe coil 8 is transmitted to a phase detector 4 through an amplifier 2 and an automatic balancer 3. When the surface of the material to be inspected has no flaw, the signal subjected to phase detection by the phase detector 4 and frequency discrimination by a frequency discriminator 5 and outputted from a flaw level setting unit 7 is very small. When the surface has a minute flaw of about 0.05 to 0.1 mm, however, the eddy current changes conspicuously since the depth of permeation is about the same with the depth of the flaw, and a signal corresponding to the amount of change is outputted from the phase detector 4, wherefrom the flaw is detected.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an eddy current flaw detection method for detecting flaws on the surface of steel materials. Regarding the method.

(Prior Art) Generally, as a method for inspecting flaws on the surface of steel materials, an eddy current flaw detection method shown in JIS standard number GO568 is known. In this eddy current flaw detection method, when a test piece is placed inside a coil through which alternating current is passed, if there is an unhealthy part in the test piece, the eddy current flowing through the test piece will change, and the impedance of the coil or the eddy current induced in the coil will change. A change occurs in the voltage, and the presence and extent of an unhealthy portion is determined from the amplitude, phase, etc. of this changing electrical signal.

The test frequency of the alternating current in this case is determined in consideration of the skin effect. Conventional test frequencies are related to magnetic and non-magnetic properties (current frequencies of 35 to 512 kHz are often used).

(Problem to be Solved by the Invention) However, when inspecting non-magnetic steel using the conventional eddy current flaw detection method, the test frequency range is as described above (as it is set in a relatively small frequency range, If the flaw is a minute flaw with a depth of about 0.05 to 0.1 mm, it is difficult to detect the flaw.

In particular, when inspecting non-magnetic steel bearing material whose surface has been cut with bar steel, etc., it is difficult to distinguish whether the detected change in eddy current is due to minute flaws or uneven striations from cutting marks. (The detection accuracy was low.

The present invention has been made to solve these problems, and an object of the present invention is to provide an eddy current flaw detection method for non-magnetic steel that reliably detects minute flaws on the surface of non-magnetic steel with high accuracy.

(Means for Solving the Problems) The eddy current flaw detection method for non-magnetic steel of the present invention applies a frequency of 1 to
It is characterized by detecting defects in the steel material by detecting the amount of change in eddy current generated on the surface of the steel material by applying a 3 MHz high frequency current nearby.

The reason why the frequency of the high-frequency current is set in the range of 1 to 3 MHz is because
This is because if it is less than I MHz, it will be difficult to accurately detect minute flaws in non-magnetic steel, and even if it exceeds 3 MHz, the detection value of the flaw detection level (flaw signal S/noise signal N) will not improve. be.

(Function) According to the eddy current flaw detection method for non-magnetic steel of the present invention, when the frequency of the alternating current used for eddy current flaw detection is set to 1 to 3 MHz, changes in eddy current due to the skin effect on the surface of the steel material determine the presence or absence of micro-flaws on the surface of the steel material. This is noticeable.

Therefore, on the surface of non-magnetic steel, 0.05 to 0.1
When micro-flaws with a depth of about mm are formed, the penetration depth of eddy currents generated on the surface of the steel material due to the skin effect is approximately the same as the depth of the micro-flaws, and the eddy current becomes noticeable depending on the presence or absence of flaws. Changes to

Therefore, the degree of flaw detection (flaw signal S/noise signal N) is approximately twice as high as that of the conventional method, and minute flaws can be detected reliably.

(Example) Embodiments of the present invention will be described below based on the drawings.

In this example, a beading material made of non-magnetic steel is used as a test piece, and the surface thereof is inspected for micro-flaws.

The surface of this beer ring material is cut with a bar steel, and minute irregularities are formed in the cutting marks on the surface.

As shown in FIG. 2, the flaw detection apparatus places a probe coil 8 close to the surface of the beading material 9, with the center line of the coil being approximately perpendicular to the surface of the test piece. The probe coil 8, which rotates in the direction of the second arrow shown in the figure, is adapted to flow an eddy current to the surface of the bearing material 9 while rotating around the axial center of the bearing material 9.

The flaw detection circuit to which the probe coil 8 is connected includes a 4M
An oscillator 1 capable of passing an alternating current with a frequency up to Hz through a probe coil 8 is provided, an amplifier 2 that receives an eddy current signal on the surface of the peeling material, an automatic balancer 3, a phase detector 4, a frequency discriminator 5, A sensitivity adjuster 6 and a flaw level setter 7 are connected in this order.

