Misread weld symbols cause structural failures, rework, and safety incidents — every mechanical engineer working with fabricated steel structures, pressure vessels, or weldments needs fluency in both AWS A2.4 and ISO 2553 weld symbol notation.
Welding is one of the most common joining methods in mechanical engineering, appearing in structural frames, pressure vessels, machine bases, piping, and countless fabricated assemblies. The specific weld type, size, location, and finishing requirements are communicated through welding symbols on engineering drawings, governed by AWS A2.4 (American Welding Society, dominant in North America) and ISO 2553 (internationally). While the two systems have similarities, they differ enough in symbol placement and notation that using one standard’s symbols on a drawing governed by the other can result in welds on the wrong side, with wrong geometry, or with missing finish requirements.
The Welding Symbol: Overall Structure
A welding symbol (note: “welding symbol” refers to the complete annotation; “weld symbol” refers to the specific geometric shape indicating weld type) consists of several elements arranged around a reference line:
- Reference line: A horizontal line — the baseline of the entire welding symbol. All other elements are positioned relative to this line.
- Arrow: A line from one end of the reference line pointing to the joint. The arrowhead indicates the joint location on the drawing.
- Weld symbol: The geometric symbol representing the weld type, placed on or above/below the reference line.
- Dimensions: Weld size and length, placed to the left (and right, for intermittent welds) of the weld symbol.
- Supplementary symbols: Weld-all-around, field weld, flush/convex/concave contour symbols.
- Finish symbols: Grinding (G), chipping (C), machining (M), or rolling (R) symbols indicating how the weld face should be finished.
- Tail: A fork shape at the end of the reference line opposite the arrow, used to add text references (WPS number, process specification, test requirements). Omitted when no tail information is needed.
Arrow Side vs Other Side: The Critical Distinction
The most important rule for reading welding symbols is the arrow side / other side convention:
In AWS A2.4:
- Weld symbols placed below the reference line apply to the arrow side of the joint (the side the arrow is pointing to)
- Weld symbols placed above the reference line apply to the other side of the joint (the opposite face)
- Symbols on both sides indicate welds on both faces
In ISO 2553: The convention is the same in the 2019 edition — below the reference line = arrow side, above = other side. (Earlier ISO 2553 editions had the opposite convention, which is a source of historical confusion — always check which edition of ISO 2553 the drawing references.)
A weld symbol placed incorrectly relative to the reference line results in a weld on the wrong face of the joint — this can be catastrophic in structural and pressure vessel applications where the weld face orientation affects load path and weld quality.
Basic Weld Symbols
Fillet Weld (△): A right-triangle symbol indicating a weld with a triangular cross-section filling the corner of a T-joint, lap joint, or corner joint. The most common weld type in structural fabrication. The weld size (leg length) is specified to the left of the symbol.
Example: 6△ below the reference line means a 6mm fillet weld on the arrow side. If a length is specified: 6△-50-100 means 6mm fillet, 50mm long, on 100mm centers (intermittent fillet weld).
Groove Welds: Used for butt joints (plate edges joined together). Several types are specified by their cross-sectional shape:
- Square groove (||): Two vertical lines — no bevel, plates butted square. Suitable for thin material.
- V-groove (\//): A V-shape indicating beveled edges on both plates. Depth of preparation and included angle are specified.
- Bevel groove (/|): One plate is beveled, the other is square. The arrow always points to the beveled plate (or “breaks” toward it for emphasis).
- U-groove (U): A U-shaped preparation — better weld quality, less distortion than V, but more expensive to prepare.
- J-groove (J): One plate only has a J-shaped bevel.
- Flare-V and Flare-bevel: Used for curved/round sections.
Groove weld dimensions: the number to the left of the symbol indicates the groove depth (S) and/or weld size (E). Example: 10(12)V means a V-groove with 10mm preparation depth and 12mm effective throat.
Plug and Slot Welds (○ and □): A circle indicates a plug weld (cylindrical hole filled with weld); a rectangle indicates a slot weld (elongated hole). The diameter or slot dimensions are specified above/below the symbol. Used to join overlapping plates or to repair cracks.
Spot and Seam Welds: An X in a circle indicates a spot weld (resistance, arc, or laser). An O with two parallel lines through it indicates a seam weld. Used primarily in sheet metal fabrication.
