Reading TTT and CCT diagrams

The C-curve and its nose

On a TTT diagram, transformation start and finish curves form the characteristic C-shape. The bend — the nose — is the fastest the diffusional transformation can happen: high enough temperature for fast diffusion, but enough undercooling for a strong driving force. Above the nose you get pearlite; below it, bainite; and if you cool fast enough to miss the nose entirely, martensite.

Martensite is athermal

Martensite does not appear on the time axis the way pearlite and bainite do. It forms by a diffusionless shear below the martensite-start line:

Ms and Mf are horizontal lines set by composition. The more carbon and alloy, the lower they sit — and if Mf falls below room temperature, some austenite is retained.

TTT versus CCT — use the right one

Real quenching is continuous cooling, not an isothermal hold, so the practical diagram is the CCT. Its curves are displaced to longer times and lower temperatures relative to the TTT, because the time spent at the most-active temperatures is shared across the cooling path.

The critical cooling rate

The slowest cooling path that just misses the nose is the critical cooling rate for a fully martensitic structure. Hardenability is, in effect, how far left the nose sits: lean steels need a drastic quench to beat it (risking distortion and cracking), while alloyed steels push the nose right so a milder quench still hardens through-section.

Why it matters downstream

The microstructure read off the CCT sets the as-quenched hardness, the residual-stress and cracking risk, and the starting point for tempering. It connects directly to the Jominy hardenability curve and to weld HAZ prediction, where the local cooling rate (often expressed as the 800→500 °C time) decides whether the HAZ turns brittle.