Kinetics Heat Treatment - Iowa State Universitybastaw/Courses/MatE271/Week8.pdf · Material...

Material Sciences and Engineering, MatE271 1

Material Sciences and Engineering MatE271 1Week8

KineticsHeat Treatment

Material Sciences and Engineering MatE271 Week 8 2

��Fe3C (cementite) rapid cooling��C slow cooling

Fe Composition wt% C

L

�+L�

��

��

��

�

��L

eutectic

eutectoid(pearlite)

��Fe(FCC)

austenite

�-Fe(BCC)ferrite

Temp.

Baking Alloying

- Ingredient - Composition (wt%) - Baking temperature - Equilibrium diagram- Baking time - Cooling time (kinetics)

Time-dependent phase transformation



Goals for this unit (Ch. 10)

� Understanding how temperature and cooling can be used to alter properties (e.g. Fe-C system).

- The TTT-diagram (Ch. 10.1-2)

- Applications: (Ch. 10.3-5)

- Hardening (Steel alloys)

- Precipitate hardening (Aluminum alloys)

- Annealing (recrystallization and grain growth)


- All previous discussion has been for “slow” cooling

- Many times, this is TOO slow, and unnecessary

- Nonequilibrium effects

- Phase changes at T other than predicted

- The existence of nonequilibrium phases at room

temperature

Nonequilibrium Cooling



- Phase diagrams only represent what should happen in equilibrium (e.g. slow cooling)

- Most materials are not processed under such conditions

-

-

- Time - temperature history required to generate a certain microstructure

- Time - temperature - transformation (TTT) diagrams

10.1 Time, the third dimension


A BComposition, %B

Melting Temp.(Pure A)

Melting Temp.(Pure B)

A + Liquid

Liquid + B

Liquid

Liquidus

Eutectic Line

A + B(both solids) Invariant Point

Tem

pera

ture

Tem

pera

ture

Time

Time effect

at 100% of A

You have to drop Temp slightly to start solidification



Transformation

- Most transformations do not take place instantaneously

e.g. to change crystal structures, atoms must diffuse

Which takes time

Energy is required to form phase boundaries between parent and product phases

Liquid

solidNucleation and growth

Surface energy +ve

volume energy -ve

Net energy

rc

Net

ene

rgy

chan

ge


� NucleationThe formation of very small particles of the new phase

Often begins at imperfection sites – especially grain boundaries

� GrowthThe nuclei increase in size

Some or all of the parent phase disappears

Complete when system reaches equilibrium

Transformation by Nucleation and Growth



10.2 The TTT Diagram

A BComposition, %B

Melting Temp.(Pure A)

Melting Temp.(Pure B)

A + Liquid

Liquid + B

Liquid

Liquidus

Eutectic Line

A + B(both solids) Invariant Point

Tem

pera

ture

Tem

pera

ture

Time

at 100% of A

1 50 100 % completion of reaction

Time required for reaction completion


-The fraction of reaction that has

occurred is measured as a

function of time

- Usually at a constant T

- Progress is usually determined

by microscopy or other physical property

- Data is plotted as fraction transformed vs. log time

Rate of Transformation

Tem

pera

ture

Time

at 100% of A

1 50 100 % completion of reaction



� Phase transformations occur when either

-

-

-

� Temperature is most common method to induce phase

transformations

� Phase boundaries are crossed during heating or cooling

Phase Transformation: when?


Phase Diagram vs. TTT Diagram

�When a phase boundary is crossed, the alloy proceeds towards equilibrium according to the phase diagram

� Most phase transformations require a finite time

�Phase diagrams cannot indicate how long it takes to achieve equilibrium

� Many times the preferred microstructure is metastable

� The required transformation time is obtained from the TTT-Diagram



� Metallic Materials are extremely versatile

- They possess a wide range of mechanical properties

� Microstructure development occurs by phase transformations

- Diffusional Transformation:

- Diffusionless Transformation

� Properties can be tailored by changing microstructure

Phase Transformation


Diffusional Transformation��Fe

(FCC)austenite

coarse pearlite

fine pearlite

lower bainite

upper bainite

spheroidite

Fe Composition wt% C

��

�

��

��eutectoid( 0.77% C )

�-Fe(BCC)ferrite

�

��Fe3C

��Fe3C



- Consider the eutectoid reaction

��(0.77 wt% C) � ��(0.22% C) + Fe3C (6.70% C)

Austenite transforms to ferrite and cementite – through

Carbon diffuses away from ferrite to cementite

Temperature affects the rate:

Construct isothermal transformation diagrams from

% transformation diagrams

Diffusional Transformation (Pearlite)


Pearlite Transformation (diffusional)

Austenite grainboundary

Austenite (�)

Austenite (�)

