When performing cold bending or hot forming on Q550NH weathering steel, the following precautions should be taken to maintain its mechanical properties and corrosion resistance:
1. Cold Bending Precautions
Minimum Bend Radius:
Typically ≥2× material thickness (for plates <16mm). Larger radii reduce cracking risk.
Material Condition:
Prefer normalized or TMCP-treated Q550NH for better ductility.
Preheating (if needed):
For thick plates (>20mm) or tight bends, preheat to 100–150°C to avoid cracking.
Avoid Over-Bending:
Excessive deformation can cause microcracks, reducing fatigue life.
2. Hot Forming Precautions
Temperature Range:
850–950°C (optimal for forming without grain coarsening).
Avoid exceeding 1100°C (risk of excessive scaling and decarburization).
Cooling Control:
Air cooling is standard; avoid water quenching (may induce residual stresses).
Post-Forming Heat Treatment:
For critical parts, normalizing (880–920°C + air cooling) restores microstructure.
3. Welding After Forming
Cold-Formed Areas:
Anneal (if heavily worked) before welding to relieve stresses.
Hot-Formed Areas:
Ensure temperature drops below 300°C before welding to avoid HAZ brittleness.
4. Corrosion Resistance Preservation
Surface Cleaning:
Remove mill scale/oxide layers post-forming (sandblasting preferred).
Avoid Contaminants:
Prevent oil, grease, or copper deposits that disrupt patina formation.
5. Standards & Testing
Follow GB/T 4171 for post-forming mechanical tests.
Conduct Charpy impact tests at service temperatures (if for low-temp use).
