Common Defects And Prevention Of Welded Steel Pipes

In actual production, welding defects in welded steel pipes are usually not caused by a single cause, but are the result of a combination of multiple factors. Today’s article will introduce in detail the common defects of welded steel pipes and how to prevent these problems.

- Inclusions

The formation mechanism of inclusion defects is that the metal oxide is not extruded with the molten metal, but is sandwiched on the welding surface.

These metal oxides usually form at a V angle on the surface of the molten metal. When the speed of the strip approaching the edge is lower than the melting speed, and the melting speed is higher than the discharge speed of the molten metal, the vertex of the V-shaped opening will form inclusions containing molten metal and metal oxides that cannot be completely extruded normally and discharged. The surface of clean metal solutions is doped with these metal oxides, creating defects during forging and welding.

This defect will cause cracks in the weld after it is flattened, and inclusions will be seen at the fracture of the weld. This defect exists in different forms, sometimes single and sometimes in chains.

Preventive measures for inclusion defects:

1. The V-shaped angle is strictly controlled within 4 to 6 degrees.

2. Adjust the unit to ensure the opening angle length is stable

3. The Mn/Si ratio in the chemical composition of strip steel is greater than 8:1

4. Reduce oxidation in the welding area

- Prearc

This defect is actually insufficient fusion caused by the front arc. Usually, burrs or oxide scale and rust on the edge of the strip form a bridge before the V-angle apex, causing a short circuit to cause a current jump and a pre-arc phenomenon. Heat is reduced at the V corner.

For defects caused by instantaneous shunting, a bright and flat plane fracture can be seen from the weld fracture. Sometimes there are no burrs, oxide scale, rust, etc. on the edge of the strip. However, too small V angle or too high voltage can also cause pre-fracture. Arcing phenomenon, which is caused by high-voltage discharge at the edge of the strip.

Pre-arc defect prevention measures:

1. The V-shaped angle is strictly controlled within 4 to 6 degrees.

2. The edge of the strip is clean, smooth and burr-free

3. Keep the cooling water clean, control the flow of the strip to the cooling water, and try to avoid the V-angle direction.

- Insufficient fusion

This defect is caused by the edges of the two steel strips being heated but not completely fused, without forming a good weld. The direct cause of insufficient fusion is insufficient heat during welding. There are many related factors that lead to insufficient welding heat, such as high-frequency power. The output power, V angle and heating length, the position of the magnetic rod, the working conditions and cooling of the magnetic rod, the size of the induction coil, the welding speed, etc., these factors interact with each other, and the combined effect leads to such defects.

Preventive measures for unfused defects:

1. Matching of welding input heat and welding speed, and characteristics of tube blank raw materials

2. Working conditions of magnetic rod

3. V angle and heating length

4. Induction coil specifications