Carbon steel can be balanced or nearly balanced by annealing and normalizing. The quenching results in non-equilibrium structure. Therefore, when studying the microstructure after heat treatment, it is necessary to refer not only to the iron-carbon phase diagram, but also to the isothermal transformation of the reference steel. Curve (C curve).
The iron carbon phase diagram can explain the crystallization process of the alloy during slow cooling and the relative amount of structure and phase at room temperature. The C curve can describe the microstructure of a certain composition of steel under different cooling conditions. The C curve is suitable for isothermal cooling conditions. The CCT curve (austenite continuous cooling curve) is suitable for continuous cooling conditions, and the C curve can be used to a certain extent, and the structural change during continuous cooling can also be estimated.