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Getting to Know Flux-core Wire

Flux-cored wire diagram

Flux-cored wires are available in self-shielded and gas-shielded varieties for welding base materials, including mild steel, low-alloy steel, stainless steel, and nickel alloys. This graphic details their operation.

No single filler metal is suitable for every job. The mechanical and chemical properties of the base material, the required welding position, the available equipment, and the welding operator's skill set, among other considerations, all factor in making the best selection.

When you are seeking to increase productivity through higher deposition rates, flux-cored wires often are a viable option. These wires are known for high performance and weld quality in industries ranging from general fabrication and manufacturing to construction, offshore, and shipbuilding.

Flux-cored wires have unique welding characteristics and requirements, advantages, and limitations. Knowing these can help you determine whether they are the right choice.

Flux Core Wire Uses, Types, and Characteristics

Flux-cored wire comes in self-shielded and gas-shielded varieties, some of which are designated for all-position welding (for example, American Welding Society [AWS] E71T-1C), or for flat and horizontal applications (E70T-1C). Both types are gas-shielded and produce a slag that helps protect the liquid weld metal during cooling, but this slag must be removed after welding and between passes.

These wires are available for various base materials, including mild steel, low-alloy steel, stainless steel, and nickel alloys. For steel welding, they are available in tensile strengths of 70 KSI for mild steel, as well as 80 to 120 KSI and higher for welding high-strength, low-alloy steel.

Self-shielded flux-cored wires (FCAW-S) produce their own shielding gas when the arc initiates, eliminating the need for an external gas cylinder and making them suitable for use in portable and remote applications. These wires tend to produce slightly higher levels of smoke and spatter than gas-shielded flux-cored wires, but many classifications offer good impact toughness, even at lower temperatures.

Self-shielded flux-cored wires are often employed in lieu of shielded metal arc welding (SMAW) electrodes as a means to increase productivity since they are continuously fed and don't require repeated downtime for changeover. They are generally available in diameters from 0.035 to 7/64 in.

Gas-shielded flux-cored wires (FCAW-G) require an external shielding gas of either 100 percent CO2 or a blend of argon and CO2. They tend to have a higher operator appeal, meaning they are easier to control and to use to produce an aesthetically pleasing weld. They typically are used in a shop setting. When used outside, FCAW-G wires may require a barrier, either a tent or some other means to protect the weld pool, so that the shielding gas doesn't blow away.

Typically available in diameters from 0.035 to 7/64in., these wires can be used instead of solid wires to enhance productivity through higher deposition rates; welding operators can add more weld metal to a joint in less time, especially in out-of-position applications.

Pound for pound, FCAW-G wires typically are less expensive than FCAW-S varieties, which contain additional core materials and alloying elements, but do not require the additional expense for the shielding gas. FCAW-S wires also have a lower efficiency, about 65 percent compared to FCAW-G, which have 75 to 85 percent efficiency. These efficiencies also are lower than solid wires', because a portion of the wire is lost in the slag-forming agents that are discarded during the welding process. These factors should be considered when choosing a welding process.

working with flux-cored wires in welding

Gaining the best results from flux-cored wires is as much a matter of selecting the right one for the job as it is having the proper equipment and training to weld with them. It’s important to know the proper welding parameters and techniques to gain the best results.

Both types of wires are classified by usability designators defined by AWS—a number from 1 to 14 or the letter G or GS, which indicates the wire's polarity and operating characteristics.

FCAW-G Shielding Gases

FCAW-G wires have varying shielding gas requirements, and each type provides specific characteristics. Wires with a "C" designation in their AWS classification — for example, E70T-1C H8 — operate with only CO2. Those with a "M" designation, such as E71T-1M, require a shielding gas mix of CO2 and argon, usually a 75/25 percent balance.

Some wires are considered dual-gas and have a "C/M" designations that allows them to be used with both types of gases.

Care should be taken when considering a shielding gas change. While the wire may operate with either shielding gas, altering the gas is considered an essential variable change that can require new welding procedures and testing before use.

