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Arc strike: what it is and how to spot it

Fillet Gauge editorial teamSources checked 4 min read

An arc strike is a small spot of melted, resolidified metal on the base metal outside the weld, left where an arc was struck or touched by accident. It can come from the electrode, the electrode holder or a poor ground clamp. The spot cools very fast, so it can be hard, brittle and cracked even though it looks minor.

A weld bead seen from above on a plate, with a small melted oval scar on the plate surface away from the bead
The bead is sound; the damage is on the plate. Original drawing, not to scale.
Use this diagram

How to recognize an arc strike

  1. Look off the weld. Arc strikes sit on the base metal, outside the joint. Check the plate either side of the weld, around the ground clamp and anywhere the welder might have rested or struck the electrode.
  2. Look for a small melted spot. It is usually a small, round or irregular scar, sometimes with a tiny crater, a bit of slag or a discolored ring around it.
  3. Look for little lines in it. Because the spot quenches so fast, it can have fine cracks. A magnifier and side light help.
  4. Check where the ground was. A loose or arcing ground clamp leaves burn marks too, and they count as arc strikes.

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Welding Inspection and Discontinuities

A convex fillet weld replica is checked with a leaf-type fillet gauge. The drawing calls for a single fillet size. The vertical leg fits the 3/8 in blade and the horizontal leg the 5/16 in blade. What size is recorded?

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How it is found and recorded

Visual inspection finds arc strikes. When one is found, the next question is whether it is cracked, so a surface method, magnetic particle or penetrant testing, often follows, especially after it has been ground. Record the location. On some work the repair is to grind the strike out and check the area, and some documents call for a hardness check or a surface method afterward. Look up what the governing document requires rather than assuming a light grind is enough.

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Every answer comes with a full explanation.

Common mix-ups

  • Arc strike or spatter. Spatter is a droplet of metal stuck on top of the plate. An arc strike is a spot where the plate itself melted.
  • Arc strike or surface porosity. Porosity is a hole in the weld metal. An arc strike is on the base metal, outside the weld.
  • Arc strike or a crater. A crater is at the end of a weld bead. An arc strike is away from the bead altogether.

Why it matters

The arc melts a tiny pool on cold, thick plate that pulls the heat away almost instantly. On many steels that creates a hard, brittle spot with no preheat to soften it, and often a small crack. In service, especially under fatigue or impact loading, that spot can start a crack in an otherwise sound member. Good practice is simple: strike the arc in the joint or on a run-on tab, keep holders insulated and secure the ground clamp.

Arc strikes on the CWI exam

On Part A, arc strikes fit Visual Examination and Welding Application and Control, where the welder's practice is covered. On Part B they fit Welding Inspection and Discontinuities, at least 34% of the practical. On Part C for the D1.1 endorsement, look under the Clause 7 workmanship rules, not only the Clause 8 visual acceptance criteria: a weld can pass Clause 8 and still break the Clause 7 arc strike rule, so read your own copy.

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Separate arc strikes from spatter and porosity in the weld discontinuities quiz. Then read the droplet look-alike, spatter, and the other quick-quench crack site, crater cracks. All types are in the weld discontinuities guide.

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Sources

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Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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