1. Qualitative Compliance Requirements (Mandatory)
Cold bending test pass: Per GB/T 4171, Q235NH must pass a 180° cold bending test with a specified bend radius (e.g., d=1.5a for t≤16 mm, where d=mandrel diameter and a=plate thickness) without cracks, delamination, or visible defects on the inner/outer surfaces. This ensures the material's residual stress after cold bending does not exceed its plastic deformation capacity and will not cause immediate failure.
No impact on structural performance: Residual stress must not reduce the steel's yield strength (≥235 MPa for t≤16 mm), tensile strength (360–510 MPa), or elongation (≥25% for t≤16 mm) below standard thresholds.
2. Industry-Accepted Residual Stress Control Ranges (Practical Guidelines)
General structural applications: Compressive residual stress ≤200 MPa; tensile residual stress ≤0.6×yield strength (≤141 MPa for Q235NH). This prevents stress corrosion cracking and ensures dimensional stability during service.
Critical components (e.g., bridges, towers): Tensile residual stress ≤0.4×yield strength (≤94 MPa) to enhance fatigue resistance and corrosion durability.
3. Factors Affecting Residual Stress After Cold Bending
Bend radius and thickness: Smaller bend radii (d/a <1.5) or thicker plates (t>16 mm) increase residual stress; thicker plates typically require a larger bend radius (e.g., d=2a for t>16 mm) to keep stress within safe limits.
Post-bending stress relief: For critical applications, stress relief annealing (550–650°C for 1–2 hours, followed by air cooling) can reduce residual stress by 30–50%, ensuring compliance with stricter stress limits.
4. Detection and Verification Methods
Non-destructive testing (NDT): X-ray diffraction or ultrasonic testing can quantify residual stress at the bend area.
Destructive testing: The hole-drilling method (per ASTM E837 or GB/T 31310) is commonly used for accurate residual stress measurement in cold-formed regions.
