What is the hardest form of steel?

Martensite is recognized as the hardest form of steel due to its unique crystalline structure that forms when austenite is rapidly cooled (quenched). This transformation involves a significant change in the arrangement of atoms, resulting in a very hard and brittle microstructure. The hardness of martensite is primarily attributed to the effects of carbon that is trapped within the body-centered tetragonal lattice during the quenching process, which significantly increases the dislocation density within the material and thereby enhances its hardness.

In contrast, the other forms of steel mentioned, such as austenite, mild steel, and ferrite, possess different characteristics. Austenite is a high-temperature phase of steel that can be soft and ductile. Mild steel, known for its malleability and low carbon content, is not designed for high hardness. Ferrite, which is another phase of iron, has a body-centered cubic structure and is also relatively soft. Thus, in terms of hardness, martensite is the clear choice as the hardest form of steel.