7018 electrodes are a staple in structural welding, valued for their high strength, low hydrogen content, and ability to produce crack-resistant welds. A common question among welders is whether these electrodes can be used with AC (alternating current) power sources. The short answer is yes, but with significant limitations that affect weld quality and performance.
The Design of 7018 Electrodes and Current Compatibility
7018 electrodes are primarily engineered for DC (direct current) welding, specifically DC reverse polarity (DCRP), where the electrode is positive and the workpiece is negative. This design aligns with the electrode's core purpose: the low hydrogen flux coating requires a stable arc to burn evenly, ensuring proper shielding and minimizing hydrogen pickup in the weld pool. DC current provides this stability, as it maintains a consistent flow of electrons, preventing arc flicker or extinguishing.
AC current, by contrast, alternates direction 50 or 60 times per second, causing the arc to restart with each cycle. This instability can disrupt the flux coating's ability to generate a continuous shield. The 7018's flux is not optimized for this repeated arc interruption-without a steady arc, the flux may burn unevenly, leaving gaps in the shield that allow atmospheric gases (oxygen, nitrogen) to contaminate the weld pool. This contamination leads to defects like porosity, slag inclusions, or reduced ductility, all of which weaken the weld.
Challenges of AC Welding with 7018
Arc stability is the most critical issue when using 7018 on AC. Even with a well-tuned machine, the alternating current can cause the arc to "hunt" (flicker or jump), making it difficult to maintain a consistent weld bead. This is especially problematic on thin materials, where a wandering arc may burn through the workpiece or create uneven penetration. On thick steel, the instability can lead to incomplete fusion between passes, a dangerous flaw in structural applications.
Hydrogen control is another concern. 7018's low hydrogen classification (H4 or H8) relies on precise arc conditions to prevent moisture absorption from the atmosphere. AC's unstable arc disrupts the flux's ability to seal the weld pool, allowing moisture to enter and react with the molten metal, increasing hydrogen levels. In thick sections or high-strength steels, this raises the risk of hydrogen-induced cracking, a silent failure mode that can occur hours or days after welding.
Weld appearance is also affected. AC welding with 7018 often produces a irregular bead profile, with spatter and uneven slag coverage. The slag may not detach cleanly, requiring excessive post-weld cleaning and increasing the risk of slag inclusions if not fully removed. In industries with strict visual inspection standards-such as bridge or pressure vessel fabrication-these imperfections can lead to weld rejection.
When AC Welding with 7018 Might Be Considered
Despite these challenges, AC welding with 7018 is sometimes attempted in field settings where only AC power sources are available, such as remote construction sites or agricultural repairs. In such cases, welders may use AC if no DC equipment is accessible, prioritizing a temporary fix over ideal conditions. However, this is not recommended for critical applications, as the risk of hidden defects is too high.
Some older AC machines with "stabilizer" features or high-frequency arc starters can improve arc stability marginally. These systems help restart the arc during each AC cycle, reducing flicker. When paired with careful technique-such as maintaining a shorter arc length and slower travel speed-they may produce acceptable welds on non-critical, low-stress joints (e.g., farm equipment repairs). Even then, the welds will not match the quality of those made with DC current.
Best Practices and Alternatives
For reliable results, 7018 electrodes should always be used with DC power when possible. DC ensures a stable arc, proper flux combustion, and low hydrogen levels-critical for meeting structural standards like AWS D1.1 (Structural Welding Code - Steel). If AC is the only option, welders should take steps to mitigate risks: use a machine with arc stabilization, preheat the workpiece to 200–300°F (93–149°C) to reduce hydrogen solubility, and limit welding to thin sections (1/4 inch or less) where cracking risks are lower.
When AC must be used for thicker materials, alternative electrodes are better suited. 6013 or 6011 electrodes, designed for AC compatibility, sacrifice some strength but offer stable arcs and better slag control. While they do not match 7018's tensile strength (60,000 psi vs. 70,000 psi), they produce more reliable welds with AC power, making them safer choices for non-critical applications.
Conclusion
While 7018 electrodes can technically be used with AC current, the practice is not recommended for critical or structural welding. AC's inherent arc instability undermines the electrode's low hydrogen benefits, increases defect risks, and compromises weld strength. For temporary, low-stress repairs in AC-only environments, it may serve as a last resort, but with strict limitations.
In professional settings, DC power remains the standard for 7018 welding, ensuring the consistent, high-quality welds required for safety-critical applications. Welders should prioritize DC equipment when working with 7018, or opt for AC-compatible electrodes when AC is unavoidable-balancing practicality with the need for reliable, defect-free joints.
