By: Hobart Brothers

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Gas-shielded flux-cored arc welding (FCAW-G) produces high-quality welds by relying on a continuously-fed tubular wire filled primarily with metallic and metallic-oxide powders. These powders act as a flux during welding to form a protective slag over the completed weld. In addition, the slag removes impurities from the weld pool to generate a discontinuity-free weld even in the presence of light rust, scale or other surface contaminants. The slag of all-position FCAW-G wires supports the molten weld metal, enabling good bead contour and relatively high deposition rates when welding out-of-position.

FCAW-G wires are similar in construction to self-shielded flux-cored arc welding (FCAW-S) wires, but do not generate enough of an inert atmosphere during welding to fully protect the weld. For this reason, it is necessary to use an external shielding gas with FCAW-G wires to achieve quality welds. Either 100 percent carbon dioxide or 75-85 percent argon/carbon dioxide blends are used, depending on the specific wire and desired welding characteristics. While the use of an external shielding gas may seem to be an inconvenience, it helps FCAW-G wires offer much greater usability compared to FCAW-S wires.

The American Welding Society (AWS) A3.0M Standard Welding Terms and Definitions defines usability as a measure of the relative ease of application of a welding filler metal to make a sound weld. Some factors that contribute to overall usability include:

  Arc force and resulting penetration
  Puddle fluidity and wetting action
  Amount of spatter
  Ease of slag removal
  Bead appearance and contour obtained via proper techniques

Most steel FCAW-G wires are classified to one of two AWS specifications: AWS A5.20  Specification for Carbon Steel Electrodes for Flux-Cored Arc Welding or AWS A5.29  Specification for Low-Alloy Steel Electrodes for Flux-Cored Arc Welding. Some example classifications in these specifications are E70T-12C and E81T1-Ni1MJ H8, respectively.

The 12 and 1 following the T in these example classifications are usability designators. 

Usability designators indicate a general grouping of electrodes that contain similar flux or core components and have similar usability characteristics. (Ref.1) But remember, wire classifications exist to allow comparison between wires, as well as to provide manufacturers with the flexibility to offer unique products. Different products, even with similar or identical classifications, may have noticeable variances in their usability that could influence the selection process.

Cross sections of welds made (left to right) with T-1, T-9, and T-12 wires similar yet different usability designators and formulas will often show minor differences in usability: bead appearance, contour, penetration profiles, and arc characteristics.

Mild steel FCAW-G wires with rutile slag
Mild steel FCAW-G wires classified under AWS A5.20 with the 1, 9 or 12 usability designators (T-1, T-9 and T-12 wires) are the most common in industry.  

All T-1, T-9 and T-12 wires contain a rutile-based (titanium dioxide) slag, which is primarily responsible for the slags formation and performance. Since all three wires have a rutile slag base, they generally offer very good usability/welding characteristics. However, there are differences in the chemical and mechanical property requirements between these mild-steel usability designators. 

Typically, a rule of thumb is: the better mechanical properties a wire offers, the greater the difference in the way it welds. In short, the elements and compounds used in the formulations to ensure a higher-performance weld do not usually help optimize the wires usability characteristics. This consideration does not mean performance will be unacceptable, just different. To offset and improve the balance between mechanical properties and the usability of a FCAW-G wire, filler metal manufacturers constantly look at ways to develop and apply new formulation techniques. 

The following sections provide a closer look at the FCAW-G wires available with rutilebased slag systems. 

Mild steel T-1 wires These wires provide some degree of toughness (a minimum of 20 ft-lbs at 0 degrees Fahrenheit), but compared to other usability designators, they do not offer the highest toughness. As a result, they are often used for less critical applications. However, T-1 wires offer very good flexibility when the filler metal manufacturer is formulating a product to offer a stable arc, smooth transfer, small globule size, good wetting action, the ability to weld over rust and scale and good slag release. 

Cross-sections of welds made with a rutile (left) and basic (right) slag types. Note the increased penetration and convexity of the weld made with a T-5 FCAW-G wire.

Mild steel T-9 wires
Versus T-1 only and T-12 wires, T-9 wires are the most common in the industry. T-9 wires tend to offer an optimal balance of welding characteristics and mechanical properties. A T-9 wire offers the same chemical composition requirements as T-1 wires, but has more stringent toughness requirements (offering a minimum of 20 ft-lbs at -20 degrees Fahrenheit). As a result, T-9 wires are also technically T-1 wires but are capable of being used in a wider range of applications (since toughness at -20 degrees Fahrenheit translates to equal or greater toughness at 0 degrees Fahrenheit).

