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Undercut of Welding: Causes, Prevention, Measurement, and Repair
Learn what causes undercut of welding, how to identify and measure it, acceptable limits, prevention methods, and practical ways to repair weld undercut.
Table of Contents
Undercut of welding is a common weld-profile defect that forms along a weld toe or root. Although it may look like a small groove, excessive undercut can reduce the effective base-metal section and create a local stress concentration.
Most welding undercut problems are related to heat input, travel speed, arc length, torch angle, and filler deposition. Stable process control also helps. Dynalasers laser welding systems provide concentrated heat input and consistent parameters that can help reduce undercut in welding when the joint and settings are properly prepared.
What Is Undercut in Welding?
What is undercut in welding? It is a groove melted into the base metal along the edge of a weld that has not been completely filled with weld metal. It commonly forms at the weld toe but can also occur near the root.
This undercut welding defect differs from a low weld bead because part of the original base-metal profile has actually been melted away.
What Does Undercut Look Like on a Weld?
An undercut on a weld usually appears as a narrow depression beside the bead. It may be continuous or limited to short sections.
A typical undercut weld example may show:
- A groove beside one or both weld toes
- A sharp transition between weld and base metal
- Deeper sections near starts, stops, or cap passes
Shallow undercut may require measurement rather than visual judgment alone.
Types of Undercut in Welding
Undercut is generally classified by where the groove forms.
External or Toe Undercut
External undercut forms where the visible weld face meets the base metal. Fillet weld undercut can occur along one or both weld toes and is normally easy to detect visually.
Internal or Root Undercut
Root undercut in welding occurs near the root side of the joint. Where backside access is limited, it may be more difficult to detect and evaluate.
What Causes Undercut in Welding?
Understanding what causes undercut in welding is more useful than automatically reducing amperage. Undercut develops when the weld edge is melted faster than the molten pool can refill it.
The main welding undercut causes include:
Cause | What Happens | Typical Adjustment |
Excessive current or voltage | Too much base metal melts at the edge | Reduce heat input |
Travel speed too fast | The pool cannot fully fill the toe | Slow slightly |
Arc too long | Heat spreads toward the edges | Shorten and stabilize the arc |
Incorrect torch angle | Too much energy reaches one sidewall | Correct the angle |
Insufficient filler | Melted edges remain unfilled | Improve filler deposition |
Excessive weaving | Too much heat reaches the edges | Reduce weave width |
Poor fit-up | Access limits proper toe filling | Improve joint preparation |
Accumulated plate heat | Later passes become progressively hotter | Control interpass temperature |
Accumulated heat is often overlooked. A root and fill pass may look acceptable while the cap develops undercut because the workpiece is already hot. In this case, reducing current for later passes or controlling interpass temperature may work better than treating the problem as a fixed machine-setting issue.
Why Is Undercut Bad for a Weld?
The severity of weld undercut depends on its depth, length, location, loading condition, and the applicable standard. A shallow groove does not automatically make a weld unacceptable.
Deeper undercut reduces local material thickness and creates a sharp transition that concentrates stress. This becomes more important in parts exposed to vibration, cyclic loading, or fatigue.
How to Measure Weld Undercut
Visual inspection can locate the welding discontinuity, but proper evaluation normally requires checking its location, length, and depth.
Check the Location and Length
Identify whether the undercut is at the toe or root, continuous or intermittent, and whether it affects one or both sides of the weld. Some acceptance criteria consider affected length as well as maximum depth.
Measure the Undercut Depth
A weld undercut gauge, bridge cam gauge, or similar inspection tool can be used to measure from the adjacent base-metal surface to the deepest part of the groove.
For how to measure undercut in welding, depth is normally more important than groove width when determining acceptance.
How Much Undercut Is Acceptable in Welding?
There is no universal value for allowable undercut in welding. Acceptance depends on the applicable code, material thickness, joint type, loading condition, defect depth, and affected length.
Weld undercut acceptance criteria may come from AWS, ASME, ISO 5817, engineering drawings, or project-specific requirements. Therefore, how much undercut is allowed in a weld should not be answered with one number unless the governing standard is known.
