Why Do Steel Bars Straighten?
Steel bars are composed of Iron Crystals and Allotropic's. Iron Crystals are atoms in a structure where the atom in the structure have a strong attraction to adjacent atoms. When stresses are placed on the iron crystals, the distance between the atoms stretch. Once the stress is removed, the atoms spring back to their original structure. However, if the yield point is exceeded, the atoms do no revert and the structure takes on a different shape.
In the straightening process the bar is worked back and forth causing planes of atoms within the structure to slip over one another which causes the bar to straighten.
In the higher alloy bars, elements such as carbon and vanadium will slip a few times but not after that. As a result, the higher the carbon the carbon levels and higher the alloys, the more difficult it is to straighten a bar.