Weld Inclusion: Causes, Types, Detection, Codes, and How to Prevent Them

What Is a Weld Inclusion? A Hidden Threat Inside the Joint

You finish a clean-looking cap on a pressure line, step back, and it looks perfect. Then the radiograph comes back with a thin dark streak running along the bevel—a slag inclusion buried exactly where you can’t see it. That invisible flaw is one of the most common reasons welds fail inspection, and understanding it is the difference between a passed joint and an expensive cut-out.

A weld inclusion is a particle of foreign, nonmetallic material trapped inside the weld metal or lodged at the weld interface. TWI’s guidance on slag inclusions describes them as nonmetallic particles—usually slag or flux—mechanically entrapped in the joint instead of melting into it. Crucially, an inclusion is not a void. It is solid contamination with its own chemistry, and that difference changes how it behaves under load.

Weld Inclusion Defined: Foreign Material, Not Empty Space

Because inclusions are foreign material, they either never melted or solidified at a different temperature than the surrounding steel, creating localized zones with very different mechanical properties from the weld around them. Think of a tiny ceramic pebble frozen in metal: the weld cools and contracts, but the pebble doesn’t move with it.

The result is a stress riser—a hard, brittle island surrounded by tougher metal. That mismatch is why an inclusion that looks trivial on a film can quietly decide where a crack begins years later.

Inclusion vs. Porosity vs. Lack of Fusion

These three defects get mixed up constantly, but each forms differently and demands a different fix.

Porosity is empty space; lack of fusion is missing bonding; an inclusion is foreign material sitting where metal should be. Contrast that with a defect like overlap, which is excess weld metal rolling over the toe without fusing—a shape problem rather than trapped contamination.

Surface-Visible vs. Subsurface Inclusions

Some inclusions sit right on the surface and show up as dark specks or streaks in the cap. Many more are fully buried. As ESAB’s welding defects guide notes, inclusions can be surface or subsurface, which is exactly why a visual once-over—even a careful one—misses most of them. Seeing the surface is not proof of a sound joint. A spatter fleck, copper eroded from a worn contact tip, or a stray tungsten shard can all end up as inclusions.

Why Inclusions Matter Metallurgically

An inclusion interrupts the metallic continuity of the weld. ESAB explains that this reduces ductility and fatigue resistance while giving cracks an easy place to start. Under cyclic loading—bridges, pressure vessels, machine frames—that tiny island becomes a nucleation site for fatigue cracking. In corrosive service, the chemistry mismatch can even drive localized attack.

Not every inclusion is an automatic reject, though. TWI notes that slag and flux inclusions are linear defects, but because they lack the sharp edges of a crack, some standards and codes allow them within defined limits. The catch: those limits are tight, size- and location-dependent, and never a reason to weld sloppily.

What’s Ahead in This Guide

This guide walks through the whole picture: the main types of inclusions, the mechanisms that create them, the processes most at risk (TIG, flux-cored, and submerged arc top the list), and why they matter to fatigue and corrosion. From there we cover detection with visual, radiographic, ultrasonic, and destructive methods, what codes actually accept, and a prevention playbook built on technique, parameters, and consumable control.

We close with the mistakes that cause most inclusions—and the repairs they trigger. Before any of that, though, it helps to tighten your fundamentals. Solid welding practice and correct polarity prevent more inclusions than any inspection method can catch.

The Main Types of Weld Inclusions Explained

Walk into any weld inspection and you’ll hear “inclusion” used as a single word for several very different flaws. They form differently, hide differently, and clear different acceptance thresholds. Knowing which one you’re looking at is the first step toward fixing the root cause—instead of just grinding and hoping.

Slag Inclusions

Slag is the classic culprit. It forms when molten flux—or the slag layer it leaves behind—gets trapped in the weld pool before it can float out and solidify on the surface. That makes it a hallmark of flux-based processes: SMAW (MMA), FCAW, and submerged arc. It can also occur in MIG welding, though less commonly (TWI). The gasless, flux-cored route covered in our guide to gasless MIG welding is a frequent offender simply because there is more slag to manage.

On a radiograph, slag reads as elongated lines—sometimes continuous, sometimes broken into dashes—running along the length of the weld. Sharp notch-like tie-in at the toes, deep weaving, and poor interpass cleaning all invite it.

Flux Inclusions

Flux inclusions are slag’s close relative—unmelted or partially fused flux particles rather than a fully formed slag line. Submerged arc welding is the usual source, where granular flux can be dragged into the pool at high travel speeds or with sloppy flux handling. Equipment like robotic submerged arc welding keeps those variables tighter, which is one reason automation helps cut the defect rate.

