Acicular ferrite
Encyclopedia
Acicular ferrite is a microstructure
of ferrite
that is characterised by needle shaped crystallite
s or grains when viewed in two dimensions. The grains, actually three dimensional in shape, 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 from the inclusions, resulting in randomly oriented short ferrite needles with a 'basket weave' appearance. This interlocking nature, together with its fine grain size (0.5 to 5 um with aspect ratio from 3:1 to 10:1), provides maximum resistance to crack propagation by cleavage
. Acicular ferrite is also characterised by high angle boundaries between the ferrite grains. This further reduces the chance of cleavage, because these boundaries impede crack propagation. It is reported that nucleation of various ferrite morphologies is aided by nonmetallic inclusion; in particular oxygen-rich inclusions of a certain type and size are associated with the intragranular formation of acicular ferrite. Acicular ferrite is a fine Widmanstätten
constituent, which is nucleated by an optimum intragranular dispersion of oxide/sulfide/silicate particles.
Composition control of weld metal is often performed to maximise the volume fraction of acicular ferrite due to the toughness it imparts. Higher alloy contents generally delay transformation, during continuous cooling transformation will then take place at lower temperatures and lead to higher hardness.
Microstructure
Microstructure is defined as the structure of a prepared surface or thin foil of material as revealed by a microscope above 25× magnification...
of ferrite
Ferrite
Ferrite may refer to:* Ferrite , iron or iron alloys with a body centred cubic crystal structure.* Ferrite , ferrimagnetic ceramic materials used in magnetic applications....
that is characterised by needle shaped crystallite
Crystallite
Crystallites are small, often microscopic crystals that, held together through highly defective boundaries, constitute a polycrystalline solid. Metallurgists often refer to crystallites as grains.- Details :...
s or grains when viewed in two dimensions. The grains, actually three dimensional in shape, have a thin lenticular shape. This microstructure is advantageous over other microstructures because of its chaotic ordering, which increases toughness
Toughness
In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing; Material toughness is defined as the amount of energy per volume that a material can absorb before rupturing...
.
Acicular ferrite is formed in the interior of the original austenitic
Austenite
Austenite, also known as gamma phase iron, is a metallic non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of ; other alloys of steel have different eutectoid temperatures...
grains by direct nucleation from the inclusions, resulting in randomly oriented short ferrite needles with a 'basket weave' appearance. This interlocking nature, together with its fine grain size (0.5 to 5 um with aspect ratio from 3:1 to 10:1), provides maximum resistance to crack propagation by cleavage
Cleavage (crystal)
Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the...
. Acicular ferrite is also characterised by high angle boundaries between the ferrite grains. This further reduces the chance of cleavage, because these boundaries impede crack propagation. It is reported that nucleation of various ferrite morphologies is aided by nonmetallic inclusion; in particular oxygen-rich inclusions of a certain type and size are associated with the intragranular formation of acicular ferrite. Acicular ferrite is a fine Widmanstätten
Widmanstätten pattern
Widmanstätten patterns, also called Thomson structures, are unique figures of long nickel-iron crystals, found in the octahedrite iron meteorites and some pallasites. They consist of a fine interleaving of kamacite and taenite bands or ribbons called lamellæ...
constituent, which is nucleated by an optimum intragranular dispersion of oxide/sulfide/silicate particles.
Composition control of weld metal is often performed to maximise the volume fraction of acicular ferrite due to the toughness it imparts. Higher alloy contents generally delay transformation, during continuous cooling transformation will then take place at lower temperatures and lead to higher hardness.
External links
- http://www.msm.cam.ac.uk/phase-trans/2007/acicular.html