Common defects in the weld zone of spiral welded steel pipes include porosity, hot cracking, and undercut. Porosity in the weld seam of spiral welded steel pipes not only affects the tightness of the weld seam, causing pipe leaks, but also becomes a corrosion initiation point, severely reducing the weld seam strength and toughness.
Factors contributing to porosity in the weld seam of spiral welded steel pipes include: moisture, contaminants, oxide scale, and iron filings in the flux; the composition and coating thickness of the weld; the surface quality of the steel plate and the edge treatment of the steel plate; the welding process; and the steel pipe forming process.
The flux composition of spiral welded steel pipes. When welding with appropriate amounts of CaF2 and SiO2, a large amount of H2 will be absorbed, generating highly stable HF that is insoluble in liquid metal, thus preventing the formation of hydrogen porosity.
Air bubbles in spiral welded steel pipes. Bubbles often occur in the center of the weld bead. The main reason is that hydrogen gas remains trapped inside the weld metal in the form of bubbles. Therefore, to eliminate this defect, it is essential to first remove rust, oil, moisture, and other substances from the welding wire and weld. Secondly, the flux must be thoroughly dried to remove moisture. In addition, increasing the current, decreasing the welding speed, and slowing down the solidification rate of the molten metal are also very effective.
The flux build-up thickness for spiral welded steel pipes is generally 25-45mm. The thickness should be adjusted based on the flux particle size and density; larger particles and lower density require a higher thickness, and vice versa. Higher current and lower welding speeds require a higher thickness, and vice versa. Furthermore, in summer or when the air humidity is high, the recycled flux should be dried before reuse. Sulfur cracking (cracks caused by sulfur) occurs when welding plates with strong sulfur segregation bands (especially soft-boiling steel), causing sulfides from the sulfur segregation bands to enter the weld metal. This is because the sulfur segregation bands contain low-melting-point iron sulfides, and hydrogen gas is present in the steel. Therefore, to prevent this situation, using semi-killed or killed steel with less sulfur segregation is also effective. Secondly, cleaning and drying of the weld surface and flux are also essential.
Surface treatment of spiral welded pipe plates. To prevent iron oxide scale and other debris from falling off during uncoiling and leveling from entering the forming process, a plate cleaning device should be installed. Hot cracking. In submerged arc welding, hot cracks can occur within the weld bead, especially at the arc initiation and extinguishing craters. To eliminate these cracks, backing plates are usually installed at the arc initiation and extinguishing points, and at the end of the plate-coil welding, the spiral welded pipe can be reversed to weld into the lap weld. Hot cracking is most likely to occur when the weld stress is high or when the Si content in the weld metal is high.
Edge treatment of spiral welded pipe plates. Rust and burr removal devices should be installed on the edges of the steel plates to reduce the possibility of porosity. The cleaning device is installed after the milling machine and disc shear. The device consists of two adjustable-gap active wire wheels on each side, pressing the plate edge firmly. Slag entrapment occurs when a portion of the slag remains in the weld metal.
Weld morphology of spiral steel pipes. If the weld formation coefficient is too small, the weld shape is narrow and deep, making it difficult for gas and inclusions to float, easily forming porosity and slag inclusions. Generally, the weld formation coefficient is controlled between 1.3 and 1.5, with a value used for thick-walled spiral steel pipes and the minimum value for thin-walled pipes. Poor weld penetration occurs when the overlap between the inner and outer weld metals is insufficient, sometimes resulting in incomplete penetration. This condition is called insufficient weld penetration.
Reducing the secondary magnetic field of spiral steel pipes. To reduce the influence of the magnetic blow, the connection position of the welding cable on 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. Undercut. Undercut is a V-shaped groove appearing along the weld centerline at the edge of the weld. Undercut occurs when the welding speed, current, voltage, or other conditions are inappropriate. Excessive welding speed is more likely to cause an undercut than unsuitable current.
Regarding the process of spiral welded steel pipes, the welding speed should be appropriately reduced or the current increased to delay the crystallization rate of the weld pool metal, facilitating gas escape. Simultaneously, if the strip delivery position is unstable, adjustments should be made promptly. Frequent fine-tuning of the front or rear axle to maintain the weld shape should be avoided, as this hinders gas escape.
Post time: Mar-06-2026