ISO 6520-1 group 1 · Cracks
ISO 6520-1 reference 1001 is a crack visible only under magnification, in weld metal, HAZ or parent material. Cause matrix, why standard production NDT alone can miss it, and where acceptance is set.
Microcrack is the only entry in the crack group whose defining feature is scale rather than orientation or location. ISO 6520-1 reference 1001 covers a crack visible only under magnification, produced by essentially any mechanism that produces a macro crack, just caught before it has grown.
That shared mechanism is why a microcrack deserves attention: hydrogen cracking, hot-cracking segregation, reheat cracking and grain-boundary liquation can all present first as a microcrack, and the conditions that produced one would produce a macro crack given more strain, hydrogen, or time.
The detection consequence is unusual here: neither visual testing nor in-process camera work can resolve a grain-scale feature, and standard production NDT is tuned for larger imperfections. Confirming one means metallographic sectioning. See crater crack and branching crack for other entries a microcrack can precede, lack of fusion for the other entry where sectioning is the real question, and undercut for the surface feature a fatigue-driven microcrack most often starts from.
| Cause | Mechanism | Control |
|---|---|---|
| Localised hydrogen below the threshold for a visible crack | Diffusible hydrogen can initiate cracking at the scale of a few grains without the strain needed to propagate further. | The same controls that prevent macro cold cracking — low-hydrogen consumables, dry storage, controlled preheat — remove the condition regardless of eventual size. |
| Grain-boundary liquation in the partially melted zone | A narrow band just outside the fusion line locally melts at the grain boundary, and strain there can open a fine intergranular crack. | Control heat input and peak temperature in that zone; qualify on the actual heat of material, which this mechanism is sensitive to. |
| Reheat (stress-relief) cracking during post-weld heat treatment | In certain creep-resistant alloys, PWHT strengthens the grain interior faster than the boundary, so stress relief concentrates as intergranular microcracking. | Select PWHT time and temperature with the alloy's reheat-cracking sensitivity in mind rather than a generic schedule. |
| A weld-toe or surface stress concentrator such as undercut, present before any service loading | A sharp toe angle, undercut or overlap concentrates strain during welding and cooling, and a microcrack can initiate there at the fabrication stage — the same feature that would drive fatigue crack growth once the part is loaded in service. | Control toe geometry and surface finish during welding, and correct any confirmed undercut or overlap before the weld enters service rather than treating it as a cosmetic finding. |
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 | Does not find it | A microcrack sits below the magnification ISO 17637 visual testing works at, aided or unaided. |
| Thermal | In-process infrared thermography | Limited | The thermal signature of the producing conditions is measurable, but the crack itself is below a thermal camera's resolution. |
| Camera | High-speed weld pool imaging | Does not find it | A grain-scale feature is well below what any weld-pool camera resolves. |
| NDT | Metallographic examination, or penetrant and magnetic particle testing of a ground and etched surface | Primary route | Standard production ultrasonic or radiographic testing is tuned to larger imperfections; sectioning is how a grain-scale crack is confirmed. |
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, with a documented qualification for microcracks distinct from the general exclusion of cracks. | Microcracks are the one crack sub-type ISO 5817 treats with a qualification rather than a blanket exclusion. |
| ISO 10042 | The equivalent quality-level structure for aluminium and its alloys. | Check the aluminium-specific treatment separately; it does not mirror the steel standard automatically. |
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"