Tungsten Inclusions

Tungsten inclusions belong almost exclusively to GTAW. They happen when the tungsten electrode touches the molten pool—often from dipping the tip during a filler dab—or when current is so high the electrode itself melts into the weld (ScienceDirect). The result is bright, high-density particles scattered through the deposit.

Tungsten’s melting point is roughly 3,422 °C, which is exactly why the electrode normally survives the arc without transferring (ESAB). Treat it as misuse, not bad luck. Correct polarity and amperage help—see our breakdown of TIG welding polarity for DCEN, DCEP, and AC.

Oxide and Millscale Inclusions

These come from the parent metal, not the filler. Weld over millscale, rust, or heavy oxide on the joint face and that layer can be trapped before it fully breaks down (ScienceDirect). They show as fine, often linear indications hugging the fusion line and can act as tiny corrosion sites. Mechanical cleaning back to bright metal is the only real cure.

Copper and Contact-Tip Inclusions

Copper inclusions are rarer but sneaky. Molten copper from an overheated, worn contact tip—or from copper backing—can transfer into the weld, leaving high-conductivity particles that behave very differently from slag or flux. This is why tip condition matters so much; our guide to contact tip welding covers wear limits and troubleshooting. Keeping wire, tip, and liner consistent—and knowing what MIG welding wire is made of—limits the risk.

Side-by-Side Comparison

The key difference is density and chemistry. Light, glassy slag and flux tend to string out and float; dense tungsten and copper particles settle and resist removal; oxide films sit flat against the fusion boundary. That behavior determines how easy each is to detect and how aggressively it must be repaired.

How Weld Inclusions Form: Causes and Mechanisms

In a healthy weld, slag is a passenger, not a prisoner. It forms on top of the molten pool, sits lighter than the metal beneath it, and rises to the surface where it can be chipped off between passes. An inclusion is simply what happens when that natural journey gets interrupted. Understanding the mechanisms is the fastest route to preventing them.

The Float-Out Principle: How Slag Should Escape

Slag is less dense than molten steel, so buoyancy pushes it upward. Whether it actually reaches the surface depends on three things: how fluid the slag is, how long the pool stays molten, and how much arc force stirs the pool. Give the slag enough heat and time and it floats clear. Freeze the pool early, starve it of heat, or trap a pocket with no escape route, and the slag solidifies inside the joint. Every inclusion cause below is a variation on that theme.

Incomplete Interpass Cleaning: The Number-One Cause

Ask any welding inspector what they see most, and the answer is slag left behind from a previous pass. ScienceDirect’s summary of slag inclusions and Klas Weman’s Welding Processes Handbook both name incomplete slag removal between passes as the most common cause. The mechanism is simple: a chip of fused slag sits on the cold face of the last bead, the next pass flows over it, and the arc can’t reach it through the metal. It’s now sealed in.

Inadequate Overlap and Sharp Notches

When two adjacent beads are laid side by side without enough overlap, they leave a valley between them. As TWI explains, slag becomes trapped when two adjacent weld beads are deposited with inadequate overlap; the next layer then seals the slag into that void. Sharp notches in joint boundaries or between passes worsen it—they’re perfect slag traps because the pool edge freezes before the slag can climb out. This is why weave width and step-over matter as much as heat. Poor technique that produces overlap defects often rides alongside hidden inclusions.

Slag Spilling Ahead of the Arc

Slag can also run ahead of the molten pool and get buried under incoming weld metal. This happens with an unfit joint configuration, poor joint design, incorrect electrode manipulation, or arc blow pushing the pool sideways. Poor joint access compounds it—if the electrode can’t reach the root of a tight corner, the welder compensates with awkward angles that splash slag forward. Maintaining the right polarity for flux core welding helps keep the arc stable and the slag trailing, not leading.

Sudden Parameter Changes and Cold Pools

Heat controls dwell time. Welding current that’s too small, or travel speed that’s too fast, gives the slag no time to float out before the pool freezes. On thin material, cranking the amperage the way you would on heavy plate—say, settings akin to 3/16 E7018 amperage—just burns through instead of helping. In electroslag welding, the stakes are higher: sudden voltage variations or excessive flux addition can cool the slag pool abruptly, decreasing penetration and leaving solid base metal sitting over molten slag that then gets trapped, according to the FHWA/Volpe report on electroslag welding defects.

