Pipe bending springs are typically made from square section spring steel.
Spring steel
Spring steel is a low alloy, medium carbon steel which has very high yield strength. The properties of spring steel allow objects made of such to return to their original state even after significant bending and twisting.
Why is it called ‘square section’ steel?
It is called ‘square section’ because it is made from a thin, square bar of spring steel.
As you can see from the diagram, the bar is coiled to make a pipe shape. If continued past this first loop the bar would become a flexible spring that still retained maximum external and internal surface areas. This is essential for its effectiveness when supporting the internal or external walls of a pipe.
A cross-section of the bar is square.
Polished finish
Pipe bending springs are given a polished finish so as to improve their corrosive resistance by preventing oxidisation and pitting. This is important not only to increase the longevity of the tool but also to make it easier to insert and remove from pipes.