Welded Structure Dimensional Tolerances
ISO 13920 controls the overall geometry of a finished welded fabrication — its dimensions, angles, straightness, and flatness. It is completely separate from ISO 5817, which governs the quality of individual weld joints. Indicate on drawing as ISO 13920-B (or with combined class designations e.g. ISO 13920-BF).
ISO 13920 — "Is the fabricated structure the correct shape and size?" Controls overall assembly dimensions, angles, flatness, and straightness of the whole construction.
ISO 5817 — "Is each individual weld joint acceptable?" Controls weld quality: porosity, cracking, undercut, incomplete fusion, excess reinforcement, etc.
Both are typically specified together on the same drawing. They answer different questions and neither replaces the other.
Linear Tolerances
Four classes: A (fine) through D (very coarse). Class B is the standard default for general fabrication.
| Class | Description | 30–120 mm | 120–400 mm | 400–1000 mm | 1000–2000 mm | 2000–4000 mm | >4000 mm |
|---|---|---|---|---|---|---|---|
| A | Fine | ±1 | ±1 | ±2 | ±3 | ±4 | ±5 |
| B | Medium — standard default | ±1 | ±2 | ±3 | ±4 | ±6 | ±8 |
| C | Coarse | ±2 | ±3 | ±5 | ±7 | ±10 | ±14 |
| D | Very Coarse | ±3 | ±5 | ±8 | ±11 | ±16 | ±22 |
Angular, Straightness & Flatness Tolerances
All values expressed as linear deviation per 1000 mm of length (mm/m).
| Class | Description | Angular deviation | Straightness | Flatness |
|---|---|---|---|---|
| A | Fine | ±2 | ±2 | ±2 |
| B | Medium — standard default | ±3 | ±4 | ±4 |
| C | Coarse | ±6 | ±8 | ±8 |
| D | Very Coarse | ±10 | ±16 | ±16 |
Class B is achievable with normal fabrication practice and standard fixturing. Class A requires controlled tacking sequences, experienced welders, and often post-weld correction. Classes C and D are for non-structural or rough fabrication only. The tolerance class applies to the whole structure — tighter local requirements must be individually dimensioned and toleranced on the drawing.
ISO 5817 — Weld Joint Quality Levels
ISO 5817 applies to fusion welded joints in steel, nickel alloys, and titanium alloys. It defines three quality levels — B, C, and D — that set the maximum permissible severity of imperfections in each individual weld joint. The level is specified on the drawing or in the Welding Procedure Specification (WPS).
For joints subject to high static and dynamic loading. Safety-critical structures. Fatigue-dominated design. Full root penetration is typically mandatory.
Pressure vessels · Lifting equipment · Offshore structures · Nuclear · Aerospace · Bridges
For joints under moderate static and limited dynamic loading. General engineering structures. The most commonly specified level in industrial fabrication.
General machinery · Vehicles · Shipbuilding · Construction steelwork · Rail vehicles
For joints under static loading only. Non-structural or non-critical components. Represents the minimum acceptable quality for any welded joint.
Non-structural brackets · Sheet metal frames · Enclosures · Furniture · Secondary supports
Material scope: Steel, nickel alloys, titanium alloys. For aluminium alloys use ISO 10042, which uses the same B/C/D level structure applied to aluminium-specific imperfections.
AWS equivalent (approximate only): Level B ≈ AWS D1.1 Class A. The two systems are not directly interchangeable — always apply the standard specified on the drawing.
Referenced by: EN 1090 (structural steel execution), EN 15085 (railway welding), EN 13480 (industrial piping), EN 13445 (unfired pressure vessels), EN 12952/12953 (boilers).
Weld Imperfections — Acceptance Criteria
ISO 6520-1 classifies imperfections into six groups: cracks, cavities, solid inclusions, lack of fusion/penetration, shape imperfections, and miscellaneous. ISO 5817 sets the acceptance limit for each at each quality level. Key imperfections are listed below.
t = nominal thickness of the thinner part being joined | a = nominal throat thickness of a fillet weld | b = weld face width | h = height of excess weld metal above the base surface. All limit values are maximums. "Not permitted" = any occurrence at any size is a reject at that quality level. This table is a reference summary — always consult the full ISO 5817 standard for production acceptance decisions.
