JPS6329245A - Ultrasonic flaw detecting method for thick-walled steel pipe - Google Patents

Abstract

PURPOSE:To enable flaw detection with a more sufficient sensitivity margin than flaw detection by a conventional longitudinal wave-transversal wave mode converting method by propagating a longitudinal wave directly to the internal surface defect of a thick-walled steel pipe. CONSTITUTION:This method is a flaw detecting method which supplies the longitudinal wave L directly to the internal surface defect by using a range of a longitudinal wave refraction angle thetaL=14-20 deg.. In this case, the acoustic pressure reciprocation passing rate of the longitudinal wave-transversal wave mode converting method is 6.8%, but that of the direct longitudinal wave method is 11.8% and 5.0% higher. Assuming that the sound pressure at a point C is 1, the sound pressure at a point C'' is about 2.8 from an ultrasonic wave beam path. Namely, flaw detection is performed by this method with a more sufficient sensitivity margin than flaw detection by the conventional longitudinal wave- transversal wave mode converting method.

Description

Detailed Description of the Invention (Industrial Field of Application) This invention particularly relates to thickness/outer diameter (hereinafter referred to as 1/1) 0
．． This invention relates to an oblique ultrasonic flaw detection method that is effective for steel pipes with an outer diameter of 80 m or more.

(Prior Art) The mainstream method of oblique ultrasonic flaw detection for steel pipes is to propagate only transverse waves through the steel to detect defects on the inner and outer surfaces of the pipe. In this case, if t/b is about 0.2 or more, the ultrasonic beam will not reach the inner surface, making it impossible to detect defects on the inner surface. t/b
A longitudinal wave-transverse wave mode conversion method has been developed as an ultrasonic flaw detection method for internal defects in steel pipes with a diameter of 0.2 or more and is generally applied (reference: Japanese Patent Publication No. 17024-1981).

However, in the above longitudinal wave-transverse wave mode conversion method, the outer diameter is
When the depth is 80° or higher, the ultrasonic beam path becomes long, and there is a problem that the reflected ultrasonic waves from the depth of about 0.3W of the inner surface defect are small and the signal/noise (hereinafter referred to as S/N) cannot be detected well. .

(Object of the Invention) In view of the above, the present invention is an oblique ultrasonic flaw detection method for steel pipes, which detects minute internal defects with a good S/N ratio in steel pipes with an outer diameter of 80 mm or more with a t/l) ) 0.2 or more. The present invention provides an ultrasonic flaw detection method.

(Problem to be solved by the invention) JIS Z2344 is often used for ultrasonic flaw detection tests on metals, and JIS GO582 is used for steel pipes.
``Ultrasonic flaw detection method for steel pipes'' is used.P1 The ultrasonic flaw detection method for steel pipes is usually the oblique ultrasonic flaw detection method.

As is well known, there are two types of waves: longitudinal waves (hereinafter referred to as "S waves") and transverse waves (hereinafter referred to as "S waves").A longitudinal wave is incident on the interface between a different first medium and a second medium. Then, a refracted longitudinal wave and a refracted transverse wave S are generated in the second medium due to the incident angle.

FIG. 3 is a diagram showing the relationship between the round trip pass rate of sound pressure and the refraction angle when the first medium is water and the second medium is steel. T and TL indicate the sound pressure round-trip passage rate of the S wave and the L wave, respectively. It can be seen from FIG. 3 that efficient flaw detection is limited to a range where the sound pressure reciprocating rate is high, that is, the refraction angle θ8=35 to 70 degrees. This is the range that has traditionally been considered effective.

To explain this in more detail, when performing flaw detection using only S waves, the limit wall thickness/outer diameter at which the ultrasonic beam reaches the inner surface of the tube is:
As shown in Figure 4, t/b = 172 (1 - blood θ), given by θ = 3
Assuming 5 degrees, the limit is t/ll = 0.213. In the case of flaw detection using only S waves, t/b is 0.213.
If it exceeds this value, the ultrasonic beam will not reach the inner surface of the tube, making it impossible to detect defects on the inner surface of the tube. This is why JIS GO 682° "Ultrasonic flaw detection method for steel pipes" stipulates that the target steel pipe is "thickness to outer diameter percentage ratio of 20 or less".For this reason, t/l)) 0.2 As an ultrasonic flaw detection method for steel pipes, the method of detecting flaws by mode conversion from longitudinal wave L to shear wave S, as shown in Fig. 5, is recommended and generally used.This method, as shown in Fig. 6, Direct transverse wave S
This is because the round-trip passage rate of sound pressure is higher when mode conversion from longitudinal wave L to transverse wave S is performed in order to apply the wave to the inner surface defect of the tube. In fact, in the actual mode conversion method, for steel pipes with small outer diameter, the depth is 0.
．． Internal defects of about 3 m can be detected with good S/'N.

