What Does Acicular Ferrite Mean?
Acicular ferrite is a microstructure of ferrite in steel that is characterized by needle-shaped crystallites or grains when viewed in two dimensions. The three-dimensional grains have a thin, lenticular shape. This microstructure is advantageous over other microstructures because of its chaotic ordering, which increases toughness.
Acicular ferrite is formed in the interior of the original austenitic grains by direct nucleation on the inclusions, resulting in randomly-oriented short ferrite needles with a "basket weave" appearance. Acicular ferrite is also characterized by high-angle boundaries between the ferrite grains. This further reduces the chance of cleavage, because these boundaries impede crack propagation.
Corrosionpedia Explains Acicular Ferrite
In the late 1970s, the importance of an acicular ferrite microstructure in optimizing strength and toughness was identified. Since then, extensive research has been done on acicular ferrite formation and its relation to oxide inclusions, weld metal hardenability and cooling conditions.
Acicular ferrite has a microstructure comprised of fine interwoven ferrite laths or plates, and was first recognized in high-strength, low-alloy (HSLA) steel weld metals. This fine interlocking structure was found to be a desirable microstructure in low-carbon steel weldments because it showed improved toughness over that of other transformation products, such as conventional bainite.
Acicular ferrite is a fine Widmanstätten constituent, which is nucleated by an optimum intragranular dispersion of:
- Silicate particles
The interlocking nature of acicular ferrite, together with its fine grain size, provides maximum resistance to crack propagation by cleavage.
Acicular ferrite has been obtained by both isothermal treatments and continuous coolings in a medium-carbon microalloyed forging steel. Lower cooling rates cause the development of other phases such as allotriomorphic ferrite or pearlite, but a high-volume fraction of acicular ferrite can be achieved using two-stage continuous cooling cycles. 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.