Defects that are prone to occur in the welding area of spiral steel pipes

Defects that are prone to occur in the welding area of spiral steel pipes include pores, hot cracks, and undercuts.

The pores in the spiral steel pipe weld not only affect the tightness of the pipe weld and cause pipeline leakage but also become an induction point for corrosion, 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 ingredients and coverage thickness, surface quality of the steel plate and steel plate side plate treatment, welding process and steel pipe forming process, etc.

Flux composition. When welding contains an appropriate amount of CaF2 and SiO2, it will absorb a large amount of H2 and generate HF which is highly stable and insoluble in liquid metal, thus preventing the formation of hydrogen pores.

bubble. Bubbles mostly 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 from the welding wire and weld. and other substances, followed by the fact that the flux must be dried well to remove moisture. In addition, increasing the current, reducing the welding speed, and slowing down the solidification rate of the molten metal is also very effective.

The accumulation thickness of flux is generally 25-45mm. The accumulation thickness of flux has a large particle size and low density, and vice versa. The accumulation thickness of large current and low welding speed should have a minimum value. In addition, in summer or when the air humidity is high, the accumulation thickness of flux is generally 25-45mm. When using, the recycled flux should be dried before use. Sulfur cracking (cracking caused by sulfur). Cracks are caused by the sulfide in the sulfur segregation zone entering the weld metal when welding plates with strong sulfur segregation zones (especially soft boiling steel). The reason is that the sulfur segregation zone contains low melting point iron sulfide and the presence of hydrogen in the steel. Therefore, to prevent this situation, it is effective to use semi-killed steel or killed steel with less sulfur segregation zone. Secondly, it is also necessary to clean and dry the weld surface and flux.

Steel plate surface treatment. To prevent iron oxide scale and other debris that falls off during unwinding and leveling from entering the molding process, a board surface cleaning device should be installed. Thermal cracks. In submerged arc welding, hot cracks can occur in the weld bead, especially in the arc starting and arc extinguishing craters. To eliminate this kind of crack, backing plates are usually installed at the arc starting and arc extinguishing points, and at the end of the plate coil butt welding, the spiral steel pipe can be reversed and welded into the overlap 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 edges of steel plates to reduce the possibility of pores. The cleaning device is installed behind the edge milling machine and disc shear. The structure of the device is two active wire wheels with adjustable upper and lower positions on one side, which press the edge of the board up and down. Inclusion of welding slag. The inclusion of welding slag means that part of the welding slag remains in the weld metal.

Weld morphology. The forming coefficient of the weld is too small, the shape of the weld is narrow and deep, gas and inclusions are not easy to escape, and pores and slag inclusions are easily formed. Generally, the weld forming coefficient is controlled at 1.3-1.5, which is the value for thick-walled spiral steel pipes and the minimum value for thin-walled spiral steel pipes. Poor weld penetration. The metal overlap of the inner and outer welds is not enough, and sometimes the welding is not penetrated. This condition is called insufficient weld penetration.

Reduce the secondary magnetic field. To reduce the impact of magnetic blowback, 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 some welding cables on the workpiece. Undercut. Undercut is a V-shaped groove appearing on the edge of the weld along the center line of the weld. Undercut occurs when welding speed, current, voltage, and other conditions are inappropriate. Among them, too high welding speed 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 delay the crystallization speed of the weld pool metal to facilitate gas escape. At the same time, if the strip delivery position is unstable, adjustments should be made in time to avoid frequent fine-tuning of the front or rear axles. The bridge remains shaped, making it difficult for gas to escape.