In Q345NH weathering steel, phosphorus (P) and sulfur (S) are both controlled to very low levels, and their effects on performance are quite different:
1. Phosphorus (P) ≤ 0.035%
Benefits
Improves atmospheric corrosion resistanceP helps form a denser, more stable protective patina on the surface, which is why weathering steels contain slightly more P than ordinary structural steel.
Harms
Severely reduces low‑temperature toughnessP segregates at grain boundaries, making the steel more brittle, especially in cold conditions.
Worsens weldability and increases cold-cracking sensitivity.
Reduces plasticity and ductility.
Balance in Q345NH
P is kept low but not zero - enough to enhance corrosion resistance, but not enough to cause serious embrittlement.
2. Sulfur (S) ≤ 0.030%
Almost entirely harmful
Forms MnS inclusions that act as crack initiation points,→ greatly reduces transverse impact toughness and ductility.
Causes non-uniform corrosion and pitting on the surface.
Worsens surface quality and hot workability.
Impairs fatigue performance.
Only tiny benefit
Sulfur can slightly improve machinability, but this is negligible for weathering steel structures.
Control purpose
S is limited as much as possible to ensure toughness and uniformity.
