Flux cored welding is an arc welding process that uses a continuously fed tubular wire filled with flux compounds instead of a solid metal wire. The flux inside the wire protects the molten weld pool from contamination, stabilizes the arc, and can add alloying elements to strengthen the finished joint. It’s one of the fastest welding methods available, capable of depositing up to 21 pounds of weld metal per hour with large-diameter wire, compared to just 2 to 5 pounds per hour with traditional stick welding.
How the Process Works
The wire used in flux cored welding looks like standard MIG wire from the outside, but it’s hollow. That hollow center is packed with fluxing agents, powdered metals, and chemical compounds. When the wire feeds through the welding gun and an electric arc forms between the wire and the workpiece, the intense heat melts both the metal sheath and the flux core simultaneously.
As the flux compounds burn, they produce gases and a layer of slag that float over the molten weld pool. This shield keeps oxygen and nitrogen in the surrounding air from reaching the liquid metal, which would otherwise cause weak, porous welds. Once the weld cools, the slag hardens into a crust on top that you chip or brush away to reveal the finished bead underneath. The process runs on a wire feeder and power supply similar to MIG welding equipment, making it semi-automatic: the machine feeds the wire at a set speed while you guide the gun along the joint.
Self-Shielded vs. Gas-Shielded Types
Flux cored welding comes in two distinct varieties, and understanding the difference matters because it determines where and how you can use the process.
Self-Shielded (FCAW-S)
Self-shielded flux cored wire relies entirely on its internal flux to protect the weld. The chemical reactions happening inside the arc generate enough shielding gas and slag coverage on their own, with no external gas bottle required. Lincoln Electric describes these wires as “a stick electrode turned inside out,” and the comparison is useful: like stick welding, the protection travels with the wire itself. This makes self-shielded wire ideal for outdoor work, field repairs, and construction sites where wind would blow away any external gas shield. The portability factor is significant too, since you don’t need to haul gas cylinders to remote job locations.
Gas-Shielded (FCAW-G)
Gas-shielded flux cored wire uses both its internal flux system and an external shielding gas, which is why it’s sometimes called “dual shield” or “double shielded” welding. The most common shielding gas is either pure carbon dioxide or a blend of 75% argon and 25% carbon dioxide. The dual protection generally produces cleaner welds with better mechanical properties, but it comes with a limitation: wind can disrupt the external gas coverage and cause porosity in the weld. You can use gas-shielded wire outdoors, but you’ll need wind screens or barriers to keep the shielding gas in place.
Where Flux Cored Welding Gets Used
Flux cored welding is built for thick materials and heavy work. The industries that rely on it most include structural steel erection, shipbuilding, bridge construction, heavy equipment manufacturing, and general construction. These are applications where you’re joining carbon steel, low-alloy steel, stainless steel, cast iron, or high-nickel alloys in large volumes, often on material that’s too thick for lighter processes to penetrate efficiently.
The speed advantage is the main draw. With deposition rates reaching 21 pounds per hour using 3/32-inch diameter wire in flat positions, flux cored welding can lay down weld metal roughly four times faster than stick welding. Even in vertical and overhead positions, where gravity works against you, flux cored wire still delivers around 10 pounds per hour. For comparison, MIG welding in spray transfer mode tops out at about 14 pounds per hour with 1/16-inch wire. On large-scale projects where hundreds or thousands of feet of weld are needed, that productivity difference translates directly into shorter schedules and lower labor costs.
How It Compares to MIG Welding
Flux cored welding and MIG welding (also called GMAW) use similar equipment, and many welding machines can run both processes with a simple swap of wire and drive rolls. But the two processes behave differently in practice.
MIG welding uses a solid wire and always requires external shielding gas. It produces less spatter, leaves no slag to clean, and generally creates a neater-looking weld. It works well on thinner materials and in controlled shop environments. Outdoors, though, MIG welding struggles because even a light breeze can scatter the gas shield and contaminate the weld.
Flux cored welding penetrates deeper into thick material, welds faster, and (in its self-shielded form) handles wind and outdoor conditions without issue. The trade-off is more spatter, a slag layer that needs removal after each pass, and higher wire costs. Flux cored wire is considerably more expensive per pound than solid MIG wire. Many fabrication shops keep both processes available, using MIG for thinner or cleaner work and switching to flux cored for heavy joints and outdoor tasks. That mixed approach often ends up being the most economical setup overall, even though it requires stocking more consumables and drive rolls.
Common Weld Defects and Their Causes
Flux cored welding produces strong, reliable joints when done correctly, but the slag system introduces some defect risks that don’t exist with MIG welding.
Slag inclusions are the most characteristic problem. These occur when bits of hardened slag get trapped inside the weld instead of floating to the surface. The four primary causes are incorrect bead placement, wrong travel angle or speed, improper heat input, and failing to clean slag from previous passes before laying the next one. On multi-pass welds, thorough slag removal between passes is essential.
Porosity shows up as small gas pockets scattered through the weld metal. Rust, paint, grease, oil, dirt, or moisture on the base metal before welding are the usual culprits. Letting too much wire stick out past the contact tip can also cause it by weakening the shielding coverage. Excessive voltage creates a specific type of porosity called wormholes, which are elongated tube-shaped voids running through the weld.
Both defects weaken the joint and can cause failures under load. The good news is they’re preventable with proper technique: clean your base metal, maintain the right stick-out distance, use appropriate voltage and travel speed, and always chip and wire-brush the slag between passes on multi-layer welds.
Equipment and Setup Basics
A flux cored welding setup includes a constant-voltage power supply, a wire feeder, a welding gun, and spools of flux cored wire. If you’re running gas-shielded wire, you’ll also need a gas cylinder and regulator. Most machines designed for MIG welding can handle flux cored wire, though you may need to swap to knurled drive rolls (which grip the softer tubular wire without crushing it) and adjust your polarity. Self-shielded wires typically run on straight polarity (electrode negative), while gas-shielded wires run on reverse polarity (electrode positive). Your wire’s packaging or data sheet will specify which.
Wire diameters range from small sizes suited for thinner material and out-of-position work up to the larger 3/32-inch wires used for high-deposition flat welding on thick plate. Choosing the right diameter depends on the thickness of the material you’re joining, the welding position, and how much penetration you need. Thicker wire deposits more metal per hour but is harder to control on vertical or overhead joints.