Die Sets

A rotary-draw die set has three components: Bend die, clamp die, and pressure die. The key material requirement for these dies is that they are tough. A tough die has hard working surfaces that can absorb a shock without breaking. So a tough die has some give. It is not brittle. For the same reason we do not use glass hammers, we should avoid materials and heat treatments for bending dies that make them brittle instead of tough and so break instead of wear out.
Consequently, the tool steels that are sometimes specified for rotary-draw die sets, such as A2, D2, or even a shock-resistant material like S7, are not the best materials in terms of cost-effectiveness. While they can be heat treated to a high level of hardness, they are designed to hold a sharp edge in metal-cutting operations. So the brittleness that comes with this hardness is acceptable for that use. However, tube-bending is, of course, not a metal-cutting process. There is no need for tooling materials that hold a sharp edge.