JP2001221781A - Method of ultrasonic flaw detection - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely inspect flaws likely to occur in stainless piping used in a confluence part of fluids of different temperatures. SOLUTION: Transverse ultrasonic wave is made incident on a material 1 to be inspected so as to make a refraction angle θ1 around a transverse wave critical angle. This transverse wave is reflected on the back side 1a of the material 1 to be converted into a longitudinal ultrasonic wave by mode conversion, and the longitudinal wave is directed to a flaw 4. Thereby a local recess and a shallow crack in a recess part in an initial stage of cracking can be accurately detected in their early stages.

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 detecting internal flaws such as internal cracks generated on the inner surface of a stainless steel pipe or a weld by a nondestructive inspection using ultrasonic waves. .

[0002]

2. Description of the Related Art Conventionally, flaw detection of an inner surface crack generated in an inner surface or a welded portion of a stainless steel pipe is generally performed by oblique flaw detection from an outer surface. Generally, it is not necessary to consider the mode change of the transverse wave, so that the angle of incidence at the time of reflection and the angle of reflection are equal, and therefore, there is an advantage that the flaw position can be easily calculated by geometric calculation. In this case, in the flaw detection of a crack extending vertically from the inner surface, as shown in FIG. 5, the transverse ultrasonic wave emitted from the transducer 11 of the probe 10 is incident on the material 12 to be inspected at the refraction angle θ, and The reflected wave at the corner 12a due to the inner surface 12a of the material and the crack 13 is detected. The refraction angle θ is set to an angle of 45 ° or more, which is sufficiently larger than the critical angle at which mode conversion occurs during reflection and a transverse wave changes to a longitudinal wave.

[0003]

By the way, it is known that an internal crack is generated on the inner surface of a stainless steel pipe where fluids of different temperatures join by thermal fatigue.
Its origin depends on: First, S (sulfur) contained in the fluid flowing through the pipe and Fe in the stainless steel
The FeS compound reacts with the (ferrite) component to form a FeS compound on the inner surface of the pipe in a film form. The FeS film is locally cracked or peeled off due to expansion and contraction of stainless steel due to fluctuations in the fluid temperature, disturbance of the fluid flow, and the like. When this is performed intermittently for a long time, the crack portion of the FeS coating becomes a local depression of stainless steel. The concave portion further becomes a stress concentration portion of the expansion and contraction of the stainless steel due to the fluctuation of the fluid temperature, and cracks are generated in the stainless steel. In addition, the grooves and steps formed by the cutting tool when the inner surface of the pipe is machined also become stress concentrated portions, and similarly become crack generation sources. For this reason, in the initial crack detection, it is necessary to detect the presence or absence of a local stainless steel dent and to confirm whether or not a crack has occurred at the tip of the dent.

However, in the conventional oblique flaw detection method, FIG.
As shown in (A), in the local concave portion 12b on the inner surface 12a side of the inspection object 12, the reflection surface does not become perpendicular to the incidence of the ultrasonic wave and does not reflect at the corner.
In relation to the angle of inclination of the concave reflecting surface and the angle of the ultrasonic incident wave,
The reflected wave does not return to the probe, making it difficult to obtain a reflected wave. Further, as shown in FIG. 6 (B), even when a crack 13 occurs at the tip of the concave portion 12b, the detected echo is low and the reflection is performed at the corner, so that the crack from the inner surface to the tip of the crack is formed. There is a problem that it is difficult to obtain a proportional relationship between the height of the reflected wave and the height of the reflected wave. Further, if the crack is small, the reflected wave at the bottom surface does not reach the cut surface, and the reflected wave cannot be obtained. Also, when the crack height is shallow, the crack height cannot be measured because the reflected wave at the corner and the reflected wave at the tip of the crack cannot be separated.
That is, the conventional shear wave oblique flaw detection has a problem that, in the detection of a crack due to thermal fatigue of a stainless steel pipe, the ability to detect a crack in an early stage is low.

