Slag inclusions are non-metallic material — solidified flux residue — trapped inside the weld. They are the signature defect of flux-shielded processes (MMA/stick, FCAW, SAW), and they almost always trace back to one procedural failure: depositing a pass over slag that wasn’t fully removed.
Unlike porosity, which is rounded and volumetric, slag inclusions are often elongated and irregular — which is why acceptance standards treat them more severely: a string of slag along a fusion line behaves mechanically like a crack-like planar flaw, not like a pore.
What is a slag inclusion?
ISO 6520-1 classifies solid inclusions as group 300:
| Type | ISO 6520-1 code | Description |
|---|---|---|
| Slag inclusion | 301 | Flux/coating residue trapped in weld metal (linear 3011, isolated 3012, clustered 3013) |
| Flux inclusion | 302 | Unfused flux (typical of SAW) |
| Oxide inclusion | 303 | Metallic oxide trapped during solidification (frequent in aluminium) |
| Metallic inclusion | 304 | Foreign metal particle, e.g. tungsten from TIG (3041) |
The process matters: solid-wire MIG/MAG and TIG cannot produce true slag inclusions — if RT shows linear inclusions in a gas-shielded solid-wire weld, suspect oxide films or lack of fusion instead, and re-read the indication before writing the NCR.
What causes slag inclusions
Interpass cleaning failures (the big one)
- Slag not fully removed between passes — particularly in the toe lines, where convex beads create a wedge the chipping hammer doesn’t reach
- Time pressure on multi-pass work: the second pass goes down while pockets of slag survive in the groove
- Difficult access (deep narrow grooves, small bevel angles) making mechanical cleaning ineffective
Technique and parameters
- Low heat input / low current: the arc fails to remelt and float residual slag out of the pool
- Electrode angle too shallow, letting molten slag run ahead of the arc and be over-run by the pool
- Travel speed too high, freezing the pool before slag separates
- Wrong electrode size for the joint, leaving unfused corners where slag collects
Joint and surface condition
- Undercut or rough profile in the previous pass trapping slag (see undercut: causes and acceptance criteria)
- Welding over rust, mill scale, or shop primer that feeds the slag system
ISO 5817 acceptance criteria
ISO 5817 sets dimensional limits on solid inclusions per quality level — Level B (stringent), C (intermediate), D (moderate). The limits cap inclusion length and height as a function of material thickness, following the same logic as other volumetric imperfections: what is tolerable at Level D is rejected at Level B.
Two principles to retain:
- Isolated, short inclusions may be acceptable at intermediate levels; linear/aligned inclusions (3011) are treated far more severely because of their planar behaviour.
- Your governing code can override ISO 5817 — pressure and fatigue applications often impose stricter sentencing on linear indications regardless of level.
Exact dimensional values are in ISO 5817 Table 1 (the standard is copyrighted — keep a controlled copy at the sentencing station). For how quality levels map to your application, see the ISO 5817 guide and the B vs C vs D comparison table.
Detection
- Radiographic testing (ISO 17636) — slag shows as irregular, angular dark indications, distinguishable from the rounded form of porosity.
- Ultrasonic testing — effective on thicker sections; linear slag gives strong planar-like responses.
- Visual testing (ISO 17637) — catches surface-breaking slag lines at the toes, and more importantly verifies interpass cleaning before the next pass hides the evidence.
- In-process monitoring — thermal imaging sees the interpass surface state and pool behaviour every pass; anomalies in solidification thermography correlate with trapped non-metallics. Overview: real-time weld quality monitoring.
Prevention checklist
- Make interpass cleaning a hold point on multi-pass WPSs — chip + wire brush (or needle gun), with attention to toe lines.
- Keep beads flat to slightly convex; avoid deep convex profiles that create slag traps.
- Respect WPS current: low-end current with a heavy-coated electrode is a slag-trap recipe.
- Use correct electrode/torch angle so slag flows behind the pool, never ahead.
- Grind out suspect areas before the next pass — 30 seconds of grinding beats an RT repair cycle.
- Document cleaning per pass in the traveler — under ISO 3834 this is exactly the kind of in-process evidence auditors ask for.
Slag inclusions are one of the defects covered in our complete map: see the welding defects guide with acceptance criteria.
Frequently Asked Questions
What is a slag inclusion in welding?
A slag inclusion (ISO 6520-1 code 301) is non-metallic solid material — residue from electrode coating or flux — trapped inside the weld metal or between weld passes. It occurs in flux-shielded processes: MMA/stick, flux-cored arc welding, and submerged arc welding. Gas-shielded solid-wire MIG/MAG and TIG produce silicate islands but not true slag inclusions.
What causes slag inclusions between weld passes?
The dominant cause is incomplete slag removal before depositing the next pass, especially in narrow or convex beads where slag wedges into the toe lines. Contributing factors: low heat input that fails to remelt residual slag, wrong electrode angle letting slag run ahead of the pool, undercut or rough profiles from the previous pass that trap slag, and welding over heavily rusted or primed surfaces.
Are slag inclusions acceptable under ISO 5817?
Small isolated inclusions can be acceptable depending on quality level; long or aligned inclusions are treated severely because they behave like planar defects. ISO 5817 limits inclusion length and height relative to material thickness, with Level B the most restrictive. Exact dimensional limits are in ISO 5817 Table 1 — verify the current edition and your governing code, which may be stricter.
How do you detect slag inclusions in a weld?
Subsurface slag inclusions are found by radiographic testing, where they appear as irregular dark indications (unlike the rounded indications of pores), or by ultrasonic testing. Surface-breaking slag at toe lines is visible in VT per ISO 17637. In production, the most effective control is procedural: enforce and document interpass cleaning before each pass is deposited.
