Why HF Weld Defects Matter
High-frequency (HF) welding is the fastest and most economical way to make longitudinal seam pipe, but the weld is made in milliseconds and its quality depends on the precise balance of forming geometry, heat input, squeeze pressure, and cooling. When the balance is wrong, the seam can contain defects that weaken the pipe under pressure or during flattening. Some of these defects are invisible to ultrasonic and eddy-current testing, which is why the industry relies on a combination of process control, inline inspection, and destructive testing. Understanding the defect families is the first step in preventing them.
The Eight Common Defect Families
| Defect | Common name | Nature |
|---|---|---|
| Inclusion | Black overburned oxide | Oxide trapped on the fusion surface, not squeezed out with the molten metal |
| Pre-arc | White overburned oxide | Overheating before the V-point, forming oxide on the edge |
| Insufficient fusion | Open seam | Edges heated but not fused; the seam springs open after the squeeze rolls |
| Insufficient edge fusion | Edge wave | Fusion missing at the outer or inner edge of the strip |
| Insufficient middle fusion | Middle cold welding | Fusion failure at the middle of the wall |
| Sticking weld | Cold welding | Edges joined by pressure without true fusion; undetectable by most NDT |
| Cast weld | Brittle welding | Molten metal not fully expelled, leaving cast metal with oxides in the seam |
| Porosity | Pinhole | Gas pockets on the fusion surface from overheating with insufficient discharge |
Inclusions and Their Prevention
Inclusions are metal oxides formed at the V-point that are not expelled with the molten metal and are clamped into the fusion surface. They occur when the edge approach speed is lower than the melting speed and the molten metal discharge is incomplete. The fracture of an inclusion is flat and lacks the fibrous appearance of a sound weld. The risk increases when the V-angle is less than about 4 degrees or when the manganese-to-silicon ratio of the strip is below about 8 to 1. Prevention focuses on controlling the V-angle, the heat input, and the strip chemistry so that the oxide forms and is expelled cleanly.
Lack of Fusion and Cold Welding
Insufficient fusion, or open seam, is the direct result of insufficient welding heat: the edges are heated and flattened but never fuse, and the seam opens after passing the squeeze rolls. Contributing factors include the heat input, the V-angle and V-length, the magnetic-rod position, the cooling conditions, and the induction-coil size. The prevention list includes matching the heat input to the material and speed, positioning the magnetic rod beyond the centre of the squeeze rolls by about 3 mm, keeping the V-length within the pipe-diameter range, limiting the V-angle to about 7 degrees, and keeping the coil-to-pipe clearance below about 6 mm. Sticking welding, or cold welding, is the most dangerous defect because the joint has no gap, transmits ultrasonic signals, and escapes electromagnetic detection, yet it cracks flat and brittle under flattening. It is prevented by sufficient welding power and by increasing the squeeze and the strip width.
Cast Welding and Porosity
Cast welding, or brittle welding, leaves cast metal containing oxides on the fusion surface because the molten metal is not completely discharged; the fracture is flat and brittle and it fails the flattening test. It is prevented by increasing the weld discharge and the strip width. Porosity, or pinhole, appears as spherical bright white spots on a fibrous fracture, caused by high-temperature welding with insufficient discharge; small pores are visible on the fusion line after the external burr is removed. Both defect families indicate that the heat-to-squeeze balance of the line is wrong and must be corrected before production continues.
Detection and Quality Control
HF seams are examined inline by ultrasonic and eddy-current methods, and the line samples are flattened and metallographically inspected. Because sticking welds escape the standard methods, mills also control the process parameters, monitor the weld temperature, and destructively test samples at defined intervals. A weld that passes the flattening test and shows a full fibrous fracture with a visible white fusion line is the target. The mill test certificate reports the NDT results, and buyers should confirm the testing programme when ordering HF pipe for pressure service.
Frequently Asked Questions
Q: What is the most dangerous HF welding defect?
Sticking welding, or cold welding, because the joint transmits ultrasonic signals and escapes electromagnetic detection, yet cracks brittle under flattening. It is prevented by sufficient power and squeeze.
Q: What causes an open seam?
Insufficient welding heat: the edges are melted but not fused, and the seam springs open after the squeeze rolls. The V-angle, heat input, magnetic-rod position, and coil clearance are the controlling factors.
Q: What is a cast weld?
Cast metal with oxides left on the fusion surface because the molten metal was not fully expelled. The fracture is flat and brittle and fails the flattening test.
Q: How are HF seams inspected?
By inline ultrasonic and eddy-current testing, plus flattening and metallographic tests on samples. Process control is essential because some defects evade NDT.
Q: What is an inclusion in HF welding?
Metal oxide trapped in the fusion surface because the molten metal was not fully expelled. The risk rises at narrow V-angles and low manganese-to-silicon ratios.
Q: How is porosity formed?
By high-temperature welding with insufficient discharge, leaving gas pockets that appear as bright spots on the fracture. Correcting the heat-to-squeeze balance prevents it.
