Weld quality levels under ISO 5817 govern imperfection limits in the weld bead itself. But a structurally sound weld placed at the wrong position, on a distorted assembly, or on a frame that has bowed beyond engineering tolerances is still a non-conformance — just one that ISO 5817 will never catch.
That is the gap ISO 13920 fills. Published by ISO as General tolerances for welded constructions — Dimensions for lengths and angles — Shape and position, the standard defines four graduated tolerance classes (A, B, C and D) for the overall geometry of welded fabrications. Understanding how to select and apply those classes is foundational for any manufacturer building to EN 1090, EN 15085, or any quality framework that references dimensional conformance of welded assemblies.
What ISO 13920 Covers
ISO 13920 applies to the overall dimensions and shape of a welded construction, not to the weld bead geometry. Its scope includes:
- Linear dimensions: lengths, widths, heights, depths, diameters, and distances between features
- Angular dimensions: deviation from a specified angle or from perpendicularity
- Form tolerances: flatness, straightness, and parallelism of surfaces or edges on the finished welded assembly
The standard deliberately excludes weld bead imperfections (covered by ISO 5817 / EN ISO 10042 for aluminium), weld joint preparation geometry (ISO 9692 series), and tight positional tolerances assigned individually on engineering drawings — those override the general tolerance class wherever they appear.
ISO 13920 is a general tolerance standard: it applies to every dimension on a welded assembly that is not individually toleranced on the drawing. Specifying the class once in the title block or welding standard reference is enough to govern the entire fabrication.
The Four Tolerance Classes
ISO 13920 defines two sets of tolerance tables — one for linear dimensions and one for angular dimensions — each with four classes. Classes are labelled A through D, from tightest to most permissive.
Linear Dimension Tolerances
Linear tolerances are given as ±mm and scale with the nominal dimension:
| Nominal dimension (mm) | Class A | Class B | Class C | Class D |
|---|---|---|---|---|
| up to 30 | ±1 | ±2 | ±3 | ±4 |
| 30–120 | ±1 | ±2 | ±4 | ±5 |
| 120–400 | ±1 | ±3 | ±5 | ±8 |
| 400–1000 | ±2 | ±4 | ±6 | ±10 |
| 1000–2000 | ±3 | ±5 | ±8 | ±12 |
| 2000–4000 | ±4 | ±6 | ±10 | ±15 |
| above 4000 | ±5 | ±8 | ±12 | ±20 |
Abbreviated — refer to the published standard for the complete tables including intermediate ranges.
Angular Dimension Tolerances
Angular tolerances apply to the shorter leg of the angle and also scale with its length (in mm):
| Shorter leg (mm) | Class A | Class B | Class C | Class D |
|---|---|---|---|---|
| up to 30 | ±20’ | ±45’ | ±1° | ±1°30’ |
| 30–120 | ±15’ | ±30’ | ±45’ | ±1°15’ |
| 120–400 | ±10’ | ±20’ | ±30’ | ±1° |
| above 400 | ±5’ | ±10’ | ±20’ | ±45’ |
Flatness and Straightness Tolerances
Shape tolerances (flatness, straightness, parallelism) follow a separate table, again with four graduated classes. At a 1000 mm measurement length, Class A allows 3 mm deviation, Class B allows 6 mm, Class C allows 12 mm, and Class D allows 24 mm.
Selecting the Right Class
Decision rule: specify the loosest class that still satisfies functional and structural requirements. Tighter classes cost more — they require more corrective passes, more fixturing, more distortion management, and longer cycle times.
The four classes map roughly to fabrication contexts as follows:
Class A is appropriate for precision assemblies where downstream machining clearances are tight, or where fit-up to mating sub-assemblies demands close control. Typical applications include machine frames, jigs and fixtures, precision structural nodes.
Class B is the most common production default for structural steel fabrication. EN 1090-2 — the execution standard for steel structures — references ISO 13920 Class B as the baseline for Execution Class 2 and 3 structures where no tighter tolerance is specified.
Class C is typical for large weldments in shipbuilding, civil structures, and storage tanks where the sheer scale of the assembly makes tighter control impractical and the structural function tolerates more variation.
Class D is used for coarse assemblies where only rough dimensional agreement matters — heavy earthmoving equipment skids, rough infrastructure frames, or non-critical bracketry.
Mapping to EN 1090 Execution Classes
EN 1090-2 links Execution Class (EXC) to ISO 13920 tolerance class as follows:
| EN 1090-2 Execution Class | Typical ISO 13920 class |
|---|---|
| EXC 1 | D or C |
| EXC 2 | B (default) |
| EXC 3 | B or A depending on engineering specification |
| EXC 4 | A, with individual tolerances specified on drawings |
For rolling-stock welding governed by EN 15085, the certification level (CL1–CL4) typically requires Class B or tighter for load-bearing joints, with Class A for safety-critical nodes such as bogie frames.
