ISO 6520-1 group 2 · Cavities
ISO 6520-1 reference 2016 is an elongated, tunnel-shaped gas cavity, common wherever flux or shielding carries moisture. Cause matrix, why radiography reads its signature, and where acceptance is set.
A wormhole is what a gas pore becomes when it cannot simply float out. Instead of a rounded cavity, the escaping gas cuts an elongated, tunnel-shaped path through the freezing metal, and on a radiograph that path shows as a distinctive tapering trace rather than a rounded indication. The designation exists because a tunnel concentrates the lost sound metal along a line instead of a point.
The cause matrix splits along process lines more sharply than for other porosity designations. Flux-based processes get there through moisture in the flux; gas-shielded processes get there through a sustained shielding failure; both routes end in the same turbulent, gas-rich pool a rounded pore does not require. Geometry that gives the gas a long, constrained escape path makes either cause worse, which is why the imperfection turns up repeatedly in deep grooves and in closely spaced additive layers alike.
Because the underlying condition — sustained gas generation into a turbulent pool — lasts for a meaningful fraction of a pass rather than an instant, it is one of the easier porosity signatures to catch while welding: continuous bubbling on the pool camera or an extended thermal anomaly both flag the same run a wormhole would eventually be confirmed on, the way the single gas pore is confirmed once it is already on the plate.
| Cause | Mechanism | Control |
|---|---|---|
| Moisture or impurities in flux | Flux-cored and submerged arc processes carry part of their shielding chemically; moisture or contamination in the flux decomposes in the arc and generates a sustained volume of gas rather than a brief pulse. | Store and condition flux and cored wire to the consumable supplier's moisture limits and re-bake where the process calls for it. |
| Excessive travel speed or current causing pool turbulence | A turbulent pool lets a gas bubble grow and travel with the solidification front instead of floating out, forming a tunnel rather than a rounded pore. | Measure travel speed and current per pass against the qualified window instead of estimating them. |
| Poor gas shielding coverage on gas-shielded processes | Where shielding gas rather than flux is the barrier, a sustained loss of coverage produces the same tunnelling behaviour as a flux moisture problem. | Monitor gas flow and nozzle condition continuously, not only at process start. |
| Deep, narrow joint or layer geometry that constrains the escape path | A gas bubble generated in a deep groove, or between closely spaced additive layers, has a longer path to a free surface and is more likely to freeze in transit as a tunnel than escape as a rounded pore. | Treat a design that repeatedly produces wormholes as an access problem, not only a parameter problem. |
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 | A wormhole that reaches the surface reads as a small surface pore or crater. The tunnel itself is subsurface and not visible from outside. |
| Thermal | In-process infrared thermography | Supporting signal | A sustained gas evolution event during a pass leaves an extended anomaly in the cooling signature rather than a single spike. |
| Camera | High-speed weld pool imaging | Supporting signal | Continuous bubbling or a visible gas plume from the pool during the run is the precursor to a tunnel-shaped cavity, distinct from an instantaneous spatter event. |
| NDT | Radiographic testing to ISO 17636 | Primary route | Radiography shows a distinctive elongated, tapering trace, sometimes in a repeating herringbone pattern, which is what distinguishes a wormhole from an elongated cavity or a row of pores. |
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 | A wormhole is assessed on its projected length and position rather than treated as a simple rounded pore. Read it in the standard. |
| AWS D1.1 | Structural welding acceptance, with separate criteria for statically and cyclically loaded work | Which criteria apply depends on the loading, so establish that before inspecting. |
| 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. |
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"
