Shipbuilding and marine fabrication
Weld monitoring for shipbuilding
High-volume welding under classification-society rules. The codes that apply, the four monitoring pain points that cost yards most, and which Therness signal addresses each.
ISO 6520-1 group 2 · Cavities
Surface pore (ISO 6520-1 ref. 2017): causes and detection
ISO 6520-1 reference 2017 is a gas pore that breaks the weld surface, the one porosity designation visual testing actually leads on. Cause matrix, detection routes, and where acceptance is set.
- ISO 6520-1 reference
- 2017
- Group
- 2 — Cavities
- Also called
- Surface pore, Surface-breaking pore, Open pore
Surface pore is the outlier in ISO 6520-1 group 2: nearly every other cavity designation is defined by what a subsurface radiograph shows, and this one is defined by what an inspector sees standing at the weld. A gas bubble reaches the surface at the moment the pool freezes and stays open instead of closing over, the same way an open toe groove stays open in continuous undercut — both are surface-breaking imperfections that visual testing is built to find.
That is also why the cause matrix reads differently from a subsurface designation. Every entry in it is a condition that puts the gas source at, or very near, the surface at the exact moment of freezing — surface contamination, a shielding lapse timed with the end of a pass, or an unfilled crater — rather than a condition present through the depth of the weld metal.
The practical value of watching the pool while welding is at its clearest here: a bubble failing to close over is visible on a high-speed camera before the bead has even finished cooling, at the point where a parameter correction still costs nothing. By the time the same pore is confirmed with a gauge or a penetrant check, it is already a rework decision rather than a process adjustment.
Typical causes
| Cause | Mechanism | Control |
|---|---|---|
| Gas evolution timed with the pool freezing at the surface | A single bubble reaches the surface just as the surrounding metal solidifies, so it cannot close over and leaves an open pore instead of a subsurface one. | Hold arc energy and travel speed inside the qualified window so the pool has time to close over gas before it freezes. |
| Contamination local to the surface being welded | Surface moisture, oil or coating on the joint releases gas at the pool surface rather than deeper in the weld metal, so the escaping gas has the shortest possible path out. | Strip the weld zone to bright, dry metal immediately before welding. |
| Shielding loss at the moment of solidification | A brief shielding interruption right as a pass finishes lets air reach the pool exactly when it is freezing, producing a surface-breaking pore at that point rather than a subsurface one. | Maintain gas post-flow and hold the torch in position briefly after the arc extinguishes. |
| Crater termination without proper fill | Gas concentrated in an unfilled crater at the end of a pass has the shortest path to a free surface and breaks through as the crater solidifies. | Use a crater-fill technique or current downslope rather than breaking the arc abruptly. |
Detection: which route finds it
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, with weld gauges for depth | Primary route | The imperfection is a single pore breaking the surface, so visual testing is the intended route and the one every acceptance level actually assumes. |
| Thermal | In-process infrared thermography | Supporting signal | The gas release that produces a surface pore disturbs the pool at the same point, which is measurable while the run is being made. |
| Camera | High-speed weld pool imaging | Supporting signal | Shows the pool failing to close over an escaping bubble at the surface, the moment the parameter or the shielding could still be corrected. |
| NDT | Radiographic testing to ISO 17636, penetrant testing to ISO 3452-1 | Supporting signal | Confirms and can help size a surface pore; neither is the route that finds it first, unlike a subsurface porosity designation. |
Where acceptance is defined
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 surface imperfections, indexed by ISO 6520-1 reference number | Graded as a surface imperfection, separately from the internal porosity designations (2011-2014). Read it in the standard. |
| ISO 10042 | The equivalent quality levels for aluminium and its alloys | Use this instead of ISO 5817 whenever the parent material is aluminium. |
| AWS D1.1 | Visual inspection acceptance, with different criteria for statically and cyclically loaded connections | Cyclically loaded work is generally treated more severely for surface-breaking imperfections. |
See the condition, not the consequence
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.