Contaminated Surfaces

Mill scale, rust, oil, paint, and moisture all feed inclusions. As they burn, they form extra oxide and gas pockets that don’t float out cleanly. Contaminants also change how the pool wets the metal, encouraging incomplete fusion and voids where slag lodges. Clean steel is not a luxury—it’s a process control. A stable wire feed and a clean, well-maintained contact tip round out the variables that decide whether slag leaves or stays.

The common thread is simple: inclusions need a pocket, a void, or a cold pool to survive. Remove the pocket, keep the pool hot, and clean between every pass—and slag has nowhere to hide.

Which Welding Processes Are Most Prone to Inclusions?

Not every process hides a flaw the same way. The risk of an inclusion is baked into the nature of the process — whether it burns flux, whether it uses a consumable electrode, and how deep the groove runs.

Flux-Based Processes: SMAW, FCAW, and SAW

Slag inclusions are typically associated with the flux processes — MMA/SMAW, FCAW, and submerged arc — but they can also occur in MIG welding, as TWI notes. The reason is simple: wherever there’s flux, there’s slag, and wherever there’s slag, there’s a chance some of it never floats free.

SMAW is the classic offender. Every rod produces a slag blanket you must chip and brush before the next pass. Skip that step in a root or fill, and you weld the slag straight into the joint.

FCAW behaves the same way. It’s a flux-based process, so gasless/flux-cored welding carries identical interpass slag-removal obligations as stick — a point we cover in our guide to gasless MIG welding. Some operators wrongly treat wire feed as “clean” and rush the interpass. That’s how a thin, continuous slag line ends up running along the bevel.

SAW uses a granular flux blanket, not a coated electrode, but it still leaves a glassy slag layer that must be removed between passes. Its deep-penetration, narrow-groove habit is a double-edged sword.

High deposition rates and tight groove geometries increase inclusion risk by impeding slag float-out, according to ESAB. The faster you pour metal in and the tighter the gap, the less time and space slag has to rise to the surface before it freezes.

GMAW: Cleaner, But Not Immune

Gas metal arc welding (MIG) uses a solid wire and shielding gas, so it has no slag to trap. Yet inclusions still occur — usually oxides, spatter, or mill scale rolled into the puddle.

A worn contact tip, poor gas coverage, or a contaminated wire liner can seed inclusions too. Keep tips and liners in spec, as we explain in our guide to contact tip welding.

GTAW: The Tungsten Risk

GTAW (TIG) uses a non-consumable tungsten electrode, so its characteristic inclusion defect is tungsten entrapment rather than slag, per ScienceDirect and ESAB. Dip the tungsten, touch the puddle, or run too much current, and a fragment breaks off and freezes in the weld.

Electrode grinding direction and polarity matter, too — a poorly prepared tip sheds tungsten more easily, which is why ESAB stresses proper sharpening. Our guide to TIG welding polarity explains how DCEN, DCEP, and AC change the arc’s behaviour at the tip.

Automation and High-Deposition Systems

Robotic and automated SAW systems achieve very high deposition but still depend on controlled flux handling and slag removal to avoid inclusions. Automation removes operator fatigue and inconsistency from the equation, but it doesn’t remove the slag — the machine still needs a reliable cleaning step between passes. That’s why our article on robotic submerged arc welding stresses flux recovery and interpass control alongside the deposition numbers.

Process-by-Process Risk Summary

The takeaway: the more flux a process uses and the tighter and faster it runs, the more disciplined your cleaning and technique must be.

GTAW’s risk sits elsewhere — it rewards a steady hand and a well-ground electrode. Whatever the process, the underlying habit is identical: keep the puddle clean, let slag and gases escape, and never weld over debris. If you’re still building the muscle memory to keep a puddle clean at speed, our guide to welding practice is a good place to start.

Why Weld Inclusions Matter: Fatigue, Corrosion, and Structural Failure

A slag inclusion a few tenths of a millimeter across sounds trivial. On paper it barely changes the joint. In service it changes everything, because an inclusion isn’t just a cosmetic blemish—it interrupts metallic continuity in the weld, leaving a discontinuity exactly where the metal should be solid and uniform. That single fact drives every serious consequence that follows.

Loss of Cross-Sectional Area

Sound metal carries load. Trapped slag, oxide, or tungsten doesn’t. Every inclusion displaces weld metal that would otherwise resist tension, shear, or internal pressure, shrinking the effective throat of the joint. The effect is the same physics behind void-type defects: AWS Inspection Trends (August 2025) notes that piping porosity and similar defects reduce weld cross-section and strength. Swap “porosity” for “slag” and the reduction in load capacity is identical—just with a sharper, more aggressive defect geometry.

