Here are the main factors that directly affect the low‑temperature impact toughness of S355J2W weathering steel, covering composition, production, structure, and service conditions.
1. Chemical composition
Carbon (C)Higher C → stronger but more brittle at low temperatures; increases cold-cracking tendency.
Manganese (Mn)Moderate Mn improves toughness by refining grains and fixing sulfur; too high may cause banding.
Phosphorus (P) & Sulfur (S)Both are strong embrittling elements; P segregates at grain boundaries, S forms brittle inclusions.Strictly limited in S355J2W to ensure −20°C performance.
Nickel (Ni)Significantly improves low‑temperature toughness and lowers ductile‑brittle transition temperature.
Microalloy elements (Nb, V, Ti)Refine grains → improve both strength and toughness.
2. Grain size
Finer grains → significantly better low‑temperature toughness.
Coarse grains (from high rolling or annealing temperature) → sharp drop in impact energy.
3. Microstructure uniformity
Banded structure (layered ferrite-pearlite) → causes directional brittleness.
Non-uniform microstructure → reduces low‑temperature impact stability.
4. Production process
Controlled rollingLower finish rolling temperature → finer grains → better toughness.
Normalizing heat treatmentEliminates segregation, homogenizes structure, stabilizes low‑temperature performance.
Cooling rateToo fast → hard, brittle microstructure; too slow → coarse grains.
5. Plate thickness
Thicker plate → slower internal cooling, coarser core grains, more segregation → worse toughness.
Thin plates generally have better and more consistent low‑temperature performance.
6. Inclusions and purity
More oxide, sulfide, or slag inclusions → more crack initiation points → lower toughness.
Higher steel purity → better low‑temperature performance.
7. Welding and thermal processing
Welding causes grain coarsening in the HAZ → local toughness drop.
Excessive heat input, improper preheating or post-weld heat treatment → brittle weld zones.
8. Service temperature
As temperature drops below −20°C, toughness decreases gradually.
S355J2W is certified for −20°C but not for deep cryogenic conditions.