The output signal of the eddy current detected from the bearing material 9 is first sent to the amplifier 2 and amplified, and the amplified output signal is sent to the automatic balancer 3. The automatic balancer 3 detects the output signal of the amplifier 2 and sends a signal obtained by removing fluctuations from the signal to the phase detector 4 in order to suppress the influence of gradual changes in the ambient temperature and maintain a balanced state. Output. Phase detector 4
converts the input signal sent from the automatic balancer 3 to the oscillator l.
The phase is adjusted by the phase shift control signal outputted from the phase shifter 10, and noise other than the flaw signal is suppressed.

Furthermore, a frequency discriminator 5 receiving the signal sent from the detector 4 performs frequency discrimination and suppresses noise other than the predetermined flaw signal. In addition, the output signal of the frequency discriminator 5 is transmitted to the sensitivity adjuster 6.
If the flaw signal exceeds a preset value, an alarm (not shown) is activated.

When inspecting the surface of the bearing material 9, the alternating current of the oscillator 1 is adjusted to a predetermined frequency in the range of 1 to 3 MHz, and the alternating current is brought close to the surface of the bearing material 9 to be inspected to generate eddy currents. flow.

If there are no defects on the surface, the signal from the probe coil 8 is transmitted to the phase detector 4 via the amplifier 2 and the automatic balancer 3. As a result of the frequency discrimination performed by the device 5, the signal output to the flaw level setting device 7 becomes extremely small. However, if there is a microscopic flaw of about 0.05 to 0.1 mm on the surface, the penetration depth will be about the same as the depth of the flaw, so the eddy current will change significantly, and the signal will change depending on the amount of change. is output from the phase detector 4, and a flaw is detected.

In addition, signals due to minute flaws can be reliably distinguished from signals due to minute irregularities on the surface cutting marks of the beading material.

Next, for the beading material with micro flaws formed at a depth of 0.07 mm and 0.1 mm, the frequency of 1 to 3 MHz was
In order to compare the detection accuracy when inspecting flaws using a frequency outside this range and a frequency outside this range, we tested the change in the degree of flaw detection depending on the frequency. The frequencies tested were IMHz, 2 MHz and 3 MHz as frequencies in the range of 1-3 MHz.
, 200KHz and 51 as frequencies below 1MHz
4 MHz was used as the frequency above 2 KHz and 3 MHz.

As a result of the test, as shown in Figure 1, flaws with a depth of 0°1 mm showed a high flaw detection degree at IMHz, and
, 07mm showed the highest flaw detection value at 3MHz. This results in 0.1 and 0.07 of this beering material.
It can be seen that flaws of 1 mm in diameter can be detected accurately by using alternating currents with frequencies of I MHz and 3 MHz, respectively.

In this example, a probe coil was used to apply an eddy current to the surface of the bearing material. However, when the non-magnetic steel material to be inspected is a steel pipe, steel bar, wire rod, etc., the steel material is passed through the coil and the surface of the steel material is A through coil may be used to flow an eddy current. In addition, when inspecting the inside of a pipe or hole,
An interpolation coil may also be used. Furthermore, the eddy current flaw detection method for non-magnetic steel of the present invention can also be applied to film thickness measurement, shape and dimension measurement, etc.

It goes without saying that the non-magnetic steel to which the present invention is applied is not limited to austenitic stainless steels such as 18-12.18-8.16-14 nickel-chromium steel and high manganese steel.

(Effects of the Invention) As explained above, according to the eddy current flaw detection method for non-magnetic steel of the present invention, the surface of the steel material is inspected using a high-frequency alternating current of 1 to 3 MHz. depth 0.0
It is possible to reliably detect minute flaws of about 5 to 0.1 mm with high precision, and it is also effective in being able to distinguish between uneven streaks of cutting marks and surface flaws on the material to be cut.

[Brief explanation of the drawing]

FIG. 1 is a characteristic diagram showing the relationship between flaw detection frequency and flaw detection degree, and FIG. 2 is a schematic configuration diagram showing a flaw detection circuit according to an embodiment of the present invention. 9...Beering material (non-magnetic steel)

Claims (1)

[Claims] (1) When detecting micro-flaws on the surface of non-magnetic steel,
An eddy current flaw detection method for non-magnetic steel, characterized in that defects in the steel are detected by detecting the amount of change in eddy current generated on the surface of the steel by applying a high frequency current with a frequency of 1 to 3 MHz to the surface of the steel.