lec_07

MME 291: Lecture 07

Iron – Iron Carbide Iron Iron Carbide Phase Diagram

Prof. A.K.M.B. RashidDepartment of MMEBUET, Dhaka

The iron – carbon phase diagram

Today’s Topics

p g

Details of iron – iron carbide phase diagram

Classification of iron – carbon alloys

© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 02

Reference:

1. SH Avner. Introduction to physical metallurgy, 2nd Ed., Ch. 7.

Allotropic Transformation of Pure Iron

Liquid

1535°C δ (delta) iron (BCC)

γ (gamma) iron (FCC)

α (alpha) iron (BCC)

α (alpha) iron (BCC)

nonmagnetic

nonmagnetic

15351401°

910°

768°

Tem

pera

ture

, °C

Cooling Curve of Pure Iron

α (alpha) iron (BCC)magnetic

Time


Iron –Carbon Phase Diagram

Iron Carbide, Fe3CAn interstitial compound, containing 6.67

Tem

pera

ture

, °C wt.% carbon.

The compound dissociates into iron and carbon, if given the opportunity:

Fe3C = Fe + 3C

Fe3C is a metastable compound, because such decomposition takes a very long time at room temperature.T

Iron Carbonwt. % carbon

Because of the presence of Fe3C, this diagran is not a true equilibrium diagram.


Fe3C6.67

1535° 1492°

Iron – Iron Carbide Phase DiagramTe

mpe

ratu

re, °

C 1401°

910°

723°

1130°

T

Fe Fe3Cwt. % carbon


0.100.50

• • •

Tem

pera

ture

, °C

0.18

0.025 0.80

2.0 4.3

• •

• •

T

0.008• •6.67

Fe Fe3Cwt. % carbon


LiquidDelta (δ) iron

Tem

pera

ture

, °C

Austenite (γ )

Ferrite (α) CemeniteT

wt. % carbonFe Fe3C

Ferrite (α) Cemenite


Laustenite + L L + cementite

Tem

pera

ture

, °C

austenite

ferrite cemenite

austenite + cementiteaustenite + ferrite

T

wt. % carbonFe Fe3C

ferrite cemeniteferrite + cementite


The Invariant Reactions

δ -iron + L

•Peritectic Point

empe

ratu

re, °

C

austenite

• Point

Te

wt. % carbonFe Fe3CFe

The Peritectic Reaction @ 1492 C

Delta-iron 0.10%C + Liquid 0.50%C Austenite 0.18%Ccoolingheating


E t ti

empe

ratu

re, °

C

austenite + cementite

liquid•

Eutectic Point

Te

wt. % carbonFe Fe3C

The Eutectic Reaction @ 1130 C

Liquid 4.3%C ( Austenite 2.0%C + Cementite 6.67%C )Ledeburite (an eutectic mixture)

coolingheating


Tem

pera

ture

, °C

austenite

ferrite + cementite

Eutectoid Point•

T

wt. % carbonFe Fe3C

The Eutectoid Reaction @ 723 C

Austenite 0.8%C ( Ferrite 0.025%C + Cementite 6.67%C )Pearlite (an eutectoid mixture)

coolingheating


liquiddelta iron

The Major Phases

Tem

pera

ture

, °C

austenite

ferrite nite

lede

-bu

rite

lite

T

wt. % carbonFe Fe3C

ferrite

cem

en

pear

l


liquidaustenite + liquid liquid +

cementite

Iron – Iron Carbide Phase DiagramTe

mpe

ratu

re, °

C

austenite

ferrite

austenite + cementiteaustenite + ferrite

cementite

earli

te

men

ite

T

wt. % carbonFe Fe3C

pearlite + cementite

pe

ferrite + pearlite

cem


Characteristics of Major Phases

Tem

pera

ture

, °C

ferrite

lede

-bu

rite

Ferrite (α)An interstitial solid solution of carbon dissolved in BCC α-iron.Carbon solubility – 0.025 wt.% max. at 723 C, 0.008 wt.% min. at 0 C.The softest structure that appears on the iron – iron carbide diagramT

wt. % carbonFe Fe3C

ferrite


diagram.Average properties: 40,000 psi TS, 40 % elong. in 2 inch, < RC 0 or < RB 90 hardness.

Austenite (γ)An interstitial solid solution of carbon dissolved in FCC γ-iron

Tem

pera

ture

, °C

austenite

dissolved in FCC γ-iron.Carbon solubility – 2.00 wt.% max. at 1130 C, 0.80 wt.% min. at 723 C.Not stable at room temperature; can be made stable under certain conditions.Average properties: 150,000 psi TS, 10 % elong. in 2 inch, RC 40 hardness, high toughness. T

wt. % carbonFe Fe3C


toughness.

Tem

pera

ture

, °C

nite

Cementite (Fe3C)An interstitial intermetallic compound of iron carbide with an orthorhombic structure.Its chemical formula is Fe3C and contains 6.67 wt.% carbon.The hardest and brittle structure that appears on the iron iron carbide diagramT

wt. % carbonFe Fe3C

cem

en


iron – iron carbide diagram.Average properties: 5,000 psi TS, high compressive strength.

PearliteThe eutectoid mixture of fine plate-like lamellar mixture of ferrite and cementite.

Tem

pera

ture

, °C

lite

Formed from austenite that contains 0.80 wt.% carbon during slow cooling at 723 C.Average properties: 120,000 psi TS, 20 % elong. in 2 inch, RC 20 hardness.

T

wt. % carbonFe Fe3C

pear

l


Tem

pera

ture

, °C

lede

-bu

rite

LedeburiteT

wt. % carbonFe Fe3C


The eutectic mixture of austenite and cementite.Formed from liquid that contains 4.30 wt.% carbon during slow cooling at 1130 C.Not stable below 723 C, where austenite of ladeburite transformed into pearlite. The structure is then called “transformed ladeburite.”

AusteniteFCC structure4 iron atoms/unit cell

FerriteBCC structure2 iron atoms/unit cell

Carbon Solubility in Iron

Dense atomic packing (74 %) Loose atomic packing (68 %)

Radius of an average carbon atom is 0.70 (x10-8) cm.

Iron expands when changes from austenite to ferrite during cooling.

Why does austenite absorb more carbon than ferrite ?

Radius of maximum void in FCC austenite is 0.52 (x10-8) cm, while that in BCC ferrite is 0.36 (x10-8) cm.

The distortion of ferrite lattice by carbon atom is much greater than in case of austenite.

Austenite has a greater solubility of C atoms than ferrite© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 19

Classification of Iron –Carbon Alloys

°C

liquidSteels are Fe-C alloys that pass through austenite

zone during cooling

Tem

pera

ture

,

austenite

ferrite

ferrite + pearlite + cementite

eutectic

eutectoid

STEELS CAST IRONShypo -

eutectoidhyper -

eutectoidhypo -

eutectichyper -eutectic

pearlite4.32.00.8


Next Class

MME 291: Lecture 08

Development of Structure in SteelsStructure in Steels

lec_07

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