Wires operating with 100 percent CO2 offer more weld penetration, but also tend to create more spatter, whereas wires for mixed gases have reduced spatter and smoke, along with a smoother bead appearance. Once again, it is worthwhile to weigh out cost when choosing between the two types of classifications and shielding gases. CO2 is less expensive, but will likely generate a weld that requires more time and labor to remove spatter. Conversely, mixed gases are more expensive, but the welds need less cleaning after completion.

Equipment Requirements for Flux Core Wire

Gaining the best results from flux-cored wires is as much a matter of selecting the right wire for the job as it is having the proper equipment and training to weld with them. Both FCAW-S and FCAW-G operate with a standard constant-voltage (CV) power source set for straight polarity (direct-current electrode negative, or DCEN) or reverse polarity (direct-current electrode positive, or DCEP), depending on the wire formulation.

A common mistake made when setting up equipment to operate FCAW-S is selecting the wrong polarity on the welding power supply. While many wire welding processes operate using DCEP, the majority of FCAW-S wires are designed to operate on DCEN. Always consult the filler metal manufacturer’s recommendations for operation.

FCAW-S wires often are paired with a voltage-sensing wire feeder. The welder can set the voltage at the power source but then control the wire feed speed (and therefore amperage) at the feeder. This feature is helpful on large job sites; fewer trips to the power source allows for more welding time. In the event that the welder varies the contact-tip-to-work distance (CTWD), voltage-sensing wire feeders also can help regulate the subsequent voltage variations.

Both wire types require V-knurled drive rolls in the wire feeder to provide smooth wire feeding and consistent weld quality. Flux-cored wire is softer than solid wire and can easily be deformed or crushed if incorrect drive rolls are used.

Proper Technique

During the welding process, welders should employ a drag technique. A good drag angle for flat, horizontal, and overhead positions is between 15 and 45 degrees. For vertical-up welds, a gun angle of 5 to 15 degrees works well. A steady and fast enough travel speed keeps the weld pool from getting ahead of the arc, which could lead to slag inclusions.

working with flux-cored wires in welding

For applications in which companies seek to gain productivity through higher deposition rates, flux-cored wires often are a viable option. These wires are available in self-shielded and gas-shielded varieties for use in the field and shop.

Welders using flux-cored wires should be sure to use the correct stick-out or electrode extension; self-shielded wires are particularly sensitive to this variable. Depending on the wire diameter and type, the manufacturer-recommended stick-out could exceed 2 in.; check the requirements for each wire.

Improper stick-out can cause issues such as burnback, worm tracking, incomplete slag coverage, and difficult slag removal. Stick-out also is crucial because it provides a level of resistive wire heating, which helps increase the deposition rate. The increase in resistance allows less current to pass through the arc, permitting higher wire feed speeds to be used, and thus increasing the deposition rate.

Storage Requirements

As with any filler metal, it's important to store FCAW-G and FCAW-S wires in a clean, dry area. Damage from moisture or other contaminants can lead to poor weld quality and likely void the product warranty.

It's a good idea to maintain the same temperature in the storage area as in the welding area. Moving wires from a cold storage room to a warmer weld cell may cause condensation to form on the wires. This condensation can cause the wire to rust and potentially cause porosity and wire feeding problems. If maintaining the same temperature in both spaces is not feasible, allow the wire to acclimate to the temperature of the weld cell for 24 hours before welding.

It also is important to keep the wires in their original vacuum- or hermetically sealed packages until ready for use. For wire already in use, take precaution to remove the spool from the wire feeder, place it in a plastic bag, and store it properly. Remember to remove any wire still inside of the gun if that equipment won't be used for a long period of time. Flux-cored wires, especially in humid climates, can rust inside the gun, requiring the removal and replacement of the entire welding gun liner.

Final Thoughts on Flux Core Wire

Training is key when using any type of filler metal. Welders new to using flux-cored wires may need additional training and/or certifications for their particular application, for example, converting from an SMAW electrode to an FCAW-S wire for a structural application.

Another example would be converting from an FCAW-G wire to an FCAW-S. While both wires are known as flux-cored, they are in different AWS classifications, and this change could potentially require additional testing and qualification.

Always follow the required work procedures and operating parameters for a given application and flux-cored wire. Combined, proper technique and operation can lead to better weld quality and productivity, while also reducing costly downtime.

About the Author

Jonathan Will

Product Manager

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Troy, OH 45373

937-332-4000