The most common rework in the bending shop is "wrong angle": the drawing calls for 90°, but the bend comes out at 85° or 95°. This is not so noticeable in carbon steel, but in stainless steel it is almost inevitable — because its work-hardening rate and yield strength are both high.
Quick judgment
Springback increases with R/t (inside bend radius / sheet thickness) and with material strength, and is smaller when bending along the rolling direction than transverse to it. Fixing the "bend radius" and "rolling direction" at the time of ordering is far more reliable than relying on the operator's feel to compensate afterwards.
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 usually requires two-stage forming or a dedicated die;
- 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 greater the span between support points 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 certainly 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;
- Die 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 must be decided according to the drawing tolerance;
- Coining: using greater pressure to press the material against the die can significantly reduce springback, at the cost of higher tonnage and faster die wear.
Two things that can be fixed at the time of ordering
- Specify the inside bend 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, repeated trial and error will follow, and even the whole batch may be 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 normal, the outer surface cracks directly;
- The bend line is parallel to the rolling direction (transverse bending) but no annealing or increased R is done, leading to batch cracking;
- Using the wrong V-groove opening: thin sheet with a large groove, the angle is never stable no matter how it is adjusted;
- Ignoring material temper: the springback of the same grade in annealed and hard temper differs by a factor of two.
Tell us the grade, sheet thickness, target inside radius, bend length and quantity, and we can give recommendations on material temper (annealed/hard) and cut-to-length and slitting according to your forming plan — half of bending cracks and springback come from the wrong material temper being selected.
