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Repair of S355 LSAW Steel Pipe Transverse Crack

Date:2018-06-07    keywords:S355 LSAW Steel Pipe
In recent years, with the development of the national economy, thick-walled LSAW steel pipe have been increasingly used in offshore platforms, high-temperature and high-pressure vessels, and wind power pipe piles. The production of such steel pipes generally uses D36 steel, S355 steel and structural steel plates with Z-direction properties. The thickness of the plates is large, the carbon content is high, and the carbon equivalent is high. The weldability is poor, and the hardenability tends to be large. The welding performance of the joints is reduced, and cold cracks are easily generated, especially the transverse cracks are more likely to appear. After the occurrence of welding cracks, in addition to the necessary control measures, it is also necessary to pay attention to the timely repair of the workpiece. Taking S355 steel as an example to illustrate the repair measures after the occurrence of transverse cracks in welding. specifically is:
1, Confirming the defect
A 100% UT inspection was performed on all welds and the location, length, depth and direction of the cracks were marked. If the entire weld has defects, it is recommended to shave the entire weld back to submerged arc welding. Local arc welding is recommended for local rework.

2, Preheating before planing
The preheating temperature is 110-170°C, and the preheating temperature detection position is not less than 150mm on both sides of the weld, and the heating range is within 500mm from the periphery of the defect.

3, Gouging
The gouging range is that the two sides of the crack defect are outward and the good weld is not less than 50mm. The outward gouging is carried out at the ends of the gouging. The transition plane and the vertical line are at least greater than 45°. Gouging carbon rod angle should be below 60 °, especially in the cracks to be reached, the angle is as small as possible.

4, Polishing
Polished to no black skin, the surface after grinding should be a smooth transition, there should be no sharp pits.

5, PT
Permeate testing (PT) after cooling to the appropriate temperature.

6, Polished
According to the PT test result, it is polished and polished until there is no red line.

7, MT
After grinding, magnetic particle testing (MT) is performed to confirm that there is no residual crack, otherwise it should be continued until MT detects no crack.

8, Preheating before welding
The recommended preheating temperature is 110-170°C. The preheating temperature detection position is not less than 150mm on both sides of the weld and the heating range is within 500mm from the weld.

9, Welding
According to the repair welding instructions, the width of the weld bead should not exceed 15mm, and the road can be divided. Arc welding or automatic submerged arc welding can be used.

10, After welding insulation, slow cooling

11, Post-weld heat treatment
Post-weld heat treatment is mainly diffusive hydrogen, reducing post-weld residual stress, for the rigidity of the weldment has a special "hydrogen treatment", "relief heat treatment." The recommended heat treatment system is: After the welding is completed, the ceramic electric blanket is used to heat it to 200° C., and the heat is turned off after being kept warm for 2 hours.

12, Post-weld testing
48 hours after the completion of welding, the NDT test was performed as required to confirm the return of the pass.

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