- Rotary piercing
A heated cylindrical billet is fed between two convex-tapered rollers, which are rotating in the same direction. The rollers are usually 6° askew from parallel with the billet's longitudinal 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 outer 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 from between the rollers and the billet is intentionally high and many times increased with knurling of the rollers. This friction sets up stresses varying radially through the billet cross section with the highest stresses are at the OD and the central axis. The stress exceeds the yield strength of the billet and cause circumferential fissures to propagate at various radii near the OD and a central longitudinal void to form at the axis. A tapered mandrel 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 various circumferential fissures and sets the initial internal diameter and OD values. The newly formed tube is then cooled and can be cold worked to further refine the diameters and to gain the desired yield strengths.
Mannesmann mills can produce tubes as large as 300 mm (12 in) in diameter.
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.
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