1. Welding Consumable Selection: Prioritize Low-Hydrogen and Low-Temperature Toughness
Low-hydrogen grade: Mandatorily use low-hydrogen electrodes/wires (e.g., SMAW: E5015-G; GMAW: ER50-G; SAW: H08Mn2NiMoA + SJ101G flux). Hydrogen content in consumables must be ≤5mL/100g (for electrodes) to avoid hydrogen embrittlement-critical for high-strength Q355NH (yield strength ≥355MPa).
Match low-temperature toughness: Consumables must have Charpy V-notch (CVN) impact values ≥27J (or higher, per design) at the service temperature (e.g., -20°C or -30°C). For example, choose Ni-alloyed wires (e.g., ER50Ni1) for -30°C environments to lower the weld's ductile-brittle transition temperature (DBTT).
Retain weather resistance: Select consumables with Cu (0.20–0.50%) and Cr (0.30–1.20%) content matching Q355NH to ensure the weld joint forms a protective rust layer (avoiding "weld corrosion priority" in low-temperature, high-humidity conditions).
2. Mandatory Preheating: Prevent Rapid Cooling and Brittle Microstructures
Preheat temperature: Base metal must be preheated to 100–150°C (measured 50mm from the weld groove). For plates ≥25mm thick or when welding in ≤-20°C, increase preheat to 150–200°C to ensure uniform heating through the section.
Preheat scope: Heat a 100–150mm wide area around the weld groove (beyond the HAZ) to avoid thermal gradients. Use induction heating or gas torches (avoid open flames directly on the surface to prevent oxidation).
Temperature monitoring: Use contact thermometers (not infrared, which is inaccurate on rusty surfaces) to ensure preheat is maintained before welding starts.
3. Strict Heat Input and Interpass Temperature Control
Heat input range: Control at 20–35kJ/cm (e.g., SMAW: current 160–220A, voltage 22–26V; GMAW: current 180–250A, voltage 23–28V). Avoid exceeding 35kJ/cm to prevent HAZ grain growth.
Interpass temperature: Maintain between 120–200°C (never below 100°C or above 250°C). Below 100°C, the weld cools too fast; above 250°C, the HAZ softens and loses strength. Use temperature stickers to monitor between passes.
4. Welding Technique: Minimize Stress and Hydrogen Trapping
Arc control: Use a short arc to reduce spatter and heat loss (critical in cold, windy environments). Avoid cold starts/stops-use arc initiation/termination tabs to prevent porosity or microcracks.
Weld sequence: Adopt "symmetric welding" (e.g., weld both sides of a butt joint alternately) to balance thermal stress. For thick plates, use multi-pass welding with small bead sizes (≤5mm per pass) to control cooling rate.
Wind and moisture protection: Weld in enclosed workspaces or use windshields (wind speed >2m/s disrupts shielding gas for GMAW/GTAW). Keep consumables dry: store electrodes in a heated oven (80–100°C) after drying (350–400°C for 1–2 hours).
5. Post-Weld Treatment: Relieve Stress and Remove Hydrogen
Slow cooling: Never force-cool welds (e.g., with water or cold air). Cover with insulation blankets to cool to ≤50°C at a rate ≤50°C/h.
Hydrogen bake-out (for critical joints): If service temperature ≤-20°C or plate thickness ≥30mm, perform bake-out at 200–250°C for 2–4 hours. This accelerates hydrogen release before it accumulates in stress zones.
Stress relief annealing (optional but recommended): For load-bearing components (e.g., bridge brackets), anneal at 550–620°C (hold 1h/25mm thickness) to reduce residual stress by 60–80%. Avoid exceeding 620°C to prevent strength loss.
6. Rigorous Post-Weld Inspection
Low-temperature impact testing: Test weld metal and HAZ specimens at the minimum service temperature (e.g., -20°C) to verify CVN toughness ≥27J.
Non-destructive testing (NDT): Conduct ultrasonic testing (UT) for internal cracks (within 24–48h post-welding, after hydrogen diffuses) and magnetic particle testing (MT) for surface cracks.
Weather resistance validation: For exposed joints, perform salt spray tests or monitor rust layer formation for 3–6 months to ensure the weld forms a uniform, dense patina with the base metal.