On a lightly loaded bracket, that lost area may never matter. On a wall-thickness-critical pressure joint, a few percent of lost area is a few percent of lost margin—and margin is precisely what you’re paying for.

Stress Concentration and Fatigue Life

Inclusions don’t just subtract area; they concentrate stress. Because most slag and oxide inclusions have sharp edges that don’t blend smoothly into the surrounding metal, they act as stress raisers. Under cyclic loading—road traffic on a bridge, pressure cycles in a vessel, thermal cycling in a pipeline—those points become crack initiators. ESAB’s defect guidance is blunt about the mechanism: inclusions reduce ductility and fatigue resistance and can act as crack initiation sites.

Fatigue is unforgiving this way. A buried inclusion can sit harmlessly for years, then a crack nucleates at its tip and grows with every load cycle until the remaining ligament can’t hold. The best-built joints rarely fail from a single overload—they fail from a flaw that was given thousands of chances to grow.

Corrosion Initiation

An inclusion is also a corrosion gateway. According to TWI’s job knowledge on slag inclusions, trapped material creates a locally different environment that can start corrosive attack—much like the crevices formed by undercut and overlap welding defects. In stainless and pipeline service, that’s a pitting or crevice-corrosion seed that eats inward, hidden beneath a smooth cap.

Isolated vs. Aligned Indications: Why Codes Get Stricter

Not every inclusion is equal. A single, small, isolated inclusion buried in a low-stress region may be judged cosmetic. But closely spaced inclusions lined up along a bevel or fusion line are treated far more severely, because they effectively link into one large planar defect. ASME BPVC Section VIII UW-51 rules, for example, treat multiple aligned indications more harshly than isolated ones for exactly this reason. Accumulation, not just size, is what turns a defect into a reject.

Where Inclusions Are Most Critical

• Pressure vessels and pipelines: internal pressure plus cyclic service means any lost area or stress riser directly erodes safety margin.
• Structural steel: bridges and building frames live under fatigue loading, where crack initiation dominates design life.
• Aerospace: the thinnest margins and the highest cycle counts make even tiny inclusions unacceptable.

In all four fields, the risk compounds progressively. A small inclusion reduces area and adds a stress riser; the riser starts a crack; the crack links with adjacent flaws; corrosion widens the path. That’s progressive failure, and it starts with something you couldn’t see from the top of the cap—which is why consistent, disciplined welding practice and proper reading of good versus bad welds matter long before inspection day.

Detecting Weld Inclusions: Visual, Radiographic, Ultrasonic, and Destructive Testing

You cannot fix what you cannot find. A weld can look flawless under the arc light and still hide slag, flux, or a stray tungsten fragment buried deep in the joint. Good detection is a layered system—each method covers what the previous one misses. Here is how the full inspection toolkit works, and what each technique can and cannot see.

Visual Inspection: Fast, Cheap, and Limited

Visual examination is always the first line of defense. A sharp eye under good light catches surface-breaking slag, exposed porosity, and obvious undercut. But visual inspection can only confirm surface defects; subsurface indications like lack of fusion or buried slag inclusions require NDT methods such as ultrasonic testing, radiography, magnetic particle, or penetrant testing, as ESAB’s welding defects guide explains. Inspectors also need to protect their eyes against arc flash and UV—the same discipline you apply on the torch applies at the inspection bench, which is why protective eyewear for welding belongs in every inspection kit.

Radiographic Testing (RT): The Slag Specialist

Radiography passes X-rays or gamma rays through the weld onto film or a digital detector. Because slag is less dense than steel, it shows up as a lighter, elongated line running along the bevel—this is why slag inclusions are so readily identified on a radiograph. As TWI notes, slag inclusions appear as elongated lines that are typically irregular. RT excels at volumetric defects—slag, porosity, and voids—but it can miss tight planar defects such as lack of fusion or cracks if they align with the beam. It also has a size threshold: rounded indications less than 0.8 mm (1/32 in.) in maximum diameter are disregarded in ASME radiography of welder performance coupons.

