FCAW Welding Best Practices: Self-Shielded Guide

Self-shielded flux core welding, or FCAW-S, is one of the most practical welding processes available for structural steel erection, bridge construction, heavy equipment repair, and outdoor field work. It runs without an external shielding gas supply, produces high deposition rates, and handles demanding, variable conditions that shop-based processes can’t always accommodate.

But FCAW welding, specifically the self-shielded variant, comes with variables that require attention. The flux chemistry that eliminates the need for gas creates slag that must be managed between passes. Out-of-position and multi-pass applications introduce additional complexity. And the margin for error on parameters like stick-out, travel angle, and voltage is narrower than many operators expect.

This guide covers the common challenges in self shielded FCAW, slag inclusions, porosity, wormtracking, fusion issues, undercutting, and penetration problems, along with wire selection guidance and a process comparison to help operations choose between FCAW-S and GMAW for the application at hand.

FCAW-S vs. GMAW: Choosing the Right Process

Self-shielded FCAW and gas metal arc welding (GMAW) are both wire-fed processes, but they serve different environments. Understanding where each belongs helps welding operations avoid using the wrong tool for the job.

FCAW-S uses a flux-filled tubular wire that generates its own shielding as the flux burns — no gas cylinder, no regulator, and no risk of shielding disruption from wind or drafts. GMAW relies on an external shielding gas (argon, CO₂, or a blend), which provides a consistent shield in controlled environments but is vulnerable in outdoor or open-structure applications.

Factor	FCAW-S	GMAW
Shielding gas	None — flux generates its own shield	External gas cylinder required
Portability	High — no gas equipment needed	Limited by gas supply
Best applications	Outdoor, structural, field work	Indoor, precision fabrication, thin metals
Wire cost	Higher per spool	Lower per spool
Deposition rate	High	Moderate to high
Slag cleanup	Required between passes	None
Weld appearance	Rougher; cleanup required	Cleaner bead profile

For structural steel, field repair, and outdoor applications where wind or mobility are factors, FCAW-S is the right choice. For precision fabrication on thinner materials in a controlled shop environment, GMAW typically delivers a cleaner result with less post-weld cleanup.

Flux Core Welding Wire Selection

Not all flux core welding wire is the same. The right wire depends on the base material, welding position, desired mechanical properties, and whether the application requires deoxidizers to handle light surface contamination.

Most self-shielded wires are classified under AWS A5.20 for carbon steel applications. Key elements to understand when specifying wire:

• T-8 wires are formulated for all-position welding and include deoxidizers. They are the most common choice for structural FCAW-S work.
• T-11 wires are single-pass wires for flat and horizontal positions. Not suited for out-of-position or multi-pass applications.
• Deoxidizers (aluminum or titanium) provide tolerance for welding over light rust, mill scale, or minor contamination — but are not a replacement for proper cleaning on critical welds.
• Wire diameter affects deposition rate and puddle control. Larger diameters (3/32″ and above) suit flat/horizontal positions; smaller diameters (3/64″, 7/64″) are used for out-of-position work.

Always follow the filler metal manufacturer’s recommended parameters, voltage ranges, wire feed speeds, and stick-out distances, for the specific wire being used. Drifting outside those parameters is one of the most consistent contributors to defects in FCAW-S applications.

Slag inclusions

Slag inclusions, where molten flux from inside the wire becomes trapped inside the completed weld, are one of the most common FCAW-S defects. They occur most often in out-of-position and multi-pass applications, and they are almost always the result of correctable technique or parameter issues.

Causes: Travel speed and gun angle are the first variables to check. When welding vertical-up, maintain a drag angle of 5 to 15 degrees. For flat or horizontal positions, use a drag angle of 15 to 45 degrees — increase this if the problem persists. Moving too slowly allows the weld puddle to get ahead of the arc, trapping slag rather than letting it float to the surface. Insufficient heat input for the wire diameter will also cause inclusions.

Cures: Clean between every pass — don’t assume slag will fuse cleanly into the next bead. Maintain the filler metal manufacturer’s recommended voltage for the wire diameter in use. In wide groove openings and root passes, leave enough space in the joint for the weld metal to fill completely. Placing the bead too tightly against the groove wall is a common setup for inclusions in multi-pass welds.

Porosity

Porosity occurs when gas becomes trapped in the solidifying weld metal. In FCAW-S, surface contamination is the primary cause, gases generated by contaminants on the base material interfere with the flux shielding and get trapped in the weld.

