Diagram Fasa

铁碳相图

Fe-Fe3C PHASE DIAGRAM

In their simplest form, steels are alloys of Iron (Fe) and Carbon (C). The Fe-C phase diagram is a fairly complex one, but we will only consider the steel and cast iron part of the diagram, up to around 7% Carbon.

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IMPORTANCE OF Fe-Fe3C SYSTEMl Of all binary alloy systems, the one that is possibly

the most important is that for iron and carbon.

l Both steels and cast irons, primary structural materials in every technologically advanced culture, are essentially iron-carbon alloys.

l The focus of this lecture is to study the phase diagram for this system and the development of microstructures.

l The relationships between heat treatment, microstructure, and mechanical properties are based on the phase diagram of this system.



WHAT DO YOU KNOW ABOUT IRON?

α-ferrite solid solution of C in BCC FeØStable form of iron at room

temperature.ØThe maximum solubility of C

is 0.022 wt%.ØTransforms to FCC γ-

austenite at 912oC

铁素体

奥氏体PDF created with pdfFactory Pro trial version www.pdffactory.com


WHAT DO YOU KNOW ABOUT IRON?γ-austenite - solid solution of C in FCC FeØThe maximum solubility of C

is 2.14 wt %.ØTransforms to BCC δ-ferrite

at 1395oCØ Is not stable below the

eutectoid temperature (727oC) unless cooled rapidly

奥氏体




δ-ferrite solid solution of C in BCC FeØThe same structure

as α-ferriteØStable only at high

T, above 1394oCØMelts at 1538oC

δ-铁素体




渗碳体

Fe3C (iron carbide or cementite)ØThis intermetallic compound

is metastable, it remains as a compound indefinitely at room T, but decomposes (very slowly, within several years) into α-Fe and C (graphite) at 650-700oC



A FEW COMMENTS ABOUT Fe-Fe3Cl C is an interstitial

impurity in Fe. It forms a solid solution with α, γ, δphases of iron.

l Maximum solubility in BCC α-ferrite is limited (max.0.022 wt% at 727oC) -BCC has relatively small interstitial positions.

l Maximum solubility in FCC austenite is 2.14 wt% at 1147oC - FCC has larger interstitial positions



MECHANICAL PROPERTIESl Cementite is very hard and brittle - can strengthen

steels.

l Mechanical properties also depend on the microstructure, that is, how ferrite and cementiteare mixed.



MAGNETIC PROPERTIESl α -ferrite is magnetic below 768oC,

l Austenite is non-magnetic.



CLASSIFICATION OF FERROUS ALLPYSCommercially pure iron

(C<0.02%)Steel

(C:0.02~2.14%)Cast Iron

(C:2.14~6.70%)




(C<0.02%)Steel

(C:0.02~2.14%)Cast Iron

(C:2.14~6.70%)

Eutectoid Steel(C=0.76%)

共析钢




(C<0.02%)Steel

(C:0.02~2.14%)Cast Iron

(C:2.14~6.70%)

Hypoeutectoid Steel(C<0.76%)

亚共析钢




(C<0.02%)Steel

(C:0.02~2.14%)Cast Iron

(C:2.14~6.70%)

Hypereutectoid Steel(C>0.76%)

过共析钢




(C<0.02%)Steel

(C:0.02~2.14%)Cast Iron

(C:2.14~6.70%)

Eutectic Cast Iron(C=4.3%)

共晶白口铸铁




(C<0.02%)Steel

(C:0.02~2.14%)Cast Iron

(C:2.14~6.70%)

Hypoeutectic Cast Iron(C<4.3%)

亚共晶白口铸铁




(C<0.02%)Steel

(C:0.02~2.14%)Cast Iron

(C:2.14~6.70%)

Hypereutectic Cast Iron(C<4.3%)

过共晶白口铸铁



IMPORTANT LINES AND REACTIONS



A

B

C

DLiquidus line

H J

E FSolidus line

A

C



A

B

C

D

H J

E F

A

C

Ledeburite莱氏体

Pearlite

珠光体A1SP k



A

B

C

D

H J

E F

A

C

Peritectic: 0.17 wt%C, 1493oCL+δ ↔ γ

L+δ

γ, Austenite



A

B

C

DLiquidus line

H J

E FSolidus line

A

C

S

Solvus line, solubility limit of C in γ.Fe3CII, proeutectoid Fe3C,二次渗碳体，由奥氏体中析出，常呈网状分布于奥氏体的晶界

