Early in my shop days, running FCAW on thick carbon steel, I couldn’t figure out why my welds looked rough and inconsistent even though my settings looked perfect on paper. That experience pushed me to really understand common FCAW welding mistakes and defects—from poor joint prep and weak arc control to choosing the wrong filler wire for the metal thickness.
A lot of welders, especially those switching between MIG vs TIG or doing the occasional stainless welding job, struggle with flux-cored details like stick-out, travel angle, and slag control. These issues aren’t just cosmetic; they directly affect penetration, weld quality, structural strength, and job-site safety, not to mention wasted wire and rework time.
If you’ve ever wondered why your FCAW welds keep cracking, undercutting, or trapping slag, this guide will walk you through practical, shop-proven fixes.
Image by kdmfab
What Causes Porosity in FCAW Welds and How Do I Stop It?
Porosity shows up as those pesky gas pockets in your weld, making it look bubbly and weak. In FCAW, it’s often from trapped gases that don’t escape properly during solidification. The flux core in the wire generates shielding gas as it melts, but if contaminants interfere, you’re in trouble.
This defect works by allowing moisture, oil, or rust to vaporize and get trapped. Self-shielded FCAW wires rely entirely on the flux for protection, while gas-shielded ones use external CO2 or argon mixes. Either way, poor prep amplifies the issue.
Use FCAW when you’re welding outdoors or on dirty steel, as it’s more tolerant than MIG. But porosity hits when welding painted or oily surfaces without cleaning. Why bother fixing it? Because porous welds lack penetration and can fail in high-stress applications like trailer frames.
In my shop, I once had a batch of porosity on a pipeline repair because the humidity was high and the wire sat out overnight.
Lesson learned: store your spools in a dry cabinet. Start by grinding or wire-brushing the joint to bare metal. For gas-shielded FCAW, check your flow rate—aim for 35-45 CFH to avoid turbulence that sucks in air.
Dial in your settings: for a 0.035-inch wire on 1/4-inch steel, run 18-22 volts and 150-250 amps, with wire feed at 250-350 IPM. Too low voltage, and the arc sputters, trapping gas. Test on scrap first. If porosity persists, switch to a wire with better deoxidizers, like E71T-1 for mild steel.
Keep your travel speed steady—about 10-15 inches per minute. Rushing creates shallow beads prone to defects. And always wear your respirator; those fumes from flux aren’t friendly.
How Can I Avoid Slag Inclusions That Ruin My Flux-Cored Beads?
Slag inclusions happen when bits of flux get trapped in the weld pool instead of floating to the top. It’s a common defect in FCAW because the process produces a lot of slag by design—to shield and clean the weld.
The mechanism is simple: as the wire melts, flux forms slag that should be chipped off between passes. But if your technique is off, it mixes in and solidifies inside.
This is ideal for multi-pass welds on thick plates, where slag protects against oxidation. Use it on jobs like shipbuilding or heavy equipment repair, where you need deep penetration without perfect cleanliness.
Why care? Inclusions create stress points that lead to cracks, especially in cyclic loading like on bridges. I’ve seen pros skip chipping and pay for it with X-ray rejects.
Shop tip: After each pass, let the weld cool slightly, then use a chipping hammer at a 45-degree angle. Don’t pound too hard, or you’ll dent the bead. For stubborn slag, a needle scaler saves time.
Joint prep is key—bevel edges to 30-45 degrees for better access. Match your electrode diameter: 0.045-inch for heavier sections to carry more flux. Amperage wise, stick to 200-300 amps for that size on 3/8-inch material to ensure the slag flows out.
If you’re self-shielded, drag the gun at 10-15 degrees; for gas-shielded, push at 5-10 degrees. Wrong angle buries slag. And store rods properly—damp flux leads to more inclusions.
Why Does My FCAW Weld Have Undercut and What Fixes It?
Undercut is that groove along the weld toe where base metal melts away without filler replacing it. In FCAW, high heat or fast travel causes it, eroding the edges.
It occurs as the arc digs too deep, often from excessive amperage or poor gun angle. The flux helps stabilize, but can’t compensate for bad parameters.
Apply this when welding fillets or laps on structural steel—it’s fast, but undercut weakens the joint by concentrating stress.
Prevent it because undercut can lead to fatigue cracks in vibrating machinery. I remember undercutting a boom arm repair; it failed after a week, costing downtime.
