Welded Assemblies: ISO 5817, ISO 3834 and EN 1090 Explained

A welded assembly is specified with three standards that answer three different questions: ISO 5817 sets how good the finished weld must be (quality level B, C or D), ISO 3834 sets what the fabricator must have in place to make it, and EN 1090 sets execution and CE marking rules for structural steel and aluminium components in buildings and civil works. On most machine and equipment drawings only the first two apply; EN 1090 applies when the part is a structural component of a construction work.

What do ISO 5817, ISO 3834 and EN 1090 each cover?

They are not alternatives. ISO 5817 judges a weld, ISO 3834 judges the workshop that made it, and EN 1090 is a product and conformity framework that pulls in both. A drawing can use the first, a contract the second, and the law the third.

Table 1. Scope of the three standards and who asks for them

Source: ISO 5817:2023; ISO 10042:2018; ISO 3834-1:2021 to -4:2021 (ISO catalogue); EN 1090-1:2009+A1:2011 and European Commission FAQ of 2014-07-04.

What do weld quality levels B, C and D mean?

ISO 5817:2023 defines three quality levels for imperfections such as undercut, porosity and linear misalignment. Level B corresponds to the highest requirement on the finished weld and D to the lowest, with C between them. The standard covers fully penetrated butt welds and all fillet welds, addresses static, thermal, corrosion and pressure loading, and gives additional fatigue guidance in its Annex B. It excludes metallurgical aspects such as grain size and hardness, and beam welding is covered by ISO 13919-1.

The level is a design decision with a price. A tighter level means more rework and more inspection, so one level for the whole drawing is rarely the right answer. Name the level per weld or per weld group, and keep B for the welds that carry the load or the fatigue.

ISO 5817 does not cover aluminium. ISO 10042:2018 does the same job for arc-welded aluminium and its alloys with the same three levels, for material above 0.5 mm.

The level controls the weld, not the dimensions of the assembly. As-welded lengths, angles, shape and position fall under ISO 13920:2023, which has four tolerance classes chosen by function. Machined features keep their own tolerances, as in ISO 2768 general tolerances.

What does ISO 3834 require from a welding fabricator?

ISO 3834 is a series of quality requirements for fusion welding of metallic materials that applies in workshops and at field installation sites. Part 1 (2021) gives the criteria for choosing the level. Parts 2, 3 and 4 define the three levels: comprehensive, standard and elementary. Part 5 (2021) lists the documents with which conformity to part 2, 3 or 4 can be claimed.

The three parts cover the same ground at decreasing depth. A weld cannot be fully verified afterwards without destructive testing, so the standard checks the process that produces it instead. ISO 3834 does not require a total quality management system, though its part 1 identifies the elements of one that complement the series.

Choosing the level is the buyer's call and depends on the use of the part: loading, fatigue, consequence of failure. Asking for part 2 on a lightly loaded bracket adds cost without adding safety. Certification by a third party is a separate matter from applying the standard. According to TWI, neither the Construction Products Regulation nor EN 1090 makes independent certification to ISO 3834 mandatory, though purchasers may demand it. Ask for the certificate's exact scope: the ISO 3834 part, the processes and the material groups it names.

When does EN 1090 apply to a welded part?

EN 1090-1:2009+A1:2011 is the harmonised standard under which structural steel and aluminium components are CE marked under Regulation (EU) No 305/2011. Mandatory CE marking under it has applied since 2014-07-01. The Commission's FAQ states three conditions, all of which must be met:

• The product is within the scope of EN 1090-1.
• It is a structural construction product: intended to be incorporated permanently in construction works (buildings or civil engineering works) and with a structural function in them.
• No dedicated European product specification covers it.

The Commission's FAQ also carries an indicative, non-exhaustive CEN list of products outside the scope. It includes castings, forgings, scaffolding, pipelines, pressure vessels without their supporting structure, workshop-fabricated steel tanks, components produced on the work site, structural components for the moving parts of cranes, and steel and aluminium stairs, walkways and fences forming an integral part of a machine. A welded frame built into a machine is not meant to be permanently incorporated in a building, so it fails the second condition. EN 1090 CE marking therefore does not apply to such a frame, though a customer may still reference EN 1090-2 as a technical specification.

When EN 1090 does apply, EN 1090-2 (steel) or EN 1090-3 (aluminium) gives the technical requirements, sorted into four execution classes, EXC1 to EXC4. The specifier chooses the class for the structure, a component or a detail, and EXC2 applies if none is named. The class then sets the welding quality system and, in the 2008 edition analysed by Staniszewski and colleagues, the ISO 5817 level.

Table 2. Execution class, ISO 3834 part and ISO 5817 level in EN 1090-2

Source: ISO 3834 mapping from TWI Job Knowledge; quality levels from Staniszewski et al., Archives of Metallurgy and Materials 58/1 (2013), Table 2, based on EN 1090-2:2008. Check both against the edition on the order.

The manufacturer's factory production control is assessed under system 2+ of the harmonised standard, and a notified body certifies it. The Construction Products Regulation was revised: Regulation (EU) 2024/3110 entered into force on 2025-01-07 and its main part has applied since 2026-01-08. According to the Commission, a product covered by a standard cited under the 2011 regulation remains under it in principle until a new standard is cited (status as of 2026-10-30).

