1. Impact on Mechanical Properties: Ductility, Stress Concentration, and Risk of Cracking
The outer surface of the bent steel is stretched excessively, and the inner surface is compressed. If the radius is too small (e.g., smaller than 1.5× the material thickness, R < 1.5t), the outer fiber may exceed SPA-H's plastic deformation limit, leading to surface cracks, micro-fractures, or even complete splitting.
Severe plastic deformation also causes work hardening in the bending zone: the yield strength and tensile strength increase, but ductility and toughness decrease sharply. This makes the bent part brittle and prone to breaking under subsequent dynamic loads (e.g., vibration in bridge structures) or temperature changes.
Example: For 10mm-thick SPA-H steel, if bent at R=5mm (R < t), the outer surface tensile strain will exceed 100%, far exceeding its plastic limit-resulting in visible cracks.
The plastic deformation of the bending zone is within a safe range (typically R ≥ 2–3t for cold bending of SPA-H), avoiding over-stretching/compression. The steel retains most of its original ductility and toughness, ensuring the bent part can withstand design loads (e.g., bending moment, shear force) without cracking.
Work hardening is mild and localized, not affecting the overall mechanical performance of the component.
2. Impact on Corrosion Resistance: Patina Formation and Localized Corrosion Risk
Cracks or micro-gaps in the bending zone destroy the continuity of the base metal. When exposed to the atmosphere, corrosive media (moisture, oxygen, salt spray) penetrate these defects and initiate localized corrosion (pitting corrosion, crevice corrosion). The patina cannot form stably in cracked areas, leading to "accelerated rusting"-the corrosion rate here can be 5–10 times faster than the intact surface.
Even if no visible cracks occur, a too-small radius may cause surface scratches or oxide film damage during bending (due to friction between the steel and the bending die). These defects become "corrosion initiation points," weakening long-term corrosion resistance.
The bending surface remains smooth and intact, allowing the alloying elements (Cu, Cr) to react uniformly with the atmosphere. A continuous patina forms across the entire surface (including the bending zone), maintaining consistent corrosion resistance with the unprocessed base metal.
3. Impact on Structural Integrity and Service Life
Cracks or work hardening in the bending zone reduce the component's fatigue resistance. Under cyclic loads (e.g., vehicle traffic on bridges, wave impact on marine structures), stress concentrates at the cracks, leading to fatigue failure (sudden breakage) after a certain number of cycles. This drastically shortens the service life of the component (from decades to a few years in severe cases).
Localized corrosion in the bending zone further expands cracks over time, forming a "corrosion-fatigue" coupling effect-accelerating structural damage.
The bending zone has no stress concentration points or corrosion defects. The component can maintain its design service life (30–50 years for SPA-H structures), as the bending process does not compromise its mechanical or corrosion performance.
