PPT shows different concepts related to acicular ferrite in Q&A format.
Includes:
1)What is acicular ferrite?
2)Desirability of this structure
3)how its microstructure looks like
4)Transformation mechanism
5)Importance in welding
6)Various influencing condition to structure
3. What is acicular ferrite?
Acicular means “shaped or pointed like needle”.
However acicular ferrite are shaped lenticular plate.
Definition: Acicular ferrite is a microstructure of ferrite in steel that is characterized by needle-
shaped crystallites or grains when viewed in two dimensions. The grains, actually three-
dimensional in shape, have a thin lenticular shape.
They are chaotic in order
Approximate aspect ratio: 10 μm long and 1 μm wide
4. What is acicular ferrite?
•Acicular ferrite transformation exhibits incomplete reaction phenomenon, an
important characteristic of bainite.
•Acicular ferrite is formed in the same temperature range as bainite by the same
transformation mechanism.
•The ferrite plates in the bainitic microstructure are nucleated at austenite grain
boundaries and/or at active interface allotriomorphic ferrite/Austanite and form
packets consisting of parallel plates having similar crystallographic orientations,On
the contrary, Acicular Ferrite particles nucleate intragranularly.
5. Bainite forms when austenite grain size is small while acicular ferrite is formed when grain size
of austenite is coarse.
REASON
When the austenite grain size is large the number density of inclusions becomes large relative
to boundary nucleation sites promoting the acicular ferrite at the expense of bainite.
6. Favorable condition for Acicular ferrite
transformation
•Acicular ferrite grows supersaturated with carbon, but the excess carbon is shortly
afterwards rejected into the remaining austenite.
•Bainitic microstructure can be replaced by one containing acicular ferrite by
increasing oxygen.
•Acicular ferrite is a fine Widmanstätten constituent, which is nucleated by an optimum
intragranular dispersion of:
◦ Oxide
◦ Sulfide
◦ Silicate particles
In 1981 Harrison and Farrar removed the inclusion by vacuum remelting a weld ; When cooled, the steel
transformed into bainite instead of the original acicular ferrite
7. Experiment of Thewlis
Thewlis in 1997 have argued that in some welds the so called acicular ferrite may predominantly
be intragrannular nucleated Widmansten ferrite rather than Bainite.
They reached conclusion by noting that the estimated Bainite start temperature was lower than
that at which coarse plates nucleated on very large inclusion(3-9 μm diameter).
Although there is uncertainty in in their calculated Bs value, the conclusion that mixed
microstructure of intragranulary nucleated Widmanstätten ferrite and intragranulary nucleated
bainite was obtained seems justified.
Hence Acicular ferrite is sometimes considered to be intragranularly nucleated Widmanstätten
ferrite on the basis of the observation of ‘steps’ at the transformation interface.
8. • The weight of evidence is that the acicular ferrite recognized in
most weld microstructure is intragranularly nucleated Bainite.
And the term acicular ferrite should be reserved for this fine
microstructure.
• If coarse Widmanstätten ferrite forms on inclusion then it can
be called intragranularly nucleated Widmanstätten ferrite.
• The name given to phase are important because they imply a
mechanism of transformation which can be used in theoretical
models
10. Is acicular ferrite structure is desired?
Short answer : Yes
The interlocking nature of acicular ferrite, together with its fine grain size, provides maximum
resistance to crack propagation by cleavage.
Explanation
The boundaries act as strong obstacles to propagation of cleavage cracks forcing these cracks to
change the microscopic propagation planes in order to accommodate the new local
crystallography. On the contrary, the low-angle boundaries are week obstacles which are not
effective hindrances in crack growth. For this reason, their influence on the toughness is
negligible.
12. Acicular Ferrite Transformation
Mechanism
•Acicular ferrite is formed in the same temperature range as bainite by the same
transformation mechanism of bainite.
•The Acicular Ferrite transformation starts through a nucleation of the primary
plates at non-metallic inclusions and progresses during the sympathetic
nucleation of secondary ferrite grains nucleated at the Austenite /primary
Acicular Ferrite plate interface. This additional mechanism of Acicular Ferrite
formation represents a second stage in Austenite decomposition.
