ISO 6520-1 group 5 · Imperfect shape and dimension
ISO 6520-1 reference 507 covers linear misalignment between welded members. Cause matrix, why direct measurement leads over any inspection method, and the standards that set the acceptance limit for your material and code.
Linear misalignment is the one imperfection in this batch that inspection does not so much find as measure: the two members are offset before the arc is ever struck, and no amount of weld quality corrects a joint that started out of plane. It is a fit-up condition carrying an ISO 6520-1 number.
That distinction matters for where the fix belongs. A welder cannot weld out an offset that tooling created, and a system watching the arc has nothing useful to say about a condition that exists before the arc starts — see overlap for an imperfection that, by contrast, forms entirely during the run. The check for misalignment belongs at tack-up, with a straightedge or a hi-lo gauge, and again immediately after tacking, because a tack made against an already-offset edge freezes the condition in rather than catching it.
On cyclically loaded joints the offset does more than look wrong: it introduces local bending into what the design assumed was a straight load path, so the same offset costs more on a fatigue-critical seam than on a static one. That is also why pipeline, structural and pressure-vessel codes each set their own tolerance rather than sharing one.
| Cause | Mechanism | Control |
|---|---|---|
| Fit-up tooling or clamping out of tolerance | The two members are held at different heights before the arc is ever struck, so the offset is a fixturing condition rather than a welding one. | Verify fit-up against the drawing at the tack stage, not after the joint is welded and the offset is fixed in place. |
| Tack welds placed without checking alignment first | A tack made against a locally sprung or distorted edge freezes the offset in before the production weld ever starts. | Check alignment immediately before and after tacking, not only at final fit-up. |
| Component tolerance stack-up from upstream cutting or forming | Plate thickness variation, camber, or an out-of-tolerance edge accumulates across a multi-part assembly until the joint faces no longer line up. | Treat a recurring offset on the same joint type as an upstream tolerance problem rather than a fit-up habit on the cell floor. |
| Insufficient restraint during welding | Shrinkage from earlier passes pulls one member out of line before later passes are made, especially on a long seam welded from one end. | Sequence and restrain long seams to control distortion rather than correcting alignment weld by weld. |
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 a straightedge or hi-lo gauge | Primary route | The offset is a direct geometric measurement, checked before, during and after welding rather than inferred from any inspection signal. |
| Thermal | In-process infrared thermography | Limited | A dimensional offset between two plates is a geometric condition, not a thermal one; thermography may show an asymmetric cooling pattern at a severe mismatch but does not replace direct measurement. |
| Camera | Weld pool imaging | Does not find it | The camera's field of view is the arc and pool. Plate-to-plate offset is a fit-up condition set before the arc starts, outside that view. |
| NDT | Radiographic testing to ISO 17636 | Supporting signal | A significant offset can show as a density step on a radiograph and complicates reading the same image for other imperfections, but neither radiography nor ultrasonic testing is how the offset itself is normally verified. |
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 linear misalignment, indexed by ISO 6520-1 reference number and read together with wall thickness | The permitted offset depends on the selected quality level and the thickness of the thinner member. Read it in the standard. |
| API 1104 | Hi-lo acceptance for pipeline girth welds, assessed independently of structural criteria | Pipeline acceptance is its own regime; do not carry a structural or vessel criterion into it. |
| AWS D1.1 | Alignment tolerance for structural butt joints, distinct from the pipeline criterion | Structural and pipeline tolerances are not interchangeable even for the same nominal thickness. |
| ASME BPVC Section VIII | Plate alignment tolerance for pressure vessel longitudinal and circumferential seams | Read the construction code, not the qualification code, for the requirement. |
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"
