Troubleshooting guide › Weld defects › Incomplete penetration
Incomplete penetration (lack of root penetration) means the weld metal did not reach the bottom of the joint. The two plates are joined only near the top, and the unwelded root acts as a crack that opens under load. It is the most common reason a butt weld fails a bend test, and the cause is almost always the joint preparation or the current.
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.
From the back of a single-sided butt weld, the root is not visible as a continuous thin bead; instead the original plate edges can still be seen with a gap or a sharp line between them. On a cross-section there is an unfused notch at the root. On a fillet, the weld bridges the corner without melting into it.
Illustration, not a photo. Orange marks the defect.
Square-edge plates butted tight together cannot be penetrated beyond about 3 mm from one side with MIG, 4-5 mm with stick. The arc melts the top and the root stays cold.
Not enough heat to melt to the root; the pool floats on top.
The pool is dragged along before it can melt down.
A 4 mm electrode or 1.6 mm wire cannot get into a narrow root.
The arc is on the top of the groove walls or on the previous pass; the root is in shadow.
A 3-4 mm land at the bottom of the bevel is too much to melt through.
High voltage widens and flattens the arc and reduces its digging action.
On a submerged arc machine the arc current follows the supply; peak-hour sag reduces penetration without any change of setting.
| Plate thickness (mild steel) | Preparation from one side | Root gap | Root face |
|---|---|---|---|
| Up to 3 mm | Square edge | 0-1 mm | – |
| 3-5 mm | Square edge, weld both sides, or single V | 1.5-2.5 mm | – |
| 5-12 mm | Single V, 60 degrees included | 2-3 mm | 1-2 mm |
| 12-20 mm | Single V with backing, or double V | 2-3 mm | 1.5-2 mm |
| Over 20 mm | Double V or U, weld alternately from both sides | 2-3 mm | 2 mm |
Typical workshop practice for mild steel. Drawings or a WPS override this table.
MIG / MAG root pass, 1.2 mm ER70S-6 wire, 10 mm plate, single V with 2.5 mm gap. A starting point only.
| Current / voltage, root | 150-180 A / 19-21 V, dip transfer |
| Current / voltage, fill | 240-290 A / 26-30 V |
| Stick-out | 10-12 mm on the root (short so the arc reaches) |
| Torch angle | 5-10 degrees push, aimed into the root |
| Travel, root | 20-30 cm/min, watch the keyhole |
| Gas | CO2 for maximum penetration, 15-18 L/min |
CO2 penetrates deeper than Ar-CO2 at the same settings, which is why it stays the common gas for thick mild steel.
Not from one side with MIG or stick. Submerged arc can penetrate 10-12 mm from one side on a square edge at 600-800 A, which is one of its main advantages.
Penetration is about reaching the root of the joint. Fusion is about melting into the plate surface anywhere along the weld. A weld can reach the root and still be unfused on the sidewalls, and both are rejected.
Only if the current was the limit. A 400 A machine with the same tight square-edge joint gives the same result. Fix the preparation first, then make sure the machine can hold the current at the duty cycle.
Penetration is set by current, so the machine must deliver the current the plate needs, continuously, at the duty cycle of the job. A 250 A machine running 6 mm plate at 100 % duty will drop into thermal protection and the operator will quietly turn it down. The product finder matches machine size to your plate thickness.
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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