Ultrasonic Testing (UT) and Phased Array

Ultrasonic testing sends high-frequency sound into the weld and listens for echoes, making it the stronger choice for planar defects. For full-penetration ferritic steel welds from 8 mm to 100 mm thick, ISO 11666 specifies ultrasonic acceptance levels AL 2 and AL 3, using probe frequencies typically between 2 MHz and 5 MHz. Slag may return a lower-amplitude, more diffuse echo than a crack, so skilled interpretation matters. Phased array UT (UT-PA) steers and focuses the beam electronically for better coverage; ISO 4761:2022 covers phased array ultrasonic testing of thin-walled steel components. Users can apply this same electronic-beam logic to automated cells, as seen in robotic submerged arc welding.

Surface Methods: Magnetic Particle and Dye Penetrant

Magnetic particle testing (MT) suits ferromagnetic steels; dye penetrant testing (PT) works on non-magnetic materials like austenitic stainless and aluminium. Both only reveal surface-breaking indications—they will never find slag buried in the root. Use them as a complement to volumetric methods, not a replacement.

Destructive Testing: Cutting to the Truth

When you need certainty, you cut. Bend tests reveal ductility and near-surface defects, tensile tests measure strength loss, and macro or micro etching exposes slag and fusion boundaries under magnification. These are standard for procedure qualification and welder coupons, and macro-etch is the fastest way to prove a technique actually works—something deliberate welding practice teaches better than any textbook.

Matching the Method to Thickness and Material

Thickness and material drive the choice. Thick ferritic sections favor UT; thin walls favor RT or phased array per ISO 4761. Austenitic stainless and aluminium scatter sound badly, so RT or specialized UT techniques take over. Whatever the process—TIG polarity work or flux-cored runs—the detection plan should be written before the first arc strikes, not after the weld fails.

Codes and Acceptance Criteria: What’s Actually Allowed

Here’s the mindset shift that separates a competent welder from a frustrated one: perfection is not the goal—conformance is. Every weld contains some level of imperfection. Codes exist precisely because zero inclusions is an unrealistic, uneconomical standard. The real question is not “is there an inclusion?” but “does this inclusion exceed what the applicable code permits?”

That answer changes depending on where the weld lives. A decorative handrail and a nuclear pressure vessel follow completely different rulebooks.

ISO 5817: The Three Quality Levels

ISO 5817 provides three quality levels—B, C, and D—where quality level B corresponds to the highest requirement on the finished weld (per ISO/TC 44/SC 10). In practice, level D is the loosest, level C is the workhorse for general fabrication, and level B is reserved for demanding, fatigue-loaded, or safety-critical joints. The standard sets permissible limits for inclusion size, length, and spacing that scale with material thickness.

ISO 11666: Ultrasonic Acceptance Levels

When ultrasonic testing is the method, ISO 11666 supplies the acceptance levels. Acceptance levels AL 2 and AL 3 correspond to ISO 5817 quality levels B and C respectively. Notably, a level corresponding to quality level D is not included, because UT is generally not required for that quality (ISO/TC 44/SC 5). So if your spec calls out UT, you’re working to B or C by default.

ASME BPVC Section VIII UW-51: Radiographic Limits

For pressure vessels, ASME BPVC Section VIII, UW-51 gives the radiographic acceptance rules. For indications characterized as slag inclusions or cavities, the weld is unacceptable when the indication length exceeds 2/3t (where t is thickness). Two hard boundaries frame everything: for all thicknesses, indications less than 1/4 in. (6 mm) are acceptable, and indications greater than 3/4 in. (19 mm) are unacceptable.

Multiple aligned indications are acceptable only when the sum of their longest dimensions does not exceed t within a length of 6t, and when adjacent indications are separated by the defined minimum distance (ASME BPVC Section VIII). This is the clause that catches long, stringer-type slag lines that individually look small.

AWS D1.1: Workmanship-Based Visual Criteria

Structural steel under AWS D1.1 leans on workmanship-based visual criteria. D1.1 includes visual acceptance limits for piping porosity and other workmanship defects, meaning much of the judgment happens before any NDT is run. A visual inspector following D1.1 is checking profile, size, and surface discontinuities—often the first gate an inclusion-laden weld has to pass. Related defects like overlap follow similar logic; see our guide to the overlap welding defect for comparison.

Why the Code, Not Your Eyeballs, Decides

Acceptance ultimately depends on three variables: the applicable code, the service condition, and the chosen quality level. The same 5 mm slag inclusion might pass on a storage tank and fail on a cyclic-loaded bridge. That’s why welders, inspectors, and engineers must agree early on which standard governs—and why “it looks fine” is never an acceptance criterion. Building the habits that meet these specs starts with disciplined repetition; our welding practice guide covers the fundamentals, while process-specific setups (like TIG polarity or contact tip welding) often determine whether you’re fighting inclusions at all.

