Welding undercut is a defect where a groove forms along the weld toe, reducing joint strength by up to 60%. AWS D1.1 allows max 0.8mm undercut for material ≤1″ thick, 1.6mm for >1″. To prevent: maintain 5-15° travel angle, 1/8″ arc length, 10-15% less current, 45-60° work angle. Use drag technique, weave bead, pause at toes. Grind & re-weld to repair. Monitor with gauges & visual inspection.
In this comprehensive guide, we will delve into the technical details of welding undercut and provide practical solutions to help you achieve high-quality welds.
What is Welding Undercut?
Welding undercut is a defect characterized by a groove that melts into the base metal adjacent to the weld toe, which is left unfilled by the weld metal. This notch reduces the cross-sectional thickness of the base metal and creates a stress concentration point, potentially compromising the integrity of the welded joint.
According to AWS D1.1 standards, the acceptable undercut depth is:
- 1/32″ (0.8mm) for material thickness up to 1″
- 1/16″ (1.6mm) for material thickness greater than 1″
Undercut defects deeper than 0.5mm are considered severe and require repair. It is important to note that undercut can reduce the fatigue strength of a weld joint by up to 40-60%, while the tensile strength is reduced in proportion to the loss in cross-sectional area caused by undercutting.
Causes of Welding Undercut
To effectively prevent welding undercut, it is essential to understand its primary causes. Some of the most common factors contributing to undercut include:
- Excessive current
- Incorrect torch angle
- Fast travel speed
- Long arc length
- Improper weld bead placement
Excessive Current
Using an excessively high welding current can cause the edges of the weld to overheat and melt away, resulting in undercut. To prevent this, reduce the current by 10-15% if undercutting is observed. For example, if you are using a welding machine with a current range of 50-220A, and you notice undercut at 200A, reduce the current to 170-180A and reassess the weld quality.
Tools and techniques to measure and adjust current:
- Use a welding machine with a digital display for accurate current settings
- Employ an ammeter or clamp meter to measure the actual current output
- Adjust the current using the machine’s controls, such as a knob or buttons
- Monitor the weld pool and bead appearance to gauge the effectiveness of current adjustments
Incorrect Torch Angle
Maintaining the correct torch angle is crucial for preventing undercut. A travel angle of 5-15° and a work angle of 45-60° help direct the arc into the joint, ensuring proper fusion and filling of the weld groove.
Tools and techniques to maintain proper torch angles:
- Use a welding gauge or angle finder to set and verify torch angles
- Practice maintaining consistent angles by welding on scrap metal
- Employ a welding table or fixture to help maintain proper positioning
- Regularly check the torch angle during welding and make adjustments as needed
Fast Travel Speed
A travel speed that is too fast may not allow enough time for the molten metal to fill the melted groove, leading to undercut. Adjust your travel speed to ensure adequate filling of the weld joint. The appropriate travel speed depends on factors such as material thickness, welding process, and desired weld size.
Tools and techniques to control travel speed:
- Use a metronome or timer to maintain a consistent travel speed
- Practice welding at different speeds on scrap metal to find the optimal range
- Employ guide rails or a welding carriage to maintain steady travel speed
- Monitor the weld pool and bead appearance to assess travel speed effectiveness
Long Arc Length
An arc length that is too long can make the arc unstable and generate spatter, causing undercut. Keep the arc length to 1/8″ or less and maintain the electrode close to the puddle for better control and minimized undercut risk.
Tools and techniques to maintain proper arc length:
- Use a welding gauge or ruler to measure and set the appropriate arc length
- Practice maintaining a consistent arc length on scrap metal
- Employ a welding helmet with a clear view of the arc and weld pool
- Regularly check the arc length during welding and make adjustments as needed
Improper Weld Bead Placement
Failing to tie in the toes of the weld can result in undercut. Ensure proper weld bead placement and overlap to create a smooth transition between the weld metal and base metal.
Tools and techniques for proper weld bead placement:
- Use a welding gauge or template to mark the desired weld bead location
- Practice welding with proper bead placement on scrap metal
- Employ a welding jig or fixture to maintain consistent bead placement
- Regularly inspect the weld bead placement during welding and make adjustments as needed
Prevention Techniques
To minimize the occurrence of welding undercut, employ the following techniques:
- Adjust wire feed speed and voltage to achieve a stable spray transfer
- Use a drag or backhand welding technique to deposit more filler metal at the toes
- Weave or pause on each side of the weld bead to allow the edges to fill in
- Clean mill scale, rust, paint, and dirt from the joint faces prior to welding
Technique | Description | Tools and Processes |
---|---|---|
Drag or Backhand Welding | This technique involves pointing the electrode in the direction of travel, which helps deposit more filler metal at the toes of the weld. | – Use a welding torch with a 10-15° drag angle – Maintain a consistent drag angle throughout the weld – Practice the technique on scrap metal to perfect the motion |
Weaving or Pausing | By weaving the electrode or pausing on each side of the weld bead, you allow the molten metal to fill in the edges, reducing the risk of undercut. | – Use a welding torch with a weaving motion or pause on each side – Maintain a consistent weave or pause pattern throughout the weld – Practice the technique on scrap metal to perfect the motion |
Joint Cleaning | Removing contaminants such as mill scale, rust, paint, and dirt from the joint faces before welding promotes arc stability and reduces the likelihood of undercut. | – Use a grinder with a wire wheel or flap disc to remove contaminants – Employ a solvent or degreaser to clean the joint faces – Inspect the joint faces for cleanliness before welding |
Repairing Welding Undercut
If an undercut defect occurs despite preventive measures, it can be repaired using the following steps:
- Grind a smooth taper adjacent to the toe of the weld, creating a shallow groove.
- Re-weld the ground area using a smaller electrode to fill in the groove.
- Ensure proper fusion and smoothness of the repaired area.
Tools and techniques for repairing welding undercut:
- Use a grinder with a carbide burr or grinding wheel to create a smooth taper
- Select a smaller diameter electrode, such as a 1/16″ or 3/32″, for repair welding
- Employ a welding torch with a lower amperage setting for better control
- Inspect the repaired area for proper fusion and smoothness, and re-grind if necessary
However, it is always better to prevent undercutting from occurring in the first place by using proper welding parameters and techniques.
Conclusion
Welding undercut is a serious defect that can compromise the strength and longevity of welded joints. By understanding its causes, employing prevention techniques, and knowing how to repair it when necessary, you can significantly improve the quality of your welds. Remember to maintain a tight arc, use a 10° drag angle, and keep a moderate travel speed to minimize the risk of undercut. Regularly inspect your welds for any signs of undercut and address them promptly to ensure the integrity of your welded structures.
References:
Undercut Welding Defect: Causes, Prevention, and Repair
Tips for Avoiding Common Flux-Cored Problems & Improving Your Fcaw Welds