JPH05119027A - Ultrasonic flaw detection of thick-wall steel pipe - Google Patents

Abstract

PURPOSE:To enable closing echo height of outside face flaw to echo hight of inside face flaw and thereby to detect the outside face flaw highly accurately by processing the artificial flaw depth of the inside face larger than a specific value and by executing compensation function of outside face echo height (DAC). CONSTITUTION:When executing a flow detection of a thick-wall steel pipe by a ultrasonic flaw detection, sensitivity difference of both comparing test pieces 5 one of which has an outer surface artificial flaw 7 of 5 % depth hOD to wall thickness of the steel pipe, and another one of which has an inner surface artificial flaw 6 of depth hID smaller than the depth of the outer surface artificial flaw 7, is calibrated so as to stay within the predetermined echo height, by using both test pieces. Then, flow detection is carried out after compensation by the use of a DAC. By processing the depth of the inside face artificial flaw 6 to be smaller than 5 %t and conducting the DAC compensation, the echo height of the outside face flaw can be made close to the echo height of the inside face flaw and therewith the outside face flaw can be detected highly accurately.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasonic flaw detection method for thick-walled steel pipes.

[0002]

2. Description of the Related Art Conventionally, defects such as flaws existing inside or on the surface of steel and the like are detected by ultrasonic flaw detection.
This ultrasonic flaw detection test utilizes the property of ultrasonic waves to be reflected at the boundary surface of substances having different densities, and ultrasonic waves with a frequency of 0.2 to 25 MHz are used for non-destructive inspection.
~ 5 MHz is frequently used. Generally, a pulse reflection method is used, but as shown in FIG. 2, a contact medium such as oil or glycerin is applied to the surface of the material 1, and an ultrasonic pulse T is made incident from the probe 2 through the contact medium. , A reflection pulse F from the defect 3 is normally received by a receiving device 4 such as a cathode ray tube oscillograph.
Then, the relationship between the received voltage (echo) and time is displayed and the presence or absence of a defect is evaluated.

The object of measurement is the position on the time axis where the echo is recognized, the height of the echo, the spread of the echo on the material surface, etc. These data are used, for example, as shown in FIG. The position and degree of the defect are evaluated by comparing with the data of the echo height F from the bottom surface, the bottom echo height B of the sound part, or another comparison test piece. Sensitivity / accuracy of the probe and device used for such defect detection are important factors for measurement, and there are regulations such as JIS standards. Also, contrast test pieces used for sensitivity adjustment are JIS standards and APIs. -There is a regulation in the SR2 standard.

That is, for example, in the case of being based on the API-SR2 standard, as shown in FIG. 4, the comparative test piece 5 of the steel pipe has artificial flaws 6, 7 having a depth of 5% with respect to its wall thickness t.
The inner surface (ID) and the outer surface (OD) are pierced respectively, and the inner gate is skipped by 0.5 and the outer gate is skipped by 1.0.

[0005]

However, when the flaws on the inner surface of the thick-walled steel pipe are to be flaw-detected by using the ultrasonic flaw detection method as described above, as shown in FIG. 5, echoes on the outer surface (OD) are flawed. The height F _OD has the drawback that it is considerably lower than the height F _ID of the internal flaw echo from the inner surface (ID). Therefore, by using the external echo height compensation function (hereinafter abbreviated as DAC) added to the flaw detector, the difference between the external flaw echo height F _OD and the internal flaw echo height F _ID , that is, the sensitivity difference Δ.
The external flaw echo height F _OD is compensated so that S is usually 10 dB (about 3 times the adjustment width of the DAC) or less, but in the case of a thick steel pipe with a wall thickness t of 40 mm or more, For example, as shown by the solid line in FIG. 6, the internal flaw echo height F _{ID is} almost
The external flaw echo height F _OD is 1 to 2 dB with respect to 26 dB, and the sensitivity difference ΔS far exceeds the limit of 10 dB.

Therefore, even if the DAC compensation is attempted, since the compensation limit is exceeded, only the external flaw echo height F _OD ′ shown by the dotted line can be obtained, and compensation of several dB at most is performed. Is. As one of the measures, DA
Although it is conceivable to increase the ability of C compensation, there is a drawback that the S / N ratio deteriorates while the cost increases. Also,
There is a method to reduce the frequency of the probe, but the resolution and S
The / N ratio will be reduced.

The present invention has been made in order to solve the above problems, and is intended for a thick-walled steel pipe which does not reduce the resolution and the S / N ratio while maintaining the current DAC compensation. The purpose is to propose an ultrasonic flaw detection method.