| Imperfection | ISO 6520 | B — Stringent | C — Intermediate | D — Moderate |
|---|---|---|---|---|
| CRACKS — Not permitted at any quality level | ||||
| Crack — all types | 100 | Not permitted | Not permitted | Not permitted |
| Crater crack | 104 | Not permitted | Not permitted | Not permitted |
| POROSITY — Gas cavities within the weld | ||||
| Single pore — max diameter | 2011 | ≤ 0.5 mm and ≤ 0.1t, max 2 mm | ≤ 0.2t, max 4 mm | ≤ 0.3t, max 5 mm |
| Linear porosity — row of pores | 2014 | Not permitted | ≤ 0.2t per pore, max 4 mm | ≤ 0.3t per pore, max 5 mm |
| Wormhole — elongated cavity | 2015 | Not permitted | Not permitted | ≤ 0.3t, max 5 mm |
| FUSION & PENETRATION — Internal joint quality | ||||
| Incomplete fusion | 401 | Not permitted | Not permitted | Not permitted |
| Incomplete root penetration | 402 | Not permitted | ≤ 0.1t, max 1.5 mm | ≤ 0.2t, max 2 mm |
| Root concavity | 515 | Not permitted | ≤ 0.1t, max 1 mm | ≤ 0.2t, max 2 mm |
| SHAPE IMPERFECTIONS — Surface and geometric | ||||
| Undercut — groove at weld toe | 5011 | ≤ 0.5 mm depth | ≤ 1 mm depth | ≤ 1.5 mm depth |
| Excess weld metal — face reinforcement | 502 | h ≤ 1 + 0.10b, max 3 mm | h ≤ 1 + 0.15b, max 4 mm | h ≤ 1 + 0.25b, max 5 mm |
| Fillet throat undersize | — | −0.3 + 0.1a mm | −0.3 + 0.1a mm | −0.3 + 0.1a mm, max −1 mm |
| Linear misalignment — plate offset | 507 | ≤ 0.1t, max 2 mm | ≤ 0.15t, max 3 mm | ≤ 0.25t, max 5 mm |
| Angular misalignment | 508 | ≤ 1° | ≤ 2° | ≤ 3° |
| MISCELLANEOUS | ||||
| Spatter — metal droplets on base | 602 | Not acceptable | Acceptable unless otherwise specified | Acceptable |
| Arc strike — outside weld zone | 601 | Not permitted | Not permitted | Permitted if no cracking |
t = material thickness (thinner part). a = fillet weld throat. b = weld face width. h = excess weld metal height above surface. All values in mm and are maximum limits. Summary only — refer to full ISO 5817 for production inspection.
Weld Symbol Anatomy
ISO 2553 defines the reference line system for encoding weld requirements on engineering drawings. All information about a weld — its type, size, length, process, quality level — is arranged around a single reference line. The actual weld symbols (graphical representations) are defined in the full ISO 2553 document; this section covers the anatomy of the system.
ISO 2553 is the international standard used in Europe, Asia, and globally. AWS A2.4 is the North American standard. The key structural difference: ISO 2553 uses a dashed identification line above or below the solid reference line to indicate which face of the joint carries the weld. AWS A2.4 uses symbol position above/below the reference line for the same purpose, without a dashed line. Always confirm which standard applies before interpreting a welding drawing from an unfamiliar source.
| Position | Element | What It Specifies |
|---|---|---|
| Below the reference line | Weld symbol | Weld on the arrow side of the joint |
| Above the reference line (dashed) | Weld symbol | Weld on the opposite side to the arrow |
| Left of the weld symbol | Size — s (depth), a (throat), z (leg) | Weld cross-section dimensions in mm |
| Right of the weld symbol | Length × pitch | Weld run length and intermittent spacing — e.g. 50×150 |
| Circle at arrow–line junction | All-around symbol | Weld continues all the way around the joint perimeter |
| Flag at arrow–line junction | Site / field weld | Weld to be made on site, not in the workshop |
| Tail at far end of reference line | Process code / standard | ISO 4063 process number (e.g. "135") or applicable welding standard |
| Supplementary indication | NDT / acceptance level | Non-destructive testing requirement or ISO 5817 quality level |
Common Weld Types — Text Reference
The following covers the most common weld joint types encountered in practice. Precise graphical symbols are defined in ISO 2553 — consult the standard document for the correct drawn form.
