SEAMLESS VS. WELDED (ERW)

The two most common methods for manufacturing carbon steel pipe are Seamless (SMLS) and Electric Resistance Welded (ERW). While both are governed by ASTM specifications, the method of manufacture dictates the pipe's pressure handling capabilities, wall thickness consistency, and cost.

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1. Seamless Pipe (SMLS)

Seamless pipe is manufactured without a welding seam. The process begins with a solid round steel "billet" which is heated and pushed or pulled over a form (piercing rod) until the steel is shaped into a hollow tube. This process is known as extrusion.

Key Characteristics:

• Pressure Integrity: Because there is no seam, seamless pipe is considered to have uniform strength around its entire circumference. It has a joint factor of 1.0 (100% efficiency).
• Rougher Finish: The extrusion process often leaves a rougher surface finish on the ID (Inside Diameter) compared to welded pipe.
• Eccentricity: It is harder to control the wall thickness precision in seamless pipe. One side of the pipe may be slightly thicker than the other.

2. Welded Pipe (ERW)

ERW pipe starts as a flat sheet of steel (called Skelp). The skelp is cold-formed into a cylindrical shape, and the two edges are heated and fused together using electric resistance. No filler material is used. The weld seam is then "scarfed" (trimmed) and heat-treated to ensure it matches the strength of the rest of the pipe.

Key Characteristics:

• Consistency: Since it starts as a flat plate, the wall thickness is extremely consistent throughout the length of the pipe.
• Cost Effective: ERW is significantly faster and cheaper to manufacture than seamless pipe.
• Seam Vulnerability: Historically, the seam was a weak point. However, modern Heat Treatment (Normalizing) makes the seam virtually as strong as the body.

Quick Comparison