Iron carbon diagram

Iron Carbon dig, describing the transformation of molten iron -carbon alloy in equilibrium to different phases.

1. IRON IRON-CARBON
DIAGRAM

2. IRON IRON-CARBON DIAGRAM
Ferrite
Austenite
Steel Cast iron
Pearlite
Pearlite and
Cementine
Pearlite and
Carbide
Eutectic
eutectoid

3. Outline
Introduction
Coolingcurveforpureiron
Definitionofstructures
Iron-Carbonequilibriumphasediagram–Sketch
TheIron-IronCarbideDiagram-Explanation
TheAustenitetoferrite/cementite
transformation
Nucleation&growthofpearlite
EffectofC%ageonthemicrostructureofsteel
Relationshipb/wC%age&mechanical
propertiesofsteel

4. Cooling curve for pure iron

5. Definition of structures
Variousphasesthatappearonthe
Iron-Carbon equilibriumphase
diagramareasunder:
•Austenite
•Ferrite
•Pearlite
•Cementite
•Martensite*
•Ledeburite

6. Unit Cells of Various Metals
FIGURE-Theunitcellfor(a)austentite,(b)ferrite,and(c)martensite.
Theeffectofthepercentageofcarbon(byweight)onthelatticedimensions
formartensiteisshownin(d).Notetheinterstitialpositionofthecarbon
atomsandtheincreaseindimensioncwithincreasingcarboncontent.
Thus,theunitcellofmartensiteisintheshapeofarectangularprism.

7. Microstructure of different phases of steel

8. Definition of structures
Ferriteisknownasαsolidsolution.
Itisaninterstitialsolidsolutionofasmall
amountofcarbondissolvedinα(BCC)iron.
stableformofironbelow912deg.C
Themaximumsolubilityis0.025%Cat
723Canditdissolvesonly0.008%Cat
roomtemperature.
Itisthesofteststructurethatappearsonthe
diagram.

9. Definition of structures
Ferrite
Averagepropertiesare:
Tensilestrength=40,000psi;
Elongation =40%in2in;
Hardness >RockwellC0or
>RockwellB90

10. Definition of structures
Pearliteistheeutectoidmixture
containing0.80%Candis
formedat723°Converyslow
cooling.
Itisaveryfineplatelikeor
lamellarmixtureofferriteand
cementite.
Thewhiteferriticbackgroundor
matrixcontainsthinplatesof
cementite(dark).

11. Definition of structures
Pearlite
Averagepropertiesare:
Tensilestrength=120,000psi;
Elongation =20%in2in.;
Hardness =RockwellC20,RockwellB
95-100,orBHN250-300.

12. Definition of structures
Austeniteisaninterstitialsolidsolutionof
Carbondissolvedin(F.C.C.)iron.
Maximumsolubilityis2.0%Cat1130°C.
Highformability,mostofheattreatments
beginwiththissinglephase.
Itisnormallynotstableatroom
temperature.But,undercertainconditionsit
ispossibletoobtainausteniteatroom
temperature.

13. Definition of structures
Austenite
Averagepropertiesare:
Tensilestrength=150,000psi;
Elongation =10percentin2in.;
Hardness =RockwellC40,
approx;and
toughness =high

14. Definition of structures
Cementiteorironcarbide,isveryhard,
brittleintermetalliccompoundofiron&
carbon,asFe
3C,contains6.67%C.
Itisthehardeststructurethatappearsonthe
diagram,exactmeltingpointunknown.
Itscrystalstructureisorthorhombic.
Itishas
lowtensilestrength(approx.5,000psi),
but
highcompressivestrength.

15. Definition of structures
Ledeburiteistheeutectic
mixtureofausteniteand
cementite.
Itcontains4.3percentCandis
formedat1130°C.

16. Definition of structures
Martensite-a super-saturated solid solution of
carbon in ferrite.
It is formed when steel is cooled so rapidly that
the change from austenite to pearlite is
suppressed.
The interstitial carbon atoms distort the BCC
ferrite into a BC-tetragonal structure (BCT).;
responsible for the hardness of quenched steel

17. The Iron-Iron Carbide Diagram
Amapofthetemperatureatwhichdifferent
phasechangesoccuronveryslowheating
andcoolinginrelationtoCarbon,iscalled
Iron-CarbonDiagram.
Iron-Carbondiagramshows
thetypeofalloysformedunderveryslow
cooling,
properheat-treatmenttemperatureand
howthepropertiesofsteelsandcastirons
canberadicallychangedbyheat-treatment.

18. Various Features of Fe-C diagram
Peritectic L + d= 
Eutectic L =  + Fe
3C
Eutectoid = a+ Fe
3C
Phases present
L
Reactions
d
BCC structure
Paramagnetic
austenite
FCC structure
Non-magnetic
ductile
aferrite
BCC structure
Ferromagnetic
Fairly ductile
Fe
3Ccementite
Orthorhombic
Hard
brittle
Max. solubility of C in ferrite=0.022%
Max. solubility of C in austenite=2.11%

19. Three Phase Reactions
Peritectic, at 1490 deg.C, with low wt% C
alloys (almost no engineering importance).
Eutectic, at 1130 deg.C, with 4.3wt% C,
alloys called cast irons.
Eutectoid, at 723 deg.C with eutectoid
composition of 0.8wt% C, two-phase mixture
(ferrite & cementite). They are steels.

