How to Tell Seamless Pipe from Welded Pipe

Steel pipe is a relatively common pipe material. It can be divided into different types such as seamless pipe and welded pipe. So what is the difference between seamless pipe and welded pipe? Next, the carbon steel pipe supplier will briefly introduce to you.

Not sure if the pipe in your hands is seamless or welded? In most cases you don't need a lab — a flashlight, a caliper, and a few minutes are enough.

a: Start with the cut end of the pipe. Point a flashlight along the inner wall at a low angle and look for a raised line or slight discoloration running the length of the pipe — that's the weld seam. A genuine seamless steel pipe won't show this under any lighting angle, since there's no seam to catch the light in the first place.

b: Grinding can hide a seam from the eye, but it rarely hides it from your fingers. Run a fingertip slowly around the inside circumference near the cut end; a welded pipe often still has a faint ridge or a slightly rougher patch where the weld was ground down, even after the outside has been polished smooth.

c: Measuring helps too, and it's worth doing if the first two checks are inconclusive. Take a caliper reading at four points around the same cross-section. Welded pipe, rolled from strip or coil, tends to hold a tighter, more even wall thickness across those four points. Seamless pipe usually shows more variation from one side to the other — a byproduct of the hot-piercing process rather than a defect.

None of these tests is 100% conclusive on its own, especially with high-quality ERW pipe that's been heat-treated and finished to look seamless. When the seamless vs welded pipe question really matters — for a pressure-rated application, for example — mill certification and test reports remain the only fully reliable answer.

Read full guide: Seamless Pipe vs Welded Pipe: What's the Difference?

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