One more thing worth internalizing: acceptance criteria are usually written into the project specification before a single arc is struck. Learn to read that spec, and you’ll know exactly how much margin you have—and when to slow down, clean a pass, or change technique before an inspector sees the joint.

Prevention Playbook: Technique, Parameters, and Consumable Control

Prevention is always cheaper than a cut-out. Nearly every inclusion traces back to three controllable inputs: a clean joint, sensible parameters, and disciplined consumable care. Get those right and you eliminate the pockets, oxide films, and stray particles that trap non-metallic material inside a weld.

Cleanliness Between Every Pass

The highest-return habit in the shop is chipping and brushing every pass. On multi-pass work, layer-by-layer slag removal stops molten metal from rolling over leftover slag and locking it into the joint. As ESAB’s welding defects guide notes, thorough interpass cleaning is core to inclusion prevention.

Design the groove for access, too. A wider bevel angle and adequate root gap let the electrode and chipping hammer reach the bottom of the joint. A groove you cannot clean is a groove that will fail inspection.

Technique That Keeps Slag Out

Set the correct welding current and travel speed for the joint, then keep them there. Too little current leaves heavy, glassy slag that is hard to remove; too fast a travel outruns the shielding and traps it. Control your weave width and do not pause too long at the toes—wide, slow weaves build the exact pockets that collect slag. Surface preparation is the other half of the job: grind or wire-brush mill scale, rust, oil, and paint from the joint faces and at least an inch beyond the toes. Moisture flashes to gas that disrupts the arc and leaves voids where slag collects, so dry the work and never weld over condensation.

Electrode manipulation matters as much as the machine. Aim for an adequate bead shape and degree of overlap so successive passes sit cleanly rather than leaving crevices. TWI’s guidance on slag inclusions stresses correct overlap; a thin or misaligned bead is a slag trap. If your profile is off, review the overlap welding defect before chasing parameters.

Process-Specific Plays

• SMAW: Use dry electrodes, correct polarity, and a drag angle that pushes slag back. Match amperage to rod size—check 3/16 E7018 amperage settings for reference.
• FCAW: Verify polarity—flux-cored runs best with the right DCEN vs DCEP setup, and gasless wire still needs clean interpass work. Keep the gun angle steep enough to hold slag behind the puddle.
• SAW: Manage flux depth and recovery. Automated setups benefit from consistent travel and flux handling—see robotic submerged arc welding.
• GTAW: Control current and gas flow, and most effectively, sharpen and polish the tungsten. Coarse grinding leaves microscopic burrs that break off into the pool, so a smooth, polished grind matters (ESAB). Keep arc length near 1–3 mm and a steady torch angle to keep tungsten out of the puddle. Confirm your current type before you strike up—the wrong waveform invites tungsten loss, so settle AC or DC for TIG welding steel first.

Consumables, Settings, and Records

Store electrodes and flux in a dry, heated cabinet, keep wire clean, and replace worn contact tips and liners that cause erratic feed. A tidy shop layout makes dry storage and clean grinding easy. Confirm polarity against your WPS—DCEN, DCEP, or AC changes how the arc deposits metal (see TIG welding polarity), and the wrong choice invites inclusions.

None of this works without skill and paperwork. Follow the WPS, train welders on each process, and record parameters, consumable lots, and interpass cleaning for every joint. If your fundamentals are shaky, invest in deliberate welding practice first. Run the playbook in order—clean, control, care—and inclusions stop being a surprise on the radiograph.

Common Mistakes That Lead to Inclusions (and How to Fix Them)

Even the most experienced welders can trap slag inclusions or tungsten fragments inside a joint. Most of these flaws come down to small errors in technique, prep, or consumable care. Catching them early will save hours of cut-outs and rework, especially when working towards the clean results seen in Good and Bad MIG Welds or the precision required for jobs like DC Welding Aluminium.

Running Too Cold

The mistake: Using amperage far below the recommended range for your electrode or wire causes a sluggish puddle that cannot float slag to the surface. For SMAW, this is one of the most frequent causes of slag entrapment with E7018, as noted in AWS discussion threads.

Immediate fix: Reference a verified setting, such as the values for 3/16 E7018 Amperage, and adjust in small increments while watching puddle fluidity.

Principle: Adequate heat keeps the weld pool molten long enough for non-metallic matter to rise, so do not sacrifice penetration for a “cool” arc.

