1. Pre-Welding Preparation: Eliminate Hydrogen Sources and Reduce Stress
Clean the Weld Joint Area Thoroughly
Remove all contaminants within 20–30 mm of both sides of the weld seam using a wire brush, grinder, or solvent:
Rust, oxide scales, and moisture (major hydrogen sources)
Oil, grease, paint, or dirt (can decompose into hydrogen during welding)
Ensure the surface is dry and free of residues before welding.Select Low-Hydrogen Welding Materials
Use low-hydrogen electrodes (e.g., E7018-G for carbon steel weathering grades, E8018-B2 for high-strength variants) - these limit hydrogen diffusion into the weld zone.
Dry the electrodes strictly: Bake at 300–350°C for 1–2 hours before use, and store them in a heated electrode holder during welding to prevent reabsorbing moisture.
For gas-shielded welding (MIG/MAG), use high-purity shielding gas (e.g., 98% Ar + 2% O₂) with low moisture content; avoid using contaminated gas.
Preheat Thick Plates to Slow Cooling
Preheating is mandatory for plates ≥16 mm thick or for welding in low-temperature environments (<5°C):
Target preheating temperature: 100–150°C (measure with a thermocouple to ensure uniform heating of the joint area).
Keep the interpass temperature between 100–200°C during multi-pass welding to prevent rapid cooling of the heat-affected zone (HAZ), which can form hard, brittle martensite.
2. Welding Process Control: Optimize Parameters to Reduce Residual Stress
Adopt Low-Heat-Input Welding Methods
Choose processes with stable arc and low heat input to avoid overheating the HAZ:
Recommended methods: Shielded Metal Arc Welding (SMAW), Gas Metal Arc Welding (GMAW), or Flux-Cored Arc Welding (FCAW).
Avoid high-heat processes (e.g., submerged arc welding) for thin plates, as they can cause grain coarsening and increased stress.
Control Welding Parameters Precisely
Use moderate welding current and voltage (e.g., 180–220 A, 22–26 V for E7018 electrodes) to ensure full penetration without excessive heat.
Weld at a steady speed (150–250 mm/min) to prevent the weld pool from solidifying too quickly (a cause of hot cracking).
For long weld seams, use intermittent welding (skip welding) instead of continuous welding to distribute heat and reduce residual stress buildup.
Use Multi-Pass Welding for Thick Joints
For plates >20 mm thick, divide the weld into multiple thin passes instead of a single thick pass:
Each pass thickness: ≤3 mm.
Grind the surface of each pass before the next to remove slag and defects, which reduces stress concentration at the weld interface.
3. Post-Weld Treatments: Eliminate Residual Stress and Hydrogen
Hydrogen Bake-Out for High-Risk Components
For critical structures (e.g., load-bearing weathering steel signs, bridge components), perform a post-weld hydrogen bake-out immediately after welding:
Heat the entire component to 200–250°C and hold for 2–4 hours, then cool slowly in air. This accelerates hydrogen escape from the weld and HAZ, eliminating the risk of delayed cold cracking.
Stress Relief Annealing for Thick or Welded Assemblies
For components with complex welds or thick plates (>25 mm), conduct stress relief annealing:
Heat the component to 550–650°C (below the critical transformation temperature of weathering steel), hold for 1–2 hours per 25 mm thickness, then cool slowly (≤50°C/hour) to room temperature.
This process reduces residual welding stress by 60–80% and lowers the risk of lamellar tearing or stress corrosion.
Avoid Rapid Cooling After Welding
Do not quench the weld zone with water or expose it to strong winds immediately after welding. Let the component cool naturally in air to prevent thermal shock.
4. Additional Quality Control Measures
Inspect Welds for Defects Promptly
Conduct non-destructive testing (NDT) within 24–48 hours after welding (to detect delayed cold cracking):
Use ultrasonic testing (UT) for internal defects (e.g., cracks, porosity) or magnetic particle testing (MT) for surface defects.
Avoid Mechanical Damage to the Weld Zone
Do not perform heavy grinding or hammering on the weld or HAZ, as this can introduce new stress concentrations and trigger cracking.
