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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