Discontinuity
Cluster porosity
A 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.
TitleCluster porosity
ClassCavity
ViewIndicative
What it looks like
- A dense knot of rounded voids separated by clean weld metal from the rest of the bead
- Often found at arc strikes, restarts, or tack welds that were not ground back
- On film, a dark blotchy patch made up of many overlapping rounded indications
- The surrounding weld frequently looks entirely normal, which makes surface inspection unreliable
Why it happens, and how it is stopped
Causes
- Restarting an arc over a crater without cleaning or reshaping it first
- A momentary loss of shielding when the welder changed position, snagged a hose, or paused mid-bead
- A patch of contamination such as a paint stripe, grease spot, or primer overspray on the joint
- Welding over a tack that was made with a different consumable or was itself porous
- Damp or damaged electrodes at the start of a pass before the covering dried out in the arc
Prevention
- Grind starts, stops, and tacks to sound metal and feather them before welding over them
- Use a run-on tab or back-step start so the unstable first moment of the arc lands outside the finished joint
- Remove localised contamination from the entire weld zone, not only the groove faces
- Keep the gas nozzle clear of spatter build-up so the shielding column stays even across the puddle
How it is found
Which examination methods can see it, and what it is about the discontinuity that makes them able to.
- RTThe clustered voids give a strong local density change, printing as a distinctly darker patch against uniform weld metal.
- UTA shifting group of small echoes appears over a short probe travel, then disappears entirely, matching the localised nature of the flaw.
- VTUseful only where the cluster reaches the cap, but restart locations are worth examining closely because that is where clusters concentrate.
How inspectors think about it
- Concentration matters as much as total quantity, because many pores in a small volume remove more load-carrying metal from one cross-section than the same count spread over a long bead.
- Inspectors weigh the size of the cluster relative to the weld cross-section and how much sound metal separates individual pores.
- A repeating pattern of clusters at every restart points at technique rather than luck, so the finding usually triggers a look at the procedure and the welder rather than a single repair.
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
Uniformly scattered porosityRounded gas pockets frozen into the weld metal and spread fairly evenly through the deposit, rather than bunched together or lined up, showing that shielding or cleanliness was marginal for the whole pass.Linear porosityGas pores lined up in a row, almost always along the weld root or between passes, because the gas source is itself a line such as an unclean root face or a contaminated interpass surface.Piping porosity (wormhole)An elongated tubular gas cavity that grew as the solidification front chased a rising gas bubble, leaving a hole shaped like a worm burrow rather than a sphere inside the weld metal.Arc strikeA 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.