1. Tensile Strength and Yield Strength
Normalization: Typically increases both tensile strength (by 5–10%) and yield strength (by 3–7%) compared to the as-rolled state. This is due to grain refinement and the formation of a more uniform ferrite-pearlite structure, which enhances interatomic bonding and resistance to deformation.
Annealing: Slightly reduces tensile and yield strength (by 2–5%) as the process softens the material by relieving internal stresses and coarsening pearlite lamellae marginally, prioritizing ductility over strength.
Stress Relief Annealing: Has minimal impact on strength, as it focuses on reducing residual stresses without altering the primary microstructure.
2. Ductility (Elongation)
Normalization: Improves elongation by 1–3% (e.g., from 22% to 24–25% in some cases). The refined, uniform grain structure allows greater plastic deformation before fracture, as fine grains distribute stress more evenly.
Annealing: Significantly enhances ductility (elongation increases by 3–5%) by softening the steel and reducing grain-boundary restrictions, making it more malleable for forming processes.
Rapid Cooling (Post-Normalization): May slightly reduce ductility if cooling is too fast, as it can introduce minor microstructural inhomogeneity (though this effect is limited in Q295GNH due to low hardenability).
3. Toughness (Impact Energy)
Normalization: Markedly improves impact toughness (e.g., Charpy V-notch energy increases by 15–25%). Fine-grained microstructure and uniform phase distribution reduce stress concentration, allowing the material to absorb more energy during impact before fracturing.
Annealing: Improves toughness moderately (5–10% increase) by relieving internal stresses that could act as fracture initiation points, though its effect is less pronounced than normalization.
Slow Cooling (Post-Normalization): Degrades toughness by promoting coarse grain growth and uneven pearlite formation, which create brittle zones prone to crack propagation.
4. Hardness
Normalization: Increases hardness slightly (by 5–10 HB) due to finer pearlite and grain refinement, which enhance resistance to indentation.
Annealing: Reduces hardness (by 10–15 HB) as the material softens, making it easier to machine or weld.
Stress Relief Annealing: Has negligible effect on hardness, as it does not alter the microstructure's phase composition.
5. Corrosion Resistance
Normalization: Enhances weathering corrosion resistance indirectly. By homogenizing the distribution of alloying elements (Cu, Cr, Ni), it promotes the formation of a denser, more uniform protective oxide film on the surface, slowing down corrosion.
Excessive Heating/Cooling: Can diminish corrosion resistance if microstructural inhomogeneity occurs (e.g., segregation of alloy elements), as it creates localized areas more susceptible to corrosion.