[0008]

According to the present invention, in detecting a thick-walled steel pipe by an ultrasonic flaw detection method, an outer surface artificial flaw having a depth of 5% of the wall thickness of the steel pipe and a depth of the outer surface artificial flaw are used. Is also calibrated so that the difference in sensitivity between them is within a predetermined echo height using a contrasting test piece having a shallow depth of internal artificial flaw, and then the external echo height compensation function is used for compensation before flaw detection. This is an ultrasonic flaw detection method for thick-walled steel pipes.

[0009]

[Operation] As a result of earnest research and experimentation on the above problems, the present inventor has obtained the following findings. That is, as shown in FIG. 6, the echo height becomes lower between the outer surface beam path and the echo height as the beam path length becomes longer, and when the outer surface artificial flaw depth is variously changed, the depth of the artificial flaw becomes larger. As the depth becomes deeper, the echo height becomes higher as shown in FIG.

Therefore, since the outer surface beam path becomes longer as the wall thickness of the steel pipe becomes thicker, the depth of the outer surface artificial flaw is made larger than the depth of the inner surface artificial defect in order to increase the echo height from the outer surface. It is necessary to deepen it. However, it is necessary to consider that the artificial flaw depth to be provided on the comparative test piece is 5% t in the API-SR2 standard as described above. As for the depth of the internal artificial flaw, the height of the internal flaw echo (for example, 26 dB) is set so that the height of the external flaw echo becomes a sensitivity difference (ΔS) within 10 dB of the DAC compensable range. To do.

According to an experiment conducted by the present inventor, the relationship between the inner surface artificial flaw depth h _ID (mm) and the wall thickness t (mm) can be expressed by the following equation. It should be noted that k is a constant determined by the steel type. For example, 1.6 is suitable for carbon steel and 1.2 is suitable for 13Cr steel. h _ID = k · 1/2 (5% t) If the internal artificial flaw depth is smaller than 5% t, the API standard may be impaired, but in this case, smaller flaws can be detected. Since it will be difficult for producers because it is possible, it can be said that it is rather a welcome measure for consumers. Further, it is possible to detect a flaw that is shallower than the reference artificial flaw on the inner and outer surfaces by making the judgment level strict. In addition, such countermeasures have a higher ultrasonic attenuation than carbon steel, for example, 13Cr.
It is more effective when applied to high alloy steel such as steel.

[0012]

EXAMPLES Examples of the present invention will be described below.
The method of the present invention was applied to a comparative test piece 5 of a thick-walled steel pipe having a wall thickness t of 40 mm shown in FIG. External artificial flaws 7 provided on the comparison test piece 5
The depth h _OD was 2 mm (5% t) according to the API standard, and the depth h _ID of the inner surface artificial flaw 6 was 1 mm (2.5% t). The probe 2 was brought into contact with this comparative test piece 5, and the echo height of each was measured and calibrated.
The internal flaw echo height for the 5 skips was 26 dB, while the external flaw echo height for the external gate 1.0 skip was 21 dB, and the sensitivity difference ΔS was 5 dB. Therefore, it was possible to perform DAC compensation to adjust the height of the external flaw echo to 26 dB, which is the same as the height of the internal flaw echo.

[0013]

As described above, according to the present invention, the internal artificial flaw depth is processed to be shallower than 5% t,
By performing DAC compensation, the height of the external flaw echo can be made close to the height of the internal flaw echo, so that the external flaw can be detected with high accuracy, which greatly contributes to the improvement of the quality assurance of the product.

[Brief description of drawings]

FIG. 1 is a schematic diagram illustrating an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a configuration of a conventional ultrasonic flaw detection method.

FIG. 3 is an explanatory diagram of the principle of the ultrasonic flaw detection method.

FIG. 4 is an explanatory diagram of a calibration example using a conventional test piece of a steel pipe.

FIG. 5 is an explanatory diagram of an echo height at the time of calibration by a conventional steel pipe comparative test piece.

FIG. 6 is a characteristic diagram showing the relationship between the external artificial flaw echo height and the wall thickness of a conventional steel pipe comparative test piece.

FIG. 7 is a characteristic diagram showing a relationship between an outer beam path length and an echo height.

FIG. 8 is a characteristic diagram showing a relationship between an external artificial flaw depth and an echo height.

[Explanation of symbols]

 2 Probe 5 Comparative test piece 6 Internal artificial flaw 7 External artificial flaw

Claims (1)

[Claims] 1. When detecting a thick steel pipe by an ultrasonic flaw detection method, an outer artificial flaw having a depth of 5% of the wall thickness of the steel pipe and an inner artificial flaw having a depth shallower than the depth of the outer artificial flaw. A thick-walled steel pipe characterized by calibrating so that the difference in sensitivity between the two is within a predetermined echo height using a contrasting test piece with and then compensating using the external echo height compensation function and then performing flaw detection. Ultrasonic flaw detection method.