Discontinuity
Uniformly scattered porosity
Rounded gas pockets frozen into the weld metal and spread fairly evenly through the deposit, rather than bunched together or lined up, showing that shielding or cleanliness was marginal for the whole pass.
What it looks like
- Small round or slightly oval voids scattered along the bead with no obvious pattern
- On a broken or sectioned face, smooth shiny walls rather than the torn look of a crack
- On film, dark rounded spots of varying size distributed through the weld image
- Surface-breaking pores show as clean round pinholes in an otherwise sound bead face
Causes
- Loss of shielding gas coverage from too little flow, too much flow causing turbulence, a leaking hose, or wind blowing across the joint
- Moisture picked up by low-hydrogen electrode coverings or flux that was stored in damp air
- Oil, grease, paint, rust, mill scale, or cutting fluid left on the joint faces
- Excessive arc length or travel speed that lets the puddle freeze before dissolved gas can escape
- Contaminated or wet base metal in humid conditions, particularly first thing in the morning on cold steel
Prevention
- Clean the joint and surrounding surfaces to bright metal and keep them dry before striking an arc
- Set and verify shielding gas flow at the nozzle, and screen the work from drafts rather than raising the flow rate
- Store low-hydrogen electrodes and flux in heated holding ovens and limit exposure time in the open air
- Hold a short arc and moderate travel speed so the puddle stays fluid long enough for gas to float out
How it is found
- VT
- Only pores that break the surface are visible, so a clean-looking cap can still sit over a porous interior.
- RT
- Gas pockets are voids with far less density than steel, so they absorb little radiation and print as sharply defined dark rounded spots.
- UT
- Scattered pores return many small, low-amplitude echoes that rise and fall as the probe moves, rather than one strong reflector.
How inspectors think about it
- Rounded discontinuities are treated far less severely than planar ones because a spherical void spreads stress around itself instead of concentrating it at a sharp tip.
- Judgement usually turns on the size of the largest pore, how many appear within a given length of weld, and how close pores sit to one another.
- Surface-breaking pores draw extra attention because they can hold moisture and start corrosion, and because they interrupt the smooth toe transition a fatigue-loaded joint depends on.
General reasoning only. The accept/reject limits for your work come from the code or specification governing the job, which this site does not reproduce.
Related discontinuities
Cluster porosityA tight group of gas pores concentrated in one localised area of the weld with sound metal on either side, marking a moment when shielding or cleanliness broke down instead of a whole-pass problem.Linear porosityGas pores lined up in a row, almost always along the weld root or between passes, because the gas source is itself a line such as an unclean root face or a contaminated interpass surface.Piping porosity (wormhole)An elongated tubular gas cavity that grew as the solidification front chased a rising gas bubble, leaving a hole shaped like a worm burrow rather than a sphere inside the weld metal.Slag inclusionNon-metallic slag trapped inside the weld instead of floating to the surface, most often left in a corner or undercut of a previous pass and then buried by the next one.
Can you name it on sight?
The gauge trainer's identification mode shows you a weld and four names. Reading about porosity and picking it out of a line-up are not the same skill.