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Chapter 8
PHASE
METALS
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Contents: Phase transformations
Kinetics of solid state reactions
Isothermal transformation diagrams
Continuous coolin transformation dia rams
Mechanical behaviour of Iron-carbon alloys
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Introduction:
Phase
Metastable system
Constitutional diagram
n erpre a on o p ase agram lever rule
Transformation types
Solid state transformations
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Kinetics of solid state transformation:
Plot of fraction reacted versus the logarithm of time typical of manysolid-state transformations in which temperature is held constant.
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Avrami equation:
(10.1)
(10.2)
e RTQ
Ar
=
For most reactions and over specific temperature ranges,
rate increases with temperature according to
(10.3)
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Percent recrystallization as a function of time and at constant
temperature for pure copper.
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10.5 ISOTHERMAL TRANSFORMATION DIAGRAMS
1600
1400
1200
L
austenite
+LL+Fe3C
T(C)
ferrite
1000
800
600
400
0 1 2 3 4 5 66.7
+Fe3CFe3C
cementite+Fe3C
+
(Fe) Co, wt% C
Eutectoid:
0.7
7
727CT
0
.022
Undercooling byT: Ttransf.< 727C
Equil. cooling: Ttransf.= 727C
TRANSFORMATIONS & UNDERCOOLING
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10.5 ISOTHERMAL TRANSFORMATION DIAGRAMS
PEARLITE
austenite ferrite + cementite
(727C) PEARLITE
cooling
heating
(0.76 wt% C) (0.022 wt% C) + Fe3C(6.7 wt% C)
Iron- Iron Carbide Eutectoid reaction
Fe
(Austenite)
Eutectoidtransformation
C FCC
Fe3C
(cementite)
(ferrite)
+
(BCC)
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10.5 ISOTHERMAL TRANSFORMATION DIAGRAMS
(PEARLITE)
Austenite ()
grainboundary
cementite (Fe3C)
ferrite ()
Diffusive flowof C needed
pearlitegrowthdirection
Carbon atoms diffuse away from the 0.022 wt% ferrite regions and to the6.7 wt% cementite layers, as the pearlite extends from the grain boundary
into the unreacted austenite grain.
Mechanism of Iron-Iron Carbide Eutectoid reaction
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10.5 ISOTHERMAL TRANSFORMATION DIAGRAMS(PEARLITE)
% Pearlite
0
50
100
Nucleationregime
Growthregime
log (time)t50
Nucleation rate increases w/ T
Growth rate increases w/ T
Reaction rate is a result of nucleation and growth of crystals.
Nucleation rate high
T just below TE T moderately below TE T way below TENucleation rate low
Growth rate high
pearlitecolony
Nucleation rate med .
Growth rate med. Growth rate low
TE = Eutectoid temperature
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10.5 ISOTHERMAL TRANSFORMATION DIAGRAMS(PEARLITE)
100
50
01 102 104
T=675Cy,
%t
ran
sformed
time (s)
400
500
600
700
1 10 102 103 104 105
0%p
earlite
100%
50%
Austenite (stable) TE (727C)Austenite(unstable)
Pearlite
time (s)
isothermal transformation at 675C
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10.5 ISOTHERMAL TRANSFORMATION DIAGRAMSor
Time-Temperature-Transformation (T-T-T) Plot(PEARLITE)
The austenite-to-pearlite transformation will occuronly if an alloy is supercooled to below the eutectoidtemperature.
austenite will be present and to the right of finishcurve , only pearlite will exist.
The time necessary for the transformation to beginand then end depends on temperature.
The transformation rate increases with decreasingtemperature.
The start and finish curves are nearly parallel and
they approach the eutectoid line asymptotically.
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10.5 ISOTHERMAL TRANSFORMATION DIAGRAMS
orTime-Temperature-Transformation (T-T-T) Plot
(PEARLITE)
The Isothermal Transformation Diagram or TTT diagramis valid only for a particular composition
These plots are accurate only for transformations in
throughout the duration of reaction.
At compositions other than eutectoid, a proeutectoidphase (ferrite or cementite) coexist with
pearlite.Additional curves for proeutectoid transformationmust be included on TTT diagrams.
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10.5 ISOTHERMAL TRANSFORMATION DIAGRAMS(PEARLITE)
T(C)Austenite (stable)
TE (727C)
Eutectoid composition, Co = 0.77wt%C
Begin at T > 727C
Rapidly cool to 625C and hold isothermally.
1 10 102 103 104 105 time (s)
500
600
Pearlite
0%p
earlite
100%
50%
Coarse pearlite
Fine pearlite
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PEARLITE MORPHOLOGY
m
Ttransfjust below TELarger T: diffusion is faster, C
atoms can diffuse relatively longdistances. --Pearlite is coarser.