Causes: Dirt, rust, grease, oil, paint, and moisture on the base material are the most common culprits. Excessive wire stick-out, beyond 1-1/4 inch from the contact tip, reduces the effectiveness of the flux shielding. Air currents from fans or outdoor wind can also disrupt the shielding envelope around the weld puddle.

Cures: Clean the full length of the weld joint before striking an arc — this cannot be compensated for with wire chemistry. Maintain consistent stick-out throughout the weld. If welding in a drafty environment, place a welding screen around the work area. Filler metals with added deoxidizers provide some tolerance for light surface contamination, but they are a buffer, not a substitute for clean base material on critical structural welds.

Wormtracking

Wormtracking refers to the marks on the surface of the weld bead, worm-like in appearance, caused by gas from the flux escaping along the surface rather than cleanly venting through the puddle. It signals that voltage is too high relative to the wire feed speed setting.

Causes: Excessive voltage for the wire feed speed setting is the root cause. Verify that the parameter combination falls within the filler metal manufacturer’s recommended range for the wire diameter being used.

Cures: Reduce voltage in increments of 1/2 volt until the surface marks stop. Don’t overcorrect; dropping voltage too far introduces slag inclusions and lack of fusion. Make small adjustments, evaluate the result, and verify that the final settings fall within the manufacturer’s recommended range before continuing.

Lack of Fusion and Undercutting

Lack of fusion and undercutting are distinct defects with some shared causes, primarily gun angle and insufficient heat input. Both affect weld integrity and need to be corrected before they contribute to cracking or joint failure.

Lack of Fusion

Causes: Incorrect gun angle is the primary cause. Contaminants on the base metal are also a factor, as is insufficient heat input in some cases.

Cures: Maintain a drag angle of 15 to 45 degrees and keep the arc on the trailing edge of the weld puddle. When using a weave technique, hold the arc on the groove sidewall, this is where fusion is most likely to be incomplete if the arc moves too quickly through the centre. Clean the base material thoroughly before welding and between passes.

Undercutting

Causes: Excessive travel speed is the primary cause, the weld metal doesn’t have enough time to fill the melted-out area at the toe of the weld. Incorrect gun angle and excessive heat also contribute.

Cures: Reduce welding current and voltage. Slow travel speed to allow the weld metal to fill the joint completely, particularly at the toes. Adjust the gun to point more directly toward the weld joint. On wider weave passes, pause briefly at each toe of the bead. Following the recommended parameters for the wire diameter eliminates most undercutting issues before they start.

Getting Penetration Right

Penetration problems in FCAW welding come in both directions, too much and too little, and both compromise weld integrity. Good joint penetration requires attention to parameters, gun position, and joint preparation.

Excessive Penetration

Causes: Too much heat, typically from voltage that’s too high or travel speed that’s too slow, causes weld metal to melt through the base material. On root passes with wide groove openings, the root opening and land width determine how much heat the joint can absorb before burn-through occurs.

Cures: Reduce voltage, lower wire feed speed, and increase travel speed. If excessive penetration recurs on root passes, verify joint dimensions before welding, the root opening may need to be tightened.

Insufficient Penetration

Causes: Shallow fusion between the weld and base metals results from insufficient heat input, improper joint preparation, or a gun position that can’t reach the bottom of the groove.

Cures: Increase voltage and wire feed speed, reduce travel speed, and verify that the gun can access the root of the joint. Wire extension and arc characteristics depend on reaching the bottom of the groove — if the gun can’t get there, penetration will be inadequate regardless of parameter settings.

Selecting the Right Self-Shielded Flux-Cored Gun

Self-shielded FCAW-S applications require a gun purpose-built for the process, not a standard MIG gun. Higher amperages, larger wire diameters, and demanding field conditions all call for equipment designed to handle them.

Our self-shielded FCAW gun lineup includes the IronPro™ built for high-amperage structural applications where durability and heat tolerance are the priorities. 

Gun ergonomics matter too. A gun that’s too heavy or poorly balanced for the application contributes to inconsistent technique, which shows up directly in weld quality, particularly on out-of-position work where gun angle is hardest to maintain.

FCAW Welding Done Right

The defects that appear in self shielded FCAW work, slag inclusions, porosity, wormtracking, fusion problems, undercutting, penetration issues, aren’t inherent to the process. They are symptoms of parameter drift, technique gaps, or equipment that isn’t matched to the application. Operators who understand what drives each defect and how to correct it spend less time reworking welds and more time putting metal in the joint.

For more on selecting the right gun for FCAW welding applications, or to explore Bernard’s full lineup of self-shielded equipment, visit our self-shielded FCAW gun resources for product details and technical support.