ISO 5817 defines three quality levels for fusion weld imperfections — B, C, and D — and the difference between them is not semantic. A weld accepted at Level D will be rejected at Level B for the same undercut depth. Specifying or auditing the wrong level is not a paperwork issue: it translates directly into fatigue life, liability, and conformity under EN 1090, ISO 3834, and sector codes.
This article puts all three levels side by side so engineers, inspectors, and QMS managers can read the comparison in one place — not scattered across 40 pages of normative tables.
What the three levels actually mean
ISO 5817 classifies weld quality based on imperfection severity, not manufacturing method or material. The underlying imperfection catalogue is drawn from ISO 6520-1, which codes every weld discontinuity type (crack, pore, undercut, lack of fusion, misalignment, excess weld metal, and so on).
The three levels are:
| Level | Name | Intent |
|---|---|---|
| B | Stringent | Safety-critical, dynamically loaded, fatigue-sensitive welds |
| C | Intermediate | Moderate fatigue loading, general engineering |
| D | Moderate | Static loading, non-critical geometry, low consequence of failure |
The letters deliberately run B → D rather than A → C: the ISO committee left room for a future tighter level (which does not yet exist in this standard, though EN 15085-3 class CP A fills that role for rail).
One invariant across all three levels: cracks, lack of fusion, and lack of root penetration are not acceptable at any level (they are imperfections in Group 1 of ISO 5817, coded as “not permitted” regardless of B/C/D). Levels govern dimensional limits on surface-breaking and volumetric imperfections, not binary presence of planar defects.
Acceptance limits: B vs C vs D side by side
The table below compares the most frequently encountered imperfections in production. All limits reference ISO 5817:2023 Tables 1–5. Where limits are thickness-dependent, t is nominal material thickness at the weld.
| Imperfection type | ISO 6520 code | Level B | Level C | Level D |
|---|---|---|---|---|
| Continuous undercut | 5011 | h ≤ 0.05t, max 0.5 mm | h ≤ 0.1t, max 1 mm | h ≤ 0.2t, max 2 mm |
| Intermittent undercut | 5011 | h ≤ 0.1t, max 1 mm | h ≤ 0.2t, max 2 mm | h ≤ 0.2t, max 2 mm |
| Root undercut | 5013 | Not permitted | h ≤ 0.05t, max 0.5 mm | h ≤ 0.1t, max 1 mm |
| Overlap (cold lap) | 506 | Not permitted | Not permitted | h ≤ 1 mm |
| Angular misalignment (butt) | 508 | α ≤ 1° | α ≤ 2° | α ≤ 3° |
| Linear misalignment (butt) | 507 | h ≤ 0.05t, max 2 mm | h ≤ 0.1t, max 3 mm | h ≤ 0.15t, max 4 mm |
| Excess weld metal (butt, face) | 502 | h ≤ 1 mm + 0.1b, max 5 mm | h ≤ 1 mm + 0.15b, max 7 mm | h ≤ 1 mm + 0.25b, max 10 mm |
| Porosity (single pore) | 2011 | d ≤ 0.5 mm or 0.05t | d ≤ 0.5 mm or 0.1t | d ≤ 0.5 mm or 0.2t, max 3 mm |
| Porosity (field of pores) | 2012 | Max 2% cross-section area | Max 4% | Max 8% |
| Crater pipe | 2025 | Not permitted | h ≤ 0.1t, max 1 mm | h ≤ 0.2t, max 2 mm |
| Incomplete root fusion | 4013 | Not permitted | Not permitted | Not permitted |
| Crack (all types) | 100 | Not permitted | Not permitted | Not permitted |
| Lack of fusion | 401 | Not permitted | Not permitted | Not permitted |
Per-defect deep dives — causes, detection, prevention — are collected in the welding defects guide.
Note:
b= weld width,d= diameter of pore,h= height/depth,α= angle. Values above are indicative references — always verify against the current edition of the standard and any overriding customer or code requirement.
The limits in ISO 5817 are lower bounds for rejection, not targets for production. A weld that just passes Level B undercut at 0.04t is not “good” — it is borderline. Process capability (Cpk) targets should keep actual imperfection distribution well inside the limit, not touch it.
Which level applies to your application?
The quality level for a weld is always specified externally — by the governing construction standard, the customer specification, or the regulatory code. It is not a choice made freely by the fabricator.
EN 1090-2 (structural steel, EXC classes)
EN 1090-2 is the primary vehicle for steel structures in Europe. It maps execution classes (EXC) to required ISO 5817 quality levels:
| EXC class | Typical structure | ISO 5817 level |
|---|---|---|
| EXC 1 | Statically loaded, consequence class CC1 | D |
| EXC 2 | General construction, CC1/CC2 | C (D for some joint types) |
| EXC 3 | Dynamically loaded, fatigue, CC2/CC3 | B or C |
| EXC 4 | Highest consequence, seismic, CC3 | B |
The execution class is assigned by the designer, not the fabricator. A steel bridge approach span will typically be EXC3 → Level B for primary welds.
ISO 3834 (welding QMS)
ISO 3834 does not prescribe a quality level directly. It requires that the fabricator’s welding QMS demonstrates capability to achieve the quality level specified by contract or governing code. The ISO 5817 level is therefore part of the quality plan derived from 3834 Part 2, 3, or 4, with inspection references to ISO 17637 (visual) and ISO 17635 (NDT method selection).
