Rotary piercing is a

hot working In metallurgy, hot working refers to processes where metals are plastically deformed above their recrystallization temperature. Being above the recrystallization temperature allows the material to recrystallize during deformation. This is impo ...

metalworking Metalworking is the process of shaping and reshaping metals to create useful objects, parts, assemblies, and large scale structures. As a term it covers a wide and diverse range of processes, skills, and tools for producing objects on every scale ...

process for forming thick-walled seamless tubing. There are two types: the Mannesmann process, invented in the 1880s, and the Stiefel process, developed two decades later.

Mannesmann process

A heated cylindrical billet workpiece is fed between two convex-tapered rollers, which are rotating in the same direction.Erik Oberg and Franklin Jones, ed.s, ''Machinery's Encyclopedia'' … , vol. 6 (New York, New York: The Industrial Press, 1917)
pages 333-334
The rollers are usually 6° askew from parallel with the

billet A billet is a living-quarters to which a soldier is assigned to sleep. Historically, a billet was a private dwelling that was required to accept the soldier. Soldiers are generally billeted in barracks or garrisons when not on combat duty, alth ...

's axis. The rollers are on opposite sides of the billet, and the surface of their largest cross sections are separated by a distance slightly smaller than the outside diameter (OD) of the original billet. The load imparted by the rollers compresses the material and the 6° skew provides both rotation and translation to the billet. The friction between the rollers and the billet is intentionally high, and is sometimes increased by using knurled rollers. This friction establishes stresses varying radially through the billet, with the highest stresses at the outer surface and the central axis. The stress exceeds the

yield strength In materials science and engineering, the yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and wi ...

of the billet and causes circumferential fissures to propagate at various radii near the outer surface, and a central longitudinal void to form at the axis. A tapered

mandrel A mandrel, mandril, or arbor is a gently tapered cylinder against which material can be forged or shaped (e.g., a ring mandrel - also called a triblet - used by jewelers to increase the diameter of a wedding ring), or a flanged or tapered or ...

is set inside and a short distance from the start of the central void. This mandrel forces the material outward and compresses the material against the back side of the tapered rollers. This compressive loading fuses the circumferential fissures and sets the initial internal and external diameter values. The formed tube is then cooled and can be cold worked to refine the diameters and to achieve the desired yield strengths. Mannesmann mills can produce tubes as large as 300 mm (12 in) in diameter.

Stiefel process

The Stiefel process is very similar to the Mannesmann process, except that the convex rollers are replaced with large conical disks. This allows for larger tubes to be formed.

References

{{Reflist Metal forming