lecture bainite, bainitic alloys and bulk nanocrystalline steel

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

mailto:[email protected]

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.

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.

• 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