JPH0820424B2 - Ultrasonic flaw detection method - Google Patents

Description

TECHNICAL FIELD The present invention relates to an ultrasonic flaw detection method for detecting defects in a steel material.

[Prior Art] An ultrasonic flaw detection method for detecting surface defects and internal defects of a material to be inspected by causing an ultrasonic wave to enter the material to be inspected such as a steel pipe using a probe and receiving a reflection echo thereof , Has been widely used in the past.

However, in the conventional ultrasonic flaw detection method as described above, the crystal grains are coarse (for example, the thickness is about 3 to 4 mm, and the grain size No. In the inspected material (5 or less), there is a problem that the ultrasonic waves are scattered at the grain boundaries and the scattering echo is significantly increased, and the attenuation of the ultrasonic waves is increased accordingly, which makes it difficult to detect fine defects. It was The degree of attenuation of such ultrasonic waves varies depending on the material and size of the material to be inspected, but generally the relationship expressed by the following equation (1) is established.

P = P ₀ e ^-2αx (1) where P ₀ : sound pressure of the source x: sound wave propagation distance P: sound pressure at the propagation distance x α: damping constant (1) Since it is proportional to the fourth power of the frequency of the sound wave, a method of performing flaw detection with a low-frequency ultrasonic wave is adopted for the material to be inspected with coarse crystal grains as described above.

By the way, in the ultrasonic flaw detection method, the sound pressure P _F when a defect having an area S is detected is expressed by the following equation (2).

However, λ: wavelength of ultrasonic wave In the formula (2), the sound pressure P _F becomes larger as the wavelength λ of the ultrasonic wave becomes shorter.

Therefore, as described above, when low frequency ultrasonic waves are used, since the wavelength is long, P _{F in} equation (2) becomes small, the defect detection performance deteriorates, and the defects to be detected are inevitably small in the thin material, There is a problem that a defect cannot be detected by using the low frequency ultrasonic wave as described above.

As a countermeasure against this, the present inventor has already proposed a method of analyzing the frequency of a received echo using a high frequency band sensor (Japanese Patent Application No. 1-6461).

[Problems to be Solved by the Invention]

However, this method has limitations in flaw detection performance, such as diameter
An austenitic stainless steel pipe (grain size No. ≈ 5) with a thickness of 38.0 mm and a thickness of 6.0 mm, and a depth of 0.3 using a 5 MHz probe.
When detecting artificial defects of mm, the S / N was about 2.5, and the detectability of shallow defects was not sufficient.

The present invention has been made in view of such circumstances, and an object thereof is to provide an ultrasonic flaw detection method capable of obtaining a good S / N ratio by capturing a received signal in two frequency bands and obtaining a difference between the two frequency bands. And

[Means for solving the problem]

The ultrasonic flaw detection method according to the present invention is an ultrasonic flaw detection method in which an ultrasonic wave is incident on a material to be inspected, a reflection echo thereof is received, and a defect is analyzed to detect a defect. Of the received reflection echo, means for detecting a defect signal based on a signal in a predetermined frequency band, means for detecting a noise signal based on a signal in a predetermined frequency band higher than the frequency band, and both of the detection Means for subtracting the other from one of the signals.

[Action]

In the method of the present invention, this can detect defective echoes in a predetermined frequency band and scattered echoes in a higher frequency band, and obtain the difference between them to reduce the influence of scattered echoes.

[Examples] The method of the present invention will be specifically described below with reference to the drawings illustrating the examples. FIG. 1 is a block diagram showing the configuration of an ultrasonic flaw detector used for carrying out the method of the present invention.

In the figure, 1 is a steel pipe as a material to be inspected, and 2 is a probe. The probe 2 that transmits and receives ultrasonic waves in a wide band is
It is provided so as to be inclined with respect to the surface of the steel pipe 1 through water W. The probe 2 has a high center frequency based on the signal from the pulse oscillator 3 that is at least twice as high as the frequency shown in the following formula (3), which is usually used for flaw detection of steel pipes, and has a wide band across the low frequency side and the high frequency side. Are transmitted and received.

