39 Removing Broken Bits or Bolts
A screw extractor is a tool used expressly for removing broken or seized screws. There are two types; one has a spiral flute structure, which is commonly known by the trademarked name Easy Out; the other has a straight flute structure. The tools are made of very hard, brittle steel; they can break off inside the screw if too much torque is applied, making the removal much more difficult. (https://en.wikipedia.org/wiki/Screw_extractorLinks to an external site.)
How to use an easy out
More methods to remove broken bolts