Supplementary Symbols
Weld All Around (circle at arrow/reference line junction): A small circle where the arrow meets the reference line indicates the weld is to be made all around the joint — continuously around the entire perimeter. This is critical for watertight or hermetic enclosures.
Field Weld (flag at arrow/reference line junction): A flag (filled triangle) at the arrow-reference line junction indicates the weld is to be made in the field (on-site), not in the fabrication shop. This distinction matters for quality control, inspection access, and assembly sequencing.
Melt-Through Symbol (●): A filled circle on the other-side of the reference line indicates that complete penetration is required when welding from one side only. The back bead must be visible and meet size requirements.
Backing Bar (rectangle with B): Indicates a backing bar is required on the other side of the weld to support the root pass. The backing bar may be permanent (specified with “R” for remains) or temporary.
Contour and Finish Symbols
The weld face profile and finishing requirement are specified above or below the weld symbol (for the respective side):
- Flush contour (—): A flat line above/below the weld symbol indicates the weld face must be flush with the base metal surface.
- Convex contour (⌒): An arc above/below indicates the weld may have a convex face profile.
- Concave contour (⌣): An inverted arc indicates a concave weld face is required (common for fillet welds in fatigue-critical joints).
Finish symbols are placed alongside the contour symbol when post-weld processing is required:
- G = Grind to contour
- C = Chip to contour
- M = Machine to contour
- R = Roll to contour
- U = Unspecified (any method achieving the contour)
A flush contour with G means: grind the weld flush with the base metal surface. This is common on fatigue-critical welds and aerodynamic surfaces.
Fillet Weld Size and Throat
For fillet welds, two size concepts are important:
- Leg size (S): The length of each leg of the weld triangle. For a standard equal-leg fillet, a single value is given (e.g., 6mm). For unequal legs, both are given.
- Effective throat (E or Z): The shortest distance from the root of the weld to the face — for a standard 45° fillet, throat = 0.707 × leg size. Throat governs the strength of the weld in shear; it is the critical dimension for structural calculations.
AWS specifies fillet welds by leg size. ISO 2553 can specify either by leg size (Z for “z-measure”) or by throat size (a for “a-measure”). An ISO weld symbol “a5” means a 5mm throat, not a 5mm leg — approximately equivalent to a 7mm leg AWS fillet. Confusing these leads to significantly undersized welds.
AWS vs ISO 2553: Key Differences
| Topic | AWS A2.4 | ISO 2553:2019 |
|---|---|---|
| Fillet weld size specification | Leg size (default) | Throat (a) or leg (z) — must specify which |
| Arrow side convention | Below reference line = arrow side | Below reference line = arrow side (2019 edition) |
| Groove depth vs weld size | S(E) format: depth left of symbol, throat in parentheses | Similar format with slight notation differences |
| All-around symbol | Circle at arrow/reference line junction | Circle at reference line turn |
| Tail | Present when reference needed | Present when reference needed |
| Effective throat for partial penetration | E specified in parentheses | Specified as “a” value |
Practical Drawing Reading Example
Consider a T-joint connection in a structural steel frame. The welding symbol shows:
- 6△ below the reference line (arrow side): 6mm fillet weld on the arrow side
- 6△ above the reference line (other side): 6mm fillet weld on the other side
- A weld-all-around circle at the reference line junction
- A flush contour symbol (—) with G (grind) on the other-side symbol only
This specifies: continuous 6mm fillet welds all around the joint on both sides; the other-side weld must be ground flush. The arrow-side weld can be left as-welded. This is a common specification for base plates and column connections requiring a clean appearance on one face.
Conclusion
Welding symbols on engineering drawings encode all the information a welder, fabricator, and inspector needs to produce and verify a weld joint — type, size, location, contour, finish, and special requirements. Fluency in both AWS A2.4 and ISO 2553 is increasingly important for engineers in global supply chains. The most critical rules to memorize: below the reference line is the arrow side, the fillet weld size basis differs between AWS (leg) and ISO (throat or leg — specified explicitly), and the weld-all-around and field weld symbols must never be confused or omitted. These details determine the structural integrity and safety of fabricated assemblies.
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