Why Weld Symbols Are Worth Mastering
Weld symbols are the most compact and precise method of communicating welding requirements on an engineering drawing. A correctly written weld symbol specifies the joint type, weld size, weld length, pitch, location (arrow side or other side), and special process requirements — all in a single annotation. A misread or incorrectly specified weld symbol can result in an undersized weld that fails in service or an over-welded joint that wastes material and introduces unnecessary distortion.
This article covers the AWS A2.4 and ISO 2553 weld symbol conventions used on industrial engineering drawings, with emphasis on the symbols most commonly encountered by mechanical designers: fillet welds, groove welds, and the additional information elements that specify practical welding requirements.
The Reference Line and Arrow: The Foundation of Every Weld Symbol
Every weld symbol is built on a reference line — a horizontal line with an arrow pointing to the weld joint location. The position of information relative to this reference line determines whether it applies to the arrow side of the joint (the side the arrow points to) or the other side (opposite the arrow).
Arrow Side vs. Other Side
- Information placed below the reference line applies to the arrow side of the joint.
- Information placed above the reference line applies to the other side of the joint.
- When the same weld is required on both sides, symbols appear both above and below the reference line.
This convention is fundamental. An engineer who places a fillet weld symbol above the line when the weld is needed below the line creates a fabrication error. Verify the arrow side before finalizing every weld callout.
Fillet Weld Symbols
Fillet welds are represented by a right triangle on the reference line. The leg size (weld size) is placed to the left of the symbol. The length and pitch of intermittent welds are placed to the right.
Fillet Weld Symbol Elements
| Element | Position on Symbol | Example |
|---|---|---|
| Weld size (leg length) | Left of triangle symbol | 6 (leg = 6 mm) |
| Weld length | Right of symbol, first number | 50 (50 mm long weld) |
| Weld pitch (intermittent) | Right of symbol, after hyphen | 50-150 (50 mm weld, 150 mm pitch) |
| All-around symbol | Circle at junction of arrow and reference line | Indicates weld all around the perimeter |
| Contour symbol | Above or below triangle | Flat (flush), convex, or concave finish |
| Finish symbol | Adjacent to contour symbol | G=grind, C=chip, M=machine |
For example, a 6 mm fillet weld, 50 mm long at 150 mm pitch, on the arrow side, flush ground: the symbol shows "6" to the left of the arrow-side triangle, "50-150" to the right, a flat contour symbol above the triangle, and a G finish designation.
Groove Weld Symbols
Groove welds are specified when the joint requires full or partial penetration through the material thickness. The basic groove types and their symbols:
- Square groove: Two parallel lines (no preparation, used for thin materials)
- V-groove: Two angled lines forming a V shape — most common for moderate to heavy plate
- Bevel groove: One angled line — only one member is beveled (asymmetric joint)
- J-groove: Curved preparation on one side — better root access, used for thicker materials
- U-groove: Curved preparation on both sides — used for very thick sections where V-groove would require excessive weld metal
For groove welds, additional information is specified in a tail flag attached to the reference line arrow end. The tail contains the welding procedure specification (WPS) number, the groove angle, the root opening, and the root face dimension for complete specification.
CJP vs. PJP Groove Welds
Complete Joint Penetration (CJP) groove welds develop the full strength of the base metal. They require back-gouging and back-welding or the use of a backing bar to ensure complete fusion at the root. The designation "CJP" appears in the tail. Partial Joint Penetration (PJP) welds penetrate to a specified depth less than the full material thickness. The effective throat dimension must be specified because it determines the weld strength.
Supplementary Symbols
Several supplementary symbols modify the basic weld symbol to specify additional requirements:
- Field weld flag: A flag at the junction of arrow and reference line indicating the weld is to be made in the field (after shipment), not in the fabrication shop.
- All-around symbol: A circle at the same junction indicating weld is required around the full perimeter of the joint.
- Melt-through symbol: A filled circle at the root side, indicating complete penetration visible on the back side is required.
- Backing bar: A rectangle symbol below a groove weld symbol indicating a permanent or removable backing bar is used.
ISO 2553 vs. AWS A2.4: Key Differences
International projects frequently use ISO 2553 symbols. The fundamental concepts are identical, but there are notation differences that can cause misinterpretation if the applicable standard is not identified on the drawing.
| Feature | AWS A2.4 | ISO 2553 |
|---|---|---|
| Fillet weld symbol | Right triangle | Right triangle (same) |
| Arrow side vs. other side | Below = arrow side, above = other side | Dashed line = other side; solid = arrow side |
| Weld size notation | Leg length | Throat dimension (a=throat, z=leg) |
| Groove weld preparation | In tail flag | Specified directly on symbol |
The most critical difference is the weld size convention. AWS uses leg size; ISO uses throat dimension. A 6 mm fillet weld in AWS notation (6 mm leg) has a throat of approximately 4.2 mm. Specifying 6 mm on an ISO drawing specifies the throat — this is a different weld size, and the difference affects both strength and weld metal cost.
FAQ
Q: How do I specify that a weld joint needs to be inspected?
Inspection requirements are specified in the tail flag of the weld symbol, referencing the applicable inspection standard and method. For example: "Inspect per AWS D1.1, UT after welding" in the tail. Alternatively, inspection requirements can be specified in drawing general notes applying to all welds on the drawing, with specific welds called out for enhanced inspection in the tail where needed.
Q: What is the correct way to specify a weld on a structural tube (HSS) corner joint?
A corner joint on an HSS tube member typically uses fillet welds on all accessible faces. Specify the fillet weld on each face separately, or use the all-around symbol if the same weld is continuous around the perimeter. For structural applications, verify that the weld size and length provide the required strength for the applied loads — do not rely on visual adequacy alone.
Q: How should I communicate weld sequence requirements when distortion is a concern?
Weld sequence is most effectively communicated in the welding procedure specification (WPS) referenced in the tail flag, not in the weld symbol itself. For cases where sequence is critical and no WPS exists, a drawing note specifying the required sequence (e.g., "Weld per indicated sequence 1→2→3→4 to minimize distortion" with numbered weld locations) is acceptable. Sequence notes on drawings are less formal than a WPS but better than no guidance at all.
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