"ERW pipe" is a straight seam electric resistance welded pipe, which is a welded pipe used to transport vapor and liquid objects such as oil and natural gas, and can also be used as an octg pipe. It can meet various requirements of high and low pressure, and occupies a pivotal position in the field of transportation pipes in the world.

Defects and their prevention during welding of erw steel pipe

1. Inclusions
The formation mechanism of inclusion defects is that metal oxides are not extruded with the molten metal, but are sandwiched on the welding fusion surface to form a defect. This type of defect is that the metal oxide is not sandwiched on the fusion surface as the molten metal is extruded. These metal oxides are formed on the surface of the V-shaped molten metal. In the V-shaped mouth, if the approach speed of the edge of the steel strip is less than the melting speed, the melting speed is higher than the molten metal discharge speed, and a narrow fan-shaped area containing molten metal and metal twist oxide is formed after the V-shaped apex. These molten metals and metal oxides cannot be completely discharged after normal extrusion, thus forming an inclusion zone.

Precaution:
1) The V-shaped angle is controlled at 4°-6°;
2) Reliable tooling and equipment installation to ensure a stable V-shaped mouth length;
3) Relatively low welding temperature obtains better weld quality;
4) Avoid the Mn/Si ratio in the chemical composition of the steel strip less than 8:1 (avoid less than 4:1 when the impact toughness requirements are low)

2. Pre-arc
This kind of defect is actually the lack of fusion caused by pre-arcing. Usually, the burrs or scales and rust on the edge of the strip fall before the apex of the V angle to form a bridge. The short circuit causes the current jump to produce a pre-arc phenomenon, the short circuit current changes the current direction, and reduces the heat at the V angle.
Precaution:
1) The V-shaped angle is strictly controlled at 4 ~ 6
2) The strip edge is clean, smooth and burr-free
3) Keep the cooling water clean, control the flow direction of the cooling water, and try to avoid flowing to the V angle

3. Insufficient fusion
This defect is due to the fact that the edges of the two strips are heated but not completely fused, and a good weld is not formed. The direct reason for the lack of fusion is insufficient heat during welding, and there are many factors related to insufficient welding heat. Such as the output of high-frequency power, V angle and heating length, the position of the magnetic rod, the working condition and cooling of the magnetic rod, the size of the induction coil, the welding speed, etc., these factors affect each other, and the combined effect leads to such defects.
Precaution:
1) The matching of welding input heat and welding speed, the characteristics of the raw material of the tube blank
2) The position of the magnetic bar exceeds the center of the squeeze roller by 1/8 in (3.18 mm);
3) The length of the V-shaped opening angle does not exceed the length of the pipe diameter;
4) The V-shaped mouth angle does not exceed 7 degrees;
5) The difference between the inner diameter of the induction coil and the outer diameter of the steel pipe is not more than 1/4 in (6.35 mm);
6) The width of the steel strip is appropriate and meets the requirements of the production pipe diameter.

4. Stick welding (cold welding)
Sticking welding defects cannot be detected by current detection methods, so they are the most dangerous welding defects in high-frequency welding. There are no gaps in the joint formed by sticking welding, and ultrasonic signals can be transmitted, which cannot be detected by electromagnetic detection (EMI), but it cracks when flattened, and the fracture is flat and brittle. Slightly fibrous compared to a fully fused weld fracture. Some gaps can be detected. If you observe the transverse metallographic section, you can see that the HAZ is very narrow, there is no white fusion line, and the rise angle of the metal flow line is very small.
Preventive measures:
1) Use sufficient welding power for materials of different specifications and welding speeds;
2) Fully extrude or increase the width of the strip.