Before accepting or repairing the weld, verify the applicable specification and permitted depth and length.
How to Prevent Undercut in Welding
The best approach to how to prevent undercut in welding is to troubleshoot in a logical order rather than changing several settings at once.
Start With Heat and Travel Speed
Check current or voltage first, then travel speed. Excessive heat can erode the weld edge, while moving too quickly may leave too little time for filler metal to wash into the toe.
On multipass welds, also consider accumulated workpiece heat.
Check Arc Length and Torch Angle
A long arc or incorrect work angle can direct excess heat toward the weld edge. Consistent torch position becomes especially important around corners, changing joint gaps, and difficult welding positions.
Make Sure the Toe Gets Enough Filler
Lowering current alone may not solve the problem. The operator may also need to slow slightly, improve filler placement, or reduce excessive weaving so the molten pool properly fills the weld toe.
For suitable applications, laser welding offers more concentrated energy delivery and lower overall heat input. Dynalasers laser welding machines provide stable process control and optional wire feeding, helping reduce conditions that can contribute to undercutting in welding.
Undercut in MIG, TIG, and Stick Welding
The defect is similar across welding methods, but common causes differ.
Process | Common Cause | Check First |
MIG welding undercut | High voltage, fast travel, poor gun angle | Voltage and travel speed |
TIG welding undercut | Excessive current or insufficient filler | Current and filler timing |
Stick welding undercut | High amperage, long arc, fast travel | Amperage and arc length |
For MIG weld undercut, voltage and wire feed should be considered together. TIG depends heavily on filler placement, while stick welding often benefits from tighter arc-length control.
How to Fix Undercut in Welding
How to fix undercut in welding first depends on whether the defect exceeds the applicable acceptance criteria. An acceptable shallow groove should not automatically be repaired because unnecessary rework introduces another heat cycle.
When undercut weld repair is required:
- Prepare and clean the affected area
- Remove unacceptable material where required
- Correct the original process problem
- Re-weld using adjusted parameters
- Inspect the repaired area again
Simply adding another pass without correcting heat input, travel speed, angle, or filler delivery can reproduce the defect.
For precision or thin-sheet applications, a controlled laser process can also help reduce recurring edge defects. Dynalasers systems provide stable, low-heat-input welding and can use wire feeding where additional filler is required.
Undercut vs Underfill vs Lack of Fusion
These welding defects can look similar but involve different problems.
Defect | What Is Wrong? | Typical Appearance |
Undercut | Base metal beside the weld is melted away and not filled | Groove along toe or root |
Too little weld metal fills the intended profile | Weld face sits too low | |
Weld metal does not properly fuse with surrounding material | May be visible or internal |
For weld underfill vs undercut, the key difference is the missing material: underfill means insufficient deposited weld metal, while undercut removes part of the original base metal.
FAQs About Welding Undercut
Can a Weld Have Undercut on Only One Side?
Yes. One-sided undercut may result from an uneven work angle, limited joint access, asymmetric geometry, or excessive heat directed toward one sidewall.
Why Does Undercut Sometimes Appear Only on the Cap Pass?
The workpiece may already be hot when the cap is deposited. Combined with insufficient filler at the toes or aggressive cap technique, this can produce undercut even when earlier passes were acceptable.
Can You Have Undercut Without Excessive Amperage?
Yes. Fast travel, a long arc, incorrect torch angle, excessive weaving, or insufficient filler can all cause undercut even when current is reasonable.
Is Undercut More Serious in Fatigue-Loaded Welds?
It can be. The groove creates a local stress concentration, which becomes more significant under repeated or fluctuating loads. Final acceptance should still follow the applicable design and welding standard.
Conclusion
Undercut of welding is usually caused by the interaction of heat input, travel speed, arc length, torch angle, filler deposition, and workpiece temperature rather than one setting alone.
Correct measurement and acceptance criteria are as important as prevention. For applications requiring stable, low-heat welding, Dynalasers laser welding systems can help reduce conditions that contribute to welding undercut, while proper setup and technique remain essential.
