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Ultrasonic Testing of Transverse Cracks in Welds of LSAW Steel Pipe

Date:2018-07-04    keywords:ultrasonic testing, transverse cracks
Transverse crack in the weld of thick-walled lsaw steel pipe
D36, D36-Z35 and S355 structural steel plates with thickness of 20~50mm are used respectively to produce marine experience exchanges through the JCOE forming method. The lsaw steel pipe of the 54.HAN GUAN welded pipe platform are 508~1372mm in diameter. The weld seam adopts X-shaped groove, the inner weld is three-wire submerged arc welding, and the outer welding is multi-layer multi-channel three-wire submerged arc welding. Ultrasonic testing of the weld was carried out after 24h or 48h of welding. Lateral scanning revealed that there were forest-like echoes in the middle of the weld wall thickness. Further testing found that sometimes the entire weld was intermittently with similar signals, and the depth was in the weld. centre position. In order to find out the nature of the defect, X-ray filming was performed on some of the places where the reflected signal was the highest, and no defects were found. Magnetic particle inspection was performed again after carbon arc gouging, and it was confirmed that the defect was a transverse crack.

Ultrasonic testing method for transverse crack defects in welds
The transverse crack is more harmful, and the ultrasonic detection is more difficult than other defects. This is mainly because the ultrasonic detection is difficult to distinguish the nature of the defect. In order to improve the detection rate of transverse cracks and accurately determine the transverse cracks, combined with the standard regulations of AWS D1.1/D1.1M and API RP 2X, the scanning methods of ultrasonic waves for transverse cracks are summarized. Practice has proved that these methods are effective for detecting and judging transverse cracks in thick-walled welds.

Ultrasonic discrimination of transverse crack
When using the above "parallel scan with high weld seam" method to detect the transverse defects of the weld, if a reflection signal is found, the weld height of the weld can be smoothed, and the defect plane, depth position, and further Defect height, defect reflection characteristics, defect orientation, defect waveform, dynamic waveform, echo envelope and other parameters, combined with groove form, material properties, welding process and welding method, etc. The actual nature of the decision is made and the location of the defect is determined. Then use the gouging to grind the anatomy and then verify the magnetic particle test. During the detection process, the steps for discriminating transverse cracks are as follows:
(1) Firstly, in the lateral scanning sensitivity (this time is 6-12 dB higher than the longitudinal scanning sensitivity), the probe is placed on the weld directly for parallel scanning;
(2) After the horizontal defects are found, select the echo position with higher amplitude from 1 to 2, and analyze the weld height after the weld is leveled. The reflection amplitude is lower than 20% of the DAC display, in principle, it is not fixed;
(3) Using a surround scan to determine whether the defect is a point defect, and if it is a point defect, it is not further characterized;
(4) Observe the static waveform and the dynamic waveform, and analyze the amplitude variation of the defect signal to determine whether it is a transverse crack. There are four basic ways of moving the probe, namely, left and right movement, back and forth movement, fixed point rotation and surround motion. The corresponding dynamic waveform also has four forms, as shown in Figure 5. Or the gouging is repaired to the depth of the defect and verified by magnetic particle inspection as a transverse crack. When the number of defects is small, the repair process can be directly performed without verification.

Transverse crack treatment
Once the defect is determined to be a transverse crack, a repair process must be performed. The transverse crack should be determined by the depth and width of the gouging and grinding before rework. To ensure complete rework, the depth determined by the ultrasonic inspection should be increased by a few millimeters. In addition, after the gouging and grinding, the magnetic powder inspection must be carried out before the repair welding repair. It should be noted that the sidewall groove is also in the detection range to ensure that the defects are completely removed, and then welded by the repair welding personnel, otherwise it is difficult to ensure the crack. Complete eradication. Low-hydrogen welding wire (strip) should be used for rework. It must be preheated before rework. After gouging, clean and repair the carbon deposit to expose the metallic luster. After rework, take necessary slow-cooling protection measures, such as welding with asbestos after welding. Slowly sew. In addition, for the re-repair inspection of transverse cracks, it must be strictly checked at 24h or 48h after welding.

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