1. Pre-Weld Preparation: Eliminate Hydrogen Sources & Optimize Base Metal
Thoroughly clean the base metal surface:
Remove oil, grease, rust, moisture, and paint from the welding zone (at least 20–30mm on both sides of the joint) using wire brushes, sandblasting, or solvent cleaning. These contaminants decompose into atomic hydrogen during welding-even thin rust (≥5μm) can double hydrogen content in the weld.Dry welding consumables (low-hydrogen materials are mandatory):
Use low-hydrogen electrodes (e.g., E7018-G) or flux-cored wires (e.g., E71T-8-Ni1) compliant with EN ISO 14341. Bake electrodes at 350–400°C for 1–2 hours (per manufacturer specs) to remove moisture, then store them in a heated electrode holder (60–100°C) to prevent reabsorbing moisture. For flux, dry it at 250–300°C for 2–3 hours before use.Preheat the base metal (critical for thick plates):
Preheat S355J2W to 80–150°C (measured 50mm from the weld groove) for plates thicker than 12mm, or when ambient temperature is below 0°C. Preheating slows the cooling rate of the weld heat-affected zone (HAZ), reducing the formation of brittle martensite and allowing hydrogen to escape early. Use oxy-acetylene torches or induction heaters for uniform heating-avoid localized overheating.
2. In-Weld Process Control: Reduce Cooling Rate & Limit Hydrogen Diffusion
Control heat input within a reasonable range:
Use moderate heat input (15–25 kJ/cm) for S355J2W. Too low heat input leads to rapid cooling (promoting martensite); too high causes grain coarsening (weakening toughness). Adjust parameters: for manual arc welding, use 120–180A current and 22–26V voltage; for MIG/MAG welding, maintain a wire feed speed of 3–5 m/min.Adopt multi-pass welding for thick plates:
For plates thicker than 20mm, use multi-pass welding instead of single-pass. Each subsequent pass reheats the previous weld and HAZ, "tempering" brittle microstructures and helping hydrogen escape. Between passes, keep the interpass temperature at 80–120°C (no lower than the preheat temperature) to avoid rapid cooling.Avoid arc strikes outside the weld groove:
Arc strikes on the base metal create small, localized hard zones (high in martensite) that act as crack initiation points. If accidental strikes occur, grind them smooth and check for microcracks with dye penetrant testing (DPT).
3. Post-Weld Treatment: Remove Hydrogen & Relieve Stress
Post-weld heat treatment (PWHT) for high-stress joints:
For critical structures (e.g., load-bearing frames, pressure vessels), perform PWHT: heat the weldment to 550–620°C, hold for 1–2 hours (per 25mm thickness), then cool slowly (≤100°C/hour) to 300°C before air cooling. PWHT relieves 60–80% of residual stress and drives out 90% of diffusible hydrogen. For non-critical joints, skip PWHT but implement a "post-heat hold."Post-heat hold (simpler alternative to PWHT):
After welding, keep the weldment at 200–250°C for 2–4 hours using insulation blankets. This "bakes out" hydrogen (accelerating its diffusion from the weld) and slows cooling, reducing martensite formation. It is especially effective for thin plates or when PWHT is impractical.Delay non-destructive testing (NDT) to allow hydrogen escape:
Wait 24–48 hours after welding before conducting NDT (e.g., ultrasonic testing, radiography) for cold cracks. This "hydrogen delay period" gives diffusible hydrogen time to escape-testing too early may miss cracks that form later.
