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Lecture 10

Dr. Javad Mola

Institute of Iron and Steel Technology (IEST)

Tel: 03731 39 2407

E-mail: [email protected]

Bainite

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Pearlite formed at

720 °C

Eutectoid Steels

Bainite isothermally

formed at 290 °C

MartensiteBainite

isothermally formed at

180 °C

The darker appearance of bainite compared to martensite is due to the presence of carbides which are absent in virgin martensite.

During the late 1920s, in the course of these pioneering studies on the isothermal transformation of austenite at temperatures above that at which martensite first forms, but below that at which fine pearlite is found, Davenpont and Bain (1930) discovered a new microstructure consisting of an “acicular, dark-etching aggregate” which was quite unlike the pearlite or martensite observed in the same steel.

Discovery of Bainite

H. K. D. H. Bhadeshia, Bainite in Steels: Theory and Practice, 3rd Edition, Maney Publishing, Leeds, 2015.E.C. Bain, Functions of the Alloying Elements in Steel, American Society for Metals, 1939.

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Pearlite formation

Bainite formation

TTT

→P P

B

→B

Ms

Bs

Tem

per

atu

re

Log time

Martensite formation

Bainite Formation Temperature Range

Pearlite reaction is essentially a high-temperature reaction. Martensite formation, on the other hand, is a low temperature reaction. There is a wide range of temperature within which neither pearlite nor martensite form. This is the region in which fine aggregates of ferrite plates (or laths) and cementite particles are formed.

Upper

Lower

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0 200 400 600 800 1000

-20

0

20

40

60

80

100

120 Ferrite

+

Perlite

BainiteCooling rate (°C/s)

Re

lative

Le

ng

th C

ha

ng

e, m

/cm

Temperature, °C

0.04

0.1

0.6

1

2

3

5

10

15

20

60

200

Cooling

Martensite

Bainite Formation Temperature Range

42CrMo4

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Displacive Mechanism (Surface Relief)

Similar to martensite and Widmanstätten ferrite formation, bainitic transformation results in surface tilting of polished specimens (surface relief).

Roughening of a polished specimen due to bainite formation

Tilting of initially straight surface scratches after bainite formation

H. K. D. H. Bhadeshia, Bainite in Steels: Theory and Practice, 3rd Edition, Maney Publishing, Leeds, 2015.

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Invariant-Plane Strain

The surface relief after bainite formation is a consequence of invariant-plane type of strain (IPS) as explained in the discussion of martensite.

Habit Plane

S (shear component)

(dilatational component)

Invariant-plane strain (IPS)

B

Displacive

Reconstructive

Approximate values of shear strain and dilatational strain for a variety of transformation products in steel.


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Microstructure

Upper bainite (typically forming at temperatures above 400 °C)The microstructure of upper bainite consists of fine plates of ferrite, each of which about 0.2μm thick. The plates grow in clusters called sheaves. Within each sheaf, the plates are parallel and almost of identical crystallographic orientation and a common crystallographic habit plane. The individual plates in a sheaf are often called the ‘sub-units’ of bainite. They are usually separated by low-misorientation boundaries or by cementite particles.Bainite grows at relatively high temperatures compared to martensite. At these temperatures, the large strains associated with the IPS shape change cannot be elastically accommodated by the soft austenite whose strength decreases as the temperature rises. The transformation strains are therefore relaxed by the plastic deformation of the adjacent austenite. The local increase in dislocation density caused by the yielding of austenite resists the movement of the glissile transformation interface. This localized plastic deformation eventually halts 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.

Lower bainite (typically forming at temperatures below 400 °C)Lower bainite has a microstructure and crystallographic features which are verysimilar to those of upper bainite. The major distinction is that carbide particlesalso precipitate inside the plates of ferrite.

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Microstructure

Schematic representation of a bainite sheaf consisting of many subunits

Bainite sub-units

Lower bainite Both inter-lath and in-lath cementite

(transition carbides at low isothermal holding temperature)

Interlath carbides In-lath carbides

Upper bainite Only inter-lath cementite

Redrawn based on: M. Takahashi, H.K.D.H. Bhadeshia, Mater. Sci. Technol. 6 (1990) 592–603.

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Microstructure

Optical light micrograph showing sheaves of lower bainite in a

partially transformed Fe-0.3C-4Cr steel. The light etching matrix

phase is martensite.

