Common Weld Defects: Porosity, Undercut & Overlap

Porosity — Trapped Gas in the Weld Metal

What it looks like: Round or elongated holes on the surface or revealed after grinding — worm tracks, pinholes, or a sponge-like fracture face. Surface porosity is easy to see; subsurface porosity shows up on RT/UT or when you grind into the bead.

Why it happens: Gas gets trapped as the puddle solidifies. Common causes: dirty/oily/wet base metal, mill scale left in the joint, wrong or insufficient shielding gas coverage, drafts blowing the gas envelope, a clogged nozzle, excessive stick-out, damp flux-core or stick electrodes, or leaks in the gas hose/regulator path.

Fixes: Clean to bright metal, verify CFH with the Shielding Gas Flow Rate Calculator, block drafts, replace worn O-rings, keep consumables dry, and do not paint over a porous bead — grind out and reweld. Heat input that is wildly low can also freeze gas in place before it escapes — check the Heat Input Calculator if your settings are suspect.

Undercut — A Groove Melted Into the Toe

What it looks like: A sharp notch or groove along the toe of the weld where base metal was melted away and not filled with weld metal. It creates a stress riser — inspectors fail undercut that exceeds code limits even when the bead looks “pretty” in the middle.

Why it happens: Excessive amperage or voltage, travel speed too fast for the heat (washing the toe without filling), wrong travel angle (arc digging into the base metal), long arc length, or weaving that spends too little time at the toes.

Fixes: Drop amps/volts into the qualified range, slow travel enough to fill the toes, use a slight push or drag appropriate to the process, shorten arc length, and pause briefly at each toe on weave beads. Joint geometry that is too sharp can make undercut more likely — review details with the Weld Joint Design Calculator and Weld Joint Design Guide.

Overlap — Weld Metal Rolling Over Without Fusing

What it looks like: Weld metal that sits on top of the base metal at the toe like a rolled edge or cold lap — a visible shelf where the bead did not wet into the parent metal. Probe with a pick: if there is a crevice under the rolled metal, it is overlap, not a smooth toe.

Why it happens: Too little heat (low amps/volts), travel too slow with a cold puddle, wrong angle dumping metal ahead of the arc, dirty base metal preventing wetting, or downhill technique that lets metal roll without fusion.

Fixes: Increase heat into the procedure range, clean the joint, adjust angle so the arc digs at the leading edge of the puddle, and do not “stack cold” to avoid burn-through — fix fit-up and heat instead. Overlap is a fusion defect; grinding the face without removing the unfused shelf leaves the defect in place.

Quick Visual ID Checklist

Porosity: holes / worm tracks. Undercut: notch into the plate at the toe. Overlap: rolled weld metal sitting on plate without fusion. Do not confuse undercut (missing base metal) with overlap (extra weld metal poorly fused) — the repair methods differ.