Two cases:
Ttransfwell below TE--Smaller T: diffusion is slower
--Pearlite is finer.
10
- Smaller T:
colonies arelarger
- Larger T:
colonies aresmaller
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10.5 ISOTHERMAL TRANSFORMATION DIAGRAMS
(PEARLITE) (Temperature Dependence)
675C(T smaller)
50
100
(%
pearlite) 0
50
600C
(T larger)650C
austen
ite
1 10 102 103
time (s)
0y
Data were collected after rapidly cooling a specimen composed of 100%austenite to the temperature indicated; that temperature was maintained
constant throughout the course of the reaction.
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Banite (Transformation product of austenite)- Temperature
range 215-540 0C
Bainite consists of ferrite and cementite phases.
Microstructural details of bainite can be broadly visible only
Banite Formation
using electron microscopy. Data were collected after rapidly cooling a specimen composed
of 100% austenite to the temperature indicated; that temperature
was maintained constant throughout the course of the reaction.
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10.5 ISOTHERMAL TRANSFORMATION DIAGRAMS
(BAINITE)
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Rate of reaction is higher at point N
Bainite transformation rate increases exponentially with rising
temperature
Important Points, ITD- Bainite
Lower Banite 200 to 300 0C
Pearlitic and Banitic transformations are competitive,
transformation between pearlite and bainite not possiblewithout first reheating to form austenite.
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marensite
Upper Banite
cementite
ferrite
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Lower Banite
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Using the Isothermal transformation diagram specify
1) Nature of final Microstructure
2) Approximate percentages of each microconstituents
for a small specimen that has been subjected to following time-temperature treatments
1) Cool rapidly to 400 0C, hold for 200 s, then quench to room
EX.1
temperature
2) Rapidly cool to 575 0C, hold for 20s, rapidly cool to 350 0C,
hold for 100 s, then quench to room temperature
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Spheroidite Formation
Heating pearlitic or bainitic microstructures in the range above
700 0C for between 18- 24 h, gives Spheroidite
This transformation has occurred by additional carbon diffusion
No change in the compositions or relative amounts of ferriteand cementite phases
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Spheroidite Microstructure
Sphere like small particles are cementite, the continuous
phase is ferrite
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Martensite Formation
Martensite is a nonequlibrium structure and transformation
does not involve diffusion
During martensitic transformation, quenching rate is rapid
enough to prevent carbon diffusion
FCC Austenite BCT MartensitePolymeric
transformationPolymeric
transformation
xx x
x
x
x potentialC atom sites
Fe atomsites
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ISOTHERMAL TRANSFORMATION DIAGRAMS
(MARTENSITE)
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Important Points, ITD- Martensite
Martensitic transformation temperature, 100 0C to 215 0C,
(Low), No carbon diffusion
Athermal transformation
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Massive martensite microstructure
(Lath)
Marensite grain as long and thin plates
For alloys containing less than 0.6Wt % C
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Plate Martensitic microstructure
(Lenticular)
Neddle or plate like martensitic grain in untranformed austenite
For alloys containing more than 0.6Wt % C
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ITD for Alloy steel
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10.5 ISOTHERMAL TRANSFORMATION DIAGRAMS
(MARTENSITE)
The presence of alloying elements other than
carbon (e.g., Cr, Ni, Mo and W) may causesignificant changes in the positions and shapes ofthe curves in the isothermal transformation
.
1. Shifting to longer times the nose of the austenite-to-pearlite transformation, and
2. The formation of a separate bainite nose.
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ITD for Iron-carbon alloy
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EX. 2
Sketch and label time temperature paths on T-T-T
diagram to produce following microstructures
1) 100% Coarse pearlite2) 50% martensite and 50% austenite
3 50% coarse earlite 25% bainite and 25%
martensite.
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EX 3
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ISOTHERMAL TRANSFORMATION DIAGRAMS
Using the isothermal transformation diagram for an
iron carbon alloy of eutectoid composition, specify thenature of the final microstructure, subjected to thefollowing time- temperature treatments:
EX.3
(a) Rapidly cool to 350
C, hold for 10
4
s, and quench tothe room temperature.
(b) Rapidly cool to 250C, hold for 100s, and quench to
the room temperature.
(c) Rapidly cool to 650C, hold for 20s, rapidly cool to400C, hold for 103s, and quench to the room
temperature.
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10.5 ISOTHERMAL TRANSFORMATION DIAGRAMSExample Problem
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Isothermal heat treatments are not most practical to conduct
Most heat treatments for steels involve the continuous cooling
of a specimen to room temperature.
