The 7 Weld Defects That Get Flagged — and How to Spot Them First

Common weld defects and their causes, from the welder's chair

Every reject costs you twice. First the time you put into laying that bead, then the time grinding it out and doing it over. And the kicker? Most of what gets flagged on a CSA job is catchable right at the bench, before anybody hauls out the NDT gear. Train your eye to spot trouble while you're still standing there with your hood up and you save yourself a world of grief.

This isn't written from the inspector's clipboard. It's written from your chair — the welder doing the work. The goal is simple: know what each defect looks like, know what causes it, and know how to fix it before it ever gets a chance to fail an X-ray. One thing up front: the actual pass/fail thresholds — how much undercut is too much, how big a pore matters — are spelled out in the CSA standards, and you should check those against the real document.

Quick vocabulary note before we dig in. A discontinuity is just a break in the uniformity of the weld — it only becomes a defect when it crosses the acceptance limit in the standard. The toe is where the weld meets the base metal, the root is the bottom of the joint, and the face or crown is the top surface you see. Keep those straight and the rest reads easy.

The 7 welding defects that get you flagged

1. Porosity

What it looks like: gas pockets in the weld. Sometimes you see them right on the surface as pinholes or pitting. Sometimes they're buried and only RT catches them. Long tunnels of trapped gas are wormholes.

Cause: contamination is the big one — oil, moisture, rust, paint, mill scale. Also losing your shielding gas to wind or low flow, a long arc, or damp stick electrodes that pulled moisture from the air.

Field fix: clean your prep down to bright metal. Check your gas flow and watch for drafts — even a shop fan can blow your coverage off. Store and condition your low-hydrogen rods properly instead of leaving them on the bench all week.

2. Undercut

What it looks like: a groove melted into the base metal at the weld toe that never got filled back in. Runs along the edge of your bead like a little ditch.

Cause: too much amperage, traveling too fast, a bad work angle, or a long arc pulling metal away from the toe.

Field fix: back your current down, slow your travel, fix your angle, and if you're weaving, dwell a beat longer at the toes so the puddle fills the edges.

3. Lack of fusion

What it looks like: weld metal sitting against the base metal or a previous pass without actually bonding to it. Nasty one, because it's often hidden under a decent-looking cap.

Cause: not enough heat to melt the sidewalls, traveling too fast, poor gun angle, or welding over dirty, oxidized surfaces.

Field fix: bump your heat input, make sure you've got real access into the joint, clean between every pass, and watch your angle so the arc is actually digging into the sidewall.

4. Cracks

What it looks like: straight-line splits in the weld or base metal. Crater cracks show up at the end of a bead where you broke the arc too quick. Hot cracks form while the weld's still solidifying; cold or hydrogen-induced cracks can show up hours later.

Cause: hydrogen in the weld, too much restraint and stress in the joint, cooling too fast, skipping preheat, or leaving the crater unfilled.

Field fix: control your preheat and interpass temps, run low-hydrogen consumables and keep them dry, always fill your craters, and don't let a thick section quench itself. Preheat and interpass numbers are usually driven by the standard and the WPS. And here's the hard rule: a crack is a crack. It doesn't get blended or buried — it gets gouged out completely and rewelded.

5. Slag inclusions

What it looks like: trapped non-metallic slag, usually stretched out in lines between passes where you didn't clean well enough.

Cause: lazy interpass cleaning, low amps that don't float the slag out, a bad bead profile that traps it in a valley, or the wrong travel angle pushing slag ahead of the puddle.

Field fix: chip and brush every pass like you mean it, sequence your beads so you're not leaving deep gaps, and run enough heat to let the slag come to the top.

6. Overlap (cold lap)

What it looks like: weld metal that rolled over the toe and sat on the base metal without fusing to it. Looks filled, but it's just metal piled on top.

Cause: traveling too slow, dumping in too much metal, low amps, or a bad angle.

Field fix: pick up your travel speed, balance your current, and fix your angle so the puddle ties in instead of flopping over. People mix this up with undercut — remember, undercut is metal missing from the base metal, overlap is unfused metal added on top of it.

7. Incomplete penetration

What it looks like: the weld didn't reach all the way through the joint the way it was supposed to. The root never fully filled, leaving a gap or unfused area down at the bottom of the joint where you can't see it from the cap.

Cause: root opening too tight, too big a root face, not enough heat, travel speed too fast, or the wrong electrode angle keeping the arc from reaching the root.

Field fix: open your fit-up to spec, get your heat and travel dialed so the arc reaches the root, and check your angle so you're actually driving into the joint. On open-root work, your technique through the root pass is everything.