1. Strict Control of Welding Ambient Temperature
Minimum ambient temperature limit: Generally, welding should not be performed when the ambient temperature is below -10°C (for unheated workpieces). If temperatures drop to -10°C to -20°C, the weld zone and its surrounding 100–150 mm area must be preheated to at least 50–100°C before welding.
Reason: Low ambient temperatures accelerate heat loss from the weld pool, leading to rapid solidification of the molten metal. This increases the risk of cold cracks (caused by high residual stress and hydrogen embrittlement) and coarse grain structures in the heat-affected zone (HAZ).
2. Mandatory Preheating of Workpieces
Preheating scope: Focus on the weld joint and adjacent base metal (100–200 mm from the joint).
Preheating temperature:
For plates ≤ 12 mm thick: Preheat to 50–80°C if ambient temp < 0°C;
For plates > 12 mm thick or high 拘束 joints (e.g., T-joints): Preheat to 80–120°C, even if ambient temp is slightly above 0°C.
Heating method: Use induction heating or flame heating (avoid local overheating > 200°C, which damages the base metal's microstructure).
3. Selection of Welding Consumables
Match toughness and corrosion resistance: Choose welding materials (electrodes, wires) that match S355J2WP's low-temperature toughness and weather resistance:
Electrodes: Use low-hydrogen electrodes (e.g., E7018-W, EN 1600: E46 4 B 32 H10) with a hydrogen content ≤ 10 mL/100g (to prevent hydrogen-induced cold cracks).
Solid wires: Use weather-resistant wires (e.g., ER70S-W, EN ISO 16834: G 46 4 M 21 2 MnMoNi) for MIG/MAG welding.
Avoid mismatched consumables: Do not use ordinary carbon steel consumables-they will reduce the weld joint's corrosion resistance and low-temperature toughness (may fail -20°C impact tests).
4. Welding Process Parameters Optimization
Control heat input: Maintain moderate heat input (typically 15–25 kJ/cm) to avoid:
Excessively low heat input: Rapid cooling, leading to brittle martensite in the HAZ;
Excessively high heat input: Coarse grains in the HAZ, reducing toughness.
Interpass temperature control: Keep the interpass temperature between 80–150°C (not exceeding 200°C). If the temperature drops below the preheating temperature during multi-pass welding, re-preheat before continuing.
Welding speed: Avoid overly fast welding (causes insufficient fusion) or overly slow welding (increases grain coarsening).
5. Post-Welding Treatment (Critical for Thick Plates/Joints)
Post-weld heat treatment (PWHT): For plates > 25 mm thick or high-stress joints, perform stress relief annealing at 550–620°C (hold for 1–2 hours per 25 mm thickness) to reduce residual stress and eliminate hydrogen.
Post-weld cleaning: Remove welding slag, spatter, and oxides immediately after welding. For weather resistance, the weld joint can be treated with a weathering steel passivator or artificial rust accelerator to promote uniform formation of the protective rust layer (avoids uneven corrosion between the weld and base metal).
6. Welding Operation Precautions
Prevent hydrogen sources: Keep workpieces, consumables, and welding areas dry. Low-hydrogen electrodes must be baked (350–400°C for 1–2 hours) and stored in a warm, dry container before use (to remove moisture, a major source of hydrogen).
Avoid cold drafts: Shield the weld pool from wind (use wind screens) to prevent accelerated cooling and porosity.