Mild steel T-12 wires While these wires have the same toughness requirements as T-9 wires, they offer restricted tensile strength (70 to 90 vs. 70 to 95 KSI) and manganese windows (1.60 vs. 1.75). Demand for the T-12 designator stemmed from the ASME BPV (American Society of Mechanical Engineers Boiler and Pressure Vessel Code) committees, specifically where the A-number 1 chemical composition designator is to be applied to welding procedure specifications. Here, restricted tensile strength and manganese content are intended to help reduce the risk of cracking due to poor ductility when welding thick materials.

T-12 wires conform to all requirements of T-1 and T-9 wires but are also typically formulated to offer best-in-class properties and marketed for use in critical applications. They are often formulated to offer mechanical properties, specifically toughness, that are above and beyond what is required. Some are specially formulated with low impurity contents to maximize mechanical performance following post-weld heat treatment (stress-relief).  

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Many T-12 wires are also formulated to provide comparatively low-hydrogen deposits often with a maximum diffusible hydrogen content of 8.0 ml/100g, and sometimes with a maximum as low as 4.0 ml/100g. The use of fluoride compounds is a popular method to control hydrogen in the weld deposit. However, many of these fluoride compounds are detrimental to usability, typically increasing spatter and producing a harsher arc. Implementing tight manufacturing controls can help avoid the overuse of fluorides that are harmful to usability, but these controls can quickly influence wire costs. To optimize welding performance while keeping diffusible hydrogen low, a combination of methods is often used. Seamless FCAW-G wires are one option, as the seamless design and production removes virtually all moisture and prevents its reabsorption.

0.15 seconds of arc footage of a standard T-5 wire shows the transfer of a large globule, as well as some spatter.Photographs of the welding arcs using (left to right) T-1, T-9, and T-12. Although very similar, subtle differences can be observed in the width of the arc cone and globule size.

FCAW-G wires with basic slag
Wires bearing a 5 designator provide the best toughness among FCAW-G classifications. This is because T-5 wires have a basic slag system, meaning the slag is formed using elements in this case, calcium (lime) fluoride which are chemically basic. This basic slag system inherently contributes less oxygen to the weld metal. Rutile-based T-1, T-9 and T-12 wires, by comparison, are comprised more so of slightly acidic compounds that contribute more oxygen to the weld metal. Oxygen levels above the very low level of 300 ppm in the weld metal are detrimental, so minimizing additional oxygen contribution is paramount to gaining optimal toughness. 

T-5 wires also inherently offer low-weld deposit diffusible-hydrogen levels, as their high fluoride content ties up the hydrogen, preventing it from contributing to stresses in the microstructure that could lead to cracking. They also provide high arc force, which greatly assists penetration into the base material. Both benefits are especially desirable when welding thick materials or performing repairs, as they ensure complete root fusion in limited access joints, and help to minimize the risk of hydrogen-induced cracking. 

For these reason, T-5 wires are most commonly used for demanding applications in the heavy equipment and offshore fabrication industries. Again, the high amount of fluorides these wires contain are quite detrimental to to achieving smooth welding characteristics. Compared to T-1,9 and 12, the difference between T-5 wires is night and day, as most provide a much more globular transfer, harsher arc, increased spatter and a more convex bead profile with noticeable solidification lines on the weld face.

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Many T-5 wires are limited to the flat and horizontal position. However, some are formulated to operate in DCEN polarity as opposed to DCEP, which can assist operators with out-of-position welding. Always consult the manufacturers data sheets to determine the recommended polarity and positional capabilities for a T-5 product.

The bead appearances of welds made with a rutile (top) and basic (bottom) slag types. Note the increased spatter and freeze lines present on the weld made with the T-5 wire.

Conclusion
Always make an honest assessment of the demands of each application before selecting which FCAW-G wire to use. More critical applications, with demanding mechanical properties, may have to sacrifice usability to meet code or other application requirements the wires needed may simply not weld as smoothly. However, if toughness or other specific mechanical properties are not required, there will be more wire options, and a wider range of arc characteristics, available. 

Also remember that not all wires are created equal. When selecting a filler metal, try different wires of both the same and different classifications. Each manufacturer has different approaches to balancing usability with the wires overall properties, and every welding operator has his or her own preference of weldability characteristics. And remember the famous adage: You get what you pay for, as the best wire option is not always the most affordable. 

Reference 1: AWS A5.20/A5.20M:, Specification for Carbon Steel Electrodes for Flux-Cored Arc Welding, paragraph B7. Miami, Fla.: American Welding Society.

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Flux Core Wire Types

Flux-cored arc welding (FCAW) is a wire-fed process like gas metal arc welding (GMAW/MIG). However, what makes FCAW a unique process is the filler metal. FCAW uses a tubular wire that generates slag during welding. While removing the slag after welding can be inconvenient, the slag is critical for the process to achieve good weld quality and provide unique benefits compared to MIG and other welding processes.

