The distance the weld metal extends from the weld face into a joint, exclusive of weld reinforcement. See GROOVE WELD SIZE.
Joint penetration is the depth of fusion of a weld from the original surface of the base metal to the point where fusion ends.
For the weld to be acceptable it is necessary that the base metal and filler metal be completely fused together to that point. A weld may be made with partial penetration, where a gap or notch exists at the root of the weld, or complete penetration where fusion is complete from top to bottom.
Complete joint penetration is normally required in welds and when it cannot be obtained from one side in one pass, several passes are used with grooved joint preparation or weld passes are made from the root surface, or both.
In a square butt joint, joint penetration and root penetration are the same. In a groove weld, root penetration is the distance from the bottom of the groove to
the point where fusion ends.
Through penetration and complete fusion and bonding of the metal are essential for successful sound welds. Through penetration is not easily accomplished in square butt joints over 6 mm (1/4 inch) thickness by most arc welding processes. A groove or bevel joint preparation is used to achieve complete penetration. A gap between two plates with square edges may help attain penetration but oxides formed on the edges could prevent complete metallurgical bonding near the root of the joint. Full penetration can be attained in square butt joints in thicker plates by the electron beam, laser and plasma arc processes.

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