| Weld Type | Key Dimensions Specified | Typical Use | Typical Thickness |
|---|---|---|---|
| Fillet weld | Leg length (z) or throat (a) — z = a × √2 | Joining two surfaces at ~90°. Most common weld type in fabrication. | Any |
| Square butt (I-groove) | Root gap | Simple full-penetration butt joint on thin material | Up to ~4 mm |
| Single V butt | Included angle (typ. 60°), root face, root gap | Full-penetration butt joint, single-side access | 6–25 mm |
| Double V butt (X-groove) | Included angle, root face | Full-penetration butt, access from both sides — less distortion than single V | >12 mm |
| Single bevel butt | Bevel angle, root face, root gap | T-joint or corner where only one plate is bevelled | 6–25 mm |
| Single U butt | Groove radius, root face, root gap | Thick sections — lower filler metal volume than V-groove | >20 mm |
| Resistance spot weld | Nugget diameter, pitch, edge distance | Sheet metal lap joints — automotive, HVAC, enclosures | 0.5–4 mm per sheet |
| Plug / slot weld | Hole or slot dimensions, fill depth | Lap joint where fillet access is impossible; additional shear reinforcement | Any |
Weld Distortion — Causes & Control
Distortion is caused by non-uniform heating and cooling that produces residual stresses. Understanding which type of distortion a joint geometry will produce is essential for designing welding sequences and fixturing strategies that minimise it.
| Type | Mechanism | Prevention | Correction |
|---|---|---|---|
| Transverse shrinkage | Weld metal contracts perpendicular to weld axis on cooling | Minimum number of passes; pre-set joint gap; balanced welds | Restraint jigs during welding; post-weld correction |
| Longitudinal shrinkage | Weld metal contracts along the weld length | Backstep welding sequence; balanced sequences from centre outward | Cold stretching; heat straightening |
| Angular distortion | Asymmetric shrinkage through the plate thickness | Double-sided welds; pre-set the joint angle; balanced pass sequence | Mechanical straightening; flame correction |
| Bowing / longitudinal bending | Weld is eccentric relative to the neutral axis of the section | Locate welds close to the neutral axis; balanced welding sequence | Press straightening; flame correction |
| Buckling | Compressive residual stress in thin plate panels | Reduce heat input; use intermittent welds; clamp to stiffeners | Stretching; planishing; controlled flame correction |
| Twisting | Asymmetric restraint or inadequate fixturing | Rigid fixturing throughout; balanced welding sequence | Reverse-torque press; flame correction |
Welding Process Reference Numbers
ISO 4063 assigns numerical codes to welding and allied processes. These codes appear in the tail of weld symbols on drawings, in WPS documents, and in welder qualification certificates.
| Code | Full Process Name | Common Abbreviation | Typical Application |
|---|---|---|---|
| ARC WELDING | |||
| 111 | Manual metal arc welding | MMA / SMAW / Stick | Site work, structural steel, maintenance, repair |
| 114 | Self-shielded flux-cored arc welding | FCAW-S | Outdoor and windy conditions — no gas shielding needed |
| 121 | Submerged arc welding (single wire) | SAW | Thick plates, pressure vessels, longitudinal seams — automated |
| 131 | MIG welding — inert shielding gas | GMAW / MIG | Aluminium, stainless steel, non-ferrous metals |
| 135 | MAG welding — active shielding gas | GMAW / MAG | Structural and fabrication steel — most common industrial process |
| 136 | Flux-cored arc welding (active gas) | FCAW-G | High deposition structural steel, heavy fabrication |
| 141 | TIG welding — inert gas | GTAW / TIG | Root passes, thin material, stainless steel, titanium, precision work |
| RESISTANCE WELDING | |||
| 21 | Resistance spot welding | RSW | Sheet metal, automotive body panels, HVAC ducting |
| 22 | Resistance seam welding | RSEW | Pressure-tight continuous seams in sheet metal |
| 23 | Projection welding | PW | Weld studs, nuts, embossed sheet metal features |
| OTHER PROCESSES | |||
| 311 | Oxy-acetylene welding | OAW | Thin sheet, artistic metalwork, repair, heating for forming |
| 52 | Laser beam welding | LBW | Precision, automotive, electronics, thin sheet, remote operations |
| 91 | Brazing | — | Copper alloys, HVAC, heat exchangers, carbide tool tips |
| 97 | Soldering | — | Electronics, copper plumbing, low-strength dissimilar metal joints |