The present invention has been made in view of the above circumstances, and it is possible to effectively detect a flaw generated inside a material to be inspected, and particularly to accurately detect an initial crack caused by thermal fatigue in a stainless steel pipe. It is an object of the present invention to provide an ultrasonic flaw detection method capable of detecting.

[0006]

Means for Solving the Problems To solve the above problems, a first aspect of the ultrasonic flaw detection method according to the present invention is directed to a method of producing a transverse ultrasonic wave so that a material to be inspected has a refraction angle near a critical shear wave angle. Is incident, and this transverse ultrasonic wave is reflected on the back side of the material to be inspected, converted into a longitudinal ultrasonic wave by mode conversion, and the longitudinal ultrasonic wave is advanced to the flaw detection position to obtain a flaw detection signal. And

In the ultrasonic flaw detection method according to the second invention, a transverse ultrasonic wave is incident on the material to be inspected so as to have a refraction angle near the critical angle of the transverse wave, and a concave portion on the back side is detected. The incident position is changed along the surface direction of the inspection target material from the ultrasonic incident position at the time, and the transverse ultrasonic wave is incident on the inspection target material, and the transverse ultrasonic wave is reflected on the back surface side of the inspection target material by mode conversion. The method is characterized in that a flaw at the tip of the recess is detected by the longitudinal wave instead of the longitudinal ultrasonic wave.

The ultrasonic flaw detection method according to a third invention is characterized in that, in the first or second invention, the material to be inspected is a stainless steel pipe, and the flaw intended for flaw detection is a thermal fatigue inner crack. .

That is, the present invention is characterized in that a secondary creeping wave is used for flaw detection. In the present invention, the ultrasonic wave transmitted from the probe is incident on the inspection object near the refraction angle which is the critical angle of the transverse wave. The critical angle depends on the material of the material to be inspected, but is about 33 ° for stainless steel.
The angle of incidence of the ultrasonic wave on the boundary of the material to be inspected is determined so as to obtain a desired refraction angle with the critical angle as a target.

Therefore, by transmitting the above-mentioned transverse wave ultrasonic wave toward the back surface side of the material to be inspected, the reflected wave is mode-converted on the back surface, and changes from a transverse wave to a longitudinal wave. This change can be obtained by setting the angle of refraction at the test object near the critical angle. This reflected wave is called a secondary creeping wave. This secondary creeping wave can be advanced in the flaw detection direction. This is different from the conventional shear wave reflected wave, and becomes a longitudinal wave reflected wave, which travels at an angle of 5 to 10 ° with respect to the back surface, passes over the concave portion, and the reflected wave cannot be obtained. On the other hand, a scratch on the concave portion can be hit at an angle almost perpendicular to the surface, and a reflected wave can be obtained. This enables both reflection at the dent and detection of a crack at the tip of the dent. Further, it is possible to estimate the crack height from the height of the reflected wave at the crack.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS. As shown in FIG. 4, the material to be inspected 1 is a stainless steel pipe used at a junction of fluids having different temperatures. In the present embodiment, the specimen 1 is generated due to thermal fatigue at a pipe or a weld near the junction. Flaw detection for the purpose of detecting cracks. At the time of flaw detection, as shown in FIG. 1, a probe 2 for generating a shear wave ultrasonic wave is installed on a material 1 to be inspected, and a flaw detection position is determined in accordance with an assumed flaw position. At the flaw detection position, the refraction angle θ1 is near the critical angle at which the transverse ultrasonic wave incident on the material 1 changes to a longitudinal wave by mode conversion when reflected by the bottom surface 1a. The refraction angle is set to 33 ° in consideration of characteristics. At this time, the angle of incidence of the ultrasonic wave on the inspection target material 1 is determined in consideration of the difference in the acoustic characteristics between the acoustic wedge 3b of the probe 2 and the inspection target material 1. The shear wave ultrasonic wave that has entered the test object 1 travels through the test object 1 without causing a mode change. This transverse ultrasonic wave, as shown in FIG.
It hits the surface of the concave portion 1b on the back surface 1a side of the material 1 to be inspected. In the concave portion 1b, the reflected wave is reflected in the direction of the probe 2 that has transmitted the ultrasonic wave as a transverse wave due to the incident angle. This makes it possible to detect the presence of the shallow recess 1b.