CONTINUOUS COOLING TRANSFORMATION
DIAGRAMS
transformation at constantly changing temperature
For continuous cooling, the time required for a reaction to
begin and end is delayed. Thus the isothermal curves are shifted
to longer times and lower temperatures
Normally banite will not form for any carbon steel when
contineously cooled to room temperature
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CONTINUOUS COOLING TRANSFORMATION DIAGRAMS
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CONTINUOUS COOLING TRANSFORMATION DIAGRAMS
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For thecontinuous
cooling ofeutectoidcomposition,there exists acritical quenchingrate, whichrepresents the
minimum rate ofquenching thatwill produce atotally martensitic
structure.CONTINUOUS COOLING TRANSFORMATION DIAGRAMS
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CCT diagram
for type 4340
alloy steel
Critical
CCT diagram for an alloy
coo ng ra e s
diminished evenby the presence
of carbon
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Sketch and label CCT diagram for a 0.35 wt% C iron carbonalloy to yield the following microstructures. The cooling start
from 850C
(a) fine pearlite and proeutectoid ferrite
(b) Martensite
(c) martensite and proeutectoid ferrite
(d) coarse pearlite and proeutectoid ferrite
EX.4
(e) martensite, fine pearlite, and proeutectoid ferrite.
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Micro constituent Trans- Temp Microstructure
Coarse pearlite 6750c Thick layers
of
& Fe3cFine pearlite 6000c Thin layers of
& Fe3c
Upper banite 5400
c Alternatethick layersof Fe3c
needle and
strips
Lower banite 3000c thin plate
and Fe3c fine
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Spheroidite(Reheatedpearlite or banite)
AT 7000c,Reactiontime high
Fe3csphericalparticles in
matrixMassivemartensite (Lath)
2150c Marensitegrain as long
< 0.6Wt % C and thinplates
Lenticular (Plate)
Martensite >0.6Wt % C
2150c Neddle or
plate likemartensiticgrain inuntranformedustenite
ec an ca e av or o e- oys
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Fine Pearlite
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PEARLITE MORPHOLOGY
m
Ttransfjust below TELarger T: diffusion is faster, C
atoms can diffuse relatively longdistances. --Pearlite is coarser.
Two cases:
Ttransfwell below TE--Smaller T: diffusion is slower
--Pearlite is finer.
10
- Smaller T:colonies arelarger
- Larger T:
colonies aresmaller
S h idi Mi
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Spheroidite Microstructure
Sphere like small particles are cementite, the continuous
phase is ferrite
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(a) Edge dislocation (b) Screw dislocation
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Fine pearlite is harder and stronger than Coarse pearlite because
Hard Cementite is reinforcing the soft ferrite in the region adjacent to the boundary
Phase boundaries serves as barriers to dislocation motion during plastic
deformation
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Effect of wt%C
100
%EL
,ft-lb)
801100YS(MPa)
TS(MPa)
Co>0.77wt%C
Hypereutectoid
Co
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240
320
hardness finepearlite
coarsepearlites heroidite
60
90
tility(%AR)
spheroidite
Hypo Hyper Hypo Hyper
Hardness: fine > coarse > spheroidite %AR: fine < coarse < spheroidite
80
160
wt%C0 0.5 1
Brin
ell
0
30
wt%C0 0.5 1
Du
finepearlite
coarse
pearlite
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Fine pearlite is harder and stronger than coarse pearlite,Coarse pearlite is more ductile than fine pearlite.
Reason
1. There is a large adherence between the two phasesacross the boundary.
.
phase. The degree of reinforcement is higher in finepearlite, because of the greater phase boundary area perunit volume of material.
3. Phase boundaries serves as barriers to dislocationmotion
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600
hardness
martensite
Hypo Hyper
Fine Pearlite vs Martensite
0
200
wt%C0 0.5 1
Brin
ell
fine pearlite
Hardness: fine pearlite
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Tempering Temperature range : 250 650C
Martensite (BCT, Single phase) ------------ Tempered martensite (F + Fe3C phases)
Microstructure
Tempered Martensite
Enhanced ductility and toughness
Mechanical properties depends on cementite particle size
TEMPERED MARTENSITE
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reduces brittleness of martensite,
reduces internal stress caused by quenching.
TEMPERED MARTENSITE
YS(MPa)
TS(MPa)
1600
1800
TS
YS
decreases TS, YS but increases %AR
800
1000
1200
30
40
50
60
200 400 600Tempering T (C)
%AR%AR
9
produces extremely small Fe3C particles surrounded by.
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gth it
yMartensite
T Martensite
Stre
n
Ducti
fine pearlitecoarse pearlite
spheroidite
General Trends