Introduction to pre-welding and welding essentials of straight seam steel pipes

Straight seam steel pipes play a vital role in modern infrastructure and industrial applications due to their high strength, dimensional accuracy, and reliability. To ensure the quality and performance of these pipes, strict control must be maintained throughout the manufacturing process—especially during the pre-welding and welding stages. This article outlines the key technical considerations involved in both pre-welding and welding operations of straight seam steel pipes, helping to guarantee product consistency, structural integrity, and long-term service performance.

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1. Pre-Welding of Straight Seam Steel Pipes

Pre-welding involves preparing the joints and welds to ensure a stable foundation for subsequent welding processes. Key technical requirements include:

Joint Alignment (Forming Seam):
The edges of the steel plates must be properly aligned with no or minimal misalignment. Typically, the offset should be ≤8% of the plate thickness, and not exceed 1.5 mm at maximum.

Weld Penetration and Height Control:
Adequate weld penetration and deposition must be ensured to avoid common welding defects such as cracking or burn-through. Weld height should be carefully controlled to minimize excess buildup, particularly on the external surface.

Continuous and Well-Formed Weld:
The pre-weld must be stable, uniform, and continuous to provide a reliable base for subsequent external welding.

Weld Defect Prevention:
The weld should be free from deviations, porosity, cracks, slag inclusions, burn-through, or back weld protrusions. The center deviation of the weld should be ≤1 mm.

Arc and Surface Protection:
No arc burns should be present, and spatter must be minimal. The integrity of the pipe end groove and external surface must remain unaffected.

Weld Compatibility:
The weld metal must match the parent material, with its mechanical and chemical properties meeting relevant standards.

2. Welding Essentials of Straight Seam Steel Pipes

The manufacturing of large-diameter straight seam welded pipes typically employs forming methods such as UOE, CFE (Roll Forming), and CE Forming. Most production lines include a post-welding diameter expansion process to optimize pipe shape and performance. This step has become a key quality assurance measure for producing straight seam welded pipes with consistent geometry and structural reliability.

Diameter Expansion Process:
Diameter expansion involves radially enlarging the pipe from the inside using hydraulic or mechanical methods. Mechanical expansion, widely adopted in global manufacturing lines, is known for its simplicity and high efficiency. It uses radially expanding fan-shaped blocks to incrementally deform the pipe along its full length in a controlled and uniform manner.

The diameter expansion process is typically divided into three main stages:

Initial Rounding Stage:
The fan-shaped blocks expand outward until they fully contact the inner wall of the pipe. At this point, the inner radius becomes consistent across the pipe's cross-section, achieving a preliminary round shape.

Nominal Inner Diameter Stage:
The fan blocks continue to move outward at a reduced speed until they reach the precise target position—corresponding to the final inner circumference of the finished pipe.

Rebound Compensation Stage:
To counteract material spring-back after pressure release, the fan blocks move slightly beyond the nominal diameter to a position predetermined by the process design, ensuring dimensional accuracy after unloading.