1. Materials Engineering – Day 13 More About Steel

2. Steel and Carbon Conent
1015 steel – plain carbon – 0.15%C
1090 steel – plain carbon – 0.90%C
What happens as carbon content increases? In general, we see more and more pearlite in slow cooled steels. More and more cementite available in all steels. Strength  up. Ductility  down.
BUT, AT A GIVEN CARBON CONTENT, WIDELY VARYING PROPERTIES ARE AVAILABLE DEPENDING ON PROCESSING.

3. Chapter 10: Phase Transformations
ISSUES TO ADDRESS...
• Transforming one phase into another takes time. Fe g
(Austenite)
Eutectoid
transformation C
FCC
Fe3C
(cementite) a
(ferrite) +
(BCC)
• How does the rate of transformation depend on
time and T ?
• How can we slow down the transformation so that
we can engineering non-equilibrium structures?
• Are the mechanical properties of non-equilibrium
structures better?

4. The Austenite to Pearlite Transformation
This takes time. Two things must happen.
Nucleation of Pearlite. (Best nucleation sites are in or near grain boundaries.) Nuclei are baby crystals. The thermodynamics of nucleation get stronger with supercooling.
Growth of Pearlite. This takes diffusion. Carbon atoms must diffuse away from the g to the Fe3C layers which are starting. Diffusion is stronger at high temperatures and weakens as the temperature decreases.
These two aspects of the transformation work against each other.

5. Eutectoid Transformation Rate
• Growth of pearlite from austenite:
Adapted from Fig. 9.15, Callister 7e. g a
pearlite
growth
direction
Austenite (g)
grain
boundary
cementite (Fe3C)
Ferrite (a)
Diffusive flow
of C needed a g
• Recrystallization
rate increases
with DT.
Adapted from Fig , Callister 7e.
675°C
(DT smaller) 50
y (% pearlite)
600°C
(DT larger)
650°C
100
Course pearlite  formed at higher T - softer
Fine pearlite  formed at low T - harder

6. C-Curves
Think of some austenite, lowered suddenly to a temperature below 727C and allowed to transform at that temperature.
At high temperature, atoms can diffuse rapidly BUT, nucleation rates are very low due to only slight undercooling. Therefore, the overall transformation tends to be lengthy.
At low temperature, nucleation is very speedy, but diffusion is slow. Therefore the transformation tends to be lengthy.
At some intermediate temperatures there must be an optimum. Thus we get a C-shaped curve.

7. Nucleation and Growth
• Reaction rate is a result of nucleation and growth
of crystals.
% Pearlite 50
100
Nucleation
regime
Growth
log (time) t
0.5
Nucleation rate increases with T
Growth rate increases with T
Adapted from
Fig , Callister 7e.
• Examples:
T just below TE
Nucleation rate low
Growth rate high g
pearlite
colony
T moderately below TE g
Nucleation rate med .
Growth rate med.
Nucleation rate high
T way below TE g
Growth rate low

8. TTT Curves for a Euctectoid Steel
Note the axes:
Time – always log scale.
Temperature
Note the curves:
Start time
50% Completion (dashed)
End time. (after crossing this there is no more austenite left to transfform.

9. Isothermal Transformation Diagrams
• Fe-C system, Co = 0.76 wt% C
• Transformation at T = 675°C.
Note the axes:
Time – always log scale.
Temperature
100
T = 675°C y,
% transformed 50
Note the curves:
Start time
50% Completion (dashed)
End time. (after crossing this there is no more austenite left to transform. 2 4 1 10 10
time (s)
400
500
600
700 1 10 2 3 4 5
0%pearlite
100%
50%
Austenite (stable)
TE (727C)
Austenite
(unstable)
Pearlite
T(°C)
time (s)
isothermal transformation at 675°C

10. The Pearlite Spectrum
The kind of pearlite that you get depends on the transformation temperature.
High temperature. Low nucleation. Higher growth. Larger coarser more separated layers in the pearlite.  Not as strong, more ductile.
Low temperature. (Nearer to the “nose.”) High nucleation. Little diffusion. Finer thinner closer spaced layers in the pearlite.  Stronger, less ductile.
Intermediate temperatures – an intermediate pearlite.

