Shipbuilding and marine fabrication
Weld monitoring for shipbuilding
High-volume welding under classification-society rules. The codes that apply, the four monitoring pain points that cost yards most, and which Therness signal addresses each.
ISO 6520-1 group 4 · Lack of fusion and penetration
Lack of penetration (ISO 6520-1 ref. 402): causes and detection
ISO 6520-1 reference 402 is a shortfall in the depth the weld metal actually reaches at the root. Cause matrix, why radiography leads detection, and the standards that govern acceptance.
- ISO 6520-1 reference
- 402
- Group
- 4 — Lack of fusion and penetration
- Also called
- Incomplete penetration, Lack of penetration, Insufficient root penetration
Reference 402 is a shortfall in depth: weld metal did not reach as far into the joint as design intended, most often at the root of a single-sided full-penetration weld. It differs from lack of fusion in mechanism: fusion is a bonding failure at an interface that was reached; penetration is a failure to reach the interface at all.
The cause matrix asks whether the arc had the energy, access and time to melt through to the intended depth. Root gap and root face set the geometry; arc energy and travel speed set whether the arc can do the job in the time available; misalignment and process selection change what “enough” means for a given joint.
Radiography reads this imperfection with unusual clarity — a continuous line along the root, one of the more recognisable indications in weld film, which is why it has long been the benchmark case for teaching radiographic interpretation. Thermal and camera evidence on the root pass earn their place earlier, as a screening signal on the run.
See reference 4013, lack of root fusion, the bonding failure this imperfection is most often confused with; reference 4011, lack of sidewall fusion; and reference 401, lack of fusion, in general.
Typical causes
| Cause | Mechanism | Control |
|---|---|---|
| Arc energy insufficient for the joint thickness | The arc melts parent metal at the surface without carrying enough energy through the section to reach the intended root depth. | Set and verify arc energy against the qualified procedure for the actual thickness, not a nominal one. |
| Root gap too small or root face too large | A tight fit-up gives the arc no path through the joint; an oversized root face leaves more material than qualified parameters were set to melt through. | Check fit-up against the drawing before welding starts; an error here is cheaper to fix than one found by radiography. |
| Travel speed too high on the root pass | The pool has less time at the root than the procedure assumes, so the melt front does not reach the intended depth. | Measure travel speed on the root pass specifically; it is often set differently, and less carefully, than on fill passes. |
| Joint misalignment or high-low at the root | A step between the two parts changes the effective root geometry, so a procedure qualified on a flush joint under-penetrates a misaligned one. | Check and record alignment at the root before welding, and treat an out-of-tolerance step as a fit-up stop. |
| Process or polarity without enough arc force for the joint design | Some process and polarity combinations produce a wide, shallow pool rather than the digging action a deep root needs. | Qualify process and polarity for the joint design being welded, not for convenience alone. |
Detection: which route finds it
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 | Limited | Confirms visible root reinforcement or melt-through only where back-side access exists. It cannot confirm the depth the weld metal reached inside the section. |
| Thermal | In-process infrared thermography | Supporting signal | A back-side or through-thickness thermal signature correlates with how far the melt front reached, which is why thermal monitoring is used as a screening signal for penetration — never as the acceptance result itself. |
| Camera | High-speed weld pool imaging | Supporting signal | Keyhole and pool shape on the root pass are a real-time proxy for penetration depth while the run is being made. |
| NDT | Radiographic testing to ISO 17636 | Primary route | The classic route: a lack of penetration reads as a dark, continuous line at the root on the film, one of the more distinctive indications in weld radiography. |
Where acceptance is defined
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 | Lack of penetration is graded under the same strict planar-imperfection rows as lack of fusion. |
| API 1104 | Acceptance for pipeline girth welds, assessed by radiographic interpretation | Pipeline acceptance is its own regime; do not carry a structural criterion into it. |
See the condition, not the consequence
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.