Growth direction Of Pearlite

Fe3Ccementite

Ferrite, �

��(0.77 wt% C) � ��(0.22% C) + Fe3C (6.70% C)Austenite Ferrite Cementite

Pearlite

Check Fig. 9.2Check Fig. 9.2P. 306P. 306



� Pearlite is a mix of cementite and ferrite ( )

- Cementite is harder but more brittle than ferrite

� Layer thickness also has an effect

- Fine pearlite is harder and stronger than coarse

Mechanical Properties of Pearlite


Fe3C in Pearlite and Bainite



Isothermal Diagrams

� Only valid for a particular composition for a particular system

- Other compositions will have different curves

� Only valid when the temperature is constant throughout the transformation


Diffusionless Transformation: Martensitic Transformation

� Crystal: � (FCC) � (BCC)� FCC accommodates C easily than BCC� C Fe3C ( )

- trapped in the FCC lattice� Form Body center tetragonal lattice, BCT



The Full Isothermal TTT

Coarse pearlite

fine pearlite

lower bainite

upper bainite

100% martensite

martensite

spheroidite


� Strongest, hardest, and most brittle

�Hardness is dependent on C content

� Martensite is not as dense - therefore when it transforms it causes stress ( )

� Tempering (heat treatment) of martensite relieves

stress - makes it tougher and more ductile

Note - other alloy system experience diffusionless (or martensitic) transformation

Mechanical Properties of Martensite



Martensite Tempering- stress reliving

Tempering temperature

martensite

Check Fig. 10Check Fig. 10--1818P. 370P. 370

M ��Fe3C (isolated particles)TemperedMartensite:


10.3 Hardenability

Hardness: surface resistance to indentation

H= F/Aprojected

F

�

Ap

Hardneability: relative ability of steel to hardenedby quenching

- Related to and of Martensitic transformation



- Cylindrical specimen is cooled from the end by a spray or water

- Specimen size, shape is specified

- Water spray and time is specified

- The hardness is measured with respect to the distance from the quenched end

- Rockwell hardness measured (a hardness scale)

Jominy End-Quench

heat to above Teutectoid cool

measure hardness




500nm

Al-alloy7150-T651(6.2Zn, 2.3Cu,2.3Mg, 0.12 Zr)

10.4 Precipitate Hardening


Precipitate Hardening

Al-Cu alloy (96% Al-4%Cu)

��

�

T

Time

�

��

Slow cooling



Age Hardening

��

�

T

Time

�

quenchaging

Fine dispersionof � particle

Coherent interface


Age Hardening

Super saturated ��solid solution

��phase precipitate

��phase growth

Aging time



Alloy load carrying capacity

Aging Time

GP zone and service life

growthcoalescence


10.5 Annealing

- Loss of hardness at high temperature- relief of residual stresses Stress = - reduction of dislocation density

ForceArea

- Link between deformation and microstructure- Cold work- Recovery- Recrystallization- Grain growth

�Deformation is measured by percentage Strain = 100% dimensional changes

�L L



Cold-working

The degree of plastic deformation is expressed as % cold worked:

%CW A AA

xo f

o�

� 100%

Ao Af

Why does this occur?

��Dislocation-dislocation strain field interactions

��Dislocation density increases with cold working -

so the average separation between dislocations decreases


��Strain hardening may be removed by annealing

(heating to higher T to allow dislocations to move)

Cold-working-cont.

Brass Cu-Zn

CW 3 sec at 580oC 4 sec

8 sec 1 hr



Recovery, Recrystallization

�Plastic deformation results in changes in

microstructure and properties

- Grain shape

- Strain hardening

- Increased dislocation density

�Original properties can be regained by

appropriate heat treatment

Recovery, recrystallization, grain growth


RecoveryRecrystallization

temperature

Brass

��Some of the stored strain energy is relieved by movement of dislocations at high T

- Number of dislocations is reduced

- Configuration of dislocation is altered



- Even after recovery, grains are still in a high energy state

(they have been deformed)

- Recrystallization is the formation of a new set of strain-free equiaxed grains.

- New grains form by nucleation and growth

Short range diffusion

- Requires time and temperature

- Recrystallization temperature: Temperature at which recrystallization reaches completion in 1 hr.

Recrystallization


• Cold Worked

• Initial Stage

• Intermediate Stage

• Complete

Recrystallization

• Grain Growth

• Grain Growth,

higher temperature

Stages of Recrystallization



Grain Growth

- Occurs in all crystalline materials - why?

- Energy is associated with grain boundaries -

– As grain size increases, total boundary area decreases

-All grains can’t grow

– Large ones grow at the expense of small ones

-Fine grains superior properties

- How to produce fine grain structure???


Reading Assignment

READ Class Notes & relevant portions of

Shackelford, 2001(5th Ed)

– Chapter 10, pp 354-389

-HW5 will be available on Friday, Oct 19

Due Friday Oct 26

Will not accept HW stashed under my door

Kinetics Heat Treatment - Iowa State Universitybastaw/Courses/MatE271/Week8.pdf · Material...

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