Pre-welding technology of LSAW steel pipe

LSAW (Longitudinal Submerged Arc Welding) steel pipes are produced by pressing (rolling) a single medium and thick plate into a tube blank in a mold or forming machine, followed by double-sided submerged arc welding and diameter expansion. Pre-welding technology, a critical part of the LSAW steel pipe welding process, involves "shallow welding" the forming seam along the entire length and is one of the specialized processes in LSAW steel pipe production.

The Process of LSAW Steel Pipe Pre-Welding

During pre-welding, the steel pipe blank is first seamed, followed by continuous gas-shielded welding. Throughout this process, the status of the welding seam and the quality of the welding are monitored and feedback is provided.

Specific Steps in a Pre-Welding Cycle

The inlet roller table accepts the tube blank.

The opening position of the tube blank is adjusted.

The tube blank is conveyed.

The tube blank is seamed.

The quality of the seam is confirmed.

The welding gun descends to prepare for welding and starts the laser tracker for tracking.

The shielding gas and cooling water valve are opened.

Welding starts (the tube blank is fed at the welding speed).

Welding stops at the terminal arc extinguishing.

Shielding gas lags off.

The welding torch rises and returns.

The tube blank is transferred to the next process.

Pre-Weld Quality

Pre-weld quality encompasses both seam quality and weld quality.

- Seam Quality:

No edge misalignment or misalignment smaller than the specified value. Generally, the edge misalignment should be ≤ 8% of the plate thickness, with a maximum not exceeding 1.5mm.

- Weld Quality:

1. Ensure suitable penetration depth and deposition amount.

2. Avoid post-weld cracking and burn-through, while controlling the height of the weld seam to prevent affecting the excess height of the external weld seam.

3. The weld bead should be continuous and well-formed to ensure high external weld quality.

4. No defects such as welding deviation, porosity, cracks, slag inclusion, burn-through, and back welding flash. The weld center deviation should be ≤1mm.

5. No arc burn, minimal spatter, and no impact on the bevel and surface quality of the pipe end.

6. The physical and chemical properties of the weld should match the base metal to meet quality requirements.

Welding Specifications

Welding specifications are typically determined by experiment. For different welding wire specifications, when the welding line energy is within a specific range, the weld seam exhibits good appearance and physical and chemical properties. For instance, with a Φ4.0mm welding wire, when the line energy is 35~40kJ/cm, the weld's appearance and physical and chemical properties are in an ideal state.