P

Q

Solvus line, solubility limit of C in α.Fe3CIII, 三次渗碳体，沿铁素体晶界，较少，略

Acm

A3

G



EQUILIBRIUM MICROSTRUCTURE

12 3 45 6



EUTECTOIDl When alloy of

eutectoid composition (0.76 wt % C) is cooled slowly it forms pearlite, a lamellar or layered structure of two phases: α-ferrite and cementite(Fe3C)



Pearlitel The pearlite exists as grains, often termed “colonies”;

within each colony the layers are oriented in the same direction, which varies from one colony to another.

ferrite

cementite

colony



Pearlitel The layers of alternating phases in pearlite are

formed for the same reason as layered structure of eutectic structures: redistribution C atoms between ferrite (0.022 wt%) and cementite (6.7 wt%) by atomic diffusion.

l Mechanically, pearlite has properties intermediate to soft, ductile ferrite and hard, brittle cementite.



HYPOEUTECTOID

l In α+γ region, the composition of αphase changes along MN, the composition of γ phase changes along MO…

先共析铁素体



HYPOEUTECTOIDl Hypoeutectoid alloys

contain proeutectoidferrite (formed above the eutectoid temperature) plus the eutectoid pearlite that contain eutectoid ferrite and cementite.

Microstructure of a 0.38wt% steel. Courtesy Republic Steel Corp.



HYPEREUTECTOID

l In γ+Fe3C region, the composition of cementite phase remains constant, the composition of γ phase changes along PO…

二次渗碳体



HYPEREUTECTOIDl Hypereutectoid alloys

contain proeutectoidcementite (formed above the eutectoid temperature) plus pearlite that contain eutectoid ferrite and cementite.

Microstructure of a 1.4wt% steel. Courtesy U.S. Steel Corp.



CALCULATION OF PHASE AMOUNTl Application of the lever rule with tie line that extends

from the eutectoid composition (0.75 wt% C) to α–(α+ Fe3C) boundary (0.022 wt% C) for hypoeutectoidalloys and to (α + Fe3C) – Fe3C boundary (6.7 wt% C) for hypereutectoid alloys.

Fraction of pearlite:

74.0022.0

022.076.0022.0 '

0'0 −

=−

−=

+=

CCUT

TWp

Fraction of proeutectoid α:

74.076.0

022.076.076.0 '

0'0

'

CCUT

UW −=

−−

=+

=α



CALCULATION OF PHASE AMOUNTl Application of the lever rule with tie line that extends

from the eutectoid composition (0.75 wt% C) to α–(α+ Fe3C) boundary (0.022 wt% C) for hypoeutectoidalloys and to (α + Fe3C) – Fe3C boundary (6.7 wt% C) for hypereutectoid alloys.

Fraction of pearlite:

94.57.6

76.07.67.6 '

1'1 CC

XVXWp

−=

−−

=+

=

Fraction of proeutectoid cementite:

94.576.0

76.070.676.0 '

1'

1'

3

−=

−−

=+

=CC

XVVW CFe



EUTECTIC

L

γ+Fe3C共晶渗碳体

Fe3CII

二次渗碳体

P+ Fe3CII +Fe3C

莱氏体Ld

变态莱氏体Ld’



HYPOEUTECTIC

L

γI+Ld(γ+Fe3C)

先共晶奥氏体

Fe3CII

二次渗碳体

γI+L

莱氏体

由γI析出由γ析出

P P

Fe3C

Fe3CII Fe3CII Fe3C+ + Ld’( + + )

Fe3CIIFe3CII



HYPEREUTECTIC

L

Ld(γ+Fe3C)

一次渗碳体

Fe3CII

二次渗碳体

L+Fe3CI

莱氏体

Fe3CIIP Fe3CLd’( + )

Fe3CIIFe3CII +Fe3CI

+Fe3CI+



INFLUENCE OF ALLOYING ELEMENTSl Addition of alloying elements affects the position of

eutectoid with respect to temperature and to carbon concentration.

l The purpose of alloying: improve corrosion resistance, and/or render amenable to heat treatment.



Diagram Fasa

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