The ERW (Electric Resistance Welding) Tube Mill Process is used to manufacture welded steel tubes from flat strips or coils using high-frequency electric resistance welding. It is one of the most efficient and widely used methods for producing structural and precision tubing.
Step-by-Step Overview
1. Coil Loading & Uncoiling
Input: Hot-rolled or cold-rolled steel coil (carbon steel, alloy steel, stainless steel).
Equipment: Decoiler/uncoiler.
Function: Feeds flat strip into the tube mill line.
2. Strip Flattening / Leveling
Removes coil-set curvature.
Ensures the strip is flat and dimensionally stable.
3. End Shearing & Welding (for coil-to-coil joining)
End of one coil is joined to the start of the next using shearing + butt welding.
Ensures continuous operation with accumulator support.
4. Accumulator (Optional)
Temporarily stores strip material to allow coil changeover without stopping the mill.
Maintains uninterrupted operation.
5. Edge Trimming (Optional)
Trims strip edges for clean and aligned welding.
Especially used when edge quality is inconsistent.
6. Forming Section
Uses a series of forming rollers to bend the flat strip into a round tube gradually.
Number of roll stands depends on tube size and thickness.
Roll design is crucial to avoid edge buckling or surface marks.
7. High-Frequency Electric Resistance Welding (HFI-ERW)
Core step in ERW tube production.
The formed tube passes through an induction coil that generates high-frequency current (typically 100–500 kHz).
Current flows along the edges of the tube → resistive heating occurs → edges reach welding temperature.
Pressure rolls (squeeze rolls) forge the heated edges together to create a solid weld - no filler material needed.
Key Benefits:
Fast, efficient, no filler material
Continuous process suitable for large-scale production
8. Weld Bead Removal (Optional)
External bead is scarfed off for a smooth finish.
Internal bead removal is more complex - used in precision or food-grade tubing.
9. Cooling Section
Tube is rapidly cooled with water spray after welding to stabilize the weld and prevent distortion.
10. Sizing / Straightening
The hot, freshly welded tube is passed through sizing rolls to:
Finalize the outer diameter.
Adjust roundness or shape (e.g., square or rectangular).
Refine wall thickness tolerance.
Final straightening is often done in a Turk's Head unit.
11. Non-Destructive Testing (NDT)
Eddy current testing or ultrasonic inspection ensures weld integrity and surface quality.
12. Length Measuring and Cutting
Flying saw or cut-off saw cuts tubes to required lengths during continuous operation.
Ensures high throughput without stopping the line.
13. Run-Out Table & Collection
Finished tubes are transported to the run-out table for:
Visual inspection
Length sorting
Cooling (if needed)
14. End Finishing (Optional)
End processing depending on application:
Deburring
Beveling
Threading
Facing
Chamfering
15. Final Inspection & Packaging
Tubes are bundled, marked, and prepared for shipment.
May include protective oiling, plastic capping, or wrapping.
Applications of ERW Tubes
| Industry | Applications |
|---|---|
| Construction | Scaffolding, structural tubing |
| Automotive | Chassis, shock absorbers, mufflers |
| Agriculture | Equipment frames, irrigation pipes |
| Furniture | Metal frames, support tubing |
| Energy | Oil and gas transmission (low pressure) |
| HVAC | Ducts, conduits |
