1. Stress Relief Annealing (Most Common)
Process parameters: Heat the HAZ (and the entire welded joint) to 550–620°C (avoid exceeding 620°C to prevent over-softening of the base metal), hold at this temperature for 1–2 hours per 25mm of plate thickness, then cool slowly (cooling rate ≤150°C/h) to room temperature.
Effects on HAZ: Reduces welding residual stress by 60–80%, eliminates brittle microstructures (e.g., martensite) in the HAZ, and improves ductility-avoiding cracking under load or environmental stress.
2. Normalization (For Severe HAZ Damage)
Process parameters: Heat the joint (including HAZ) to 880–920°C (above Q355NH's Ac3 transformation temperature), hold for 0.5–1 hour to ensure uniform austenitization, then cool in air (faster than annealing but slower than quenching).
Effects on HAZ: Refines coarsened grains in the HAZ, restores a uniform ferrite-pearlite microstructure, and balances strength and toughness-though it may slightly reduce yield strength (controllable within acceptable ranges).
3. Post-Weld Tempering (Supplemental to Stress Relief)
Process parameters: After stress relief annealing, reheat the HAZ to 400–500°C, hold for 1 hour, then cool in air.
Effects on HAZ: Further softens brittle phases, stabilizes the microstructure, and enhances long-term dimensional stability-critical for structures exposed to cyclic loads or temperature fluctuations.
Key Notes
Not all Q355NH welds need PWHT: For thin plates (<12mm) or non-load-bearing structures (e.g., decorative parts), PWHT may be omitted if welding parameters are well-controlled (low heat input, matching consumables).
Avoid overheating: Exceeding 650°C will destroy Q355NH's alloy distribution, further weakening the HAZ's weather resistance.