EN 15085-3 (rail vehicles)
Railway rolling-stock uses its own weld class system (CP A, B, C, D) that maps roughly as follows to ISO 5817:
| EN 15085-3 class | Approximate ISO 5817 equivalent |
|---|---|
| CP A | Tighter than B (non-normative) |
| CP B | B |
| CP C2 / CP C3 | C |
| CP D | D |
EN 15085-3 also mandates specific NDT extent and personnel certification (ISO 9712) beyond ISO 5817’s acceptance limits, so a simple equivalence is insufficient — both documents apply.
Decision framework: choosing the right level
Quick-reference checklist for quality level selection:
- Is a governing code specified (EN 1090, ASME, EN 13445, EN 15085)? → Use the code’s required level. Stop here.
- Does the customer specification name a level? → Use it. Document the reference.
- Is the joint subject to fatigue or cyclic loading? → Level B minimum.
- Is the joint safety-critical or with high consequence of failure? → Level B.
- Is the joint statically loaded, non-primary structural function? → Level C or D based on consequence class.
- Is the joint non-structural (bracket, gusset, temporary attachment)? → Level D acceptable if not load-bearing.
A common mistake is applying Level C across a whole fabrication without checking if primary butt joints in a fatigue-loaded zone require B. Mixed-level drawings are the correct approach — assign per joint, not per component.
How inspection scope changes across levels
ISO 5817 acceptance limits alone do not define the inspection programme. The standard pair is ISO 17635 (NDT method selection) and ISO 17637 (visual testing), both of which scale inspection intensity with quality level.
For Level B, practical implications include:
- 100% visual testing per ISO 17637 by a certified inspector (CWI or equivalent, ISO 9712 Level 2 VT)
- Volumetric NDT (RT or UT) on a defined percentage of butt welds — typically 10–100% depending on joint category and governing code
- Surface NDT (MT or PT) for fatigue-critical welds
For Level C, 100% visual is still standard, but volumetric NDT extent is typically reduced (5–10% spot-check for EXC2 steel structures).
For Level D, visual inspection is usually sufficient for non-critical joints, with NDT only on specific contract or code-defined welds.
Real-time monitoring as a complement to acceptance testing
In-process thermal monitoring — continuous measurement of weld pool temperature distribution and heat input during welding — does not replace ISO 5817 acceptance testing. It addresses a different question: process stability, not post-weld conformity.
What real-time monitoring contributes is a reduction in the defect population that reaches the acceptance gate. If heat input excursions, start/stop anomalies, or inter-pass temperature violations are caught in-cycle, the fraction of welds presenting undercut, crater pipes, or porosity at final inspection drops sharply. For Level B fabrication under ISO 3834 Part 2, this supports the manufacturer’s obligation to maintain process capability and traceability — not as a substitute for inspection, but as documented evidence of process control.
The Therness HeatCore AI system integrates thermal monitoring with WPS-linked parameter limits, logging every weld pass against the applicable acceptance requirements and generating traceability records aligned with ISO 3834 and EN 1090 audit needs.
Summary: B vs C vs D at a glance
| Criterion | Level B | Level C | Level D |
|---|---|---|---|
| Strictness | Highest | Intermediate | Lowest |
| Typical EXC class (EN 1090-2) | EXC3/EXC4 | EXC2/EXC3 | EXC1/EXC2 |
| Cracks / lack of fusion | Not permitted | Not permitted | Not permitted |
| Continuous undercut limit | 0.05t, max 0.5 mm | 0.1t, max 1 mm | 0.2t, max 2 mm |
| Linear misalignment limit | 0.05t, max 2 mm | 0.1t, max 3 mm | 0.15t, max 4 mm |
| Root undercut | Not permitted | 0.05t, max 0.5 mm | 0.1t, max 1 mm |
| Field porosity | ≤2% cross-section | ≤4% | ≤8% |
| NDT extent (typical) | High (100% VT + volumetric) | Medium (100% VT, spot UT/RT) | Low (VT, code-defined NDT) |
For a complete imperfection-by-imperfection breakdown with all limits, see the ISO 5817 complete guide.
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Book a demoFrequently Asked Questions
What is the difference between ISO 5817 Level B, C, and D?
Level B is the most stringent (smallest tolerated imperfections), used for dynamically loaded, fatigue-critical, or safety-critical welds — aerospace, pressure equipment, EXC3/EXC4 steel structures. Level C is intermediate, for components subject to fatigue loading (EXC2/EXC3). Level D is the most permissive, for statically loaded non-critical structures (EXC1/EXC2). As a rule, acceptance limits scale with material thickness t: for continuous undercut, h ≤ 0.05t (B), 0.1t (C), 0.2t (D).
How do I choose the right ISO 5817 quality level?
The quality level is usually specified by your governing standard or customer, not chosen freely. EN 1090-2 maps execution classes (EXC1–EXC4) to required ISO 5817 levels: EXC1 → D, EXC2 → C (D for some), EXC3 → B or C, EXC4 → B. For pressure equipment, EN 13445 and EN 12952/12953 prescribe the required level based on joint category. If no code applies, start from the consequence of failure: fatigue or dynamic loading → B; moderate service → C; static non-critical → D.
Can ISO 5817 Level B and Level C be mixed on the same component?
Yes. Weld drawings can specify different quality levels for different weld seams on the same component — a flange butt weld might require Level B while a non-structural attachment weld requires Level C. Each weld must be individually marked on the drawing and inspection records must reference the applicable level. This is common practice in EN 1090-3 aluminium structures and in rolling-stock (EN 15085-3) where different weld classes (CP A–D) are assigned per joint.