How to Specify ISO 13920 in Drawings and WPS
The standard defines a concise notation for the title block of engineering drawings:
ISO 13920 - BThis single line governs all un-toleranced linear and angular dimensions on the drawing. Where a specific feature requires a tighter or looser class, an individual tolerance annotation on that dimension takes precedence over the general class.
In the Welding Procedure Specification (WPS) or the welding plan, reference to ISO 13920 class should appear alongside the quality level reference (ISO 5817 / EN ISO 10042). A complete quality reference block typically looks like:
Weld quality: ISO 5817 - Level B
Fabrication tolerances: ISO 13920 - Class BThis separation is important: ISO 5817 Level B governs the weld bead; ISO 13920 Class B governs the finished assembly dimensions. Both must be satisfied independently.
Inspection and Verification
Verifying conformance to ISO 13920 requires dimensional measurement of the finished assembly. Common methods include:
- Manual measurement: steel rules, digital calipers, height gauges, angle gauges — adequate for Class C and D, labour-intensive for Class A
- Coordinate Measuring Machines (CMM): precise, traceable, suited for Class A and B in batch production
- Laser tracker and total station: preferred for large structures (shipbuilding, civil, wind tower) where CMM access is impractical
- Structured light scanning: fast surface capture for flatness and profile deviation, useful for Class B automotive and rolling-stock frames
Measurement records must be traceable to calibrated instruments per ISO 17662 and archived as part of the welding quality records required under ISO 3834.
Measuring a cold assembly is insufficient if the fabrication process introduces residual distortion that only becomes apparent after heat treatment or load application. Measure at the agreed reference condition (typically ambient temperature, unrestrained) specified in the engineering drawing.
Where Thermal Monitoring Supports Dimensional Conformance
Dimensional non-conformance in welded structures almost always originates during the welding process itself: uncontrolled heat input, non-uniform bead sequence, inadequate fixturing, or thermal gradients that drive angular distortion and longitudinal shrinkage.
Real-time thermal monitoring addresses these root causes at the source rather than discovering deviations at final inspection:
Heat input tracking — Monitoring the thermal signature of each pass verifies that heat input stays within the WPS envelope. Excess heat input is the primary driver of distortion in multi-pass joints. Integrating heat input control directly into the welding cycle reduces post-weld straightening and the dimensional variability that pushes assemblies outside ISO 13920 Class B limits.
Interpass temperature surveillance — ISO 13916 requires interpass temperature measurement for qualified weld procedures. Uncontrolled interpass temperature leads to thermal gradients between passes that accumulate as angular distortion — especially in long, thin plate sections.
T-joint and fillet distortion detection — Fillet welds on T-joints are among the highest-risk geometries for angular distortion. Thermal cameras mounted on-axis to the joint can detect asymmetric bead profiles and flag conditions that predict deviation from flatness limits before they compound across multiple joints.
Sequence verification — Welding sequence is the single most effective tool for managing residual distortion. Thermal imaging of the assembly as a whole during fabrication provides a spatial distortion map in near-real-time, allowing operators to adjust sequence or add restraint before deviations exceed the ISO 13920 class limits.
This proactive approach is particularly valuable for EXC 3 and EXC 4 structures or EN 15085 CL1 assemblies where Class A tolerances are required and rework costs are high.
Common Non-Conformance Scenarios
Three scenarios account for the majority of ISO 13920 non-conformances in production:
Bowing of long weldments — Longitudinal shrinkage across multiple passes bows beams and columns. Cause: inconsistent heat input or incorrect bead sequence. Prevention: balanced welding from the neutral axis outward, verified by thermal monitoring.
Angular distortion of plate-to-plate T-joints — Transverse fillet shrinkage rotates the vertical plate relative to the baseplate. Cause: single-side welding without counterbalancing. Prevention: alternate-side weld sequence or pre-set angular offset, monitored thermally.
Accumulated positional error in multi-joint assemblies — Individual joints within Class B tolerance compound to put a final feature outside limits. Cause: no intermediate check at sub-assembly stage. Prevention: stage inspection at defined assembly checkpoints with measurement records per ISO 3834 quality plan.
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ISO 13920 is the general tolerance standard for welded constructions. Its four classes — A (tightest) through D (most permissive) — govern linear dimensions, angular dimensions, and shape tolerances for the finished assembly, independent of weld bead quality limits under ISO 5817. Class B is the production default for structural steel under EN 1090-2 EXC 2–3; Class A applies to precision and safety-critical assemblies in EN 15085 and EXC 4.
Specifying the correct class at design stage, referencing it in the title block and WPS, and verifying compliance through calibrated dimensional inspection with traceable records are the three non-negotiable steps for a conformant welding quality system.
Thermal monitoring of heat input, interpass temperature, and bead geometry during production is the most cost-effective way to prevent dimensional non-conformances at source — before they require costly rework or, worse, disposition under a non-conformance report.
