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 5 · Imperfect shape and dimension
Burn-through (ISO 6520-1 ref. 510): causes and detection
ISO 6520-1 reference 510 covers burn-through, collapse of the weld pool through the joint. Cause matrix, the thermal precursor that lets you stop it before it happens, and the standards that govern acceptance.
- ISO 6520-1 reference
- 510
- Group
- 5 — Imperfect shape and dimension
- Also called
- Burn-through, Melt-through
Burn-through is the only imperfection in this ISO 6520-1 group that removes the joint entirely rather than weakening it: the pool loses its solid support and collapses, leaving a hole where the load path should be. It concentrates on thin sheet, root passes and thickness transitions, for the same underlying reason in each case — the parent material available to hold the pool back is thinner than the arc energy assumes.
Unlike most of this group, burn-through gives a genuine precursor rather than only a post-weld finding. Pool width and temperature climb measurably before collapse, a signature an in-process thermal or high-speed camera system can see while the run can still be stopped or the parameter corrected — the difference between a flagged pass and a hole that has to be ground out and re-welded under a qualified repair procedure. Excess penetration is the milder relative of the same heat-versus-thickness problem.
The cause matrix is a heat-input-versus-section-thickness problem in every entry: excess current or slow travel for the gauge, a root gap too wide for the material, heat stacked up from earlier passes, or a procedure carried across a thickness step without a matched parameter set on the thin side.
Typical causes
| Cause | Mechanism | Control |
|---|---|---|
| Excess local heat input relative to section thickness | Current too high or travel too slow for the gauge delivers more energy than the section can conduct away, and the pool loses its solid support. | Match heat input to the thinnest section in the joint, not to the thicker member, and verify travel speed rather than trusting the panel. |
| Root gap or fit-up wider than qualified for the thickness | Excess gap reduces the parent metal available to support the pool at exactly the point where support matters most. | Hold root gap to the qualified tolerance on thin material and treat a widening trend as a fixturing problem. |
| Heat accumulation from repeated passes or short weld lengths | A thin section that has not cooled between passes reaches collapse temperature at a lower arc energy than a cold start would need. | Track interpass condition on thin material and sequence welds to avoid stacking heat in one area. |
| Thickness transition welded without a matched procedure | A procedure qualified for the thicker member, applied across a step down to thinner material, delivers more energy than the thin side can support. | Qualify and apply a separate parameter set either side of a thickness transition rather than carrying one procedure across it. |
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 | Primary route | The hole or heavily sagged root is directly visible once the weld is made, which is why post-weld visual inspection is the definitive route. |
| Thermal | In-process infrared thermography | Supporting signal | Pool width and temperature climb measurably before collapse, a precursor an in-process system can see while the run can still be stopped or corrected. |
| Camera | High-speed weld pool imaging | Supporting signal | Shows the pool losing its solid support in real time, immediately ahead of the collapse itself. |
| NDT | Radiographic testing to ISO 17636 | Limited | The hole is already visible without it; radiography adds little beyond confirming the extent of metal loss for repair sizing. |
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 | Acceptance of burn-through, indexed by ISO 6520-1 reference number in the same group 5 table as the other shape imperfections here | Read the specific requirement directly in the standard rather than assuming it follows the same pattern as a volumetric imperfection. |
| API 1104 | Burn-through and melt-through acceptance for pipeline girth welds | Pipeline acceptance is its own regime, assessed separately from structural or vessel criteria. |
| ASME BPVC Section VIII | Examination and repair requirements for pressure vessel welds | Read the construction code, not the qualification code, for the requirement and the repair route. |
| AWS D1.1 | Weld soundness requirements for structural welds | Structural, pipeline and vessel codes each treat a through-thickness loss of the joint on their own terms. |
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.
