Ultrasonic flaw detection method for inner wall defects of ultra thick-walled steel pipes

This paper introduces an ultrasonic flaw detection method for longitudinal inner wall defects of ultra-thick-walled steel pipes--the modified shear wave oblique projection method. The incident angle of the probe is selected within the range less than the first critical angle, and the modified shear wave generated by the oblique projection of the refracted longitudinal wave on the outer wall of the steel pipe is used to detect the longitudinal inner wall defects of the steel pipe. The design of the probe incident angle, the adjustment of the flaw detection sensitivity, and the waveform judgment, when the ultra-thick-walled steel pipe is flawed by this method, are described, and the effectiveness of this method is verified by the flaw detection example of the Ф121mm×36mm specification steel pipe.

Introduction

In the ultrasonic flaw detection of steel pipes, steel pipes with a wall-to-diameter ratio of t/D∧0.2 are usually called ultra-thick-walled steel pipes. For the flaw detection of such steel pipes, if the conventional shear wave reflection method is used, it is no longer possible to detect its longitudinal inner wall defects, and a special method is required for detection. There have been cases where a large number of longitudinal inner wall defects were found in the use of ultra-thick wall drill collar pipes that had not been inspected, resulting in project delays and huge economic losses. Therefore, it is urgent to develop a method for detecting longitudinal inner wall defects of ultra-thick wall steel pipes. This study uses a large number of tests and analyses on the longitudinal inner wall defects of ultra-thick wall steel pipes by ultrasonic flaw detection and uses modified shear waves to detect its longitudinal inner wall defects so that the detection range of the steel pipe wall thickness is increased. After practical verification, good results have been achieved.

1. Principle of shear wave reflection method for steel pipe flaw detection

The shear wave reflection method for steel pipe flaw detection is that when the ultrasonic wave is obliquely incident on the surface of the steel pipe, refraction, and wave type conversion are generated at the interface between the organic glass and the steel pipe, and the directional relationship between the refracted wave and the incident wave conforms to Snell's law. When the incident angle α is selected between the first critical angle αⅠ and the second critical angle αⅡ, only a single shear wave is generated in the steel pipe, thereby realizing the simultaneous detection of defects on the inner and outer walls of the steel pipe. The selection of the incident angle must meet the following two conditions: ① After the sound beam is incident, only refracted shear waves are generated in the steel pipe (that is, αⅠ≤α<αⅡ is required); ② The refracted shear wave sound beam can scan the inner wall of the steel pipe. Therefore, when a single shear wave is used to detect a steel pipe, the ratio of the wall thickness to the outer diameter of the steel pipe should at least meet t/D≤0.2, that is, the refracted shear wave should at least be tangent to the inner wall of the steel pipe to ensure simultaneous detection of defects on the inner and outer walls of the steel pipe. Then, when inspecting ultra-thick-walled steel pipes with t/D∧0.2, if the incident angle is selected between αⅠ and αⅡ, the refracted shear wave in the steel pipe will not be able to detect defects on the inner wall.

2. Ultra-thick wall steel pipe flaw detection method

Analysis of the reflection, refraction, and wave type conversion phenomenon, when the ultrasonic wave is incident obliquely, shows that when the incident angle is less than αⅠ, the ultrasonic wave in the steel pipe is a refracted longitudinal wave and a refracted transverse wave. The refracted longitudinal wave undergoes wave-type conversion on the outer wall of the steel pipe, generating a reflected transverse wave (i.e., a deformed transverse wave) that is projected onto the inner wall of the steel pipe, thereby detecting the inner wall defects of the ultra-thick wall steel pipe (the deformed transverse wave can be made tangent to or intersect with the inner wall of the steel pipe by changing the incident angle of the probe). It can be seen from the acoustic pressure reciprocating transmittance of ultrasonic waves obliquely incident on the plexiglass/steel interface that when the incident angle α is less than the first critical angle (27.6°), the acoustic pressure reciprocating transmittance TLS of the incident wave converted to the refracted shear wave is very low, with a maximum of less than 10%, that is, the intensity of the refracted shear wave transmitted into the steel pipe is very weak, and the effect of detecting the inner wall defects of ultra-thick-walled steel pipes is extremely poor; while the acoustic pressure reciprocating transmittance TLL of the incident wave converted to the refracted longitudinal wave is relatively high, with a maximum of about 25%, which means that most of the energy in the refraction process exists in the refracted longitudinal wave, and the modified shear wave generated by the refracted longitudinal wave after reflection on the pipe wall also has high energy, so the flaw detection sensitivity of the inner wall defects is significantly higher than that of the refracted shear wave. Using the modified shear wave oblique method to detect the inner wall defects of ultra-thick-walled steel pipes is a relatively ideal method, which has been fully proved in practice.

