Welding Symbols ISO 2553 — Complete Guide with Examples
Introduction
ISO 2553:2019 establishes the symbolic representation system for welded joints on drawings and technical documents. Every welding professional — from welder to International Welding Engineer (IWE) — must master this symbology to correctly interpret fabrication drawings and WPS (Welding Procedure Specifications).
This guide covers the complete ISO system: symbol structure, elementary symbols, supplementary symbols, dimensioning, and practical interpretation examples.
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Welding Symbol Structure (ISO 2553)
The ISO welding symbol consists of the following elements, from outer to inner:
| Element | Function |
|---|---|
| Arrow line | Points to the joint or edge to be welded |
| Reference line (continuous) | Support for symbols — the arrow side |
| Identification line (dashed) | Opposite side to the arrow |
| Elementary symbol | Type of weld (V, X, fillet, etc.) |
| Dimensions | Weld measurements (throat, length, spacing) |
| Supplementary symbols | Weld profile, all-around, field weld, etc. |
| Tail | Additional information (process, standard, position) |
Fundamental placement rule:
- Symbol on the reference line (continuous) → weld executed on the arrow side
- Symbol on the identification line (dashed) → weld executed on the other side (opposite to arrow)
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Elementary Symbols
Elementary symbols represent the type of joint preparation and the weld cross-section:
| Weld type | Symbol | Description |
|---|---|---|
| Square butt weld | ǀ | Square edges, no bevel |
| Single-V butt weld | V | V-groove, one side |
| Double-V butt weld (X) | X | V-groove both sides |
| Single-bevel butt weld | ∨ (half) | Single bevel on one member |
| Single-U butt weld | U | Curved groove (J-groove both sides) |
| Single-J butt weld | J | Curved groove, one side only |
| Fillet weld | △ | Triangular weld on T-joints or lap joints |
| Spot weld | ○ | Resistance spot welds |
| Plug/slot weld | ▭ | Hole or slot filled with weld |
| Surfacing (overlay) | ═ | Material deposited on a surface |
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Supplementary Symbols
| Symbol | Meaning |
|---|---|
| ◠ (convex arc) | Convex weld profile / reinforcement |
| ◡ (concave arc) | Concave weld profile |
| — (flat line) | Flush / flat weld profile |
| ○ at arrow-reference junction | All-around weld (weld around entire joint) |
| 🏴 flag | Field weld / site weld |
| M | Mechanical finish (grinding, machining) |
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Dimensioning the Weld
Dimensions are placed alongside the elementary symbol in the following order:
| Dimension | Position | Example |
|---|---|---|
| a (throat thickness) | Before the symbol | a5 △ = fillet with 5 mm throat |
| z (leg length) | Before the symbol | z7 △ = fillet with 7 mm leg |
| s (penetration depth) | Before symbol, butt welds | s10 = 10 mm penetration |
| l (length) | After the symbol | △ 50 = 50 mm weld run |
| n (number of welds) | Prefix | 3 × 50 = 3 welds of 50 mm |
| e (spacing) | After length | 50(100) = 50 mm every 100 mm (intermittent) |
| α (groove angle) | Inside V symbol | V 60° = 60° groove angle |
Fillet weld formula: a ≈ 0.7 × z (throat is approximately 70% of leg length)
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ISO 2553 vs AWS A2.4 — Key Differences
| Concept | ISO 2553 | AWS A2.4 |
|---|---|---|
| Arrow side | Symbol on continuous line | Symbol below reference line |
| Other side | Symbol on dashed line | Symbol above reference line |
| Tail content | Process, standard, position | Process, specification |
| All-around weld | Circle at junction | Circle at junction (identical) |
| Field weld | Flag | Flag (identical) |
Critical warning: The placement convention is inverted between ISO and AWS. In ISO, the symbol on the continuous line = arrow side. In AWS, the symbol below the line = arrow side. This generates frequent errors in international projects.
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Practical Interpretation Examples
Example 1 — Single-V butt weld, full penetration: V symbol on the continuous line with s = full thickness, angle 60°, process 135 in the tail. Interpretation: Single-V butt weld on the arrow side, full penetration, 60° groove, MAG 135 process.
Example 2 — Double-sided intermittent fillet weld: Two triangles (one on each line), dimensions z5 – 50(100). Interpretation: Double-sided fillet, 5 mm leg, 50 mm welds every 100 mm, alternating (staggered if Z appears between dimensions).
Example 3 — All-around fillet weld: Triangle with circle at the arrow-reference junction. Interpretation: Continuous fillet weld around the entire joint (typical for pipes, nozzles, flanges).
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Common Errors
| Error | Consequence |
|---|---|
| Confusing continuous and dashed lines | Weld executed on the wrong side |
| Mixing ISO and AWS conventions | Side inversion |
| Omitting throat "a" on fillets | Welder decides the dimension — may be insufficient |
| Not indicating full penetration | Welder may not achieve complete penetration |
| Omitting process in the tail | Risk of using wrong process for the material |
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*Article based on ISO 2553:2019, AWS A2.4:2020, Welding Handbook (AWS, Vols. 1-3), Tecnologia Mecânica (V. Chiaverini). Published at WeldFC Academy — the technical reference in metalworking.*