The Core Principle: Strain Concentration
Cracking during cold bending occurs when the outer fibers of the steel are stretched beyond their ductility limit. The sharper the bend (smaller the bend radius), the more severe this stretching becomes.
Key Factors Influencing Cracking at the Bend
Bend Radius (The Most Critical Factor)
Rule of Thumb: For weathering steels like Q295NH, the minimum inside bend radius should be at least 2 to 3 times the material thickness (2t - 3t).
Example: For a 10 mm thick plate, the minimum safe inside radius is 20-30 mm.
A "sharp angle" with a radius smaller than this is highly prone to cracking.
Material Properties & Directionality
Ductility: Q295NH has good ductility, but it is higher strength and less ductile than mild carbon steel (like Q235). This means it has less "stretch" before failing.
Grain Direction: Always bend perpendicular to the rolling direction (across the grain). Bending parallel to the grain significantly increases the risk of cracking along the grain boundaries.
Material Condition
Surface Defects: Any notches, scratches, or slag on the surface facing tension (the outside of the bend) will act as stress concentrators and initiate cracks.
Mill Scale: It is highly recommended to remove the mill scale by abrasive blasting before bending. The brittle mill scale will crack and can lead to tears in the underlying base metal.
Bending Process
Tooling: Use sharp, clean, and properly sized punches and dies. A punch with a worn or incorrect radius will promote cracking.
Speed: Bend slowly. A slow, controlled speed allows the material to flow and redistribute stress plastically.