20. How to read the Fe-C phase diagram
Ferrite
Austenite
Steel Cast iron
Pearlite
Pearlite and
Cementine
Pearlite and
Carbide
Eutectic
eutectoid

21. The Iron-Iron Carbide Diagram
Thediagramshowsthreehorizontallineswhich
indicateisothermalreactions(oncooling/
heating):
Firsthorizontallineisat1490°C,whereperitectic
reactiontakesplace:
Liquid+d↔austenite
Secondhorizontallineisat1130°C,where
eutecticreactiontakesplace:
liquid↔austenite+cementite
Thirdhorizontallineisat723°C,whereeutectoid
reactiontakesplace:
austenite↔pearlite(mixtureofferrite&
cementite)

22. Delta region of Fe-Fe carbide diagram
Liquid + d↔ austenite

23. Ferrite region of
Fe-Fe Carbide
diagram

24. Simplified Iron-Carbon phase diagram
austenite ↔ pearlite (mixture of ferrite & cementite)

25. The Austenite to ferrite / cementite transformation in
relation to Fe-C diagram

26. The Austenite to ferrite / cementite
transformation in relation to Fe-C diagram
Inordertounderstandthetransformation
processes,considerasteeloftheeutectoid
composition.0.8%carbon,beingslowcooled
alonglinex-x‘.
Attheuppertemperatures,onlyausteniteis
present,withthe0.8%carbonbeingdissolved
insolidsolutionwithintheFCC.Whenthesteel
coolsthrough723°C,severalchangesoccur
simultaneously.

27. The Austenite to ferrite / cementite
transformation in relation to Fe-C diagram
Theironwantstochangecrystal
structurefromtheFCCaustenitetothe
BCCferrite,buttheferritecanonly
contain0.02%carboninsolidsolution.
Theexcesscarbonisrejectedand
formsthecarbon-richintermetallic
knownascementite.

28. Pearlitic structure
Thenetreactionatthe
eutectoidistheformation
ofpearliticstructure.
Sincethechemical
separationoccursentirely
withincrystallinesolids,
theresultantstructureisa
finemixtureofferriteand
cementite.

29. Schematic picture of the formation and
growth of pearlite
Ferrite
Cementite
Austenite
boundary

30. Nucleation & growth of pearlite

31. The Austenite to ferrite / cementite transformation in
relation to Fe-C diagram
Hypo-eutectoidsteels:Steelshavinglessthan
0.8%carbonarecalledhypo-eutectoidsteels
(hypomeans"lessthan").
Considerthecoolingofatypicalhypo-eutectoid
alloyalongliney-y‘.
Athightemperaturesthematerialisentirely
austenite.
Uponcoolingitentersaregionwherethestable
phasesareferriteandaustenite.
Thelow-carbonferritenucleatesandgrows,
leavingtheremainingaustenitericherincarbon.

32. The Austenite to ferrite / cementite
transformation in relation to Fe-C diagram
Hypo-eutectoid steels-
At 723°C, the remaining
austenite will have assumed
the eutectoid composition
(0.8% carbon), and further
cooling transforms it to
pearlite.
The resulting structure, is a
mixture of primary or pro-
eutectoid ferrite(ferrite that
forms before the eutectoid
reaction) and regions of
pearlite.

33. The Austenite to ferrite / cementite transformation in
relation to Fe-C diagram
Hyper-eutectoidsteels(hypermeans
"greaterthan")arethosethatcontainmore
thantheeutectoidamountofCarbon.
Whensuchasteelcools,asalonglinez-z',
theprocessissimilartothehypo-eutectoid
steel,exceptthattheprimaryorpro-eutectoid
phaseisnowcementiteinsteadofferrite.

34. The Austenite to ferrite / cementite transformation in
relation to Fe-C diagram
Asthecarbon-richphasenucleatesandgrows,
theremainingaustenitedecreasesincarbon
content,againreachingtheeutectoid
compositionat723°C.
Thisaustenitetransformstopearliteuponslow
coolingthroughtheeutectoidtemperature.
Theresultingstructureconsistsofprimary
cementiteandpearlite.
Thecontinuousnetworkofprimarycementite
willcausethematerialtobeextremelybrittle.

35. The Austenite to ferrite / cementite transformation in
relation to Fe-C diagram
Hypo-eutectoid steel showing primary cementite along grain
boundaries pearlite

36. The Austenite to ferrite / cementite
transformation in relation to Fe-C diagram
Itshouldbenotedthatthetransitions
asdiscussed,areforequilibrium
conditions,asaresultofslowcooling.
Uponslowheatingthetransitionswill
occurinthereversemanner.

37. The Austenite to ferrite / cementite transformation in
relation to Fe-C diagram
Whenthealloysarecooledrapidly,entirely
differentresultsareobtained,sincesufficient
timemaynotbeprovidedforthenormalphase
reactionstooccur.
Inthesecases,theequilibriumphasediagram
isnolongeravalidtoolforengineering
analysis.
Rapid-coolprocessesareimportantintheheat
treatmentofsteelsandothermetals(tobe
discussedlaterinH/Tofsteels).

38. Principal phases of steel and their
Characteristics
Phase
Crystal
structure
Characteristics
Ferrite BCC Soft, ductile, magnetic
Austenite FCC
Soft, moderate
strength, non-
magnetic
Cementite
Compound of Iron
& Carbon Fe
3C
Hard &brittle

39. 24
• Teutectoidchanges: • Ceutectoidchanges:
Alloying Steel with more Elements

40. Cast Irons
-Iron-Carbon alloys of
2.11%C or more are cast
irons.
-Typical composition: 2.0-
4.0%C,0.5-3.0% Si, less
than 1.0% Mn and less
than 0.2% S.
-Si-substitutes partially for C
and promotes formation of
graphite as the carbon
rich component instead
Fe
3C.

41. Applications
It is used tailor properties of steel and to heat
treat them.
It is also used for comparison of crystal
structures for metallurgists in case of rupture
or fatigue

42. Conclusion

43. Thanks