Pre-welding technology of LSAW steel pipe

LSAW (Longitudinal Submerged Arc Welded) steel pipes are produced by rolling a single medium or thick steel plate into a cylindrical shape using a mold or forming machine, followed by double-sided submerged arc welding. This process not only joins the edges of the plate but also expands the pipe diameter. One critical aspect of the LSAW steel pipe production process is the pre-welding stage, where the forming seam is subjected to "shallow welding" along the entire length of the pipe. This specialized technique is essential to ensure the integrity of the final welded pipe.

Pre-Welding Process for LSAW Steel Pipes

During the pre-welding stage, the steel pipe blank undergoes a series of steps to prepare it for further welding processes. The process is performed in a continuous cycle:

- Tube Blank Feeding
The tube blank is initially accepted on the inlet roller table, where its position is adjusted to align with the next steps.

- Tube Blank Seaming
The edges of the tube blank are seamed together, creating the initial weld joint.

- Welding Process
After seaming, the welding gun descends to begin the welding process. A laser tracker is used to ensure accurate tracking of the welding seam.

- Gas Shielding and Cooling
Shielding gas is released, and the cooling water valve is opened to maintain optimal temperature control during welding.

- Continuous Welding
The welding process proceeds as the tube blank is fed at a consistent speed. The weld continues until the terminal arc is extinguished, signaling the end of the welding process.

- Post-Welding Process
After welding, the shielding gas is turned off, and the welding torch is retracted. The tube blank is then transferred to the next stage in the production line.

This pre-welding cycle ensures that the tube blank is properly prepared and welded, providing the foundation for the subsequent welding processes that will finalize the pipe's integrity.

Pre-Welding Quality Control

Ensuring high-quality pre-welding is essential for the overall performance and safety of the LSAW steel pipe. The quality of the pre-weld includes both the seam quality and the weld quality. Key quality requirements include:

- Edge Alignment
The edges of the tube blank should align properly, with any misalignment (or "wrong edge") not exceeding 8% of the plate thickness, and the maximum deviation should be no greater than 1.5mm.

- Welding Penetration and Deposition
The welding seam must achieve the right penetration depth and deposition amount. It is essential that no cracking or burn-through occurs, and that the external weld height is controlled to avoid excess material.

- Continuous, Well-Formed Weld Bead
The weld bead should be smooth and continuous, ensuring a strong and uniform final external weld.

- Defect-Free Welds
The weld should be free from defects such as welding deviation, porosity, slag inclusion, cracks, burn-through, or back welding flash. The center deviation of the weld should be kept within 1mm.

- Minimal Arc Burn and Splash
The weld should not cause arc burns, and the amount of welding splash should be minimal, ensuring that the pipe's bevel and surface quality remain unaffected.

- Physical and Chemical Compatibility
The physical and chemical properties of the weld must match those of the base metal and the welding material to meet the necessary quality standards.

Welding Specifications and Quality Control

The welding specifications for LSAW steel pipes are typically determined through experimental testing. For welding wires of various specifications, optimal welding results are achieved when the line energy falls within a certain range. For example, with a 40mm welding wire, ideal results are obtained when the line energy is between 35 and 40 kJ/cm. In this range, the weld exhibits excellent appearance, as well as superior physical and chemical properties.

By maintaining strict control over these variables, manufacturers ensure that the final LSAW steel pipe meets the rigorous demands of industries requiring high-strength, durable piping solutions, such as those in oil and gas transportation and other heavy-duty applications.