Continuous Cooling Transformation¶

A material-specific time–temperature map that overlays continuously decreasing thermal paths on transformation-start, fraction, finish, and martensite boundaries to predict which microstructural products form during practical cooling from a parent phase.

Version: v2 · 2026-09-19 · History
Domain-specific #: 1551
Domain group: Natural Sciences
Origin domain: Chemistry & Materials Science
Subdomains: Physical Metallurgy, Steel Phase Transformation Kinetics → Chemistry & Materials Science
Aliases: CCT diagram, Continuous-cooling-transformation diagram, Continuous cooling diagram

Core Idea¶

A Continuous Cooling Transformation diagram, or CCT diagram, maps how a material's parent phase transforms while temperature decreases continuously along specified thermal paths. In steels, specimens are commonly austenitized, cooled at controlled rates, and monitored by dilatometry or complementary metallography and hardness measurements. Results from multiple paths are assembled on temperature versus logarithmic time axes, with curves or regions marking the start and finish of ferrite, pearlite, bainite, or other diffusional products; martensite start and sometimes fraction or finish information; and the microstructure or hardness obtained at the end.

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Cooling Speed Map

When very hot steel cools down, its insides can turn into different kinds of tiny patterns. Cool it slowly and you get one kind, cool it super fast and you get another. A CCT chart is a map that shows which pattern you get for each way of cooling.

Steel Cooling Chart

Steel heated until it glows has a special inside arrangement. As it cools, that arrangement changes into new ones, and which new ones appear depends on how fast it cools. Scientists cool many samples at different speeds and watch when each change starts and finishes. They put all those results on one chart of temperature versus time. To use it, you trace your own cooling path on the chart and see which regions it passes through, which tells you what the steel will be like when cold.

Continuous-Cooling Transformation Diagram

A Continuous Cooling Transformation (CCT) diagram shows how a material, usually steel, transforms while its temperature keeps falling along a specified cooling path. Samples are first heated to form austenite, then cooled at controlled rates while instruments detect when new structures such as ferrite, pearlite, bainite, or martensite begin and finish forming. The results are plotted as temperature versus time on a logarithmic scale. Slow cooling crosses the ferrite and pearlite regions; medium cooling may produce bainite; very fast cooling can skip those and reach the martensite-start temperature with much austenite left. A CCT diagram is not an equilibrium phase diagram, whose horizontal axis is composition: its lines are timing events measured under one test procedure. It also differs from a TTT diagram, which holds samples at a single temperature instead of cooling them continuously.

A Continuous Cooling Transformation diagram is a path-indexed kinetic map: for a specified alloy and prior parent-phase condition, it records which transformations occur, and when, along continuously decreasing temperature-time histories. In steels, specimens are austenitized and cooled at controlled rates, with transformation events detected by dilatometry and checked by metallography and hardness testing. Data from many cooling paths are assembled on temperature versus log-time axes, with start and finish curves for diffusional products (ferrite, pearlite, bainite), the martensite-start line and sometimes fraction information, and the final microstructure or hardness per path. Reading the diagram means following a cooling curve from the austenitizing condition through every boundary it crosses. Slow paths allow ferrite and pearlite before martensite is possible, intermediate paths give bainite or mixed products, and sufficiently fast paths avoid diffusional regions and reach Ms with much austenite untransformed. The boundaries are kinetic events under a defined protocol, not equilibrium phase limits, and the time axis replaces the composition axis of a phase diagram. Unlike an isothermal TTT diagram, CCT traverses a continuum of temperatures; because transformation progress depends on accumulated history, overlaying a cooling curve on a TTT chart is only an approximation.

Scope of Application¶

CCT diagrams support selection of steel grade, austenitizing practice, quench medium, section size, and cooling schedule for heat treatment. They help estimate hardenability—the capacity to form martensite through a section under a given cooling severity—and explain why a surface and core can acquire different microstructures. They also guide normalization, controlled rolling, forging cool-down, welding heat-affected-zone analysis, and additive-manufacturing thermal-cycle studies when the appropriate prior condition and cooling histories are represented.

Clarity¶

“Transformation start” and “finish” are operational thresholds, not infinitely sharp physical instants. A start curve may correspond to a detectable small fraction; a finish curve may correspond to a chosen large fraction. Different instruments and thresholds can shift them. Fraction curves make this convention more explicit.

Manages Complexity¶

Steel transformation couples thermodynamics, diffusion, nucleation, interface motion, elastic strain, composition partitioning, heat transfer, and initial microstructure. CCT converts many laboratory trajectories into a compact visual decision surface. An engineer can ask whether a proposed curve intersects a transformation nose, when a reaction begins, which products coexist at room temperature, and how sensitive the outcome is to cooling delay.

Abstract Reasoning¶

If a diffusional start curve shifts to longer time, a slower cooling rate can bypass it and still reach martensite start.
If cooling slows enough to cross both start and finish of ferrite/pearlite before M_s, little austenite remains to form martensite.
If a path enters a bainite region but exits before finish, later martensite can produce a mixed bainite–martensite microstructure.

Knowledge Transfer¶

The full identity transfers among alloys that undergo path-dependent transformations during continuous cooling, provided each system has its own parent phase, products, detection criteria, and preparation. It does not license using a steel CCT chart for a different composition or prior condition. Welding-CCT and deformation-CCT diagrams are recognized variants because the thermal or mechanical preparation changes the relevant kinetic map.

Relationships to Other Abstractions¶

Current abstraction Continuous Cooling Transformation Domain-specific

Parents (1) — outgoing typed relations

Continuous Cooling Transformation is a kind of Phase Diagram Prime

CCT is a kinetic, time-bearing species of state map, not an equilibrium composition–temperature chart.

Hierarchy paths (3) — routes to 2 parentless roots

Continuous Cooling Transformation → is a kind of Phase Diagram → presupposes Classification

Show alternative paths (2)

Neighborhood in Abstraction Space¶

Continuous Cooling Transformation sits in a sparse region of the domain-specific corpus (93^rd percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.

Family — Unclustered & Miscellaneous (2769 abstractions)

Nearest neighbors

Computed from structural-signature embeddings · 2026-10-25