How to avoid bubbles in welded carbon steel pipes?

It is common for welded carbon steel pipes to develop air bubbles in the weld, particularly in large-diameter carbon seamless steel pipes. These weld pores can compromise the tightness of the pipeline, leading to leakage, and act as initiation points for corrosion, significantly reducing the weld's strength and toughness. Various factors contribute to the formation of weld porosity, including moisture, dirt, oxide scale, and iron filings in the flux, welding components, and covering thickness, surface quality of the steel plate, treatment of the steel plate edge, welding process, steel pipe forming process, and flux composition. The presence of appropriate amounts of CaF2 and SiO2 in the flux can react and absorb a large amount of H2, forming HF, which is highly stable and insoluble in liquid metal, thus preventing the formation of hydrogen pores.

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Bubbles typically form in the center of the weld bead due to hydrogen remaining within the weld metal as bubbles. To eliminate this defect, it is crucial to first remove rust, oil, moisture, and other contaminants from the welding wire and weld area. Additionally, the flux must be thoroughly dried to remove moisture. Increasing the welding current, reducing the welding speed, and slowing down the solidification rate of the molten metal are also effective measures.

The thickness of the flux layer should generally be 25-45mm. The maximum thickness is used for flux with large particle size and low density, while the minimum thickness is used for flux with small particle size and high density. High welding current and low welding speed require thicker flux layers, while the opposite conditions require thinner layers. In high humidity conditions, recovered flux should be dried before use to prevent moisture-related defects.

Sulfur cracking, caused by sulfides in sulfur segregation bands entering the weld metal, can occur when welding plates with strong sulfur segregation bands, particularly in soft-boiling steel. This type of cracking is due to the presence of hydrogen in iron sulfide and low melting point steel in the sulfur segregation zone. To prevent sulfur cracking, it is effective to use semi-killed steel or killed steel with fewer sulfur-containing segregation bands.