The defects that are easy to occur in the welding area of the spiral steel pipe include air holes, thermal cracks, and undercuts.
Spiral steel pipe weld porosity not only affects the tightness of the pipe weld, causing pipeline leakage but also becomes a corrosion induction point, seriously reducing the strength and toughness of the weld. The factors that cause pores in the weld are moisture, dirt, oxide scale, and iron filings in the flux, welding composition and coverage thickness, surface quality of the steel plate, edge plate treatment of the steel plate, welding process, and steel pipe forming process, etc.
Flux composition. When the welding contains an appropriate amount of CaF2 and SiO2, it will react and absorb a large amount of H2 to generate HF with high stability and is insoluble in liquid metal, thereby preventing the formation of hydrogen pores.
bubble. Most of the bubbles occur in the center of the weld bead. The main reason is that hydrogen is still hidden in the welded metal in the form of bubbles. Therefore, the measure to eliminate this defect is to first remove the rust, oil, water, and moisture of the welding wire and the weld. and other substances, followed by the fact that the flux must be dried well to remove moisture. In addition, it is also effective to increase the current, reducing the welding speed, and slow down the solidification rate of the molten metal.
The accumulation thickness of flux is generally 25-45mm. The flux particle size is large and the density is small, and the accumulation thickness is taken as the minimum value; the recovered flux should be dried before use. Sulfur cracking (cracks caused by sulfur). When welding plates with a strong sulfur segregation zone (especially soft boiling steel), the sulfide in the sulfur segregation zone enters the weld metal and produces cracks. The reason is that iron sulfide with a low melting point in the sulfur segregation zone and the presence of hydrogen in the steel. Therefore, to prevent this situation, it is also effective to use semi-killed steel or killed steel with less sulfur-containing segregation zone. Secondly, it is also necessary to clean and dry the weld surface and flux.
Steel surface treatment. To prevent sundries such as iron oxide scale falling off from uncoiling and leveling from entering the forming process, a plate surface cleaning device should be installed. hot crack. In submerged arc welding, thermal cracks can occur in the weld bead, especially at the arc start and arc extinguishing craters. To eliminate such cracks, backing plates are usually installed at the arc starting and extinguishing points, and at the end of the coil butt welding, the spiral steel pipe can be reversed to weld into the stitch welding. Hot cracks are most likely to occur when the weld stress is high, or when the si in the weld metal is high.
Steel plate edge treatment. Rust and burr removal devices should be installed on the edge of the steel plate to reduce the possibility of air holes. The position of cleaning device is installed behind the edge milling machine and the disc cutter. The structure of the device is two active wire wheels with adjustable gaps on one side, which press the edge of the plate up and down. Involvement of welding slag. Weld slag entrainment is the residual part of welding slag in the weld metal.
Weld morphology. The forming coefficient of the weld seam is too small, the shape of the weld seam is narrow and deep, the gas and inclusions are not easy to float out, and it is easy to form pores and slag inclusions. Generally, the weld forming coefficient is controlled at 1.3-1.5, the value is taken for thick-walled spiral steel pipes, and the minimum value is taken for thin-walled steel pipes. Poor penetration. Insufficient overlap of inner and outer weld metal, sometimes incomplete penetration. This condition is called insufficient penetration.
Reduce the secondary magnetic field. To reduce the influence of magnetic deflection, the connection position of the welding cable on the workpiece should be as far away as possible from the welding terminal, to avoid the secondary magnetic field generated by part of the welding cable on the workpiece. Undercut. Undercutting is the appearance of a V-shaped groove along the centerline of the weld at the edge 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 if the current is not suitable.
Craftsmanship. The welding speed should be appropriately reduced or the current should be increased to delay the crystallization speed of the weld pool metal so that the gas can escape. The bridge remains in shape, making gas escape difficult.
Post time: Mar-22-2023