Bending and Forming Steel Plate: Radius, Grain Direction and Springback

What happens at a bend

Bending stretches the outside of the bend and compresses the inside. Cracking begins when the outer-fibre strain exceeds what the steel can deliver at that thickness — which is why every bending rule is expressed as a minimum inside radius relative to thickness (r/t).

Minimum radius: grade decides

Indicative press-brake practice (mill data sheets govern; values grow with thickness):

Material	Typical minimum inside radius
S355 structural plate	~1–2 × thickness (bend axis across grain at the low end)
S690 high-strength	~2–3 × thickness
NM400 / AR400 wear plate	~3–4 × thickness
NM500 / AR500 wear plate	~4–6 × thickness or more

Wear plate radii surprise fabricators used to structural steel: hardness and bendability trade directly against each other, and a radius that S355 takes easily will crack NM500. Thickness makes it worse — the same grade needs a larger r/t at 40 mm than at 10 mm, because strain concentrates and through-thickness quality margins shrink.

Grain direction

Rolled plate is anisotropic: ductility is lowest in the through-thickness and short-transverse directions, best along the rolling direction. The rule follows:

• Bend line perpendicular to the rolling direction (bend axis across the grain) tolerates the tightest radii — the outer fibres stretch along the grain.
• Bend line parallel to the rolling direction needs a larger radius, commonly a step up of roughly 50–100% in r/t, and is the orientation in which edge cracks initiate.

Add two edge rules: use plate with sound, dressed edges in the bend zone (thermal-cut edge cracks and notches are crack starters, especially on hard plate), and position holes and cut-outs well away from the bend line — commonly at least ~2 × thickness plus the radius — because they distort and concentrate strain.

Springback

When the punch retracts, elastic recovery opens the bend angle. Springback:

• Increases with strength: S355 might spring back a few degrees; wear and high-strength plates noticeably more. It also increases with larger radius-to-thickness ratios and wider die openings.
• Is compensated by overbending — driving past the target angle so recovery lands on it — by bottoming/coining (at high tonnage cost), or by angle measurement and CNC correction on modern press brakes.
• Varies within a plate batch with yield strength scatter, which is why first-piece verification and per-batch adjustment are normal on Q&T plate.

Cracking diagnosis

Outer-surface cracks at the bend almost always mean one of: radius too tight for the grade/thickness, bend axis parallel to grain, a defective or hardened cut edge in the bend zone, or bending below the maker's recommended minimum temperature (cold weather thickens the risk — many makers want hard plate at least at room temperature, and preheating the bend line for the heaviest work). Hot forming wear plate is not a field improvisation: heating above the original tempering temperature permanently softens it.