AWS A2.4 §7.3 · D1.1:2025 Figure 5.1

U-開先溶接 Symbol

How to read a U-groove 溶接記号 — U-shape on the 基線, root radius in cross-section detail only. Symmetric preparation, 事前認定 CJP joint details per D1.1:2025.

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U-Groove Weld Symbol — Anatomy

Single-U — 45°, D=3/4"

Joint cross-section

Double-U groove

Tail references radius detail

Key rule (A2.4 §7.3.3): Root radius (r) and ルート面 (f) cannot appear on the symbol line. They must be specified in a cross-section detail, drawing note, or other reference in the tail of the 溶接記号表示. This applies to both U-groove and J-groove welds.

How to Read

U-Groove Symbol Dimensions

The U-groove symbol looks like a U sitting on the reference line — a curved bottom with two vertical sides. Unlike bevel and J-groove welds, the U-groove prepares both members symmetrically, so no broken arrow is needed.

What Goes on the Symbol

Root opening (R) — INSIDE the U symbol.

Groove angle (α) — above or below the weld symbol as appropriate per A2.4 §7.3.2.

Groove depth (D) — to the LEFT for PJP (partial joint penetration) welds.

Weld サイズ (S) — in parentheses to the LEFT for PJP. Omitting D and S specifies CJP.

What Does NOT Go on the Symbol

Root radius (r) and root face (f) — per A2.4 §7.3.3, these MUST appear in a cross-section detail or other data with reference in the tail. Not on the symbol line. This is unique to U-groove and J-groove welds and is a common drafting error.

Arrow Side vs. Other Side

When the U-groove symbol sits below the reference line, the preparation is on the 矢の側 of the joint. Above the line means 反対側. Both above and below (double-U) means both members are prepared from both sides.

No broken arrow: Unlike J-groove and bevel-groove welds, a U-groove symbol uses a straight arrow. Both members get the curved preparation, so there is no ambiguity about which member to prepare.

D1.1:2025 Figure 5.1

Prequalified U-Groove CJP Joints

D1.1:2025 Figure 5.1 Type 6 (single-U) specifies prequalified CJP 突合せ継手 details. All prequalified U-groove CJP joints require backgouge from the second side before depositing the back weld.

Parameter	SMAW	GMAW / FCAW
Joint designation	B-U6	B-U6-GF
Groove angle (α)	45° (pos. A) · 20° (pos. F, OH)	20° (pos. A)
Root face (f)	1/8" (3 mm)	1/8" (3 mm)
Root radius (r)	1/4" (6 mm)	1/4" (6 mm)
Root opening (R)	0–1/8" (0–3 mm)	0–1/8" (0–3 mm)
Backgouge	Required	Required

Economy note: U-groove preparation reduces 溶接金属 volume significantly compared to double-V on thick plate. The curved walls converge tighter, meaning less 溶加材, less 入熱, and less 溶接ひずみ. The cost is machining — U-groove cannot be flame-cut like a V形開先.

Common Questions

U-Groove Weld Symbol FAQ

What is the difference between a U-groove and a V-groove weld?

Both prepare both members and are used for butt joints. A V-groove uses two straight angled cuts meeting at a point or small root face. A U-groove uses curved preparations — each member gets a rounded (U-shaped) cut instead of a straight bevel. This reduces weld metal volume significantly on thick plate because the curved walls converge tighter than straight V-faces. The tradeoff is that U-groove preparation requires machining, while V-groove can be flame-cut. The symbol looks like a U sitting on the reference line.

Can root radius and root face appear on the U-groove symbol?

No. Per AWS A2.4 §7.3.3, the groove radii and root faces of U-groove welds SHALL be specified by a cross-section, detail, or other data with reference in the tail of the welding symbol. They cannot appear on the symbol line itself. This rule applies to both U-groove and J-groove welds and is a common source of drafting errors.

What are the prequalified U-groove joint details in D1.1:2025?

D1.1:2025 Figure 5.1 Type 6 (single-U) specifies prequalified CJP butt joint details. The joint designation is B-U6 for SMAW and B-U6-GF for GMAW/FCAW. For SMAW: groove angle α = 45° (position A) or 20° (positions F, OH). For GMAW/FCAW: α = 20° (position A). Root face f = 1/8" (3 mm). Root radius r = 1/4" (6 mm). Root opening R = 0 to 1/8". All prequalified U-groove CJP joints require backgouge.

Why use a U-groove instead of a V-groove or J-groove?

A U-groove prepares both members (versus J-groove which prepares one member only). Use a U-groove when both sides of the joint need the curved preparation for symmetry or when access from one side is limited. Compared to a double-V groove, a U-groove uses less weld metal and causes less distortion on very thick plate (typically over 1"). The curved root geometry gives better fusion control at the root. The main disadvantage over V-groove is the need for machining or gouging equipment.

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