introduction to engineering materials engr2000...

Introduction to Engineering MaterialsENGR2000

Chapter 10: Phase Transformations in Metals(10.1-2, 10.5-9)

Dr. Coates

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10.2 Basic Concepts

• Phase transformations involve alteration of themicrostructure– Diffusion dependent transformations

– Diffusionless transformations

1. No change in number or composition of phases present2. Change in phase or composition

1. A metastable phase is produced




Terminology for microstructures in Fe-C alloys




Steels

• Plain carbon steels– Steels in which C is the primary alloying element

• Alloy steels– Contain significant concentrations of alloying elements

such as Cr, Ni, Mo, W, etc.




-ferrite- low in C and for most cases, can be considered

pure Fe




10.5 Isothermal Transformation Diagrams

• IT diagrams– Represent time & temperature dependence of

transformations




• Besides temperature, what other parameters can bevaried to induce a phase transformation?

• External pressure• Time• composition




Eutectoidreaction

CwtCFeCwtCwt .%7.6.%022.0.%76.0:reaction Eutectoid

3

Iron-Iron carbide phase diagram




Temperature dependence of an iron-carbon alloy of eutectoid composition

Date collected after rapidly cooling a specimen of 100% austenite




Isothermal Transformation diagram for the eutectoid alloy (0.76 wt % C)




Points to note on using IT diagrams

• Diagrams are valid only for specific alloycompositions.

• Diagrams are accurate only for transformations inwhich the temperature of the alloy is held constantthroughout the duration of the reaction.




Pearlite- a two-phase mixture with layers of alternating

ferrite & cementite• Cooling 0.76 wt. % C alloy to just below the eutectoid

temperature…single phase austenite transforms to pearlite




Heat treatment




• How does temperature affect transformation rate?

• transformation rate increases with decreasing temperature

• What phase exists at temperatures above eutectoid

• Austenite only

• Austenite only

• What phase is present to the left of the transformationstart curve?

• What phase is present to the right of the transformationfinish curve?

• Pearlite only




Coarse pearlite and fine pearlite

• Coarse pearlite– At temperatures just below the eutectoid, relatively

thick layers of both the -ferrite and cementite phasesare produced. (high diffusion rates)

• Fine pearlite– With decreasing temperature ~ 540 C, the carbon

diffusion rate decreases and the layers become thinner.(low diffusion rates)




Microstructure of pearlite

Why should we care about lamellar thickness?




Bainite-the ferrite phase is in the form of plates with

cementite in-between

• Cooling 0.76 wt. % C alloy to further below the eutectoidtemperature (below 550 C) … single phase austenitetransforms to bainite (a two-phase mixture of ferrite &cementite)

pearlite

bainite




No proeutectoid phase forms with bainite!




Where does maximum rate of transformation occur?




Microstructure of bainite




Pearlite and Bainite transformations

• Once a portion of an alloy has transformed toeither pearlite or bainite, transformation to othermicrostructure is not possible without reheating toform austenite.




Spheroidite• If a steel alloy with either pearlitic or bainitic

microstructures is heated to a temperature belowthe eutectoid and left at that temperature for a longperiod of time…– At 700 C for 18 to 24 hrs– Cementite phase appears as sphere-like particles

embedded in a continuous -ferrite matrix.

• Transformation occurs by additional carbondiffusion with no change in compositions of relativeamounts of ferrite and cementite phases!




• Which is more stable, the pearlite or spheroiditicmicrostructure?

Spheroiditic microstructures are more stable than pearlitic ones. Since pearlite transforms to spheroidite, the latter is more stable




Martensite

• Quenching (cooling rapidly) 0.76 wt. % C alloy to~200 C…– amount of transformed phase depends solely on

temperature & not on time– same composition as the parent phase– Diffusionless transformation– A non-equilibrium phase, therefore does not appear on

the iron-iron carbide phase diagram




Martensite• The FCC austenite is transformed to BCT (BCC

that has been elongated along one of itsdimensions) martensite




Martensite- a plate-like or needle-like microstructure




Microstructure of martensite




Horizontal lines- transformation is independent of time




Consider an alloy of eutectoid composition rapidly cooled from a temperature above 727 C to 165 C…what% austenite immediately transforms to martensite?

• Cite two major differences between martensitic andpearlitic transformations

1) atomic diffusion is necessary for the pearlitic transformation, whereas the martensitic transformation is diffusionless; and 2) relative to transformation rate, the martensitic transformation is virtually instantaneous, while the pearlitic transformation is time-dependent.

