In short: Yes, you can run 7018 welding rods on AC (alternating current) in some cases, but it is not ideal, and performance will be limited compared to DC (direct current). The 7018 is primarily designed for DC use, and its behavior on AC depends on factors like rod design, power source capabilities, and welding goals. Below is a detailed breakdown.
Why 7018 Is "DC-First"
The 7018 is a low-hydrogen, iron powder electrode (the "18" in its classification indicates low hydrogen, while "70" refers to its 70,000 psi tensile strength). Its coating is formulated to work best with DC reverse polarity (DCRP), where the electrode is positive and the workpiece is negative. This polarity:
Stabilizes the arc, ensuring smooth metal transfer.
Generates enough heat in the electrode to melt the coating evenly, releasing shielding gases (like carbon dioxide) that protect the weld pool from contamination.
Minimizes spatter and produces a clean, consistent bead.
AC, by contrast, alternates between positive and negative cycles (60 times per second in North America). This fluctuation disrupts the 7018's arc stability and coating performance-issues that make AC a secondary option.
When 7018 Can Work on AC
While not preferred, 7018 can function on AC under specific conditions:
Using "AC-rated" 7018 rods
Some manufacturers produce 7018 variants explicitly labeled for AC use (e.g., "7018-AC" or "7018AC"). These rods have modified coatings with additives (like more iron powder or potassium compounds) that improve arc stability during AC's polarity shifts. They are less common than standard 7018 but exist for situations where only AC power is available.
With a high-quality AC power source
Older or basic AC welders may struggle to maintain a steady arc with 7018, but modern AC machines with features like arc force control or stabilized output can reduce fluctuations. This helps counteract AC's natural tendency to weaken the arc during polarity reversals.
For light, non-critical work
AC can work for simple, low-stress welds (e.g., repairs on thin steel, non-structural joints) where perfect bead appearance or maximum strength isn't essential. The weld may have more spatter or uneven penetration, but it will hold under moderate loads.
Problems with Running 7018 on AC
Even in the best AC scenarios, 7018 performance suffers compared to DC. Key issues include:
Poor arc stability: AC's polarity reversal causes the arc to "flicker" or extinguish momentarily, leading to irregular metal transfer. This results in spatter, uneven bead shape, and gaps in the weld.
Incomplete shielding: The coating may not melt evenly, reducing the release of shielding gases (critical for low-hydrogen rods to prevent porosity). Moisture or contaminants can enter the weld pool, weakening the joint.
Reduced penetration: AC typically delivers less focused heat than DC, especially on thicker materials. Welds may lack fusion with the base metal, a major flaw in structural or high-strength applications.
Hydrogen risk: While 7018 is low-hydrogen, unstable arcs on AC can cause incomplete combustion of the coating, trapping more hydrogen in the weld. This increases the risk of cracking, defeating the rod's primary advantage.
When to Avoid 7018 on AC
AC is not suitable for 7018 in these cases:
Critical applications: Structural welding (bridges, buildings), pressure vessels, or machinery (where weld failure could be dangerous) requires DC. Codes like AWS D1.1 explicitly recommend DC for 7018 in certified work.
Thick materials: AC's weaker heat input struggles to penetrate steel over 1/4 inch thick, leading to cold lap or incomplete fusion.
High-quality standards: If you need smooth, clean beads (for appearance or to avoid post-weld grinding) or maximum tensile strength, DC is mandatory.
Alternatives If You Only Have AC
If DC isn't available but you need a low-hydrogen, strong weld, consider these options instead of forcing 7018 on AC:
AC-specific low-hydrogen rods: Some electrodes (e.g., 7024, though it's not low-hydrogen, or specialized AC-rated low-hydrogen rods like 8018-AC) are designed for AC. They sacrifice some strength but offer better stability.
Use a different rod type: For non-critical AC work, 6013 or 6011 rods (AC-friendly) are easier to use, though they don't match 7018's strength.
Final Verdict
While 7018 can technically run on AC with the right equipment and for light tasks, it's a compromise. The rod's design prioritizes DC (especially DCRP) for stability, penetration, and low-hydrogen performance. For reliable, high-quality, or critical welds, always use 7018 on DC. Reserve AC use for temporary, non-structural jobs-and expect some trade-offs in weld quality.
