Iron iron carbide diagram By Hariprasad

All the equilibrium phases are shown in the diagram

1. Hari Prasad
Presentation
by
Hariprasad(Asst. Professor)

2. Steels
L+Cementite
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0.18
0.008

3. Pureiron,uponheating,experiencestwochanges
incrystalstructurebeforeitmelts.
Atroomtemperaturethestableform,called
ferrite,oriron,hasaBCCcrystalstructure.
Ferriteexperiencesapolymorphictransformation
toFCCaustenite,oriron,at912
0
C.
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4. Thisaustenitepersiststo1394
o
C,atwhich
temperaturetheFCCausteniterevertsbackto
aBCCphaseknownasferrite,whichfinally
meltsat1538
0
C
Allthesechangesareapparentalongtheleft
verticalaxisofthephasediagram.
The�-ferriteisvirtuallythesameas??????-ferrite,
exceptfortherangeoftemperaturesover
whicheachexists.
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5. (a) Ferrite (90X) (b) Austenite (325X)
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6. Maximumsolubilityofcarboninvarioussingle
phases:
�−????????????��
(0.1%)
�−????????????��
(2.1%)
??????−????????????��
(0.008%)
??????????????????????????????????????????
(0.025%)
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Room temp solubility
Stable at only high temp

7. Thesolubilityofcarbonvariesindifferentforms
ofiron.
Indeltairon,maximumsolidsolubilityofcarbon
is0.1%
Ingammairon,themaximumsolidsolubilityof
carbonis2.03%
AusteniteisasolidsolutionofcarboninFCC
ironandsoluteatomsoccupyinterstitial
positionsinthislattice.
Inalphairon,carbonhasalimitedsolid
solubilityofabout0.008%atroomtemperature
Themaximumsolubilityofcarboninferriteis
0.025%.
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8. Description of the phases in Iron-Iron
Carbide system
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9. Steels
L+Cementite
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10. •Interstitialsolidsolutionofsmallamountscarbon
dissolvedinBCC-Iron
•Max.solubilityofcarbonis0.025%at727
o
Cand
0.008%atroomtemp
•Softestamongallthephases
??????-Ferrite
•InterstitialsolidsolutionofcarboninFCCiron.
•Maxsolubilityofcarbonis2.1%
•Itisnotstablebelow727
o
C
•Non-magnetic
Austenite
•InterstitialsolidsolutionofcarboninBCCiron
•UponheatingAustenitechangestoBCCandgets
??????-Ferrite
•Maxsolubilityofcarbonis0.1%
??????-Ferrite
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11. •ThisisalsoknownasIronCarbide(Fe
3C)
•It’sachemicalcompoundbecauseitcontainsa
fixedpercentageofcarbon(6.67%)
•Ithasanorthorhombiccrystalstructure(a≠b≠c)
Cementite
•Itistheeutecticlamellarmixtureofaustenite
(lightphase)andcementite(darkphase)
•Itisunstablebelow727
o
Candtransformsinto
??????-ferriteandcementite.
Ledeburite
•Austenitecontaining0.8%Cformspearliteupon
slowcoolingbelow727
o
C
•Itslamellarstructureof??????-ferriteandcementite.
•Itisverybrightinappearance(likeapearl)
•Ithasafingerprintlikeappearance
Pearlite
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12. Therearethreereactionswhichoccuriniron–
cementitephasediagram.
1.Peritectic
2.Eutectic
3.Eutectoid
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13. Steels
E
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14. Peritecticreaction:Inthealloycontaining0.18%
carbon,theinitialcrystalsofdeltasolidsolution
andthewholeofliquidphaseiscompletely
transformedtofromausteniteoncoolingat
1493
0
C
L+��(austenite)
Eutecticreaction:alloywithcarboncontent
4.33%,theliquidistransformedintoaustenite
andcementiteoncoolingat1147
0
C
L �(austenite)+cementite
cooling
heating
cooling
heating
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15. “Theeutecticofausteniteandcementiteisknownas
ledeburite”
Eutectoidreaction:inIron–carbonalloywith
0.8%carbon,theausteniteistransformedinto
ferriteandcementitebyeutectoidreactionon
coolingin727
0
C
Gamma(austenite) alpha+cementite
Theeutectoidofferriteandcementiteisknownas
“Pearlite”
cooling
heating
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16. Ifthecarbon%islessthan2.14,thattypeofalloy
comesunderthecategoryofsteels.
Ifthecarbon%greaterthan2.14,thatalloy
comesunderthecategoryofcastirons
STEELS:
Therearemajorcategoriesofsteels.
i.Hypoeutectoidsteels
ii.Hypereutectoidsteels
iii.Eutectoidsteels
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17. Steelswithcarboncontentfrom0.025%to0.8%
arecalledhypoeutectoidsteels.
Steelswithacarboncontentof0.8%isknownas
eutectoidsteels
Steelswithacarboncontentgreaterthan0.8%
arecalledhypereutectoidsteels
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18. Hypoeutectoid steels
Therearesolidstatetransformationsinthissteels.
Theyarethetransformationofgammairontoalpha
ironandthedecompositionofaustenite.
Thelimitingcompositionforgettingpearliteis
0.0025%C.
Withcarboncontentlessthanthisamount,nopearlite
willbeformed.Thealloywillcontainonlyferrite
grains.
Steelscontainingcarbonbetween0.025-0.8%would
containvaryingamountofferriteandpearliteand
theirrelativeproportionsdependoncarboncontent
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19. Schematic representations of
the microstructures for an
iron–carbon alloy of
hypoeutectoidcomposition
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20. PointC:Atabout875
0
C,pointc,the
microstructurewillconsistentirelyofgrainsof
the??????-phase,asshowninfig.
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21. PointD:about775
o
C,whichiswithinthe??????+
??????phaseregion,boththesephaseswillcoexistasin
theschematicmicrostructure.
Mostofthesmallparticleswillformalongthe
original??????grainboundaries
??????
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22. Micrograph of hypoeutectoid steel (0.34%C)
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23. Hypereutectoid steels
Ateutectoidtemperature,thecompositionof
austeniteis0.8%carbon
Onfurthercooling,theentireamountof
austenitewilltransformtopearlite
Hence,thefinalmicrostructureconsistsof
pearliteandproeutectoidcementite
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24. Schematic representations of the
microstructures for an iron–
carbon alloy of hypereutectoid
composition
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25. Micrograph of hypereutectoid steel
(1.34%C)
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Pearlite + proeutectoid cementite

