Troubleshooting guide › Weld defects › Arc strikes and crater cracks
The two ends of a weld cause more rejections than the middle. An arc strike outside the joint leaves a tiny hardened, often cracked, spot on the plate. A crater left at the end of the weld shrinks as it freezes and cracks in a star pattern. Both are small, both are easy to miss, and both are where fatigue cracks start.
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.
Arc strike: a small pitted mark or a splash of metal on the plate away from the weld, sometimes with a hairline crack through it. Crater crack: a star- or line-shaped crack in the hollow at the end of the bead, or at every stop in a multi-run weld. On aluminium and stainless the crater is a visible dimple that cracks across.
Illustration, not a photo. Orange marks the defect.
Scratch-starting a stick electrode on the plate, or touching the MIG wire or TIG tungsten to the plate away from the joint. The spot heats and quenches in a fraction of a second, leaving hard martensite that cracks.
The arc is cut while the pool is still large; the centre of the pool is the last to freeze and shrinkage tears it.
Pulling the MIG torch or TIG tungsten upwards lengthens the arc and widens the crater before it freezes.
Every stop is a crater; the next start does not always re-melt it.
Aluminium and fully austenitic stainless weld metal crack easily at the crater; quenched steel cracks at arc strikes.
A loose or dirty earth clamp arcs against the plate and leaves arc strikes where no one is looking.
Typical values on an inverter MIG or TIG with 4T trigger. A starting point only.
| MIG crater fill | Down-slope 0.5-1.5 s; crater current 50-70 % of welding current |
| MIG hot start | Start current 110-130 % for 0.3-0.5 s on cold plate |
| TIG down-slope | 2-5 s on steel, 3-8 s on aluminium; end current 10-20 % of weld current |
| TIG post-flow | 1 s per 10 A of welding current, minimum 5 s |
| Stick | Strike inside the joint; back-step 10-15 mm at the end |
| Run-on / run-off tabs | 50 x 50 mm pieces of the same plate tacked at each end of a butt weld, cut off afterwards |
On a foot-pedal TIG, ease off the pedal while adding a last drop of filler to the crater.
The metal under it was heated to melting and quenched by the cold plate in less than a second. On medium-carbon or high-strength steel that makes a hard, brittle spot with a crack-like edge. Under fatigue load the crack grows from there.
If you restart on the crater and re-melt it fully, yes. If the crack is deep it may survive underneath. Grinding the crater to a taper first is the safe way.
Press the trigger to start (with hot start), release and the weld continues, press again to go into crater fill, release to stop. It gives a proper start and finish without holding the trigger for the whole weld.
A machine with crater fill (down-slope at the end of the weld) and a 4T trigger mode prevents most crater cracks on MIG and TIG, provided the filler, the settings and the finish technique are right; it is a help, not a guarantee. For stick, a hot start that does not need the electrode scratched across the plate reduces stray arc strikes. These are standard on inverter machines and worth insisting on if you weld anything that will be inspected.
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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