1. Core Heat Treatment: Normalizing (+N) – Mandatory for Most Scenarios
Heating Temperature: 890–950°C (specifically, 900±20°C for optimal results). This temperature ensures complete austenitization of the steel, dissolving carbides and homogenizing the microstructure.
Holding Time: 15–30 minutes (adjusted based on plate thickness; thicker plates require longer holding to ensure uniform heating through the cross-section).
Cooling Method: Air cooling (not water quenching). Slow air cooling promotes the formation of a fine-grained ferrite-pearlite microstructure-this structure balances strength (yield strength ≥355 MPa, as required by Q355 grade) and low-temperature toughness, while also facilitating the formation of a dense, adherent rust layer (the key to weathering resistance).
2. Supplementary Heat Treatment: Stress Relieving – For Specific Applications
Welding (e.g., large steel structures like bridges, containers).
Cold forming (e.g., bending, stamping with high deformation).
These processes introduce residual stresses into the steel, which can accelerate local corrosion or cause deformation during service. The stress relieving process for Q355GNH is:
Heating Temperature: 550–650°C (below the Ac1 transformation temperature, to avoid altering the normalized ferrite-pearlite structure).
Holding Time: 1–2 hours (per 25 mm thickness).
Cooling Method: Slow furnace cooling (≤50°C/hour below 500°C) to minimize reintroduction of stress.
3. Heat Treatment Prohibitions: Processes to Avoid
Quenching + Tempering (Q+T): Rapid water quenching after austenitization would form hard martensite, which increases brittleness and makes the rust layer porous (reducing corrosion resistance). Tempering cannot fully restore the ideal ferrite-pearlite structure for weathering.
Overheating (>950°C): Excessive heating causes grain coarsening, which degrades low-temperature toughness and slows the formation of the protective rust layer.
Intercritical Annealing: Heating between Ac1 and Ac3 temperatures disrupts the ferrite-pearlite balance, leading to uneven strength and corrosion resistance.
4. Link to Weathering Performance: Heat Treatment's Role in Rust Layer Formation
The uniform distribution of alloying elements (Cu, Cr, Ni in Q355GNH) in a fine-grained matrix promotes the formation of a dense, compact rust layer (α-FeOOH) on the surface.
This rust layer acts as a barrier to oxygen and water, slowing further corrosion. Improper heat treatment (e.g., coarse grains) would result in a loose, porous rust layer that fails to protect the base steel.
