What is represented by a filled (black) semi-circle appearing on a reference line?

What is represented by a filled (black) semi-circle appearing on a reference line?

• A. A convex weld surface
• B. Melt-through
• C. A back weld
• D. A backing bar

Answer: (B)

In welding symbols, the marks on the reference line tell you how the weld should behave in the thickness of the material. A filled (black) semicircle on that line is the standard symbol for melt-through, meaning through-penetration weld. It indicates the weld must burn through the entire thickness so the weld fuses from one side to the other. This is different from a shape that would indicate the bead’s appearance (a convex weld surface), a back weld on the opposite side, or a backing bar, which are depicted with different symbols or notes. So the filled semicircle specifically communicates through-penetration (melt-through) of the joint.

In welding symbols, the marks on the reference line tell you how the weld should behave in the thickness of the material. A filled (black) semicircle on that line is the standard symbol for melt-through, meaning through-penetration weld. It indicates the weld must burn through the entire thickness so the weld fuses from one side to the other. This is different from a shape that would indicate the bead’s appearance (a convex weld surface), a back weld on the opposite side, or a backing bar, which are depicted with different symbols or notes. So the filled semicircle specifically communicates through-penetration (melt-through) of the joint.