An arc strike (also called an arc mark or stray arc; ISO 6520-1 code 601) is the small scar left when the welding arc touches the parent metal outside the joint — a careless electrode tap, a dragged torch, a poor earth-clamp contact arcing under the clamp.
It looks trivial: a fingernail-sized rough spot beside the seam. Metallurgically, it’s one of the more insidious imperfections in welded fabrication — which is why standards treat it far more severely than its size suggests.
Why a tiny mark is a real defect
When the arc touches bare plate for a fraction of a second, it melts a shallow pool a few millimetres wide. Unlike a weld bead, this spot has no preheat, no controlled heat input, and no filler — and the surrounding cold mass quenches it at an extreme cooling rate.
On carbon and low-alloy steels the consequences stack up:
- Untempered martensite: the self-quenched spot can hit very high hardness — brittle and crack-susceptible.
- Micro-cracks: the combination of hardness and thermal shrinkage stress frequently produces cracking invisible to the naked eye.
- Notch effect: the rough, re-solidified surface is a stress concentrator.
- Fatigue initiation: under cyclic loading, arc strikes are classic crack starters — this is why bridge, crane, and offshore codes are unforgiving about them.
The same physics that governs cold cracking applies here, concentrated in a spot nobody planned, recorded, or inspected. Background on the cooling-rate/microstructure link: heat input, cooling rate and t8/5.
What causes arc strikes
| Cause | Typical scenario |
|---|---|
| Arc initiation outside the joint | Striking the electrode on the plate “to get the arc going” instead of in the groove or on a run-on tab |
| Accidental torch/electrode contact | Moving between welds with a live electrode; cramped access |
| Defective earth (ground) connection | Loose or dirty clamp arcing under load — strikes appear under the clamp, often unnoticed |
| Live cable or holder contact | Damaged insulation touching the workpiece |
| Tack-weld practice | Tacking with excessive open-circuit “scratching” |
ISO 5817 acceptance criteria
ISO 5817 (imperfection 601) is blunt:
| Quality level | Acceptance |
|---|---|
| B (stringent) | Not permitted |
| C (intermediate) | Not permitted |
| D (moderate) | Permitted only if the properties of the parent metal are not affected |
In practice, on hardenable structural steel “properties not affected” is difficult to demonstrate without hardness testing — so most fabricators treat every arc strike as repairable damage regardless of level. As always, verify the current edition of the standard; codes for dynamically loaded structures (EN 1090 EXC3/EXC4, bridge and crane codes) and pressure equipment impose removal and verification. For how quality levels are assigned, see the ISO 5817 quality levels guide.
Detection and repair
Detection is visual (ISO 17637) — but only if inspectors look beyond the weld: along torch paths, at earth-clamp positions, and on faces that were “just resting” against live equipment. Arc strikes under clamps and fixtures are the classic escape.
Repair is a defined sequence, not a quick grind:
- Remove the damaged spot by grinding to sound metal, blending smoothly.
- Verify remaining thickness against the design minimum.
- On hardenable or fatigue-critical material, perform MT or PT (magnetic particle testing) to confirm no cracks remain.
- If below minimum thickness: weld repair under a qualified procedure, then re-inspect.
- Record the repair against the joint or component ID — under ISO 3834, unrecorded repairs are audit findings.
Prevention
- Strike the arc in the joint or on a run-on tab — never on free plate.
- Maintain earth clamps: clean contact area, tight connection, positioned on the part being welded.
- Inspect cable and holder insulation in the daily equipment check.
- In robotic cells, verify approach paths cannot drag a live wire across the part — and monitor arc-on signals against programmed weld positions; an arc event outside a programmed seam is exactly the kind of anomaly real-time monitoring flags automatically.
Arc strikes are one of the defects mapped in the complete welding defects guide with ISO 5817 acceptance criteria.
Frequently Asked Questions
What is an arc strike in welding?
An arc strike (ISO 6520-1 code 601, also called arc mark or stray arc) is a localized spot where the arc was initiated or accidentally touched the parent metal outside the weld joint. The brief arc melts a small area that self-quenches at an extreme cooling rate, leaving a hard, brittle martensitic spot with possible micro-cracks — a metallurgical notch, not a cosmetic blemish.
Why are arc strikes dangerous?
The tiny melted spot cools almost instantly because the surrounding cold plate acts as an infinite heat sink. On hardenable steels this produces untempered martensite — locally very hard, brittle, and prone to micro-cracking. Under fatigue or impact loading, arc strikes act as crack initiation sites. Failures of dynamically loaded structures have been traced to arc strikes that were never repaired.
Are arc strikes acceptable under ISO 5817?
At quality levels B and C, arc strikes are not permitted. At level D they may be tolerated only if the properties of the parent metal are not affected — which on hardenable structural steels is rarely demonstrable without testing. Codes for dynamically loaded structures typically require removal by grinding plus verification. Verify against the current edition of ISO 5817 and your governing code.
How do you repair an arc strike?
Grind the affected spot to sound metal with a smooth transition, verify minimum wall thickness is preserved, and check for cracks — by penetrant or magnetic particle testing on critical or hardenable materials. If grinding goes below minimum thickness, a weld repair under a qualified procedure is required. Simply welding over an arc strike does not remove the hardened, possibly cracked HAZ beneath.
