Why martensite is not in Fe-C diagram?

Martensite is not explicitly represented in the Fe-C phase diagram because it is a non-equilibrium phase that forms during rapid cooling (quenching) of austenite, rather than through equilibrium phase transformations. The Fe-C diagram primarily depicts phases that exist under equilibrium conditions, like ferrite, cementite, and austenite. Since martensite forms through a diffusionless transformation, it does not have a stable temperature-composition relationship represented in the phase diagram. Instead, martensite is often discussed in the context of the transformation kinetics and microstructural features rather than equilibrium phases.