Welded or seamless tube: how they are made and how they differ

Every steel tube belongs to one of two families: welded, made from a strip of steel rolled into a tube and welded along its length, and seamless, made from a solid steel bar that is pierced and then rolled. They look almost the same, but they are made in completely different ways, cost different amounts and are not used for the same jobs. Below you can see step by step how each one is made, how to tell them apart and which one to choose.

Contents: in short · how welded tube is made · how seamless tube is made · how to tell them apart · which one to choose · myths · FAQ

In short

How welded tube is made

Most tubes used in construction, including almost all square and rectangular hollow sections, are welded. The process is continuous: steel strip goes in at one end, and cut lengths of tube come out at the other.

1. The strip. It starts with a coil of rolled steel, slit lengthwise into strips. The width of the strip equals the circumference of the finished tube, plus a small allowance.
2. Forming. The strip passes through a series of roll pairs that bend it gradually until it becomes a cylinder with its edges almost touching. All of this is done cold.
3. Welding. The edges are heated with high-frequency current to just below melting point and pressed together by two rolls. No electrode or filler metal is used; the process is called ERW or HFW (high-frequency electric resistance welding).
4. Removing the bead. The metal squeezed out by the weld forms a bead, which a cutting tool scrapes off the outside straight away. On the inside the bead is often left, as a thin line along the length.
5. Sizing and inspection. The tube passes through sizing rolls, and the weld is checked continuously with ultrasound or eddy current.
6. Cutting. A “flying” saw that travels with the tube cuts it into 6 or 12 m lengths without stopping the line.

Square and rectangular tube usually starts out as round welded tube too. After welding, the sizing rolls gradually “square” it. That is why it has rounded corners, and why the simple weight formula gives slightly more than the table (see how to calculate the weight of rectangular tube).

Large pipes for pipelines (from a few hundred mm upwards) are welded differently: with submerged arc welding, either along a straight seam (thick plate pressed into a U shape, then an O) or along a spiral seam, from strip wound in a helix. This is how pipes more than a metre in diameter are made.

How seamless tube is made

The idea sounds impossible: how do you make a hole along the whole length of a solid bar? The answer came in the 1880s from the brothers Reinhard and Max Mannesmann in Germany, who patented the skew rolling process that bears their name in 1886.

1. The billet. A solid round steel bar is cut to length and heated in a rotary furnace to about 1,250 °C, until it glows and turns soft.
2. Piercing. The billet enters between two rolls set at an angle to each other, which spin it and push it forward in a spiral. Spinning between the rolls opens up the core of the bar, and a pointed plug placed in its path widens it into a hole. The result is a “hollow shell”, a short tube with thick walls.
3. Elongation. The shell is threaded onto a long mandrel bar and passed through several rolling stands, which thin the wall and make it several times longer. An older variant, the pilger mill, works “forwards and backwards”, like a pilgrim’s step.
4. Sizing. The tube, still hot, goes through a sizing or stretch-reducing mill that gives it its final outside diameter.
5. Finishing. Then come cooling, straightening, end cutting, sometimes heat treatment, and non-destructive testing along the whole length.

For precision tubes, for example for hydraulic cylinders, the hot-rolled tube is also cold drawn through a die over a mandrel. This gives tolerances of a few hundredths of a millimetre and a smooth surface. It is also why “drawn tube” is sometimes used, wrongly, for any seamless tube.

How to tell them apart

• Look inside. A welded tube usually shows a straight line along its whole length where the seam is. On galvanised or painted tube it is harder to see.
• Read the marking. Marked tubes carry the standard and the type: SMLS means seamless, while ERW, HFW or SAW means welded. EN 10216 covers seamless pressure tubes, EN 10217 welded ones, and EN 10219 cold-formed welded hollow sections.
• Measure the wall. On a seamless tube the thickness may vary a little from one side to the other (eccentricity). On a welded tube, made from rolled strip, the wall is very uniform.
• Compare the price. For the same size, seamless tube is noticeably more expensive.

Which one to choose?

Welded tube is the normal choice for nearly all steel fabrication: gates, fences, stairs, halls, shelving, railings, furniture. Modern high-frequency welds are inspected along their whole length and have strength comparable to the rest of the tube. Welded tube is cheaper, more uniform and available in many more sizes, especially in square and rectangular sections.

Seamless tube is worth it where a seam would be a point of risk: high pressure (hydraulics, high-pressure air, boilers), high temperatures, drilling and oil and gas lines, parts machined from thick-walled tube. For very thick walls at small diameters it is practically the only option, while at very large diameters it is the other way round and almost all pipe is welded.

Common myths

• “Welded tube cracks along the seam.” With modern, inspected tube this is rare. The real risk is when bending, if the seam ends up on the stretched side of the bend. Practical tip: when bending, put the seam on the side, on the neutral line, where the material is neither stretched nor compressed.
• “All seamless tube is drawn.” No: most of it is hot rolled. Only part of it, the precision tube, is cold drawn.
• “Seamless means better steel.” The quality of the steel is set by its grade (for example S235 or S355), not by how the tube was made. Two tubes of the same grade have the same material properties.

How much it weighs and where to buy it

Weight is calculated the same way for both types: a Ø48.3 × 3.2 tube weighs 3.56 kg/m, welded or not. For any size, use the metal weight calculator or the round tube table. Below are products from the MatHaus catalogue, with current prices.

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Frequently asked questions

What does seamless tube mean?

A tube made from a solid steel bar, hot pierced and rolled, with no seam along its length.

What do SMLS and ERW mean on a tube?

SMLS means seamless. ERW (electric resistance welded) means a tube welded along its length by resistance welding, usually with high-frequency current.

Can square tube be seamless?

It exists, but it is rare and expensive. Almost all square and rectangular tube on the market is welded and then formed from round tube.

Can welded tube take pressure?

Yes, within the limits of the standard it is certified to (for example EN 10217 or EN 10255). For high pressures and temperatures, seamless tube is preferred.