In the manufacturing of ERW (Electric Resistance Welded) pipes and tubes, there is a critical yet often overlooked process: Tube Scarfing.
When a metal strip is rolled and welded into a tube, the intense heat and pressure create excess molten metal at the seam. This protrusion is known as the weld bead or flash. If this bead is not precisely removed, the resulting tube will fail to meet the standards required for precision industrial applications.
This article explores Scarfing Tools, how they work, and how to select the right tooling to ensure superior tube quality.
A Scarfing Tool is a specialized cutting instrument used on a Tube Mill. Its core function is to act like a planer, shaving off the protruding weld bead while the seam is still in a red-hot state (typically immediately after welding and before cooling).
The goal is to make the surface of the pipe smooth and flush with the parent metal. This process is divided into two specific applications:
OD Scarfing: Removing the weld bead from the Outside Diameter.
ID Scarfing: Removing the weld bead from the Inside Diameter.
2. OD vs. ID Scarfing: Process Comparison
While the objective is the same, the complexity and equipment required for treating the outside versus the inside of the tube differ significantly.
OD Scarfing (Outside Diameter)
This is the more straightforward process. The tooling is mounted externally, allowing the operator to easily monitor the cut.
Structure: Typically consists of a rigid Tool Holder and a replaceable Carbide Insert.
Key Factor: The insert usually features a specific Radius (R) on the cutting edge. This curvature must perfectly match the tube’s outside diameter.
If the Radius is too large: The insert will only cut the top of the bead, leaving a flat spot.
If the Radius is too small: The insert will gouge the parent metal, causing wall thinning.
ID Scarfing (Inside Diameter)
This is the technical bottleneck of tube production because the cutting happens inside the tube within a restricted space.
Structure: Relies on a Tow Bar (or Mandrel) system. This long bar is anchored upstream (before the weld point), extending cantilever-style inside the tube through the welding zone.
The Cut: A specialized ring-shaped tool, known as a Cutting Ring, is mounted on the end of the tow bar.
Chip Removal: The removed internal bead forms a long, curled metal strip (the “scarf”). If not removed immediately, this hot metal can clog the tube or damage the tooling.
3. Core Components: Why Tungsten Carbide?
Scarfing is an extremely demanding cutting operation. The tools must operate continuously under high heat and withstand intermittent impact from hard spots in the weld. Therefore, material selection is the key to success.
Material: The vast majority of scarfing inserts are made from Tungsten Carbide. This material offers exceptional “hot hardness” (the ability to retain hardness at high temperatures).
Coating Technology: To further extend tool life, premium inserts often use CVD or PVD coatings (such as TiN or Al2O3). These coatings increase surface hardness and act as a thermal barrier, protecting the insert from thermal shock.
Geometry: OD scarfing often uses standard ISO inserts (like SNMX, SOUB, SPUB, and SNG) with custom radii, while ID scarfing typically requires proprietary Ring tools.
4. Troubleshooting Common Issues
Even with the best tooling, production issues can arise. Below are common pain points and their solutions:
Issue
Possible Cause
Solution
Chatter (Vibration)
Tow bar is too long/thin (lack of rigidity); Insert is dull.
Check tow bar stiffness; Replace the insert; Add damping.
Chipping
Wrong carbide grade selected (too brittle); Impurities in the weld.
Switch to a tougher carbide grade; Check weld quality.
Steps / Flat Spots
The radius of the insert does not match the tube OD.
Re-measure tube OD and switch to the correct radius insert.
Incomplete Removal (ID)
Cutting ring height is too low; Insufficient hydraulic/pneumatic pressure.
Adjust tow bar height to ensure the ring presses against the ID wall.
5. Why is High-Quality Scarfing Important?
While some structural tubing may skip ID scarfing, it is non-negotiable for high-end applications.
Fluid Dynamics: For pipes transporting oil, gas, or water, an internal weld bead creates turbulence, restricting flow rate and increasing pumping energy costs.
Post-Processing: If the tube needs to undergo Bending or Flaring, a protruding internal bead will interfere with the mandrel, causing the tube to jam or split.
Corrosion & Hygiene: In food processing or chemical pipelines, rough weld seams are breeding grounds for bacteria and starting points for stress corrosion. A smooth surface significantly extends pipeline lifespan.
Caption: Left: Unprocessed weld with visible protrusion. Right: Scarfed weld, perfectly flush with the tube profile.
6. Buying Guide: How to Choose the Right Scarfing Tool
When sourcing inserts or holders, ensure you have the following parameters ready:
Tube OD Range: Determines the required Radius (R) for OD inserts.
Tube ID Size: Dictates the maximum diameter and strength of the Tow Bar system.
Material Type: Stainless steel welds are harder and “stickier” than carbon steel, requiring specific chip breakers and coatings.
Line Speed: High-speed mills require inserts with superior wear-resistant coatings to reduce downtime.
Conclusion
The Scarfing Tool may seem like a small consumable, but it is the “gatekeeper” of tube quality. From selecting high-grade Carbide Inserts to precisely calibrating the Tow Bar system, every detail impacts the final product’s marketability. For tube manufacturers aiming for excellence, investing in a high-performance scarfing system is essential.