However, when this longitudinal wave → transverse wave S mode conversion method is applied to a steel pipe with an outer diameter of 80 mm or more, the ultrasonic beam path becomes long, and the sound pressure of the reflected ultrasonic wave from a defect with a depth of about 0.3 mm increases. It was found that the S/N ratio was low and it was not possible to detect defects well. and,
No effective flaw detection method has been developed for this problem.

(Means for Solving the Problems) The flaw detection method of the present invention was developed to overcome the current situation, and is an ultrasonic angle flaw detection method for thick-walled steel pipes that directly detects defects on the inner surface of thick-walled steel pipes vertically. This is an ultrasonic flaw detection method for thick-walled steel pipes, which is characterized by propagating a wave L and thereby detecting defects on the axial inner surface of the steel pipe with a good signal-to-noise ratio.

The present invention is particularly suitable for cases where the wall thickness/outer diameter of the steel pipe to be tested is 0, 2 or more,
This is an effective ultrasonic flaw detection method for objects with an outer diameter of 80 m or more.

Figure 1 (a) and (b) are 76.3mφx 20. O
m t (t/b = 0.262) and 168.3 coφ
It is a diagram when internal defect detection is performed by longitudinal wave-transverse wave mode conversion method with longitudinal wave refraction angle θ, = 43' for X44.1 exposure t (t/'D=o, 2s2).

In this case, the sound pressure round trip passage rate is the same for both.

However, if we compare the sound pressure at point C/point where the ultrasonic beam reaches the inner surface in the figure, we can see that since sound pressure is inversely proportional to the square of the distance, if the sound pressure at point 0 is 1, then A-B- The beam path length of A'-B'-C' is 2.2 times that of beam path C, and the sound pressure at 67 points is about 0.2. 0 points and 67
If the same defect exists at a point, in order to detect it in the same way, it is necessary to increase the amplifier sensitivity of the flaw detector by as much as 15 dB for a diameter of 168.3 mm.Generally, 8/N is poor and cannot be detected.

(Example) FIG. 2 is a diagram when flaws were detected using the method of the present invention.

In the method of the present invention, the longitudinal wave refraction angle θ1 is preferably 14 to 20.
This is a method of detecting flaws by applying longitudinal waves directly to defects on the inner surface of steel pipes using a range of degrees. FIG. 2 is an explanatory diagram when the longitudinal wave refraction angle θ1 is set to 16 degrees.

In this case, when comparing the sound pressure round-trip passage rate, it is found that the longitudinal wave-transverse wave mode conversion method has a coefficient of 6.8, while the direct longitudinal wave method has a coefficient of Fill,
8 section and 5.0 section become higher. Also, from the ultrasonic beam path
When comparing the sound pressure at point C and point C, the sound pressure at point 0 is 1
In this case, the sound pressure at point 01 is about 2.8. It can be seen that in the method of the present invention, flaw detection can be performed with a sufficient margin of sensitivity compared to flaw detection using the conventional longitudinal wave-transverse wave mode conversion method.

Experimental results show that even in ultra-thick-walled steel pipes as shown in Table 1, by using this flaw detection method that applies direct longitudinal waves to the inner surface of the steel pipe, defects with an inner surface depth of about 0.3 m can be easily detected with a good S/N ratio. .

Table 1: Ultra-thick-walled steel tube This method of detecting flaws by directly propagating longitudinal waves to the inner surface of the tube is effective for detecting defects on the inner surface of ultra-thick inner pot tubes, and can be said to be an appropriate flaw detection method.

[Brief explanation of the drawing]

FIG. 1(&)#(b) is an explanatory diagram showing the difference in flaw detection between a small outer diameter and a large outer diameter in the longitudinal wave-transverse wave mode conversion method, and FIG. 2 is an explanatory diagram of the present invention. Figure 3 is a graph schematically showing a sound pressure reciprocating vehicle with an incident angle θ4 and a refraction angle θ8 when the first medium is water and the second medium is steel, and Figure 4 is a graph showing the refraction angle θ and t/ An explanatory diagram showing the limits of 1), Figure 5 is t/b) 0
．． FIG. 6 is an explanatory diagram of the longitudinal wave-transverse wave S-mode conversion method used in the steel pipe No. 2, and FIG. 6 is a graph schematically showing the sound pressure round-trip passage rate in the longitudinal wave-transverse wave mode conversion method. □ □ Figure 6 #1f CJA front south θ5 0 10 1520@530 a, 506
0ri0ma support yiho θL

Claims (1)

[Claims] An ultrasonic angle flaw detection method for thick-walled copper pipes, which is characterized in that a longitudinal wave L is propagated directly to the thick-walled steel pipe.