Prevention and causes of undercutting of submerged arc welded steel pipes

Causes and preventive measures of a single unilateral undercut: In the production process of submerged arc welded steel pipes, especially in high-speed welding, the undercut has become a major defect in the production process of spiral submerged arc welded steel pipes. The defect of undercut mainly has the following surface forms: single unilateral undercut, single bilateral undercut, unilateral continuous undercut, and bilateral continuous undercut. Among them, unilateral continuous undercut and bilateral continuous undercut are opposite. Welding seams are very harmful, and they become fatal defects of welding seams like welding 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 a single unilateral undercut is sporadic and has no regularity. Excessive changes in the forming seam, small burrs or small gaps on the edge of the strip, and misformed edges may form a single undercut. For a single single-sided undercut that is within the scope of the standard, the undercut can be eliminated by grinding, and for the undercut that exceeds the standard range, it can be eliminated by repair.

2. Preventive measures: Due to the uncertainty caused by such defects, we do not need to make major adjustments. We can make the edge of the strip as smooth as possible and maintain stable molding if conditions permit. For the problem of smooth edges of the strip steel, an edge milling machine can be used instead of disc shears to solve the problem. For the problem of forming stability, it can be solved by improving the shape adjustment.

Causes and preventive measures of a single bilateral undercut:

1. Causes of formation:
①Welding wire joint: Due to changes in the diameter and smoothness of the welding wire joint, the wire feeding speed of the welding wire joint will suddenly change when passing through the wire feed wheel, which will cause instantaneous changes in the welding voltage and melting speed and a sudden change in the welding molten pool. The widening and insufficient replenishment of molten metal can result in a single double-sided undercut at this solder joint.
②Welding specifications: Under normal circumstances, the welding specifications will not change significantly during the continuous production process. Therefore, there will be no undercuts during the normal production process. However, under the influence of an external power supply, the welding current and voltage may also undergo sudden changes, and the sudden changes will eventually lead to the generation of undercuts.
③Instant short circuit: Sometimes due to burrs on the edge of the board or metal burrs mixed in the flux, an instant short circuit will occur at the contact tip during normal welding. The instant short circuit will cause instantaneous changes in the welding current and voltage, eventually leading to undercuts. The treatment of a single double undercut is the same as the treatment of a single single undercut and can be processed by grinding or repairing.

2. Preventive measures:
① Handle the welding wire joints: For welding wire manufacturers, the joints should be heat treated so that their hardness is consistent with that of the entire welding wire. When butt-welding wires in the continuous production process of submerged arc welded steel pipes, the joints should be ground after the butt-joining is completed to achieve a smooth transition and uniform true diameter, and an oxyacetylene gun should be used to bake the front and rear welding wires at the joints and allow them to cool naturally. , to achieve the purpose of consistent hardness.
② Strictly control the welding specifications: Pay attention to the changes in current and voltage during the welding process. If the changes are too large, find the cause of the power supply and the power supply outside the system to control the changes in specifications within the allowable range.