Discontinuity
Arc strike
A small localised melted spot where the arc touched the base metal outside the joint, quenched instantly by the surrounding cold plate into a hard, brittle, crack-prone patch.
TitleArc strike
ClassSurface
ViewIndicative
What it looks like
- A small pit, blister, or discoloured spot on the plate away from the weld
- Often with a rim of remelted metal and a hard shiny centre
- May contain fine cracks or porosity within the tiny remelted zone
- Frequently found near the joint on the side where the welder rests the electrode or clamp
Why it happens, and how it is stopped
Causes
- Striking the arc on the base metal instead of in the joint or on a run-on tab
- Accidental contact between the electrode, wire, or nozzle and the workpiece
- A poor work lead connection arcing between the clamp and the workpiece
- Careless handling of a live electrode holder or gun between passes
- Dragging cables or the gun across the work while the machine is energised
Prevention
- Strike arcs only within the joint, on run-on tabs, or on scrap that will be removed
- Attach the work lead securely to clean bare metal close to the joint
- Keep live electrodes and guns clear of the work when not welding
- Remove any arc strike by grinding it out and blending the area smooth, then re-examine it
How it is found
Which examination methods can see it, and what it is about the discontinuity that makes them able to.
- VTThe strike is a surface mark, so direct examination of the plate around the joint is the primary and usually sufficient method.
- MTReveals whether the hard, quenched spot has cracked, which is the real hazard rather than the cosmetic mark itself.
- PTFinds cracking in an arc strike on non-magnetic material where magnetic particle inspection is not applicable.
How inspectors think about it
- The concern is metallurgical rather than dimensional: a tiny volume of metal quenched by a cold plate can form a hard brittle spot that starts a crack in service.
- Because of that, arc strikes are usually treated as unacceptable on the base metal of fracture-critical and cyclically loaded components regardless of how small they look.
- After a strike is ground out, an inspector still expects the area to be examined for cracking and checked for adequate remaining thickness.
General reasoning only. The accept/reject limits for your work come from the code or specification governing the job, which this site does not reproduce.
Related discontinuities
SpatterDroplets of molten metal thrown out of the arc and stuck to the plate around the weld, mostly a cleanliness and coating problem but occasionally a source of local hard spots.Crater crackA small star-shaped or linear crack in the depression left where the arc was broken, formed because the last shallow pool of metal cooled fast and shrank into a cavity it could not fill.Cluster porosityA tight group of gas pores concentrated in one localised area of the weld with sound metal on either side, marking a moment when shielding or cleanliness broke down instead of a whole-pass problem.Toe crackA crack starting at the junction of the weld face and the base metal, where the abrupt change in section concentrates stress and the heat-affected zone is at its hardest and least forgiving.