The defects that are prone to occur in the welded area of the spiral steel pipe include pores, hot cracks, and undercuts.
The porosity of the welded seam of the spiral steel pipe not only affects the tightness of the welding seam of the pipeline and causes the leakage of the pipeline, but also becomes the induction point of corrosion, which seriously reduces the strength and toughness of the welding seam.
The factors that cause porosity in the weld are moisture, dirt, oxide scale and iron filings in the flux, welding components and covering thickness, surface quality of the steel plate and the treatment of the edge plate of the steel plate, 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, and generate HF with high stability and insoluble in liquid metal, which can prevent 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 inside the weld metal in the form of bubbles. Therefore, the measure to eliminate this defect is to first remove rust, oil, moisture, and moisture from the welding wire and weld. and other substances, followed by the flux must be well dried to remove moisture. In addition, it is also effective to increase the current, reducing the welding speed, and slowing down the solidification rate of the molten metal. The accumulation thickness of the flux is generally 25-45mm. The accumulation thickness of the flux is large and the density is small, and vice versa, the minimum value; the accumulation thickness of high current and low welding speed is the value, and vice versa, the minimum value is used. In addition, in summer or when the air humidity is high When the flux is recovered, the flux should be dried before use. Sulfur cracking (cracks caused by sulfur). Cracks are caused by sulfides in the sulfur segregation band entering the weld metal when welding plates with strong sulfur segregation bands (especially soft-boiling steel). The reason for this is the presence of hydrogen in iron sulfide and steel with a low melting point in the sulfur segregation zone. Therefore, to prevent this from happening, it is also effective to use semi-killed steel or killed steel with less sulfur-containing segregation bands. Secondly, it is necessary to clean and dry the weld surface and flux.
Steel plate surface treatment. To avoid sundries such as iron oxide scale falling off from the uncoiling and leveling from entering the forming process, a board surface cleaning device should be installed. hot cracks. In submerged arc welding, hot cracks can occur in the weld bead, especially in the arc starting and arc extinguishing craters. To eliminate such cracks, backing plates are usually installed at the arc starting and arc extinguishing, and at the end of the coil butt welding, the spiral steel pipe can be reversed and welded 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. A rust and burr removal device should be installed on the edge of the steel plate to reduce the possibility of air holes. The position of the removal device is installed after the edge milling machine and the disc shear. The structure of the device is two active wire wheels with adjustable gaps at the upper and lower positions on one side, which press the edge of the plate up and down. Inclusion of welding slag. Engaging in welding slag means that a part of the welding slag remains in the welded metal.
Weld shape. The forming coefficient of the weld is too small, the shape of the weld is narrow and deep, the gas and inclusions are not easy to float, and it is easy to form pores and slag inclusions. Generally, the welding seam forming coefficient is controlled at 1.3-1.5, the thick-walled spiral steel pipe takes the value, and the thin wall takes the minimum value. Poor penetration. Insufficient overlap of internal and external weld metal, and sometimes incomplete penetration. This condition is called insufficient penetration.
Reduce the secondary magnetic field. To reduce the influence of magnetic bias blowing, the connection position of the welding cable on the workpiece should only be far away from the welding terminal, to avoid the secondary magnetic field generated by some welding cables on the workpiece. Undercut. The undercut is the appearance of a V-shaped groove at the edge of the weld along the centerline of the weld. An undercut is produced when conditions such as welding speed, current, and voltage are not appropriate. Among them, too high welding speed is more likely to cause undercut defects than unsuitable 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 delivery position of the strip is unstable, it should be adjusted in time to prevent frequent fine-tuning of the front axle or rear axle. The bridge remains in shape, making it difficult for the gas to escape.