Identification and Prevention of Weld Undercut

weld undercut is a groove or depression formed along the toe of the weld in the base material due to improper welding parameters or operational techniques during the welding process. It reduces the effective cross-sectional area of the base material and may lead to stress concentration or cracking. In simple terms, it appears as an irregular, notch-like defect along the edges of the weld.

1. Definition and Characteristics
   Weld undercut refers to a continuous or intermittent groove formed along the weld toe due to excessive arc heat or improper operation, where the melted base material is not adequately filled by the deposited metal. Key features include:

• Location: Often occurs in vertical, horizontal, and overhead welding positions.

• Morphology: Can be classified as external undercut (on the groove face) or internal undercut (at the root of the groove), and may appear as continuous, intermittent, or inter-pass undercut.

Harm: Reduces the effective cross-section of the base material, causes stress concentration, and may initiate cracks in high-strength low-alloy steels due to edge hardening.

2. Causes
   Primary causes relate to welding parameters and operational techniques:

• Current and Arc: Excessive current or prolonged arc length leading to overmelting of the base material.

• Improper Manipulation: Incorrect electrode angle (e.g., deviation to one side) or inappropriate weaving technique (too short or too long dwell time at edges).

• Other Factors: Unsuitable shielding gas (e.g., in MAG welding), excessive leg size (>9mm in single-pass vertical welding), or arc blow (in DC welding).

3. Prevention Measures
   Solutions summarized from authoritative sources:

• Parameter Optimization: Reduce current, shorten arc length, and use AC welding instead of DC where applicable.

• Operational Standards: Correct welding posture (e.g., short arc welding), adjust travel speed (slower at edges, faster at center).

• Process Improvement: Apply multi-pass welding, reduce weaving width, direct arc toward thicker plates, and ensure uniform groove processing and fit-up clearance.

4. Repair Notes
   During repair, control heat input to avoid excessive cooling rates due to small weld size, which increases the risk of hydrogen-induced cracking. It is recommended to use small-diameter electrodes (φ2.0~3.0mm) for overlay welding to ensure proper fusion at the root.