ISO 6520-1 group 1 · Cracks
ISO 6520-1 reference 101 covers cracks running parallel to the weld axis, most often a solidification crack. Cause matrix, why detection depends on whether it breaks the surface, and where acceptance is set.
Longitudinal crack is what a welder usually means by “centreline crack.” ISO 6520-1 reference 101 covers any crack running parallel to the weld axis — in the weld metal, at the fusion line, in the heat-affected zone or in the parent material.
Most instances are solidification (hot) cracks: the last liquid to freeze along the centreline is enriched in low-melting segregates, and if the joint cannot contract freely, shrinkage strain opens that still-weak plane. A narrower, deeper pool makes this worse. A minority are delayed hydrogen cracks running the same axis for an unrelated reason, which is why the cause has to be established before the fix is chosen.
See transverse crack and crater crack for the other members of this ISO 6520-1 group, and lack of fusion for the planar imperfection sharing its detection route.
| Cause | Mechanism | Control |
|---|---|---|
| Narrow, deep ('finger') weld pool shape | A pool deeper than it is wide solidifies from both sides toward a single centreline plane, trapping segregated, low-melting metal there. | Set parameters for a wide, shallow pool and verify the achieved bead profile, not only the panel settings. |
| High sulphur or phosphorus content, or excessive dilution | Low-melting eutectic films stay liquid after the surrounding metal solidifies and cannot carry shrinkage strain along the bead axis. | Control filler and base material chemistry through certified records, and manage dilution through joint prep and parameters. |
| High restraint from joint design, fit-up or fixturing | A joint that cannot contract freely along its length concentrates shrinkage strain in the still-solidifying centreline. | Sequence and stagger welds to release restraint progressively, and qualify fixturing for the joint thickness. |
| Diffusible hydrogen with a hardenable microstructure and restraint | Hydrogen diffusing to the fusion line or HAZ boundary can open a delayed crack there hours after welding. | Control consumable hydrogen and preheat/interpass temperature. |
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 | Supporting signal | Finds a crack that has broken the surface; a subsurface centreline or hydrogen crack is invisible to it. |
| Thermal | In-process infrared thermography | Supporting signal | Reveals a pool shape or cooling rate outside the qualified window before the crack opens. |
| Camera | High-speed weld pool imaging | Supporting signal | Shows the pool's trailing edge narrowing into an unfavourable shape while the run is still being made. |
| NDT | Penetrant/magnetic particle testing for a surface-breaking crack, ultrasonic or phased array testing to ISO 17640 and ISO 13588 for a subsurface one | Primary route | A tight, planar crack gives too little radiographic contrast to register reliably, so ultrasonic or phased array testing carries detection; where hydrogen cracking is suspected, delay inspection long enough — up to 48 hours — for a delayed crack to have formed. |
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 indexed by ISO 6520-1 reference number; cracks are excluded from every level. | A crack is not sized against a permitted extent — its presence is itself the finding. |
| ISO 10042 | The equivalent quality-level structure for aluminium and its alloys. | Use this instead of ISO 5817 whenever the parent material is aluminium. |
| ASME BPVC Section VIII | Pressure vessel acceptance, with examination methods in Section V and repair requirements for any crack found. | A crack found by any qualified examination method is addressed through the code's repair provisions, not graded. |
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.
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Priorità I, Obiettivo Specifico a), Azione 4 – "Sostegno alla nascita delle start up"
