What is undercutting?

Short Answer:

Undercutting is a common welding defect that occurs when the base metal along the edge of the weld is melted away and not properly filled with the weld metal. It appears as a groove or depression at the weld toe, reducing the strength of the joint. Undercutting usually happens due to high welding current, fast travel speed, or incorrect electrode angle.

This defect weakens the welded structure because it acts as a stress concentration point where cracks may develop under load or vibration. Proper control of welding parameters, electrode angle, and welding speed helps in preventing undercutting during the process.

Detailed Explanation :

Undercutting

Undercutting is a groove or depression formed at the edge of a weld bead that occurs when the base metal melts away and is not replaced by filler metal. It is a type of welding defect that negatively affects the mechanical strength and appearance of the welded joint. The groove created along the weld toe acts as a weak point that can lead to cracks or even complete failure under stress or fatigue loading.

The main cause of undercutting is improper control of welding parameters. If the welding current is too high, the base metal melts excessively, and the molten pool becomes too fluid to retain its shape, leading to erosion of the base edges. Similarly, if the welding speed is too fast, there isn’t enough time for the filler material to properly fill the melted area, causing a hollow or undercut edge.

Causes of Undercutting

  1. High Welding Current:
    When the current is too high, the excessive heat melts more of the base metal than necessary. As a result, molten metal flows away, leaving behind a groove near the weld toe.
  2. Excessive Welding Speed:
    High welding speed reduces the time available for filler metal deposition, preventing the molten pool from fully filling the edges. This often creates a narrow, deep undercut.
  3. Incorrect Electrode Angle:
    Holding the electrode at an improper angle can direct the arc force away from the correct position, causing uneven melting of the base metal and leaving a depression.
  4. Improper Manipulation of Electrode:
    Lack of steady hand movement or poor control of arc length can lead to irregular heating and localized undercutting.
  5. Low Arc Voltage or Long Arc Length:
    A longer arc length or incorrect voltage produces unstable heat distribution, leading to incomplete fusion and surface defects like undercutting.
  6. Improper Joint Design:
    Poor joint preparation or incorrect bevel angle may make it difficult to access edges properly, increasing the risk of undercut formation.

Effects of Undercutting

Undercutting significantly affects both the strength and durability of a welded joint. Some important consequences are:

  • Reduced Strength: The groove at the weld toe reduces the cross-sectional area, making the joint weaker.
  • Crack Initiation: The sharp edges of an undercut act as stress concentrators where cracks can easily start under loading.
  • Poor Fatigue Resistance: In components exposed to cyclic loads, undercutting accelerates fatigue failure.
  • Poor Aesthetic Appearance: Visually, undercutting makes the weld look uneven and of low quality.
  • Corrosion Risk: The depressions trap moisture and other contaminants, increasing the risk of corrosion, especially in outdoor or chemical environments.

Methods to Prevent Undercutting

  1. Control Welding Current:
    Maintain the correct current as per the electrode and material type. Avoid using excessively high current settings.
  2. Optimize Welding Speed:
    Do not move the electrode too quickly. Allow sufficient time for filler material to deposit properly into the joint.
  3. Maintain Proper Electrode Angle:
    The electrode should be held at the recommended angle, generally 10°–15° from vertical, depending on the process.
  4. Ensure Proper Arc Length:
    Keep the arc length short and steady to maintain stable heating and good filler deposition.
  5. Use Correct Electrode Size:
    Match electrode size to the thickness of the metal being welded. Oversized electrodes may cause excessive heat and undercutting.
  6. Employ Weaving Technique:
    A small side-to-side movement of the electrode during welding can help distribute heat evenly and fill edges better.
  7. Clean Workpiece Properly:
    Dirt, rust, or oil on the base metal can disturb the arc stability, leading to improper melting and undercut formation.
  8. Proper Training and Supervision:
    Skilled welders with experience can adjust parameters on the spot to avoid defects like undercutting.

Inspection and Detection

Undercutting is easily visible to the naked eye during visual inspection. It appears as a sharp groove along the weld bead. In some cases, when the defect is small, magnifying tools or non-destructive testing (NDT) methods like dye penetrant inspection or ultrasonic testing can be used to identify it precisely.

Repair of Undercutting

Minor undercuts can be repaired by grinding and re-welding the affected area with a lower current setting and slower travel speed. It’s essential to clean and prepare the surface before re-welding to ensure good fusion and a smooth finish.

Conclusion

Undercutting is a serious welding defect that weakens the joint and reduces its life. It is mainly caused by high current, incorrect speed, and poor electrode handling. Preventing undercutting requires proper control of welding parameters, joint preparation, and skilled operation. Regular inspection and maintenance ensure strong, defect-free welds that perform safely under load and stress conditions.