Ttt

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436-221/223/281 Unit 2: Engineering Materials Dr. K. Xia

Phase Transformations in Fe-C System• Fe-C system is chosen to

illustrate some typicaltransformations and resultingmicrostructures, not to mention itsimportance in engineeringapplications of steels

Isothermal TransformationDiagrams

• Such a diagram plots the fractionof transformation versus timewhen the alloy is heldisothermally (i. e. at a constanttemperature) at varioustemperatures

• Example: formation of pearlite ina eutectoid alloy (0.76C) from theeutectoid reactionγ = α + Fe3C (at T = Ti)

– fraction of transformation vs timecurves at various temperatures

Reading: 10.5-10.9 (5th & 6th ed)

y = 1− exp( −kt n )Avrami equation

start time: t0

completion time: t100

half time:t50

Corresponding to each temperature,we can find t0, t50 and t100.

436-221/223/281 Unit 2: Engineering Materials Dr. K. Xia

Phase Transformations in Fe-C System (2)– plot temperature versus

log t0, t50 and t100,respectively

• connecting all thepoints for t0 = thestart curve

• connecting all thepoints for t50 = thehalf curve

• connecting all thepoints for t100 = thecompletion curve

– these curves are alsocalled TTT curves or Ccurves

– the rate of transformationis the fastest at anintermediate temperature(the nose)

675650

no reaction above Te

t50 = ~28 hat ~720°C

t50 < 5 sat 600°C

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436-221/223/281 Unit 2: Engineering Materials Dr. K. Xia

Phase Transformations in Fe-C System (3)

ABCD = a real heat treatment route

due to highdiffusion rate

due to lowdiffusion rate

436-221/223/281 Unit 2: Engineering Materials Dr. K. Xia

Phase Transformations in Fe-C System (4)Coarsepearlite

Finepearlite

Micro-structures

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436-221/223/281 Unit 2: Engineering Materials Dr. K. Xia

Phase Transformations in Fe-C System (5)• Extended (down to ~215°C) TTT diagram for the eutectoid steel

– at T < ~540°C, a new microstructure - bainite - forms as a result ofphase transformation

– Bainite takes a needle or a plate shape and consists of elongatedcementite in a matrix of ferrite

cementite

ferrite

bainite

~540°C

pearlite

austenite

nose -fastesttrans-formation

436-221/223/281 Unit 2: Engineering Materials Dr. K. Xia

Phase Transformations in Fe-C System (6)• Complete TTT diagram

for the eutectoid steel– when cooled so fast

that no C diffusion ispossible, a newphase - martensite -forms (at even lowertemperatures) fromthe austenite phase

– the martensite (M)transformation isdiffusionless, andthus takes virtully notime (the amount of Mdepends on T only,NOT on time)

Above M(start),no M forms

At M(90%), 90% of Atransforms into M

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436-221/223/281 Unit 2: Engineering Materials Dr. K. Xia

Phase Transformations in Fe-C System (7)– martensite

• Fe forms a body-centredtetragonal (BCT) unit cell

• C atoms remain in theinterstitial sites (no formation ofcementite)

• a non-equilibrium phase (notpresent in the phase diagram)

Austenite(FCC)

Martensite(BCT)

c > a

Dark phase: martensite of needle shape

Light phase: untransformed austenitehardandbrittle

furthertransfor-mationpossible

436-221/223/281 Unit 2: Engineering Materials Dr. K. Xia

Phase Transformations in Fe-C System (8)Example 10.1• Fe-C of eutectoid composition• starting microstructure: γ at

760°C• Find: microstructures and

percentages of eachmicroconstituent after thefollowing heat treatments– (a) rapid cooling to 350°C,

holding for 10000 s, andquenching to RT

– (b) rapid cooling to 250°C,holding for 100 s, andquenching to RT

– (c) rapid cooling to 650°C,holding for 20 s, rapidcooling to 400°C, holdingfor 1000 s, and quenchingto RT

No Mforms asthe startingphase is Bor P

50%B 100%B100%A A to Bstarts

100%A 100%A

A to M starts50%M

90%M

100%A

A to Pstarts 50%P

50%A

no moretransformation

A to Bstarts

A to Bcompletes

These phasesand structurescannot beread from theequilibriumphasediagram!!

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436-221/223/281 Unit 2: Engineering Materials Dr. K. Xia

Phase Transformations in Fe-C System (9)• Effect of composition

– C: non-eutectoid compositions• e. g. a hypereutectoid alloy (Co)

– proeutectoid cementite mayappear

Co

T'

T'

ProeutectoidFe3C

A to Cstarts A & C coexist

C C+

A to Cstarts

A to Ccompletes

A to Pstarts

A to Pcompletes

436-221/223/281 Unit 2: Engineering Materials Dr. K. Xia

Phase Transformations in Fe-C System (10)– other alloying elements

• shape and positionof the starting, halfand completion linesmay be different

Fe-0.76C

4340 alloy steel: Fe-0.4C-1.8Ni-0.7Mn-0.8Cr-0.2Si-0.25Mo

proeutectoidferrite

HW: follow isothermal phase transformationsof alloy steel 4340 at 350, 550, 650 and 750°C

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436-221/223/281 Unit 2: Engineering Materials Dr. K. Xia

Phase Transformations in Fe-C System (11)Continuous cooling

transformation (CCT)diagrams

• In practice, the cooling is oftencontinuously carried out at acertain rate to roomtemperature, rather than rapidcooling to a certain temperatureand holding at the temperaturefor a period of time when theisothermal transformationdiagrams are applicable.

• For continuous cooling, thetransformation diagrams aredifferent: various curves areusually shifted to lowertemperatures and longer times.Also, some phases may notappear at all.

continuouscooling

isothermalholding

436-221/223/281 Unit 2: Engineering Materials Dr. K. Xia

Phase Transformations in Fe-C System (12)• Example: continuous cooling of a

eutectoid steel

A

P

P + MM

A to Pstarts

A to Pfinishes:100% P

A to P stops:P +remaining A

remainingA to Mstarts

A to Mstarts

A to Ptransfor-mationstops atline AB

Cooling rate determines whatmicrostructures will be presentfollowing heat treatment

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436-221/223/281 Unit 2: Engineering Materials Dr. K. Xia

Phase Transformations in Fe-C System (13)• Things can certainly get

much more complicatedwith alloying, but thediagram can be read inthe same way.

4340 alloy steel

HW: Try to reach thefinal microstructuresfollowing cooling atdifferent rates