Post on 15-Sep-2020
1
Eutectoid transformation rate
Diffusive flow of C needed
α
α
γγ
α
• Growth of pearlite from austenite:
Adapted fromFig. 9.15,Callister 7e.
γαααα
α
α
pearlite growth direction
Austenite (γ)grain boundary
cementite (Fe3C)Ferrite (α)
γ
• Recrystallization rate increases with ΔT.
Adapted fromFig. 10.12,Callister 7e.
675°C (ΔT smaller)
0
50
y (%
pea
rlite
)
600°C (ΔT larger)
650°C
100
Coarse pearlite formed at higher T - softerFine pearlite formed at low T - harder
2
Transformations and undercooling
• Can make it occur at: ...727ºC (cool it slowly) ...below 727ºC (“undercool” it)
• Eutectoid transformation (Fe-C System): γ ⇒ α + Fe3C0.76 wt% C
0.022 wt% C6.7 wt% C
Fe3C
(cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
γ (austenite)
γ+L
γ +Fe3C
α +Fe3C
L+Fe3C
δ
(Fe) Co , wt%C
1148°C
T(°C)
αferrite
727°C
Eutectoid:Equil. Cooling: Ttransf. = 727ºCΔT
Undercooling by ΔTtransf. < 727°C
0.76
0.02
2
Adapted from Fig.9.24,Callister 7e. (Fig. 9.24adapted from Binary AlloyPhase Diagrams, 2nd ed.,Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International,Materials Park, OH, 1990.)
3
Isothermal transformation diagrams
Adapted from Fig. 10.13,Callister 7e.(Fig. 10.13 adapted from H. Boyer (Ed.)Atlas of Isothermal Transformation andCooling Transformation Diagrams,American Society for Metals, 1977, p.369.)
• Fe-C system, Co = 0.76 wt% C• Transformation at T = 675°C.
100
50
01 102 104
T = 675°C
y,
% tr
ansf
orm
ed
time (s)
400
500
600
700
1 10 102 103 104 105
0%pearlite
100%
50%
Austenite (stable) TE (727°C)Austenite (unstable)
Pearlite
T(°C)
time (s)
isothermal transformation at 675°C
4
Effect of cooling history
• Eutectoid composition, Co = 0.76 wt% C• Begin at T > 727°C• Rapidly cool to 625°C and hold isothermally.
Adapted from Fig.10.14,Callister 7e.(Fig. 10.14 adapted fromH. Boyer (Ed.) Atlas ofIsothermal Transformationand CoolingTransformation Diagrams,American Society forMetals, 1997, p. 28.)
400
500
600
700
0%pearlite
100%
50%
Austenite (stable) TE (727°C)Austenite (unstable)
Pearlite
T(°C)
1 10 102 103 104 105
time (s)
γ γ
γ
γ γ
γ
5
Transformations with proeutectoid materials
Hypereutectoid composition – proeutectoid cementite
α
CO = 1.13 wt% C
TE (727°C)
T(°C)
time (s)
A
A
A+C
P
1 10 102 103 104
500
700
900
600
800
A+
P
Adapted from Fig. 10.16,Callister 7e.
Adapted from Fig. 9.24,Callister 7e.
Fe3C
(cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
γ (austenite)
γ+L
γ +Fe3C
α +Fe3C
L+Fe3C
δ
(Fe) Co , wt%C
T(°C)
727°CΔT
0.76
0.02
2
1.13
6
Nonequilibrium transformation products
• Bainite: --α lathes (strips) with long rods of Fe3C --diffusion controlled.• Isothermal Transf. Diagram
Adapted from Fig. 10.18, Callister 7e.(Fig. 10.18 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and CoolingTransformation Diagrams, American Society for Metals, 1997, p. 28.)
(Adapted from Fig. 10.17, Callister, 7e. (Fig.10.17 from Metals Handbook, 8th ed.,Vol. 8, Metallography, Structures, and PhaseDiagrams, American Society for Metals,Materials Park, OH, 1973.)
Fe3C(cementite)
5 µm
α (ferrite)
10 103 105
time (s)10-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0% 100%
50%
pearlite/bainite boundary
A
A100% bainite
100% pearlite
7
Spheroidite
• Spheroidite: --α grains with spherical Fe3C --diffusion dependent. --heat bainite or pearlite for long times --reduces interfacial area (driving force)
(Adapted from Fig. 10.19, Callister, 7e.(Fig. 10.19 copyright United StatesSteel Corporation, 1971.)
