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
Overlap (ISO 6520-1 ref. 506): causes and detection
ISO 6520-1 reference 506 covers overlap, weld metal folded over the joint face without fusing to it. Cause matrix, why the fold hides a fusion problem underneath, and the standards that set the acceptance limit for your material and code.
- ISO 6520-1 reference
- 506
- Group
- 5 — Imperfect shape and dimension
- Also called
- Overlap, Toe overlap, Root overlap, Cold lap
Overlap happens when the weld pool arrives at the toe or root faster than it can wet into the parent material: the excess metal folds over a surface it never metallurgically bonded to. Seen from outside, the fold can look like sound weld metal sitting proud of the joint; the actual discontinuity is the unfused interface underneath it, which is why the ISO 5817 acceptance criteria grade overlap with the severity of a planar imperfection, not a cosmetic one.
That unfused interface is also why overlap rarely appears alone. A cold, sluggish pool that folds at the toe instead of wetting it is frequently also failing to fuse the sidewall beneath the fold, so a visual finding of overlap is a reasonable trigger to check the same run with ultrasonic testing rather than accepting the surface call as the whole story — see lack of fusion for the underlying mechanism.
The cause matrix is a fluidity and aim problem: travel too slow, current too low, the arc pointed at the pool instead of the leading edge. All of it is visible on a high-speed weld pool camera while the run is still being made, which is the only point at which any of it can still be corrected without a repair.
Typical causes
| Cause | Mechanism | Control |
|---|---|---|
| Travel speed too slow relative to current | The pool grows faster than the arc advances, and the excess metal folds over the toe onto unmelted parent material instead of fusing into it. | Measure travel speed against the qualified range; overlap is one of the clearest signs travel is lagging current. |
| Current or wire feed speed too low to keep the pool fluid | A cooler, more viscous pool loses the fluidity to wet the toe and instead piles at the edge of the joint. | Log current and wire feed speed per pass; a cold pool produces overlap and lack of fusion together more often than either appears alone. |
| Incorrect torch angle or excessive weave amplitude | The arc is aimed away from the leading edge of the pool, so metal is deposited beside the joint rather than into it. | Qualify torch angle and weave for the position and verify it in the cell rather than trusting the procedure sheet alone. |
| Welding position working against the pool | In horizontal and overhead work gravity pulls the pool down and over the lower toe before it has fused. | Position welds where practical and qualify separately for positions that cannot be avoided. |
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 | Primary route | Overlap is a surface fold, open and directly measurable, which is why visual testing is the intended route and the one every code assumes. |
| Thermal | In-process infrared thermography | Supporting signal | A pool that is losing fluidity and starting to pile at the toe shows in the cooling signature before the fold is visible on the finished bead. |
| Camera | High-speed weld pool imaging | Supporting signal | Shows the pool failing to wet the toe and folding over it while the run is being made, which is the moment travel speed or aim can still be corrected. |
| NDT | Magnetic particle or penetrant testing to ISO 17638 and ISO 3452-1, ultrasonic testing to ISO 17640 | Supporting signal | Surface methods confirm the extent of the fold; ultrasonic testing checks whether a lack-of-fusion pocket sits beneath it, which a visual call alone cannot rule out. |
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 overlap, indexed by ISO 6520-1 reference number | Overlap is generally treated with the severity of a planar imperfection rather than a cosmetic one. Read it in the standard. |
| AWS D1.1 | Visual acceptance criteria distinguishing statically and cyclically loaded structural connections | Which criteria apply depends on the loading, so establish that before inspecting. |
| ISO 10042 | The equivalent quality levels for aluminium and its alloys | Use this instead of ISO 5817 whenever the parent material is aluminium. |
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.
