Tube Mill Product

Overview

A tube mill is an integrated production system that converts flat steel coil into welded circular pipe in a single continuous pass. The incoming strip is gradually bent through progressive roll stands into an open O-shape, its edges are heated by high-frequency induction to fusion temperature (~1,700 °C), and they are forged together by closing rolls. The weld is immediately cooled, and the tube passes through sizing stands that remove eccentric diameter variation introduced during forming. Finally, a flying saw severs pieces to length. The process is fast (50–100 m/min), precise (±0.5 mm OD tolerance), and economical for commodity diameters: a tube mill can produce 5–10 tonnes per hour of finished welded tube.

The incoming Entry Section & Uncoiler unwinds the wide coil and supplies the flat strip at constant tension. The Progressive Forming Stands progressively narrow the strip until it forms a continuous spiral tube with an open seam. At the point where the seam edges meet, the High-Frequency Induction Welder heats them by induction coupling, and the Weld Closing Roll forge them together. Post-weld, the Post-Weld Cooling spray hardens the weld zone, and the Sizing Section rounds any diameter eccentricity. The Cutoff System flying saw then severs the tube to the programmed length.

Forming stands and incremental bending

Each Individual Forming Stand is a pair of profile-shaped rolls: the lower Bottom Forming Roll is driven and fixed, the upper Top Forming Roll floats on spring cartridges. As the strip feeds through, friction gradually bends it. The first stand makes a light dent, the second more, and by the 6th–8th stand, the edges are nearly touching, forming an open tube. The upper roll follows the bottom roll profile because its springs yield to match.

All lower rolls are synchronized by the Forming Drive Shaft, a long drive shaft with intermediate Helical Gear Pair stages connecting each roll bearing. The Forming Drive Motor drives this shaft at constant speed (rotational), so every stand rotates at the same rpm. Linear speed (m/min) increases as the tube diameter grows — the first stand feeds flat material, the last stand feeds nearly-closed tube — but the rotational speeds are constant, so there is no slip and no wrinkles.

Spring pressure on the Top Forming Roll units is adjusted based on material. Mild steel (< 250 MPa) runs at low pressure; high-strength (HSLA 500 MPa) and stainless steel demand stiffer springs to prevent chatter. Aluminium requires gentle forming to avoid edge cracking; the roll radii are generous (R > 1 × thickness).

High-frequency induction welding

The High-Frequency Induction Welder uses electromagnetic coupling to heat the open seam edges. The Induction Weld Coil is a copper coil (or sometimes a cable) placed just before the tube edges meet. An oscillating current at 300–500 kHz is supplied by the HF Power Supply, creating a changing magnetic field. The edges of the tube, being conductive ferrous metal, have eddy currents induced in them, and those currents dissipate I²R heating. The seam edges heat to 1,600–1,800 °C in less than a second.

At the exact moment the edges are at fusion temperature, the Weld Closing Roll squeeze them together under 50–500 tonnes of force (depending on tube size and wall thickness). The pressure is applied by Closing Cylinder, usually pneumatic for small tubes and hydraulic for large ones. The closing force forges the softened metal edges together, displacing the liquid-film boundary layer and forming a solid weld nugget. The entire heating and closing cycle is 0.1–0.5 seconds, so thermal penetration is shallow and heat-affected zone is narrow.

The Weld Monitor Sensor is a thermocouple or proximity sensor in the coil area that monitors weld temperature and adjusts the HF Power Controller HF power to maintain a setpoint. Too little power and the edges don't fuse; too much and the tube overheats and becomes brittle. The controller modulates HF output continuously.

Sizing and diameter control

Fresh from the weld, the tube may be slightly eccentric or out-of-round because the forming rolls have mechanical clearance and the seam itself creates a slight diameter step. The Sizing Section contains 2–4 Sizing Roll Pair, each a pair of rolls that are smaller diameter than the tube and contoured to set the OD and roundness. The last sizing stand is often slightly smaller in diameter, calibrating the final OD to ±0.5 mm. The sizing rolls run on the same drive shaft as the forming section, so they turn synchronously.

Cutoff and length control

The Cutoff System is positioned at a fixed location downstream of sizing. A Cutoff Timing Sensor sensor (encoder or proximity switch) monitors tube position. When the PLC counts the tube to the setpoint length, it triggers the Cutoff Motor. For saws, the motor spins the blade into the tube; for shears, it closes the blades. Clamps (usually Tube Clamp pneumatic or hydraulic) hold the tube on both sides of the cut point to prevent movement.

Post-weld cooling

Immediately downstream of the weld closing rolls, the Post-Weld Cooling system sprays water onto the hot weld zone. This rapid cooling (from 800 °C to room temperature in seconds) harnesses the latent heat of the steel, raising hardness — the weld is harder and stronger than the base metal. Without this cooling, the weld would cool slowly in air, remaining soft. The Cooling Water Pump circulates 10–30 L/min through Spray Nozzle positioned above and below (or all around) the tube. The Coolant Tank sump is sized for thermal capacity; for high-speed mills, a heat exchanger or cooling tower may be added.

Drive synchronization

All forming rolls, sizing rolls, and the HF coil position are driven by the Main Drive System. The Main Drive Motor is a large induction motor (15–45 kW) regulated by a VFD to control line speed (50–100 m/min). The Main Reduction Gearbox is a helical reducer ensuring all rolls run in synchronization. The Drive Coupling is a flexible coupling that slips under excessive torque (jam protection). If the tube jams in the sizing rolls, the coupling protects the forming stands from shock.

Changeover for different tube sizes

Changing from one tube OD to another (e.g., 25 mm to 40 mm) requires a complete roll change. The forming stands and sizing rolls are roll-pairs that are physically swapped with a pre-machined set for the new size. This is typically a 4–8 hour job done by experienced technicians: the old roll set is removed from the Individual Forming Stand and Sizing Roll Pair, the new rolls are installed and aligned, the spring cartridges are adjusted, and the machine is run at low speed to verify roundness. The HF coil position may also need adjustment for the new seam location.

Material-specific behavior

Mild steel welds cleanly at standard HF settings. High-strength grades (HSLA 350–500 MPa) weld harder (higher hardness differential) and can crack if cooled too rapidly; cooling water temperature is raised slightly (40–50 °C preheat). Stainless steel (304, 316) is harder and less conductive, demanding higher HF power (often 20–30 % more than mild steel for the same tube size). Aluminium cannot be welded by HF (it is non-ferrous); tube mills for aluminium use resistance seam welding instead, or mechanical locking (like a standing seam on a roof).