Many key factors besides alloying elements strongly affect the low‑temperature impact toughness of S355J0WP.
1. Rolling & production process
Controlled rolling / finishing temperatureLower finishing temperature → finer grains → much better low‑temperature toughness.Too hot rolling makes coarse grains → brittle at low temperature.
Cooling rate after rollingFast uniform cooling refines structure; slow cooling causes coarse ferrite.
2. Steel purity (impurities & inclusions)
Sulfur, oxygen → form elongated non-metallic inclusionsThese act as crack starters and sharply reduce impact energy in cold conditions.
Internal porosity, slag, segregation also lower toughness.
3. Plate thickness effect (size effect)
Thicker plate = worse low‑temperature toughnessThicker material has heavier segregation, slower cooling, coarser core structure.
Thin sheets always perform better in low‑temp impact.
4. Microstructure uniformity
Banded structure, uneven ferrite/pearlite distribution → raise brittleness.
Homogeneous fine microstructure improves toughness greatly.
5. Residual stress
From hot rolling, cutting, bending, welding.
High residual stress accelerates crack opening at low temperature → reduces impact resistance.
6. Welding thermal cycle (for fabricated parts)
Heat-affected zone (HAZ) easily becomes coarse and brittle.
Even if base metal is good, welding can drop low‑temp toughness significantly.
7. Service environment factors
Lower testing temperature naturally reduces impact energy.
Moisture, aging, and long-term load also degrade toughness over time.