TEM micrograph of the specimen at left showing carbides within subunits of

lower bainite.

In-lath carbide

Bainite sheaf

Martensitic matrix


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Inter-lath retained

austenite (dark)

TEM micrograph of a sheaf of upper bainite in a partially transformed Fe-0.43C-2Si-3Mn alloy. Retained austenite exists in the inter-lath (inter-subunit) regions. Silicon addition has suppressed the precipitation of inter-lath carbides resulting in carbide-free bainite.

Microstructure


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Carbide Types

Transition carbides (𝜀/𝜂) form at low transformation temperatures. At relatively high temperatures, cementite (θ) will form instead of transition carbides. Transition carbides and cementite may coexist at certain temperatures.

Atom probe map of elements for lower bainite in a 100Cr6 alloy isothermally heat treated at 260 °C for 2500 sec.

𝜀: Fe2.4C

θ: Fe3C

W. Song, J. von Appen, P. Choi, R. Dronskowski, D. Raabe, W. Bleck, Acta Mater. 61 (2013) 7582–7590.

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Diffusionless hypothesis: bainitic ferrite grows without any diffusion of carbon resulting in C-supersaturated plates of bainitic ferrite. Immediately after bainitic transformation, carbon supersaturation is relieved by partitioning to austenite, and/or through carbide precipitation.

Diffusional hypothesis (para-equilibrium): bainitic ferrite growth is associated with carbon diffusion, and is not distinctly different in character from Widmanstätten ferrite (carbon diffusion out of growing ferrite plates so that ferrite is never supersaturated). At high temperatures, the carbon diffusivity is high enough to prevent the carbide formation inside ferrite plates so that carbides only precipitate in the C-enriched inter-lath regions. In the case of lower bainite, on the other hand, the carbon diffusivity is not sufficiently high to keep pace with the b growth velocity, therefore carbides regularly precipitate during growth of bainitic plates. These carbides will appear as in-lath carbides after transformation.

Diffusionless or Diffusional?

𝛾 → 𝛼𝐵(𝑠𝑢𝑝𝑒𝑟𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑑) + 𝛾𝑢𝑛𝑡𝑟𝑎𝑛𝑠𝑓𝑜𝑟𝑚𝑒𝑑

→ 𝛼𝐵(𝑢𝑛𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑑) + (θ/ε/𝜂) + 𝛾𝑒𝑛𝑟𝑖𝑐ℎ𝑒𝑑

𝛾 → 𝛼𝐵(𝑢𝑛𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑑) + (θ/ε/𝜂) + 𝛾𝑒𝑛𝑟𝑖𝑐ℎ𝑒𝑑

cementite transition carbides


M. Hillert, A. Borgenstam, Metall. Mater. Trans. A 43 (2012) 4487–4495.

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Diffusionless or Diffusional?

Diffusionless growth requires that transformation occurs at a temperature below T0, where the free energy of bainite becomes less than that of austenite of the same composition. Growth without diffusion can only occur if the carbon concentration of austenite lies to the left of the T0 curve. The increase in the free energy of bainite due to the transformation strains (recall the displacive nature of bainite formation) shifts the T0 curve to lower C contents, i.e. to the T0 curve. Suppose that a plate of bainite is forming and any excess carbon is rejected into the residual austenite. The next plate of bainite then has to grow from carbon-enriched austenite.

For diffusionless bainite growth, this process must stop when the austenite carbon concentration reaches the T0 curve.

For diffusional bainite growth, this process must stop when the austenite carbon concentration reaches the Ae3 curve because driving force for diffusional growth is still available in the T0-Ae3

range.


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Diffusionless Hypothesis

Carbon in austenite

Tem

per

atu

re

Ae3T0

ഥ𝑿

T1

Max. C in retained

for diffusionless

growth

Max. C in retained

for diffusional

growth

A series of experimental results

reporting the estimated C content of retained

austenite after bainitictransformation in an Fe-0.43C-3Mn-2.12Si

steel free of precipitates

Proponents of diffusionless transformation hypothesis argue that experimental observations similar to the one shown at right indicate that the bainitic transformation stops when the carbon content of austenite reaches the T0 line.

