Why Does the Bending Radius of a 316 Stainless Steel Heater Sheath Decrease Nonlinearly as Wall Thickness Increases Below 1.2 Millimeters?

For the manufacturing engineer designing custom-shaped electric immersion heaters, the minimum bending radius is often the factor that determines if the heater will fit into a vessel nozzle or have to be designed with a completely different configuration. The idea that bigger walls permit tighter bends is wrong for 316 stainless steel sheaths. In fact, as wall thickness is reduced below 1.2 mm, the possible bend radius goes down dramatically, although not linearly. This article quantifies the connection of sheath wall thickness to minimum bend radius to provide reasonable limitations for forming operations without cracking or excessive wall thinning.

The mechanics of tube bending of thin-walled 316 sheaths
When a tube is bent, tensile force is applied to the outside radius and compressive force to the inner radius. For a given bend radius, the thinner walls are less likely to buckle on the compression side as the wall thickness-to-diameter ratio, t/D ratio governs stability. In fact, a smaller t/D ratio promotes bendability to a certain extent. For 316 sheaths with outer diameters in the range of 8 to 12 mm the critical transition is at a wall thickness of approximately 1.2 mm. With t/D ratios above 0.12, which means wall thicknesses above 1.2 mm, it is getting more and more difficult to bend. The reason is that the thick wall does not want to plastically deform and so needs larger forces. This in turn leads to higher springback and a higher danger of cracking on the tension side. For t/D less than 1.2 mm, the ratio is under 0.10 and the sheath becomes more and more formable. For a wall thickness of 1.0 mm, the minimum bend radius is 2.5 times the outer diameter. The minimum bend radius is 2.0 times the outer diameter at 0.8 mm. On the 0.7-mm manufacturing floor, bend radii as tight as 1.5 x the outer diameter are realisable with suitable tooling and internal mandrel support.

Practical Bend Radius Limits for Typical Wall Thicknesses
For a normal 10 mm outer diameter 316 sheath, the following minimum bend centerline radii apply for production bending without internal mandrel support. For 0.8 mm wall thickness minimum bend radius is 20 mm (2× OD) Wall thickness 1.0 mm, min. radius 25 mm (2.5″ OD). Minimum radius 35 mm (3.5× OD) for 1.2 mm wall thickness Minimum radius for 1.5 mm wall thickness is 50 mm (5x OD). Outer wall cracking can occur at 10% wall thinning when bending below 80 mm radius (8x OD) for 2.0 mm wall thickness. These values are for one bend in annealed 316 tube. Even when the wall thickness is small, several bends in close proximity require higher radii. Engineers specifying bent heaters should not assume that conventional minimum bend radii apply to all manufacturers. They should ask for the minimum bend radius for the wall thickness they have selected. Even if the sheath is cracked from over bending, the cost is much more than altering the heater shape or increasing the available space.