Why are U-shaped heat exchange tubes not suitable for hot bending?

During cold bending, the metal undergoes work hardening; as a result, the metal tube after cold bending is much harder than one that has been bent using heat. However, cold bending does not alter the original properties of the metal. After cold bending, there is no need for cleaning or removal of scale, and thermal deformation does not occur. Compared to hot bending, cold bending requires more power to bend the tube, and both springback and residual stress increase significantly. Moreover, cold bending cannot be used to form sharp bends with very small radii of curvature. Thermal bending possesses an adaptability that cold bending cannot match. For example, the straight-line distance between two adjacent elbows on a pipe can be kept very small; it is even possible to make continuous bends without any straight sections in between ； It is capable of processing materials with very poor cold formability into elbows ； It is capable of processing elbows that require a large amount of mechanical energy for cold bending, and it can bend ductile materials that are prone to cracking during cold bending into the desired shape. Thermal bending can be used to form elbows with a small radius on pipes. For carbon steel pipes and most alloy steel pipes, the bending radius for hot bending is much smaller than that for cold bending; it can be as low as 0.7 to 1.5 times the outer diameter of the pipe. Thermal bending still has the following disadvantages: complex equipment, high processing costs, low production efficiency, and poor surface finish.

The second floor explained it in great detail; thumbs up. There is also an issue with reducing the book value.

Could you explain in detail the issue of the thinning amount?

There must be significant residual stress in cold bending. . . . . .

Cold bending induces residual stresses; in some stress-corrosion environments, stress-relief heat treatment is required, followed by a hydrostatic test