Only steel that is high in carbon can be hardened and tempered. If a metal does not contain the necessary quantity of carbon, then its crystalline structure cannot be broken, and therefore the physical makeup of the steel cannot be altered. Frequently, the term "hardening" is associated with tempered steel.

What material Cannot break?

Researchers claim the material, known as Proteus, cannot be cut. In fact, it's so tough it destroys any tool that attempts to slice into it. Natural materials like diamond and sapphire are strong because of the dense lattice of atomic bonds, but they're inflexible.

What are the hardest metals to break?

10 Strongest Metals in the World