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T (Time) T (Temperature) T (Transformation) diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition.

Diagram TTTDiagram TTT

It is used to determine when transformations begin and end for an isothermal (constant temperature) heat treatment of a previously austenitized alloy.

TTT diagram indicates when a specific transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved.

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Demonstration of how an isothermal transformation diagram (bottom) is generated from percent transformation versus logarithm of time measurements (top).

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TTT diagram for a hypoeutectoid composition (0.5 wt % C) compared with the Fe–Fe3C phase diagram.

α + γ

Ms is higher for this low C-steel.

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TTT diagram for eutectoid steel shown in relation to the Fe–Fe3 C phase diagram (see Figure 9.38). This diagram shows that, for certain transformation temperatures, bainite rather than pearlite is formed. In general, the transformed microstructure is increasingly fine grained as the transformation temperature is decreased. Nucleation rate increases and diffusivity decreases as temperature decreases. The solid curve on the left represents the onset of transformation (~1% completion). The dashed curve represents 50% completion. The solid curve on the right represents the effective (~99%) completion of transformation. This convention is used in subsequent TTT diagrams.

γ

α + Fe3C

γ

~1% ~99%

(α+FexC)

DIFFUSIONAL TRANSFORMATIONS

50%

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TTT diagram for a hypereutectoid composition (1.13 wt % C) compared with the Fe–Fe3 C phase diagram

γ + (Fe3C)proeut

γ

Ms is lower than that of the eutectoid steel.

proeutectoid

kusharjanto, metalurgi-unjani 8AISI 10110

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TTT Diagram and microstructures obtained by different types of cooling rates

Full TTT DiagramFull TTT Diagram

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The complete TTT diagram for an iron-carbon alloy of eutectoid composition.

A: austenite

B: bainite

M: martensite

P: pearlite

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Isothermal transformationdiagram for analloy steel (type 4340):A, austenite; B, bainite;P, pearlite; M, martensite;F, proeutectoid ferrite.

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Perhatikan empat garis pendinginan yang berbeda, terlihat bahwa struktur mikronyapun berbeda.

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Perlit kasar (Coarse Pearlite) Perlit halus (Fine Pearlite)

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Martensite, 1220X.Bainite, 15000X.

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Example Problem

(a) Rapidly cool to 250°C, hold for 100s, and quench to room temperature

At 760°C: in the austenite region (g)— 100% austenite

Rapidly cool from 760°C to 250°C: 100% austenite

Hold for 100 seconds at 250°C: 100% austenite

Quench to room temp.: 100% martensite

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Example Problem

(b) Rapidly cool to 600°C, hold for 104 s, and cool to room temperature

At 760°C: in the austenite region (g)— 100% austenite

Rapidly cool from 760°C to 600°C: 100% austenite

Hold for 104 s at 250°C: 100% pearlite

Quench to room temp.: 100% pearlite

So What’s a CCT Diagram?

• Phase Transformations and Production of Microconstituents takes TIME.

• If you don’t hold at one temperature and allow time to change, you are “Continuously Cooling”.

• Therefore, a CCT diagram’s transition lines will be different than a TTT diagram.

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Superimposition ofisothermal andcontinuous coolingtransformationdiagrams for aeutectoidiron–carbon alloy.

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Moderately rapidand slow coolingcurves superimposedon a continuouscoolingtransformationdiagram for aeutectoidiron–carbon alloy.

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Continuous coolingtransformation diagramfor a eutectoidiron–carbon alloy andsuperimposed cooling curves, demonstratingthe dependence of the final microstructure on the transformations that occur during cooling.

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Continuous coolingtransformation diagramfor an alloy steel (type 4340) and severalsuperimposed cooling curves demonstratingdependence of the final microstructureof this alloy on thetransformations that occur during cooling.

Slow Cooling

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Time in region indicates amount of microconstituent!

Medium Cooling

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Cooling Rate, R, is Change in Temp / Time °C/s

Fast Cooling

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This steel is very hardenable… 100% Martensite in ~ 1 minute of cooling!

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Time-temperature transformation (TTT) diagramsmeasure the rate of transformation at a constant temperature. In other words a sample is austenitised and then cooled rapidly to a lower temperature and held at that temperature whilst the rate of transformation is measured.

Continuous cooling transformation (CCT) diagramsmeasure the extent of transformation as a function of time for a continuously decreasing temperature. In other words a sample is austenitised and then cooled at a predetermined rate and the degree of transformation is measured.

Difference between cct and ttt phase diagram?