If a joint requiring complete penetration is accessible from only one side, which symbol is used?

If a joint requiring complete penetration is accessible from only one side, which symbol is used?

• A. back weld
• B. melt-through
• C. backing weld
• D. backing bar

Answer: (B)

When a joint must have complete penetration but you can only access it from one side, use the melt-through symbol. This indicates that the root weld is meant to penetrate through the full thickness and exit on the opposite face, achieving full penetration from the single-sided access. Backing material or a backing weld would require access to the far side to place or weld against, which isn’t available here, so they aren’t appropriate. Melt-through conveys the needed through-thickness fusion from the accessible side.

When a joint must have complete penetration but you can only access it from one side, use the melt-through symbol. This indicates that the root weld is meant to penetrate through the full thickness and exit on the opposite face, achieving full penetration from the single-sided access. Backing material or a backing weld would require access to the far side to place or weld against, which isn’t available here, so they aren’t appropriate. Melt-through conveys the needed through-thickness fusion from the accessible side.