Which metallurgical phase contributes to the hardness of steel when carbon content is high?

Martensite is a critical microstructural phase in steel that significantly impacts its hardness, especially when carbon content is elevated. When steel is rapidly cooled from its austenitizing temperature, the carbon atoms do not have enough time to diffuse out of the face-centered cubic (FCC) structure of austenite. As a result, they become trapped in the body-centered tetragonal (BCT) structure of martensite, leading to an increase in hardness due to its high internal stress and dislocation density.

This transformation, known as quenching, produces a very hard and brittle structure, making martensite essential for applications where high wear resistance and hardness are required. The high carbon content in the steel facilitates the formation of martensite during this process, enhancing the material's overall strength and hardness.

Other phases like austenite, ferrite, and cementite contribute to the mechanical properties of steel in different ways. While austenite can lead to some hardness, it is not stable at lower temperatures and does not provide the high levels of hardness that martensite does. Ferrite is typically softer and ductile, which contrasts with the functionality of martensite. Cementite, while contributing to hardness and strength, does not