Prevention and causes of undercuts in submerged arc welded steel pipes

Causes and preventive measures for the formation of single one-sided undercuts: In the production process of submerged arc welded steel pipes, especially in high-speed welding, undercuts have become one of the main defects in the production process of spiral submerged arc welded steel pipes. The undercut defect mainly has the following surface forms: single one-sided undercut, single two-sided undercut, one-sided continuous undercut, and two-sided continuous undercut. Among them, one-sided continuous undercut and two-sided continuous undercut are harmful to the weld. They are fatal defects of welds like weld cracks and incomplete penetration. Therefore, we should attach great importance to taking measures to prevent the occurrence of such defects.

1. Causes of formation

Generally speaking, the formation of single one-sided undercuts is sporadic and irregular. Excessive changes in the forming seam, small burrs or small notches on the edge of the strip, and forming misalignment may form single undercuts. For single one-sided undercuts within the permitted range of the standard, the undercuts can be eliminated by grinding, and for undercuts beyond the standard range, they can be eliminated by repairing.

2. Preventive measures

Due to the uncertainty of such defects, we do not need to make major adjustments. We can make the edges of the strip as smooth as possible and keep the forming stable if conditions permit. For the problem of smooth edges of the strip, the edge milling machine can be used instead of the disc shear to solve the problem. For the problem of stable forming, the adjustment method can be adopted to solve it.

Causes and preventive measures for the formation of single double-sided bite edge

1. Causes

① Wire joint: Due to the change in diameter and smoothness at the wire joint, the wire feeding speed of the wire joint will suddenly change when passing through the wire feeding wheel, which will cause instantaneous changes in welding voltage and melting speed. The sudden widening of the welding pool and insufficient replenishment of molten metal can cause a single double-sided bite edge at this weld.

② Welding specifications: Under normal circumstances, the welding specifications will not change significantly during continuous production. Therefore, there will be no bite edge in normal production. However, under the influence of an external power supply, the welding current and voltage may also change suddenly, and the result of the sudden change will eventually lead to the formation of a biting edge.

③ Momentary short circuit: Sometimes due to burrs on the edge of the plate or metal burrs mixed in the flux, a momentary short circuit will occur at the conductive nozzle during normal welding. The momentary short circuit will cause instantaneous changes in welding current and voltage, and eventually lead to the generation of undercuts. The treatment of a single double undercut is the same as that of a single undercut, which can be treated by grinding or repairing.

2. Preventive measures

① Treat the welding wire joint well: For welding wire manufacturers, the joint should be heat treated to keep its hardness consistent with the hardness of the entire coil of welding wire. When butt welding wires in the continuous production process of submerged arc welding steel pipes, the joint should be ground after the butt welding is completed to achieve a smooth transition and uniform true diameter, and the front and rear welding wires at the joint should be roasted with an oxyacetylene gun and allowed to cool naturally to achieve the purpose of uniform hardness.

② Strictly control welding specifications: Pay attention to the changes in current and voltage during welding. If the changes are too large, find the reasons for the power supply and the power supply outside the system to control the changes in specifications within the allowable range.