An equilibrium phase diagram says which phases are favored at a given temperature and composition, but it does not say how fast a transformation occurs. Time-temperature-transformation (TTT) and continuous-cooling-transformation (CCT) diagrams add kinetics.

A TTT diagram records the start and finish of transformations during isothermal holding. Curves often have a C-shaped form: transformation is slow near the equilibrium transformation temperature because the thermodynamic driving force is small, and slow again at low temperature because diffusion is sluggish. Intermediate temperatures can transform fastest.

To use a TTT diagram, follow the actual thermal path. If an alloy is rapidly cooled to a holding temperature and kept there, the horizontal time spent at that temperature determines which transformation-start and finish curves are crossed.

A CCT diagram performs the analogous role for continuous cooling, which is more typical of industrial heat treatment. Its boundaries differ from TTT curves because the material experiences a sequence of temperatures rather than one isothermal hold.

For steel, these diagrams distinguish products such as pearlite, bainite—a fine transformation product that forms at temperatures below typical pearlite formation—and martensite, which forms by a rapid diffusionless transformation when cooling suppresses the diffusional alternatives.

TTT and CCT diagrams therefore bridge equilibrium phase diagrams and practical heat treatment by adding the missing time scale.