Rethreader shanks are usually made from high speed steel.
High-speed steel is extra hard and will not lose this hardness with temperature changes.
It is called high-speed steel because it is capable of maintaining its shape and working at high speeds.
It is important for the threads to be hard so that they will not deform, but are also capable of altering the damaged threads.
Rethreader handles are typically made from plastic. Plastic is strong and hardwearing, but has a smooth feel to it, which makes it more pleasing to hold and use. Sometimes, a rubber grip is added to the handle for extra comfort and security.