Seamless Pipe Piercing Mill

Overview

A seamless pipe piercing mill turns a solid hot steel billet into a thick-walled hollow shell without any welded seam. It is the first hot-forming step in the seamless tube route: a round billet is heated to about 1200 to 1280 degrees, charged into the mill, and forced between two driven rolls that grip it, spin it, and drive it onto a fixed piercing point that opens a bore down the center. The shell that leaves the mill is later rolled and sized in separate stands to reach the final pipe wall and diameter, but the bore itself is made here. Because the hole is formed by deformation rather than drilling, the mill works the metal hot and fast, and one bite produces the full length of the shell.

The machine is built around the Piercing Roll Set, fed by the Entry Table and Billet Charging and worked against the Plug and Mandrel Bar. The Guide Shoes and Diescher Discs closes the sides of the pass, the Main Drive Train turns the rolls, the Hydraulic Adjustment System sets their position, and the Water Cooling System keeps the tooling from softening. All of it is framed by the Mill Housing and Stand and sequenced from the Electrical and Control Room.

The Mannesmann effect

The mill exploits a behavior of metal under cross-roll loading that the Mannesmann brothers found in the 1880s. The two work rolls are not parallel: each is tilted to a feed angle and set at a cross angle so that, as they turn, they both rotate the billet and screw it forward along the mill axis. The rolls bite the surface hard, but the surface and the core do not see the same stress. Near the center the material is pulled in alternating tension as the billet rotates under the squeeze, and after a few turns this cyclic tensile stress tears open a small cavity along the axis. That cavity is the start of the bore. The Piercing Plug is held on the axis just behind the point where the cavity forms, so the metal opens onto the plug and is shaped into a round, smooth bore rather than a ragged crack. The art of piercing is timing the roll geometry so the cavity opens exactly at the plug nose and not before, since a cavity that runs ahead of the plug leaves a rough inner surface.

How the mill works

A heated billet rolls down the Driven Roller Table to the Billet Stop, where the Centering Guides line it up with the roll gap. The Charging Pusher then drives the billet into the bite. The two Barrel Work Rolls, each a barrel with an entry cone, a central gorge, and an exit cone, grip the billet and spin it while feeding it onto the Piercing Plug. The plug sits on the Mandrel Bar, which is held back by the Thrust Block against the heavy axial force of piercing. The Guide Shoes or driven Diescher Discs back the shell on the two sides the rolls do not touch, keeping it round. The shell screws over the plug and off the back of the bar in one pass.

Major subsystems

The Piercing Roll Set carries the rolls in Roll Chocks on tapered Roll-Neck Bearings, with a Roll Tilt Drive to set the feed angle and a Roll Skew Drive to set the cross angle. The Main Drive Train runs from a large Main Drive Motor through a Pinion Gearbox that splits torque to both rolls, then through Drive Couplings and Universal Spindles that reach the roll necks across the tilt and skew range. The Hydraulic Adjustment System uses a Hydraulic Power Pack, a Valve Block of Servo Valves, and Adjustment Cylinders to set the roll gap and hold it under load. Water from the Water Cooling System floods the rolls through Spray Nozzles and runs through the bar to cool the plug. The Electrical and Control Room runs the Mill PLC and the Drive Cabinet that hold every set point.

Materials

The tooling lives in a brutal place: it is squeezing steel near its forging temperature while abrading against scale. The Barrel Work Rolls are forged hot-work steel ground to the cone profile, washed continuously to limit thermal fatigue. The Piercing Plug is a chromium-nickel-molybdenum cast steel run with a controlled oxide layer that acts as a thermal barrier, and it is cooled between billets through the Mandrel Bar because its nose can reach the billet temperature. The Housing Posts and Tie Rods are sized for the large separating force, since piercing pushes the rolls apart with great load. The Universal Spindles and Pinion Shafts are case-hardened alloy steel to carry the drive torque through the universal joints.

Use

Piercing mills sit at the head of seamless tube plants that supply line pipe, casing and tubing for wells, boiler and pressure tube, and mechanical and bearing tube. The shell from this mill goes on to a plug mill, a mandrel mill, or a pilger mill to reach the wall, then to a stretch-reducing or sizing mill for the final diameter. The seamless route is chosen where a welded seam is a liability: high pressure, fatigue loading, or sour service. A Piercing Plug and roll pass are changed to suit each billet size and grade, so the mill is set up by recipe for each campaign.