The impact toughness of Q550NH weathering steel exhibits temperature-dependent behavior typical of high-strength low-alloy steels, with notable transitions across different temperature ranges:
1. Room Temperature (20°C)
High toughness: Charpy V-notch (CVN) values typically ≥60 J (per GB/T 4171), suitable for most structural applications.
2. Low Temperatures (-20°C to -40°C)
Gradual reduction: Toughness decreases but remains ≥27 J (standard minimum for weathering steels), ensuring ductility in cold climates.
Microstructure role: Fine-grained ferrite-pearlite (from TMCP/normalizing) delays brittle transition.
3. Very Low Temperatures (< -50°C)
Brittle transition risk: CVN may drop sharply if the steel lacks nickel (Ni) micro-alloying; Ni-modified grades perform better.
4. Elevated Temperatures (100–200°C)
Stable toughness: No significant loss, but long-term exposure may affect corrosion resistance.
Key Influencing Factors
Processing: TMCP improves low-temperature toughness vs. as-rolled.
Alloying: Higher Cu/Cr/Ni content enhances corrosion resistance but may slightly reduce sub-zero toughness.
Thickness: Thicker sections show more pronounced toughness anisotropy.
