Seamless, by definition, means “perfectly consistent,” but many people get confused by terms like “seam-free” and “welded-and-drawn” when it comes to selecting tube. Seamless tubing, welded tubing, and seam-free tubing all appear to be consistently free of seams, but a closer examination reveals big differences.

Characteristics

316 stainless tube offers increased corrosion resistance, because it is 100% seam-free without any longitudinal or orbital welds. Seamless stainless tube begins as molten steel, which is refined and cast in to a billet.Plug drawing tube diagram The billet is extruded into a seamless pipe that is then reduced to a smaller size using a pilger mill, which utilizes a mandrel to support the internal diameter. After the reduced pipe is cleaned and annealed, it goes through a cold-drawing process. This process uses a die to reduce the outer diameter and a plug to maintain the inner diameter. After the reduced pipe is cleaned and annealed, it goes through a cold-drawing process. This process uses a die to reduce the outer diameter and a plug to maintain the inner diameter. After cleaning and annealing, the cold-drawing process is repeated many times until the tube reaches the desired dimensions, still perfectly seamless.

To produce a welded tube, flat strip is roll-formed and joined with a longitudinal seam. The process can be done several different ways, including tig welding or laser alignment. Producing welded tube is simpler and therefore less expensive than seamless. However, the weld area is dimensionally inconsistent with the strip and causes areas of stress concentration.

For tubes exposed to high internal pressures, inconsistencies in the tubing can be a major concern. Coiled tubing on spool crevices can cause cracks, which can propagate and cause the welded tube to rupture. As a result, ASME B31.3, Process Piping Code, de-rates the working pressures of welded tubes by 15 percent compared to seamless tubes of the same dimensions.  

Furthermore, these crevices, which exist on both sides of the weld seam, create opportunities for corrosion. Metallurgical contamination during heat treatment can also lead to corrosion, as residue from the manufacturing process may become trapped in these crevices. In some applications, where a medium is cyclically pumped through the tube, the weld seam can flake off and affect the performance of the overall system, and that can be detrimental. Longitudinal welds present an opportunity cross-sectional micrograph of a welded tube for defect, which cannot be tolerated in critical applications.

Some manufacturers use different methods in an attempt to minimize the dimensional variation and integrate the weld seam. A common method used is to re-draw the welded tubing through a die. This often includes a plug on the inside of the tube for support, but it can also be done without a plug. These products are known by many names including: welded-and-drawn, seam-integrated and seam-free tubing. Redrawing “as-welded tubing” with a die and plug and subsequently annealing helps homogenize the weld. However, in order to fully homogenize the weld, these operations need to be repeated several times, increasing the overall cost of the finished product. Seamless tubing, in comparison, does not have any welds. And stainless coil tubing that is seamless is always the best choice.