ISO 6520-1 group 1 · Cracks
ISO 6520-1 reference 104 is the crack that forms in an unfilled weld crater at arc stop. Cause matrix, why crater-fill technique is the real control, and where acceptance is set.
Crater crack is the one crack-group imperfection with a single, well-understood trigger: the arc was broken before the crater was filled. ISO 6520-1 reference 104 covers the crack that forms in that unfilled depression at the end of a weld run.
The physics is ordinary solidification (hot) cracking concentrated in the worst possible geometry: a concave pool with the least surrounding material to resist shrinkage, cooling fastest exactly where restraint is highest. That is why the fix is procedural — a programmed current taper or a back-step technique gives the last metal time to fill the depression.
Because the stop is discrete and predictable, it is easy to watch for directly, which matters most on parts with many stop-starts by design, such as wire-arc additive manufacturing builds. See longitudinal crack and microcrack for the other hot-cracking entries in this group.
| Cause | Mechanism | Control |
|---|---|---|
| Arc broken abruptly without filling the crater | The last molten metal in an unfilled crater has the least surrounding material to resist shrinkage, concentrated by the crater's concave shape. | Use a programmed crater-fill or current-taper function, or a deliberate back-step technique, before extinguishing the arc. |
| High current or fast travel speed maintained into the stop | A larger, hotter pool at arc break leaves more shrinkage volume concentrated in the crater with no time to backfill. | Reduce current and travel speed approaching a planned stop, and verify the technique is actually used. |
| High sulphur or phosphorus content, or excessive dilution | The low-melting segregation that drives centreline hot cracking concentrates at the crater, where cooling is fastest. | Control filler and base metal chemistry, and manage dilution at the stop as for the rest of the run. |
| Repeated stop-start welding | Every arc stop is a crater, so a technique or geometry forcing many short runs multiplies crack-prone stops. | Minimise unnecessary stops and treat crater technique as mandatory on every stop, not only the final one. |
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 crater is at the surface by definition, so visual testing with a gauge is the intended route and finds most instances. |
| Thermal | In-process infrared thermography | Supporting signal | The cooling signature at a stop shows whether the crater filled normally or retained an abnormal hot spot. |
| Camera | High-speed weld pool imaging | Supporting signal | The stop is a discrete, predictable event: whether crater-fill technique was executed is visible as it happens. |
| NDT | Penetrant or magnetic particle testing to ISO 3452-1 and ISO 17638 | Supporting signal | Confirms and delineates what visual testing already found; rarely the leading route for a surface-visible imperfection. |
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, including crater cracks, are excluded from every level. | A crater crack is graded the same as any other crack: found means repaired, not sized against a permitted extent. |
| ISO 10042 | The equivalent quality-level structure for aluminium and its alloys. | Aluminium is frequently more crater-crack prone than steel, so technique is qualified separately for it. |
| AWS D1.1 | Structural welding acceptance, treating any crack, including a crater crack, as a mandatory-repair finding regardless of loading. | There is no static-versus-cyclic distinction for cracks the way there is for undercut. |
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"
