A groove melted into the base metal adjacent to the weld toe or weld root and le$ unfilled by weld metal.

Causes: Excessive welding current; improper electrode technique; mismatch between electrode design and weld position.

Corrections: use a moderate welding current and proper welding speed; use an electrode that produces a puddle of the proper size; proper weaving technique; proper positioning of the electrode relative to a horizontal fillet weld.

The term undercut is used to describe either of two situations. One is the melting away of the sidewall of a weld groove at the edge of the bead, thus forming a sharp recess in the sidewall in the area in which the next bead is to be deposited. The other is the reduction in thickness of the base metal at the line where the beads in the final layer of weld metal tie into the surface of the base metal (e.g., at the toe of the weld).

Both types of undercut are usually due to the specific welding technique used by the welder. High amperage and a long arc increase the tendency to undercut. Incorrect electrode position and travel speed are also causes, as is improper dwell time in a weave bead. Even the type of electrode used has an influence. The various classifications of electrodes show widely different characteristics in this respect. With some electrodes, even the most skilled welder may be unable to avoid undercutting completely in certain welding positions, particularly on joints with restricted access.

Undercut of the sidewalls of a weld groove will in no way affect the completed weld if the undercut is removed before the next bead is deposited at that location. A well-rounded chipping tool or grinding wheel will be required to remove the undercut. If the undercut is slight, however, an experienced welder who knows just how deep the arc will penetrate may not need to remove the undercut.

The amount of undercut permitted in a completed weld is usually dictated by the fabrication code being used, and the requirements specified should be followed because excessive undercut can materially reduce the strength of the joint. This is particularly true in applications subject to fatigue. Fortunately, this type of undercut can be detected by visual examination of the completed weld, and it can be corrected by blend grinding or depositing an additional bead.

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