Incomplete penetration is a weld that didn’t go deep enough: the joint was designed to be fused through its full thickness, but the root was left unfused or unfilled. The drawing says “full penetration butt weld”; the cross-section says otherwise.
Its danger is structural arithmetic plus fracture mechanics: the effective throat is smaller than design and the unfused root is a planar, crack-like gap positioned exactly where bending stress in many joints peaks.
What is incomplete penetration?
ISO 6520-1 group 402 (lack of penetration):
| Type | Code | Description |
|---|---|---|
| Incomplete penetration | 402 | Weld fails to extend through required thickness |
| Incomplete root penetration | 4021 | Root face of a single-side butt weld not fused/filled |
Related but distinct: incomplete root fusion (4013) — metal reached the root but didn’t bond (lack of fusion family).
One critical clarification before sentencing anything: partial-penetration joints are not defective by design. A fillet or partial-pen butt weld specified with throat a or penetration s on the drawing is judged against that requirement. Incomplete penetration is a defect only relative to the specified penetration — which is why the weld symbol and WPS must travel with the inspection record.
ISO 5817 acceptance criteria
For joints specified as full penetration, ISO 5817 distinguishes by joint configuration:
- Incomplete penetration (402, double-side accessible joints): not permitted at Level B; at C and D only short imperfections within dimensional caps may be tolerated.
- Incomplete root penetration (4021, single-side butt welds): not permitted at Levels B and C; at Level D only short imperfections up to h ≤ 0.2t (max 2 mm).
- Incomplete root fusion (4013) is not permitted at any level.
- Exact wording and caps are in ISO 5817 Table 1; verify the current edition. Pressure, pipeline and fatigue codes (e.g. ASME B31.3, API 1104) impose their own root criteria, frequently stricter.
Quality-level context: ISO 5817 guide · B vs C vs D comparison.
What causes incomplete penetration
Energy deficit
- Current too low for root pass thickness — the most common cause
- Travel speed too high: energy per unit length insufficient (see heat input calculation)
- Arc length too long, dissipating energy at the surface
Joint geometry
- Root face too thick or root gap too tight for the process
- Bevel angle too narrow, restricting arc access to the root
- Misalignment shifting the arc off the root centreline (compare linear misalignment limits)
Process selection and technique
- Electrode/wire diameter too large to enter the root opening
- Push angle excessive, deflecting penetration forward
- In single-side welding without backing: pool control errors leaving the root cold — the inverse failure of burn-through, and a sign the parameter window is too tight for open-loop control
Detection
| Method | Notes |
|---|---|
| Visual (root side) | Only when the root is accessible — single-side pipe welds, open structures |
| Radiography (ISO 17636) | Root gap = sharp dark line; the classic method for pipeline girth welds |
| Ultrasonic / PAUT | Locates and sizes the unfused root; preferred on thick sections |
| Macro sections | Destructive — qualification and periodic sampling, not 100% control |
| Thermal monitoring | Penetration tracks the thermal field: a cold root has a measurable signature. Per-weld, non-destructive — see penetration depth control with thermal monitoring |
The production problem with the first four: they’re either sample-based or post-weld. A parameter drift that costs 1 mm of penetration produces a day of defective parts before the next RT sample. Continuous thermal monitoring turns penetration from a sampled destructive property into a per-weld measured one.
Prevention
- Qualify the root pass at the low end of the current range — if penetration is marginal at WPS minimum, the window is wrong.
- Control fit-up: root gap and root face within WPS tolerance, every joint — fit-up drift is penetration drift (see joint fit-up verification).
- Match wire/electrode diameter to the root opening.
- Keep the arc on the leading edge of the pool, driving into the root.
- Monitor every weld: penetration is invisible from the cap side — exactly the kind of defect that 100% in-process monitoring exists for.
Incomplete penetration is one of the root defects mapped in the welding defects guide with ISO 5817 acceptance criteria.
Frequently Asked Questions
What is incomplete penetration in welding?
Incomplete penetration (ISO 6520-1 code 402) is weld metal that fails to extend through the required joint thickness — the root of the joint is left partially unfused or unfilled. In a full-penetration butt weld this leaves a planar gap at the root; the actual throat is thinner than design, and the root gap acts as a built-in crack under load.
What is the difference between incomplete penetration and lack of fusion?
Incomplete penetration (402) means the weld did not reach deep enough — the root was never filled. Lack of fusion (401) means weld metal reached a surface but failed to bond with it. They can coexist at the root, and the distinction matters for root-cause analysis: penetration is mostly an energy/geometry problem, fusion is mostly an arc-placement problem.
Is incomplete penetration acceptable under ISO 5817?
For joints designed as full penetration, ISO 5817 does not permit incomplete penetration at Level B. For single-side butt welds, incomplete root penetration (4021) is also not permitted at Level C — only Level D tolerates short imperfections up to h ≤ 0.2t (max 2 mm). Incomplete root fusion (4013) is not permitted at any level. Partial-penetration joints are different: there, the unfused portion is a design feature, not a defect. Verify the current edition and your code.
How do you verify weld penetration without destructive testing?
Volumetric NDT: radiography shows a root gap as a sharp dark line; ultrasonic testing locates and sizes the unfused root. In production, penetration correlates strongly with the thermal field of the weld — root-side or face-side thermal monitoring tracks penetration indirectly on every weld, replacing the sample-based macro section as the day-to-day control.
