20140205 - Fe-C Diagram
Transcript of 20140205 - Fe-C Diagram
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Introduction to Fe-C equilibrium phase
diagramChia-Chang Shih
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Outline
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Divergence among pure iron, steeland cast iron
0.02wt% C 2wt% C
pure iron steel cast iron
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Pure iron
1. Melt point of pure iron : 1538
2. Crystalline transformation of allotrope appears twice under the equilibrium condition.
3. Three transformations on crystalline structure in pure iron from room temperature to
melting point:a. -Fe: Which shows the crystalline structure of body centered cubic (BCC) at the
temperature raging from 1394 to 1538 .
b. -Fe: Face centered cubic (FCC) is the corresponding crystalline structure while
temperature controlled at 912 -1394 .c. -Fe: Crystalline structure transformed into BCC once temperature is below 912 .
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Crystalline structures
(1) triclinic(2) (3) monoclinic(4)-(7) orthorhombic
(8)-(9) tetragonal(10) trigonal(11) hexagonal(12)-(14) cubic
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Comparisons between BCC and FCC
N = 2CN = 8APF = 0.68Close packed planes : {110} {112} {123}Close packed directions : Slip system : 48
N = 4CN = 12APF = 0.74Close packed planes : {111}Close packed directions : Slip system : 12
ferrite austenite
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Introduction to Fe-C alloy
1. Carbon atoms tend to insert into the interstitial sites of Fe matrix to form theinterstitial solid solution
2. The solubility of carbon in -Fe is about 0.02%. -Fe reveals the ferromagneticunder 770 and this temperature is the so-called A2.
3. The solubility of carbon in -Fe is about 2.04%.4. Cementite, ie Fe 3C, which shows a tendency to be formed once carbon content is
higher than the solubility in Fe matrix.5. Ferrite and Austenite reveal the maximum interstitial space of 0.036nm and 0.053nm,
respectively.
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1. Generally, we could obtain the information about phases andmicrostructures in a specific alloy by an equilibrium phasediagram.
2. All phases shown in equilibrium phase diagram can bedefined as equilibrium phases, and their transformation musttake place under a condition of extreme slow cooling speed.
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What are the phases within materials?
Difference among the phases can be distinguishedthrough the crystalline structure and chemicalcomposition.
Case 1 : different crystalline structure + identical compositeex. Low-carbon ferrite and austenite
Case 2 : different composite + same crystalline structureex. and
phases within stainless steels
Case 3 : both composite and crystalline structure are differentex. ferrite and cementite
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Binary phase diagram
binary isomorphous system
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Binary phase diagram
eutectic system : L +
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Binary phase diagram
intermediate phases
intermediate solid solutionsterminal solid solutions
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Binary phase diagram
Eutectoid system +
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Binary phase diagram
Peritectic system : L +
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Binary phase diagram-invariant reaction
system reaction phase diagram
eutectic L +
eutectoid +
Peritectic + L
peritectoid +
monotectic L1 + L2
L
L
L2L1
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What can be achieved from phase diagram?
1. Corresponding composites and temperatures of the phasesunder equilibrium condition.
2. Solid solubility3. Corresponding solidification temperature of alloys under
equilibrium condition.4. Corresponding temperature and composition ranges for
respective equilibrium phases.
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The effects of composition, process andmicrostructure on
mechanical properties of steels
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Fe-C equilibrium phase diagram
symbol name Crystallinestructure
L liquid
-Fe BCC
austenite FCC
ferriteBCC
Fe3C cementite OrthorhombicA1 : + Fe 3CA2 : ferromagnetic transitive temperatureA3 : + Acm : + Fe 3C
A1
Acm A3
Bao tinh
Cng tch
Cng tinh
Cementite cng tinh
A2 T768Ko cat Austenite
Cemeitite I
Cemeitite II
Cemeitite III
0.09%C
0.5%C
0.006%C
0.18%C
0.02%C
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Microstructures in pure iron
ferrite austenite
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Microstructures in pure iron-IF steel
Optical microscopy Scanning electron microscopy
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Peritectic
L
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Eutectoid reaction
Fe 3C
1. Pearlite had transformed from austenite byeutectoid reaction, which is one kind of lammer-mixture structure that consists of the ferrite andFe3C.
2. Generally, corresponding temperature and carboncontent of eutectoid reaction was about727 (A1) and 0.77%, respectively.
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Eutectoid structure the formation of pearlite
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Eutectoid structure the formation of pearlite
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N-W relationship: (110) bcc//(111) fcc , [001] bcc //[-101] fcc(Nishiyama-Wasserman relationship)
K-S relationship: (110) bcc //(111) fcc , [1-11] bcc //[0-11] fcc(Kurdjumov-Sachs relationship)
Coherent interfaceSemicoherent interfaceIncoherent interface
Interface energy: CI < SI < II
Eutectoid structure the formation of pearlite
h ff f li
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The effect of temperature on pearliteinterlamellar spacing
Th ff f li
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** Spacing between ferrite and Fe 3C within pearlitestructure shows an inverse proportion to T.
**T = T eutetoid -T
The effect of temperature on pearliteinterlamellar spacing
Th ff f ll l
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The effect of alloy elements on pearlite interlamellar spacing
Ferrite stabilized elements favor the decrease of interval between ferrite and Fe 3Cwithin pearlite structure.
Th ff f ll l id
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The effects of alloy elements on eutectoidreaction
causes : 1. ferrite stabilized elements2. dislocation atmosphere
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Proeutectoid reaction
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Proeutectoid structure
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Hypereutectoid reaction
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Hypereutectoid structure
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Eutectic
L Fe 3C
A complex structure consists of iron andFe3C results from the emergency of eutectic reactioncan be called as Ledeburite.
Eutectic reaction appears at eutectic point withtemperature of about 1148 and carbon content of 4.3%.
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Eutectic reaction
1. Liquid iron with carbon content of 4.3% will be transformed into a complex structurewhich consists of -Fe and Fe 3C at 1148 under extremely slow cooling rate.
2. -Fe comes from the eutectic reaction will be transformed into pearlite whiletemperature decreasing until 727 .
L
Fe3C
Fe3C
(1148 ) (727 )
Equilibrium cooling
Fe3C
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Proeutectic reaction
1. -Fe solidifies firstly from liquid iron with carbon content of 2 - 4.3% in theregion of L + A .
2. When temperature decreased to 1148 (mark 2), remaining liquid iron must betransformed into Ledeburite through eutectic reaction.
3. Secondary Fe 3C will be precipitated from -Fe.4. All the -Fe will be transformed into pearlite by eutectoid reaction once
temperature decreasing until to 727 (mark 3).
L
L
Fe3C(1148 ) (727 )
Fe3C
Fe3CFe3C
Equilibrium cooling
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Hypereutectic reaction
1. Cementite (Fe 3C) firstly solidifies in the liquid iron with carbon content of 4.3%-6.67% (regionof mark 1 to 2).
2. All the remaining liquid iron must solidify and be transferred into Ledeburite through eutecticreaction when temperature decreasing until 1148 (mark 2) under isothermal condition.
LFe3C
(1148 ) (727 )
Fe3C
Fe3CFe3C
L
Fe3C Fe3C
Equilibrium cooling