Welding symbolisation is the system of standardised symbols used on engineering drawings to define a weld joint. In Europe EN ISO 2553 is used; in the USA AWS A2.4 applies. There are key reading-direction differences between the two systems.
Basic Symbol Structure
- Reference line: Horizontal main line
- Arrow: Points to the joint location
- Basic symbol: Geometric shape indicating weld type
- Supplementary symbols: Surface finish, all-around weld, field weld, etc.
Basic Weld Symbols
| Symbol | Meaning | Description |
|---|---|---|
| I | Square groove | No root opening, thin plate |
| V | V-groove | Single-side V preparation |
| X (double V) | Double V | Thick plate, symmetric |
| U | U-groove | Very thick plate, less weld metal |
| K (triangle) | Fillet weld | T-joint, fill weld |
| Circle (closed) | All-around weld | Welds full perimeter |
| Flag | Field weld | Applied away from the shop |
Symbol Placement: EN ISO 2553
- System A (European): Arrow-side symbol above the reference line, other-side below
- System B (ISO standardised): Dual reference line (dashed on top) — preferred for international projects
Difference with AWS A2.4: In AWS the arrow-side symbol is always written below the reference line — opposite to EN ISO 2553. Never mix the two standards on one drawing.
Dimensional Information
- a: Throat thickness (fillet weld)
- z: Leg length (fillet weld)
- s: Weld depth (penetration depth for butt weld)
- L: Weld length (intermittent welds)
- e: Pitch between welds (intermittent)
- n: Number of welds (intermittent)
Practical Examples
Example 1: Double Fillet Weld at T-Joint
Symbol: a5 ▽ ⌒
Meaning: 5 mm throat, continuous fillet weld on both sides.
Example 2: Intermittent Fillet Weld
Symbol: a4 ▽ 6×50 (100)
Meaning: 4 mm throat, six welds 50 mm long, 100 mm pitch.
All-Around and Field Weld Symbols
- Circle (○): All-around weld (full perimeter)
- Flag (▶): Field weld — to be applied at the assembly site
Surface Finish and Tail References
- Flat line (—): Flush finish (grinding)
- Concave arc (∪): Concave surface
- Convex arc (∩): Convex surface
In the tail of the reference line: process number (EN ISO 4063), WPS number, NDT type and rate, acceptance level.
Best practice: Combining process number, WPS number and acceptance level in the tail gives the fabricator all the information at a glance. Example: 135 / WPS-022 / EN ISO 5817-B
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