Why does undercut frequently occur in submerged arc welded steel pipes during production

Undercutting is a common defect in the production of submerged arc welded steel pipes, especially during high-speed welding. It can easily occur if not carefully monitored. Undercutting has become one of the main defects in the production of spiral submerged arc welded steel pipes. This defect mainly manifests in the following surface forms: single-sided undercutting, continuous undercutting on one side, and continuous undercutting on both sides. Among these, continuous undercutting on one side and continuous undercutting on both sides are the most detrimental to the weld, becoming as fatal as weld cracks and incomplete penetration. So, how can we prevent undercutting in submerged arc welded steel pipes?

First, the causes and preventive measures for undercut on a single side of a submerged arc welded steel pipe:

1. Causes of undercut on a single side of a submerged arc welded steel pipe: Generally, undercut on a single side occurs sporadically and without a regular pattern. Excessive variations in the forming seam, small burrs or notches on the strip edge, and misalignment during forming can all lead to an undercut. For an undercut within the permissible range of the standard, it can be eliminated by grinding. For an undercut exceeding the standard range, it can be eliminated by repair.

2. Preventive measures for undercut on a single side of a submerged arc welded steel pipe: Due to the uncertainty of this defect, we do not need to make major adjustments. Where conditions permit, we can smooth the strip edge as much as possible and maintain forming stability. For the problem of smoothing the strip edge, a milling machine can be used instead of a disc shear to trim the edge. For the problem of forming stability, the support of the forming tool can be increased.

Second, Causes and Prevention Measures of Undercut in Single Double-Sided Submerged Arc Welded Steel Pipes

1. Causes of Undercut in Single Double-Sided Submerged Arc Welded Steel Pipes

① Wire Joint: Due to changes in the diameter and smoothness of the wire joint, the wire feeding speed changes suddenly when the wire joint passes through the wire feed roller. This causes instantaneous changes in welding voltage and melting speed, resulting in a sudden widening of the weld pool and insufficient replenishment of molten metal, which can lead to a single double-sided undercut at this weld point.

② Welding Specifications: Under normal circumstances, welding specifications do not change significantly during continuous production; therefore, undercut will not occur during normal production. However, under the influence of external power sources, welding current and voltage may change abruptly, ultimately leading to an undercut.

③ Instantaneous Short Circuit: Sometimes, due to burrs on the plate edge or metal burrs mixed in the flux, an instantaneous short circuit may occur at the contact tip during normal welding. This instantaneous short circuit causes instantaneous changes in welding current and voltage, ultimately leading to an undercut. The treatment of a single double undercut is the same as that of a single single undercut; it can be handled by grinding or repair.

2. Preventive measures for a single double-sided submerged arc welded steel pipe undercut

① Properly handle the welding wire joint: For welding wire manufacturers, the joint should be heat-treated to ensure that its hardness is consistent throughout the entire coil of welding wire. During the continuous production of submerged arc welded steel pipes, when butt welding wires, the joint should be ground after butt welding to achieve a smooth transition and uniform diameter. The welding wires before and after the joint should be heated with an oxy-acetylene torch and allowed to cool naturally to achieve consistent hardness.

② Strictly control welding specifications: During welding, pay attention to changes in current and voltage. If the changes are too large, investigate the cause in the power supply and external power sources to ensure that the changes in specifications are kept within the allowable range.