50%




The effect of alloying

• Alloying elements other than C such as Cr, Ni,Mo, W, etc. cause significant changes in theshapes of the curves in the IT diagrams.




Example 10.1

• Using the IT diagram for an Fe-C alloy ofeutectoid composition, specify the nature of thefinal microstructure (in terms of the microconstituents present and approximate percentages)of a small specimen that has been subjected to thefollowing time-temperature treatments. Assumethat the specimen begins at 760 C and that it hasbeen held at this temperature long enough to haveachieved a complete and homogenous austenitestructure.




(a) Rapidly cool to 350 C, hold for 10,000 s and quench to room temperature




(b) Rapidly cool to 250 C, hold for 100 s and quench to room temperature




(c) Rapidly cool to 650 C, hold for 20 s, rapidly cool to 400 C, hold for 1000 s, and quench to room temperature




Example 8.3-1

• When a eutectoid steel is heat-treated in the austenite phasefield, quenched and isothermally held at temperaturesbelow 727 C, different morphologies of ferrite andcementite are possible. Describe these microstructures andhow they are produced. Austenite

Examples from reference: The Science and Design of Engineering Materials by James P. Schaffer, Ashok Saxena, Stephen D. Antolovich, Thomas H. Sanders, Jr. and Steven B. Warner (Second Edition)




Example 8.3-1- pearlite

• Quenching from above727 C and holding attemperatures below 727 C,but above 550 C results inpearlite.– layered structure of ferrite

and cementite– lower the hold temperature,

finer the layers.




Example 8.3-1- bainite

• Quenching from above727 C and holding attemperatures below 550 C,but above ~200C results inbainite.– cementite needles in a

ferrite matrix– lower the hold temperature,

finer the structure.




Example 8.3-1- spheroidite

• Tempering– spheroidite




Example 8.3-2

• Prepare a sample of pearlite from a sample ofeutectoid steel with a bainitic structure…

• Hint: The only way pearlite, bainite or martensitecan be produced is through the decomposition ofaustenite.

Austenite




Example 8.3-2

• Transform the bainite toaustenite– above 727C

• Quench to a temperaturebelow 727C but above550C– at 600C hold for 10s to

transform the austenite topearlite.




Example 8.3-3

• Describe the microstructures that develop when aeutectoid steel is subjected to the following heattreatments…(a) The steel is rapidly quenched from above 727C to

650C, held at that temperature for 12s, then quenchedto room temperature.

(b) The steel is rapidly quenched from above 727C to600C, held at that temperature for 3s, quenched to350C, held at that temperature for 1000s, thenquenched to room temperature.




Example 8.3-3(a)

• The steel is rapidlyquenched from above727C to 650C, held at thattemperature for 12s, thenquenched to roomtemperature.

pearlite

bainite

martensite

austeniteeutectoid temperature

a 50-50 mixture of pearlite & martensite




Example 8.3-3(b)

• The steel is rapidlyquenched from above727C to 600C, held at thattemperature for 3s,quenched to 350C, held atthat temperature for1000s, then quenched toroom temperature.

pearlite

bainite

martensite

austeniteeutectoid temperature

a 50-50 mixture of pearlite & bianite




48

Adapted from Fig. 10.25,Callister & Rethwisch 8e.

Continuous Cooling Transformation Diagrams

Conversion of isothermal transformation diagram to continuous cooling transformation diagram

Cooling curve




fig_10_26




fig_10_27




fig_10_28




53

Isothermal Heat Treatment Example Problems

On the isothermal transformation diagram for a0.45 wt% C, Fe-C alloy, sketch and label thetime-temperature paths to produce the followingmicrostructures:a) 42% proeutectoid ferrite and 58% coarse pearliteb) 50% fine pearlite and 50% bainitec) 100% martensited) 50% martensite and 50% austenite




54

Solution to Part (a) of Example Problem a) 42% proeutectoid ferrite and 58% coarse pearlite

Isothermally treat at ~ 680ºC

-- all austenite transforms to proeutectoid and coarse pearlite.

A + B

A + P

A + aA

BP

A50%

0

200

400

600

800

0.1 10 103 105

time (s)

M (start)M (50%)M (90%)

Adapted from Fig. 10.29,Callister 5e.

Fe-Fe3C phase diagram, for C0 = 0.45 wt% C

Wpearlite C0 0.0220.76 0.022

= 0.45 0.0220.76 0.022

= 0.58

W = 1 0.58 = 0.42

T (ºC)




55

b) 50% fine pearlite and 50% bainiteSolution to Part (b) of Example Problem

T (ºC)

A + B

A + P

A + aA

BP

A50%

0

200

400

600

800

0.1 10 103 105

time (s)

M (start)M (50%)M (90%)



Then isothermally treat at ~ 470ºC

– all remaining austenite transforms to bainite.

