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The Iron Iron Carbide (Fe Fe3C) Phase Diagram

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Peritectic L = at T=1493 oC and 0.18wt%C

Eutectic L = Fe 3Cat T=1147 oC and 4.3wt%C

Eutectoid = Fe3Cat T=727 oC and 0.77wt%C

Phases Present

L

Reactions

ferrite deltaBcc structureParamagnetic

austeniteFcc structure

Non magneticductile

ferriteBcc structure

FerromagneticFairly ductile

Fe 3C cementiteOrthorhombicHard, brittle

Max. solubility of C in ferrite=0.022%in austenite=2.11%

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Phases in FeFe 3C Phase Diagram ferrite solid solution of C in BCC Fe

Stable form of iron at room temperature. Transforms to FCC gaustenite at 912 C

austenite solid solution of C in FCC Fe Transforms to BCC ferrite at 1395 C

Is not stable below the eutectic temperature (727 C)

unless cooled

rapidly.

ferrite solid solution of C in BCC Fe It is stable only at T, >1394 C. It melts at 1538 C

Fe3C (iron carbide or cementite) This intermetallic compound is metastable at room T. It

decomposes (very slowly, within several years) into Fe

and C (graphite) at 650 700 C FeC liquid solution

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Comments on FeFe 3C systemC is an interstitial impurity in Fe. It forms a solid solution with , , phases of ironMaximum solubility in BCC ferrite is 0.022 wt% at 727 C. BCC: relatively small interstitial positions

Maximum solubility in FCC austenite is 2.14 wt% at 1147 C FCC has larger interstitial positionsMechanical properties : Cementite (Fe3C is hard and brittle:

strengthens

steels).

Mechanical

properties

also

depend

on

microstructure: how ferrite and cementite are mixed.Magnetic properties : ferrite is magnetic below 768 C, austenite is non magnetic

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Classification.Three types of ferrous alloys:Iron : < 0.008 wt % C in ferrite at room TSteels : 0.008 2.14 wt % C (usually < 1 wt % )

ferrite

+

Fe3C at

room

T

Cast iron : 2.14 6.7 wt % (usually < 4.5 wt %)

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3

1

3

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Eutectoid steel

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Hypoeutectoid steel

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Hypoeutectoid alloys contain proeutectoidferrite (formed above the eutectoid temperature) plus

the eutectoid perlitethat contain eutectoid ferrite and

cementite.

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Hypereutectoid alloys contain proeutectoid

cementite (formed above the eutectoid temperature) plus

perlite that contain eutectoid ferrite and cementite.

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How to calculate the relative amounts of proeutectoid phase ( or Fe 3C) and pearlite?

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Use the lever rule and a tie line that extends from the eutectoid composition (0.75 wt% C) to (0.022 wt% C) for hypoeutectoidalloys and to Fe 3C (6.7 wt% C) for hypereutectoid alloys .

Example: hypereutectoid alloy, composition C1

( )( )

( )

( )

( )

( )76.07.6

76.0CementiteidProeutectoof Fraction

76.07.6

7.6Pearliteof Fraction

13

1

=

+

==

=+==

C

X V

V W

C

X V

X W

C Fe

P

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Example

For alloys of two hypothetical metals A and B , there exist an , A rich phase and a , B rich phase. From the mass fractions of both phases of two different alloys, which are at the same temperature, determine the composition of the phase boundary (or solubility limit) for both

and at this temperature.

Alloy Composition

Fraction of phase

Fraction of phase

60wt%A 40wt%B

0.57 0.43

30wt%A 70wt%B

0.14 0.86

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The problem is to solve for compositions at the phase boundariesfor both and phases (i.e., C and C ). We may set up two

independent lever rule expressions, one for each composition, interms of C and C as follows:

o C C

C =C C

C C =.=W

60570

11

o C C

C =C C C C =.=W

30140 22

In these expressions, compositions are given in weight percent A . Solving for C and C from these equations, yield

C = 90 (or 90 wt% A10 wt% B)

C = 20.2 (or 20.2 wt% A79.8 wt% B)

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Heat Treatment of SteelsOn

slowly

cooling

the

steels,

the

properties

of

the

steel

are

dependent mainly on the percentage carbon.Different percentage carbon implies different percentage of

microconstituents and phases

pearlite and ferrite pro eutectoid for the hypo eutectoid steels

pearlite and cementite pro eutectoid for the hyper eutectoid steels.The temperature is high enough and the time at high temperature

is long

enough,

for

the

atoms

to

diffuse

and

attain

equilibrium

conditions

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Isothermal Transformation

Consider a rapid cooling from A to T1 in the diagram for an eutectoid steel and keeping the steel at this temperature for the eutectoid reaction to occur.

The eutectoid reaction will take place isothermally(0.78 wt%C) (0.02%C) + Fe3C(6.70%C)

Austenite, on time, will transform to ferrite and cementite.

Carbon diffuses away from ferrite to cementite Temperature affects the rate of diffusion of carbon..

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At temperature T1 the transformation will occur

gradually, following a S shape curve

)exp(1 nkt y =

Avrami equation

A

T1

Avrami relationship Describes the fraction of a transformation that

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poccurs as a function of time. This describes most solid state transformations that involve diffusion, thus martensitic transformations are not described.

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For most reactions rate increases with temperature:

RT Q

Ae r /

=

Rate of transformation

5.0

1

t

r =

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The rate of a phase transformation is the product of the growth rate

and nucleation

rate

contributions,

giving

a maximum

transformation

rate at a critical temperature (a). Consequently, there is a minimum time (t min ) required for the transformation, given by the C curve.

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Time Temperature Transformation DiagramTTT diagrams are isothermal transformations (constant T the material is cooled quickly to a given temperature THEN the transformation occurs)

The thickness

of

the

ferrite

and

cementite layers

in

pearlite is

~

8:1. The absolute layer thickness depends on temperature of transformation. The higher the temperature, the thicker the layers.

At higher T. Sshaped curves shifted to longer times transformation dominated by slow nucleation and high atomic diffusion. Grain growth is controlled by atomic diffusion.At higher temperatures, the high diffusion rates allow for larger grain growth and formation of thick layered structure of pearlite(coarse pearlite ).

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At low temperatures, the transformation is controlled by a rapidnucleation but slow atomic diffusion. Nucleation is controlled by supercooling.

Slow diffusion at low temperatures leads to fine grained microstructure with thin layered structure of pearlite ( fine pearlite ).

At compositions other than eutectoid, a proeutectoid phase (ferrite or cementite) coexists with pearlite.

If transformation temperature is low enough (540 C) below the nose of the TTT curve the diffusion rates are greatly reduced

Under such conditions is not possible to form pearlite and a different phase, bainite, is formed.

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TTT Curve Eutectiod Steel

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TTT Curve - 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

P s P f

Bs B f

M f

M s

Pearlite

Bainite

Martensite

Problem:

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Determine the activation energy for recrystallization of cold worked copper,

given the fraction of recrystallization with time at different temperatures.

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RT Q Ae r / =

B i it th d t f t it

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Bainite another product of austenitetransformation

Needles or plates needles of ferrite separated byelongated particles of the Fe 3C phase

Bainite forms as shown on the T-T-T diagram attemperatures below those where pearlite forms

Pearlite forms 540 to 727 0C

Bainite forms 215 to 540 0C

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Martensite

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Martensite

Athermal transformation Massive martensite

Plate martensite

Body centered tetragonal (BCT)

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Body centered tetragonal (BCT)structure.

M s stands for Martensite StartTemperature andM f stands for Martensite

Finished Temperature .

Due to the fast cooling, diffusion of

carbon is restricted. To make roomfor the carbon atoms, the latticestretches along one crystal direction.

Martensite is a metastable phase.

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Martensite is a supersaturated solution ofcarbon in iron.

Due to the high lattice distortion,martensite has high residual stresses.The high lattice distortion induces highhardness and strength to the steel.However, ductility is loss (martensite is toobrittle) and a post heat treatment isnecessary.

FCC BCC BCT

Heat treatment

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Heat treatment

Examples

(a) rapidly cool to 350 oC; hold 10 4 s andthen quench to room temp.

(b) rapidly cool to 250 oC; hold 100s and

then quench to room temp.

(c) rapidly cool to 650 oC; hold 20s andrapidly cool to 400 oC, hold 103s, andand then quench to room temp.

(a)

(b)

(c)

50% pearlite + 50% bainite

100% martensite

100% bainite

Austenite

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Austenite

Diffusionless process

Reheat Body-centeredtetragonal

Temper embrittlement : reduction of toughness after tempering

Continuous Cooling Transformation + cooling curve

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g g

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Control of final structures

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Common Industrial Heat Treatments

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o Annealing (slow cooling from austenitizing temperature.Product: coarse pearlite)

o Normalizing (air cooling from austenitizing temperature.

Product: fine pearlite)o Quench & Temper. Quench (fast water or oil coolingfrom austenitizing temperature. Product: martensite).

Tempering (heating to T < A 1 to increase ductility ofquenched steels)

o Martempering. (Quench to T above M s and soak until allthe steel section is at that temperature, then quench toambient temperature. Product: Martensite)

o Austempering. (Quench to T above M s and soak until phasetransformation takes place. Product: Bainite

Annealing

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Stages of annealing: Heating to required temperature Holding (soaking) at constant temperature

Cooling

The time at the high temperature (soaking time) is long

enough to allow the desired transformation to occur.Cooling is done slowly to avoid warping/cracking of dueto the thermal gradients and thermo-elastic stresses

within the or even cracking the metal piece.

Annealing

Purposes of annealing:

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Relieve internal stressesIncrease ductility, toughness, softnessProduce specific microstructure

Process Annealing - effects of work-hardening (recovery andrecrystallization) and increase ductility. Heating is limited to avoidexcessive grain growth and oxidation.

Stress Relief Annealing minimizes stresses due toPlastic deformation during machining

o Nonuniform coolingo Phase transformations between phases with different densitiesAnnealing temperatures are relatively low so that useful effects of

cold working are not eliminated.

Examples of heat treatment

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Lower critical temperature A1 below which austenite does not exist

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Upper critical temperature lines, A 3 and A cm above which all

material is austenite Austenitizing complete transformation to austenite

Full annealing: austenizing and slow cooling (severalhours). Produces coarse pearlite (and possible proeutectoidphase) that is relatively soft and ductile. It is used to softenpieces which have been hardened by plastic deformation,and which need to undergo subsequent machining/forming .Temperatures for full annealing:

Hypoeutectoid steel : A 3 + 50 oCHypereutectoid steel : A 1,3 + 50 oC

Normalizing

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Annealing heat treatment just above the upper criticaltemperature to reduce grain sizes (of pearlite and proeutectoidphase) and make more uniform size distributions.The interlamellar distance of pearlite decreases as the cooling

rate increases.Slow cooling (annealing furnace cooling) coarsepearlite interlamellar spacing of 4.5 m hardness of

200BHNMedium to slow cooling (normalizing still air cooling) normal pearlite interlamellar spacing of 3.0 m

hardness of 220BHNMedium to Fast (normalizing forced air cooling) finepearlite interlamellar spacing of 2.0 m hardness of

300BHN

Normalizing

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Normalizing

temperatures:Hypoeutectoid steel :A3 + 50 oC

Hypereutectoid steel :Acm + 50 oC

Spheroidizing: prolonged heating just below the eutectoidtemperature which results in the soft spheroidite structure

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temperature, which results in the soft spheroidite structure.

This achieves maximum softness needed in subsequent formingoperations.

Pearlite or

bainite

Heating

18~24hbelow the eutectoid temp.

Spheroidite

Reduction of /Fe 3C GB area

Hardness and ductility

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Quenching

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The most common method to harden a steel.

It consists of heating to the austenizing temperature (hypoeutectoid steel) andcooling fast enough to avoid the formation of ferrite, pearlite or bainite, toobtain pure martensite.

Martensite ( ) has a distorted BCT structure. It is the hardest of thestructures studied. The higher hardness is obtained at 100% martensite.

Martensite hardness depends solely of the carbon content of the steel. The

higher the carbon content, the higher the hardness.Martensite is very brittle and can not be used directly after quench for anyapplication.

Martensite brittleness can be reduced by applying a post-heat treatmentknown as - tempering .

Austenizing temperature:Hypoeutectoid steel : A 3 + 50 oCHypereutectoid steel : A + 50 oCT(

oC)

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Hypereutectoid steel : A 1,3 + 50 C( )

Time (t)

a bc

(a) Cooling in oil

(b) Cooling in water

(c) Cooling in brine

Cooling depends on the geometry and mass of the component.

External surfaces are cooled faster than the inner core of thecomponent.

Tempering of Martensitic Steels

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Martensite is too brittle to serve engineering purpose Tempering is necessary to increase ductility and toughness of

martensite. Some hardness and strength is lost.

Tempering consist on reheating martensitic steels (solutionsupersaturated of carbon) to temperatures between 150-500 C toforce some carbide precipitation.1st stage 80-160 C

(low C martensite) + carbide (Fe 2.3 C)2nd stage 230-280 C

retained bainite

3rd stage 160-400 C + carbide + Fe 3C(tempered martensite)

3rd stage (cont) 400-700 oCGrowth and spherodization of cementite and other carbides

Martempering T ( oC)

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A process to prevent the formation ofquench cracks in the steel.

Cooling is carried out, as fast as

possible, to a temperature over theMS of the steel.

The steel is maintained at T>M S until

the inner core and outer surface ofthe steel component is at the sametemperature.

Log (time)

Surface

InnerCore

M S

M F

The steel is then cooled below the M F to obtain 100%martensite. There is a need to increase the ductility of this steel

by tempering.

T

T ( oC)Austempering

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Log (time)

Surface

InnerCore

M S

M F

Bainite

A process to prevent theformation of quench cracks inthe steel.

Cooling is carried out, as fastas possible, to a temperatureover the M S of the steel.

The steel is maintained atT>M S until the austenitetransforms to 100% bainite.

There is no need of tempering post treatment.

T

Hardenability

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Hardenability is the ability of the Fe-C alloy to transform tomartensite during cooling. It depends on alloy compositionand quenching media.Hardenability should not be confused with hardness. Aqualitative measure of the rate at which hardness decreaseswith distance from the surface because of decreasedmartensite content.High hardenability means the ability of the alloy to produce ahigh martensite content throughout the volume of specimen.

Hardenability is measured by the Jominy end-quench test performed in astandard procedure (cylindrical specimen, austenitization conditions,quenching conditions - jet of water at specific flow rate and temperature).

Hardenability

Hardenability Ability to be hardened by the formationof martensite as a result of heat treatment

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Jominy End-Quench Test Jominy End-Quench Test

Distance from quenched end

Rockwell Hardness Different Cooling rate !! Different Cooling rate !!

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High carbon content, better hardenability

High carbon content, better hardenability

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