Materiale Dhe Perpunim Termik Chapter11

7/29/2019 Materiale Dhe Perpunim Termik Chapter11

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Transformation and

Treatment

Chapter 11


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In the last chapter

We looked at some fairly simple 2

component phase diagrams in some detail

We explored more complicated phasediagrams

We did not examine how the phase change

from one solid phase to another or othersoccurs


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Solid State Reactions

The change from one solid to another has a

lot in common with the solidification

processIt does not happen instantly

Need nucleation

Need time for growth


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Recall

For solidification

DG = 4/3 p r3DGv +4 p r2s

Volume free energy + surface energy

For one solid phase changing to another

DG = 4/3 p r3DGv +4 p r2s + 4/3 p r3e

Volume energy + surface energy + strain energy

Because the new solid does not take up the samevolume as the old solid


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Nucleation

Nucleation usually occurs at grain

boundaries

Unlike solidification, it isnt too hard to geta nucleus going

However, the nucleation rate increases as

the temperature goes down


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Growth

The nucleus grows as material diffuses to

the site

Diffusion is a function of temperatureIf you cool the material off immediately, it

is hard for diffusion to occur

Supersaturated non-equilibrium structurescan occur


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Kinetics

Nucleation and growth determine how fast

the transformation will occur.

Avrami relationshipf=1-exp(-ctn)

f is the fraction converted

t is timec and n are constants for a given temperature


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Avrami Plot

FractionConverted

Time (sec)

Conversion is50% Complete

t is the time

required for50% conversion

| Incubation Time |


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Growth Rate

Often expressed as 1/t

The growth rate is a function of temperature

Often, the higher the temperature, the fasterthe solid transforms

Why?

Diffusion dominates in many systems

Not always true thoughfor example..


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Effect of Temperature on Phase

Transformation

GrowthRate

Nucleation

Rate

Overall

Transformation

RateTemperature

Rate

Equilibrium transformation

temperature


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Effect of Temperature on Phase

Transformation

Time

Time for 50%

Transformation

Minimum Time required

for Transformation


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C-curve

Typical of many metals, ceramics, glasses

and polymers

Ex. Iron changes phase this way


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What if growth dominates?

For some metals nucleation occurs readily

The only factor that changes with

temperature then is the growth ratewhichis diffusion controlled

For these metals, the solid to solid phase

change always occurs faster at highertemperatures


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Effect of Temperature on Copper

Fract

ionTransform

ed

Time

135 C

120 C

80 C


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Growth rate follows an Arrhenius

Relationship

Growth rate = A exp(-Q/RT)

Growth rate is proportional to overall

transformation rate


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How Does the Solid Form?

Liquid

L + a

a

a + b

This iswhat we

would liketo happen


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How Does the Solid Form?

Liquid

L + a

a

b

a

This is what

typically

happens

We want to

avoid thisstructure,

which is

caused by

slow cooling


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Age hardening or Precipitation

Hardening

A treatment used on non optimum alloy

structures

Produces a uniform dispersion ofFine

Hard

Coherent PrecipitateIn a softer, more ductile matrix


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#1 Solution Treatment

Reheat the alloy up to a temperature where only one solid

phase exists (above the solvus)

This dissolves the second solid phase (bfor example) into

the primary phaseDont exceed the eutectic temperature

L

a a + q

a + L


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#2 Quench

Rapidly cool to room temperature or below

This results in a supersaturated

nonequilibrium structureThe second phase does not form, because

diffusion is so slow!! L

a a + q

a + L


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# Aging

Reheat to a temperature below the solvus

Diffusion is still slow, so the atoms can only

diffuse a short distance

Results in a fine precipitate

There is an optimum aging timeL

a a + q

a + L


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Types of Precipitates

Coherent Non Coherent


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Coherent Precipitates

Form First

Eventually grow until they snap out of

solutionProduce more hardening

If you over agethe strength goes down

becausePrecipitate goes from coherent to noncoherent


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Aging

See the animations on the CD

Artificial agingelevated temperatures

Natural agingroom temperatureNot suitable for use at high temperature

Why?

