1. Limited hardenability during quenching
Quenching (rapid cooling from austenitizing temperature) fails to form significant martensite (the hard phase in high-alloy steels). Instead, it primarily forms fine ferrite and pearlite, similar to normalization but with slightly higher strength due to faster cooling.
Strength gains are minimal (tensile strength increases by ≤5% compared to normalization) and not cost-effective for structural applications.
2. Tempering after quenching: Marginal improvements
Relieves minor residual stresses from quenching, slightly improving toughness (Charpy impact energy increases by 5–10%).
May reduce any slight hardness gained from quenching, but since quenching itself does not significantly harden the steel, the overall effect is negligible.
3. Key drawbacks
Reduced corrosion resistance: Abnormal heat treatment (e.g., overheating during quenching) can disrupt the uniform distribution of Cu, Cr, and Ni, impairing the formation of the protective oxide film critical for weathering performance.
Increased brittleness risk: Rapid quenching may introduce thermal stress, leading to microcracks in thick sections, especially if tempering is insufficient.
Economic inefficiency: Quenching and tempering require more energy and stricter process control than normalization but provide no meaningful performance gains for Q355GNH's intended use (structural components requiring weather resistance).
4. Optimal alternative: Normalization
Refines grains and homogenizes microstructure, balancing strength and toughness.
Preserves the uniform distribution of alloying elements, ensuring stable formation of the protective patina.
Is more cost-effective and reliable for structural applications.
