In Q345NH weathering steel, carbon (C) content is strictly controlled within ≤0.16%, and its influence on strength and toughness follows a clear trade‑off relationship:
1. Effect on Strength
Carbon is the most effective strengthening element in steel.
As carbon content increases, it forms pearlite and increases solid‑solution strengthening, which significantly raises yield strength and tensile strength.
For Q345NH, a moderate carbon level ensures it reaches the required yield strength ≥ 345 MPa.
If carbon is too low, strength may fall below the standard; if too high, brittleness increases sharply.
2. Effect on Toughness (especially low‑temperature impact toughness)
Higher carbon content significantly reduces toughness and raises the ductile‑brittle transition temperature (DBTT).
Excess carbon promotes brittle microstructures and increases hardness, making the steel more likely to fracture under impact at low temperatures.
It also increases sensitivity to welding cold cracking, further weakening toughness in welded components.
3. Combined effect in Q345NH
The standard limits carbon to ≤0.16% (typically around 0.12–0.15%) to achieve:
Sufficient strength to meet Q345 grade requirements
Acceptable low‑temperature toughness and good weldability
If carbon were increased to boost strength, toughness would drop to an unsafe level for outdoor and cold‑environment use.