Problems with welding


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Requirements for Age Hardening

Must have a phase diagram that exhibits a

change from a single solid phase to two

solid phases (a->a + b)Matrix should be soft and ductile

Precipitate should be hard and strong

Must be quenchableMust have a coherent precipitate


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Aluminum - Copper Aging

a

q

a

#1 Solution

Treatment#2 Quench

as

s

#3 Aging

a +

q


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Eutectoid Reaction

One solid phase transforms to two different

solid phases

The ironcarbon phase diagram has aeutectoid

This diagram is the basis for iron and steels


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400 C

1400

C

1200

C

1000

C

800

C

600 C

1600

C

Fe 1% C 2% C 3% C 4% C 5% C 6% C 6.70% C

Iron-Iron Carbide Phase Diagram

Cementite

(Fe3C

Liquid

a, ferrite

g,

austenite

d, ferrite

g + Cementite

L + g L + cementite

Eutectic

Eutectoid


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Pure Iron

Solidifies first as d ferrite

which is BCC

Then, as it cools, it goes

through an allotropicphase transformation to g

austenitewhich is FCC

Finally, it changes to a

ferrite, which is BCC

40

0 C

14

00

C

12

00C

10

00

C

80

0

C

60

0 C

16

00

C

F

e

1

%

C

2%

C

3

%

C

4%

C

5%

C

6%

C

6.70

% C


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Carbon is significantly

more soluble in

austenite than in

ferrite, because of thecrystal structure

As the austenite cools,

the carbon eventuallycomes out of solution

as cementite40

0

C

14

00

C

12

00

C

10

00

C

8

0

0C60

0

C

16

00

C

F

e

1

%

C

2%

C

3

%

C

4

%

C

5%

C

6%

C

6.70

% C


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Dispersion Strengthened Iron

In this region the iron

is dispersion

strengthened

Solid a converts to a

+ cementite

a a+Cementite


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Hypoeutectoid Iron

g

ag

aPearlite

Ductile

Continuous

Phase


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Hypereutectoid Iron

g

gFe3C

Fe3C

Pearlite

Brittle

Continuous

Phase

Used inBall

Bearings


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Is it Iron or Steel?

Steel is an iron-carbon alloythat may

contain other alloying elements

Low , Medium and High Carbon SteelUsually less than about 1%

Alloy Steels such as Stainless contain other

elements such as Chromium


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Cast Iron

Cast Iron has more than 2.14% C

Usually has 3 or 4% Iron

400C

1400 C

12

00

C

100

0 C

80

0

C

600

C

160

0 C

F

e

1%

C

2%

C

3%

C

4%

C

5% C 6% C 6.70

% C

L

g

a

d

Note the low

melting point,

which is an

advantage for

casting

2.14%C


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i


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Time -Temperature

Transformation

The rate of transformation depends on how

much you undercool the metal

Time

Time for 50%

Transformation

Minimum Time required

for Transformation


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Iron

The microstructure depends on how muchyou undercool the iron

If transformation occurs at a hightemperature (near the equilibrium phasechange temperature) the microstructure willbe course

At cooler temperatures, a finermicrostructure is formed


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Other Microstructures of Iron

If the transformation temperature of iron is

kept above about 550 C, a lamellar

microstructure resultswhich we call

pearlite

At temperatures below 550 C diffusion is

very slow. The resulting microstructure

changes to round particles of cementite in a

ferrite matrix. Its called bainite


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Why Does Bainite form?

Extremely thin lamellar layers result in a lot

of surface area at the boundary between the

cementite and ferrite

This results in high total surface energy

too high

The surface energy is reduced by switchingto more rounded particles


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Affect of Temperature on Bainite

Transformation temperature affects the

bainite microstructure, just like it affects

pearlite

Lower temperatures result in smaller

cementite particles in the ferrite matrix

Wh h if


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What happens if you get

REALLY COLD

At really low transformation temperaturesdiffusion basically stops

Neither bainite or pearlite can form

The crystal trys to change from the FCCaustenite phase to the BCC ferrite phase,but it traps the excess C in the matrix

The result is a BCT crystal structurecalled Martensite


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Martinsitic Reaction

Diffusionless

Not time dependant

Not an equilibrium structureSteel Martinsite is very hard and brittle


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Tempering Steel Martinsite

If you warm martinsite back up diffusion

can occur

At least some of the carbon forms cementiteBy controlling the tempering temperature

and time, a wide range of properties can be

produced

C t l th


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Can you control the

microstructure that forms?

Yes, by controlling the transformation

temperature and time

You can get pearlite, bainite or martinsiteOr.. Combinations of the different

microstructures

Consider the following TTT diagram for aeutectoid steel (0.77% C)


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Time-Temperature-Transformation for a Eutectiod

Steel

Equilibrium Phase Change Temperature

100

200

300

400

500

600

700

0.1 1 10 100 1000 10 000 seconds

Start TimeFinish Time

PsPf

Bs Bf

Mf

Ms

Pearlite

Bainite

Martinsite


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Try the quiz

On the CD


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Friday

Jominy End Quench Test

Each part of the sample of steel is cooled at

a different rateWhat kind of structure do you expect?

How does that relate to strength and

hardness?

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Materiale Dhe Perpunim Termik Chapter11