1. Pre-Welding: Targeted Prep to Avoid Contamination & Cracks
Surface Cleaning: Q355NH's surface forms a dense oxide film (for weather resistance), but oil, rust, or paint within 20–30 mm of the groove edge will contaminate the weld pool (causing porosity or reducing corrosion resistance). Must clean the area via wire brushing, sandblasting, or acetone wiping until metallic luster is exposed.
Mandatory Preheating: Preheating is non-negotiable to slow the weld zone's cooling rate (prevents cold cracks). The temperature depends on plate thickness and ambient conditions:
Plates ≤16 mm (ambient temp >0°C): 50–80°C;
Plates 16–32 mm: 80–120°C;
Plates >32 mm or cold environments (<0°C): 120–150°C.
Use a thermal imager to ensure uniform heating-no local cold spots.
2. Welding: Precision Parameter Control to Protect Alloy Elements
Heat Input Limit: Keep heat input ≤2.5 kJ/mm (e.g., 140–180A/22–26V for manual arc welding with E501NiCu-G electrodes; 180–240A/24–28V for GMAW). Exceeding this causes HAZ brittleness.
Interpass Temperature: During multi-layer welding, keep the temp between preheating temp and 250°C. If it drops too low, re-preheat before welding the next layer.
3. Post-Welding: Stress Relief & Performance Inspection
Selective PWHT: For plates >30 mm, or structures used in low temps (-20°C+) or high stress, do stress relief heat treatment: 550–600°C (avoid >620°C to prevent toughness loss), hold 1–2 hours, cool slowly (≤200°C/h).
Special Inspections: Beyond routine NDT (X-ray/ultrasonic for cracks), test:
Low-temperature impact toughness: Weld/HAZ samples must reach ≥34J at the design temp (e.g., -40°C for Q355NH-40);
Weather resistance: For coastal/industrial projects, do salt spray tests to ensure weld corrosion rate matches the base metal.
