Ch-8 Compatibility Mode

7/28/2019 Ch-8 Compatibility Mode

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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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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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(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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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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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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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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(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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(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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(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

Ch-8 Compatibility Mode

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