11. Two Pearlites. Which formed at the higher Temperature?
Which is stronger?
Which is more ductile?

12. What Happens as we Transform at Lower and Lower Temperatures?
T (C)
727
Coarse P
Fine P
???
Log time

13. The Answer is “Bainite”
Bainite is relatively rare. The austenite is changed to pearlite before the bainite transformation can start in most practical cooling processes. But it is good stuff.
Strong, hard, some ductility.
Upper (above 300) 
Lower (below 300) 
Upper B. Needles of ferrite with strips of cementite
Lower B. Very thin ferrite plates. Ultra thin cementite rods

14. What Happens if we Transform at an even lower Temperature?
The answer is that we get a phase known as martensite.
Martensite is BC tetragonal with C interstitially.
It forms instantaneously. No nucleation or diffusion is required.
Martensite is one of the hardest substances known to man. It is also totally brittle like a ceramic.
The production of martensite is an important step, but not the final step, in the production of high strength steels. (Quench and tempered)

15. Here is the TTT Curve for a Euctectoid Steel.
Coarse P
Fine P
Upper B
Lower B
M (Martensite)

16. Martensite: Fe-C System
--g(FCC) to Martensite (BCT)
Martensite needles
Austenite
60 m x
potential
C atom sites
Fe atom
sites
(involves single atom jumps)
(Adapted from Fig , Callister, 7e.
• Isothermal Transf. Diagram 10 3 5
time (s) -1
400
600
800
T(°C)
Austenite (stable)
200 P B TE 0%
100%
50% A
M + A
90%
(Adapted from Fig , Callister, 7e. (Fig courtesy United States Steel Corporation.)
Adapted from Fig , Callister 7e.
• g to M transformation..
-- is rapid!
-- % transf. depends on T only.

17. Martensite Formation  (FCC)  (BCC) + Fe3C slow cooling quench
M (BCT)
tempering
M = martensite is body centered tetragonal (BCT)
Diffusionless transformation BCT if C > 0.15 wt%
BCT  few slip planes  hard, brittle

18. Tempering Martensite • • • reduces brittleness of martensite,
• reduces internal stress caused by quenching.
YS(MPa)
TS(MPa)
800
1000
1200
1400
1600
1800 30 40 50 60
200
400
600
Tempering T (°C)
%RA TS YS
Adapted from Fig , Callister 7e. (Fig adapted from Fig. furnished courtesy of Republic Steel Corporation.)
Adapted from Fig , Callister 7e. (Fig copyright by United States Steel Corporation, 1971.)
9 mm
produces extremely small Fe3C particles surrounded by a. • •
decreases TS, YS but increases %RA

19. Mechanical Prop: Fe-C System (1)
• Effect of wt% C
Pearlite (med)
Pearlite (med) C
ementite
ferrite (soft)
(hard)
Co < 0.76 wt% C
Co > 0.76 wt% C
Adapted from Fig. 9.30,Callister
7e. (Fig courtesy Republic Steel Corporation.)
Adapted from Fig. 9.33,Callister 7e. (Fig copyright 1971 by United States Steel Corporation.)
Hypoeutectoid
Hypereutectoid
300
500
700
900
1100
YS(MPa)
TS(MPa)
wt% C
0.5 1
hardness
0.76
Hypo
Hyper
wt% C
0.5 1 50
100
%EL
Impact energy (Izod, ft-lb) 40 80
0.76
Hypo
Hyper
Adapted from Fig , Callister 7e. (Fig based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, p. 9.)
• More wt% C: TS and YS increase
, %EL decreases.

20. Mechanical Prop: Fe-C System (2)
• Fine vs coarse pearlite vs spheroidite 80
160
240
320
wt%C
0.5 1
Brinell hardness
fine
pearlite
coarse
spheroidite
Hypo
Hyper 30 60 90
wt%C
Ductility (%AR)
fine
pearlite
coarse
spheroidite
Hypo
Hyper
0.5 1
Adapted from Fig , Callister 7e. (Fig based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, pp. 9 and 17.)
• Hardness:
fine > coarse > spheroidite
• %RA:
fine < coarse < spheroidite

21. Mechanical Prop: Fe-C System (3)
• Fine Pearlite vs Martensite:
200
wt% C
0.5 1
400
600
Brinell hardness
martensite
fine pearlite
Hypo
Hyper
Adapted from Fig , Callister 7e. (Fig adapted from Edgar C. Bain, Functions of the 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 )
• Hardness: fine pearlite << martensite.

