TTT Curves and Microstructural Transformations in Fe-C Alloys - Examples and Factors

Q: What does a TTT diagram show for a steel alloy?

A TTT diagram plots temperature versus logarithm of time and indicates the start and finish of transformations. It also shows the percentage of transformation of austenite achieved at a given temperature.

Q: How are pearlite and martensite related to eutectoid and cooling conditions?

Pearlite forms via eutectoid reaction: above the eutectoid temperature only austenite exists, while below it nucleation and growth produce pearlite with thickness depending on temperature. Martensite forms when austenitized Fe-C alloys are rapidly cooled (quenched) to a relatively low temperature, producing a non-equilibrium phase.

Q: Which factors affect the shape and results of a TTT diagram?

The diagram is influenced by steel composition (carbon content and alloying element content), austenite grain size, and the heterogeneity of austenite.

Oct 26, 2026

Upload by Jasmine

TTT (time-temperature-transformation) curves describe temperature-dependent transformation behavior in steel alloys by relating temperature to the logarithm of time. The notes explain how isothermal transformations define the start and finish of phase changes and how the fraction of transformed austenite depends on holding temperature and time. Key microstructures are summarized, including pearlite and martensite, with links to eutectoid reactions and layer thickness trends. The material also lists what TTT diagrams provide and which factors (composition, grain size, austenite heterogeneity) control transformation outcomes, followed by worked examples for final structures under different cooling paths.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages9

Reading Time23 min

AccessFree to download

TTT Curves: Definition and Purpose

Pearlite Formation

Martensite Formation

Interpreting the Isothermal TTT Diagram

What TTT Diagrams Provide

Factors Affecting TTT Diagrams

Mechanical Properties of Fe-C Systems

Fine Pearlite vs Martensite

Worked Examples: Determining Final Microstructures

Time-Temperature-Transformation (TTT) Curves TTT diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. 􀁸 TTT diagram indicates a specific transformation starts and ends and it also shows what percentage of transformation of austeniteat a particular temperature is achieved. 􀁸 The aims of TTT diagrams is determined type of structure for and portion in the curve and to obtained on specific properties . It is also called isothermal transformation diagram Pearlite 􀁸 The eutectoid reaction is fundamental to the development of microstructures in steel alloys. (0.76 wt% C) ⇌ (0.022 wt% C) + Fe3C (6.70 wt% C) -Pearlite is the microstructural product of this transformation. -Above eutectoid temperature: only austenite exists -Below eutectoid temperature: nucleation + growth -The percentage of the transformation product is related to the holding temperature and holding time. -The thickness of the ferrite/cementite layers in pearlite depends on the temperature. With decreasing temperature, the layers become progressively thinner. -At temperatures just below eutectoid 􀁯 relatively thick – layers 􀁯 coarse pearlite. -In the vicinity of 540 􀁱 C 􀁯 relatively thin layers 􀁯 fine pearlite Martensite -Martensite is formed when austenitized Fe-C alloys are rapidly cooled (or quenched) to a relatively low temperature . -Non-equilibrium single phase. -Transformation ofFCC to BCT (body-centered tetragonal) . -The martensite grains nucleate and grow at a very rapid rate . (FCC) Austenite phase (BCT) Martensite (body-centered tetragonal) The time-temperature transformation curves correspond to the start and finish of transformations which extend into the range of temperatures where austenite transforms to pearlite. 􀁸 Above 550 C, austenite transforms completely to pearlite. 􀁸 Below 550 C, both pearlite and bainite are formed. 􀁸 below 450 C, only bainite is formed. From The below fig. :- 􀁸 The horizontal line C-D that runs between the two curves marks the beginning and end of isothermal transformations. 􀁸 The dashed line curves that represents the time to transform half the austenite topearlite. NOTES 􀁸 The thickness of the ferrite and cementite layers in pearlite phase is ~ 8:1. 􀁸 The absolute layer thickness depends on the temperature of the transformation. 􀁸 The higher the temperature, the thicker the layers. The complete isothermal transformation diagram for an iron-carbon alloy of eutectoid composition. A: austenite , B: bainite , M: martensite , P: pearlite TTT diagram gives 1-Nature and type of transformation . 2-Rate of transformation. 3-Stability ofphases under isothermal transformation conditions. 4- Temperature or time required to start or finish transformation . 5- Qualitative information about size scale of product . 6-Hardness of transformed products. Factors affecting TTT diagram 1-Composition of steel-(a) carbon wt%,(b) alloying element wt% 2-Grain size of austenite 3-Heterogeneity of austenite. Mechanical Properties of Fe-C Systems For example :- Fine Pearlite vs Martensite structures Hardness: fine pearlite << martensite Example (1) :- Eutectoid steel cooled from austenite phase at 800 C ,then cooled as (TTT diagram ) as below , find the final structure if cooling path as following : 1-rapid cooling to (350 C) and hold to( 104 sec) , then rapid cooling to room temperature . 2-rapid cooling to (650 C) and hold to( 20 sec) , then, rapid cooling to )400 C) and hold at (103 sec ) , then cooled to)room temperature . ANS.:- From TTT diagrams below 1-Austenite transform to 100% bainite . 2-Austenite transform to 50% bainite and 50% perlite . Example (2) Using the isothermal transformation diagram for an iron-carbon alloy of eutectoid composition, specify the nature of the final microstructure of a small specimen that has been subjected to the following time-temperature treatments.

FAQs

What does a TTT diagram show for a steel alloy?

A TTT diagram plots temperature versus logarithm of time and indicates the start and finish of transformations. It also shows the percentage of transformation of austenite achieved at a given temperature.

How are pearlite and martensite related to eutectoid and cooling conditions?

Pearlite forms via eutectoid reaction: above the eutectoid temperature only austenite exists, while below it nucleation and growth produce pearlite with thickness depending on temperature. Martensite forms when austenitized Fe-C alloys are rapidly cooled (quenched) to a relatively low temperature, producing a non-equilibrium phase.

Which factors affect the shape and results of a TTT diagram?

The diagram is influenced by steel composition (carbon content and alloying element content), austenite grain size, and the heterogeneity of austenite.

Oct 26, 2026

Upload by Jasmine