Diagram Fasa 6ir3x

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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铁碳相图

IMPORTANCE OF Fe-Fe3C SYSTEM l 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. PDF created with pdfFactory Pro trial version www.pdffactory.com

WHAT DO YOU KNOW ABOUT IRON?

铁素体 α-ferrite  solid solution of C in BCC Fe ØStable form of iron at room temperature. ØThe maximum solubility of C is 0.022 wt%. ØTransforms to FCC γaustenite at 912oC

奥氏体


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

奥氏体


ØThe maximum solubility of C is 2.14 wt %. ØTransforms to BCC δ-ferrite at 1395oC ØIs not stable below the eutectoid temperature (727oC) unless cooled rapidly


δ-铁素体

δ-ferrite solid solution of C in BCC Fe ØThe same structure as α-ferrite ØStable only at high T, above 1394oC ØMelts at 1538oC



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

渗碳体 PDF created with pdfFactory Pro trial version www.pdffactory.com

A FEW COMMENTS ABOUT Fe-Fe3C l C is an interstitial impurity in Fe. It forms a solid solution with α, γ, δ phases of iron. l Maximum solubility in BCC α-ferrite is limited (max.0.022 wt% at 727oC) BCC has relatively small interstitial positions. PDF created with pdfFactory Pro trial version www.pdffactory.com

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

MECHANICAL PROPERTIES l Cementite is very hard and brittle - can strengthen steels. l Mechanical properties also depend on the microstructure, that is, how ferrite and cementite are mixed.


MAGNETIC PROPERTIES l α -ferrite is magnetic below 768oC, l Austenite is non-magnetic.


Commercially pure iron (C<0.02%)

Steel (C:0.02~2.14%)

Cast Iron (C:2.14~6.70%)

CLASSIFICATION OF FERROUS ALLPYS



Steel (C:0.02~2.14%)

Cast Iron (C:2.14~6.70%)


Eutectoid Steel (C=0.76%)

共析钢



Steel (C:0.02~2.14%)

Cast Iron (C:2.14~6.70%)


Hypoeutectoid Steel (C<0.76%)

亚共析钢



Steel (C:0.02~2.14%)

Cast Iron (C:2.14~6.70%)


Hypereutectoid Steel (C>0.76%)

过共析钢



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



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



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 H

J

B

Liquidus line D E

Solidus line


C

F

A H

J

B

D E C

F

莱氏体 Ledeburite

P

S

A1

珠光体 Pearlite


k

A H

J

B

L+δ

D

E C

γ, Austenite

Peritectic: 0.17 wt%C, 1493oC L+δ ↔ γ PDF created with pdfFactory Pro trial version www.pdffactory.com

F

A H

J

B

Liquidus line D E

Acm

G

A3 P

S

Solidus line

C

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

Solvus line, solubility limit of C in α. QFe3CIII, 三次渗碳体，沿铁素体晶界，较少，略 PDF created with pdfFactory Pro trial version www.pdffactory.com

F

EQUILIBRIUM MICROSTRUCTURE 2 1 3


5

4

6

EUTECTOID l 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)


Pearlite l 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. colony

ferrite

cementite PDF created with pdfFactory Pro trial version www.pdffactory.com

Pearlite l 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. PDF created with pdfFactory Pro trial version www.pdffactory.com

HYPOEUTECTOID

l In α+γ region, the composition of α phase changes along MN, the composition of γ phase changes along MO… PDF created with pdfFactory Pro trial version www.pdffactory.com

先共析铁素体

HYPOEUTECTOID l Hypoeutectoid alloys contain proeutectoid ferrite (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… PDF created with pdfFactory Pro trial version www.pdffactory.com

二次渗碳体

HYPEREUTECTOID l Hypereutectoid alloys contain proeutectoid cementite (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 AMOUNT l 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 hypoeutectoid alloys and to (α + Fe3C) – Fe3C boundary (6.7 wt% C) for hypereutectoid alloys. Fraction of pearlite: T C0' − 0.022 C0' − 0.022 Wp = = = T + U 0.76 − 0.022 0.74

Fraction of proeutectoid α: U 0.76 − C0' 0.76 − C0' Wα ' = = = T + U 0.76 − 0.022 0.74


CALCULATION OF PHASE AMOUNT l 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 hypoeutectoid alloys and to (α + Fe3C) – Fe3C boundary (6.7 wt% C) for hypereutectoid alloys. Fraction of pearlite: X 6.7 − C1' 6.7 − C1' Wp = = = V + X 6.7 − 0.76 5.94

Fraction of proeutectoid cementite: WFe C ' 3


V C1' − 0.76 C1' − 0.76 = = = V + X 6.70 − 0.76 5.94

EUTECTIC L

Fe3CII

莱氏体Ld

γ+Fe3C

共晶渗碳体

二次渗碳体

P+ Fe3CII +Fe3C 变态莱氏体Ld’ PDF created with pdfFactory Pro trial version www.pdffactory.com

HYPOEUTECTIC L γI+L

先共晶奥氏体

Fe3C) C Fe3CII γI+Ld(γ+Fe 莱氏体二次渗碳体

P+Fe3CII + Ld’( P + Fe3CII+ Fe3C ) 由γI析出 PDF created with pdfFactory Pro trial version www.pdffactory.com

由γ析出

HYPEREUTECTIC L 一次渗碳体 L+Fe3CI Ld(γ+Fe3C) +Fe3CI

Fe3CII 二次渗碳体

莱氏体

Ld’( P + Fe3CII +Fe3C ) +Fe3CI


INFLUENCE OF ALLOYING ELEMENTS l 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.


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