Key takeaways
- ISO 5817 sets acceptance limits for imperfections in arc-welded joints in steel, nickel, titanium and their alloys.
- There are three quality levels: B (stringent), C (intermediate) and D (moderate).
- The level is chosen by the designer or the application standard, not by the inspector. For steel structures, EN 1090-2 links the execution class to the quality level.
- Higher quality levels cost more to achieve and inspect, so specify B only where it is really needed.
“Welds to be ISO 5817 quality level B” is one of the most common lines in a fabrication specification, and one of the most expensive if applied without thinking. This guide explains what the ISO 5817 weld quality levels B, C and D mean, how they are chosen, how they relate to EN 1090 execution classes, and how they are verified by visual inspection and NDT.
What does ISO 5817 cover?
ISO 5817 gives acceptance limits for imperfections in fusion welds made by arc welding in steel, nickel, titanium and their alloys. It covers butt and fillet welds and lists imperfections such as cracks, porosity, inclusions, lack of fusion, undercut, excess weld metal, misalignment and incorrect throat thickness, with a limit for each quality level. For aluminium, the equivalent standard is ISO 10042. The imperfections themselves are defined in ISO 6520-1.
ISO 5817 deals with the geometry of imperfections. It does not cover metallurgical properties such as hardness or toughness; those come from the welding procedure qualification.
Quality levels B, C and D explained
| Level | Symbol | Meaning | Typical use |
|---|---|---|---|
| Stringent | B | Smallest imperfections allowed | Pressure equipment, fatigue-loaded and safety-critical structures, EXC3 and EXC4 |
| Intermediate | C | Moderate limits | General steel structures, EXC2, machinery frames |
| Moderate | D | Widest limits | Lightly loaded, non-critical parts, EXC1 |
The difference between levels is in the size and number of imperfections allowed. For example, level B does not accept some imperfections at all that level D tolerates within limits, and allows much smaller undercut, porosity and excess weld metal. Cracks are not acceptable at any level.
How ISO 5817 relates to EN 1090 execution classes
For steel structures, EN 1090-2 sets the weld quality level from the execution class (EXC) of the structure:
- EXC1: quality level D.
- EXC2: quality level C, with some imperfections at level B.
- EXC3: quality level B.
- EXC4: quality level B with additional requirements, often called “B+”.
Structures under fatigue loading may also use the fatigue-related requirements given in an annex of ISO 5817. The execution class and any additional requirements must be stated in the execution specification.
Who chooses the quality level?
The quality level is a design decision. It should be chosen by the designer or set by the application standard, based on the load, the consequences of failure, fatigue and the service environment. The inspector’s job is to verify compliance with the level specified, not to choose it. Asking for level B “to be safe” on every weld increases welding time, repair rates and the extent of NDT without any benefit where the load does not require it.
How ISO 5817 compliance is verified
- Visual testing to ISO 17637 checks the weld profile, undercut, excess weld metal, misalignment and surface imperfections with gauges.
- Surface NDT (MT or PT) finds surface-breaking cracks and lack of fusion.
- Volumetric NDT (UT, PAUT, TOFD or RT) finds internal imperfections. Each NDT method has its own acceptance standard, which is matched to the ISO 5817 quality level, for example ISO 11666 for UT and ISO 10675-1 for RT.
The NDT method, extent and acceptance levels should be written into the Inspection and Test Plan before production. For the common imperfections behind each limit, see our guide to 10 common weld defects. The official text of the standard is available from ISO.
Practical tip: when a drawing only says “ISO 5817” without a level, ask for clarification before welding. Assuming level B or D both lead to disputes at final inspection.
Our CSWIP 3.1 welding inspectors and ISO 9712 NDT personnel inspect to the specified quality level and document every result.
Frequently asked questions
Which ISO 5817 level is the highest?
Level B is the most stringent, level C is intermediate and level D is the least stringent.
Can different welds on one structure have different quality levels?
Yes. The designer can specify different levels for different joints, or even for individual imperfections, depending on their importance.
Does ISO 5817 apply to aluminium?
No. For aluminium and its alloys, ISO 10042 sets the equivalent quality levels.
