Which welding standard to put on a machine frame drawing

Three standards come up constantly in heavy fabrication quotes and they are routinely confused, including by people selling against them. They are not competing versions of the same thing. They certify different objects, and knowing which is which stops you both over-specifying and accepting a certificate that does not cover what you assumed.

What each one actually is

AWS D1.1 — Structural Welding Code, Steel. A technical code. It tells you how to qualify a welding procedure, how to qualify a welder, what joint geometries are pre-qualified, what acceptance criteria apply to visual inspection and NDT. It governs the welding itself. It is the dominant reference in North America and widely used elsewhere by convention.

ISO 3834 — Quality requirements for fusion welding. A quality system standard. It does not tell anyone how to weld. It sets out what a fabricator must have in place to be trusted to weld: procedure control, welder qualification records, equipment calibration, material traceability, inspection personnel, non-conformance handling. It comes in three levels — Part 2 comprehensive, Part 3 standard, Part 4 elementary. Part 2 is what export-capable heavy-fab shops hold.

EN 1090 — Execution of steel structures. A market access standard, and specifically a CE-marking requirement for structural steelwork placed on the European market. It builds on EN 1090-2 for execution and requires a Factory Production Control system, which in turn requires ISO 3834 certification at the appropriate level. It is organised into Execution Classes EXC1 to EXC4 by consequence of failure.

The nesting matters: EN 1090 requires ISO 3834; ISO 3834 requires welding to be done to a technical code such as EN ISO 15614 or AWS D1.1. They stack rather than compete.

Which one your frame actually needs

This is where over-specification happens, so it is worth being precise.

A machine base or frame supplied as part of a machine is, in the great majority of cases, not a structural steelwork product placed on the EU market. It is a component of machinery, covered by the Machinery Regulation, and CE marking flows through the machine as a whole rather than through EN 1090 on the frame. Specifying EN 1090 EXC3 on a machine base is common, expensive, and frequently unnecessary.

Where EN 1090 genuinely applies: load-bearing structures — gantries and supporting steelwork that form part of a building or a permanent installation, platforms, support frames anchored to a structure. If your frame is that, EXC2 covers most of it and EXC3 is for higher consequence.

For a machine base that is not load-bearing structural steelwork, a sound and proportionate specification is:

All welding to AWS D1.1 (or EN ISO 15614-1). Fabricator to hold ISO 3834-2 certification or demonstrate equivalent procedure and welder qualification records. WPS and welder qualification certificates to be supplied on request.

That buys you qualified procedures, qualified welders, and a shop with a functioning quality system — which is the substance — without buying a CE-marking regime you do not need.

What “certified” needs to mean in practice

A certificate on a wall is not the same as a controlled process. Three documents turn a claim into a fact:

• WPS — Welding Procedure Specification. The instruction sheet for a specific joint: process, consumable, current range, preheat, interpass, travel speed, number of passes.
• PQR — Procedure Qualification Record. The test evidence that this WPS produces sound welds, supported by mechanical test results.
• Welder qualification certificates — evidence that the specific people welding your part are qualified for the process, position and material thickness range involved, and that the qualifications are current.

Ask for all three at quote stage for the joints that matter. A shop with a real system produces them in an afternoon. A shop without one produces excuses, and that answer is worth more to you than the quote.

The International Institute of Welding framework that sits behind ISO 3834 also defines welding coordination personnel requirements — an IWE or IWT on staff or retained. For thick-section machine frames this is a meaningful signal, because someone has to own the procedure decisions.

Where the standard interacts with your design

Two design decisions get constrained by the code you invoke, and it is better to know before release.

Pre-qualified joints. AWS D1.1 lists joint geometries that can be used without running a procedure qualification test. Designing to those geometries removes a cost and a delay from the project. Designing an unusual groove that falls outside the pre-qualified set means a qualification test, which is several thousand dollars and a few weeks.

Minimum fillet size. Codes specify a minimum fillet leg as a function of the thicker plate in the joint, to avoid excessively fast cooling and the hard, crack-prone microstructure that follows. If your calculated fillet is below that minimum, the code minimum wins. This occasionally surprises designers who sized purely on load — and it interacts with distortion, because the extra metal has to go somewhere. The trade-offs are in controlling distortion on large weldments.

Acceptance criteria, which the code gives you for free

One of the most useful things a code provides is a defensible definition of “acceptable”. Rather than negotiating with a shop about whether a particular undercut or porosity cluster is a problem, you cite the acceptance class and the argument is over.

For machine frames, visual inspection to the code on all welds plus volumetric or surface NDT on a named list of critical welds is the proportionate shape. Which method for which joint is covered in NDT for machine frame welds.

A note on Vietnamese and regional shops

Export-oriented heavy-fab shops across Southeast Asia commonly hold ISO 3834-2, and a smaller number hold EN 1090 for structural work aimed at European projects. AWS D1.1 procedure and welder qualification is widespread because so much regional work serves American customers.

What varies much more than certification is whether the system is used daily or produced for audits. The way to tell is to ask for a WPS for a joint resembling yours and a recent welder qualification record, and to see how long it takes. The full vetting sequence is in how to audit a heavy fabrication shop.

The specification to copy

Welding. All welding to AWS D1.1, latest edition. Fabricator shall hold ISO 3834-2 certification or shall demonstrate equivalent control: qualified WPS supported by PQR for each joint type, and current welder qualification certificates covering process, position and thickness range. WPS, PQR and welder certificates to be supplied with the first article. Visual inspection to code acceptance criteria on 100% of welds. NDT per the critical weld table on sheet 2.

Short, proportionate, and it names the evidence rather than only the certificate.

If you want a frame package reviewed for whether the standards you have specified match what the part actually needs, send it through — the review comes back within 24 business hours.

See also: does EN 1090 apply to a machine frame and welding thin sheet and tube to AWS D1.3.