A welded tube has at least one longitudinal weld running its length, where the edges of a formed flat sheet are joined. A seamless tube is made from a solid billet extruded into a hollow, with no weld seam.
On thicker-section welded tube fabricated from plate, there may also be radial (belly) welds to build longer tubes from shorter plates.
Seamless tubing
Seamless tubing starts as a solid billet extruded into a seamless pipe, sometimes called a hollow or mother tube. The pipe is reduced to smaller sizes by pilgering and/or cold drawing.
Pilgering, also called cold rolling, uses two grooved rollers to control the outside diameter reduction and a mandrel to control the internal diameter reduction. It is extremely fast and can reduce cross sections by up to 90%, but it carries limitations on smaller diameter tubing and is best suited to high volume manufacturing because of the difficult change-over times.
Welded tubing
Welded tubing is made from flat strip roll formed into a circular cross section, then joined by TIG welding or laser welding. Once welded, the seam or bead can be left 'as welded' or refined by cold working, using rolling and forging methods.
Welded tubing can also be drawn after welding, much like seamless tubing, giving an even finer weld seam, tighter tolerances and a superior surface finish.
Which performs better
The weld, or the area next to it, can be a weak point for mechanical or physical properties, so seamless tube is seen as the higher performance product. It also tends to offer better concentricity, dimensional tolerances and finish.