Which editions are current in October 2026?

ISO 5817 and EN 1090-2 have both changed edition in recent years, so an undated reference on an old drawing can point to a withdrawn text. Status as of 2026-10-30:

Table 3. Edition and status of the standards in this article

Source: ISO catalogue pages for each standard; DIN Media; Czech Office for Standards harmonised standards database; EVS (Estonian Centre for Standardisation) for the dates of EN 1090-2 and EN 1090-3.

What should a welded assembly drawing and order state?

A welded drawing leaves little room for interpretation when it carries these entries:

• Weld quality level per ISO 5817:2023 (or ISO 10042:2018 for aluminium), per weld or weld group, with the year.
• The ISO 3834 part the fabricator must work to, if the use of the part justifies one.
• General tolerance standard for as-welded dimensions (ISO 13920:2023) with its class, next to the individual tolerances for machined faces.
• Whether machining follows welding, and whether the frame is stress-relieved before it.
• For structural components in construction works: EN 1090-2 or EN 1090-3, the execution class and the CE marking documents.
• Material certificates and weld records the order expects, with the drawing revision.

Welded assemblies are one of the processes in our Bursa network, described on welded assemblies. How these entries fit a one-off frame is covered in machined and welded parts for special machine builders, and the technical drawing checklist lists the remaining title block entries. For what a certificate does and does not prove, see supplier certificates.

Frequently asked questions

What is the difference between ISO 5817 and ISO 3834?

ISO 5817 sets acceptance limits for imperfections in a finished weld, in quality levels B, C and D. ISO 3834 sets quality requirements for the fabricator's welding process, in three levels: comprehensive (part 2), standard (part 3) and elementary (part 4). One is a property of the weld, the other of the workshop.

Do I need EN 1090 for a welded machine frame?

Usually not. CE marking under EN 1090-1 applies only to structural components permanently incorporated in buildings or civil engineering works, according to the European Commission's FAQ. A frame that is part of a machine does not meet that condition. The drawing may still call up EN 1090-2 or ISO 3834 by contract.

Which weld quality level should a drawing specify?

The designer chooses it from the loading, the consequences of failure and the cost of inspection. Level B is the highest requirement on the finished weld, D the lowest. ISO 5817 does not assign levels to part types; for structural components EN 1090-2 links them to execution classes.

Is ISO 3834 certification mandatory?

Neither the Construction Products Regulation nor EN 1090 requires independent certification to ISO 3834, according to TWI. A contract or customer may still ask for it. ISO 3834 itself does not demand a full quality management system.

Which ISO 3834 part belongs to which EN 1090 execution class?

EXC1 uses ISO 3834-4 (elementary), EXC2 uses ISO 3834-3 (standard), and EXC3 and EXC4 use ISO 3834-2 (comprehensive), as summarised by TWI. Confirm the mapping against the edition of EN 1090-2 on the order.

Sources

1. ISO 5817:2023, Fusion-welded joints in steel, nickel, titanium and their alloys, quality levels for imperfections (ISO catalogue)
2. ISO 10042:2018, Arc-welded joints in aluminium and its alloys, quality levels for imperfections (ISO catalogue)
3. ISO 3834-1:2021, Part 1: criteria for the selection of the appropriate level of quality requirements (ISO catalogue)
4. ISO 3834-2:2021, Part 2: comprehensive quality requirements, confirmed 2026-09-22 (ISO catalogue)
5. ISO 3834-3:2021, Part 3: standard quality requirements (ISO catalogue)
6. ISO 3834-4:2021, Part 4: elementary quality requirements (ISO catalogue)
7. ISO 3834-5:2021, Part 5: documents with which it is necessary to conform (ISO catalogue)
8. ISO 13920:2023, General tolerances for welded constructions (ISO catalogue)
9. DIN EN ISO 5817:2023-07, German adoption of ISO 5817:2023 (DIN Media)
10. EN 1090-1:2009+A1:2011, harmonised under Regulation (EU) No 305/2011, assessment system 2+ (Czech Office for Standards, harmonised standards database)
11. Commission FAQ of 2014-07-04 on the scope of EN 1090-1, with CEN's list of products not covered (CEN/TC 135 document N 686)
12. Construction Products Regulation, 2024 revision: dates and transition (European Commission, DG GROW)
13. EN 1090-2:2018+A1:2024, technical requirements for steel structures, valid from 2024-05-15 (EVS, Estonian Centre for Standardisation)
14. EN 1090-3:2019, technical requirements for aluminium structures (EVS, Estonian Centre for Standardisation)
15. Structural steel CE marking and ISO 3834 (TWI Job Knowledge)
16. Staniszewski, Sikora, Czuchryj, Radiographic quality assessment of welded joints to EN 1090-2, quality level B+ (Archives of Metallurgy and Materials 58/1, 2013)
17. Execution of Steel Structures, Steel Industry Guidance Note SN32 (BCSA, 2009)

Questions on a drawing? sales@ertas.engineering

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