Practical fix: Lower amps by 10-20% if you see melting edges. For 0.052-inch wire, try 220-280 amps on 1/2-inch plate. Slow your travel to fill the groove.
Gun technique: Maintain a 15-20 degree drag angle for self-shielded to direct heat properly. Weave slightly if needed, but not too wide.
Joint fit-up matters—gaps over 1/16-inch invite undercut. Grind smooth post-weld, but better to avoid it. Safety note: Undercut hides cracks, so inspect with dye penetrant on critical jobs.
Dealing with Lack of Fusion in Flux-Cored Welding: Signs and Solutions
Lack of fusion means the weld doesn’t bond fully to the base metal or previous passes. In FCAW, low heat input or contamination causes it, leaving cold laps.
The process relies on the arc melting both filler and base; if voltage is too low, it just deposits without fusing.
Use it for vertical or overhead positions where FCAW’s slag supports the pool. Ideal for construction sites with wind.
Why fix? Poor fusion leads to delamination under load, like in pressure vessels. I’ve pulled apart test plates showing this after rushing a job.
Tip: Boost voltage to 24-28 volts for better arc length. Amperage at 180-240 for 0.035 wire ensures penetration without burn-through.
Clean joints thoroughly—rust blocks fusion. Preheat thicker steels to 150-200°F to aid bonding.
Step-by-step: Set machine, strike arc on scrap, adjust until bead wets edges smoothly. Weld in stringers for control. If fusion lacks, grind out and reweld.
Compare to SMAW: FCAW is faster but needs finer voltage tuning.
Burn-Through Issues: When Your FCAW Heat Is Too Hot for Thin Materials
Burn-through punches holes in thin gauge metal from excessive heat. FCAW’s high deposition rate amplifies this on sheets under 1/8-inch.
It happens as amps overwhelm the material, melting through before the pool solidifies.
Stick to thicker stocks with FCAW, or use pulsed modes on advanced machines like Lincoln’s Power Wave for thin stuff.
Avoid because it ruins parts, increases distortion, and wastes filler. On a car frame repair, I burned through once—had to patch it, doubling time.
Solution: Drop amps to 120-180 for 0.030 wire on 16-gauge. Use short-circuit transfer if available.
Pulse the trigger for control. Backing bars dissipate heat. Pros: Lowers distortion. Cons: Slower on thin work compared to TIG.
Bird-Nesting and Wire Feed Problems in FCAW Setup
Bird-nesting tangles wire in the drive rolls from poor tension or liner issues. Common in FCAW due to tubular wire’s stiffness.
It disrupts feed, stopping the arc mid-weld.
Happens on long runs or with kinked hoses. Use when spools are large for efficiency.
Fix it to avoid downtime. I’ve cleared nests mid-job, losing momentum.
Check drive roll tension—firm but not crushing. Clean liners quarterly. Use knurled rolls for flux-cored.
Step-by-step: Unplug, remove tangle, reload wire straight. Test feed without gas.
Incorrect Gas Shielding: Mistakes That Lead to Oxidation in Gas-Shielded FCAW
For gas-shielded FCAW, wrong flow or mixes cause oxidation, turning welds brittle.
Gas displaces air; too little lets oxygen in.
Use 75/25 argon/CO2 for mild steel indoors.
Why? Oxidized welds crack easily. Shop story: Wrong regulator setting oxidized a batch—scrap.
Set 30-40 CFH, shield from drafts. Nozzles clean for even flow.
Pros: Better beads than self-shielded. Cons: Needs cylinders.
Choosing the Wrong Electrode Diameter for Your FCAW Job
Electrode size affects penetration and deposition. Too small for thick metal lacks fill; too big burns thin stuff.
Diameter dictates amp range—0.035 for general, 0.062 for heavy.
Match to thickness: 0.045 for 1/4-1/2 inch.
Wrong size? Poor beads, defects. I switched mid-job once—fixed arc stability.
Table of diameters:
| Diameter (inch) | Amperage Range | Best For | Pros | Cons |
|---|---|---|---|---|
| 0.030 | 100-200 | Thin sheet | Fast, less heat | Low deposition |
| 0.035 | 150-250 | General fab | Versatile | Needs good setup |
| 0.045 | 200-300 | Medium thick | Good penetration | Heavier spools |
| 0.052 | 220-350 | Heavy plate | High output | More spatter |
Select based on machine capacity—most US welders like Miller or ESAB handle up to 0.052.
Overlooking Joint Preparation: A Fast Track to FCAW Defects
Poor prep invites all defects. Joints need cleaning, beveling.
