Discontinuity
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.
TitlePiping porosity (wormhole)
ClassCavity
ViewIndicative
What it looks like
- A long, tapered tube of open metal, often angled toward the bead surface at the direction of travel
- May break the surface as a small hole with a tail, or stay wholly buried
- On film, a dark elongated indication with rounded ends rather than the sharp ends of a crack
- On a sectioned face the cavity walls are smooth and oxidised, not torn
Why it happens, and how it is stopped
Causes
- A steady, concentrated gas source such as moisture in flux or a leaking gas line feeding one point of the puddle
- High travel speed that steepens the solidification front so bubbles are overtaken instead of escaping
- Deep narrow joints where the puddle is tall and gas has a long path to the surface
- Contaminants that decompose slowly, such as heavy paint or galvanising, releasing gas throughout solidification
- Excessive current in submerged arc or flux cored welding, which drives a deep puddle that freezes from the bottom up
Prevention
- Dry consumables properly and check the gas train for leaks and moisture before starting production welds
- Reduce travel speed or current so the puddle stays open long enough for gas to break through the surface
- Remove coatings from a generous band on both sides of the joint rather than only the weld faces
- Adjust joint preparation to a wider included angle where deep narrow puddles are trapping gas
How it is found
Which examination methods can see it, and what it is about the discontinuity that makes them able to.
- RTThe tube's length makes it obvious on film, and the rounded ends distinguish it from the sharp tips of a crack.
- UTThe cavity is a large volumetric reflector, so it returns a strong echo that persists over a length of probe travel.
- VTSurface-breaking wormholes are visible as elongated openings, often with a trailing tail pointing back along the direction of travel.
How inspectors think about it
- Elongated cavities are weighed more heavily than round pores because length in the direction of stress removes a continuous strip of load-carrying section.
- The assessment generally considers the length of the cavity against the thickness of the material and whether it breaks through to a surface.
- A wormhole that vents to the cap is treated as a surface discontinuity as well as a volumetric one, since it can trap moisture and start corrosion from the inside out.
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
Uniformly scattered porosityRounded 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.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.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.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.