Misalignment defect of ERW steel pipe

1. Misalignment Defect (Lap Welding)

The misalignment defect in ERW (Electric Resistance Welded) steel pipe, also known as lap welding, occurs when the edges of the tube blank are not aligned correctly during welding. Even after the internal and external burrs are removed, traces of misalignment can still be seen on both the inner and outer surfaces of the steel pipe. The primary issue with misalignment is the reduction of the effective wall thickness of the steel pipe. According to the APISPEC5L standard, the combined misalignment and residual burr of an electric welded steel pipe should not exceed 1.5mm (0.060 in). Additionally, misalignment can affect ultrasonic and X-ray inspections and may serve as a starting point for chemical corrosion during use.

2. Causes of Misalignment Defects

Poor Forming Quality

ERW steel pipes go through a series of rolls (rough forming, roll string, fine forming) to gradually shape the coil into a cylindrical form. The quality of this forming process significantly impacts welding quality. If the steel strip's center shifts during forming, it causes uneven edges and misalignment. This misalignment is usually in one direction, making it easier to identify. Incorrect settings on the forming roller parameters can also cause uneven edge heights before the extrusion roller, leading to local misalignments with unpredictable directions.

Effect of Extrusion

ERW steel pipes are welded by heating the strip's edges and applying pressure using the extrusion roller. The extrusion force is a crucial parameter in the welding process, greatly affecting the pipe's quality. Insufficient extrusion force results in a weak weld due to fewer eutectics and residual oxides, while excessive extrusion force can extrude all metal reaching welding temperature, also weakening the weld. Overly high extrusion forces are prone to causing misalignment, which is difficult to adjust by merely reducing the extrusion force. Typically, extrusion force is expressed as the difference in circumference of the tube blank before and after passing through the extrusion roller, usually equating to half the wall thickness of the steel pipe.

Influence of Rolls and Tooling

- Rolls: Fine-forming rolls and squeeze rolls mainly influence misalignment. Excessive wear on fine-forming rolls, uneven wear on guide pieces, and damage to the squeeze roll edges (especially the upper squeeze roll) can cause periodic misalignment.

- Roll Bearings: Damaged extrusion roller bearings can cause uneven rotation or non-rotation, leading to uneven extrusion forces and misalignment. Significant wear or gaps in bearings can also result in uneven rotation and misalignment.

- Roll Tooling Installation: Poor installation quality of precision forming rolls and side extrusion rolls, large gaps between extrusion roll shafts and bearings, and between shafts and roll seats, can cause misalignment.

3. Judging and Confirming Misalignment

Misalignment is primarily identified through visual inspection. Key indicators include:

- Outer burrs: Weld extrusions are not centered in the outer burr.

- Outer weld bead: The bead appears uneven after burr removal, with changing width and shape.

- Inner burrs: Intermittent splits or cracks in inner burrs may indicate misalignment.

4. Adjusting and Eliminating Misalignment

To address misalignment, identify the cause and take appropriate measures:

- Ensure the strip steel's center does not deviate during forming. Adjust fine-forming diameter reduction and small roll reductions to eliminate edge waves and bulges.

- Select suitable process parameters, particularly the welding extrusion force.

- Grind excessively worn rolls and repair damaged squeeze rolls.

- Replace bearings with significant wear or large clearances, and ensure proper lubrication.

- Ensure rolls are installed horizontally and eliminate excessive gaps between the extrusion roll shaft and bearings during lane changes.

5. Conclusion

Misalignment is a common issue in ERW steel pipe production, especially with thin-walled pipes. Identifying the cause and taking corrective measures is crucial. During normal production, raw material butt welds can cause significant bulges, leading to misalignment when passing through the extrusion roller. Fine-tuning the upper extrusion roller can help eliminate this misalignment.

By understanding the causes and solutions for misalignment defects, manufacturers can ensure the quality and reliability of ERW steel pipes, maintaining their structural integrity and performance in various applications.