ISO 6520-1 group 4 · Lack of fusion and penetration
ISO 6520-1 reference 4011 is a planar imperfection at the joint bevel face that visual inspection cannot see. Cause matrix, why ultrasonic testing leads detection, and the standards that govern acceptance.
Reference 4011 narrows reference 401 to one specific location: the sidewall, the bevel face of the joint rather than the previous run. It is the form of lack of fusion most associated with narrow-groove and mechanised welding, where a tighter bevel saves filler metal and time — and restricts the exact arc access the sidewall needs.
The cause matrix asks one question from different angles: did the arc reach the bevel face with enough energy and dwell time to fuse it. A narrow groove restricts access geometrically. High travel speed and low arc energy restrict it thermally. A torch aimed at the pool instead of the sidewall restricts it by technique. Any one produces the same buried, planar result.
Pairing arc energy and travel speed records with a pool camera view of sidewall wetting does not confirm fusion after the fact, but shows whether the conditions that produce 4011 were present on the pass ultrasonic testing will later have to find them on.
See reference 401, lack of fusion, in general; reference 4013, lack of root fusion, the sibling at the other end of the joint; and reference 402, lack of penetration.
| Cause | Mechanism | Control |
|---|---|---|
| Arc not aimed at the bevel face | A torch, gun or weave pattern centred on the pool rather than dwelling on the sidewall leaves the bevel face itself below fusion temperature even while the pool looks normal. | Qualify torch angle and weave pattern for the joint geometry and verify positioning in the cell, not only on the procedure sheet. |
| Groove angle too narrow for the process | A narrow bevel, common in narrow-gap and mechanised welding for cost, restricts arc access to the sidewall at any parameter setting. | Treat repeated sidewall findings on one joint design as a design or access problem, not an operator problem. |
| Travel speed too high on fill passes | The pool advances faster than it can wet the sidewall, laying metal over a bevel face that never reached fusion temperature. | Measure travel speed rather than estimating it from bead appearance; on mechanised work it is the parameter most likely to drift with fixturing. |
| Arc energy below the qualified window | Insufficient energy per unit length fails to melt the bevel face before the adjacent pool solidifies against it. | Compute arc energy from measured current, voltage and travel speed per pass instead of nominal figures. |
| Mill scale or oxide left on the bevel face | A surface film prevents metallurgical bonding at the sidewall even when the thermal conditions are otherwise correct. | Enforce joint-face cleaning as a hold point ahead of each pass and record that it happened. |
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 imperfection sits at the bevel face under the cap. A visually acceptable bead gives no information about sidewall bonding underneath it. |
| Thermal | In-process infrared thermography | Supporting signal | Sees the conditions that produce it — low arc energy, high travel speed, an arc not dwelling on the sidewall — rather than the imperfection itself. |
| Camera | High-speed weld pool imaging | Supporting signal | Shows whether the pool is actually wetting the bevel face and where the arc is aimed relative to it, while the run is being made. |
| NDT | Ultrasonic and phased array testing to ISO 17640 and ISO 13588 | Primary route | The leading route for a planar imperfection at a known geometric location. Radiography can miss one entirely when the plane is unfavourable to the beam. |
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 | Sidewall 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. |
| ASME BPVC Section VIII | Pressure vessel acceptance, with examination methods in Section V | Read the construction code, not the qualification code, for the limit. |
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"
