ISO 6520-1 group 4 · Lack of fusion and penetration
ISO 6520-1 reference 4013 is a planar imperfection at the root of the joint, buried the moment the fill passes go on. Cause matrix, detection routes, and the standards that govern acceptance.
Reference 4013 narrows reference 401 to the root: the pass with the most restricted access, the tightest tolerance for heat input, and the least room to recover once it is covered. It differs from reference 402, lack of penetration, in one precise way — penetration asks whether the arc melted through the parent metal at all, while 4013 asks whether the root pass then fused to the root faces or to backing.
Most causes trace to one operator pattern: running the root pass cold on purpose to guard against burn-through, which trades one imperfection for another. The rest are fit-up conditions — gap, backing alignment, root face land — that restrict arc access or push toward that same cold-running response.
Root-pass monitoring earns unusual attention because it is the one point in the joint where a camera view of pool wetting watches the exact event a later reference number will describe, rather than a proxy for it.
See reference 401, lack of fusion, in general; reference 4011, lack of sidewall fusion, the counterpart at the bevel; and reference 402, lack of penetration.
| Cause | Mechanism | Control |
|---|---|---|
| Root pass run cold to avoid burn-through | Under-heating the root as a safety margin against melt-through can leave the root face unfused rather than merely under-penetrated. | Qualify and hold a root-pass arc energy window instead of letting operators reduce heat ad hoc; verify against measured values. |
| Root gap outside the qualified range | A gap too tight restricts arc access to the root faces; a gap too wide invites the operator response above. | Check and record fit-up gap before the root pass, and treat an out-of-range gap as a stop, not something to compensate for. |
| Backing bar or strip misaligned or poorly fitted | A gap or step at the backing disrupts the intended root geometry and can leave a cold, unfused start along the joint. | Verify backing alignment and contact as a fit-up check, separate from the root gap check. |
| Restricted access position for the root pass | Fixed-position pipe welding and other low-access geometries make it harder to see and correct root wetting in real time. | Qualify the procedure in the actual access position, and monitor it there. |
| Excessive root face land for the joint design | Too much unbevelled land leaves more material for the arc to fuse than the qualified parameters were set to handle. | Verify root face dimensions against the drawing before welding starts; an error found here is cheaper than one found by ultrasonic testing. |
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 | The root is buried the moment the first fill pass goes on. Even where an open root was visible before that, root fusion is not the same thing as root visibility. |
| Thermal | In-process infrared thermography | Supporting signal | A cold root pass shows a distinct cooling signature at the moment it is made, before it is covered by any fill pass. |
| Camera | High-speed weld pool imaging | Supporting signal | The root pass is the one place in the joint where camera visibility of pool wetting and arc position is most directly tied to a named imperfection, which is why orbital and mechanised root-pass monitoring is built around this view. |
| NDT | Ultrasonic and phased array testing to ISO 17640 and ISO 13588 | Primary route | Reaches the root from either surface, without the back-side access a visual root check needs and without radiography's dependence on a favourable beam-to-plane angle. |
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 planar imperfections, indexed by ISO 6520-1 reference number | Root lack of fusion is treated under the same strict planar-imperfection rows as the parent reference 401. |
| ISO 11666 | Ultrasonic acceptance levels for welds, paired with the quality level chosen | The examination standard and the quality-level standard are selected together, not independently. |
| API 1104 | Acceptance for pipeline girth welds, where root fusion draws particular attention | Pipeline acceptance is its own regime; do not carry a structural criterion into it. |
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"
