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Weld Discontinuities: 12 Common Types, Causes, and Inspection
Learn what weld discontinuities are, 12 common types, their causes and fixes, inspection methods, and when a discontinuity becomes a weld defect.
Table of Contents
A weld does not have to look perfect to pass inspection. The opposite is also true: a smooth, even bead can hide problems below the surface. This is why weld discontinuities are better judged by what they are, where they occur, and whether they meet the requirements of the joint.
That applies to arc welding and laser welding alike. Dynalasers gives operators control over the welding process, but good equipment does not replace fit-up, preparation, technique, or inspection.
What Is a Weld Discontinuity?
A weld discontinuity is an irregularity or interruption in the expected structure or shape of a weldment. It might be a pore in the weld metal, an unfused area along a sidewall, a crack, or simply an incorrect weld profile.
The important part of the weld discontinuity definition is that a discontinuity is not automatically a defect. Industry standards distinguish between an imperfection or discontinuity and one that exceeds the applicable acceptance limit.
Weld Discontinuity vs Weld Defect
This distinction matters in practice. A discontinuity tells you that something differs from the ideal weld. A defect tells you that the difference is not acceptable for that job.
Type | Weld Discontinuity | |
What it describes | An irregularity in the weldment | An unacceptable discontinuity |
Always rejected? | No | Yes |
Always repaired? | No | Normally requires action |
Judgment depends on | Type, size, position, distribution | Applicable acceptance criteria |
So when discussing weld defects and discontinuities, avoid assuming that every visible pore or profile change means the weld has failed. Some standards, for example, permit certain inclusions or penetration imperfections within defined limits, while cracks are treated much more strictly.
Is a Missing Weld a Weld Discontinuity?
A missing weld is slightly different. It normally means a weld required by the drawing or weld symbol was omitted, misplaced, or left incomplete. Most welding discontinuities, by comparison, are irregularities in or around a weld that is actually present. A missing weld is still a fabrication nonconformance, but it should be checked against the drawing rather than treated as another form of porosity or lack of fusion.
12 Common Types of Weld Discontinuities
There is no universal rule saying there are exactly 12 discontinuities in welding. In shop work, however, the following group covers many of the conditions that come up repeatedly during welding and inspection. They include planar, volumetric, and weld-shape irregularities.
1. Porosity
Porosity is gas trapped in the weld metal as it freezes. Sometimes it appears as a few obvious surface holes. Other times it is internal and only shows up during inspection. Wormholes are an elongated form of the same basic problem.
What usually causes it
- Contaminated material
- Moisture
- Poor shielding
- Gas leaks
- Coatings
- Unstable weld pool
What to do
- Start with cleaning and shielding before changing machine settings
- Check the gas path
- Remove oil or moisture
- Make sure drafts are not disturbing the shielding zone
2. Cracks
Cracks deserve more attention than a small rounded pore. Their sharp geometry makes them effective stress raisers, and they may occur in the weld metal, HAZ, crater, or base material. Some form during solidification; others appear later.
What usually causes it
- High restraint and residual stress
- Hydrogen contamination
- Unsuitable metallurgy or material selection
- Rapid cooling rates
- Poor welding procedure or heat control
What to do
- Do not simply cover the crack with another weld pass
- Remove the cracked section completely before repair
- Identify and eliminate the root cause (stress, hydrogen, or cooling issues)
- Apply correct preheat or post-weld heat treatment if required
- Re-weld only after conditions are corrected
3. Slag Inclusions
Slag inclusions are nonmetallic material trapped in the weld or between passes. Naturally, they are mainly a concern in processes that actually produce slag.
What usually causes it
- Inadequate cleaning between passes
- Poor bead shape or incorrect welding technique
- Restricted joint access preventing proper fusion
- Slag trapped in grooves or under previous passes
What to do
- Clean thoroughly between every pass
- Ensure proper joint access for the torch or electrode
- Adjust bead shape to avoid trapping slag
- Use correct welding sequence to allow slag to float out
4. Incomplete Fusion
Incomplete fusion, or lack of fusion, means two surfaces that should have fused did not. It can occur at the joint sidewall, between passes, or around the root.
