Flux core welding, formally flux-cored arc welding (FCAW), is a wire-feed process like MIG, except the wire is a hollow tube filled with flux. In the self-shielded version that flux makes its own shielding gas as it burns, so you can weld outdoors and on slightly dirty steel without a gas bottle.
It is a favorite for farm repairs, fences, trailers and site work on structural steel, and it is what many entry-level “MIG” machines actually run out of the box. This guide covers how it works, the two types of wire, polarity, how to read the wire label, technique, and where flux core is and is not the right choice.
How Flux Core Welding Works
A wire feeder pushes a continuous tubular electrode through a gun. When the wire touches the work, current flows, an arc forms, and the wire melts into the joint as filler metal. The flux inside the wire melts at the same time. Part of it turns to gas that pushes air away from the molten pool, and part of it forms a slag that floats to the top of the weld and protects it as it cools. You chip that slag off afterward, just as with stick welding.
Self-Shielded vs Gas-Shielded Flux Core
There are two families of flux-cored wire, and they behave quite differently.
| Feature | Self-shielded (FCAW-S) | Gas-shielded (FCAW-G, “dual shield”) |
|---|---|---|
| Shielding gas bottle | Not needed | Needed, usually CO2 or an argon/CO2 mix |
| Outdoor use | Good; tolerates wind | Poor in wind, like MIG |
| Usual polarity | DCEN (electrode negative) | Usually DCEP; check the wire label |
| Typical users | Hobbyists, farm, field and site repair | Heavy shop fabrication, structural shops |
| Common mild steel example | E71T-11 | E71T-1 |
Miller’s guide to flux-cored welding basics for mild steel (checked November 2026) describes self-shielded wire as the outdoor and beginner option, and notes that gas-shielded wire is mostly used in heavy industrial fabrication and not usually recommended for hobby work. When people say “flux core” without qualification, they almost always mean self-shielded.
Polarity: Why Your Flux Core Welds Look Bad
The most common beginner mistake is running self-shielded wire on the wrong polarity. Machines sold for MIG are usually set up DCEP (electrode positive) for solid wire. Self-shielded flux-cored wire needs DC electrode negative, also called straight polarity, and Miller’s guide calls this out as a setup step. The polarity leads are often inside the machine near the drive rolls.
Wrong polarity shows up as heavy spatter, a rough, ropey bead and poor penetration. Swap the leads before blaming the wire. Gas-shielded wires are different, so always follow the polarity printed on the spool label.
How to Read a Flux-Cored Wire Label
Carbon steel flux-cored wires are classified under AWS A5.20, and the code follows a pattern like E71T-11:
- E means electrode.
- 7 is the minimum tensile strength of the weld metal in units of 10 ksi, so 70,000 psi.
- 1 is the position: 1 means all positions, 0 means flat and horizontal only.
- T means tubular (flux-cored) wire.
- -11 (or -1, -8 and so on) is the usability designator, which tells you the shielding type, polarity and typical use. That suffix is what separates a self-shielded wire from a gas-shielded one.
Miller names E71T-11 as a good general-purpose, all-position self-shielded wire for mild steel, and suggests .030 in wire as a good all-around size, .035 in for thicker material, and .045 in only if your machine supports it. Some self-shielded wires carry limits on plate thickness or on single-pass versus multipass use, so read the manufacturer’s data sheet before using a wire on anything structural.
Flux Core Welding Technique
Set up the machine
- Drive rolls: use knurled rolls. Flux-cored wire is softer than solid wire and smooth V-groove rolls can crush it.
- Tension: just enough to feed without slipping; too much deforms the wire.
- Ground clamp: clamp to clean metal. A poor ground adds resistance and gives an erratic arc.
- Settings: start from the chart inside the machine door for your wire diameter and plate thickness, then tune with test welds on scrap.
Gun angle, stickout and travel
- Drag, don’t push. Point the gun back toward the finished weld and pull it away. The rule of thumb is “if there’s slag, you drag.”
- Travel angle: Miller recommends 5 to 15 degrees from vertical in all positions; past about 20 to 25 degrees you get more spatter and less penetration.
- Stickout: about 3/4 in of wire beyond the contact tip, roughly twice what you would use for MIG.
- Work angle: about 45 degrees into a T-joint fillet, 90 degrees to a flat butt joint.
- Out of position: for vertical work, drop voltage and wire speed somewhat from your flat settings, and weld uphill on thicker plate for better penetration.
Chip and wire-brush the slag between passes. Slag left on the bead gets trapped under the next pass as an inclusion, which is a defect.
Flux Core vs MIG vs Stick
Flux core sits between MIG and stick. It feeds wire continuously like MIG, so it is fast and easy to learn, but it shields itself and leaves slag like stick, so it works outdoors. MIG with solid wire and gas gives cleaner welds indoors with no slag, and it handles thin sheet better. Stick needs the least equipment and is the most forgiving of rust and position, with 7018 rod the standard for structural work. For how MIG and TIG compare, see MIG vs TIG welding, and for the full family of processes, our guide to the types of welding.
Flux Core on Structural Steel
Both flux-cored processes are used on structural steel such as A36 steel plate and wide-flange beams. Code welding on buildings in the US is governed by AWS D1.1, the Structural Welding Code – Steel (checked November 2026). Under that code, the procedure, the filler metal and the welder’s qualification all have to be documented, so “it looked like a good bead” is not enough. Any weld that carries structural load, such as a beam connection, a column splice or a repair to a damaged member, should be designed by a licensed structural engineer and made to a qualified procedure.
Fumes and Safety
Flux-cored welding produces more smoke than solid-wire MIG, so ventilation matters. Wear a welding helmet with the right shade, leather gloves, a flame-resistant long-sleeve jacket and leather boots, and remove flammables from the area. Welding galvanized or zinc-coated steel adds zinc fumes; OSHA’s construction rule on ventilation and protection in welding, 29 CFR 1926.353 (checked November 2026) requires mechanical or local exhaust ventilation when welding zinc-bearing metals in enclosed spaces. Our guide on welding galvanized steel covers that case in more detail.
Flux Core Welding FAQ
Is flux core welding as strong as MIG?
A sound flux core weld made with a 70-series wire has weld metal rated for at least 70,000 psi tensile strength, which matches common MIG wire. Real strength depends on fusion, penetration and the absence of defects, so technique matters more than process.
Do you need gas for flux core welding?
Not with self-shielded wire such as E71T-11, which is the kind most hobby machines use. Gas-shielded flux-cored wire does need a shielding gas.
Why is my flux core weld so spattery?
Check polarity first: self-shielded wire needs DCEN. Then check stickout, travel angle and voltage, and make sure the ground clamp is on clean metal.
Can you flux core weld thin sheet metal?
You can with small wire, low settings and fast travel, but it burns through easily. For thin sheet, MIG with solid wire and gas is usually easier.
Do you push or pull with flux core?
Pull (drag). Point the gun back at the weld pool and drag it away from the finished bead, keeping the travel angle around 5 to 15 degrees.
Reference information only, not engineering advice. Disclaimer.