Removes contaminants for better fusion.
Always for critical welds.
Tip: Use angle grinder for bevels, acetone for oils.
Step-by-step: Inspect, clean, fit-up tight, tack.
Travel Speed Errors: Too Fast or Too Slow in Flux-Cored Welding
Speed controls bead shape. Too fast: narrow, undercut. Too slow: convex, slag trap.
Aim 8-12 IPM for flats.
Adjust for position—slower vertical.
I’ve slowed for overhead, avoiding drips.
Amperage and Voltage Mismatches: Dialing In for Clean FCAW Welds
Settings balance penetration and spatter.
Voltage arcs, amps heat.
For E71T-8 self-shielded: 20-24V, 180-280A.
Test on coupons.
Comparison to SMAW: FCAW hotter, continuous.
Spatter Overload: Reducing Mess in Your FCAW Process
Spatter from unstable arc, wrong settings.
Anti-spatter spray helps.
Lower voltage slightly.
Clean nozzle often.
Distortion Control: Minimizing Warpage in FCAW Projects
Heat input warps thin parts.
Use clamps, intermittent welding.
Preheat evenly.
Shop fix: Backstep technique.
Safety Oversights That Amplify FCAW Mistakes
Fumes, UV, heat—wear PPE.
Ventilate for flux gases.
Ground properly to avoid shocks.
I’ve seen burns from skipped gloves.
Material Compatibility: Matching Filler to Base in Flux-Cored
Mismatch causes cracks.
Use E70T for mild, E308T for stainless.
Check AWS specs.
Wrong? Brittle welds.
Multi-Pass Strategies: Building Strong FCAW Welds Layer by Layer
For thick, layer properly.
Chip slag each pass.
Overlap 50%.
Ensures fusion.
Machine Maintenance: Keeping Your FCAW Rig Defect-Free
Dirty contacts cause erratic arc.
Clean weekly.
Replace liners yearly.
Prevents feed issues.
Position-Specific Tips: FCAW in Flat, Vertical, and Overhead
Flat: Easy, high speed.
Vertical: Uphill, lower amps.
Overhead: Short arcs, drag.
Adjust settings 10-15%.
Comparing Self-Shielded vs. Gas-Shielded FCAW: Picking the Right One
Self-shielded: Outdoor, no gas.
Gas: Cleaner indoors.
Pros self: Portable. Cons: More smoke.
Table:
| Type | Gas Needed | Best Environment | Bead Quality | Fume Level |
|---|---|---|---|---|
| Self-Shielded | No | Windy, outdoor | Good, slag heavy | High |
| Gas-Shielded | Yes | Controlled shop | Excellent | Lower |
Choose based on job site.
Fixing Cracks from FCAW Defects: Grind and Reweld Right
Cracks from stress, hydrogen.
Grind out, preheat, reweld.
Use low-hydrogen wire.
Prevents propagation.
Wrapping Up
You’ve got the complete guide to now to spot porosity before it bubbles up, dial in amps to avoid undercut, and choose the right wire diameter for solid fusion. This knowledge equips you to tackle jobs with confidence, reducing rework and boosting your efficiency in the shop or on site.
Remember, the best welds come from practice on scrap—always test your setup. When in doubt on a multi-pass weld, drop your interpass temp below 500°F to minimize heat-affected zone issues and keep distortion in check.
FAQ’s
What amperage should I use for 0.045 FCAW wire on 3/8-inch steel?
Stick to 200-280 amps with 22-26 volts. Start mid-range, adjust for smooth arc. Too high? Burn-through. Test on scrap for penetration.
How do I fix bird-nesting in my FCAW wire feeder?
Check drive roll tension—snug but not tight. Clean the liner with compressed air. Straighten hose loops. Reload wire carefully, clipping the end square.
Why is my FCAW weld cracking after cooling?
Likely hydrogen from moisture or wrong filler. Dry your wire, preheat base to 200°F, use low-hydrogen E71T-12. Grind cracks fully before rewelding.
What’s the best gas for gas-shielded FCAW on mild steel?
75% argon/25% CO2 at 35-45 CFH. It gives good penetration with less spatter than straight CO2. Shield from wind to avoid porosity.
How can I reduce spatter in self-shielded FCAW?
Lower voltage by 1-2 volts, keep stick-out at 3/4-inch. Use anti-spatter gel on the nozzle. Clean contacts for stable arc.