3.3 Porosity and Inclusions
Key Takeaways
- Porosity is gas entrapment during solidification — caused by moisture, contamination, inadequate shielding, or wind
- Four porosity types: uniformly scattered, cluster, linear, and piping (wormholes)
- Slag inclusions result from inadequate interpass cleaning — always remove all slag before the next pass
- Tungsten inclusions occur when the GTAW electrode touches the weld pool
- Porosity and inclusions are volumetric discontinuities — less severe than planar (cracks) for the same size
- AWS D1.1 acceptance criteria differ for cyclically loaded vs. statically loaded structures
3.3 Porosity and Inclusions
Porosity and inclusions are volumetric discontinuities — they occupy volume within the weld metal rather than forming sharp, planar surfaces. While generally less severe than cracks, they can cause rejection when they exceed code limits.
Porosity
Porosity is a cavity-type discontinuity formed by gas entrapment during solidification. As molten weld metal cools, dissolved gases attempt to escape; if they cannot escape before solidification, they form trapped gas pockets.
Types of Porosity:
| Type | Description | Typical Cause |
|---|---|---|
| Uniformly scattered | Small pores evenly distributed | Slightly contaminated base metal or consumable |
| Cluster porosity | Group of pores in a localized area | Localized contamination or arc start/stop issue |
| Linear porosity | Pores aligned in a row | Contamination along a specific interface (tack weld, root) |
| Piping porosity (wormholes) | Elongated, tubular pores perpendicular to weld surface | Gas evolving from base metal during solidification |
Common Causes of Porosity:
| Cause | Source of Gas |
|---|---|
| Moisture on base metal, electrodes, or flux | Hydrogen (H₂), water vapor (H₂O) |
| Oil, grease, paint on joint surfaces | Hydrogen, carbon compounds |
| Rust or mill scale | Oxygen (O₂), carbon monoxide (CO) |
| Inadequate shielding gas coverage | Nitrogen (N₂), oxygen (O₂) |
| Excessive arc length in SMAW | Atmospheric gases enter the arc zone |
| Wind or drafts blowing gas shield away | Atmospheric contamination |
| Excessive moisture in flux (SAW, FCAW) | Hydrogen |
| Welding over tack welds with porosity | Trapped gas from tack weld porosity |
Inclusions
Inclusions are solid non-metallic materials trapped in the weld metal or between weld passes.
| Type | Description | Cause |
|---|---|---|
| Slag inclusions | Trapped slag (from electrode coating or flux) | Inadequate slag removal between passes, improper bead sequence, too fast travel speed |
| Tungsten inclusions | Particles of tungsten electrode embedded in weld | Tungsten electrode touching the weld pool (GTAW), excessive current on small tungsten |
| Oxide inclusions | Metal oxides trapped in weld | Inadequate shielding, contaminated joint surfaces |
Slag Inclusions — Prevention:
- Clean thoroughly between passes — remove all slag before depositing next bead
- Use proper bead sequence to avoid creating undercut pockets that trap slag
- Maintain proper travel speed — too fast can cause slag to get ahead of the puddle and be trapped
- Ensure proper arc manipulation technique
- Use proper interpass temperature — not so hot that slag becomes difficult to remove
Acceptance Criteria (AWS D1.1 — General Guidance)
AWS D1.1 Table 8.1 provides visual acceptance criteria for discontinuities:
| Discontinuity | Cyclically Loaded | Statically Loaded |
|---|---|---|
| Piping porosity | Not permitted | ≤ specific limits per joint type |
| Scattered porosity | Sum of diameters ≤ 3/8" in any linear inch | More lenient limits |
| Slag inclusions | Various limits based on length and depth | More lenient limits |
| Tungsten inclusions | Per RT acceptance tables | Per RT acceptance tables |
For the Exam: Know the causes and prevention of each porosity type. The most common exam scenario: "A welder is getting porosity — what is the most likely cause?" Answer based on the specific situation (wind = gas coverage, moisture = hydrogen, etc.).
A welder is producing welds with uniformly scattered porosity when welding outdoors with GMAW. The most likely cause is:
What is the primary cause of slag inclusions between weld passes?
Piping porosity (wormholes) in a weld are typically caused by: