Unit3-1

MSE 271 Unit 7 1

Kinetics - Heat Treatment

Nonequilibrium Cooling

All of the discussion up till now has been for slow cooling

Many times, this is TOO slow, and unnecessaryNonequilibrium effects

Phase changes at T other than predictedThe existence of nonequilibrium phases at room temperature

MSE 271 Unit 7 2

Time, the third dimension

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

Most materials are not processed under such conditions

Time constraintsDesirable characteristics of nonequilibriummicrostructures

Heat treatment

Time - temperature history required to generate a certain microstructure

Kinetics - the science of time-dependent phase transformations

Time - temperature - transformation (TTT) diagrams are used to indicate the microstructure

MSE 271 Unit 7 3

Solid State Reactions

Most transformations do not take place instantaneously

e.g. to change crystal structures, atoms must diffuse - Which takes timeEnergy is required to form phase boundaries between parent and product phases

Phase Transformations

Metallic Materials are extremely versatileThey possess a wide range of mechanical properties

Microstructure development occurs by phase transformations

Properties can be tailored by changing microstructure

MSE 271 Unit 7 4

Stages of Solid State Reactions

NucleationThe formation of very small particles of the new phaseOften begins at imperfection sites - especially grain boundaries*

GrowthThe nuclei increase in size Some or all of the parent phase disappearsComplete when system reaches equilibrium

(may never be complete)

Rate of Transformation

The fraction of reaction that has occurred is measured as a function of time

Usually at a constant TProgress is usually determined by microscopy or other physical property

Data is plotted as fraction transformed vs. log time

MSE 271 Unit 7 5

Plot of solid state reactions

At constant T

G=Cexp(-Q/RT)

t0.5Nucleation Growth

Fr

ac

ti

on

o

f

Tr

an

sf

or

mat

io

n,

y

0

0.5

1.0

Log of Heating Time, t

Multiphase Transformations

Phase transformations occur whenTemperature changes orComposition changes orExternal pressure changes

Temperature is most common method to induce phase transformations

Phase boundaries are crossed during heating or cooling

MSE 271 Unit 7 6

Phase diagrams

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

Most phase transformations require a finite timePhase diagrams cannot indicate how long it

takes to achieve equilibrium Many times the preferred microstructure is

metastable

Property changes in Fe-C alloysExamples of kinetic principles can be found in the

Fe-C systemPearlite transformationMartensitic transformation

MSE 271 Unit 7 7

Pearlite transformation

Consider the eutectoid reactiong(0.77 wt% C) a(0.22% C) + Fe3C(6.70% C)Austenite transforms to ferrite and cementite -

changing iron content Carbon diffuses away from ferrite to cementiteTemperature affects the rate:

Construct isothermal transformation diagrams from % transformation diagrams


Growth of Direction of

Pearlite

Austenite Grain Boundary Austenite (g)

Austenite (g)

Ferrite, a

Cementite Fe3C

MSE 271 Unit 7 8


Interpretation of IsothermalDiagrams

Eutectoid T is a horizontal lineThe start and finish curves are nearly parallelAustenite exists to the left (not stable)Pearlite exists to the right

MSE 271 Unit 7 9

Isothermal Diagrams

500

600

700

Tem

pera

ture

, T,

C

Time, t, s

1 10 102 103 104 105 106

727C, Eutectoid Temperature- Austenite (stable)

AustenitePearlite Transformation

Fine Pearlite

Coarse Pearlite

Validity of 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

MSE 271 Unit 7 10

Martensite formation

Other microstructures form if there is a temperature profile other than isothermal

Martensite (fast quenching - prevents C diffusion -plate or needle like) -

Since it is diffusionless - it is almost instantaneous Only dependent on T to which it is quenched

Martensite

Tempering of Steel

MSE 271 Unit 7 11

Mechanical Behavior of Alloys

PearliteCementite is harder but more brittle than ferriteThus, the more Fe3C in an alloy the stronger and harder the material

However, also makes it less ductile and not as tough (low impact resistance)

Layer thickness also has an effect Fine pearlite is harder and stronger than coarse

Martensite Mechanical Behavior

Strongest, hardest, and most brittleHardness is dependent on C contentMartensite is not as dense - therefore when it

transforms it causes stressTempering (heat treatment) of martensite

relieves stress - makes it tougher and more ductile

MSE 271 Unit 7 12

Hardenability The ability of a steel alloy to transform to martensite is

its hardenabilityDependent on:

CompositionQuenching medium

There is a relationship between mechanical properties and cooling rate

Hardenability refers to the degree to which is transforms to martensite and to the depth an alloy may be hardened

HardenabilityJominy End-QuenchCylindrical specimen is cooled from the end by a

spray or waterSpecimen size, shape is specifiedWater spray and time is specified

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

Flat is ground along lengthRockwell hardness measured

MSE 271 Unit 7 13

Jominy End-Quench Results

The quenched end is cooled most rapidly and has highest hardness

100% martensiteCooling rate decreases away from end and the

hardness decreasesA hardenable steel retains large hardness

values for long distancesEach steel has unique hardenability curves

Cold Working

A ductile material can become harder and stronger as it is plastically deformed

Strain hardening = work hardening = cold working (meaning the strain is imposed at low temperatures)

Most metals strain harden at room temperature.

MSE 271 Unit 7 14

Cold Working

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

%CW = Ao - Ad

Ao

x 100

AAoo and Aand Add are the cross sectional areas before andare the cross sectional areas before andafter plastic deformation occursafter plastic deformation occurs

Cold Working

Why does this occur?Dislocation-dislocation strain field interactionsDislocation 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)

MSE 271 Unit 7 15

Recovery, Recrystallization

Plastic deformation results in changes in microstructure and properties

Grain shapeStrain hardeningIncreased dislocation density

Original properties can be regained by appropriate heat treatment

Recovery, recrystallization, grain growth

Recovery

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

Number of dislocations is reducedConfiguration of dislocation is alteredProperties return to their pre-cold-worked state

MSE 271 Unit 7 16

Recrystallization

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 growthShort range diffusion

Recrystallization

Recrystallization depends onTimeTemperature

Recrystallization temperatureDefined as the temperature at which recrystallizationreaches completion in 1 hr.

MSE 271 Unit 7 17

Stages of Recrystallization

Cold Worked

Initial Stage

Intermediate Stage

Complete Recrystallization

Grain Growth

Grain Growth, Higher Temperature

Grain Growth

After recrystallization, grains will continue to grow

Occurs in all crystalline materials - why?Energy is associated with grain boundaries - As grain size increases, total boundary area decreases

All grains cant grow Large ones grow at the expense of small ones

MSE 271 Unit 7 18

Grain Growth

Fine grained materials usually have superior properties to coarse-grained materials

If an alloy is coarser than desired, refinement may be accomplished by physically deforming and then recrystallizing.

Summary

TTT diagrams are not used for nonmetallic materials

Grain growth is especially important in ceramics Heat treatment to bond powder particles and

reduce porosity is called sintering for ceramics

Unit3-1

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