Porosity — What It Is and Why It Matters
Porosity refers to gas pockets trapped within a solidified weld bead. These voids form when dissolved gases (hydrogen, nitrogen, oxygen) cannot escape the weld pool before it solidifies. Porosity weakens the weld by reducing its effective cross-sectional area and acting as stress concentration points.
Common causes include insufficient shielding gas coverage (empty tank, wind, clogged nozzle, excessive flow creating turbulence), contaminated base metal (oil, grease, moisture, paint, rust), wet or contaminated filler metal, and excessive arc length that draws air into the shielding envelope.
Porosity appears in several forms: uniformly distributed (scattered small pores throughout the weld), cluster porosity (groups concentrated in one area), linear porosity (pores aligned along the weld axis, often at the start or stop), and wormhole porosity (elongated tube-shaped voids). Surface porosity is visible on inspection; subsurface porosity requires radiographic or ultrasonic testing to detect.
Frequently Asked Questions
How do I fix porosity in my welds?
Identify and eliminate the gas source. Check shielding gas flow rate and supply, clean the base metal thoroughly, replace contaminated or wet filler metal, shorten arc length, and check for drafts. If using stick welding, ensure low-hydrogen electrodes have been properly stored. Existing porosity must be ground out and rewelded.
Is a small amount of porosity acceptable?
Some codes allow limited porosity depending on the application. AWS D1.1 for structural steel permits small scattered pores within specific size and distribution limits. Critical applications (pressure vessels, aerospace, nuclear) have much tighter acceptance criteria. When in doubt, consult the applicable code or specification.