•A two-stage continuous cooling cycle is the best heat treatment to produce
acicular ferrite with a low-volume fraction of other phases such as
allotriomorphic ferrite, pearlite, and/or martensite.
15. Silicon segregation in an ADI microstructure
Etchant : Klemm's I reagent
Magnification : 200X
Austempered ductile iron C-3.6%
Si-2.5%
P-0.06%
Ni-1.5%
Cu-0.7%
CB – Cell Boundries
H – Ferritic halo around graphite nodule
Acicular Ferrite - Orange
16. Silicon segregation in an ADI microstructure
Etchant : Beraha-Martensite reagent
Magnification : 1000X
Austempered ductile iron C-3.6%
Si-2.5%
P-0.056%
Mg-0.052%
Cu-0.7%
AF – Acicular Ferrite
FM – Fine Martensite
Acicular Ferrite and bainite - Blue
18. Acicular ferrite in welding
On welds where the heat due to welds produces a gradient of austenite grain size in the heat
affected zone , with the largest grains adjacent to the fusion surface.
When steels containing appropriate inclusions are welded, the ratio of acicular ferrite to bainite
is highest in heat affected zone nearest the fusion boundary where the austenite grain size is at
maximum.
19. Acicular ferrite in welding
Changes in the microstructure of the heat affected zone of welds, as a function of heat input
during welding.
Steel containing
Titanium Oxide(TiO)
Ordinary Steel
21. Influence of Cooling rate
•It is commonly accepted that the transformation of acicular ferrite is situated between bainite
and coarse-grained ferrite. However, the optimum cooling rate to achieve maximum acicular
ferrite depends strongly on the production route and the composition of the steel
•Following table gives overview of recent results in the literature regarding CR for acicular ferrite
during heat treatments with or without deformation.
Cooling rate between 800-500 °C described in the literature as suitable for the formation of acicular ferrite during heat
treatments by HT-LSCM
22. Cooling rate between 800-500 °C described in the literature as suitable for the formation of acicular ferrite during heat
treatments without deformation
23. Cooling rate between 800-500 °C described in the literature as suitable for the formation of acicular ferrite during thermo-
mechanical treatments
25. Influence of alloying elements on the formation of acicular ferrite
Non-metallic inclusions (NMI)
Acicular Ferrite (AF)
26. Lower Acicular Ferrite
•Like conventional lower bainite in wrought steels, there ought to exist a lower acicular ferrite
microstructure, in which the intragranulary nucleated plates of αa contain plates of cementite
inclined at an angle of about 60 to the habit plane.
•The transition of upper bainite to lower bainite occurs when the partitioning of carbon from
supersaturated bainitic ferrite into austenite becomes slow compared with the precipitation
of carbides in the ferrite.
•Consequently, if the carbon concentration of a steel weld is increased sufficiently then for
similar welding conditions, the microstructure should undergo a transition from acicular
ferrite to lower acicular ferrite.
•Lower acicular ferrite is only found when the weld carbon concentration is large enough to
permit the precipitation of carbides from the acicular ferrite, before much of the carbon can
partition into the residual austenite.
•Lower acicular ferrite has been detected in a laser welded high-carbon steel.
27. Glance at Acicular ferrite and bainite
• There is no substitutional solute partitioning during the growth of either bainite or acicular
ferrite (Strangewood,1987 ; Chandrasekharaiah 1994)
• Both reaction stop when the austenite carbon concentration reaches a value where it becomes
thermodynamically impossible to achieve diffusion less growth (Yang and Bhadeshia 1987)
• Acicular ferrite only forms below bainite start temperature.
• There is large and predictable hysteresis in the temperature at which austenite formation
begins from a mixed microstructure of acicular ferrite and austenite , or bainite and austenite (
Yang and Bhadeshia 1987)
28. • The removal of inclusions from a weld deposit, without changing any other feature, causes a change
in the microstructure from acicular ferrite to bainite (Harrison and Farrar, 1981).
• AN increase in number density of austenite grain surface nucleation sites causes transition from
acicular ferrite to bainite (Yang and Bhadeshia, 1987)
• The elimination of austenite grain surfaces by decoration with inert allotriomorphic ferrite leads to a
transition from bainite to an acicular ferrite microstructure (Babu and Bhadeshia 1990)
