ISO 6520-1 group 2 · Cavities
ISO 6520-1 reference 2012 is gas porosity spread evenly through the weld metal rather than concentrated at one point. Cause matrix, why radiography defines the pattern, and where acceptance is set.
Uniformly distributed porosity is the “background” member of ISO 6520-1 group 2: pores of roughly the same size, spread fairly evenly through the length of the weld rather than gathered in one place or lined up along one path. The designation matters because the pattern itself is the diagnosis. A radiograph showing an even scatter points at a condition present for the whole run — a marginal gas flow, a damp consumable, a light contamination film — not at a single event.
That is also why the cause matrix reads differently from a localized defect. Nothing in it is a one-off mistake; every entry is a condition that, once present, stays present for as long as the parameter, the consumable batch or the surface state does not change. A weld that comes back with scattered porosity across its full length is telling the process engineer to look at a setting, not at a moment.
The practical argument for in-process evidence follows from the same logic: a steady condition leaves a steady signature. Gas flow logged at the torch and a cooling trend that drifts over an entire pass are both readable while the weld is being made, and both point at exactly the kind of cause this designation represents — long before the radiograph confirms the pattern days later.
| Cause | Mechanism | Control |
|---|---|---|
| Shielding gas flow marginally under the qualified rate for the whole pass | A flow slightly below the qualified setting lets a small, steady fraction of the arc atmosphere reach the pool for the full length of the run, rather than failing badly at one point. | Monitor flow at the torch continuously rather than checking it once at the start of the shift. |
| Light, uniform surface contamination along the whole joint | A thin film of oil, mill scale or rust present along the entire joint length releases gas at a steady low rate rather than in a single burst. | Treat surface preparation as a per-joint checklist, not a per-batch inspection. |
| Consumable moisture pickup | Flux-cored wire or electrode coating that has absorbed atmospheric moisture releases dissolved gas for as long as it is used, so the whole weld carries the same background pore population. | Control consumable storage and re-drying, and treat an opened package past its exposure window as suspect. |
| Base-metal degassing spread across the joint | Gas already dissolved in the parent material evolves at a similar rate wherever the arc passes, rather than concentrating at a single defect. | Verify material certification and preheat where the base metal is known to carry dissolved gas. |
The column that matters most is the last one. An inspection family marked as not finding this imperfection will not find it however carefully it is applied, and planning around that is cheaper than discovering it during a final examination.
| Family | Method | Capability | What it sees |
|---|---|---|---|
| Visual | Visual testing to ISO 17637 | Limited | Catches only the pores that happen to sit on the surface. The uniform pattern itself is a volumetric characteristic and is invisible from outside. |
| Thermal | In-process infrared thermography | Supporting signal | A pool carrying gas throughout the run does not cool like a clean one; a differential building steadily over a whole pass is a workable screening signal, not a location. |
| Camera | High-speed weld pool imaging | Supporting signal | Persistent, low-level arc and pool disturbance across the whole run is visible while welding rather than at one instant. |
| NDT | Radiographic testing to ISO 17636 | Primary route | Radiography is where the pattern is actually named: an even scatter of small rounded indications across the film is what earns the 2012 designation rather than 2013 or 2014. |
Therness pages never reproduce a standard's acceptance values: those tables are copyrighted and belong in the copy you hold. What follows is the pointer — which document governs, and which part of it to open.
| Standard | Clause topic | Note |
|---|---|---|
| ISO 5817 | Quality levels for imperfections in steel, nickel and titanium joints, indexed by ISO 6520-1 reference number | The permitted extent depends on the selected quality level and on joint geometry. Read it in the standard. |
| ISO 10042 | The equivalent quality levels for aluminium and its alloys | Reach for this instead of ISO 5817 whenever the parent material is aluminium. |
| ASME BPVC Section VIII | Radiographic acceptance for pressure vessels, with examination methods in Section V | Acceptance lives in the construction code, not in the qualification code. |
Most weld imperfections are produced by conditions that exist while the arc is running. Therness records those conditions on every weld, so a suspect run is known on the shift that made it rather than in a batch report.
Progetto cofinanziato nell'ambito del PR Piemonte FSE+ 2021-2027,
Priorità I, Obiettivo Specifico a), Azione 4 – "Sostegno alla nascita delle start up"