60 µm
α(ferrite)
(cementite)Fe3C
8
Martensite
• Martensite: --γ(FCC) to Martensite (BCT)
Adapted fromFig. 10.22,Callister 7e.
(Adapted from Fig. 10.21, Callister, 7e.(Fig. 10.21 courtesy United StatesSteel Corporation.)
• Isothermal Transf. Diagram
• γ to M transformation.. -- is rapid -- % transf. depends on T only.
(Adapted from Fig.10.20, Callister, 7e.
Martensite needlesAustenite
60 µ
m
10 103 105 time (s)10-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%50%
A
A
M + AM + A
M + A
0%50%90%
xx x
xx
x potential C atom sites
Fe atom sites
(involves single atom jumps)
9
Cooling curve
Adapted fromFig. 10.25,Callister 7e.
plot temp vs. time
10
Mechanical properties 1
Adapted from Fig.10.29, Callister 7e.(Fig. 10.29 based ondata from MetalsHandbook: HeatTreating, Vol. 4, 9thed., V. Masseria(Managing Ed.),American Society forMetals, 1981, p. 9.)
Adapted from Fig. 9.30,Callister7e. (Fig. 9.30 courtesy RepublicSteel Corporation.)
Adapted from Fig. 9.33,Callister 7e.(Fig. 9.33 copyright 1971 by UnitedStates Steel Corporation.)
• More wt% C: TS and YS increase, %EL decreases.
• Effect of wt% C
Co < 0.76 wt% CHypoeutectoid
Pearlite (med)ferrite (soft)
Co > 0.76 wt% CHypereutectoid
Pearlite (med)Cementite
(hard)
300
500
700
900
1100YS(MPa)TS(MPa)
wt% C0 0.5 1
hardness
0.76
Hypo Hyper
wt% C0 0.5 10
50
100%EL
Impa
ct e
nerg
y (Iz
od, f
t-lb)
0
40
80
0.76
Hypo Hyper
11
Mechanical properties 2
Adapted from Fig. 10.30, Callister 7e.(Fig. 10.30 based on data from MetalsHandbook: Heat Treating, Vol. 4, 9thed., V. Masseria (Managing Ed.),American Society for Metals, 1981, pp.9 and 17.)
• Fine vs coarse pearlite vs spheroidite
• Hardness:• %RA:
fine > coarse > spheroiditefine < coarse < spheroidite
80
160
240
320
wt%C0 0.5 1
Brin
ell h
ardn
ess
fine pearlite
coarse pearlitespheroidite
Hypo Hyper
0
30
60
90
wt%CD
uctil
ity (%
AR)
fine pearlite
coarse pearlite
spheroidite
Hypo Hyper
0 0.5 1
12
Mechanical properties 3
• Fine Pearlite vs Martensite:
• Hardness: fine pearlite << martensite.
Adapted from Fig. 10.32,Callister 7e. (Fig. 10.32 adaptedfrom Edgar C. Bain, Functions ofthe Alloying Elements in Steel,American Society for Metals,1939, p. 36; and R.A. Grange,C.R. Hribal, and L.F. Porter,Metall. Trans. A, Vol. 8A, p.1776.)
0
200
wt% C0 0.5 1
400
600
Brin
ell h
ardn
ess martensite
fine pearlite
Hypo Hyper
13
Tempered martensite
• reduces brittleness of martensite,• reduces internal stress caused by quenching.
Adapted fromFig. 10.33,Callister 7e.(Fig. 10.33copyright byUnited StatesSteelCorporation,1971.)
• decreases TS, YS but increases %RA• produces extremely small Fe3C particles surrounded by α.
Adapted fromFig. 10.34,Callister 7e.(Fig. 10.34adapted fromFig. furnishedcourtesy ofRepublic SteelCorporation.)
9 µ
m
YS(MPa)TS(MPa)
800
1000
1200
1400
1600
1800
30405060
200 400 600Tempering T (°C)
%RA
TS
YS
%RA
14
Summary
Adapted fromFig. 10.36,Callister 7e.
Austenite (γ)
Bainite(α + Fe3C plates/needles)
Pearlite(α + Fe3C layers + a proeutectoid phase)
Martensite(BCT phase diffusionless
transformation)
Tempered Martensite (α + very fine Fe3C particles)
slow cool
moderate cool
rapid quench
reheat
Stre
ngth
Duc
tility
Martensite T Martensite
bainite fine pearlite
coarse pearlite spheroidite
General Trends