The porosity of the spiral steel pipe weld not only affects the sealing of the pipe weld and causes pipeline leakage, but also becomes a corrosion induction point, seriously reducing the strength and toughness of the weld.
The factors that cause pores in welds include: moisture, dirt, oxide scale and iron filings in the flux, welding components and coverage thickness, steel plate surface quality, steel plate edge plate treatment, welding process and steel pipe forming process, etc.
Flux composition. When welding contains an appropriate amount of CaF2 and SiO2, it will react and absorb a large amount of H2 to generate HF that is highly stable and insoluble in liquid metal, thus preventing the formation of bubbles in the hydrogen holes. Most bubbles appear in the center of the weld. The main reason is that hydrogen is still hidden in the weld metal in the form of bubbles. Therefore, the measure to eliminate this defect is to first remove rust, oil, water and moisture from the welding wire and welding seam. and other substances, followed by the fact that the flux must be sufficiently dry to remove moisture. In addition, increasing the current, reducing the welding speed, and slowing down the solidification speed of the molten metal are also effective.
Spiral welded pipe(2)
The flux accumulation thickness is generally 25-45mm. The accumulation thickness of flux with large particle size and low density should be the maximum value, and the accumulation thickness of flux with small particle size and low density should be the minimum value; when the current is high and the welding speed is low, the accumulation thickness should be the maximum value, and vice versa. In addition, in summer or when the air humidity is high, the recycled flux should be dried before use. Sulfur cracking (cracking caused by sulfur). When welding plates with strong sulfur segregation zones (especially soft boiling steel), cracks are caused by sulfides in the sulfur segregation zones entering the weld metal. The reason is the presence of hydrogen in iron sulfide and steel with low melting points in the sulfur segregation zone. Therefore, in order to prevent this from happening, it is also effective to use semi-killed steel or killed steel with fewer sulfur-containing segregation areas. Secondly, the weld surface and flux must be cleaned and dried.
Steel plate edge treatment. Rust and deburring devices should be installed on the edges of steel plates to reduce the possibility of pores. The cleaning device is best installed behind the edge milling machine and disc cutting machine. The structure of the device is two active wire wheels with an adjustable gap on one side that press up and down the edge of the plate. Involves welding slag. Slag entrainment is the portion of welding slag that remains in the weld metal.
Welding form. The weld forming coefficient is too small, the weld shape is narrow and deep, gas and inclusions are not easy to float out, and pores and slag inclusions are easily formed. The weld forming coefficient is generally controlled at 1.3-1.5, with the maximum value for thick-walled spiral steel pipes and the minimum value for thin-walled spiral steel pipes. Poor penetration. There is insufficient overlap of internal and external weld metal and sometimes incomplete penetration. This condition is called insufficient penetration.
Reduce the secondary magnetic field. In order to reduce the influence of magnetic deflection, the connection position of the welding cable and the workpiece should be as far away from the welding terminal as possible to avoid the secondary magnetic field generated by part of the welding cable on the workpiece. weaken. An undercut is a V-shaped groove at the edge of the weld along the centerline of the weld. Undercutting occurs when conditions such as welding speed, current and voltage are inappropriate. Among them, if the welding speed is too high, it is more likely to cause undercut defects than inappropriate current.
Craftsmanship. The welding speed should be appropriately reduced or the current should be increased to slow down the crystallization speed of the weld pool metal and allow gas to escape. The bridge is still intact and the gas has difficulty escaping.