Isothermally treat at ~ 590ºC – 50% of austenite transforms

to fine pearlite.




56

Solutions to Parts (c) & (d) of Example Problem

c) 100% martensite – rapidly quench to roomtemperature

d) 50% martensite & 50% austenite

-- rapidly quench to ~ 290ºC, hold at this temperature

T (ºC)

A + B

A + P

A + aA

BP

A50%

0

200

400

600

800

0.1 10 103 105

time (s)

M (start)M (50%)M (90%)



d)

c)




10.7 Mechanical Behavior of Iron-Carbon Alloys

• Cementite is much harder but more brittle thanferrite.

• Fine pearlite is harder & stronger than coarsepearlite.

• Spheroidite is extremely ductile & tough.• Bainite is stronger & harder than pearlite.• Martensite is the hardest & strongest & most

brittle.




58

Mechanical Props: Influence of C Content

Adapted from Fig. 9.30, Callister & Rethwisch 8e.

• Increase C content: TS and YS increase, %EL decreases

C0 < 0.76 wt% CHypoeutectoid

Pearlite (med)ferrite (soft)

Adapted from Fig. 9.33, Callister & Rethwisch 8e.

C0 > 0.76 wt% CHypereutectoid

Pearlite (med)Cementite

(hard)

Adapted from Fig. 10.29, Callister & Rethwisch 8e. (Fig. 10.29 based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, p. 9.)300

500

700

900

1100YS(MPa)TS(MPa)

wt% C0 0.5 1

hardness

0.76

Hypo Hyper

wt% C0 0.5 10

50

100%EL

Impa

ct e

nerg

y (Iz

od, f

t-lb)

0

40

80

0.76

Hypo Hyper




59

Mechanical Props: Fine Pearlite vs. Coarse Pearlite vs. Spheroidite

Adapted from Fig. 10.30, Callister & Rethwisch 8e. (Fig. 10.30 based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, pp. 9 and 17.)

• Hardness:• %RA:

fine > coarse > spheroiditefine < coarse < spheroidite

80

160

240

320

wt%C0 0.5 1

Brin

ell h

ardn

ess

fine pearlite

coarsepearlitespheroidite

Hypo Hyper

0

30

60

90

wt%CD

uctil

ity (%

RA

)

fine pearlite

coarsepearlite

spheroidite

Hypo Hyper

0 0.5 1




60

Mechanical Props: Fine Pearlite vs. Martensite

• Hardness: fine pearlite << martensite.

Adapted from Fig. 10.32, Callister & Rethwisch 8e. (Fig. 10.32 adapted from Edgar C. Bain, Functions of the Alloying Elements in Steel, American Society for Metals, 1939, p. 36; and R.A. Grange, C.R. Hribal, and L.F. Porter, Metall. Trans. A, Vol. 8A, p. 1776.)

0

200

wt% C0 0.5 1

400

600

Brin

ell h

ardn

ess martensite

fine pearlite

Hypo Hyper




61

Tempered Martensite• tempered martensite less brittle than martensite• tempering reduces internal stresses caused by quenching

Adapted from Fig. 10.33, Callister & Rethwisch 8e. (Fig. 10.33 copyright by United States Steel Corporation, 1971.)

• tempering decreases TS, YS but increases %RA• tempering produces extremely small Fe3C particles surrounded by .

Adapted from Fig. 10.34, Callister & Rethwisch 8e. (Fig. 10.34 adapted from Fig. furnished courtesy of Republic Steel Corporation.)

9 mm

YS(MPa)TS(MPa)

800

1000

1200

1400

1600

1800

30405060

200 400 600Tempering T(ºC)

%RA

TS

YS

%RA

Heat treat martensite to form tempered martensite




62

Summary of Possible Transformations Adapted from Fig. 10.36, Callister & Rethwisch 8e.

Austenite (g)

Pearlite( + Fe3C layers + a proeutectoid phase)

slow cool

Bainite( + elong. Fe3C particles)

moderatecool

Martensite(BCT phase diffusionless

transformation)

rapid quench

Tempered Martensite ( + very fine

Fe3C particles)

reheat

Stre

ngth

Duc

tility

Martensite T Martensite

bainite fine pearlite

coarse pearlite spheroidite

General Trends




Shape Memory Alloys




64

Practice Problems: 10.18, 10.19, 10.20, 10.21

ANNOUNCEMENTSReading: 10.1-10.2, 10.5-10.9




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