Martensiittiä

Martensiittiä is the Finnish term for martensite, a very hard and brittle phase of steel. It is formed during a rapid cooling process, known as quenching, of steel. When steel is heated to a high temperature and then cooled quickly, the carbon atoms within the iron structure do not have enough time to diffuse into a more stable arrangement, such as pearlite. Instead, they become trapped in interstitial positions within the body-centered tetragonal crystal lattice of the martensite. This distorted crystal structure is responsible for martensite's exceptional hardness.

The formation of martensite is a diffusionless transformation, meaning that the atoms do not move long distances. The transformation occurs almost instantaneously upon cooling below a critical temperature, known as the martensite start temperature (Ms). The amount of martensite formed depends on the carbon content of the steel and the cooling rate. Higher carbon content generally leads to higher hardness and greater brittleness.

While martensite is very hard, its brittleness makes it unsuitable for many applications in its as-quenched state. To improve its toughness and ductility while retaining some of its hardness, martensite is typically tempered. Tempering involves reheating the steel to a lower temperature, allowing for some controlled diffusion of carbon and the formation of fine carbide precipitates within a less distorted iron matrix. This process significantly reduces brittleness without a drastic loss of hardness. Martensitic structures are crucial in achieving desired mechanical properties in various steel components, from tools and springs to structural parts.