Defect Analysis of ERW Steel Pipe Welding

Common Defects in welding of ERW steel pipes and their prevention

1. Inclusions
The formation mechanism of the inclusion defect is that the metal oxide is not extruded with the molten metal, but is sandwiched in the welding fusion surface to form a defect. A defect of this type is that the metal oxide is not trapped on the fusion surface as the molten metal is extruded. These metal oxides are formed on the molten metal surface of the V-shaped mouth. 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 the narrow fan-shaped region of metal twisted oxide, these molten metal and metal oxide cannot be completely discharged by normal extrusion, thus forming an inclusion zone.

Precaution:
1) The V-shaped angle is controlled between 4 degrees and 6 degrees;
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 being less than 8:1 (avoid less than 4:1 when the impact toughness requirement is low)

2. Pre-arc
This defect is actually caused by insufficient fusion caused by the pre-arc. Usually, the burrs, oxide skin and rust on the edge of the strip form a bridge before the vertex of the V angle, causing the short-circuit to cause the current to jump and produce a pre-arc phenomenon. The short-circuit current changes the current direction and reduces the heat at the V corner.

Precaution:
1) The V-shaped angle is strictly controlled at 4 ~ 6;
2) The strip edge is clean, smooth and free of burrs;
3) Keep the cooling water clean, control the flow direction of the cooling water, and try to avoid the flow 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 fully fused, and a good weld is not formed. The direct reason for the lack of fusion is the lack of heat during welding. There are many related factors that lead to insufficient welding heat, such as high-frequency power. Output, V angle and heating length, position of magnet bar, working condition and cooling of magnet bar, size of induction coil, welding speed, etc., these factors influence each other, and the combined effect leads to such defects.

Precaution:
1) The matching of welding input heat and welding speed, and the characteristics of the raw material of the tube blank
2) The position of the magnet 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 angle of the V-shaped mouth 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 suitable and meets the requirements of the production pipe diameter.

4. Stick welding (cold welding)
Adhesion welding defects cannot be detected by current inspection methods, so they are the most dangerous weldability defects in high frequency welding. There is no gap on the surface of the joint formed by stick welding, which can transmit ultrasonic signals, which cannot be detected by electromagnetic inspection (EMI), but crack when flattened, and the fracture is flat and brittle. Compared with the fully fused weld fracture, it is slightly fibrous. Some gaps can be detected. If the transverse metallographic section is observed, it can be seen that the HAZ is very narrow, there is no white fusion line, and the rising angle of the metal streamline is very small.

Prevention:
1) Use sufficient welding power for different specifications of materials and welding speeds;
2) Fully squeeze or increase the width of the steel strip.

Read more: Seamless vs ERW Welded pipe