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Aug 29, 2017

What to Know: Usability and Gas-Shielded Flux-Cored Wires


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 slag’s 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 wire’s 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 Engineer’s 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).  

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.

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 manufacturer’s 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 wire’s 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:2015, Specification for Carbon Steel Electrodes for Flux-Cored Arc Welding, paragraph B7. Miami, Fla.: American Welding Society.