time temperature transformation (ttt) or isothermal

8/10/2019 Time Temperature Transformation (TTT) or Isothermal

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Time Temperature Transformation (TTT) or

Isothermal Transformation (IT) Diagrams

by

Wahyono Suprapto


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Transformations & Undercooling

For transformation to occur, mustcool to below 727C

Eutectoidtransformation (Fe-Fe3C system):+ Fe3C

0.76 wt% C0.022 wt% C6.7 wt% C

Fe3

C(ce

mentite)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

(austenite)

+L

+Fe3C

+Fe3C

L+Fe3C

(Fe)C, wt% C

1148C

T(C)

ferrite

727C

Eutectoid:

Equil. Cooling: Ttransf.= 727C

T

Undercooling by Ttransf.< 727 C

0.7

6

0.0

22


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Coarse pearlite formed at higher temperaturesrelatively soft

Fine pearlite formed at lower temperaturesrelativelyhard

Transformation of austenite to pearlite:

pearlitegrowth

direction

Austenite ( )grainboundary

cementite (Fe3C)

Ferrite ( )

For this transformation,

rate increases with ( T)

[TeutectoidT ].675C

(T

smaller)0

50

%pearlite

600C

( T larger)650C

100

Diffusion of Cduring transformation

Carbon

diffusion

Eutectoid Transformation Rate ~ T


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Rate is a result of nucleation and growth of crystals.

Examples:

Nucleation and Growth


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6

Rate of Phase Transformation

Avrami equation => y= 1- exp (-ktn)

transformation complete

log t

Frac

tiontransformed,y

Fixed T

fractiontransformed

time

0.5

By convention rate= 1 / t0.5

Fraction

transformed

depends on

time

maximum rate reachednow amountunconverted decreases so rate slows

t0.5

rate increases as surface area increases

& nuclei grow

Avrami relationship- the rate is defined as the inverse of the time to complete half of the

transformation. This describes most solid-state transformations that involve diffusion.


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In general, rate increases as T

r= 1/t0.5=Ae-Q/RT

R= gas constant

T= temperature (K)

A= preexponential rate factor

Q= activation energy

ris often small so equilibrium is not possible.

Arrhenius expression

Adapted from Fig. 10.11,

Callister 7e. (Fig. 10.11

adapted from B.F. Decker and

D. Harker, "Recrystallization in

Rolled Copper", Trans AIME,

188, 1950, p. 888.)

135 C 119 C 113 C 102 C 88 C 43 C

1 10 102

104

Temperature Dependence of

Transformation Rate


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Generation of Isothermal Transformation Diagrams

The Fe-Fe3C system, for Co= 0.76 wt% C A transformation temperature of 675 C.

100

50

01 102 104

T = 675C

%tr

ansformed

time (s)

400

500

600

700

1 10 102 103 104 105

Austenite (stable)TE(727 C)Austenite

(unstable)Pearlite

T(C)

time (s)

isothermal transformation at 675C

Consider:


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Isothermal TransformationDiagrams2 solid curves are plotted:

one represents the time

required at eachtemperature for the start of

the transformation;

the other is for

transformation completion.

The dashed curve

corresponds to 50%completion.

The austenite to pearlite

transformation will occur

only if the alloy is

supercooledto below the

eutectoid temperature(727C).

Time for process to complete

depends on the temperature.


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Eutectoidiron-carbon alloy; composition, Co= 0.76 wt% C

Begin at T > 727C

Rapidly cool to 625C and hold isothermally.

Isothermal Transformation Diagram

Austenite-to-Pearlite


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Continuous cooling

diagram for a 4340 steelalloy and several cooling

curves superimposed.

This demonstrates the

dependence of the final

microstructureon the

transformations thatoccur during cooling.

Alloying elements used to

modify the critical cooling

rate for martensite are

chromium, nickel,

molybdenum,manganese, silicon and

tungsten.


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Hardness versus tempering time for a water-quenched eutectoid plain carbon steel (1080) that

has been rapidly quenched to form martensite.

Rockwell C and Brinell Hardness


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In commercial alloy steels, which are multi component systems, alloying elements

can be found

(1) in the free state;

(2) as intermetallic compounds with iron or with each other; ZrFe2, Zr3Fe, FeZn

phases

(3) as oxides, sulfides, and other nonmetal inclusions;

(4) in the carbide phase as a solution in cementite or in the form of independent

compounds with carbon (special carbides); or

(5) as a solution in iron.

Generally hardenability is determined by the distance below the surface at which

50% M + (25%B + 25%P) or 50HRC is obtained.Andhardenability depends on:

Carbon content.

The amount of alloying elements dissolved in austenite during the austenitizing

treatment. The austenite grain size.


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time temperature transformation (ttt) or isothermal

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