Porosity
Porosity is a volumetric weld defect consisting of gas-filled cavities trapped in the solidifying weld pool, classified as scattered, clustered, linear, or piping (wormhole) porosity depending on distribution and morphology.
Definition
Porosity results from contamination, inadequate shielding gas, moisture, or incorrect welding parameters. It is volumetric and typically less critical than planar flaws but is reject-by-code when it exceeds size or distribution limits in AWS D1.1 or ASME Section VIII.
How this damage forms
Gas trapped during weld solidification, leaving rounded voids — isolated, clustered, aligned or elongated (piping/wormhole) forms, each with its own acceptance treatment.
How it is detected
Radiography sees porosity superbly (rounded dark indications); UT sees it poorly by comparison — small spherical voids scatter rather than reflect, which is one reason RT persists for porosity-prone processes.
What causes it, and what prevents it
Porosity is a process message: moisture in consumables or shielding loss (scattered), contamination (clustered), poor technique at starts and stops. Acceptance criteria tolerate rounded indications far more than planar ones because voids concentrate stress mildly — but ALIGNED porosity can shadow a lack-of-fusion path and deserves suspicion beyond its area count.
Codes that govern assessment
Acceptance per the construction code: AWS D1.1 porosity charts, ASME VIII/B31.3 rounded-indication charts (Appendix 4 practice).
The common reporting error
Chasing porosity counts while missing the trend. Three films in a row with increasing scattered porosity is a gas-supply or consumable-storage problem about to produce a rejectable weld — the finding worth reporting is the trajectory.
Where Porosity fits in an inspection programme
A term is only useful when it connects to a decision. Porosity appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.
Related terms
- Slag Inclusion — Slag inclusion is a non-metallic solid trapped in the weld metal during shielded-metal-arc or flux-cored welding, typically from incomplete inter-pass cleaning, appearing as irregular dark indications on a radiograph.
- Undercut — Undercut is a groove melted into the base metal adjacent to the weld toe that is not filled with weld metal, reducing wall thickness and acting as a stress concentrator.
- Overlap — Overlap is the protrusion of weld metal beyond the weld toe without fusion to the base metal, producing a notch-like discontinuity that reduces fatigue performance.
Further reading
weld defect classification and detection methods
More defect terms
Crack · Lack of Fusion · Lack of Penetration · Burn-Through · Weld Spatter · Lamination · Lamellar Tearing · Hydrogen-Induced Cracking · Stress Corrosion Cracking · Sulfide Stress Cracking
Where this comes up in practice
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.