The pressure-bearing welds of nuclear island main equipment face challenges of high quantity and stringent quality requirements. Traditional single-wire narrow-gap submerged arc welding (SAW) suffers from low efficiency

difficulties in heat input control

and reliance on imported welding materials. Addressing the welding demands of thick-walled low-alloy steel (SA-508Gr.3Cl.1) pressure shells in fourth-generation nuclear power high-temperature gas-cooled reactor steam generators

this study proposes a fully domestic double-wire narrow-gap submerged arc welding (DNG-SAW) process

integrating innovations in equipment

materials

and parameter optimization. A tandem dual-wire single-pool configuration was adopted

where the leading wire with direct current electrode positive (DCEP

500-650 A) ensures penetration depth

while the trailing wire with alternating current electrode positive (ACEP

470-600 A) expands the weld width

achieving a welding speed of 500~600 mm/min and 1.5~2× efficiency improvement over single-wire SAW. A dedicated low-alloy steel welding wire (H15Mn2NiMoHR) and alkaline flux (SJ16HR) were developed

exhibiting a mean Charpy impact energy of 161.7 J at -20 ℃ for deposited metal

with sulfur and phosphorus contents controlled to 0.0013% and 0.0030%

respectively

meeting low-hydrogen and high-toughness requirements. Process qualification tests demonstrated joint tensile strengths of 690-720 MPa

a mean heat-affected zone (HAZ) impact energy of 127.7 J at -20 ℃

RTNDT ≤ -20 ℃

and a microstructure of bainite with minor ferrite. Non-destructive testing yielded 100% qualification rates. This technology was successfully applied to

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3866 mm × 95 mm steam generator shell circumferential welds

enabling full-position automatic welding with dual-pass per layer

defect-free fusion

and absence of undercut or porosity. The results confirm that DNG-SAW outperforms conventional methods in efficiency

performance

and localization

providing a robust solution for nuclear thick-walled component manufacturing

with potential applications in petrochemical and heavy equipment industries.