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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