Next, when it is assumed that a crack exists in the concave portion 1b, the probe 2 is moved in a direction away from the position where the reflected wave from the concave portion 1b is detected (FIG. 1).
Solid line position). Then, the transverse ultrasonic waves traveling on the inspection target material 1 are reflected on the back surface 1a of the inspection target material 1 as shown in FIGS. 1 and 2B. At the time of this reflection, since the refraction angle at the time of incidence of the material to be inspected is near the critical angle, mode conversion occurs, and the ultrasonic wave changes from a transverse wave to a longitudinal wave. Since this longitudinal ultrasonic wave (secondary creeping wave) travels at an angle of 5 to 10 ° with respect to the back surface, if a flaw 4 exists in this traveling direction, it will be detected by the longitudinal wave reflected wave. . That is, according to the above, even when the flaw 4 exists at the tip of the concave portion 1b, the longitudinal ultrasonic wave surely hits the flaw 4 unlike the conventional method.
The mode conversion of the longitudinal ultrasonic wave hitting the flaw 4 further occurs, and the reflected ultrasonic wave changes into a shear wave. The shear wave reflected wave travels to the surface side and is received by the probe 2 as it is, or is reflected by the surface side of the inspection target material 1 as shown in FIG.
Received at. The transverse wave reflected on the surface side undergoes further mode conversion, and is received as a longitudinal wave reflected wave. By using the secondary creeping wave in this way,
It is possible to accurately detect the dent and the crack generated at the tip of the dent. Therefore, it is possible to determine the nature of the flaw from the appearance of the peak of the reflected wave when the probe is scanned back and forth and the propagation distance of the ultrasonic wave to the reflected wave.

In the above embodiment, after detecting the dent, the probe is shifted to detect a flaw at the tip of the dent. It is also possible to detect a flaw by obtaining a wave. in this case,
The flaw detection position is determined such that the longitudinal wave reflected wave hits the flaw in consideration of the refraction, reflection, and mode conversion of the ultrasonic wave.

Further, the reflected wave is a reflection from the surface of the flaw, and there is a proportional relationship between the height of the reflected wave at the peak and the depth of the crack. As shown in FIG. 3A, the probe 2
Using the height of the reflected wave from the test side hole 5 as a reference,
The crack depth can be predicted from the reflected wave height at that time. FIG. 3B illustrates the relationship between the height of the reflected wave and the height of the crack obtained using the lateral hole 5 having a diameter of 2.4 mm, and a correlation is recognized between the two. When the height from the inner surface of the crack is larger than the wavelength of the used probe frequency, the scattered wave at the crack tip and the reflected wave at the corner between the base and the crack can be separated on the time axis Thus, it is possible to detect the scattered wave at the crack tip using the shear wave oblique probe and measure the height thereof. In the method of measuring the crack height from the scattering at the crack tip, the distance on the time axis between the reflected wave from the corner of the crack and the reflected wave from the crack tip can also be determined by scanning the probe back and forth. Since there is no difference, the probe is scanned back and forth,
By searching for a reflected wave that moves in conjunction with the reflected wave from the corner of the crack, it is possible to identify the reflected wave from the tip of the crack. By using the secondary creeping wave in this way, by reading the distance to the peak position of the reflected wave, it is possible to determine whether the dent is inherent or has a crack at the tip of the dent. As explained above, the method of the present invention comprises:
It is extremely effective in detecting a dent caused by thermal fatigue inside a stainless steel pipe and a crack generated at the tip of the dent. However, the present invention is not limited to this application, and can be used for flaw detection of a material to be inspected for another application.

