Main Ways to Identify Welded Steel Pipes and Seamless Steel Pipes

1. Visual Inspection

Welded pipe is produced by forming steel plate or strip into a tube and joining the edges, which leaves a longitudinal or spiral weld seam. This seam may be on the outer surface, the inner surface, or both, and its visibility depends on the process and finishing.
Seamless pipe has no longitudinal weld seam, since the pipe wall is formed directly from a solid billet rather than joined edges.
 

That said, weld seams on ERW pipe and other welded products are often ground, polished, or otherwise finished, and can become very difficult to see with the naked eye. Visual inspection alone is not always sufficient to reliably identify a welded pipe — the absence of a visible seam does not automatically confirm that a pipe is seamless.

2. Check Pipe Marking and MTC

2.1 Check Pipe Marking

Most pipes carry stenciled or stamped markings that can be checked against the order specification, including:

• Manufacturer
• Pipe size
• Grade
• Standard
• Heat number
• Other product information

2.2 Check the Mill Test Certificate

The MTC issued by the manufacturer typically confirms:

• Manufacturing method (welded or seamless)
• Applicable standard
• Steel grade
• Dimensions
• Heat number
• Inspection and test results

For commercial verification, pipe markings and the MTC are generally more reliable than visual inspection alone. That said, documentation should still be checked for consistency with the physical pipe, since markings or paperwork can occasionally be mismatched or incomplete.

3. Metallographic Examination

Metallographic examination looks at the pipe's microstructure to confirm its manufacturing method.

• Welded pipe shows a distinct weld zone, a heat-affected zone (HAZ) adjacent to it, and the surrounding base metal. The weld zone typically has a different grain structure than the base metal, reflecting the melting and cooling that occurred during welding.
• Seamless pipe shows a continuous microstructure around the full circumference, with no weld zone or HAZ.

This method is generally reserved for cases where documentation is missing, disputed, or where a definitive answer is required, since it involves sample preparation and laboratory equipment rather than a quick check.

4. Chemical or Etching Examination

Controlled etching may reveal differences between the weld zone and base metal by making the microstructure more visible under examination. This is a supplementary technique rather than a standalone identification method, and results should be interpreted alongside other evidence such as documentation or metallographic findings.

This method should be performed by qualified personnel using appropriate laboratory safety procedures, as it involves chemical reagents and controlled conditions.

5. Identify by Manufacturing Process

Welded pipe: Steel plate/strip → Forming → Welding → Finishing
Seamless pipe: Round billet → Heating → Piercing → Rolling/Drawing → Finishing

Because welded pipe is formed by joining the edges of plate or strip, it inherently has a weld seam. Seamless pipe is formed from a solid billet with no edges to join, so it has no longitudinal weld seam by nature of the process — not because of any finishing step.

Read a full guide: How to Identify Welded and Seamless Steel Pipes

Article source: https://article-realm.com/article/Business/84875-Main-Ways-to-Identify-Welded-Steel-Pipes-and-Seamless-Steel-Pipes.html