What usually causes it
- Low effective heat input
- Excessive travel speed
- Incorrect torch or electrode angle
- Contamination on fusion surfaces
- Poor joint access
What to do
- Check heat direction and arc placement first, not just power settings
- Reduce travel speed if needed
- Improve torch/electrode angle for better sidewall access
- Clean joint surfaces before welding
- Ensure the joint design allows proper access for fusion
5. Incomplete Joint Penetration
Incomplete penetration is often confused with incomplete fusion, but they are not quite the same.
What usually causes it
- Insufficient root gap or excessive root face
- Low heat input or incorrect settings
- Poor joint preparation or fit-up
- Incorrect electrode or torch positioning
What to do
- Improve fit-up and root opening before welding
- Adjust heat input appropriately for full root reach
- Recheck joint preparation standards
- Avoid compensating only with higher heat without correcting geometry
6. Undercut
Undercut is a groove alongside the weld toe or root that has been melted away but not filled back with weld metal.
What usually causes it
- Excessive heat input
- High travel speed
- Incorrect torch or electrode angle
- Poor control of weld pool
What to do
- Reduce excessive heat or travel speed
- Adjust torch angle to better support the weld toe
- Allow sufficient filler metal to fill the edge
- Repair deep undercut if it exceeds acceptance limits
7. Overlap
Overlap looks almost like the opposite of undercut. Weld metal extends over the base metal at the toe, but the metal underneath has not properly fused.
What usually causes it
- Cold or sluggish weld pool
- Poor manipulation technique
- Contamination on the base metal
- Excess filler metal without proper fusion
What to do
- Improve heat input to ensure proper melting at the toe
- Clean base material before welding
- Adjust travel speed and technique for better fusion
- Ensure the weld pool ties into the base metal, not just sits on top
8. Underfill
Underfill means there is simply too little weld metal in part of the finished profile. The face or root sits below where it should be.
What usually causes it
- Excessive travel speed
- Insufficient filler metal deposition
- Poor bead placement or technique
- Low deposition efficiency
What to do
- Balance travel speed with filler addition
- Increase deposition rate where needed
- Rebuild the weld profile with additional passes if required
- Maintain consistent bead placement
9. Excessive Reinforcement
More weld metal is not always better. Excessive reinforcement produces a bead that sits higher than the intended face or root profile.
What usually causes it
- Slow travel speed
- Excessive filler metal deposition
- Poor control of weld size per pass
What to do
- Reduce filler metal input to match specification
- Increase travel speed slightly if appropriate
- Grind or machine excess reinforcement if required by code
- Maintain correct weld profile per procedure
10. Root Concavity
Root concavity, sometimes called suck-back, is a recessed area on the root side of the weld.
What usually causes it
- Excessive root heat input
- Inconsistent or wide root gap
- Poor control of molten root pool
- Improper welding sequence
What to do
- Stabilize root gap and fit-up before welding
- Adjust heat input to avoid excessive melt-through
- Improve control of root pass technique
- Maintain consistent travel speed and arc position
11. Arc Strikes
An arc strike is accidental arc contact outside the intended weld area. It may look minor, but it is not the same thing as loose spatter because the base material has actually been locally heated.
What usually causes it
- Accidental electrode or torch contact
- Poor handling of electrode during setup or repositioning
- Inadequate work discipline or procedure control
What to do
- Avoid striking the arc outside the weld zone
- Follow proper start and stop procedures
- Remove or repair arc strikes according to code requirements
- Inspect affected areas if required by specification
12. Misalignment
Misalignment starts before the weld pool forms. The two parts are offset from the position intended by the joint design.
