Why cant mild steel not be hardened?

Mild steel cannot be hardened significantly because it has a low carbon content, typically around 0.05% to 0.25%. This low carbon level limits its ability to form the hard microstructure known as martensite during heat treatment processes. Instead, mild steel primarily retains a softer ferrite-pearlite structure, which provides ductility but not hardness. Consequently, while mild steel can be surface-hardened through processes like carburizing, its overall hardness remains limited compared to higher carbon steels.