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Understanding Welding Undercuts: Causes, Prevention, and Solutions

Posted on September 23, 2023 by Philbert Jackson

What is an Undercut in Welding?

Undercut at top of weld

An undercut in welding refers to a recessive groove that runs along the weld bead. It appears as a long trough in the base metal at the edge of the weldment, capable of trapping liquid. Undercuts weaken the joint in several ways. Notably, they result in an absence of material where it is needed, compromising the structural integrity of the joint. Additionally, undercuts reduce the strength of the joint by trapping water and dirt, leading to premature mechanical failure.

Types of Undercuts

External Undercut

External undercut in fillet weld

External undercuts are visible at the toe of the weld, commonly seen in fillet welds. They occur more frequently because the front surface is closer to the weld gun.

Internal Undercut

Internal undercut in butt joint

Internal undercuts are formed on the underside of a butt joint, at the core of the weldment. They are also known as root undercuts.

10 Quick Tips for Preventing Undercuts in Welding

Here are some effective tips and tricks to prevent undercuts in welding:

  1. Use the appropriate current: If undercuts are a recurring issue, try reducing the current to achieve proper fusion without overheating.
  2. Find the right voltage: Adjust the voltage knob to establish a balance between achieving a smooth finish, uniform bead profile, and preventing undercuts.
  3. Avoid haste: Stir the stick slowly to ensure sufficient time for mixing and prevent rapid solidification.
  4. Choose compatible fillers: Consult a chart or an experienced welder to select the correct filler material.
  5. Maintain a good working angle: Deposit the filler evenly into the weld pool by holding a consistent angle. Check for any differences in each side of the bead.
  6. Opt for the right electrode size: Use an electrode that fills the groove properly and delivers energy evenly.
  7. Use suitable shielding: Make an informed choice based on the desired weld and ensure the gas mixture is appropriate for the job.
  8. Weave with a steady hand: When weaving, slightly delay your stay on the edges to ensure proper fusion and reduce the chance of undercuts.
  9. Keep a steady arc: Excessive splash or resistance during welding indicates an incorrect arc length. Adjust it to maintain a steady arc throughout the weld.
  10. Proper edge preparation: Maintain desired fillet grooves, ensure their smoothness, and clean them like a professional.

Welding in progress

Main Causes of Undercuts in Welding

Undercutting is mainly caused by:

  • Wrong selection of welding parameters: Inappropriate settings, such as high current, high travel speed, and high arc voltage, can lead to undercuts.
  • Electrodes and shielding: Improper electrode material, electrode angle, electrode size, and incompatible shielding gas can contribute to undercuts.
  • Welding techniques: Improper knitting techniques and excessive arc length can result in undercuts.
  • Lack of advance preparation: Dirt or grit on welding surfaces can block heat transfer and cause undercuts.

Acceptable Amount of Undercut

The acceptability of an undercut depends on visual inspection, joint type, thickness, and material strength. Generally, undercuts below 0.5 mm (.02 in) are considered safe. However, for more formal evaluations, refer to guidelines set by the American Welding Society. According to the AWS D1.1 code, undercuts below 1/32″ are harmless. Above this threshold, close inspection is necessary. Any undercut deeper than 1/16″ renders the part rejected. For undercuts between 1/32″ and 1/16″, the total length of undercuts falling in this range should not exceed 2″ in a 12″ section for the part to pass.

Removing Undercuts in Welds

When undercuts are deemed unacceptable, there are several ways to address them. One method involves filling the undercuts with new weld beads using stringing or weaving techniques. Another option is grinding the defect using an angle grinder. However, this should only be done if the joint is thick enough to withstand the reduction in strength. For thinner weldments, adding a filler layer after grinding can restore the original thickness.

Frequently Asked Questions

Q: What is the main reason for undercut in welding?

Undercutting can occur due to various reasons, including excessive heat from high current and voltage settings, fast travel speed, and inadequate surface preparation.

Q: Does undercut weaken a weld?

Undercuts have the potential to undermine the structural integrity of a joint. They create low thickness areas that are prone to cracking under loading and reduce the joint’s strength by trapping water and dirt, leading to corrosion.

Q: How do you fix undercuts in welding?

Undercuts can be fixed by filling them with new weld beads using stringers or weaves. Alternatively, grinding the defect with an angle grinder is an option, followed by adding a filler layer if necessary.

Conclusion

Avoiding undercuts in welding can be challenging, even with experience. However, understanding the causes and following prevention methods can minimize the occurrence of this welding defect. Always practice and strive for continuous improvement to minimize undercuts. We hope this article has provided valuable insights and information. If you have any questions or comments, feel free to reach out. At Toolsworking, we are your go-to source for all welding information.

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