What usually causes it
- Poor fit-up before welding
- Weak or incorrect fixturing
- Inaccurate tack welds
- Movement during welding due to thermal distortion
What to do
- Correct alignment before welding begins
- Improve fixturing and clamping methods
- Use proper tack welding techniques
- Control distortion through balanced welding sequence
Surface vs Internal Weld Discontinuities
Location gives a useful first clue about how a weld should be inspected. Undercut, overlap, arc strikes, and some cracks can often be seen at the surface. Porosity, inclusions, incomplete fusion, and penetration problems may remain buried inside the joint.
That division is not absolute. Porosity and cracks can be either surface-breaking or internal. A good-looking bead therefore tells you something about workmanship, but not everything about the joint.
How Are Weld Discontinuities Detected?
Inspection starts with what can reasonably be found from the outside, then moves to other methods when the application requires more information.
Method | Common Use |
Visual Testing (VT) | Visible profile and surface conditions |
Liquid Penetrant Testing (PT) | Surface-breaking discontinuities |
Magnetic Particle Testing (MT) | Surface and near-surface indications in ferromagnetic material |
Ultrasonic Testing (UT) | Internal discontinuities |
Radiographic Testing (RT) | Internal conditions, particularly volumetric indications |
The important point is not to treat NDT as a simple “inside versus outside” choice. Material, joint geometry, access, and the orientation of the expected flaw all affect which technique works best.
When Does a Weld Discontinuity Become a Defect?
Finding something is only the first half of inspection. The next questions are: What is it? How large is it? Where is it? And what does the applicable requirement allow?
In practice, the sequence is roughly:
Detect → Identify → Locate → Size → Evaluate → Accept or Reject
This is also why how to prove weld integrity has no useful answer based only on bead appearance. A small inclusion may be permitted under one set of acceptance criteria, while a sharp crack may be unacceptable even when it is much smaller.
Why the Welding Process Matters
The list of likely problems changes with the process. Slag inclusions make sense for slag-producing arc processes. They are not a normal issue in autogenous laser welding.
Laser welding brings other sensitivities. Research and practical guidance from TWI show that shielding, contamination, keyhole stability, beam-to-joint position, fit-up, and process settings can influence porosity, fusion, penetration, and weld profile. A beam that drifts away from the joint line can even produce lack of root fusion or incomplete penetration while the process continues running.
With a handheld laser welder, that makes steady travel, clean surfaces, accurate beam placement, and realistic joint gaps important. Dynalasers systems let operators tune the welding parameters, but parameter control works best when the joint itself is prepared properly.
FAQs
Can Several Weld Discontinuities Have the Same Root Cause?
Yes. Poor fit-up is a good example. It may contribute to incomplete penetration, fusion problems, and an irregular bead profile at the same time. Treating only the visible symptom can leave the real cause unchanged.
Does Grinding Always Fix a Weld Discontinuity?
No. Grinding can remove some surface conditions, but it cannot correct an internal fusion problem or a crack that extends below the ground area. The repair method should match the actual discontinuity and its extent.
Can Automated Welding Still Produce Discontinuities?
Yes. Automation removes some operator variation, but it does not eliminate dirty material, joint mismatch, wrong parameters, poor shielding, or incorrect joint tracking. Repeatable welding is only useful when the process is repeatably correct.
Can Repair Welding Make the Joint Worse?
It can. A repair adds another thermal cycle and another chance to introduce cracking, porosity, or incomplete fusion. TWI has also noted cases where poorly executed weld repairs have themselves contributed to integrity problems.
Conclusion
Weld discontinuities are easier to deal with when they are treated as engineering conditions rather than simply “good” or “bad” welds. Identify what you are seeing, find the likely cause, and then check whether it actually exceeds the requirements for that joint.
The same approach applies to a conventional arc weld or a Dynalasers laser weld: control the joint and the process first, then use inspection to confirm the result.