When it is time to select a flux-cored wire for your application, you may find that there is an extensive list of wires to choose from. Selecting the best wire&#;one that is easy-to-use and produces a high-quality weld deposit with suitable mechanical and chemical properties&#;requires a basic understanding of the basic wire types and capabilities.

Primary FCAW Wire Types: Self-Shielded and Gas-Shielded

In addition to producing a slag during welding that helps to remove base metal impurities, flux-cored wires generate gases that serve to displace the atmosphere around the molten weld metal. This displacement protects the weld metal from atmospheric gases such as oxygen and nitrogen that can lead to porosity or harm mechanical properties.

Self-Shielded Flux Cored Welding (FCAW-S)

Self-shielded flux-cored wires produce sufficient gaseous shielding, so no assistance (external shielding gas) is required. Self-shielded wires are suitable even in breezy conditions, such as outdoor construction and repairs. Remember that using these wires indoors can be challenging from an environmental health and safety perspective, even when strong ventilation is used.

Some self-shielded wires offer sufficient weld toughness&#;an ability to absorb energy before fracture&#;that makes them well-suited for structural fabrication in seismic areas and demand-critical welds on bridges and similar structures.

Of course, flux-cored wires also exist for &#;light-duty&#; applications around the house, garage, or farm and ranch. While these wires may not be as tough, they often provide more-than-adequate mechanical properties and are easier to use and available in diameters that complement commercial and light-industrial equipment. In addition to the local welding supply, it is often possible to find these wires at hardware and farm supply stores.

Gas-Shielded Flux Cored (FCAW-G)

Not all flux-cored wires are self-shielded, meaning that an external shielding gas (supplied in cylinders similar to those used when MIG welding) is required to achieve acceptable weld quality; these gas-shielded wires are not capable of providing sufficient atmospheric displacement on their own. The process using these wires is often abbreviated as FCAW-G (for gas-shielded).

The trade-off to needed external shielding gas is that the gas-shielded wires are often much more user friendly than the self-shielded wires while still providing improved tolerance to poor base metal conditions&#;rust, weldable primer, and mill scale&#;than MIG welding. FCAW-G is especially popular in the railcar, shipbuilding, and heavy equipment fabrication industries.

Common shielding gases for gas-shielded include 100% carbon dioxide and 75% argon/25% carbon dioxide. 100% carbon dioxide is a lower-cost option that typically offers improved base metal penetration, while the argon/carbon dioxide shielding gases usually provide a smoother arc and reduced weld spatter.

In-Position & All-Position Flux Cored Wires

In certain applications, it is possible to position the work (by hand, sometimes using equipment) so that the weld joint is roughly parallel with the ground. This is known as welding &#;in position.&#; Since the effect of gravity is not as detrimental, it is often possible to weld at higher amperages. This translates to improved deposition rates, welding travel speeds, and in many cases, improved welding productivity.

Some flux-cored wires are limited to welding in position only. The slag of these wires tends to freeze slower than those wires that are capable of welding in the flat, horizontal, vertical, and overhead positions, but a slow freezing slag often provides a very smooth weld bead contour and good penetration. Typically, these &#;flat and horizontal only&#; wires are available in larger diameters to allow the use of very high currents. A 3/32&#; wire, for example, is typically used between 350 and 500 amps!

If work cannot be positioned, resulting in weld joints in the vertical and overhead positions, an &#;all-position&#; flux cored wire must be used. Here, the slag is designed to freeze quickly to support the molten metal and prevent it from dripping or sagging when reasonable welding parameters are used.

Weld Deposit Composition: So Many FCAW Wires!

Some flux-cored wires are suitable for welding lower-strength carbon steels. In contrast, others are designed for welding higher-strength low alloy (HSLA) steels that derive strength and toughness from elements such as nickel, chromium, and molybdenum, among others. Specialty wire manufacturers even make flux-cored wires designed for welding stainless steels, exotic nickel-based alloys, and tool steel compositions.

Some available alloys are even designed for surfacing instead of joining so that exposed areas of base metals are more abrasion or impact resistant.

Conclusion

Flux-cored arc welding is an expansive process with many wire types: gas-shielded, self-shielded, all-position, in-position, carbon steel, HSLA steel, stainless steel, and more. If you ever feel overwhelmed by the sheer number of flux-cored wires in the welding marketplace, consider turning to AWS or CWB filler metal specifications. Often, these technical documents have electrode classification systems that help you compare products and descriptions of the intended uses of certain wire classifications.

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