Discontinuity
Slag inclusion
Non-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.
What it looks like
- Irregular dark shapes inside the weld, elongated along the direction of travel
- Often lies along the fusion line or in the sharp corner between a pass and the groove face
- On film, dark indications with less sharply defined edges than a gas pore and irregular outlines
- On a sectioned macro, a glassy or dull non-metallic pocket rather than a void
Causes
- Inadequate interpass cleaning leaving slag from a previous pass in place
- Bead profiles that create sharp corners or undercuts where slag can hide from the next arc
- Travel speed too slow, letting slag run ahead of the puddle and be welded over
- Incorrect electrode angle allowing slag to flow forward instead of trailing behind the arc
- Groove angles too narrow for the slag to float clear of the joint before the metal freezes
Prevention
- Chip, grind, and brush every pass clean before the next one, including into the groove corners
- Keep bead profiles slightly convex and free of undercut so slag has nowhere to hide
- Maintain travel speed and electrode angle so the puddle leads and the slag trails behind
- Open the groove angle where deep narrow joints repeatedly trap slag at the sidewalls
How it is found
- RT
- Slag is much less dense than steel, so it absorbs less radiation and prints as an irregular dark indication distinguishable from a round pore.
- UT
- The inclusion is an acoustic mismatch that scatters sound, giving a moderate, broad echo rather than the sharp reflection of a planar flaw.
- VT
- Slag left on the surface or in a visible groove corner during in-process inspection warns of what is being buried by the next pass.
How inspectors think about it
- Slag inclusions are treated more leniently than planar flaws because they are volumetric, but elongated and aligned inclusions are judged closer to the planar scale.
- The reasoning generally looks at the length of individual inclusions, the total amount within a length of weld, and how close they sit to one another.
- Repeated slag findings point directly at interpass cleaning practice, so an inspector's most useful response is in-process observation rather than more film.
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
Incomplete fusionWeld metal that lay against the joint face or a previous bead without melting into it, leaving a tight unbonded plane that carries no load even though the joint looks completely filled.Tungsten inclusionA fragment of tungsten electrode broken off into the weld pool during gas tungsten arc welding, leaving a hard, brittle, very dense particle embedded in the deposit.Oxide inclusionSurface oxide folded into the weld pool rather than broken up by the arc, most troublesome on aluminium where the oxide skin melts far above the metal it covers.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.
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.