Arc strike

An arc strike happens away from the joint that is actually being welded — a momentary arc from an electrode brushing the surface, a ground clamp arcing where it makes poor contact, or a welder resting a live torch on the workpiece for an instant. The energy involved is small compared with a production weld, but it still melts a small volume of parent material and lets it re-solidify almost immediately, with no filler, no shielding, and no control over the result.

That combination — a fast, unplanned heating and quenching cycle on bare, unprepared metal — is metallurgically closer to an uncontrolled miniature weld than to a scratch. On steels susceptible to hardening, the rapid self-quench can leave a small hard, brittle region at the surface, and the geometric notch the strike leaves behind concentrates stress on top of that. The risk this creates is disproportionate to how small the mark looks, which is why standards such as ISO 5817 treat arc strikes as a defect condition in their own right rather than as cosmetic damage, independent of the weld’s own quality.

Because an arc strike sits on parent material rather than in the weld, it is found by visual inspection of the surrounding surface, not only of the seam — a sweep that is easy to skip if inspection attention stays fixed on the joint itself. Repair is not simply cosmetic grinding: removing visible discoloration does not confirm that the affected, potentially hardened material and any resulting crack have actually been removed, so a ground-out strike on a critical structure is typically verified afterward with the same NDT route that would find a crack, rather than being closed out on appearance alone. Adjacent spatter from the same stray contact is common and worth checking at the same time.