TTT diagram

TIME-TEMPERATURE-TRANSFORMATION DIAGRAM

Dr. Ken Lewis ISAT 430 2Spring 2001

0.8C

Phase diagram and TTT diagram

• Phase diagram :– Describes equilibrium microstructural development that is

obtained at extremely sow cooling or heating conditions. – Provides no information on time to take to form phase and on

shapes, size and distribution of phase → importance of kinetics

• TTT diagram– For a given alloy composition, the percentage completion of a

given phase transformation on temperature-time axes is described.

Which informations are obtained from phase diagram or TTT diagram?

Nucleation And Growth• For a eutectic reaction :

L (XE) → + at TE

(experiment)

(1) Quench the liquid from Tm to some lower temperature

(2) Measure the time for solidification, to go to completion at that temperature

• TTT diagram–The time required for the liquid to

transform to the eutectic microstructure is function of time

Description of new phase from melt

local atomic fluctuation

formation of many small nuclei

growth of nuclei with critical size or greater

• Homogeneous nucleation : occurs within a homogeneous medium.

• Heterogeneous nucleation : nucleation occurs at some structural imperfection such as foreign surface, and hence with reduced surface energy

For the transformation of liquid to solid ;

L → S ;

and for forming a spherical nucleus

GT = total free energy change

r = radius of embryo or nucleus = specific surface free energyGV = volume free energy change

Change in free energy for homogeneous nucleation

mV T

THG

23 43

4rGrG VT Solid-liquid

interfaceLiquid

Solid

n* : number of spherical nucleus of radius r*

0dr

Gd T at r = r*

VG

rr

2*

2

2

2

3*

3

16

)(3

16

TH

rT

G

rG m

V

)exp(*

*

kT

Gsn

24 r

VGrr 32

3

44 TG

Nuclei larger than criticalsize (r*) are stable and can continue to grow.

Nucleation rate• Nucleation rate, : number of nuclei / unit volume / unit time

)exp()exp(*.

kT

E

kT

GN D

,where G* : energy barrier to form a nucleus stable to grow.

ED : activation energy for diffusion

.

N

• At T just below Tm, – Diffusion rate is rapid but very few

nuclei are formed.

∵ G* ↑• At very low T (T ↑)

– Diffusion rate is extremely low but many nuclei are formed

∵ G* ↓• At intermediate T

– Max..

N

22* 1

)(

1

TTTG

m

Growth of nuclei

• Growth of Nuclei– Growth of nuclei is a diffusional process

, where QD : activation energy for self diffusion

• Transformation rate of a phase :

RTQD

ceG

.

..

GN

TTT Diagram• Temperature-time-transformation curve• TTT diagrams represent specific thermal

histories for the given microstructure.

Isothermal transformation of eutectoid steel

TTT diagram for eutectoid steel

Transformations of austenite : → + Fe3C

A. Diffusional transformations

1) At slightly lower T below 727 : ℃ T <<• Coarse pearlite

: nucleation rate is very low.: diffusion rate is very high.

2) As the Tt (trans. temp.) decreases to 500 ℃

• Fine pearlite: nucleation rate increases.: diffusion rate decreases.

Strength : (MPa) = 139 + 46.4 S-1 S : intermetallic spacing

655 ℃

600 ℃

534 ℃

487 ℃

pearlite

A. Diffusional transformations

3) 250 < T℃ t < 500 , below the nose in TTT diagram.℃

• Driving force for the transformation ( → + Fe3C) is very high.• Diffusion rate is very low.• Nucleation rate is very high.

→ + Fe3C : Bainite ; cementite in the form of needle type.

495 ℃ 410 ℃

bainite

B. Diffusionless Transformations - Martensitic trans.

: When the austenite is quenched to temp. below Ms → ’ (martensite)

: Driving force for trans. of austenite → extremely high. Diffusoin rate is extremely slow. : instead of the diffusional migration of carbon atoms to produce separate and Fe3C phases, the matensite transformation involves the sudden reorientation of C and Fe atoms from the austenite (FCC) to a body centered tetragonal (bct) solid solution.

→ ’ (martensite), a solid solution

: super saturated carbon atoms in ’ + shearlike transformation → very hard and brittle phase

martensite

1) Diffusionless transformation → no compositional change during transformation.

2) The trans. of → ’ starts at Ms temp. and finishes at Mf temp.

3) → ’ (BCT) ; c/a increases as the carbon content increases.


The Time – Temperature – Transformation Curve (TTT)

• At slow cooling rates the trajectory can pass through the Pearlite and Bainite regions

• Pearlite is formed by slow cooling– Trajectory passes through Ps

above the nose of the TTT curve

• Bainite– Produced by rapid cooling to a

temperature above Ms– Nose of cooling curve avoided.



• If cooling is rapid enough austenite is transformed into Martensite.– FCC > BCT– Time dependent diffusion

separation of ferrite and iron carbide is not necessary

• Transformation begins at Ms and ends at Mf.– If cooling stopped it will

transition into bainite and Martensite.



• Composition Specific– Here 0.8% carbon

• At different compositions, shape is different

Full TTT Diagram

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

A: austenite

B: bainite

M: martensite

P: pearlite

Martensite

• Diffusionless transformation of FCC to BCT (more volume!)

• Lenticular structure• Very hard & very brittle.

TTT Diagrams

TTT diagram for Hypo-eutectoid steel.

TTT diagram for a hypereutectoid Steel (1.13 wt% C)

So What’s a CCT Diagram?

• Phase Transformations and Production of Microconstituents takes TIME.

• Higher Temperature = Less 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.

Slow Cooling

Time in region indicates amount of microconstituent!

Medium Cooling

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

Fast Cooling

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

Continuous cooling transformation diagram for eutectoid steels

• Normalizing : heat the steel into region → cool it in air → fine pearlite

• Annealing : heat the steel into region → cool it in furnace (power off) → coarse pearlite

• THANKS

Technology

TTT diagram