Troubleshooting guide › Weld defects › Weld cracks
A crack is the one defect no inspector accepts, because it grows under load. There are three common kinds: hot cracks that form while the weld is freezing, cold cracks that appear hours later from hydrogen and restraint, and crater cracks at the end of a bead. Each has a different cause and a different fix.
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.
Hot cracks: a line down the centre of the bead, often visible right after welding. Cold cracks: a crack in the heat-affected zone next to the weld, sometimes appearing a day later. Crater cracks: a small star-shaped crack in the dip at the end of the bead.
Illustration, not a photo. Orange marks the defect.
Hydrogen is trapped in the weld and forces a crack as the joint cools under restraint.
Fast cooling makes the heat-affected zone hard and brittle.
The joint cannot shrink, so it tears.
A bead that is deeper than it is wide freezes from the sides and tears down the centre.
The crater freezes with a hollow centre that cracks.
Mild-steel filler on high-strength or dissimilar steel.
| Process | Typical reason on this process | What to do |
|---|---|---|
| ARC / MMA with low-hydrogen electrodes | Hydrogen cracking: damp E7018, high-strength or thick steel, no preheat, high restraint. Cracks appear hours after welding. | Bake and store electrodes, preheat thick or high-carbon steel, use a soft filler run first, check welds 24-48 h later. |
| MIG / MAG | Crater cracks at every stop, centreline cracks on deep narrow beads, cracks on high-carbon steel welded cold. | Fill craters, keep the bead width at least equal to its depth, preheat alloy steel. |
| TIG on stainless | Hot cracking in fully austenitic weld metal, crater cracks. | Use 308L/316L filler that gives a little ferrite, taper off with the pedal, fill the crater. |
| Submerged arc | Centreline hot cracking from a deep narrow bead and high sulphur in the plate; cracks at run-on/run-off. | Lower current or widen the bead with voltage, use run-on and run-off tabs, low-sulphur wire. |
| Aluminium | Hot cracking from the wrong filler or a crater left unfilled. | See the aluminium guide: 4043 filler where allowed, crater fill, run-off tabs. |
ARC (MMA), E7018 4 mm on 20 mm mild steel, multi-pass. A starting point only; confirm on a test piece of the same plate.
| Electrode | baked E7018 from a hot box |
| Pre-heat | 100-150 degrees C, check with a temperature crayon |
| Current | about 140-180 A |
| Interpass | keep below 250 degrees C, do not let it go cold |
| Finish | fill each crater; slow cool |
Typical values for mild steel. Your wire, plate condition and machine will move them. Our technical team can set them with your operator on a video call.
A crack is never acceptable. Remove it completely by grinding or gouging, confirm with dye penetrant that it is gone, and reweld with the cause corrected.
That is the signature of hydrogen cracking. Hydrogen from damp electrodes or dirty plate diffused into the hard heat-affected zone while the joint cooled under restraint. Dry consumables and preheat are the fix.
Preheat is decided by the steel grade (carbon and carbon equivalent), the plate thickness and restraint, the hydrogen level of the consumable and the heat input, and on code work it is written into the WPS. The signs that a steel needs it: carbon above about 0.25 %, carbon equivalent above about 0.4, thick plate, or welding below 5 degrees C. The temperature itself (often 75-150 degrees C for structural grades, higher for alloy steels) must come from the steel grade data or the engineer, not from a rule of thumb.
A deep, narrow bead freezes from both sides and the centre is the last to solidify; shrinkage pulls it apart. Lower the current or raise voltage so the bead is wider than it is deep.
Crater-fill and down-slope settings on an inverter MIG or TIG machine remove crater cracks almost entirely; for ARC welding, the operator must back-step by hand.
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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