Dragging the Arc or Welding Downhill

The mistake: Dragging a long arc, holding the wrong rod angle, or welding downhill can let molten slag run ahead of the puddle and solidify over it. This traps inclusions before the filler can cover the joint.

Immediate fix: Keep a tight, consistent arc, use the correct travel angle for your process, and avoid downhill on joints that require uphill control unless the procedure allows it.

Principle: The molten pool must always lead the slag. If slag leads, entrapment is almost guaranteed.

Skipping Chipping, Brushing, or Interpass Cleaning

The mistake: Failing to remove slag between passes leaves a brittle, glass-like layer that new filler cannot fuse through. This is a common error across SMAW, FCAW, and multi-pass work.

Immediate fix: Chip, then wire-brush each pass to bright metal before restarting. For flux-cored work, follow the guidance in Polarity for Flux Core Welding to avoid compounding cleaning issues.

Principle: Clean metal is a prerequisite for fusion, not an optional finishing step.

Welding Over Rust, Millscale, Paint, or Moisture

The mistake: Trapped moisture, oxides, or coatings break down in the arc and form non-metallic pockets. AWS Inspection Trends notes that moisture between lap joint members can directly produce piping porosity and inclusions.

Immediate fix: Remove coatings to clean metal, dry the joint, and seal the work area if humidity is high. This is a baseline for any safe Welding Facilities setup.

Principle: If it does not belong in the base metal, it does not belong in the weld.

Reusing Wet or Damaged Electrodes

The mistake: Wet, chipped, or flux-voided SMAW electrodes can introduce hydrogen and non-metallic matter. AWS forum discussion links improper storage to porosity and related defects.

Immediate fix: Discard damaged rods, and recondition only those allowed by procedure in a proper oven. For wire processes, verify feedstock quality as outlined in What Is MIG Welding Wire Made Of?.

Principle: Consumables are part of the weld chemistry. Treat them as controlled materials.

Continuing to Weld After Dipping the Tungsten

The mistake: Dipping the tungsten into the puddle erodes it and can leave tungsten inclusions in GTAW. Excessive current accelerates this thermal damage.

Immediate fix: Stop, remove the contaminated tungsten, re-grind to the correct profile, and re-establish the arc. Review polarity for your job with TIG Welding Polarity or AC or DC for TIG Welding Steel.

Principle: The electrode must never contact the molten pool. If it does, restart with a clean electrode.

Repair, Rework, and Key Takeaways

Finding an inclusion on a radiograph is never welcome news, but it is not the end of the joint. The standard remedy, as outlined in ESAB’s welding defects guide, is straightforward: grind or excavate the affected region, clean it thoroughly, and re-weld using improved sequencing and interpass practice. What separates a professional repair from a repeat failure is discipline in the steps around that weld.

Step 1: Characterize Before You Cut

Never start grinding blindly. A weld inspector—or a competent person—needs to know what the defect actually is, how deep it runs, and whether it’s isolated or part of a pattern. Slag and flux inclusions in particular tend to sit buried near the bevel, so a visual once-over tells you almost nothing. Confirm the extent with the same NDT method that found it, or a more sensitive one, before you touch the surface. In AWS’s Inspection Trends treatment of the subject, the emphasis is clear: determining the root cause of an unacceptable weld—rather than just patching it—is the better approach. That mindset starts here.

Step 2: Excavate With a Margin

Remove the defect by grinding or gouging, and go beyond it. Take out sound metal on all sides so the repair cavity is clean, well-shaped, and free of any trace of the original flaw. Inclusions often taper or run discontinuous, so an excavation that stops “just past” the visible indication frequently leaves a remnant that shows up on the next film. For deeper work, gouge with carbon arc or plasma, then grind the groove smooth and remove all carbon contamination before welding.

Step 3: Clean, Re-Weld, Re-Examine

After excavation, clean the cavity and surrounding metal thoroughly, then re-weld under controlled conditions with an approved repair procedure. Re-set your parameters—current, polarity, shielding gas, travel speed—and re-establish proper interpass cleaning rather than repeating whatever caused the original flaw. Poor technique across a whole joint is often a shop-wide issue, so it’s worth reviewing welding facility setup and workflow if inclusions keep recurring.

Once welded, the repair must be re-examined with the same NDT method used originally. Under ASME BPVC Section VIII UW-51, defective weld areas in pressure vessels must be repaired under UW-38 and the repaired area re-examined. That closing loop matters: a repair that is welded but never verified is just a hope. Where all radiographically disclosed defects must be eliminated, 100% radiography is required; spot radiography can still miss defects sitting between shots.

