Phase Diagrams - WordPress.com › 2017 › 02 › 2-iron-carbon-diag… · 02-02-2017 · Introduction to Iron-Carbon phase diagram by RDP. Though carbon is present in relatively

Phase Diagrams: The Iron- Iron Carbide (Fe-Fe3C)Diagram or

Iron-carbon (Fe-C) equilibrium diagram

Presented by:

Dr. R. D. Palhade,

Professor, Department of Mechanical Engineering,

SSGMCE, Shegaon

Pin-444203

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Introduction to Iron-Carbon phase diagram

by RDP

(Fe-C) equilibrium diagram

Iron, AllotropyIf the change in crystal structure is reversible then thatpolymorphic change is called as allotropy

This is the ability of some elements to exist in more than onecrystalline structure. Steel can exist as BCC when cold and asFCC when heated above a certain temperature, depending on itscarbon content.

Polymorphism: in which change of crystal structure occur due toeither increase in pressure or temperature is called aspolymorphism

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Cooling curve for pure Iron

Allotropy of pure Iron:

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Pure iron when heated experiences 2 changes in crystal structure before it melts.

At room temperature the stable form, ferrite (α iron) has a BCC crystal structure.

Ferrite experiences a polymorphic transformation to FCC austenite (γ iron) at 912 ˚C (1674 ˚F).

At 1394˚C (2541˚F) austenite reverts back to BCC phase δ ferrite and melts at 1538 ˚C (2800 ˚F).

Iron carbide (cementite or Fe3C) an intermediate compound is formed at 6.67 wt% C.

Typically, all steels and cast irons have carbon contents less than 6.67 wt% C.

Carbon is an interstitial impurity in iron and forms a solid solution with the α, γ, δ phases.

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Objectives: Iron-carbon (Fe-C) equilibrium diagram

1. Explain and use the terms hypoeutectoid, hypereutectoid,

proeutectoid, and pearlite.

2. Identify the major phases on the Fe-C diagram and explain their

chief characteristics.

3. Identify the proeutectoid phase for a given alloy.

4. Steel with other elements in small amounts are called plain

carbon steel and their structure and properties can be

discussed with the help of Fe-C diagram

5. Some time some elements added in steels to increase some of

required properties, these steels are called alloy steel-this

discussed with modified Fe-C diagram

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General classes of steels

1. Low carbon (mild steels): <0.3% C - high ductility, lowstrength, for general use, sheets, plate.

2. Medium carbon steel: 0.3-0.6% C – higher strength, higherhardness, less ductility, gears, axles, railroad, etc.

3. High carbon steels :>0.6% C – hard, strong, brittle, tool steel,springs, cutting tools

Heat Treatments Both microstructure and composition affect a material’s

properties. Heat treatment is one way to manipulatemicrostructure.

These changes to microstructure are caused by phasetransformations and changes in grain size. These effects areboth thermodynamically and kinetically driven.

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The Iron-Carbon Diagram

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Sr.

No.

Critical temp Temperature

in 0C

Significance during heating

1. A0

{critical temp of Fe3C}

210 Cementite changes ferromagnetic to

paramagnetic character

2. A1

{lower critical temp}

727 Pearlite starts transforming into

austenite

3. A2

{curie temp of ferrite}

768 Ferrite becomes paramagnetic

4. A3

{Upper critical temp. for

hypo-eutectoid steel}

727-912 Completion of ferrite to austenite

transformation

5. Acm

{upper critical temp for

hyper-eutectoid steel}

727-1147 Completion of cementite to austenite

(100%)

6. A4 1400-1492 Completion of austenite to δ-ferrite

transformation

Critical Temperature:

The temperature at which the phase changes occur during heating and

cooling are called critical temperature

The critical temperature at different stages shown on diagram A0, A1,

A2, ---etc.

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An iron-carbon equilibrium diagram forms a basis for

differentiating among

Iron (0.008 % C or less)

Hypo-eutectoid steels (0.008 to 0.8 % C)

Hyper-eutectoid steel (0.8 to 2.0 % C)

Hypoeutectic cast iron (2 to 4.3 % C) &

Hypereutectic cast iron (above 4.3 % of C)

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Temperature

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Wt. % of carbon

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General Properties:

1. α -Ferrite - Iron with a little Carbon. Fairly Soft.

2. γ -Austenite - FCC Iron with much Carbon Ductile.

3. Fe3C -Cementite - Hard ceramic material.

4. δ-Ferrite- This has no real practical significance in

engineering.

Micro-constituents of Iron and Steel (The phased diagram

includes four solid phases)

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The various phases existing in Fe-C diagram are as:

Micro-constituents of Iron and Steel (The phased diagram includes

four solid phases)

(a) Ferrite (α)

Ferrite is an interstitial solid solution of carbon in low temp. BCC

α- iron phase with very limited solubility for carbon.

At 0% C this is pure iron.

The maximum solubility of carbon in iron is 0.025% at 727oC &

At 0 oC temperature the solubility falls to 0.008%.

Ferrite is the softest structure that appears on the Fe-C

equilibrium diagram.

It can be extensively cold worked without cracking.

The carbon atoms are located in the crystal interstices.

Ferrite has:

tensile strength 2800 kg/cm2 (approx.)

Elongation 40 % in 50 mm

Hardness less than Rockwell C 0 or Rockwell B 90

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(b) Austenite (γ)

Austenite is the interstitial solid solution of carbon and/or

other alloying elements (e.g. Mn, Ni, etc.) in FCC γ- iron.

Carbon is in interstitial solid solution where as Mn, Ni, Cr, etc.

are substitutional solid solution with iron.

This has a FCC crystal structure with a high solubility for

carbon compared with α ferrite.

The solubility reaches a maximum of 2.14 % at 1147 oC The

solubility decreases to 0.8 % at 727 oC.

The difference in solubility between the austenite and α

Ferrite is the basis for the hardening of steels.

Austenite is normally not stable at room temperature.

Austenite has:

Tensile strength 10500 kg/cm2

Elongation 10% in 50 mm

Hardness Rockwell C 40 (Approx.)2/6/2017 17


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Though carbon is present in relatively low concentrations, it

significantly influences the mechanical properties of ferrite:

(a) α ferrite, (b) austenite.

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(c)Cementite (Fe3C)

Cementite or iron carbide, chemical formula Fe3C.

This is an intermetallic compound of iron and carbon,

which contains 6.67 % C and 93.3 % Fe.

Cementite is a hard and brittle interstitial compound of low

tensile strength.

Its crystal structure is orthorhombic crystal structure with

each unit cell has 12 Fe atoms and 4 C atoms.

It is ferromagnetic up to 2100C and becomes paramagnetic

Cementite has:

Tensile strength 350 kg/cm2 (Approx.)

High compressive strength

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(d) δ Ferrite

This is a interstitial solid solution of carbon in iron and has a

BCC crystal structure.

The maximum solubility or C in Fe is 0.1% at 1492oC.

This has no real practical significance in engineering.

(e) Ledeburite

Ledeburite is the eutectic mixture of austenite & cementite.

It contains 4.3 % carbon, it is formed at about 11470c

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(f) Pearlite

The Pearlite micro-constitute consists of alternate lamellae of

ferrite and cementite.

Pearlite is the product of austenite decomposition by an

eutectoid reaction.

Pearlite is an eutectoid mixture containing about 0.8 % C, is

formed 7270C

Pearlite has:

Elongation 20 % in 50 mm

Hardness Rockwell C 20

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Result: Pearlite =

alternating layers of

a and Fe3C phases

120 mm

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(h) Martensite

Martensite is a metastable phase of steel, formed by

transformation of austenite below Ms temperature.

Martensite is a interstitial supersaturated solid solution of

carbon in α-iron and has BCC tetragonal lattice.

Martensite, normally is a product of quenching.

Martensite possesses an acicular or needle like structure.

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(i) Troostite

Troostite (Nodular) is a mixture of radial lamellae of ferrite

and cementite and therefore differs from pearlite only in the

degree of fitness and carbon content which is the same as

that in the austenite from which is formed.

In steel heat treatment, the troostite, i.e. the microstructure,

consisting of ferrite and finely divided cementite is

produced on tempering martiensite below apporx. 450 C

(g) Bainite Bainite is the constituent produced in a steel when austenite

transforms at a temperature below that at which pearlite is

produced and above that at which martensite is formed.

Bainite is decomposition product of austenite, consisting of

an aggregate of ferrite & carbide.

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(a) Upper bainite (gray, feathery plates) ( 600).

(b) Lower bainite (dark needles) ( 400). (From ASM

Handbook, Vol. 8, (1973), ASM International, Materials Park, OH 44073.)

(j) Sorbite

Sorbite is microstructure consisting of ferrite and finely

divided cementite, produced on tempering martensite

above approximately 4500C.

The constituent also known as Sorbitic Pearlite, is produced

by the decomposition of austenite when cooled at a rate

slower than that which will yield a troostitic structure and

faster that that which will produce a pearlitic structure.

Difference between Pearlite, Sorbite and Troostite: are all

ferrite-cementite mixtures having a lamellar structure and

distinguishable from each other in eutectoid steel only by

theirs of dispersion.

The diagram contains three different transformations:

Invariant Reactions

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%C →

T →

Fe Fe3C6.74.30.80.16

2.06

Peritectic

L + →

Eutectic

L → + Fe3C

Eutectoid

→ a + Fe3C

L

a

L +

+ Fe3C

1492ºC

1147ºC

727ºC

Fe-Cementite diagram

0.025 %C

0.1 %C

a + Fe3C

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The diagram contains three different transformations:Invariant Reactions:

1. Peritectic reaction

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2. Eutectic reaction

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3. Eutectoid reaction

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Microstructural development for eutectoid steel (of composition 0.77

wt % C). The resulting (low-temperature) sketch

γ

α + Fe3C

Explain the microstructural

change during transformation

upon equilibrium cooling.

Calculation of each phase.

Explain the mechanical

properties of cast iron.

L

Fe3C

727oC

Iron-Carbon (Fe-C) Phase Diagram

• 2 important points

-Eutectoid (B): a +Fe3C

-Eutectic (A):L +Fe3C

Fe

3C

(ce

me

ntite

)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

(austenite)

+L

+Fe3C

a+Fe3C

L+Fe3C

(Fe) Co, wt% C

1148°C

T(°C)

a727°C = Teutectoid

A

SR

4.30

Result: Pearlite = alternating layers of a and Fe3C phases

120 mm

R S

0.76

Ce

ute

cto

idB

Fe3C (cementite-hard)

a (ferrite-soft)

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Microstructural development for a slowly cooled hypoeutectoid steel (of

composition 0.50 wt % C).

(α is the continuous phase) 727oC

γγ

γ

γ+αpri

pearlite

αpri

αpri+eut + Fe3Ceut

As C ↓, αpri+eut↑, weaker or softer.

Study of various plain carbon steel on Fe-C Equilibrium Diagram

A] Hypo-eutectoid steel containing 0.32% C:

Fe

3C

(ce

me

ntite

)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

(austenite)

+L

+ Fe3C

a+ Fe3C

L+Fe3C

(Fe) Co, wt% C

1148°C

T(°C)

a727°C

(Fe-C

System)

C0

0.7

6

proeutectoid ferritepearlite

100 mmHypoeutectoid

steel

R S

a

wa =S/(R+S)

wFe3

C

=(1-wa)

wpearlite = wpearlite

r s

wa =s/(r+s)

w =(1- wa)

a

aa

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1. At very high temperature at point 1 the entire steel is in

liquid condition.

2. As the temperature decreases when the steel reduces at a

point 2 solidification starts which gives nuclei of δ-iron

phase. As the temperature further decreases the amount of δ

-iron goes increasing.

3. At any point 3, the amount of liquid and δ -iron can be

calculated by Appling the lever rule as % liquid = AB/AC &

% of δ-iron = BC/AC

4. Further decrease in temperature brings the steel at point 4

5. The properties of δ-iron transforms into austenite due to

hyperperitectic reaction. At any point 5, the steel in now

having liquid + austenite.

6. At point 6 all the liquid get converted into austenite. Further

decrease in temperature only shows austenite contents as

shown by point 7.

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7. At point 8, the austenite tries to convert into ferrite i.e. few

grains will now show the properties of ferrite.

8. At point 9 the steel shows the contents of austenite and ferrite

can calculated by applying lever rule, % of austenite =

XY/XZ. The ferrite which is available in this region is called

as pro-eutectoid ferrite.

9. At point 10, the steel is having pro-eutectoid ferrite with

austenite. This austenite then gets transforms into pearlite

(α+Fe3C), due to hypo-eutectoid reaction.

10. at point 11, α + α + Fe3C

(proecutectoid ferrite white Pearlite-dark).

11. After point 11, no further phase change takes place and only

the temperature is allow to come up to room temperature

hence, at room temperature also the microstructure will show

proeutectoid ferrite and pearlite.

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Microstructural development for a slowly cooled hypereutectoid steel

or high carbon steel(of composition 1.13 wt % C).

L

αeut + Fe3Cpri+eut

γ + Fe3C

γ

Fe3C

Primary Fe3C

Secondary or eutectoid Fe3C

γ

γ

γ

(Fe3C)primary

αeut(Fe3C)eut

0.77

• 727oC(727+1)oC

(727-1)oC

primary

Amount of phase calculation is important because it makes the

understanding of the mechanical property of the alloy. Explain the

property of hypereutectoid steel & with carbon.

eutectoid α

Primary or

Primary or

(Fe3C is the continuous phase)

As C ↑, Fe3Cpri+eut ↑→ stronger & brittle

Hypereutectoid Steel

Fe

3C

(ce

me

ntite

)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

(austenite)

+L

+Fe3C

a +Fe3C

L+Fe3C

(Fe) Co, wt%C

1148°C

T(°C)

a

(Fe-C

System)

0.7

6 Co

proeutectoid Fe3C

60 mmHypereutectoid steel

pearlite

R S

wa =S/(R+S)

wFe3C=(1-w a)

wpearlite = wpearlite

sr

wFe3C=r/(r+s)

w =(1-w Fe3C)

Fe3C

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Hypoecutectoid Line a-e; Cool From a to d.

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Proeutectoid – Forms before eutectoid

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Eutectoid Steel

NO Proeutectoid phase!

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Pearlite

Ferrite (white)

Cementite (dark)

Eutectoid Steel

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Proeutectoid Cementite

(white)

Pearlite (striped)

Hypereutectoid Steel

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Effects of Carbon Content

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Phase Diagrams - WordPress.com › 2017 › 02 › 2-iron-carbon-diag… · 02-02-2017 · Introduction to Iron-Carbon phase diagram by RDP. Though carbon is present in relatively