Porosity — Different Limits for Groove & Fillet, Static & Cyclic

What is Porosity?

Porosity is a gas pocket or void in the weld metal. The gas (typically hydrogen, nitrogen, or argon) becomes trapped as the weld metal solidifies, leaving a spherical or elongated bubble. Visible porosity indicates poor shielding, contamination, or cooling rate issues.

• Visible surface porosity — Gas pores that break through the weld face. Appear as small rounded depressions or holes.
• Subsurface porosity — Gas pores trapped inside the weld. Not visible without radiography or ultrasonic testing.
• Porosity clustering — Multiple pores in the same area or along the weld length. More severe than isolated pores.
• Cavity size — Measured by diameter (3/8" means 0.375" diameter pore). Larger pores are more critical.

AWS D1.1 Table 6.1 Acceptance Limits — Porosity

Common Porosity Causes

• Contaminated shielding gas — Moisture, nitrogen, or oxygen in the gas creates oxide and gas entrapment. Check gas purity and flow.
• Insufficient gas flow rate — GMAW/FCAW shield gas flow too low. Arc is not properly protected; air enters the puddle and traps as pores.
• Dirty base metal surface — Mill scale, rust, or oil evolves hydrogen when heated. Hydrogen dissolves in the molten weld metal and forms pores as it cools.
• High travel speed / cooling too fast — Gas doesn't have time to escape during solidification. Slower cooling allows gas to bubble out.
• Wrong electrode or wire type — Some electrodes are more prone to porosity (especially old or contaminated stock).

How to Inspect & Document for the Report

• Visual examination under bright light — Look for small rounded holes or depressions in the weld face. Use 10x magnifier to confirm gas pores vs. surface irregularities.
• Measure pore diameter — Measure the largest dimension of each visible pore. Document size and location (e.g., "center of bead, 1/4" from start").
• Radiography or ultrasonic testing — If interior porosity is suspected or if the weld is critical. X-ray or UT reveals subsurface pores.
• Record service category and weld type — Note whether groove or fillet, and whether statically or cyclically loaded. This determines which limit applies.
• Mark decision — If porosity exceeds limit: REJECT and mark for re-weld. If within limit: ACCEPT and document size.

CWI Inspection & Prevention Tips

• Cyclic loading (bridges, cranes) is unforgiving — zero visible porosity. For static structures, 3/8" is allowed. Know the service category.
• Porosity prevention starts with gas and cleanliness. Check gas cylinders for moisture (date regulators, use dry gas), and clean base metal before welding.
• Slow travel speed and full heat input (per WPS) help gas escape. Rushing the pass traps gas.
• If porosity is detected, investigate the root cause before allowing re-weld. Bad gas, dirty surface, or cold base metal is the issue, not just the welder's skill.