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

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

The pores in the weld of spiral steel pipes not only affect the compactness of the pipeline welds and cause pipeline leakage but also become the inducing point of corrosion, seriously reducing the strength and toughness of the welds.

The factors that cause pores in the welds include: moisture, dirt, oxide scale and iron filings in the flux, welding composition and coverage thickness, surface quality of steel plates and 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, which is highly stable and insoluble in liquid metal, thereby preventing the formation of hydrogen pores.

Bubbles: Bubbles mostly occur in the center of the weld. The main reason is that hydrogen is still hidden in the welded metal in the form of bubbles. Therefore, the measures to eliminate this defect are to first remove rust, oil, moisture, and humidity from the welding wire and weld, and secondly, 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 the flux is generally 25-45mm. When the flux particle size is large and the density is small, the accumulation thickness is taken as the value, otherwise, the minimum value is taken; when the current is large and the welding speed is low, the accumulation thickness is taken as the value, otherwise, the minimum value is taken. In addition, in summer or when the air humidity is high, the recovered flux should be dried before use. Sulfur cracks (cracks caused by sulfur). When welding plates with strong sulfur segregation bands (especially soft boiling steel), sulfides in the sulfur segregation band enter the weld metal and cause cracks. The reason is that the sulfur segregation band contains low-melting iron sulfide and there is hydrogen in the steel. Therefore, to prevent this situation from occurring, it is effective to use semi-killed steel or killed steel with fewer sulfur segregation bands. Secondly, cleaning and drying of the weld surface and flux are also very necessary.

Surface treatment of steel plates: To prevent the iron oxide scale and other debris that falls off during unwinding and leveling from entering the forming process, a plate surface cleaning device should be set. Thermal cracks. In submerged arc welding, thermal cracks can occur in the weld, especially in the arc pits at the start and end of the arc. To eliminate such cracks, pads are usually installed at the start and end of the arc, and the spiral steel pipe can be reversed and welded into the overlap weld at the end of the plate coil welding. Thermal cracks are most likely to occur when the weld stress is large or the si in the weld metal is very high.

Steel plate edge treatment: Rust and burr removal devices should be set on the steel plate edge to reduce the possibility of pores. The position of the removal device is installed after the milling machine and the disc shear. The structure of the device is that there are two active wire wheels with adjustable gaps on one side, which press the plate edge up and down. Slag inclusion. Slag inclusion means that a part of the 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, and the gas and inclusions do not easily float out, which makes it easy to form pores and slag inclusions. Generally, the weld forming coefficient is controlled at 1.3-1.5, with the thick-walled spiral steel pipe taking the value and the thin-walled taking the minimum value. Poor penetration. The inner and outer weld metals do not overlap enough, and sometimes they are not fully penetrated. This situation is called insufficient penetration.

Reduce the secondary magnetic field: To reduce the influence of 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 some welding cables on the workpiece. Undercut. Undercut is a V-shaped groove along the center line of the weld at the edge of the weld. Undercut is caused by inappropriate conditions such as welding speed, current, and voltage. Among them, too high a welding speed is more likely to cause undercut defects than unsuitable current.

Process: 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, it should be adjusted in time to prevent frequent fine-tuning of the front or rear axle to maintain the forming, which makes it difficult for gas to escape.