Discontinuity
Linear porosity
Gas 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.
What it looks like
- A string of rounded voids following the weld axis rather than scattering randomly
- Most often located at the root or along a fusion face where two passes meet
- On film, a chain of dark rounded spots aligned with the joint centreline
- Individual pores usually stay round, which distinguishes the row from a fine crack
Causes
- Contamination concentrated along the root face or land, such as cutting oxide, rust, or moisture in the joint gap
- Inadequate interpass cleaning that leaves a film of slag or oxide along one fusion line
- Backing bars or backing rings that trap air or moisture behind the root pass
- Purge gas that is missing, insufficient, or contaminated when welding the inside of pipe
- A joint geometry so tight that gas generated at the root cannot escape upward through the puddle
Prevention
- Clean and dry root faces and land surfaces immediately before fit-up, not the day before
- Grind or wire-brush each pass fully before depositing the next one
- Establish and verify a proper purge on materials that need one, and hold it until enough weld thickness is in place
- Open the root slightly or adjust the land so gas has a path out of the puddle instead of being sealed in
How it is found
- RT
- The aligned voids print as a recognisable chain of rounded dark spots, and the alignment itself is the diagnostic feature.
- UT
- Echoes repeat at a consistent depth as the probe scans along the weld, showing the flaw follows the joint rather than sitting at one point.
- VT
- Where the root is accessible, a row of pinholes along the root bead can be seen directly and confirms the purge or cleanliness problem.
How inspectors think about it
- Aligned rounded indications are judged more critically than scattered ones because a row of voids acts closer to a continuous plane of weakness than isolated bubbles do.
- The reasoning usually considers the total length of the aligned group, the size of individual pores, and the ligament of sound metal between them.
- Because linear porosity clusters at the root, its position within the load path often matters more than its size, particularly on joints loaded in fatigue or bending.
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.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.Incomplete joint penetrationA groove weld that did not reach through the joint as the design required, leaving unfused root faces and an effective throat smaller than the drawing calls for.Root crackA crack in or immediately beside the root pass, the thinnest and most highly restrained part of a groove weld, where shrinkage stress from every later pass concentrates on the least metal.
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.