C content

of from

experiments


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Diffusional Hypothesis Experimental observation of slow growth rates of bainite plates/sheaves seems to be

compatible with carbon diffusion during growth. No kinetic discontinuity can be seen between Widmanstätten ferrite and bainite (they are

mechanistically the same), so that bainite is simply an extension to lower temperatures of Widmanstätten ferrite formation. Widmanstätten ferrite formation is well-known to involve carbon diffusion, so the same should also apply to the bainite formation.

Temperature, °CTemperature, °C

Bain

ite

gro

wth

rate

,

m/s

ec

Ba

init

e g

row

th r

ate

,

m/s

ec

10

1

0.1

0.01

0.1

0.01

0.001

0.0001455399 343 288 232 177399 343 288510 232

Fe-0.69C-0.7Si-0.85Mn-

1.8Ni-0.85Cr-0.81Mo steel

Fe-0.5C-0.36Si-0.68Mn-8.7Ni

steel

R.H. Goodenow, S.J. Matas, R.F. Hehemann, Trans. Metall. Soc. AIME 227 (1963) 651–658.

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Diffusional Nature of Bainite Nucleation

Curves at right represent the volume free energy

change necessary in order to obtain a detectable

nucleation rate for ferrite forming by shear in

commercial steels.

Case (a) Volume free energy change is

calculated with the assumption that the

ferrite nucleus inherits the parent austenite’s

carbon content (diffusionless).

Case (b) Volume free energy change is

calculated with the assumption that the

ferrite nucleus at all times contains only an

equilibrium carbon content (diffusional).

According to the diffusionless hypothesis, the alloys

located below the T0 line in (a) must be identified as

bainite (T0 marks a driving force of 400 J/mol, which is

commonly used as the critical driving force for bainite

formation) and those above this line must be

Widmanstätten ferrite. When the driving force of the

alloys in (a) is recalculated assuming a diffusional

nature for the transformation (b), all alloys follow a

common line (universal nucleation function) indicating

that the nucleation of the bainitic ferrite should not be

different in nature from that of the Widmanstätten

ferrite, i.e. it should also be diffusional.

T0

T0

400 J/mol

Case (a)

Case (b) Ae3Universal diffusional nucleation function

According to the following discussion, both hypotheses agree that the nucleation of bainite

involves diffusion.Ferrite nucleus inherits the parent austenite C content

Ferrite nucleus contains an equilibrium C content

F > 0

H.K.D.H. Bhadeshia, Acta Metall. 29 (1981) 1117–1130. M. Hillert, A. Borgenstam, Metall. Mater. Trans. A 43 (2012) 4487–4495.

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Bainite Start (Bs) Temperature

According to the diffusionless growth hypothesis, the Bs temperature must be slightly below T0. Usually a driving force of 400 J/mol is required for the bainitic transformation. At T0 , the driving force for bainite formation is zero. T0 is the temperature at which a driving force of 400 J/mol is available for the diffusionless bainitic transformation. A number of empirical relationships have been proposed for the compositional dependence of Bs

temperature, for instance:

Bs(°C)= 830-270C-90Mn-37Ni-70Cr-83Mo (concentrations in wt%)

Exp

an

sio

n d

ue t

o

b

tra

ns

form

ati

on

, %

Time, sec

0 200 400 600 800

0.7

0.5

0.3

0.1

As the isothermal holding temperature increases, the transformation proceeds to a smaller degree, i.e. a smaller bainite fraction. No bainitic transformation will occur above the Bs temperature (almost 470 °C for this steel).

Fe-0.34C-1.47Si-0.54Al-0.24Mo-1.20Mn-0.95Cr-0.91Ni-0.10V-1.04Co-0.99W

S. Khare, K. Lee, H.K.D.H. Bhadeshia, Metall. Mater. Trans. A 41 (2010) 922–928.

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Carbide-Free Bainite

Certain elements such as Si, Al, and P are used to prevent the carbide formation in bainite. These elements are almost insoluble in cementite, so when present, the formation of cementite which requires the diffusion of these elements out of the growing cementite will be hindered. This is the case because substitutional elements are almost immobile at temperatures where bainite forms (para-equilibrium conditions). The inhibition of cementite formation in austenite means that more C will be enriched in the austenite and that higher retained austenite fractions will be obtained after final cooling to room temperature.

Whereas the effectiveness of Si, Al, and P in suppressing cementite formation in austenite is well-documented, they are not as effective in suppressing cementite formation in ferrite/bainite/martensite.

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