7018 welding electrodes, a staple in structural steel welding for their high strength (70,000 psi tensile strength) and low-hydrogen properties, are primarily designed for direct current (DC) use-specifically DC reverse polarity (DCEP: Direct Current Electrode Positive). While technically possible to use with alternating current (AC) in limited cases, AC is not recommended due to significant drawbacks in weld quality and arc stability.
Why DC is Preferred for 7018
The key reason lies in the 7018's low-hydrogen iron powder type (E7018) flux coating. This coating is formulated to minimize hydrogen in the weld metal, critical for preventing cold cracking in high-strength steels. To achieve this, the electrode requires a stable, concentrated arc to:
Maintain a protective atmosphere: The flux coating releases gases (like CO₂) and forms a slag layer to shield the molten weld pool from atmospheric hydrogen, oxygen, and nitrogen. A stable DC arc ensures consistent gas release and slag formation, whereas AC's alternating current disrupts this balance, increasing the risk of porosity or hydrogen pickup.
Ensure proper fusion: DC reverse polarity (DCEP) concentrates heat at the electrode tip, allowing the flux to melt uniformly and the electrode core to deposit filler metal smoothly. This creates deeper penetration and better fusion with the base metal-essential for structural welds. AC, with its fluctuating current, causes erratic arc behavior, leading to uneven penetration, spatter, or incomplete fusion.
Control hydrogen levels: Low-hydrogen electrodes like 7018 are sensitive to moisture (a source of hydrogen). A stable DC arc prevents excessive sputtering, which can break the protective slag layer and expose the weld pool to moisture in the air, negating the "low-hydrogen" benefit.
Can 7018 Be Used with AC?
In theory, some modern 7018 variants (often labeled "AC/DC" or with modified flux) may work with AC, but performance suffers. AC's alternating current causes the arc to extinguish and reignite 120 times per second (in 60Hz systems), leading to:
Arc instability: Frequent arc outages disrupt heat input, resulting in uneven bead shape and poor slag coverage.
Increased porosity: Interrupted shielding allows atmospheric gases to enter the weld pool.
Higher hydrogen pickup: Moisture absorption due to unstable flux activation raises the risk of cold cracking, especially in thick sections or high-carbon steels.
Thus, AC is only advisable for non-critical, thin-gauge applications where DC equipment is unavailable-and even then, weld quality is compromised.
DC Polarity for 7018: Reverse (DCEP) is Key
For optimal results, 7018 requires DC reverse polarity (electrode positive, workpiece negative). This polarity:
Focuses heat on the electrode, ensuring the flux melts properly to form a protective slag.
Enhances arc force, driving filler metal into the joint for strong, defect-free fusion.
Minimizes spatter, keeping the weld clean and reducing post-weld cleanup.
Conclusion
7018 electrodes are not designed for AC use; they perform best with DC reverse polarity (DCEP). While AC may work in emergencies, it risks arc instability, porosity, and cold cracking-critical flaws in structural welding. For reliable, code-compliant results (e.g., AWS D1.1 structural steel standards), DC is the only recommended power source for 7018.
