Troubleshooting guide › Weld defects › Slag inclusion
Slag inclusion is flux residue that froze inside the weld instead of floating to the top. It happens with every process that uses a flux: stick (MMA), flux-cored wire and submerged arc. It is almost always a cleaning or technique problem, and it is the easiest defect to prevent once the operator knows where it comes from.
Reviewed by the Cruxweld technical team, October 2026. Settings are starting points to confirm on a test piece; acceptance is decided by the code or drawing the job is built to. Checks marked “call” are for our technical team.
Dark, glassy or irregular pockets inside the weld when it is cut or ground, or long lines of slag along the toe of a multi-pass weld. On the surface it shows as a bead that will not clean up, with slag stuck in the toe or between stringer beads. On a radiograph it appears as irregular dark lines or blotches.
Illustration, not a photo. Orange marks the defect.
On a multi-pass weld the previous bead is covered in slag. The next pass melts over it and traps whatever was left, especially in the groove at the toe.
Welding downhill, holding the electrode angle too flat, or travelling too slowly lets molten slag flow in front of the arc, and the weld freezes over it.
A cold arc does not melt the slag fully; it freezes in lumps before it can float out.
A 4 mm electrode in a narrow V cannot reach the root; the arc skips and leaves slag in the corners.
A convex, ropey bead leaves deep valleys at its toes. Slag sits in the valley and the next pass cannot melt it out.
On SAW the wire is off the joint centre so one side of the groove is not melted and flux is trapped against it.
Moist flux or electrode coating gives a slag that is thick and sticky and will not float.
| Process | Typical reason | What to do |
|---|---|---|
| ARC / MMA | Interpass cleaning skipped, downhill welding, flat electrode angle, low current | Chip and brush every pass; uphill; 60-70 degree angle; current in range |
| Flux-cored wire | Voltage too low so the slag is thick and the bead convex; weaving too wide on a fillet | Raise voltage one step; stringer beads; brush between passes |
| Submerged arc | Wire off-centre on the final pass; slag not removed between layers; flux layer too deep | Align the head; clean every layer; 25-40 mm flux depth |
| Welding carriage with flux-cored wire | Consistent speed set too slow so slag floods ahead | Raise speed to the leg-size table; keep the torch drag angle 10-15 degrees |
ARC / MMA, E6013 or E7018 electrode on 10 mm mild steel, multi-pass. A starting point only; confirm on a test piece.
| Root pass | 3.2 mm at 100-120 A, no weave |
| Fill passes | 4 mm at 150-170 A, stringer beads or a weave no wider than 2.5 times the core wire |
| Electrode angle | 60-70 degrees to the plate, dragging |
| Arc length | About the core wire diameter |
| Between passes | Chip, wire-brush until bright, inspect the toes |
| Polarity (E7018) | DC+ (electrode positive) |
Current ranges are typical for general-purpose electrodes. Follow the range printed on the electrode packet.
Repair: grind or gouge out to sound metal, confirm by dye penetrant, and reweld with the passes cleaned properly.
The arc cannot reach the root corner, so the slag there is never melted. Point the electrode into the corner, use a smaller electrode for the root, and slow down slightly at the corners of a weave.
Yes, MIG with solid wire has no slag, which is why many shops move production fillet welding to MIG. Stick stays better for outdoor work, repair and thick plate in position.
Surface slag that chips off cleanly is normal. Slag that stays after chipping is embedded and must be ground out.
Slag inclusion is a technique and cleaning defect, not a machine fault. A machine with a stable arc and good arc force (so the arc does not go out when the electrode is pushed into the joint) makes it easier to keep the slag behind the pool, but no machine removes slag between passes for you.
Send a photo of the weld or cut, your machine model, consumable and plate thickness. Our technical team replies on WhatsApp with the likely causes and the next checks to make.
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