Which of the following is harder, martensite or fine pearlite?

Martensite is indeed harder than fine pearlite due to its unique microstructural characteristics. Martensite forms when austenite is rapidly cooled or quenched, leading to a non-equilibrium phase that has a distorted body-centered tetragonal lattice structure. This distortion effectively increases the material's hardness and strength due to the trapped carbon atoms that prevent dislocation movements, which are essential for deformation.

Fine pearlite, on the other hand, is a microstructure resulting from a slower cooling process, where alternating layers of ferrite and cementite form. While fine pearlite is stronger and harder than coarser pearlite, it does not possess the same level of hardness as martensite due to its more stable and less distorted structure.

The significant difference in hardness between martensite and fine pearlite is crucial in applications where material strength and wear resistance are essential, further supporting why martensite is preferable in many engineering settings.