Metallic seamless tube rolling processes

Tubes, long hollow bodies, are omnipresent in mechanical technologies applied to all fields of industry. Originally used to transport fluids, they can now be used as containers (from pharmaceuticals to nuclear fuel tubes), lightweight structural parts, reactors and more. Materials have diversified (all metals and metal alloys, polymers, cement, glass and ceramics); sizes range from the nanometer (carbon nanotubes) to the meter (pipeline), via the micrometer (capillary tubes, needles, etc.) and the millimeter or centimeter scale. If we immediately think of the "smooth" tubes that surround us, we shouldn't forget that cross-sections can be much more complex (finned tubes for heat exchangers, tubes with circumferential, longitudinal or helical grooves, etc.).

This diversity of materials and uses is matched by a huge variety of manufacturing processes. In the remainder of this article, we will concentrate on metal tubes of simple geometric cross-section, obtained by mass-produced forming processes (rolling, extrusion, drawing, etc.).

When shaping metal tubes, the aim is, as always, to give a blank material a precise shape, adequate mechanical properties via a controlled microstructure, and a surface finish that is ideal for the product's use. Tubes are unique in that they have both an external and an internal surface, which often requires several tools to calibrate them. Between the two surfaces, the wall is often thin, which in certain respects brings this shaping process closer to that of sheet metal.

Large tubes and pipes can be produced by centrifugal casting (in the case of cast-iron pipes). Rolling-welding, using a flat-rolled strip, produces steel or aluminum tubes with diameters ranging from 10 mm to 2 m. The diameter can then be reduced much further, to less than a millimeter, by successive drawing. Finally, if seamless tubes are required to eliminate a potential weakness, extrusion, rolling or drawing can be used.

This article is in two parts. The first is a brief overview of the main tube forming processes. It also draws on Roberts' work, which remains fundamental for steel rolling processes in general, and on the more recent work by Brensing and Sommer, which focuses more specifically on steel tube forming.

In the second part, we take the example of a little-known or poorly understood process, pilger rolling, to carry out an in-depth analysis of the mechanical conditions of deformation. Some of the results are of general application to tube rolling, while others are specific to this very particular process; these points will be clarified.