Troubleshooting guide › Weld defects › Arc blow

Arc blow: when magnetism pushes the arc off the joint

Arc blow is the arc being pushed sideways or forwards by the magnetic field that the welding current creates in the plate. It shows up at the ends of plates, in corners, in box sections and near the earth clamp, and it gets worse with high DC current. The weld wanders, spatters, undercuts on one side and traps porosity, and the operator blames the machine.

Seen with: DC stick and MIG at high current; submerged arc on long seams and thick plate; welding inside box sections and near corners. Rare on AC.

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.

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Causes, most likely first, and the fix for each Nine things to try, in order Prevention checklist Questions we get asked

What it looks like

The arc leans or flares to one side or ahead of the electrode, sometimes with a roaring sound. The bead is wider on one side, with spatter thrown in one direction, undercut on the side the arc leans away from, and porosity or lack of fusion where the arc skipped. It appears or disappears as you move along the joint, and changes when you move the earth clamp.

Illustration, not a photo. Orange marks the defect.

Causes, most likely first, and the fix for each

1Earth clamp at one end of the joint

Current flows through the plate from the arc to the clamp; the magnetic field is strongest on the clamp side and pushes the arc away from it.

Fix: Move the earth clamp to the other end, or to the side, or connect two earths at both ends. Weld away from the clamp.

2Welding at a plate end or into a corner

The field cannot spread into the air beyond the plate edge, so it bunches up and pushes the arc back towards the plate.

Fix: Use run-off tabs so the magnetic path continues past the end of the weld. Weld towards the heavy tacks.

3Magnetised plate

Plate that has been lifted by a magnet, stored near a DC machine or left after magnetic particle testing carries its own field.

Fix: Demagnetise by wrapping the earth cable several turns around the plate in the opposite direction, or weld with AC. Check with a compass or a gaussmeter.

4Very high DC current

Blow force rises with the square of the current.

Fix: Split the weld into more passes at lower current, or use AC if the electrode allows.

5Long arc

A long arc is a soft arc that is easily deflected.

Fix: Shortest arc possible; on stick, hold the electrode almost touching.

6Welding inside a box section or near heavy steel

The surrounding steel concentrates the field.

Fix: Angle the electrode against the direction of blow, weld in short lengths from the middle outwards, use heavy tacks.

7Earth cable coiled or run alongside the joint

A coiled earth cable is an electromagnet; a cable along the seam adds its field to the plate.

Fix: Uncoil cables, route the earth away from the joint.

8Wrong polarity for the application

Some electrodes blow less on one polarity.

Fix: Try the other polarity if the electrode allows it; switch to AC as the last step.

Nine things to try, in order

Shorten the arc.
Move the earth clamp to the opposite end of the joint, then try the side.
Fit a second earth lead at the other end.
Tack heavily and weld towards the tacks.
Add run-on and run-off tabs.
Angle the electrode 10-20 degrees against the blow.
Weld from the middle of the seam outwards in short lengths (back-step).
Reduce current and add a pass.
Switch to AC or demagnetise the plate.

Prevention checklist

Earth clamp on bright metal, close to the weld, away from the end of the plate.
Earth cable uncoiled and routed away from the joint.
Run-off tabs on seams that end at a plate edge.
Plates kept away from lifting magnets and DC cables before welding.
AC machine available for heavy magnetised steel.
Check for blow on the first joint of a new fixture; fixtures made of steel concentrate the field.

Questions we get asked

Why does it only happen at the end of the seam?

Because the field cannot continue past the edge of the plate, it crowds up there and pushes the arc back. A run-off tab of the same thickness continues the path and the blow disappears.

Is it my inverter?

No. Arc blow is created by the current in the plate and happens on every DC machine. The inverter only shows it faster because its arc responds faster.

Can arc blow cause porosity?

Yes. When the arc is pushed sideways the gas or slag cover no longer sits over the pool, and air gets in.

Does the machine matter?

An AC welding machine does not suffer arc blow, which is one reason transformer AC arc welders stay in use for heavy stick welding on thick, magnetised steel. On DC machines the cure is in the earth connection and the sequence, not in the machine. A DC machine with arc force control recovers faster from the arc instability that blow causes.

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