Cold Lap — What It Is and Why It Matters
Cold lap (also called overlap or rollover) occurs when weld metal flows over the base metal surface without fusing to it. The weld bead sits on top of the base metal like a glob of paint rather than melting into it. Cold lap creates an unfused interface that acts as a notch and stress concentrator.
Cold lap is caused by insufficient heat input, excessive weld pool size, improper torch angle, or the weld pool advancing ahead of the arc. It is most common on fillet welds where too large a weld bead rolls over the bottom toe without achieving fusion, and in vertical-down welding where the pool runs ahead of the arc.
Prevention involves maintaining adequate heat input, controlling weld pool size, directing the arc into the base metal (not just into the existing weld pool), and ensuring travel speed keeps the arc at the leading edge of the puddle. Cold lap at the toes must be ground and rewelded.
Frequently Asked Questions
Is cold lap the same as lack of fusion?
Cold lap is a specific type of lack of fusion that occurs at the weld toe — the weld metal overlaps onto the base metal surface without bonding. Lack of fusion is the broader term covering any location where weld metal fails to fuse with base metal or previous passes, including sidewall and inter-run locations.
How do I identify cold lap?
Cold lap appears as a smooth, rounded weld toe where the weld bead seems to sit on top of the base metal rather than blend smoothly into it. Running a fingernail or scriber along the toe — if it catches at a distinct edge where the weld sits on unfused base metal, that is cold lap.