How to Prevent Porosity in the Weld Seam of Large-Diameter Spiral Pipes

Porosity in the weld seam of large-diameter spiral pipes not only affects the weld tightness, leading to potential leaks, but also serves as a corrosion initiation point, significantly reducing the weld strength and toughness. Factors contributing to porosity include moisture in the flux, contaminants, oxidation, iron filings, welding composition, flux coverage, steel plate surface quality, edge preparation, welding technique, and pipe forming technology.

Preventative Measures:

1. Reduce Secondary Magnetic Fields:** To minimize magnetic blow, ensure the welding cable connection point on the workpiece is as far away from the welding terminal as possible, avoiding secondary magnetic fields.

2. Technique: Adjust welding speed and current to control the crystallization rate of the weld pool, facilitating gas escape. Ensure stable strip steel feed position, avoiding frequent micro-adjustments that hinder gas release.

3. Flux Composition: Use flux with appropriate amounts of CaF2 and SiO2 to absorb hydrogen, preventing the formation of hydrogen pores.

4. Flux Deposition Thickness: Generally, the flux deposition should be 25-45mm. Adjust thickness based on grain size and density—larger grains and lower density require thicker deposition. In humid conditions, dry recovered flux before use.

5. Steel Plate Surface Treatment: Install equipment to clean the plate surface to prevent debris, such as iron oxide scale, from entering the forming process.

6. Steel Plate Edge Treatment: Install equipment to remove rust and burrs from the steel plate edges, ideally positioned after the milling machine and disc shears, with adjustable steel wire wheels to remove contaminants.

7. Weld Profile: A small weld shaping factor results in a narrow and deep weld shape, making it difficult for gases and inclusions to escape, leading to porosity and slag inclusion. The weld shaping factor should generally be controlled between 1.3-1.5, with thicker wall pipes at the upper limit and thinner ones at the lower limit.