2.1 Determination of the incident angle of the probe

The most fundamental prerequisite for ultrasonic flaw detection is to determine the basic parameters of the probe, especially the incident angle of the probe, which is crucial to ensure the accuracy of the flaw detection results. As mentioned above, to detect the inner wall defects of ultra-thick-walled steel pipes, the incident angle of the probe must be reduced, and the incident angle should be selected within the range of less than the first critical angle so that the deformed shear wave is at least tangent to the inner wall of the steel pipe. At this time, the intensity of the refracted shear wave is very weak, and most of the energy is concentrated in the refracted longitudinal wave.

2.2 Selection of Flaw Detection Frequency

Usually, low-frequency flaw detection is used to detect coarse-grained materials or steel pipes; fine-grained materials or steel pipes can use higher flaw detection frequencies. According to the requirements of the GB/T5777-2008 standard, the probe frequency can be selected between 1 and 15MHz. In actual detection, the situation of the ultra-thick-walled steel pipe to be detected should be comprehensively considered, and a 2.5MHz probe can be selected.

2.3 Selection of Standard Artificial Defects

To select a standard artificial defect suitable for ultra-thick wall steel pipe flaw detection and determine its flaw detection sensitivity, a sample was taken from the steel pipe to be detected with a size of Ф121mm×36mm and processed into two shapes of standard reflectors, namely, transverse holes and rectangular grooves. Longitudinal standard artificial defects were processed on the sample according to the requirements.

Two different forms and sizes of artificial defects were tested using a probe with an incident angle of 18°. The instrument used was a CTS-23B analog ultrasonic flaw detector, and the benchmark sensitivity was 80% of the full amplitude of the oscilloscope screen. Table 2 is a comparison of the flaw detection sensitivity of two different artificial defects.

3. Waveform position analysis

The waveform display of the modified shear wave oblique projection method for detecting ultra-thick wall steel pipes is different from that of the conventional shear wave reflection method. The biggest feature of its waveform display is that the external injury is in front and the internal injury is in the back (front and back are relative to the initial pulse, close to the initial pulse is in front, and far from the initial pulse is in the back), while the waveform display of the conventional shear wave reflection method is that the internal injury is in front and the external injury is in the back. In actual detection, to make the modified shear wave propagate in a zigzag shape in the pipe wall, the incident angle should usually be slightly smaller than the maximum allowable incident angle so that the modified shear wave intersects with the inner wall. For example, the maximum incident angle of the probe allowed for detecting Φ121mm×36mm steel pipes is 20.02°, and the incident angle of the probe can be selected as 18° in actual detection. The following takes the detection of Φ121 mm×36 mm ultra-thick wall steel pipes with an incident angle of 18° as an example to analyze the position of the inner and outer wall defect waves on the oscilloscope screen when using the modified shear wave oblique projection method for detection.

