1. Chemical Composition
Carbon content: Every 0.1% increase in carbon content raises the ductile-brittle transition temperature by approximately 10℃. The ideal range is 0.25%-0.45%.
Alloying elements: Every 0.1% increase in manganese (Mn) content lowers the ductile-brittle transition temperature by 2-3℃; adding 0.15%-0.25% molybdenum (Mo) refines the grain size.
Impurity control: Sulfur (S) should be ≤0.02% and phosphorus (P) ≤0.015% to avoid grain boundary segregation leading to intergranular fracture.
2. Manufacturing Process
Hot rolling/cold rolling: Cold-rolled tubes have finer grains, with a low-temperature impact energy (Akv) ≥34J, superior to hot-rolled tubes.
Heat treatment: Normalizing treatment can eliminate banded structures, and thermomechanical controlled processing (TMCP) refines the grain size to ASTM grade 10 or higher.
Welding process: Low-hydrogen welding consumables and preheating can reduce the degradation of toughness in the heat-affected zone.
3. Environment and Load
Temperature: Below the ductile-brittle transition temperature (DBTT), the impact toughness of Q345 steel may decrease by 50%.
Stress state: Thick plates (plane strain) are more prone to brittle fracture than thin plates (plane stress), and the Charpy V-notch impact test is more stringent.
Corrosive media: Stress corrosion cracking propagates faster at low temperatures; corrosion-resistant materials (such as 316L) should be selected.
4. Microstructure
Non-metallic inclusions: When the level of type D inclusions is >2, the risk of cracking increases significantly.
Banded structure: ≥4th grade or Widmanstätten structure ≥3rd grade will reduce toughness and needs to be improved by annealing.
