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AWS A2.4 §6.14 · D1.1 Clause 10

Melt-Through Symbol

How to read the melt-through symbol — the filled (solid) semicircle indicating CJP with visible root reinforcement from one side. Tubular connections, HSS joints, and one-sided CJP applications.

Melt-Through Symbol — Anatomy
filled = melt-through
V-groove + melt-through
1/8 reinforcement height left
With 1/8" root reinforcement
filled = melt-thru open = back/backing
Filled vs open semicircle
Two conditions: Melt-through requires both CJP (complete joint penetration) AND visible root reinforcement on the back side. If you need CJP without root reinforcement, do not use the melt-through symbol.
How to Read

Melt-Through Symbol Placement

Per AWS A2.4 §6.14, the melt-through symbol is a supplementary symbol — a filled (solid black) semicircle placed on the opposite side of the reference line from the groove weld.

Root Reinforcement Height

Shown to the LEFT of the melt-through symbol, on the opposite side from the groove weld. When omitted, visible root reinforcement is required but no specific height is mandated.

One-Sided Welds Only

The melt-through symbol applies only to welds made from one side. If you can weld from both sides (with backgouge or back weld), melt-through is not the correct symbol — use the back/backing weld symbol instead.

Common Applications

Pipe/tube penetrations welded from the outside only. HSS through-plate connections where inside access is impossible. One-sided CJP joints without backing bars. Edge welds on flanged joints combined with the melt-through symbol for CJP.

D1.1 Clause 10: Tubular connections are the primary D1.1 application for melt-through. HSS-to-plate and round-to-round connections where the inside surface is inaccessible frequently specify CJP with melt-through.
Common Questions

Melt-Through Symbol FAQ

What does the melt-through symbol mean?
The melt-through symbol is a filled (solid black) semicircle placed on the opposite side of the reference line from the groove weld. It indicates two conditions must be met: the weld achieves complete joint penetration (CJP) AND there is visible root reinforcement on the back side. It applies only to welds made from one side — where the welder cannot access the reverse side to deposit a separate back weld.
What is the difference between melt-through and backing weld symbols?
Both sit on the opposite side of the reference line from the groove weld, but they look different and mean different things. The melt-through symbol is a FILLED (solid black) semicircle — it means the primary weld melts completely through with visible root reinforcement. The backing/back weld symbol is an OPEN (unfilled) semicircle — it means a separate weld operation on the reverse side (either before or after the groove weld).
Can you specify the root reinforcement height?
Yes. Per AWS A2.4 §6.14, the root reinforcement height appears to the LEFT of the melt-through symbol. This is placed on the opposite side of the reference line from the groove weld dimensions. When the height is omitted, visible root reinforcement is required but no specific amount is mandated.
When is the melt-through symbol used in D1.1?
D1.1 Clause 10 governs tubular connections where melt-through is common. Typical applications include pipe or tube penetrations welded from the outside only, HSS (hollow structural section) through-plate connections, and one-sided CJP joints where backing bars are impractical. The melt-through symbol can also combine with edge weld symbols for CJP flanged joints welded from one side.
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