The most common rework in the bending shop is "wrong angle": the drawing calls for 90°, but it comes out 85° or 95°. This is not so noticeable with carbon steel, but with stainless steel it is almost inevitable — because its work-hardening rate and yield strength are both high.
Quick judgment
Springback increases with R/t (bend inside radius / sheet thickness), increases with material strength, and is smaller when bending along the rolling direction than transverse to it. Setting the "bend radius" and "rolling direction" at the time of order is far more reliable than relying on the operator's feel for compensation afterward.
Where springback comes from
During bending, the outer layer is in tension and the inner layer in compression; after unloading, the elastic portion recovers, and the angle "springs back." The problem with stainless steel is:
- Fast work hardening: the work-hardening exponent of 304 is significantly higher than that of low-carbon steel, and the deformation zone hardens quickly;
- High yield strength: the amount of elastic deformation that can recover after unloading is greater;
- So for the same R/t and sheet thickness, the springback of stainless steel is usually more than twice that of carbon steel.
Four variables (ranked by influence)
- R/t ratio: the greatest influence. The larger the R/t, the greater the springback. Empirically, when R/t < 1 the springback is smaller but cracking is easy; R/t = 1–3 is the common range; when R/t > 5 the springback is very significant and often requires two-stage forming or special tooling;
- Material strength: 304 < 316L < 2205 < 2507, with springback increasing in that order; cold-rolled hard temper (such as 1/2H) is even greater than annealed temper;
- V-groove opening width: the larger the opening, the larger the fulcrum span, and the greater the springback. The common V-groove opening is about 6–8 times the sheet thickness (V = 6t–8t); using a 12t groove to bend thin sheet will definitely give an inaccurate angle;
- Rolling direction: bending along the rolling direction (bend line perpendicular to the rolling direction) gives smaller springback and is less prone to cracking; transverse bending gives greater springback and higher cracking risk, especially when R/t is small.
How to compensate
- Overbend: directly make the upper die angle 2–5° smaller so that after springback it lands on the target angle; the specific value must be determined by **trial bending**, do not copy someone else's table;
- Tooling compensation: add shims or modify the lower die to change the effective bend angle;
- Adjust R/t: making the inside radius a little larger can reduce the cracking risk, but springback will increase — the two are a trade-off, and you need to choose according to the drawing tolerance;
- Coining: using greater pressure to press the material against the tooling can significantly reduce springback, at the cost of higher tonnage and faster tool wear.
Two things you can fix at the time of order
- Specify the bend inside radius: do not just say "bend 90°." If R is too small it will crack; if too large it will spring;
- State the rolling direction: especially for long strip bent parts, forming results along the rolling direction and transverse to it are completely different.
If these two points are clearly written, the shop can determine the parameters with one trial bend; if not, there will be repeated trial and error, and even whole batches scrapped.
Common pitfalls
- Applying carbon steel springback compensation values to stainless steel;
- When the inside radius is less than the sheet thickness (R < 1t) still bending as ordinary bending, the outer surface cracks directly;
- Bending line parallel to the rolling direction (transverse bending) without annealing or increasing R, leading to batch cracking;
- Using the wrong V-groove opening: thin sheet with a large groove, the angle is unstable no matter how you adjust it;
- Ignoring material temper: for the same grade, annealed and hard temper differ by a factor of two in springback.
Tell us the grade, sheet thickness, target inside radius, bend length and quantity, and we can give material temper (annealed/hard) and cut-to-length/slitting recommendations according to your forming plan — half of bending cracks and springback come from choosing the wrong material temper.
