Weld quality levels B, C and D to ISO 5817
The same imperfection can be acceptable and unacceptable at once — it depends on the quality level written into the documentation. Below is the full table of limits for the three levels and where the level comes from.
Short answer
ISO 5817 splits the requirements into three levels:
- B — stringent. The tightest limits, normally on critical structures and under fluctuating load.
- C — intermediate. The commonest in ordinary building structures.
- D — moderate. Lightly loaded components.
The level is not a mark for the welder or a measure of care taken — it is an acceptance criterion. The documentation chooses it, not whoever is looking at the weld.
The permitted limits
The permitted value is normally the smaller of two: the result of the
formula and the ceiling given beside it. t is the material
thickness, b the width of the weld or of the cap.
| No. | Imperfection | B — stringent | C — intermediate | D — moderate |
|---|---|---|---|---|
| 100 | Crack | not permitted | not permitted | not permitted |
| 401 | Lack of fusion | not permitted | not permitted | not permitted |
| 402 | Incomplete penetration (lack of root penetration) | not permitted | not permitted | 0.2·t max 2 mm |
| 2011, 2017 | Isolated gas pore | 0.2·t max 2 mm | 0.25·t max 3 mm | 0.3·t max 4 mm |
| 5011, 5012 | Continuous undercut | 0.05·t max 0.5 mm | 0.1·t max 0.5 mm | 0.2·t max 1 mm |
| 502 | Excess weld metal (butt weld) | 1 + 0.1·b max 5 mm | 1 + 0.15·b max 7 mm | 1 + 0.25·b max 10 mm |
| 503 | Excessive convexity (fillet weld) | 1 + 0.1·b max 3 mm | 1 + 0.15·b max 4 mm | 1 + 0.25·b max 5 mm |
| 504 | Excess penetration | 1 + 0.2·b max 3 mm | 1 + 0.6·b max 4 mm | 1 + 1·b max 5 mm |
| 507 | Linear misalignment | 0.1·t max 3 mm | 0.15·t max 4 mm | 0.25·t max 5 mm |
| 511 | Incompletely filled groove | not permitted | 0.1·t max 1 mm | 0.25·t max 2 mm |
| 515 | Root concavity | 0.05·t max 0.5 mm | 0.1·t max 1 mm | 0.2·t max 2 mm |
The standard describes several dozen kinds of imperfection and distinguishes thickness ranges and special cases on top of that. The table holds the ones that actually reach inspection reports. For a specific thickness and width the acceptance limits calculator works out the figure — the same numbers as here. The full wording of the requirements, with its notes and exceptions, always stays in the standard itself.
What no level ever allows
Three rows above carry "not permitted" in all three columns, and that is not strictness for its own sake:
Cracks (100) and lack of fusion (401) are planar defects. Their danger comes not from size but from shape: they act as a notch, concentrate stress and grow under fluctuating load. A pore of the same length is far less dangerous, because its edges are rounded and it spreads stress instead of gathering it.
Incomplete penetration (402) is permitted only at level D, and only as a short imperfection. At B and C it does not appear at all — for the same reason.
The practical consequence: the size of a planar defect is not an argument to have with an inspector. "You can barely see it" does not change the classification.
Where the level in the documentation comes from
In steel structures under EN 1090-2 the level follows straight from the execution class:
| Execution class EN 1090-2 | Quality level | Typical use |
|---|---|---|
| EXC1 | D | Lightly loaded structures, farm and utility buildings |
| EXC2 | C | Ordinary building structures — much the commonest case |
| EXC3 | B | Structures where failure endangers people: public halls, bridges |
| EXC4 | B+ | Critical structures where failure has severe consequences |
B+ at EXC4 is not a fourth level of the standard but level B with additional requirements that EN 1090-2 adds on top of ISO 5817 for some imperfections. Outside construction the level comes from the product standard: for pressure equipment, vessels and pipework it is set by the relevant standard of that family or by the customer's specification.
How to use this at inspection
The order never changes. First read the quality level off the documentation. Then give the imperfection a number to ISO 6520-1 — without a number there is nothing to discuss, because "a sort of hollow" is not an entry in any table. Only with the number and the level in hand do you look up the limit and measure the defect.
The imperfection numbers — the ISO 6520-1 table. What the defects look like and where they come from — the weld defect catalogue. How a quality level differs from the range of a procedure qualification — WPQR.
Frequently asked questions
Who picks the quality level?
Not the inspector and not the welder — the documentation. The level comes from the design, the product standard or the contract; in structural steelwork it follows straight from the execution class EXC of EN 1090-2. If the paperwork does not state a level, that is a gap to raise before welding, not something to guess at afterwards.
What undercut passes at level C on 10 mm plate?
Up to 0.5 mm. The formula gives 0.1·t, that is 1 mm, but a ceiling of 0.5 mm also applies — you take the smaller of the two. At level B it would be 0.05·t = 0.5 mm with the same ceiling; at level D, 0.2·t = 2 mm but no more than 1 mm.
Does ISO 5817 apply to aluminium?
No. ISO 5817 covers fusion-welded joints in steel, nickel, titanium and their alloys from 0.5 mm thickness. For aluminium and its alloys the equivalent standard is ISO 10042, with its own levels and its own figures.
Updated: