Slag inclusions rank among the most common internal weld discontinuities in structural fabrication and the most preventable. They appear in NDE reports when interpass discipline breaks down — a pass started over uncleaned slag from the previous bead, a missed slag pocket in a tight root opening, or an electrode angle that traps slag at the fusion line. By the time RT or UT finds them, the joint is complete and repair is expensive.
Understanding the mechanics of slag formation, the WPS parameters that increase or decrease risk, and the AWS D1.1 acceptance criteria enables CWIs and QC managers to intercept the problem at the weld station rather than the NDE table.
What a Slag Inclusion Is
Slag is the solidified flux byproduct that forms on top of the weld pool during SMAW, FCAW, and SAW processes. Under normal conditions, slag floats to the surface as the pool solidifies and can be removed by chipping and brushing before the next pass. A slag inclusion forms when slag fails to float — when it is mechanically entrapped in the weld metal or pushed against the fusion boundary by the arc.
The result is a non-metallic, elongated or irregular solid mass inside the weld deposit. Slag inclusions reduce effective weld cross-section and introduce stress concentration points, particularly at their ends. Under cyclic loading, elongated slag inclusions oriented transverse to the load can initiate fatigue cracks.
Slag inclusions are distinct from porosity (gas voids), cold lap (incomplete fusion), or lack of penetration — though the prevention strategies overlap. The AWS D1.1 acceptance criteria treat them separately because their shape, orientation, and effect on structural performance differ from spherical gas porosity.
How Slag Gets Trapped
Improper interpass cleaning is the primary cause. If a welder starts the next pass before removing all slag from the previous bead, the fresh weld metal can flow over the remaining slag and lock it in place. This is most common at the toes of previous passes, at the edge of weave beads, and in the center of wide beads that cup at the edges.
Trapped slag in tight fit-ups or root openings. When the root opening is at the lower end of the WPS-qualified range, there may not be enough access to completely fuse through the root, and slag from the root pass can remain trapped at the root tip.
Incorrect electrode or wire angle. In SMAW, a steep drag angle can push slag ahead of the pool rather than allowing it to trail behind, resulting in slag inclusions at the fusion line. The WPS specifies work angle and travel angle; deviations contribute to slag entrapment.
Undercutting from adjacent pass. If the previous pass created undercut at the weld toe, the subsequent pass may bridge over the undercut groove without fully filling it, trapping slag in the recess. This is a fusion-type slag inclusion at the fusion boundary rather than in the weld metal interior.
Flux damp or damaged in SAW. Submerged arc welding flux that has absorbed moisture or has been partially used, reconditioning issues aside, can produce slag with altered viscosity that does not float cleanly. Flux handling requirements under AWS D1.1 address moisture control for this reason.
For the broader context of interpass cleaning requirements in AWS D1.1, see interpass cleaning and slag removal in structural welding.
WPS Parameters That Influence Slag Risk
Travel speed is among the most important WPS parameters for slag control. Travel speed that is too high leaves a narrow, peaked bead with poor sidewall fusion and inadequate time for slag to float. Travel speed that is too low produces a wide, flat bead with excessive slag generation. The WPS-qualified travel speed range should reflect what actually produces acceptable beads for the specific joint geometry.
Electrode/wire diameter and current. Undersized electrode diameter for the joint thickness reduces heat input, increases the likelihood of incomplete fusion at sidewalls, and increases slag entrapment risk. The WPS specifies electrode diameter as an essential variable; using a smaller diameter than qualified can trigger requalification requirements.
Interpass temperature minimum. While the primary slag control is mechanical cleaning, maintaining the minimum interpass temperature keeps the base metal at a temperature that supports good fusion and allows slag to flow more freely. A cold joint — one that has dropped below minimum preheat before the next pass — is more prone to both lack of fusion and slag entrapment.
Weld position. Overhead and vertical positions significantly increase slag inclusion risk compared to flat and horizontal positions. The weld pool is less stable, slag management is more difficult, and operator technique has more influence on outcome. WPS position qualification limits exist partly for this reason — a procedure qualified in the flat position cannot be used overhead without separate qualification or prequalification under AWS D1.1.
Bead profile and weave width. Excessively wide weave beads, particularly in the fill and cap passes of thick groove welds, can trap slag at the edges. The WPS can limit maximum weave width; many structural WPS documents specify stringer or limited-weave beads for fill passes in groove welds for this reason. See stringer versus weave bead WPS requirements under AWS D1.1.