Worm Tracks in Welding: Causes, Fixes, and When They’re Actually a Defect

What Are Worm Tracks in Welding? Definition, Appearance, and Terminology

You lay a clean cap pass, chip the slag, and brush it down. Then you see them: a trail of tiny parallel grooves crawling along the bead like something burrowed through the metal before it froze. The weld looks rough. Your inspector, though, barely looks up. That gap between “ugly” and “unacceptable” is what makes worm tracks so confusing, so let’s define them properly.

Worm tracks are a specific form of porosity found in flux-cored arc welding (FCAW). By AWS A3.0 terminology, they are gas voids whose length exceeds their width and which run approximately parallel to the weld axis. In plain language, they are elongated, surface-breaking gas channels. The defining feature is shape: round gas pockets are ordinary porosity, while stretched-out, surface-breaking rows are worm tracks.

The look is unmistakable once you know it. Picture several short, shallow indentations lined up side by side, each longer than it is wide, all trailing along the weld’s long axis. At a glance, they can resemble a machining mark or a greasy streak more than a classic weld flaw. Depending on severity, they read as faint silver streaks or dark, opened-up grooves.

Welders rarely call them “elongated porosity.” Trade names include gas marks, chicken tracks, and wagon tracks. Lincoln Electric uses the term “gas marks” for these features and classifies them as a cosmetic imperfection rather than a weld defect. That wording matters, because it hints at the argument this article untangles. Other references treat them more soberly, as surface discontinuities that inspection may or may not reject.

The One Clue That Matters: Slag

Here is the detail that separates worm tracks from ordinary porosity: they belong to slag-producing processes. That means FCAW in both its gas-shielded (FCAW-G) and self-shielded (FCAW-S) forms, the same family of wire covered in our guide to gasless flux-cored welding. The Fabricator notes worm tracks are considered unique to slag systems, which is why you will not find them in solid-wire MIG. If your process lays down a slag blanket, it can produce worm tracks. If it does not, blame something else. The polarity for flux core welding (DCEN vs DCEP) you select also shapes how that gas escapes.

Location offers more clues. Worm tracks most often appear on the face or cap of the weld, in the flat and horizontal positions, and on larger single-pass beads. They show up frequently with all-position, fast-freezing electrodes such as E71T-1, where the slag solidifies quickly and can lock gas at the surface before it escapes.

So are they a defect, or just ugly? That is the real question, and honestly, the process does not help you answer it. The term “cosmetic imperfection” sounds forgiving. Codes are less sentimental, judging elongated gas marks by size, distribution, and the acceptance criteria of whatever standard governs your job. A worm track can be a harmless surface mark or a genuine discontinuity that fails inspection, and the difference often comes down to three variables most welders never check. Later sections walk through those variables one by one, from voltage and wire feed speed to moisture in your gas.

Why Worm Tracks Form: The Gas-Entrapment Mechanism Explained

Worm tracks aren’t a random cosmetic glitch. They’re the visible result of a race between two things: gas trying to escape the molten weld pool, and the slag sitting on top of it freezing solid first. Understand that race, and the defect stops being mysterious.

The Soda Can and the Lid

Picture a can of soda you’ve just shaken. The liquid holds dissolved carbon dioxide, and the moment you crack the lid, that gas wants out. Open it slowly and the gas hisses away harmlessly. Now imagine snapping a lid onto that can the instant you shake it. The gas still needs to escape, but the lid traps it against the surface. It pushes, bulges, and finds the weakest paths it can.

A weld pool works much the same way. The molten metal absorbs and generates gas, and the slag layer on top acts as the lid. Worm tracks are what happens when that lid sets before the gas underneath has finished escaping.

One detail makes the analogy even tighter. The gas doesn’t have to escape all at once; it only needs the metal to stay fluid long enough for the last bubbles to reach the surface. Freeze too early and even a tiny remaining gas volume becomes a groove. Freeze at the right moment and that same gas vents quietly.

Where the Gas Comes From

Every arc weld is a small chemical reactor. Extreme heat breaks down moisture, flux ingredients, coatings, and shielding gas into their component elements, and some of those elements dissolve into the molten steel.

Hydrogen is the usual culprit. It comes from moisture on the plate, in the flux, or carried in the shielding gas itself. That’s why gas suppliers hold shielding gas to a dew point below -40°F (-40°C); anything wetter feeds hydrogen straight into the pool. Learn how moisture control fits into the bigger picture in our guide to building real welding skills.

