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Kwame Nkrumah University of

Science & Technology, Kumasi, Ghana

MSE 260

Phase Transformations

Ing. Anthony Andrews (PhD)Department of Materials Engineering

Faculty of Mechanical and Chemical Engineering

College of Engineering

Website: www.anthonydrews.wordpress.com www.knust.edu.gh

An Fe-C Alloy of Eutectoid CompositionExample problem:

Specify the nature of the final microstructure (in terms of

microconstituents present and approx. percentage) of a small

specimen that has been subjected to the following time-temperature

treatments. The specimen begins at 760oC and has been held at this

temperature long enough to have a complete austenitic structure.

(a) Rapid cool to 350oC hold for 104s and quenched to Tr

(b) Rapid cool to 250oC hold for 100s and quenched to Tr

(c) Rapid cool to 650oC hold for 20s rapidly cooled to 400oC, hold

for 103s, and quenched to Tr

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(a) Rapidly cool to 350°C, hold

for 104 s, and quench to room

temperature.

The sample starts as austenite

and the transforms from ~10 s

through 500 s to bainite.

=> At 104 s, 100% bainite is

obtained. No further

transformation possible though

line passes through martensite

region

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(b) Rapidly cool to 250°C, hold

for 100 s, and quench to

room temperature.

100% austenite remain at

250°C but when cooled the

austenite converts to martensite

(starting at about 215°C)

=> Microstructure at room

temperature is 100% martensite

(c) Rapidly cool to 650°C, hold for

20 s, rapidly cool to 400°C,

hold for 103 s, and quench to

room temperature.

The sample begins to transform to

pearlite after 7s; to about 50%

after 20s.

Very little transformation takes

place during rapid cooling to

400oC

At 400oC, the remaining austenite

converts to bainite.

In the end, you have 50% pearlite

and 50% bainite.

Practical considerations

inside is slow cooled (~0.1°C/sec)

inside, slow

cooled - pearlite

outside is fast cooled (~800°C/sec)

outside, fast cooled -

martensite

Hard case on a ductile interior

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Continuous Cooling

Transformation Diagrams• Isothermal heat treatment are not the most practical to conduct

– Alloy must be rapidly cooled to and maintained at high

temperature from a higher temperature above the eutectoid

• Most heat treatment of steels involve continuous cooling of a

specimen to room temperature

– Diagram must be modified for transformations that occur as

the temperature is constantly changing

• For continuous cooling, the time required for a reaction to begin

and end is delayedwww.knust.edu.gh

Continuous Cooling Transformation

Diagrams

• A modified curves are

called continuous

cooling transformation

(CCT) diagrams

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Continuous Cooling Transformation Diagrams

• Two cooling curves:

moderately fast and

slow rates

• Microstructures => fine

and coarse pearlite

• Plain carbon steels will

not form bainite

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Slower cooling curves

allow for equilibrium

microstructures to appear.

There appears a critical

cooling rate at which

the material bypasses

all the equilibrium

phases

Steel alloys

Iron–carbon alloys

containing less than about

0.25 wt% carbon are not

normally heat-treated to

form martensite

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Dynamic Phase Transformation

On the isothermal transformation diagram for 0.45 wt% C

in Fe-C alloy, sketch and label the temperature-time paths to

produce the following microstructures:

a) 42% proeutectoid ferrite and 58% coarse pearlite

b) 50% fine pearlite and 50% bainite

c) 100% martensite

d) 50% martensite and 50% austenite

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Example problem for Co = 0.45 wt%

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Example problem for Co = 0.45 wt%

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Example problem for Co = 0.45 wt%

Kwame Nkrumah University of

Science & Technology, Kumasi, Ghana

Mechanical Behaviour of

Iron – Carbon Alloys

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Introduction

• For all the microstructures (except martensite), two

phases are present

– Ferrite

– Cementite

• Cementite is harder and more brittle than ferrite =>

increasing cementite fraction makes harder, less ductile

material

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Mechanical Properties – Influence of C

content

0.76

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Mechanical Properties –

Influence of C content

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Mechanical Properties – Fine Pearlite vs.

Coarse Pearlite vs. Spherodite

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Mechanical Effects of the Microstructure -Bainite

Because bainite steels have finer structure (i.e. smaller α-ferrite and

Fe3C particles), they are stronger and harder than pearlitic ones

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Mechanical Properties of Martensite

Tempered martensite phase

transforms to Fe3C and α-Fe

As it phase transforms the

soft matrix of α-Fe leads to

the drop in hardness.

Tempered Martensite

Micrograph of tempered martensite

• Produces extremely small Fe3C particles surrounded by α ferrite

• Decreases TS, YS but increases % RA

Summary: Austenite Transformation

Solid lines are diffusional transformations,

dashed are diffusionless martensitic

transformation

Summary: Processing Options

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Bainite

coarse fine

Austenite

Martensite

Moderate cooling (AS)

Isothermal treatment (PCS)

Tempered

Martensite

Pearlite

AS: Alloy Steel

PCS: Plain-carbon Steel

Slow

Cooling

Rapid

Quench

Spheroidite

Re-heat

Re-heat

Summary of microstructures and mechanical properties of Fe-C alloys