26. Eutectoid steel
Oncoolingateutectoidpoint(0.8%C-727
0
C),all
austenitewilltransforminto100%pearlite.
So,themicrostructureatroomtemperaturewill
revealalternatelayersofferriteandcementite,
calledpearlite
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27. Schematic representations of the
microstructures for an
iron–carbon alloy of eutectoid
composition (0.76 wt% C) above and
below the eutectoid temperature.
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28. Themicrostructureforthiseutectoidsteelthatis
slowlycooledthroughtheeutectoidtemperature
consistsofalternatinglayersorlamellaeofthetwo
phases(??????andFe
3C)tatformsimultaneouslyduring
thetransformation
Pointb,iscalledpearlitebecauseithastheappearance
ofmotherofpearlwhenviewedunderthemicroscope
atlowmagnifications
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29. Photomicrograph of a
eutectoid
steel showing the pearlite
microstructure consisting of
Alternating layers of -ferrite
(the light phase) and Fe
3C
(thin layers most of which
appear dark).
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30. Microstructure of a eutectoid steel
The pearlite exists as grains, often termed
‘‘colonies’’
The thick light layers are the ferrite phase,
and the cementite phase appears as thin
lamellae most of which appear dark.
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31. Hari Prasad
The evolution of the microstructure of hypoeutectoid and
hypereutectoid steels during cooling, in relationship to the Fe-Fe3C
phase diagram

32. Steels
E
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Cast iron

33. Cast Irons
Hypoeutectic cast iron
Eutectic cast iron
Hypereutectic cast iron
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34. Hypoeutectic cast iron
Inthiscase,astructurebelow1147
0
Cconsistsof
proeutecticausteniteandledeburite(eutectic
mixtureconsistingofausteniteandcementite).
Onfurthercooling,inthetemperaturerange
1147
0
C–723
0
C,excesscarboncomesoutas
cementitefromproeutecticandeutecticaustenite.
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35. Hari Prasad
Cementite
Austenite
Within the temp range of 1147 to 727
o
C upon cooling

36. Hari Prasad
Cementite network
Ferrite
When it cools to room temp

37. Eutectic cast iron (ledeburite
structure @1147
o
C)
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Austenite + Cementite

38. Whenthesamecastironiscooledtoroomtemp,austenite
transformstopearlitewithcementite
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Pearlite formed from austeniteCementite

39. Hypereutectic Cast Iron
Inthiscasethestructurejustbelow1147
0
C
consistsofproeutecticcementiteandledeburite.
Onfurthercoolinginthetemperaturerange
1147
0
C–727
0
C,excesscarboncomesoutas
cementitefromtheeutecticaustenite.
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40. Calculation of relative amounts of
phases in Fe-Fe
3C diagram
The relative amount of proeutectoid ferrite and
pearlite in 0.2 percent carbon steel:
0.025%C 0.8%C0.2%C
X Y
Z
Percent of ferrite:
��
��
���= (
�.�−�.�
�.�−�.���
)100 =77.4%
Percent of Pearlite:
��
��
���= (
�.�−�.���
�.�−�.���
)100 =22.6%
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41. Calculation of relative amounts of
phases in Fe-Fe
3C diagram
The relative amount of pearlite and cementite
in 1.2 percent carbon steel:
0.8%C 6.67%C1.2%C
X Y
Z
Percent of pearlite:
��
��
���=
�.��−�.�
�.��−�.�
100 =?%
Percent of cementite:
��
��
���=
�.�−�.�
�.��−�.�
100 =?%
Hari Prasad