In self-shielded flux-cored welding, nitrogen joins in. The flux contains denitriders meant to lock up nitrogen, but they only work within limits. Run the arc too long by pushing voltage too high, and the arc draws in more atmospheric nitrogen than the flux can neutralize. The excess dissolves into the pool and must come out later. Some wire systems also generate carbon dioxide as they burn, adding yet another gas hunting for an exit.

Why the Slag Freezes First

All-position electrodes like the E71-type family are engineered for out-of-position work. Their slag is designed to freeze fast, which is exactly what holds a vertical or overhead bead in place. On a wide, flat, horizontal bead, that same fast-freezing slag turns into a liability.

The slag solidifies while the weld metal beneath it is still liquid and still degassing. Escaping gas rises, slams into the underside of the frozen slag, and gets pinned there. It then creeps sideways along the easiest route, carving a groove that mirrors its path. That’s the worm-like look. As The Fabricator explains, the cause is the outer slag layer solidifying before the weld metal has solidified and completed its degassing phase.

How This Differs From Ordinary Porosity

Porosity and worm tracks often get lumped together, but they start from different problems. Porosity can appear in any arc process, including those with no slag at all. Gas bubbles simply get trapped as the metal freezes around them, leaving rounded voids scattered through the bead or lurking just under the surface.

Worm tracks are process-specific and timing-specific. They’re a slag and degassing problem, not general gas entrapment. That distinction matters when you diagnose them, and it’s why the fix usually lives in your parameters and technique rather than your base metal.

When you see scattered round voids, think gas that never left. When you see shallow parallel grooves, think gas that left too late. That also explains why the problem shows up most on flat, high-deposition passes with all-position consumables, and why it can vanish entirely the moment you change position. The physics didn’t change. The freeze rate did.

Parameter Causes: Voltage, Wire Feed Speed, CTWD, and Travel Speed

Before you blame the wire, the gas, or the phase of the moon, check your machine. Most worm tracks trace straight back to four dials you control: voltage, wire feed speed, contact-tip-to-work distance, and travel speed. Get these into the right window and the grooves usually vanish on the next pass.

Excessive Arc Voltage: The Usual Suspect

If you only adjust one thing, adjust voltage. Excess arc voltage is the single most common cause of worm tracks, because a long, soft arc gives the shielding envelope room to swirl and traps gas at the trailing edge of the puddle.

Hobart Brothers, in its guide to troubleshooting self-shielded FCAW problems, recommends the fix plainly: lower voltage in one-half-volt increments and run a test bead after each change until the tracks disappear. Don’t dive a full volt at once. Half-volt steps let you find the exact threshold where the arc tightens up without going so cold that you lose fusion or start stubbing the wire.

Voltage and Wire Feed Speed: Set Them in the Right Order

Voltage and WFS move together. Bump voltage up and the arc lengthens; bump WFS up and the arc shortens because more wire feeds into the same arc. They’re a balancing act. Match the setting to your wire diameter and material thickness.

Here’s the part that trips people up: set the machine by wire feed speed, not by amperage. Amperage in wire welding isn’t a dial you set. It’s a result of WFS and electrical stickout. If you chase a target amperage reading and then change your electrode extension, you’ve quietly changed the actual welding current. A too-short extension raises current and can push you toward the porosity and gas-tracking zone.

This is also why you should understand the differences between flux core and MIG before you swap processes. They do not want the same stickout.

Contact Tip to Work Distance

Contact-tip-to-work distance (CTWD), also called electrical stickout, is the distance from the contact tip to the work. Keep it between 3/4 in. and 1-1/4 in. for most FCAW work.

FCAW needs a longer stickout than MIG. That extra extension lets resistance pre-heat the wire so the flux is already activated before it drops into the weld pool. The result is better shielding behavior and fewer gas pockets. Shorten the stickout too much and you both steal that preheat and drive current up. Lengthen it past the top of the range and the arc wanders, the shielding suffers, and worm tracks come back.

Because the tip position matters so much, it pays to keep your consumables healthy. A worn or spattered tip changes how smoothly wire feeds and how consistent your extension stays. Our guide to contact tip selection and maintenance covers the wear signs to watch for.

Travel Speed and Bead Size

Travel speed is the sneaker. Move too fast and the arc outruns the shielding gas, so gas gets trapped and freezes into gas marks along the edges. Move too slow and you build an oversized, wide, convex bead that holds heat and gas longer, with a rough ropy surface that looks like tracks even when it isn’t.

Excessive current in a single-pass weld does something similar. Push too much heat into a small joint and the puddle gets big and sluggish. Speed up just a little and tighten your arc length and the bead flattens and cleans up.

How Fast-Freezing Slag Changes the Math

All-position electrodes make this trickier. Their fast-freezing slag sets almost immediately, which is great for vertical and overhead work but leaves the puddle less time to let trapped gas escape before it solidifies. That short window means you have to hold your parameters tighter. A voltage that’s fine on flat work can produce tracks on a vertical up pass.

Our welding practice guide walks through deliberately dialing in these variables so the settings become muscle memory.

The takeaway: treat voltage, WFS, CTWD, and travel speed as one connected system. Change one, test, and change the next. Most worm tracks die at the first half-volt.

Moisture, Contamination, and Consumable Causes

Worm tracks love a dirty, damp environment. Before you start swapping settings, remember that the arc can only work with what you feed it: the plate, the wire, the gas, and the air around the joint. If any of those carries moisture or contamination into the puddle, hydrogen and slag get trapped, and the grooving follows. Most “mystery” worm tracks trace back to something wet or something dirty.

Moisture: The Most Common Trigger

Water is the enemy because it breaks down in the arc and releases hydrogen, which gets trapped as the weld freezes. Common sources include humid air, condensation on cold plate, wet or frosted joints, and rain or snow blowing into an outdoor weld. Bring cold steel into a warm shop and it sweats. Weld a damp pipe or a joint that sat in the rain, and you have handed the arc a hydrogen supply.

Outdoor and cold-weather jobs are the worst offenders. Steel that sat through a rain shower, or plate hauled in from a freezing yard, will hold surface moisture even on a day that looks dry. A quick wipe-down and a warm pass with a torch before you strike an arc removes a surprising amount of trouble.

Flux-cored wire is even more sensitive. The flux core inside FCAW wire is hygroscopic, meaning it absorbs moisture from the air over time. Wire stored in a damp shop, or a spool left open on the machine for weeks, will pick up water and produce worm tracks even when every other variable is perfect. As The Shed Journal points out, storage method and wire age both matter, and when you’re in doubt the safest move is simply to replace the spool.

Base-Metal Contamination

Everything on the surface of the steel goes into the puddle. Rust, grease, oil, paint, primer, cutting fluids, and heavy mill scale all contribute hydrogen, carbon, and slag-forming compounds that encourage worm tracks. The fix is boring but effective: clean the base metal to remove rust, grease, paint, coatings, oil, moisture, and dirt before welding.

• Rust and mill scale: light surface rust is usually fine for FCAW; flaking, layered rust is not.
• Oil, grease, and cutting fluids: wipe with solvent and let it flash off before welding.
• Paint, primer, and coatings: grind back to bare metal along the joint.

That said, FCAW is forgiving. It tolerates light rust and a thin film of mill scale that would ruin a TIG weld. What it does not tolerate is gross contamination: flaking rust, pooled oil, thick primer, or a joint that was lubricated with cutting fluid and never wiped down. If you can scrape it off with a finger, clean it off. For a refresher on keeping consumables and torch parts in shape, see our guide to contact tip welding.

Using Preheat as a Diagnostic

When you suspect surface moisture but can’t see it, preheat. According to The Fabricator, preheating the weld joint to at least 225 degrees F can determine whether surface moisture is contributing to the problem. Heat the joint evenly, then run a test bead. If the worm tracks disappear, moisture was your culprit. If they persist, look deeper at the wire, the gas, or the base metal.

Baking or drying practices follow the same logic. Electrodes and flux that have absorbed moisture can often be dried in a holding oven, but flux-cored wire is not generally rebakeable the way stick electrodes are. For wire, replacement is the reliable answer.

When Worm Tracks Plus Piping Porosity Appear

Watch for worm tracks accompanied by piping porosity, the elongated, worm-like gas voids that tunnel into the weld. That combination signals a deeper contamination problem rather than simple surface moisture, and it must be addressed at the source before any cosmetic mitigation. Piping porosity points to a steady stream of gas or contaminant feeding the puddle: a gas line pulling air, a wire full of absorbed water, or a seriously dirty joint. Fix the input, not the symptom.

Good habits with consumables, storage, and cleanliness do more for your worm-track problem than any knob on the machine. Build them into your welding practice, and the grooving becomes a rare visitor instead of a weekly headache.

Shielding Gas, Polarity, Process, and Electrode Selection

Sometimes the cure for worm tracks isn’t a dial you turn at the machine — it’s the process and consumables you chose before you ever struck an arc. Flux-cored welding has two very different personalities, and which one you’re running changes how likely those little tunnels are to appear.

Gas-Shielded FCAW vs. Self-Shielded FCAW

Gas-shielded FCAW (FCAW-G) draws its protection from an external bottle, usually a CO2-rich blend. Because shielding gas is supplied from outside, the arc is sensitive to airflow — a draft, a fan, or a bad nozzle angle can strip the shield and let nitrogen and hydrogen into the puddle. Self-shielded FCAW (FCAW-S) instead generates its own protective atmosphere from the flux inside the wire, so it stands up to wind and outdoor work far better. That built-in armor is one reason self-shielded wires show up on pipelines and structural jobs where the weather won’t cooperate. Hobart Brothers keeps a handy troubleshooting guide for common self-shielded problems. If you want the full rundown on that side of the family, see our guide to gasless MIG welding.

Why 100% CO2 Beats Argon Blends for Gas Marks

Here’s a counterintuitive one: swapping to a fancy argon blend can make worm tracks worse. According to Lincoln Electric, gas-shielded FCAW has higher susceptibility to gas marks with argon/CO2 blends than with straight CO2. The reason is transfer mode.

Pure CO2 runs a globular, chunky transfer with large droplets. Mix in argon — C25 (75% argon/25% CO2) is the classic — and the arc shifts toward a spray-type transfer with smaller, more numerous droplets. More droplets means more surface area, and more surface area means more dissolved gas riding into the puddle. When that metal freezes before the gas escapes, you get worm tracks.

Shielding Gas Flow: Enough, Not Too Much

Flow rate matters as much as gas type. The Fabricator recommends 35–45 CFH for GMAW and FCAW, dropping to 25–30 CFH with a small tapered nozzle on a root pass. Crank it too high and you create turbulence that pulls atmospheric gas into the stream — the opposite of what you wanted. A shop fan, an open bay door, or a draft does the same thing, adding nitrogen and hydrogen that feed worm tracks. Set flow with the trigger held and the nozzle at working distance, not against a closed regulator; what the arc sees is what counts.

Electrode Choice and Low-Hydrogen Wires

The wire itself is a powerful lever. Switching an all-position electrode for a large-diameter flat/horizontal wire like 3/32-in. E70T-1C with 100% CO2 gives you a slower-freezing slag, and slow-freezing slag buys time for trapped gas to escape. These wires also deposit faster, which rewards flat-position production work. The trade-off is position: you’re now limited to flat and horizontal work — fine on a bench, useless overhead.

Hydrogen control matters too. Most modern FCAW electrodes carry low diffusible-hydrogen ratings of H8 or even H4 — a maximum of 8 or 4 mL of hydrogen per 100 g of weld metal. Less hydrogen in the arc means fewer gas marks and far lower cracking risk. Keeping consumables dry and clean protects that rating; a damp wire or a dirty joint can quietly undo it.

Polarity Basics

Most gas-shielded FCAW runs electrode positive (DCEP), and that’s typically what the data sheet lists for E70T-1C. Self-shielded wires often run the opposite polarity (DCEN), so always check the wire spec rather than assuming. Getting polarity backward changes penetration, spatter, and arc stability — and an unstable arc invites gas entrapment. For a deeper dive, see our guide to polarity for flux core welding. Keep your contact tip in good shape too: a worn tip wanders the arc and adds its own instability.

Bottom line: match the process, gas, and wire to your position and your code before you blame your technique.

Worm Tracks vs. Porosity vs. Other Defects — and What the Codes Say

Worm tracks often get tossed into the same bucket as porosity because both are gas-related. That’s a mistake. They form differently, sit in different places, and carry very different repair consequences. Classify them wrong and you’ll either grind out a sound weld or sign off on a leaker.

The cleanest distinction is straightforward: worm tracks are elongated surface marks, while porosity is trapped gas pockets, either on the surface or buried inside the weld. Same root cause, very different outcomes.

Worm Tracks Versus the Porosity Family

Worm tracks are shallow, parallel grooves that crawl along the cap in the direction of travel. They break the surface but stay cosmetic in most cases. Porosity behaves differently depending on type:

That third-to-last row causes the most confusion. Piping porosity, often called a wormhole, is the most severe porosity type because it creates a continuous leak path through the weld. For liquid- or gas-containing vessels, that’s a failure waiting to happen, not a cosmetic blemish you can argue your way past.

Don’t confuse worm tracks with overlap either. Overlap is liquid metal rolling past the toe without fusing; worm tracks are gas escaping the surface. Both look ugly, but only one is a fusion defect.

Slag inclusions and undercut round out the picture but won’t be confused with worm tracks once you know the tell. Slag inclusions come from entrapped flux or oxide; they’re solid, dense, and typically internal, with a gas-porosity mismatch. Undercut is a groove melted into the base metal along the weld toe. Neither is caused by escaping shielding gas, so neither belongs in a worm-track discussion — except that welders sometimes mistake one for the other when reading a radiograph or a visual.

What the Codes Actually Say

Here’s the part that surprises people: a worm track that isn’t associated with other porosity is generally not considered a defect, unless a Welding Procedure Specification precludes it. The gas simply got out. It left a mark, not a void.

Once porosity is involved, acceptance criteria kick in. Per The Shed Journal, worm tracks are handled under gas-mark rules rather than porosity limits, which is why an inspector can wave one through. Structural work falls under AWS D1.1 Table 6.1, which governs piping porosity and elongated porosity acceptance; repair decisions follow the table’s dimensions rather than a blanket “grind it all out.” Elongated porosity itself is defined in AWS B1.11, the Guide for the Visual Examination of Welds, which is the reference inspectors use for visual sizing and classification.

Why does any of this matter beyond paperwork? ESAB’s welding defects guide notes that porosity degrades tensile strength and impact toughness and can cause leaks; in stainless and aluminum it can even initiate corrosion. So severity and location drive everything. A cosmetic mark on a mild-steel fillet is one thing. The same indication on a leak-tight pressure vessel or a dynamically loaded structure faces far lower tolerance, and codes tighten accordingly.

For visual inspection, most shops lean on AWS B1.11, and good lighting plus the right protective eyewear makes surface discontinuities far easier to call correctly. Get the classification right, and the repair decision usually follows on its own.

Step-by-Step Diagnosis: How to Track Down the Real Cause

Worm tracks rarely announce their source, and they usually appear only after you’ve laid an otherwise clean cap pass, which makes them easy to misread. The fastest fix is a disciplined sequence that runs from cheap, quick checks to deeper investigation — one that changes only one variable at a time. Skipping ahead to exotic causes first is how most welders lose a shift.

First, Confirm What You’re Actually Looking At

Before touching a dial, confirm you have isolated worm tracks and not porosity mixed with tracks. Clean the bead and inspect it under strong light, ideally with a magnifier. True worm tracks are the fine, parallel elongated grooves riding the surface. Scattered pinholes or rounded voids mean porosity is present too, and that widens your search. Treating the wrong defect burns hours, so a sharp eye here matters — it’s the same skill behind judging good and bad MIG welds.

Work the Checklist From Cheap to Deep

Run these in order. Most cases resolve in the first three steps.

1. Verify parameters against the exact data sheet. Don’t stop at the AWS classification. Different brands and trade names meeting the same spec can require different voltage, wire feed speed, and CTWD. Pull the manufacturer’s sheet for your specific wire diameter and trade name; a generic chart for E71T-1 is a starting point, not a recipe.
2. Confirm CTWD and stickout. Excess or wandering stickout changes resistive heating and how the wire melts. Worn contact tips and liners add their own instability — see our guide to contact tip welding.
3. Inspect wire packaging, spool age, and storage. Damp, aged, or rusty wire, and opened spools left in a humid shop, feed moisture straight into the arc.
4. Check base-metal cleanliness and preheat. Mill scale, oil, rust, and surface moisture all contribute. Preheat also doubles as a diagnostic, driving off moisture before it can be trapped.
5. Verify shielding gas type, flow, leaks, and drafts. Confirm the correct gas, check hoses for leaks, and watch for drafts. Excess flow can create turbulence that actually displaces shielding gas. Lincoln Electric covers related gas marks on FCAW welds.
6. Revisit electrode selection and position. Travel angle, work angle, and drag versus push all change how gases escape the puddle.

Two flags deserve special attention. A sudden change after switching wire brands points straight back to step one and the data sheet. A gas hose leak or a draft is easy to overlook, so confirm flow at the nozzle, not just at the regulator.

Test One Variable at a Time

The temptation is to change gas, voltage, and technique all at once. Don’t. If the weld improves, you won’t know which change fixed it, and you can’t repeat the result.

Don’t chase a moving target. If you adjust voltage, gas flow, and wire brand together, you’ve reset the puzzle and learned nothing. Write down every change and the result before you touch anything else.

Start simple: reduce voltage in half-volt increments. This single adjustment is the most common fix for worm tracks. Run a coupon, inspect it, then step down another half volt if needed. To test whether surface moisture is the root cause, preheat the joint and weld the identical setup. If the tracks vanish, moisture was the culprit.

When in doubt, follow the CWB Group’s tackling worm tracks guidance and stay strictly inside your qualified WPS ranges; a fix that pushes you outside the window isn’t really a fix. That discipline keeps your procedure qualified and your inspections predictable. Build these testing habits through deliberate welding practice so diagnosis becomes instinct. Sequence beats guessing every time.

Prevention and Best Practices: Parameters, Technique, and WPS Discipline

Diagnosis tells you why worm tracks showed up on this weld. Prevention is about making sure they never show up again. The good news is that the fixes are ordinary shop habits, not exotic tricks. Work through the checklist below in order, because the cheapest corrections come first.

Start With the Parameters

The single most effective prevention method is following the manufacturer’s recommended parameters for your specific wire diameter. Voltage, wire feed speed, and travel speed all interact to control how the puddle freezes and whether gas can escape. Before you blame the wire or the gas, confirm you are actually running the settings printed on the box or data sheet for that diameter and position. On FCAW, polarity and gas flow join the list; the wrong polarity alone can carve a groove down the bead center.

1. Set voltage and WFS to the manufacturer’s range for the wire diameter.
2. Control CTWD (stickout) deliberately. Too long a stickout preheats the wire and destabilizes the arc; too short buries the tip in spatter. Hold it consistent, pass to pass.
3. Store flux-cored wire in its original packaging in a dry, enclosed environment when not in use. Moisture pickup is a silent worm-track generator.
4. Clean the joint. Mill scale, oil, rust, and paint all release gas into the puddle.
5. Manage moisture in the shop, consumables, and base metal before welding.
6. Confirm shielding gas type, flow rate, and polarity match the procedure.

If you run gasless wire, the rules for storage and polarity are even less forgiving, so it’s worth reviewing the specifics in our guide to flux-cored welding without gas. And because a worn or wrong-sized tip disturbs the whole arc, keep an eye on your contact tip as part of routine maintenance.

Technique-Level Prevention

Parameters get you close; technique finishes the job.

• Drag, don’t push. Use a drag (backhand) technique with any slag-producing filler metal to prevent slag entrapment at the root. Pushing rides slag ahead of the puddle where it can get trapped.
• Stay on the leading edge. Keep the arc at the front of the puddle so gas escapes before the metal freezes.
• Don’t over-size a single bead. Large 3/8-in. fillet welds are better made with at least three passes than one oversized all-position single pass. Each pass is shallow enough to let gas out.
• Match the electrode to position. Flat, horizontal, and vertical-up all want different characteristics. Choose a filler designed for the position you’re actually welding.

WPS Discipline and the Prevention Matrix

A procedure is only as good as its specificity. Document the exact brand and trade name of the wire, not just the classification, since two E71T-1 wires can behave very differently. Then tighten your ranges within the manufacturer’s limits instead of welding at the ragged edge of the allowable window. Finally, train operators on the why, not just the numbers — welders who understand gas entrapment spot it earlier. Cycle out old or unknown spools, and verify settings with a test coupon before running production parts.

Manufacturer troubleshooting guides, like Hobart’s self-shielded FCAW guide, and resources such as the CWB’s discussion of worm tracks all point to the same conclusion. Controlling moisture and stickout often solves problems that parameter tweaks alone cannot. Build those habits into your routine practice, and worm tracks stop being a recurring surprise.

Repair, Removal, and When to Rework

Once worm tracks exist, the first question isn’t how to grind them out. It’s whether they need to come out at all. That call hinges on your code, your WPS, and what the tracks actually are beneath the surface.

When a Repair Is Actually Required

Not every track is a defect. Some are cosmetic surface gas marks that only affect appearance. Break out the grinder or the gouger when any of these apply:

• The governing code or your WPS precludes surface discontinuities outright.
• The tracks are tied to porosity rather than a thin trapped film.
• Elongated porosity exceeds the acceptance limits in AWS D1.1 Table 6.1 — at that point the weld should be repaired.
• The joint is leak-critical: pressure vessels, vacuum lines, or fluid containment where a pinhole tells over time.

If none of those conditions applies and the marks are purely cosmetic, an inspector may pass the weld as-is. One cue deserves special caution, though. A deep pit at the very start of a track is not a shallow gas mark. It signals elongated porosity rooted deeper in the bead, and that material has to be fully excavated rather than skimmed.

Removal Methods, From Gentle to Aggressive

Shallow cosmetic worm tracks often disappear with very little grinding — a light pass with a flap disc is frequently enough to erase them and leave a serviceable surface. For deeper or porosity-linked marks, you have a ladder of options. The standard tools for correcting wormholes are grinding, air carbon-arc gouging, and rewelding, chosen by depth, position, and how much sound metal surrounds the defect. Access often decides between a grinder and a gouger.

1. Light grinding for cosmetic tracks that meet acceptance criteria once the mark is removed.
2. Grinding or gouging to excavate tracks that run deeper or repeat along the bead.
3. Full excavation for tracks showing that telltale starting pit, cutting back to sound metal on all sides.

Reweld Under Controlled Conditions

The best fix is worthless if you weld the repair with the same settings that created the defect. Before striking an arc, remove the affected region, correct the underlying contamination, moisture, or gas issue, then re-weld under controlled conditions and verify with NDT if required — the approach ESAB outlines in its welding defects guide. That means re-checking the variables that got you here. Reconfirm polarity, since a mix-up on flux-cored wire invites trapped gas. Inspect the contact tip for wear that alters current transfer, and dial your travel and voltage back within WPS range.

Verification matters as much as the repair. Depending on the code, that may mean visual inspection, dye penetrant (PT), ultrasonic (UT), or radiographic (RT) testing to confirm the fix is clean. For critical joints, a qualified inspector signs off before the part moves on.

Documentation and the Real Cost

Every repair carries a record: the defect type, its location, the removal method, the re-weld parameters, and the NDT results. Log it the same day, while details are fresh. That traceability protects you if the joint is questioned later. It also exposes the true cost of rework — excavation, rewelding, retesting, and lost production hours that dwarf the few seconds it takes to dry an electrode or fix a gas leak. Solid welding practice is far cheaper than chasing worm tracks through a finished part.

Conclusion: Turning a Cosmetic Nuisance Into a Process Advantage

Worm tracks look dramatic. Those neat parallel grooves marching down a cap pass can make a sound weld look like something crawled through it. But by now the pattern should feel familiar. Worm tracks are almost never a curse or a mystery. They are a process-control signal, and a signal you can read, trace, and fix.

Most worm-track problems trace back to three familiar suspects: excessive voltage, moisture in the wire or joint, and insufficient electrical stickout. Beyond those, contaminated consumables and a slag system that freezes at the wrong moment round out the usual lineup. The fixes are equally unglamorous: parameter discipline, dry wire, the right shielding gas, sound technique, and a weld procedure you actually follow. Here’s the part that surprises people: controlling moisture and stickout often solves cases that parameter changes alone cannot touch. None of these levers is expensive compared to the cost of a rejected joint or a scarred part.

Read the Signal, Not Just the Symptom

Symptoms rarely lie, but they can mislead you about their origin. A self-shielded wire that has absorbed shop humidity will throw worm tracks no matter how carefully you set volts. That’s why troubleshooting guides for self-shielded FCAW keep hammering moisture control — with gasless wires, the wire itself is part of the shielding system.

Why It Pays to Fix This at the Source

Reducing rework and scrap is the main economic payoff of solving worm tracks at the source. Every pass you don’t have to grind out is labor, consumables, and downtime you get to keep. But the benefits go further than the wallet. Cleaner caps mean better bead appearance, leak-tight welds on pressure and fluid work, and a weld procedure that holds up consistently instead of depending on luck.

Shops that chase this down — including operations running robotic submerged arc welding at high deposition — treat appearance discontinuities as a quality metric precisely because they reflect process stability. When the worm tracks vanish, everything downstream usually improves too.

Your Action List for This Week

1. Inspect and replace worn or oversized contact tips so stickout stays consistent.
2. Move wire off the shop floor and store it sealed and dry.
3. Verify your flux-core polarity and gas mix match the WPS.
4. Recheck voltage and CTWD against the procedure sheet.
5. If you run gasless MIG, confirm the self-shielded settings match the wire.

Then spend a little time on deliberate welding practice to lock in the muscle memory.

Here’s the encouraging truth: worm tracks are among the most solvable weld-appearance problems a welder can face. They respond to logic, not luck. Start with stickout and moisture this week, watch the tracks fade, and you’ll never again mistake a fixable setting for a fatal flaw. Understand the mechanism, follow your WPS, and a nuisance becomes proof that your process is dialed in — the difference between fearing the finish pass and expecting it to come out clean.

Frequently Asked Questions

Are worm tracks a defect or just cosmetic?

By themselves, worm tracks are generally treated as a cosmetic surface imperfection rather than a true defect. They only become a defect when a Welding Procedure Specification precludes them, when they’re associated with other porosity, or when they exceed the applicable code’s acceptance criteria.

What is the single most common cause of worm tracks?

Excessive arc voltage is the most common cause. When voltage runs too high for the wire feed speed and diameter, the added heat and dissolved gas overwhelm the slag’s ability to vent them, leaving trapped gas marks. Reducing voltage in half-volt increments is the standard first fix.

Which welding processes produce worm tracks?

Worm tracks are characteristic of slag-producing processes, mainly gas-shielded and self-shielded flux-cored arc welding (FCAW-G and FCAW-S). They’re rarely seen in MIG, TIG, or stick welding because those processes don’t create the same fast-freezing slag layer that traps gas.

Can contaminated or old wire cause worm tracks?

Yes. Flux-cored wire absorbs moisture from the air, especially if its packaging is damaged or a spool sits open in a humid shop. That absorbed hydrogen turns into trapped gas during welding. Store wire in its original sealed packaging in a dry area, and replace suspect spools.

Does shielding gas choice affect worm tracks?

It can. With gas-shielded FCAW, argon/CO2 blends such as C25 tend to produce more gas marks than 100% CO2. Argon promotes a spray-type transfer with more, smaller droplets and more dissolved gas. The wire manufacturer’s data sheet specifies the correct gas for each electrode.

How does contact tip to work distance affect worm tracks?

FCAW needs a longer electrical stickout than MIG, generally about 3/4 in. to 1-1/4 in., so the flux preheats and activates before it reaches the puddle. If the stickout is too short, the flux doesn’t activate properly, gas isn’t fully removed, and tracks can form.

How do I repair a weld that has worm tracks?

If repair is required, remove the affected area by light grinding, grinding, or air carbon-arc gouging, then re-weld under controlled conditions using correct parameters. Shallow cosmetic tracks often disappear with minimal grinding, but any associated deep piping porosity must be fully excavated.

What code governs worm track acceptance?

AWS D1.1 Table 6.1 provides acceptance criteria for piping porosity and elongated porosity, and the AWS B1.11 Guide for the Visual Examination of Welds defines how to classify these indications. Always defer to the project’s governing code and WPS.

How can I prevent worm tracks in production?

Follow the manufacturer’s parameters for the exact wire brand, diameter, and position; control CTWD; store wire dry; clean the joint and preheat if moisture is suspected; use the correct shielding gas and flow; and keep procedure ranges tight in the WPS.

Authoritative Sources & Further Reading

• https://www.hobartbrothers.com/resources/technical-articles/troubleshooting-common-self-shielded-fcaw-problems/
• https://www.hobartbrothers.com/resources/technical-articles/qa-why-are-my-welds-ugly/
• https://www.lincolnelectric.com/en/welding-and-cutting-resource-center/weld-solutions/gas-marks-on-fcaw-process-welds
• https://www.thefabricator.com/thewelder/blog/consumables/understanding-porosity-and-weld-cracking-in-fcaw
• https://www.thefabricator.com/thewelder/article/consumables/consumables-corner-diagnosing-the-obvious-and-not-so-obvious-causes-of-porosity
• https://www.cwbgroup.org/resources/videos/flux-core-arc-welding-tackling-worm-tracks
• https://conference.cwbgroup.org/resources/articles/fundamentals-of-surface-discontinuities
• https://esab.com/gb/eur_en/esab-university/articles/welding-defects-guide/
• https://openwa.pressbooks.pub/lwtechwirefeed/chapter/fcaw-defects-and-discontinuities/
• https://app.aws.org/forum/topic_show.pl?tid=31469
• https://www.theshedjournal.com/worm-tracks/
• https://www.osha.gov/welding-cutting