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
Gas pore (ISO 6520-1 ref. 2011): causes and detection
ISO 6520-1 reference 2011 covers the single gas pore. Cause matrix, which inspection route actually finds it, and the standards that set the acceptance limit for your material and code.
- ISO 6520-1 reference
- 2011
- Group
- 2 — Cavities
- Also called
- Gas pore, Blowhole, Porosity
The single gas pore is the mildest member of ISO 6520-1 group 2 and the one most often mislabelled on a report. “Porosity” on the shop floor covers several distinct designations — uniformly distributed, clustered, linear, elongated, wormhole and surface — and each is graded on its own terms. Getting the designation right before grading is not pedantry; the standard is indexed by it.
Almost every root cause reduces to one of two questions: where did the gas come from, and why could it not get out. Contamination, coatings and lost shielding answer the first. Freezing rate answers the second, which is why the same joint prepared the same way can be clean in August and porous in a cold shop in January.
That split explains why in-process evidence is worth having. Gas flow at the torch, arc voltage per pass and the cooling signature of the bead are all measurable while the weld is being made — and a weld flagged there is one the radiographer can be pointed at first, rather than found three days later in a batch report.
Typical causes
| Cause | Mechanism | Control |
|---|---|---|
| Loss of shielding gas at the arc | Draught, a blocked or spattered nozzle, a leaking hose or a flow rate set for a different torch lets air reach the molten pool. | Screen the cell, monitor gas flow at the torch rather than at the regulator, and log the reading with the weld. |
| Moisture and hydrocarbon contamination | Water, oil, cutting fluid or condensation dissociates in the arc and the dissolved gas comes out of solution as the pool freezes. | Control consumable storage and re-drying, degrease the joint, and treat overnight condensation as a process variable. |
| Surface coatings on the parent material | Primer, paint, rust, scale and zinc vaporise ahead of the arc and inject gas into the pool faster than it can escape. | Strip the weld zone to bright metal; where a coating must stay, qualify the procedure with it present. |
| Arc length or voltage above the qualified range | A long arc widens the gas envelope and entrains air, and it changes droplet transfer at the same time. | Record voltage per pass against the qualified range instead of trusting the panel setting. |
| Freezing rate too fast for the gas to escape | Low arc energy or a heavy chill from thick or cold parent material traps bubbles that would otherwise rise out. | Hold arc energy and preheat inside the qualified window and verify both from measured values. |
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 | Limited | Finds surface pores and only surface pores. A subsurface gas pore is invisible to it by definition. |
| Thermal | In-process infrared thermography | Supporting signal | A cavity perturbs the cooling signature of the bead, which flags the weld for inspection. It is a screening signal, never an acceptance result. |
| Camera | High-speed weld pool imaging | Supporting signal | Shows the precursor rather than the pore: gas disturbance, arc wander and pool agitation while the weld is still being made. |
| NDT | Radiographic testing to ISO 17636, ultrasonic testing to ISO 17640 | Primary route | Radiography is well suited to volumetric imperfections. Ultrasonic testing detects them but sizing a rounded cavity is harder than sizing a planar one. |
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 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 and ultrasonic acceptance for pressure vessels, with examination methods in Section V | Acceptance lives in the construction code, not in the qualification code. |
| 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. |
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.