Some defects should not be locally repaired at all. Centerline cracking in electroslag welds, per the FHWA/Volpe electroslag report, calls for complete removal and replacement. Covering a defect—”capping over” it—fails for a simple reason: slag inclusions create a sharp, non-fused interface that acts as a stress raiser. The new cap does nothing to remove it, and the flaw stays in the joint exactly where fatigue and corrosion will find it first.

The Bottom Line

Inclusions are common, but they are also largely preventable and always fixable when handled properly. Know your types, fix your technique, inspect properly, and follow the code. Get the basics right on every pass—clean interpass surfaces, correct polarity, stable parameters—and reference resources like our guide to overlap welding technique versus defect when a good-looking bead hides a bad one. The weld that passes is the one you never have to excavate.

Frequently Asked Questions

What is a weld inclusion?

A weld inclusion is any solid, nonmetallic or foreign particle trapped inside the weld metal or at the weld interface. Common examples are slag, flux, tungsten, oxides, and copper particles. Unlike porosity, which is a void, an inclusion is actual foreign material that interrupts the metallic continuity of the joint.

What causes slag inclusion in welding?

The most common cause is failure to completely remove slag from one pass before depositing the next. Inadequate bead overlap, poor joint access, slag spilling ahead of the arc, and running too cold or too fast so slag cannot float out all contribute. Flux-based processes such as SMAW, FCAW, and SAW are most prone.

How are weld inclusions detected?

Surface-breaking indications may be found visually or with penetrant or magnetic particle testing, but buried inclusions require volumetric methods. Radiography readily shows slag as elongated lines, while ultrasonic testing—including phased array—is used for thicker sections. Destructive bend and tensile tests are also used on coupons.

Are weld inclusions always a defect?

No. Codes such as ISO 5817 and ASME BPVC define acceptable sizes, lengths, and spacing. Slag and flux inclusions lack the sharp edges of cracks, so limited amounts may be permitted depending on quality level, thickness, and service. Always work to the applicable code and WPS.

What is a tungsten inclusion and how is it prevented?

A tungsten inclusion is a particle of the tungsten electrode trapped in the weld, occurring with the GTAW/TIG process. It is caused by dipping the tungsten in the pool, touching the filler to the electrode, exceeding the electrode’s current limit, or using a cracked or badly ground electrode. Prevention means controlling current and gas flow, keeping proper arc length, and sharpening and polishing the tungsten.

Which welding process is most prone to inclusions?

Flux-based processes—SMAW, FCAW, and submerged arc—are the most prone to slag and flux inclusions because molten slag is part of the process. GTAW is not flux-based but is uniquely prone to tungsten inclusions. MIG/GMAW can also suffer inclusions under high deposition and poor cleaning.

How do you repair a weld with inclusions?

First characterize the extent of the defect, usually with NDT. Then grind or excavate the affected region with a margin, clean the excavated area, and re-weld using an approved repair procedure and improved technique. The repaired area must be re-examined to confirm the defect has been removed.

Can you weld over a slag inclusion?

You should not. Welding over a buried inclusion simply buries it deeper and can seal it into the joint, where it remains a stress raiser and potential crack initiator. The correct approach is to excavate the defect completely before re-welding, then verify with inspection.

Authoritative Sources & Further Reading

• https://www.twi-global.com/technical-knowledge/job-knowledge/defects-imperfections-in-welds-slag-inclusions-043
• https://www.sciencedirect.com/topics/engineering/slag-inclusion
• https://www.twi-global.com/technical-knowledge/job-knowledge/tungsten-inert-gas-tig-or-gta-welding-006
• https://esab.com/us/nam_en/esab-university/articles/gtaw-tungsten-electrode-types-selecting-an-electrode-for-tig-welding-ami/
• https://esab.com/gb/eur_en/esab-university/blogs/how-advanced-methods-for-sharpening-tungsten-electrodes-helps-prevent-inclusions-ami
• https://esab.com/gb/eur_en/esab-university/articles/welding-defects-guide
• https://www.aws.org/magazines-and-media/inspection-trends/it-aug-25-the-answer-is
• https://app.aws.org/forum/topic_show.pl?tid=15921
• https://www.iso.org/standard/80209.html
• https://committee.iso.org/cms/live/live/en/sites/isoorg/contents/data/standard/09/09/90916.html
• https://www.sulzer.com/en/-/media/files/q_documents_pe/tp/tp-0008.pdf
• https://rosap.ntl.bts.gov/view/dot/68175/dot_68175_DS1.pdf