4. Actual detection

4.1 Adjusting the flaw detection sensitivity

Adjust the instrument depth range knob and horizontal shift knob, adjust the initial wave to the 0 grid position of the horizontal scale of the oscilloscope screen, and adjust the through wave (double-transmit and double-receive detection method) to the 10 grid position of the horizontal scale of the oscilloscope screen. Place the probe on the sample tube with a standard reflector (rectangular groove), align the main sound beam of the probe with the main reflection surface of the groove, move the probe to find the highest reflected echo of the defect, adjust it to 80% full-screen height, record the position and amplitude, and the reading of the attenuator on the flaw detector at this time is the initial flaw detection sensitivity. The inner and outer wall defects should be adjusted accordingly according to the above method. During normal flaw detection, to facilitate defect detection, 6 to 10 dB can be increased as the search sensitivity based on the initial sensitivity. After the defect is found, the attenuator is turned back to the reading position of the initial sensitivity for comparison. After adjusting the flaw detection sensitivity, the steel pipe is scanned circumferentially. Although there are both refracted longitudinal waves and refracted transverse waves in the steel pipe at this incident angle, as analyzed in the previous article, the intensity of the refracted transverse wave is very weak, so the deformed transverse wave generated by the refracted longitudinal wave obliquely incident on the outer wall of the steel pipe is used to detect the inner wall defects of the steel pipe. Since the positions of the inner and outer wall defects are different, the corresponding sound paths are also different, so the positions of the defect waves on the baseline of the oscilloscope screen are also different. According to the position of the defect wave on the oscilloscope screen, the operator can easily make a correct judgment on the inner and outer wall defects.

4.2 Determine the flaw detection coverage

After the probe and the standard sample are determined, first adjust the flaw detection sensitivity, and then determine the coverage of the flaw detection steel pipe. It is necessary to ensure a full scan of 110% of the circumference of the steel pipe to detect batches of steel pipes. Due to the thick wall thickness of the steel pipe, the attenuation of the ultrasonic wave increases when it propagates in the steel pipe, and the span of the zigzag propagation is large, resulting in a large zigzag leakage point surface in the circumferential direction of the steel pipe. To ensure that the ultrasonic wave scans 110% of the defects in the steel pipe, the range of movement of the probe along the circumferential direction of the steel pipe should be increased accordingly during the flaw detection, and the entire circumference should be divided into several detection surfaces according to the size of the outer diameter of the steel pipe. According to experience, at least 3 circumferential surfaces should be detected to ensure that all longitudinal defects on the entire circumference of the entire steel pipe are detected. In actual detection, before each scan, the 1/3 circumference of the pipe end should be marked, the scanning range of each scan is 1/3 of the circumference of the steel pipe, and each scan should have a 10% coverage.

5. Application effect

In the actual flaw detection of 45MnCrMo steel drill collar pipe with a specification of Ф121mm×36mm, the scrap rate of longitudinal inner wall defects is high. The samples were taken for physical and chemical inspection and analysis, and the results showed that the inclusion defects exceeded the standard, which confirmed the effectiveness of this flaw detection method. At the same time, the defect analysis results also provide a theoretical basis for improving the production process. By taking effective measures in production, the generation of batch scraps is avoided. The local morphology of the B coarse inclusions near the inner wall of the steel pipe after magnification 100 times.

6. Conclusion

(1) For ultra-thick-walled steel pipes with t/D∧0.2, the modified shear wave oblique projection method can well detect the longitudinal inner wall defects in the steel pipe. The probe incident angle can be designed by the method of detecting the inner wall with refracted shear waves, but the propagation path of ultrasonic waves in ultra-thick-walled steel pipes must be clear. The key lies in waveform recognition to make accurate judgments on the inner and outer wall defects.

(2) Since the sawtooth leakage point of the shear wave beam is large when it propagates in the steel pipe, to avoid missed detection, multiple scans should be performed on the entire circumference during flaw detection.

(3) Practice has proved that the modified shear wave oblique projection method is very effective in detecting the longitudinal inner wall defects of ultra-thick-walled steel pipes. The instrument is easy to adjust and simple to operate. The defect wave has good repeatability and stability, and there is no obvious noise influence. It can meet the needs of on-site flaw detection.