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Common Defects in Welding Zone of Spiral Submerged Arc Welded Pipe
The defects in submerged arc welding zone are gas hole, hot crack and undercut.
1. Bubbles. Bubbles mostly occur in the center of the weld bead. The main reason is that hydrogen is still hidden in the weld metal in the form of bubbles. Therefore, the measures to eliminate this defect are first to remove rust, oil, moisture and moisture from the wire and weld, and secondly to dry the flux well to remove moisture. In addition, it is also effective to increase the current, reduce the welding speed and slow down the solidification speed of molten metal.
2. Sulfur cracking (crack caused by sulfur). Cracks occur when sulphur segregation zone enters weld metal in plates with strong sulphur segregation zone (especially soft boiling steel). The reason is that hydrogen exists in iron sulfide and steel with low melting point in sulphur segregation zone. Therefore, in order to prevent this situation, the use of semi-killed steel or killed steel with less sulphur segregation band is also effective. Secondly, it is necessary to clean and dry the weld surface and flux.
3. Hot crack. In submerged arc welding, hot cracks can occur in weld bead, especially in arc initiation and arc extinguishing pits. In order to eliminate this kind of crack, the cushion plate is usually installed at the starting and extinguishing arcs. At the end of the coil butt welding, the spiral welded pipe can be reversed and welded into overlay welding. Hot cracks are most likely to occur when the stress in the weld is very high or when the silicon in the weld metal is very high.
4. Involvement of welding slag. Involving welding slag is the residual part of welding slag in weld metal.
5. Poor penetration. The metal overlap between internal and external welds is insufficient, sometimes not penetrated. This is called insufficient penetration.
6. Bite edge. The undercut is a V-shaped groove along the center line of the weld seam at the edge of the weld. Edge undercutting occurs when welding speed, current, voltage and other conditions are inappropriate. Too high welding speed is more likely to cause undercutting defects than unsuitable current.
