Why is Seamless Stainless Steel and Nickel Tubing Difficult to Manufacture at Small Diameters?

Often, an engineer or purchasing manager will specify “seamless” tubing on their drawing or request. Why? For certain applications with high pressures or extreme environments, this selection is appropriate where component failure could be catastrophic. However, seamless tubing still appears on many requests at very small diameters  (<3/64” OD) where the application for such miniature tubing is much more ordinary. In the following paragraphs, we aim to describe why seamless tubing at these small diameters can be difficult to produce and why it may not be necessary for some applications.

There is a perception among some manufacturing professionals that seamless tubing is better than welded and drawn. However, properly welded and drawn material is robust and meets mechanical property testing (like tensile strength, flare tests, flange tests) that demonstrate it is on par with seamless tubing of equivalent diameters. At miniature tube sizes, engineers should review the application carefully to determine if seamless tubing suited for very demanding applications is necessary, and here’s why: seamless tubing becomes much harder to manufacture (and in turn the cost goes up) at very small diameters.

The reason for the difference lies in how welded and drawn stainless and nickel tubing is constructed compared to seamless. Welded and drawn tubing starts with flat strip stock which is processed through a rolling mill, and then welded in an inert environment into a continuous coil. Because the incoming strip is rolled to very precise tolerances, the wall of the tube is also very consistent. It’s important to note that the welded tube is plug drawn after welding, blending the ID weld bead so it does not protrude into the tube ID. More information on welded and drawn tubing can be found here.

In contrast, seamless tubing starts with a solid billet which is pierced in the center to create a tube and then progressively drawn down to smaller sizes. The precision of the tooling and machinery is critical to maintaining the center of the tube ID is critical. Any manufacturing process has tolerances, and that’s true in this case as well. The center of the ID of the seamless tube relative to the center of the OD (concentricity) always has some variation.

The images below show the differences between a concentric and a non-concentric tube (non-concentric image is shown with very poor concentricity for illustration purposes only)

Even at high levels of precision, the concentricity of the seamless tube struggles to match that of welded and drawn tube. As tubing is drawn to smaller diameters, the concentricity matter shows up in the form of wall thickness variation---one side of the tube might be considerably thinner than the opposite side of the wall. Eventually at thin walls, this leads to the wall breaking through during the manufacturing process, and that is the root of the problem manufacturing miniature seamless stainless steel tubing. 

At Vita Needle, diameters less than 1/32” OD and walls less than .005” for seamless products add additional challenges, and there are limitations that may lead us to quote welded and drawn products only for reasons outlined above. Additionally, 17-7 is not available in seamless form in part due to economic demand for it and partly because of formability. A very similar precipitation hardening alloy, 17-4 is available in seamless because it has improved hot ductility which allows it to be pierced into seamless tube from solid billets.

In summary, careful consideration should be given to process selection. If the seamless process is not needed to meet desired performance characteristics, welded and drawn might be more cost effective and manufacturable especially at the smallest tubing diameters.