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BainiteWidmenstatten Ferrite

Acicular Ferrite

Lecture Series –B-3

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Bainite

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Displacive decomposition of Austenite

log{time}

Tem

pera

ture

/ °C

Ms

ferritepearlite

bainite

martensite

Ae3

upper bainite

lower bainite

Widmanstatten ferriteBs

Ws

Reconstructive

Displacive

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Optical Micrograph PEARLITE

Microstructure Relationship Property

Electron Micrograph BAINITE

Fig. - Carbon steel wire (0.78% C, 0.58% Mn) of 4.6mm dia.

Microstructure Relationship Property

Table – Comparison of

Austemperd

and Q&T 1090 steel

Microstructure Relationship Property

AusTempering - Advantages

Lower rate of distortion than for Q&T.

Improved toughness than Q&T steel.

In many cases strength and wear resistance can also be improved.

Increased fatigue strength.

Better resistance to shock loads.

Resistance to hydrogen and environmental embrittlement.

No need of final tempering.

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Upper and Lower Bainite (Microstructure)

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Upper bainite

Upper bainite in Fe–0.095C–1.63Si–2Mn–2Cr wt% steel transformed isothermally at 400◦C.

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Lower bainite

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By using Atomic Force microscope or Scanning Tunneling Microscope in order to study at higher Magnification.

Surface Relief Shape Change:

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Microstructure of lower bainite. Optical micrograph, Fe–0.8C wt% steel transformedat 300◦C, showing sheaves of lower bainite.

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Carbides in Lower Bainitic Ferrite platesSince long range diffusion is not allowed at lower temp, so only iron carbides (like ε, η, κ, or cementite) precipitates.

Orientation RelationshipThe O.R. b/w cementite and ferrite of bainite is similar to than found in tempered martensite i.e. Bagaryastki relationship:

Another O.R. observed by Isaichev is also close to Bagaryastki:

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The Shape Change

Bainitic Transformation

TAe3Ae1

o

Carbon Concentration

Tem

pera

ture

Free

Ene

rgy

T1

T1

To is the temp. where thefree energy and

composition of α and γ are same as a function of C concentration

To Concept

Diffusionless transformation is thermodynamically impossible if the C conc. of the γ exceeds the T0 curve.

Bainitic Transformation

TAe3Ae1

o

Carbon ConcentrationTe

mpe

ratu

reFr

ee E

nerg

y

T1

T1

All bainite forms below the To temp.Diffusionless growth requires that transformation occurs at a temperature below To.

Below To, the free energy of bainite

is less than γ of samecomposition

Bainitic Transformation Suppose that the plate of bainite forms without diffusion, but that any excess C is soon afterwards rejected into the residual γ.The next plate of bainite then has to grow from C–enriched γ.This process must cease when the γ carbon concentration reaches the T0 curve.The reaction is said to be incomplete, since the γ has not achieved its equilibrium composition (given by the Ae3 curve) at the point the reaction stops.If on the other hand, the α grows with an equilibrium C concentration then thetransformation should cease when the γ carbon concentration reaches the Ae3 curve.

Bainitic Transformation

Fig. - Illustration of the incomplete reaction phenomenon. During isothermal transformation, a plate of bainite grows without diffusion, then partitions its excess C into the residual γ. The next plate therefore has to grow from C–enriched γ. This process continues until diffusionless transformation becomes impossible when the γ composition eventually reaches the T0 boundary. (b) Experimental data showing that the growth of bainite stops when the γ C conc.reaches the T0 curve.

Bainitic TransformationIt is found experimentally that the transformation to bainite does definitely stop at the T0 boundary.The balance of the evidence is that the growth of bainite below the BS temperature involves the successive nucleation and martensitic growth of sub–units.The possibility that a small fraction of the C is partitioned during growth cannot entirely be ruled out.But there is little doubt that the bainite is at first substantially supersaturated with C.

* Bainitic ferrite is dislocation-rich. Therefore it is harder than normal ferrite.

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Widmanstätten Ferrite (WF) ORWidmanstätten ferrite plates or laths OR

Sideplates OR Intragranular plates

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•Widmanstatten ferrite is a phase formed by the transformation of austenite below Ae3. • Widmanstatten ferrite which grows by a displacive

transformation mechanism, maintains an atomic correspondence between the parent and product phases. • On an optical scale, Widmanstatten ferrite has triangular

shape and it is called as thin wedge (Fig.), The actual shape being somewhere between that of a plate and a lath.

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• Fig. 1.11: Optical micrograph showing the classical wedge shape of Widmanstiitten ferrite in • Fe-0.22C-2.05Si-3.07Mn-

0.7Mo (wt. %) steel transformed at 700 0C @ 25 days after austenitisation at 1100 0 C @ 10 min.

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•Widmanstatten ferrite can nucleate directly from austenite grain boundaries, called "Widmanstatten ferrite primary side plates" or • it can nucleate from previously formed grain boundary allotriomorphic ferrite, "Widmanstatten ferrite secondary side plates", both morphologies are shown in Fig. 1.12.

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•This does not etch dark as in case of Bainite or Martensite. Because Bainite and Martensite has lot of structure inside what we see microscopically.

•Widmenstatten Ferrite αw looks white and clean.

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Acicular Ferrite (AF)

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• The most exciting recent developments in wrought and welded steel technology have involved ‘acicular ferrite’.• The term acicular means shaped and pointed(sharp) like a needle. • It has three dimension morphology of thin, lenticular plates.• In two-dimensional sections, the acicular ferrite always appears like a

plate rather than a section of a rod.• Serial sectioning experiments which have a depth resolution of about

0.5μmhave confirmed that the shape is between that of a lath or plate, with the length, width and thickness normally less than about 36, 6 and 3μm, respectively• The plates of acicular ferrite nucleate heterogeneously on small non-

metallic inclusions and radiate in many different directions from these ‘point’ nucleation sites

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Fig: Replica transmission electron micrograph of acicular ferrite plates in martensite matrix, in a steel weld deposit which was partially transformed and quenched

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• Although the plates are nucleated heterogenously on non-metallic inclusions, the chance of observing an inclusion in any given plate is relatively small.• The probability is approximately the ratio of the inclusion volume to

that of a ferrite plate.• Acicular ferrite and bainite are in many respects similar in their

transformation mechanisms. • Their microstructures differ in detail because bainite sheaves grow as

a series of parallel platelets emanating from austenite grain surfaces, whereas acicular ferrite platelets nucleate intragranularly at point sites so that parallel formations of plates cannot develop.• There are manykinds of non-metallic inclusions which are effective in

stimulating intragranular nucleation, but some titanium compounds are found to be particularly potent. The exact mechanism of nucleation remains to be resolved.

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End of Displacive Transformation