The Ultimate Buyer's Guide for Purchasing Flux Cored Wire Self-shielded

By: Hobart Brothers

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Gas-shielded flux-cored arc welding (FCAW-G) produces high-quality welds through a continuously-fed tubular wire filled primarily with metallic and metallic-oxide powders. These powders function as a flux during welding, forming a protective slag over the completed weld while also removing impurities from the weld pool, resulting in a clean, discontinuity-free weld even in the presence of light rust, scale, or other surface contaminants. The slag associated with all-position FCAW-G wires supports the molten weld metal, allowing for good bead contour and relatively high deposition rates when welding in less-than-ideal positions.

FCAW-G wires resemble self-shielded flux-cored arc welding (FCAW-S) wires in construction but do not provide enough of an inert atmosphere during welding to protect the weld. Hence, an external shielding gas is essential when utilizing FCAW-G wires to ensure weld quality. Commonly used gases include 100 percent carbon dioxide or 75-85 percent argon/carbon dioxide blends, depending on the wire and desired welding properties. While using an external gas might seem cumbersome, it endows FCAW-G wires with significantly enhanced usability compared to FCAW-S wires.

The American Welding Society (AWS) A3.0M Standard Welding Terms and Definitions describes usability as the measure of how easy it is to apply a welding filler metal to achieve a sound weld. Some factors influencing 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 conform 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. Example classifications under these specifications include E70T-12C and E81T1-Ni1MJ H8, respectively.

The numbers 12 and 1 that follow the T in these classifications serve as usability designators, indicating general categories of electrodes that share similar flux or core components and usability characteristics. However, it's essential to remember that wire classifications facilitate comparison between different wires while allowing manufacturers to create unique products. Variability in usability among products, even those with similar classifications, can significantly affect the selection process.

Cross sections of welds made (left to right) with T-1, T-9, and T-12 wires exemplify the subtle differences in usability, including 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 usability designators of 1, 9, or 12 (i.e., T-1, T-9, and T-12 wires) dominate the industry.

These wires all contain a rutile-based (titanium dioxide) slag, crucial for their formation and performance. Although they generally offer superior usability and welding characteristics, distinctions exist in the chemical and mechanical property requirements among these mild-steel usability designators.

A useful guideline to consider is that wires with better mechanical properties might differ significantly in their welding behavior. In essence, the elements and compounds formulated to ensure high-performance welds may not always optimize usability characteristics. Manufacturers consistently seek ways to enhance the balance between mechanical properties and usability in FCAW-G wires by developing innovative formulation techniques.

Mild Steel T-1 Wires

T-1 wires offer moderate toughness (a minimum of 20 ft-lbs at 0 degrees Fahrenheit) but may not provide the highest toughness in comparison to other usability designators. Therefore, they typically find application in less critical environments. T-1 wires excel when manufacturers formulate products for stable arc characteristics, smooth transfer, small globule size, effective wetting action, and robust slag release, enabling welding over rust and scale.

Mild Steel T-9 Wires

Compared to T-1 and T-12 wires, T-9 wires prevail in the industry due to their balance of welding properties and mechanical characteristics. T-9 wires fulfill the same chemical composition requirements as T-1 wires but impose stricter toughness requirements (minimum of 20 ft-lbs at -20 degrees Fahrenheit), making them suitable for a broader array of applications without compromising performance.

Mild Steel T-12 Wires

While T-12 wires meet the same toughness requirements as T-9 wires, they have limited tensile strength (70 to 90 vs. 70 to 95 KSI) and manganese windows (1.60 vs. 1.75). The demand for T-12 arose from ASME BPV (American Society of Mechanical Engineers Boiler and Pressure Vessel Code) committees, particularly for welding procedure specifications. The restriction on tensile strength and manganese content aims to minimize cracking risks when welding thick materials.

T-12 wires meet all T-1 and T-9 requirements while typically featuring enhanced properties marketed for critical applications. They are designed to provide superior mechanical properties, particularly toughness, that surpass baseline requirements. Some are engineered with low impurity levels to optimize performance following post-weld heat treatment.

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Numerous T-12 wires also come designed with comparatively low-hydrogen deposits, often maintaining a maximum diffusible hydrogen content of 8.0 ml/100g, with some as low as 4.0 ml/100g. Fluoride compounds are frequently employed to regulate hydrogen within the weld deposit. However, it is noteworthy that many of these compounds can adversely affect usability, leading to increased spatter and a harsher arc. Effective manufacturing control can minimize the impact of harmful fluorides, but it may also influence wire costs. To optimize welding performance while keeping diffusible hydrogen levels low, a combination of production strategies is often used.

FCAW-G Wires with Basic Slag

Wires designated T-5 provide superior toughness among FCAW-G classifications because of their basic slag systems, formulated using elements like calcium (lime) fluoride that are chemically basic. This fundamental slag system introduces less oxygen into the weld metal, crucial for achieving optimal toughness. By contrast, rutile-based T-1, T-9, and T-12 wires consist mainly of slightly acidic compounds, which can contribute more oxygen to the weld metal. Maintaining oxygen levels below 300 ppm in weld bonds is vital since higher levels can compromise mechanical properties.

T-5 wires also inherently ensure low diffusible-hydrogen levels due to the high fluoride content that binds hydrogen, reducing stresses that could lead to cracking. They also boast high arc force, enhancing penetration into base materials, which is especially beneficial when welding thicker materials or conducting repairs.

Consequently, T-5 wires are favored for demanding applications in heavy machinery and offshore fabrication. The fluorides found in these wires can detract from smooth welding properties, unlike T-1, T-9, and T-12, which offer distinct performances.

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Although many T-5 wires are restricted to flat and horizontal positions, some are formulated to operate under DCEN polarity instead of DCEP, aiding operators in out-of-position welding. Always check the manufacturer’s data sheets for recommended polarity and positional capabilities of a T-5 product.

Conclusion

It is crucial to assess the specific demands of each application before selecting the appropriate FCAW-G wire. For more critical applications that require robust mechanical properties, usability may need to be compromised to meet necessary codes or application standards, potentially resulting in less smooth welds. However, if toughness and certain mechanical properties aren't essential, a broader selection of wire options and varied arc characteristics are available.

Remember that not all wires are equal. When choosing a filler metal, experiment with several wires of identical or varying classifications. Different manufacturers employ diverse strategies to balance usability and overall wire properties, and every welding operator has unique preferences concerning weldability traits. Lastly, don't forget the age-old saying: you get what you pay for, as the best wire choice may not always be the cheapest.

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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