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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
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 03
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.
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 04
Fe3C6.67
1535° 1492°
Iron – Iron Carbide Phase DiagramTe
mpe
ratu
re, °
C 1401°
910°
723°
1130°
T
Fe Fe3Cwt. % carbon
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 05
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
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 06
LiquidDelta (δ) iron
Tem
pera
ture
, °C
Austenite (γ )
Ferrite (α) CemeniteT
wt. % carbonFe Fe3C
Ferrite (α) Cemenite
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 07
Laustenite + L L + cementite
Tem
pera
ture
, °C
austenite
ferrite cemenite
austenite + cementiteaustenite + ferrite
T
wt. % carbonFe Fe3C
ferrite cemeniteferrite + cementite
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 08
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
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 09
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
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 10
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
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 11
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
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 12
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
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 13
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
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 14
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
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 15
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
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 16
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
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 17
Tem
pera
ture
, °C
lede
-bu
rite
LedeburiteT
wt. % carbonFe Fe3C
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 18
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
© Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron – Iron Carbide Phase Diagram P 20
Next Class
MME 291: Lecture 08
Development of Structure in SteelsStructure in Steels
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