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Every welded product must meet codes and standards for strength, quality, and reliability. But why do some welds fail? The truth is that all welds contain flaws, known as weld discontinuities, but only those that exceed a product’s allowable limits are classified as defects.
Understanding welding discontinuities and defects is something every welder needs to take seriously. A weld that looks finished can still fail inspection, waste hours of work, and in certain industries, create real safety risks.
What is a Discontinuity in Welding?
A weld discontinuity can appear in the weld itself, the base metal, or the heat-affected zone, and may be caused by technique errors, material conditions, or equipment problems.
The number or extent of the discontinuities that is acceptable for a given product depends on its standards. More discontinuities would be permissible for a welded gate than for an airplane, for instance, because the latter is subject to much more critical usage. Lives could be lost if the welds in a plane failed.
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What Is the Difference Between a Discontinuity and a Defect in Welding?
When a product has more discontinuities than its code of standards allows, the product is deemed defective and unfit for use.
How does an organization know when a product has crossed the line from having discontinuities to being defective? A measurement called tolerance is used.
Tolerance is the difference between the ideal, or perfect, version of the product and a version of the product that is still fit for use despite its discontinuities. Other factors that determine if a product’s discontinuities are great enough to be categorized as defects are the size, location and type of the discontinuity.[1]
To put it plainly: all defects are discontinuities, but not all discontinuities are defects. A discontinuity only becomes a defect when it exceeds the tolerance defined by the applicable welding code or standard, such as those set by the American Welding Society (AWS).
What Are Some Common Welding Discontinuities?
Below are six of the most common welding discontinuities, along with their possible causes. If these discontinuities surpass a given product’s standards, they are defects.
1. Porosity
Porosity occurs when gas becomes trapped in the weld pool, forming permanent bubbles as the metal cools from a liquid state back to a solid. Extensive porosity can result in a loss of weld strength.
Causes
- Failing to properly shield the weld from atmospheric contamination
- Neglecting to remove impurities like dirt from the base metal
- Striking too long of an arc
2. Inclusions
Inclusions occur when slag, oxides or other nonmetallic materials become trapped between the bases metal and the weld, between the beads of the weld or in the weld metal. The structural integrity of the weld can be compromised by the presence of inclusions.
Causes
- Leaving heavy mill scale, rust, or preexisting deposits of slag in the material
- Improperly cleaning prior welds
- Manipulating the arc incorrectly
3. Inadequate Joint Penetration
When a weld does not penetrate the joint enough to achieve a sound fusion of the metals, the problem can be described as an inadequate joint penetration. This discontinuity becomes a defect when it serves as a source of stress in the weld that can result in fatigue failure.
Causes
- Misdirecting the arc or other improper welding techniques
- Not preheating thick metals or using enough welding current to penetrate the joint
- Poor design, fit, or detail of the joint
4. Incomplete Fusion
Two types of incomplete fusion can happen: interpass cold lap and lack of sidewall fusion.
- Interpass cold lap is when the filler metal and previously deposited weld metal do not fuse sufficiently.
- Lack of sidewall fusion is when the joint face and weld metal fail to fuse properly.
Causes
- Leaving slag or oxides on joint surfaces or neglecting to break up oxide layers on the base or filler metals
- Traveling at an improper speed or using the wrong electrode angle
- Selecting the wrong welding process for the metal
5. Arc Strikes
When the arc contacts areas outside the intended joint, it melts the surrounding metal and leaves small, localized discontinuities on the surface. These marks might look minor, but arc strikes are one of the more serious welding defects because they act as initiation points for cracks and fatigue failure over time. In structural and pressure vessel applications, arc strikes outside the weld zone are often grounds for automatic rejection under AWS and ASME codes.[2]
Causes
- Improper or careless technique that causes the arc to strike the wrong location
- Defective ground connections.
6. Overlap
Weld deposits that flow over the surface of the base metal because they are larger than the specified joint design are referred to as overlap. They can prevent the metals from fusing.
Causes
- Traveling too slowly
- Misdirecting the arc
- Improperly sizing the weld[3][4]
| Discontinuity | What It Is | Primary Cause |
|---|---|---|
| Porosity | Gas bubbles trapped in the weld pool as the metal cools | Atmospheric contamination, dirty or unprepared base metal, arc length too long |
| Inclusions | Slag, oxides, or nonmetallic materials trapped in or between weld passes | Improper cleaning between passes, heavy mill scale, incorrect arc manipulation |
| Inadequate Joint Penetration | Weld fails to penetrate the joint deeply enough to fully fuse the metals | Wrong current settings, improper technique, poor joint design or fit-up |
| Incomplete Fusion | Weld metal and base metal, or adjacent weld passes, fail to bond properly | Slag or oxides on joint surfaces, incorrect travel speed or electrode angle, wrong welding process |
| Arc Strikes | Unintended arc contact melts metal outside the weld joint, leaving surface discontinuities | Careless or improper technique, defective ground connections |
| Overlap | Weld metal flows over the base metal surface beyond the intended joint | Travel speed too slow, misdirected arc, weld sized larger than the joint design calls for |
Welding Quality Assurance Enforces Productivity and Safety
No weld is perfect, but all welds must be fit for the service they’re placed into. Recognizing and controlling weld discontinuities is one of the most practical skills a welder can develop, whether you’re working in structural fabrication, pipefitting, or manufacturing. Quality assurance isn’t just about passing inspection. It’s about making sure the work you produce holds up under real conditions and reflects the level of skill you bring to the job.
If you’re looking to build a solid foundation in welding technique, inspection, and trade skills, Tulsa Welding School offers welding programs that cover hands-on training from day one. Programs can be completed in as few as seven months, with flexible scheduling for students balancing work or family.
Reach out today to talk more with admissions and learn more about scaling your career in the welding industry.
[1] – Title: Welding Principles and Applications; Author: Larry Jeffus; Delmar Cengage Learning; Seventh Edition; Textbook page 567-568
[2] – http://www.thefabricator.com/article/arcwelding/arc-welding-discontinuities
[3] – http://www.lincolnelectric.com/assets/us/en/literature/wc514.pdf
[4] – Title: Welding Principles and Applications; Author: Larry Jeffus; Delmar Cengage Learning; Seventh Edition; Textbook page 568-573