[0015]

As described above, according to the ultrasonic flaw detection method of the present invention, a transverse ultrasonic wave is made incident on a material to be inspected so as to have a refraction angle near a critical shear wave angle, and the transverse ultrasonic wave is applied. Is reflected on the back side of the material to be inspected and converted into longitudinal ultrasonic waves by mode conversion, and the longitudinal ultrasonic waves are advanced to the flaw detection position to obtain a flaw detection signal. It becomes possible to detect by catching with ultrasonic waves.

Further, a transverse ultrasonic wave is incident on the material to be inspected so as to have a refraction angle close to the critical angle of the transverse wave to detect a dent on the back side, and the ultrasonic wave incident position at the time of the detection is detected. By changing the incident position along the surface direction of the inspection material, the transverse ultrasonic wave is incident on the material to be inspected, and the transverse ultrasonic wave is reflected on the back surface side of the material to be inspected, and is converted into a longitudinal ultrasonic wave by mode conversion,
If the flaws at the tip of the dent are detected by the longitudinal ultrasonic waves, the dent and the flaw at the tip of the dent can be reliably detected both in the presence and in the properties. Therefore, it is possible to detect from a local dent stage which is a precursor stage of a crack, it is possible to determine whether only a dent or a crack is present in a dent, and it is possible to estimate a crack height. If the above method is used to detect a thermal fatigue inner surface crack generated in a stainless steel pipe, high detection can be obtained even in the initial stage of the crack.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a reflection path of flaw detection according to an embodiment of the present invention.

FIG. 2A is an enlarged cross-sectional view showing a reflection path when a concave portion on the back side is provided, and FIG. 2B is a reflective path when a concave portion is further damaged.

FIG. 3A is a diagram showing a flaw detection state when a relationship between a reflected wave height and a crack height is obtained using a standard flaw, and FIG. 3B is a graph showing the result.

FIG. 4 is a perspective view showing the state of use of a material to be inspected.

FIG. 5 is a sectional view showing a reflection path of flaw detection in a conventional flaw detection method.

FIG. 6A is an enlarged cross-sectional view showing a reflection path on the back side concave portion, and FIG. 6B is a view showing a reflection path on the back side.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Inspection material 1a Back surface 1b Depression 2 Probe 3a Vibrator 3b Sound wedge 4 Scratches

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Mitsuo Yotsuji 37-24, Nitta-cho, Chuo-ku, Chiba-shi, Chiba F-term (reference) 2G047 AA07 AB01 AB07 BA02 BC07 CB01 CB02 CB06

Claims (3)

[Claims] 1. A transverse ultrasonic wave is incident on a material to be inspected so as to have a refraction angle near a shear wave critical angle, and the transverse ultrasonic wave is reflected on the back side of the material to be inspected, and the longitudinal wave is converted by mode conversion. An ultrasonic flaw detection method characterized in that the flaw detection signal is obtained by advancing the longitudinal ultrasonic wave to a flaw detection position instead of ultrasonic waves. 2. A transverse wave ultrasonic wave is incident on a material to be inspected so as to have a refraction angle near a shear wave critical angle to detect a dent on the back surface side, and to detect a concave portion from the ultrasonic wave incident position at the time of the detection. By changing the incident position along the surface direction of the inspection material, the transverse ultrasonic wave is incident on the material to be inspected, the transverse ultrasonic wave is reflected on the back surface side of the material to be inspected, and is converted into a longitudinal ultrasonic wave by mode conversion. Ultrasonic flaw detection method characterized by detecting a flaw at the tip of a concave portion by longitudinal wave ultrasonic waves 3. The ultrasonic flaw detection method according to claim 1, wherein the material to be inspected is a stainless steel pipe, and the flaw intended for flaw detection is a thermal fatigue inner surface crack.