22. Summary: Processing Options
Austenite (g)
Bainite
(a + Fe3C plates/needles)
Pearlite
(a + Fe3C layers + a
proeutectoid phase)
Martensite
(BCT phase
diffusionless
transformation)
Tempered
(a + very fine
Fe3C particles)
slow
cool
moderate
rapid
quench
reheat
Strength
Ductility
T Martensite
bainite
fine pearlite
coarse pearlite
spheroidite
General Trends
Adapted from Fig , Callister 7e.

23. Effect of Cooling History in Fe-C System
• Eutectoid composition, Co = 0.76 wt% C
• Begin at T > 727°C
• Rapidly cool to 625°C and hold isothermally.
400
500
600
700
0%pearlite
100%
50%
Austenite (stable)
TE (727C)
Austenite
(unstable)
Pearlite
T(°C) 1 10 2 3 4 5
time (s)
Adapted from Fig ,Callister 7e.
(Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.) g g

24. Transformations with Proeutectoid Materials
CO = 1.13 wt% C
TE (727°C)
T(°C)
time (s) A + C P 1 10
102
103
104
500
700
900
600
800
Adapted from Fig , Callister 7e.
Adapted from Fig. 9.24, Callister 7e.
Fe3C (cementite)
1600
1400
1200
1000
800
600
400 1 2 3 4 5 6
6.7 L g
(austenite) +L
+Fe3C a
L+Fe3C d
(Fe)
Co , wt%C
T(°C)
727°C DT
0.76
0.022
1.13
Hypereutectoid composition – proeutectoid cementite

25. Non-Equilibrium Transformation Products: Fe-C
• Bainite:
--a lathes (strips) with long
rods of Fe3C
--diffusion controlled.
• Isothermal Transf. Diagram
Fe3C
(cementite) a
(ferrite) 10 3 5
time (s) -1
400
600
800
T(°C)
Austenite (stable)
200 P B TE 0%
100%
50%
pearlite/bainite boundary A
100% pearlite
100% bainite
5 mm
(Adapted from Fig , Callister, 7e. (Fig from Metals Handbook, 8th ed.,
Vol. 8, Metallography, Structures, and Phase Diagrams, American Society for Metals, Materials Park, OH, 1973.)
Adapted from Fig , Callister 7e.
(Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.)

26. Spheroidite: Fe-C System
(Adapted from Fig , Callister, 7e. (Fig copyright United States Steel Corporation, 1971.)
60 m a
(ferrite)
(cementite)
Fe3C
• Spheroidite:
--a grains with spherical Fe3C
--diffusion dependent.
--heat bainite or pearlite for long times
--reduces interfacial area (driving force)

27. Phase Transformations of Alloys
Effect of adding other elements
Change transition temp.
Cr, Ni, Mo, Si, Mn
retard    + Fe3C
transformation
Adapted from Fig , Callister 7e.

28. Cooling Curve plot temp vs. time
Actual processes involves cooling – not isothermal
Can’t cool at infinite speed
Adapted from Fig ,
Callister 7e.

29. Dynamic Phase Transformations
On the isothermal transformation diagram for 0.45 wt% C Fe-C alloy, sketch and label the time-temperature paths to produce the following microstructures:
42% proeutectoid ferrite and 58% coarse pearlite
50% fine pearlite and 50% bainite
100% martensite
50% martensite and 50% austenite

30. Example Problem for Co = 0.45 wt%
42% proeutectoid ferrite and 58% coarse pearlite
first make ferrite
then pearlite
course pearlite
 higher T
A + B
A + P
A + a A B P
50%
200
400
600
800
0.1 10
103
105
time (s)
M (start)
M (50%)
M (90%)
T (°C)
Adapted from Fig ,
Callister 5e.

31. Example Problem for Co = 0.45 wt%
50% fine pearlite and 50% bainite
first make pearlite
then bainite
fine pearlite
 lower T
A + B
A + P
A + a A B P
50%
200
400
600
800
0.1 10
103
105
time (s)
M (start)
M (50%)
M (90%)
T (°C)
Adapted from Fig ,
Callister 5e.

32. Example Problem for Co = 0.45 wt%
100 % martensite – quench = rapid cool
50 % martensite
and 50 % austenite
A + B
A + P
A + a A B P
50%
200
400
600
800
0.1 10
103
105
time (s)
M (start)
M (50%)
M (90%) c)
T (°C) d)
Adapted from Fig ,
Callister 5e.