1. Reduces Residual Stress (Critical for Structural Safety)
Allowing atomic diffusion in the HAZ to rearrange internal stress fields.
Softening local hard zones to release stress concentration.
This reduces the risk of delayed cracking (e.g., cold cracking in low temperatures) and prevents stress corrosion in outdoor environments-critical for Q355NH's long-term structural stability.
2. Improves Toughness (Reduces Brittleness)
Stress relief annealing: Converting brittle martensite to ductile ferrite-pearlite, increasing impact energy (e.g., from ≤27 J to ≥47 J, meeting Q355NH's standard requirement).
Normalization (for severe grain coarsening): Refining oversized grains in the HAZ into fine, uniform ferrite-pearlite, further boosting toughness and resistance to fracture under dynamic loads (e.g., wind, vibration).
3. Adjusts Strength (Balances Strength and Ductility)
Over-hardening (in the quenched zone): Brittle martensite raises hardness but reduces ductility.
Over-softening (in the over-tempered zone): Excessive heating lowers yield/tensile strength.
Stress relief annealing softens the over-hardened zone (reducing hardness by 20–30 HV) while slightly recovering strength in the over-softened zone-keeping the HAZ's yield strength (~345 MPa) and tensile strength (470–630 MPa) close to the base metal.
Normalization may slightly reduce overall strength (by 5–10%) but improves ductility (elongation increases by 2–3%), which is acceptable for non-high-load structures.
4. Enhances Ductility and Plasticity
Breaks down brittle phases and refines grains (via normalization or tempering), allowing the HAZ to deform more before fracture.
Typical effect: Elongation of the HAZ increases from ~10% (post-welding) to ~18–22% (after PWHT), matching Q355NH's base metal ductility-critical for structures needing to absorb energy (e.g., seismic resistance).
