Martensiit

Martensiit is a very hard, brittle phase of steel. It forms when steel is cooled rapidly from a high temperature, a process called quenching. This rapid cooling prevents the normal transformation of austenite, a high-temperature phase of iron and carbon, into softer phases like pearlite. Instead, the iron atoms are trapped in a body-centered tetragonal crystal structure. This distorted structure causes internal stresses, contributing to martensite's hardness.

The formation of martensite is a diffusionless transformation, meaning the atoms do not move significant distances relative to each other. The carbon atoms, which are too large to fit easily into the distorted iron lattice, are trapped in interstitial positions, further increasing the lattice strain and hardness. The amount of martensite formed depends on the carbon content of the steel and the severity of the quench.

While martensite is extremely hard, it is also brittle, meaning it can fracture easily. To achieve a balance of hardness and toughness, martensitic steels are often tempered. Tempering involves reheating the quenched steel to a lower temperature for a period of time. This process allows some carbon atoms to precipitate out as fine carbides, relieving internal stresses and increasing toughness while retaining a significant portion of the hardness. Martensitic microstructures are found in many tools, knives, and structural components where high strength and hardness are required.