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MM-501 Phase Transformation in Solids
Fall Semester - 2016
Engr. Muhammad Ali SiddiquiAssistant Professor, [email protected] Department of Metallurgical Engineering
NED University of Engineering & Technology, Pakistan
Bainite Formation
Upper and Lower Bainite (Microstructure)
3
Upper bainite
4Upper bainite in Fe–0.095C–1.63Si–2Mn–2Cr wt% steel transformed isothermally at 400◦C.
5
Lower bainite
By using Atomic Force microscope or Scanning Tunneling Microscope in order to study at higher Magnification.
6
Surface Relief Shape Change:Surface Relief Shape Change:
Intense dislocation debris at a bainite/austenite interface. TEM
• Bainite grows at relatively high temperatures compare to Martensite.
• The large strains associated with the shape change cannot be sustained by the austenite, the strength of which decreases as the temperature rises.
• These strains are relaxed by the plastic deformation of the adjacent austenite.
• The local increase in dislocation density caused by the yielding of the austenite blocks the further movement of the glissile transformation interface.
• This localized plastic deformation therefore stops the growth of the ferrite plate so that each sub-unit only achieves a limited size which is much less than the size of an austenite grain.
8
Optical micrograph: Microstructure of lower bainite. Fe–0.8C wt% steel transformedat 300◦C, showing sheaves of lower bainite.
• Black line is not a single plate it is actually a cluster of plate (thousand of plates).
• Each plate is stopped by plastic accommodation..
• This produce fine structure than martensite.
9
Introduction to Bainitic Alloys
Carbides in Lower Bainitic Ferrite plates
Since long range diffusion is not allowed at lower temp, so only iron carbides (like ε, η, κ, or cementite) precipitates.
Harmful Effect of Cementite θ : how can be avoided?
Brittle Fracture = initiates cleavage cracks.Ductile Fracture = initiate nucleation of voids.
As consequence of carbides there is reduction in Toughness.
Bainitic Alloys: (Carbide Free Alloys)
1 Fe - 0.4C 2Si 3Mn2 Fe - 0.2C 2Si 3Mn3 Fe - 0.4C 2Si 4Ni
Wt%
?? Suppress the θ Precipitation *
Mn/Ni for hardenability.Stop other transformation product
* Al can do the same job as Si, but presently don’t have any prove. [2004]
Role of Si, Mn & Ni =?
Microstructure of Bainitic Alloy
• Ferrite + Carbon enriched films of
Austenite.• No carbide particles in that material
• Both strength and toughness are depend
upon the scale of bainitic ferrite & films of
C-enriched-γ
Advantage as a Results1. Can achieve very fine structure just by phase transformation. Fine Plates of Bainite 0.2μm thick and 10μm in length.
2. Got a mixture of ferrite & films of austenite.3. Each ferrite plate is only about 10μm long b/c of plasticity associated with shape deformation; stop it from growing, once it reaches about that length. So actually finer than martensite.
0.2 μm
10 μm
3. Tougher than all structure; strength is due to fine structure. (it is considered as an ideal microstructure; grain refinement is only mechanism for increase both strength and toughness).
4. Due to austenite in the microstructure; “H” embrittlement problem would be solved. (diffusion rate of hydrogen in austenite is slow)
Now have a look on toughness
Notice that the impact transition temp is more than 100oC, so that completely unacceptable for any engineering material that below 100oC one can get fracture by cleavage.So, something is very wrong in our science?
• As soon as we apply stress over here the austenite is transformed into untampered martensite which is extremely hard and brittle.
• Why do we have these large region of austenite left in our material; we have transformned isothermally?
Bainite Sheaf
Untransformed high carbon Austenite
Microstructure of that Alloy
TAe3Ae1
o
Carbon Concentration
Tem
pera
ture
Free
Ene
rgy
T1
T1
How many ways can one increase volume fraction of bainitic alloys?
1. Reduce the average
carbon concentration; shift
to the Y-axis (means
lowering the “carbon”)
2. Addition / modify of
substitutional solute Mn
etc. shift/move the To
curve to higher carbon.
3. Lower the transformation
temperature but this is
limited to Ms temperature.
Changed
Original
Fe-0.2C -3Mn-2Si
Fe-0.4C -4Ni-2Si
Product of these alloy
Fig: Section of railway line
What is the normal structure of Sections?
Microstructure of Pearlite
Tunnel b/w Britain and France ;under the sea
Talk about World first Bulk Nanostructured steel
ever created
Bulk Nanocrystalline Steel
• Imagine, a steel
1.Exceptionally strong, = GPa
2.Be made in large chunks = bulk crystalline
3.Easy to manufacture
4.Low cost which is affordable = cheap
How ?
Problem: to design a bulk nanocrystalline steel which is very strong,
tough, cheap ….
Before describing this novel
material, it is important to
review the meaning of strength,
• Put an apple on 1 m2 = 1 pa • 100 MPa = I00 million apples on 1 m2
• 1GPa = billion apples on 1 m2
• 1TPa = 1000 billion apples on 1 m2
Understanding unit
Brenner, 1956
10 GPa
Theoretical Strength • Brenner achieved
tensile strength =
greater than 13 GPa
in an iron whisker
about 1.5 mm in
length.• Theoretically =
possible to achieve a
tensile strength of 21
or 22 GPa in ideal
crystals of iron.
• The strength of a crystal increases sharply as it is made
smaller because the probability of avoiding defects
increases.
• Note these are the crystals only.
• Strength collapses as we make bigger in size because of
defects increases.
• Now remember Aim ~ 22 Gpa, if we eliminating the defects in
the materials.
1. Strengthening by Deformation
• It has been possible for some time to obtain commercially, steel wire which has an ultimate tensile strength of 5.5 GPa and yet is very ductile in fracture.
• made by Kobe Steel Japan.
Scifer, Scientific Iron
• See strength 5.5 GPa and ductility (tie knot)
• We can not make a knot with Carbon fiber which has 3.3GPa
strength & virtually zero ductility.
• Scifer, as the wire is known is made by drawing a dual-
phase microstructure of martensite and ferrite in Fe–0.2C–
0.8Si–1Mn (wt-%) steel.
• So can we make a cable bridge from this = ?
1 Denier: weight in grams, of 9 km of fibre or yarn.
50-10 Denier
Scifer is 9 DenierSo we can use it for cutting semi conductors
not for making bridge cables
Figure: Comparison of size-sensitivity of single-crystals whiskers of iron and Scifer
2. Strengthening of Carbon Nanotubes
Carbon nanotube to catalyze to grow
Morinobu Endo, 2004
Claimed strength of carbon nanotube is 130 GPaEdwards, Acta Astronautica, 2000
Claimed modulus is 1.2 TPa (1000 GPa) 6X greater than SteelTerrones et al., Phil. Trans. Roy. Soc., 2004
Space-elevator concept (originally due to Arthur C. Clark), requiring a cable 120 000 km in length.2 Cable would be launched in both directions from geosynchronous orbit at a height of 36 000 km
People starting research to built an Space elevator (Russian Concept)
What is wrong with this ?
as soon as make it big the strength collapses due to increase in the defects as we scale up[as we know that about Fe in 1956. (22 GPa) ]Equilibrium number of defects (1020)Strength of a nanotube rope 2 mm long is less than 2000 MPa.
Limit of Nanotube
•Strength produced by deformation limits shape: wires, sheets...
•Strength in small particles relies on perfection. Doomed as size increases.
Summary
So far; we are unsuccessful to produce Bulk Nanocrystalline Steel
3. Thermomechanical processing
• Smallest size possible in polycrystalline substance? • Back in 1960 (Micro alloying = dramatic change in
grainsize improves the quality of steel)• 10 billion tons of steel are in service today by
micro alloying only. (HSLA steels)
Yokota & Bhadeshia, 2004
Limit of Thermomechanical Treatment
Thermomechanical processing limited by recalescence
Summary
Need to store the heat Reduce rate Transform at low temperature
Heating up the steel by itself
Courtesy of Tsuji, Ito, Saito, Minamino, Scripta Mater. 47 (2002) 893.
Howe, Materials Science and Technology 16 (2000) 1264.
Another problem = ?
