1. Welding Material Selection: Prioritize "Weather-Resistant Matching"
Manual arc welding: Choose electrodes like E4303-G (GB/T 5117) or E4316-G-these have Cu-rich cores to match Q235NH's weather resistance.
Gas metal arc welding (MIG/MAG): Use solid wires such as ER49-1-G (GB/T 8110), with matching weather-resistant flux-cored wires if using FCAW.
Submerged arc welding: Match with weather-resistant flux (e.g., HJ431-G) and wire (e.g., H08MnA-G) to avoid "corrosion weak spots" in the weld.
2. Pre-Welding Preparation: Control Cleanliness & Avoid Hydrogen Induction
Remove oil, rust, paint, or moisture from the weld area (20–30mm on both sides) using wire brushes, sandblasting, or acetone-moisture/rust will increase hydrogen content in the weld, leading to cracks.
If the base metal thickness exceeds 12mm, preheat to 80–150°C (measured 50mm from the weld groove) to slow cooling, reduce internal stress, and prevent cold cracks.
3. Welding Process: Control Heat Input & Cooling Rate
Keep heat input moderate: For manual arc welding, use a current of 120–180A (dependent on electrode diameter, e.g., 3.2mm electrodes use ~120A); avoid long arc welding (it causes alloy element burnout, reducing corrosion resistance).
Control interpass temperature: Do not exceed 250°C (for multi-pass welding) to prevent overheating the weld zone, which would soften the metal and weaken mechanical properties.
Avoid forced cooling (e.g., spraying water on hot welds)-let the joint cool naturally to 50°C below before proceeding to the next pass.
4. Post-Welding Treatment: Focus on Stress Relief & Rust Layer Continuity
Stress relief annealing (if needed): For thick plates (>16mm) or high-stress components (e.g., bridges), perform post-weld heat treatment at 550–620°C (hold for 1–2 hours, cool slowly) to eliminate residual stress and reduce cracking risk.
Restore the weather-resistant rust layer: After welding, grind the weld surface smooth (remove spatter or slag) and use a weather-resistant repair paint (matching Q235NH's composition) on the weld area-this ensures the protective patina forms uniformly across the entire structure (uncoated welds may corrode faster than the base metal).
5. Quality Inspection: Emphasize Crack & Corrosion Resistance Checks
Non-destructive testing (NDT): Use ultrasonic testing (UT) or radiographic testing (RT) to check for internal cracks; use magnetic particle testing (MT) to detect surface cracks (focus on the weld root and heat-affected zone, HAZ).
Corrosion resistance verification (for critical projects): Conduct a salt spray test on weld samples to confirm the weld joint's corrosion rate is consistent with the base metal (should be ≤0.05mm/year after patina formation).
