What does weld penetration refer to?

• A. The depth to which the filler metal fuses with the base metal; complete penetration means fusion along the full joint thickness.
• B. The amount of filler material used.
• C. The width of the weld bead.
• D. The color of the weld surface.

Answer: (A)

Penetration is about how deep the molten weld metal fuses into the base metal. It describes the depth of fusion, not how much filler is used, how wide the weld bead is, or the weld’s surface color. Full penetration means the weld metal fuses through the entire thickness of the joint, creating a bond from one face to the other. Partial penetration means fusion only goes partway into the thickness, leaving some base metal unfused at the root. This distinction matters for joint strength and integrity, since full penetration offers a through-thickness bond, while partial penetration may be weaker.