Fine crystals by transformation
1. Introduce work-hardening capacity--- How …
2. Need to store the heat3. Reduce rate 4. Transform at low temperature
Requirement for Scale up:
Design criteria for Bulk Nanocrystalline Steel
1. It should ideally be possible to
manufacture components which are large
in all dimensions, not simply in the form of
wires or thin sheets.
2. There are commercially available steels
in which the distance between interfaces is of the order of 250–
100 nm. The novelty is in approaching a structural scale in
polycrystalline metals that is an order of magnitude smaller.
3. The material concerned must be cheap to produce. A good
standard for an affordable material is that its cost must be similar
to that of bottled water when considering weight or volume.
• The following conditions are required to achieve this:
1.
2.
3.
4.
• All of these conditions can in principle be met by
the phase transformation of austenite into
bainite, partly because the reaction is
particularly amenable to control by either
isothermal or continuous cooling heat treatment.
• Furthermore, the transformation is displacive,
i.e., it leads to a shape deformation which is
macroscopically an invariant plane strain with a
large shear component, as illustrated in figure.
There is in principle no lower limit to the temperature at
which bainite can be generated.
How the bainite-start BS and martensite-start MS
temperatures vary as a function of the carbon
concentration?
0
200
400
600
800
0 0.2 0.4 0.6 0.8 1 1.2 1.4Carbon / wt%
Tem
pera
ture
/ KFe-2Si-3Mn-C wt%
BS
MS
Temperature?
1.E+00
1.E+04
1.E+08
0 0.5 1 1.5Carbon / wt%
Tim
e / s
Fe-2Si-3Mn-C wt%
1 month1 year
Take 100 year to produce bainite at room temperature
• On the other hand, the rate at which bainite
forms slows down drastically as the
transformation temperature is reduced, as
shown by the calculations in the right plot of
Fig.
• It may take hundreds or thousands of years
to generate bainite at room temperature.
• For practical purposes, a transformation time
of tens of days is reasonable.
C Si Mn Mo Cr V P0.98 1.46 1.89 0.26 1.26 0.09 < 0.002
wt%
Low transformation temperatureBainitic hardenabilityReasonable transformation timeElimination of cementiteAustenite grain size controlAvoidance of temper embrittlement
Tem
per a
ture
Time
1200 oC2 days
1000 oC15 min
Isothermal transformation
125oC-325oChours-monthsslow
cooling
Air cooling
Quench
AustenitisationHomogenisation
X-ray diffraction results
0
20
40
60
80
100
200 250 300 325Temperature/ oC
Per
cent
age
of p
hase
bainitic ferrite
retained austenite
200 Å
Caballero, Mateo, Bhadeshia
Transformation took 10 days at 200 oC
C Nano tube same X
Low temperature transformation: 0.25 T/Tm
Fine microstructure: 20-40 nm thick plates Harder than most martensites (710 HV)Carbide-freeDesigned using theory alone
Effect of Elongation due to increase in volume fraction of austenite
Strain is uniform
“more serious battlefield threats”
ballistic mass efficiency consider unit area of armour
200 Å
Very strong 2.5GPa, 710HVHuge uniform ductility
No deformationNo rapid coolingNo residual stresses
CheapUniform in very large sections
Chatterjee & Bhadeshia, 2004
Fe-1.75C-Si-Mn wt%
2104
Further Reading
Cobalt (1.5 wt%) and aluminium (1 wt%) increase the stability of ferrite relative to austenite
Refine austenite grain size
Faster Transformation
C Si Mn Mo Cr V P0.98 1.46 1.89 0.26 1.26 0.09 < 0.002
Original 5h 3/ 4d 63 550Co 4h 11h 77 640
Co + Al 1h 8h 76 640
200oC
250oC
300oC
Steel Beginning End % Bainite HVOriginal 4d 9d 69 618
Co 2d 5d 79 690Co+ Al 16h 3d 78 690
Original 2.5h 1/ 2d 55 420Co 1h 5h 66 490
Co + Al 0.5h 4h 66 490
original
Co
Co+Al
References
• H.K.D.H Bhadeshia (Online Lectures)
Thanks