ISO 6520-1 group 2 · Cavities
ISO 6520-1 reference 2014 is gas pores aligned in a row along the weld axis, easy to mistake for a planar flaw on a radiograph. Cause matrix, detection routes, and where acceptance is set.
Linear porosity is the designation most often confused with something more serious. A row of small rounded indications running parallel to the weld axis can look, at first glance, like a planar flaw such as lack of fusion. Radiography usually resolves the difference, but a second view or an ultrasonic check is the honest answer when the first radiograph leaves doubt.
What produces the row, almost without exception, is a fixed feature of the joint rather than a general process condition: a root gap, a tack weld, a restart point, or the root pass itself. Gas escaping along that feature has one direction available to it on every pass, which is why the pores line up instead of scattering.
That is also the useful reading of the pattern for process control. A single row found on radiography is a joint-preparation or root-technique question, not a whole-procedure question — and a repeating anomaly at the same relative position across several passes, seen in the cooling record or the pool image while welding, points at exactly that feature before the weld is finished.
| Cause | Mechanism | Control |
|---|---|---|
| Gas escaping along a fixed joint feature | A root gap, a tack weld, or a fit-up irregularity sits at the same relative position pass after pass, and gas escaping along that feature forms a row of pores parallel to the weld axis. | Treat a linear pattern found on radiography as a fit-up signal and check the joint preparation, not only the welding parameters. |
| Trapped gas tracking along the fusion line | Gas released at the boundary between the weld pool and a previous pass, or the parent material, tracks along that boundary instead of dispersing through the pool, leaving a row. | Enforce interpass cleaning and correct torch position at the fusion line to reduce the gas available to track. |
| Consumable moisture pickup with a single escape path | Dissolved gas from a damp consumable evolves during freezing as it does for scattered porosity, but when the joint geometry gives it one direction to travel, the pores align along it instead of scattering. | Control consumable storage and re-drying regardless of which pattern eventually shows on the radiograph. |
| Root-pass gas entrapment | A narrow root pass freezes quickly and gas generated there has only one direction to travel, producing a row close to the root rather than a scatter through the fill passes. | Verify root-pass shielding and technique separately from the fill passes. |
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 | Linear porosity usually sits below the surface; visual testing finds it only where the row happens to break through. |
| Thermal | In-process infrared thermography | Supporting signal | A disturbance repeating at the same relative position pass after pass — a root gap, a restart point — can leave a repeating pattern in the cooling record. |
| Camera | High-speed weld pool imaging | Supporting signal | A gas source tied to a fixed feature of the joint disturbs the pool at the same relative position on every pass, which is visible while welding. |
| NDT | Radiographic testing to ISO 17636, ultrasonic testing to ISO 17640 | Primary route | Radiography shows the row directly; alignment along the weld axis separates 2014 from a cluster or a scatter. A second view or ultrasonic testing confirms it is porosity, not lack of fusion. |
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. |
| API 1104 | Acceptance for pipeline girth welds, treated separately from structural work | Pipeline acceptance is its own regime; do not carry a structural criterion into it. |
| ASME BPVC Section VIII | Radiographic and ultrasonic 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"