5% ≦ d / λ ≦ 20% (3) However, d: Depth of defect The output of the pulse oscillation circuit 3 for exciting the probe 2 is input to the probe 2 at regular intervals. Each time, the probe 2 oscillates a wideband ultrasonic wave by the signal from the pulse oscillator 3, the ultrasonic wave is incident on the steel pipe 1 through the water W, and the reflection echo is detected from the steel pipe 1 through the water W. Child 2
And is input to the preamplifier 4. Preamplifier 4
The flaw detection signal input to is amplified and output to the defect echo detection circuit 5 and the scattered echo detection circuit 6.

The defect echo detection circuit 5 detects a defect signal based on a flaw detection signal of a predetermined band F ₁ set in advance out of a wide band flaw detection signal, while the scattering echo detection circuit 6 similarly detects the defect signal of the wide band flaw detection signal. A noise signal (scattering signal) is detected based on a flaw detection signal in a predetermined band F ₂ (F ₁ <F ₂ ) on the higher frequency side than the band F ₁ defined for the echo detection circuit 5.

The setting of this frequency band is determined as follows using the gate circuit 7 and the frequency analysis device 10. That is, prior to the flaw detection work of the steel pipe 1, preliminary flaw detection is performed using a test material having artificial defects on the inner and outer surfaces of a part of the steel pipe 1, and the preamplifier 4 detects a defect echo detection circuit 5 and a scattered echo. Detection circuit 6
The defect signal and the scatter signal are detected from the wide band flaw detection signals inputted to the defect signal / scatter signal, that is, the band where the S / N ratio is maximum is F ₁ , the defect signal / scatter signal,
That is, the band in which the S / N ratio is the minimum is F ₂ .

Then, the operator inputs the frequency bands F ₁ and F ₂ as the set frequency regions of the defect echo detection circuit 5 and the scattered echo detection circuit 6.

The defect signal and the scattered signal detected by the defect echo detection circuit 5 and the scattered echo detection circuit 6, respectively, are the main amplifiers 8 and 9, respectively.
Output to The main amplifier 8 is adjusted so that the defective signal has a prescribed magnitude in a dynamic state, and the main amplifier 9 has a minimum noise signal. The defect signal and the scattered signal input to the main amplifiers 8 and 9 are respectively amplified and then output to the subtractor 11.

The subtractor 11 uses the defect signal input from the main amplifier 8 to
The scattered signal input from the main amplifier 9 is subtracted, and the subtracted signal is output to the gate circuit 12.

The gate circuit 12 recognizes a signal having a predetermined value or more as a defect signal at a position on the time axis where the artificial inner surface defect and the outer surface defect can be detected by preliminary flaw detection, and outputs it as an analog output to an external device. It has become.

Next, according to the method of the present invention and the conventional method, the diameter is 6.5 mm and the wall thickness is
SUS3 with 0.47 mm, austenite grain size GS No. ≒ 7.0
04 An artificial defect with a depth of 23 μm was machined into a steel pipe and ultrasonic flaw detection was performed to determine the S / N ratio. As a result, S / N ≈ 2.8 with the conventional method.
On the other hand, it was confirmed that S / N≈6 in the method of the present invention.

[Effect] As described in detail above, in the method of the present invention, detection of fine defects in a material having coarse crystal grains can be inspected at a high S / N ratio, high quality assurance is possible, and product safety is also improved. High, etc. The present invention has excellent effects.

[Brief description of drawings]

FIG. 1 is a block diagram of an apparatus for carrying out the method of the present invention. 1 ... Inspected material, 2 ... Probe, 5 ... Defect echo detection circuit, 6 ... Scattered echo detection circuit, 7 ... Gate circuit,
8, 9 ... Main amplifier, 10 ... Frequency analyzer, 11 ... Subtractor, 12 ... Gate circuit

Claims (1)

[Claims] 1. An ultrasonic flaw detection method in which an ultrasonic wave is incident on a material to be inspected, a reflection echo thereof is received, and a defect is analyzed and analyzed to detect a defect. Means for detecting a defect signal based on a signal in a predetermined frequency band among echoes, means for detecting a noise signal based on a signal in a predetermined frequency band on the higher frequency side of the frequency band, and one of the two detection signals An ultrasonic flaw detection method comprising: