What causes worm tracks in flux core welding? Most often it is too much voltage for the wire-feed speed you have set. Damp or damaged wire, the wrong stickout, dirty metal, and (on gas-shielded wire) lost shielding can also leave the narrow, winding grooves that show up on the bead after you chip the slag.
This guide covers each cause, how to tell them apart, and what to check first. For the repair steps, see our guide on how to fix worm tracks in flux core welding.
Quick Answer
Worm tracks (also called gas marks) form when gas from the flux cannot escape before the slag and weld metal freeze. The usual trigger is excessive voltage. Check voltage against the wire data sheet first, then wire moisture, stickout, joint cleanliness, and shielding.
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What Are Worm Tracks?
Worm tracks are thin, winding lines or grooves on the weld face that you see after the slag comes off. They are caused by gas produced in the arc and flux that gets trapped under the slag as the weld freezes.
They are often shallow and mostly cosmetic. But the same conditions can also cause porosity, so you cannot judge how deep the problem goes from the surface alone. Do not confuse them with undercut (a groove along the weld toe) or slag inclusions (dark lines between passes).
What Causes Worm Tracks in Flux Core Welding?
| Cause | Why it leaves tracks | First check |
|---|---|---|
| Voltage too high for the wire-feed speed | Long arc and excess gas that cannot escape before the slag freezes | Compare settings with the wire data sheet |
| Moisture in the wire | Water in the core turns to steam and gas in the arc | Inspect the spool for rust or white residue and check how it was stored |
| Wrong stickout | Changes current and how the core burns off | Measure from the contact tip to the work |
| Dirty or coated base metal | Oil, paint, rust, and moisture add gas to the puddle | Grind or brush to bright metal |
| Lost shielding (gas-shielded wire only) | Drafts, leaks, or wrong flow let air into the arc | Check gas type, flow, hoses, and nozzle |
| Technique | Wrong travel angle or speed upsets the puddle and slag | Drag the gun at a steady, moderate angle |
Excessive Voltage
This is the most common cause. Hobart Brothers says the gas causes worm tracking when voltage is too high for a given wire-feed setting, and recommends lowering voltage in half-volt steps until the tracks disappear. Start from the manufacturer’s parameter range for your wire diameter rather than a generic chart.
Moisture and Damaged Wire
Flux-cored wire left out in a damp shop can pick up moisture, which can add to tracking and porosity. Keep unused wire in its original bag and box, and replace spools that are rusty or visibly damaged. Do not bake a spool in an oven unless the wire maker publishes an approved reconditioning procedure, because heat can damage the spool and wire coating.
Wrong Stickout
Flux core normally runs a longer stickout than solid-wire MIG. Miller suggests about 3/4 inch as a general starting point for flux-cored wire, and some self-shielded wires have their own range. Hobart notes that too much extension can contribute to porosity on its self-shielded wires. Follow the data sheet for your wire and hold the distance steady.
Contaminated Metal
Oil, grease, paint, moisture, heavy rust, and cutting residue all release gas in the arc. Flux does not replace cleaning. Clean the joint and let any cleaner evaporate before you strike an arc, and never weld where chlorinated degreaser vapors can reach the arc.
Shielding Problems on Gas-Shielded Wire
Gas-shielded wire needs the exact gas on the label at the flow the maker specifies. A draft, a leak, a clogged nozzle, or too much flow can all disrupt the shield. Turning the flow up does not fix wind; it can add turbulence and pull air in.
Technique
Drag the gun with a modest travel angle (Miller suggests about 5 to 15 degrees) and keep a steady travel speed. Very steep angles can upset the arc and trap slag.
Self-Shielded vs Gas-Shielded: Which Gets Worm Tracks?
Both can. Self-shielded wire relies entirely on its core for shielding, so voltage, stickout, and wire condition matter a lot. Gas-shielded wire adds the risk of a damaged gas shield. Neither type is immune, so work through the same checklist for either one.
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How to Tell Which Cause You Have
Change one thing at a time on a scrap coupon that matches the job, and chip the slag after each test bead.
| What you see | Likely cause | Test |
|---|---|---|
| Tracks appear on every bead, even on clean steel | Voltage or stickout | Lower voltage a half volt at a time and re-check stickout |
| Tracks plus pinholes, worse after the spool sat out | Wire moisture | Try a sealed spool of the same wire with the same settings |
| Tracks only over rusty or painted areas | Dirty metal | Clean one section to bright metal and compare |
| Tracks appear only on a breezy day (gas-shielded) | Lost shielding | Block drafts, check hoses and nozzle, confirm gas and flow |
| Grooves along the toe instead of the center | Undercut, not worm tracks | Check voltage, travel speed, and gun angle |
Products Worth Considering
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Prevention Checklist
- Set voltage and wire-feed speed from the wire data sheet or machine chart
- Verify polarity on the wire label
- Measure and hold the recommended stickout
- Store wire dry and sealed, and replace damaged wire
- Clean the joint and the work-clamp spot to bright metal
- For gas-shielded wire, confirm gas type, flow, and a draft-free work area
- Drag the gun at a steady angle and run a test bead on scrap first
Are Worm Tracks a Weld Defect?
Light surface tracks may be acceptable on a noncritical weld, but they can signal conditions that also cause porosity. For anything that carries load, such as trailers, frames, lifting points, or pressure parts, the drawing, code, or inspector decides what is acceptable. A smooth bead does not prove a sound weld, and a marked bead does not always mean a bad one.
Related reading: how to fix worm tracks in flux core welding, slag inclusion in flux core welds and how to reduce spatter when flux core welding.
Frequently Asked Questions
What is the most common cause of worm tracks?
Excessive voltage for the wire-feed speed is the most commonly cited cause. Check your settings against the wire data sheet and lower voltage in small steps while testing on scrap.
Do worm tracks mean my weld is weak?
Not necessarily. Surface tracks can be cosmetic, but the same conditions can cause porosity inside the weld. For load-bearing or code work, have the weld judged against the applicable acceptance criteria.
Can damp wire cause worm tracks?
Yes. Moisture in flux-cored wire can add gas in the arc and contribute to tracks and porosity. Store wire sealed and dry, and replace rusty or damaged wire.
Are worm tracks the same as porosity?
They are related but not the same. Worm tracks are elongated surface marks under the slag, while porosity is usually rounded cavities that can be inside the weld or break the surface.
Does self-shielded flux core get worm tracks more than gas-shielded?
Either type can. Self-shielded wire depends on its core for shielding, so voltage and wire condition are important. Gas-shielded wire can also be affected by drafts, leaks, or wrong flow.
Will pushing the gun instead of dragging cause worm tracks?
Flux core is normally run with a drag technique so slag stays behind the arc. A poor angle can upset the puddle and trap slag, so use a steady drag angle unless the wire maker says otherwise.
Sources
- Hobart Brothers: Troubleshooting Common Self-Shielded FCAW Problems – voltage, porosity, stickout, and angles
- Miller: Flux-Cored Welding Basics for Mild Steel – stickout, drag technique, and travel angle
