Acicular ferrite: Difference between revisions
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'''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, |
'''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]].<ref>{{Citation | last = Bhadeshia | first = Harshad Kumar Dharamshi Hansraj | last2 = Honeycombe | first2 = Robert William Kerr | title = Steels: microstructure and properties | page = 155 | publisher = Butterworth-Heinemann | year = 2006 | edition = 3rd | url = http://books.google.com/books?id=8okWjoJhP5AC&pg=PA155 | isbn = 9780750680844}}.</ref><!-- It consists of a fine structure of interlocking ferrite plates (sometimes called 'dark etching'). --> |
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Acicular ferrite is formed in the interior of the original [[Austenite|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 (crystal)|cleavage]]. Acicular ferrite is also |
Acicular ferrite is formed in the interior of the original [[Austenite|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 (crystal)|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 pattern|Widmanstätten]] constituent, which is nucleated by an optimum intragranular dispersion of oxide/sulfide/silicate particles. |
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Composition control of weld metal is often performed to |
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. |
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==See also== |
==See also== |
Revision as of 12:25, 21 June 2010
This article needs additional citations for verification. (October 2009) |
Acicular ferrite is a microstructure of ferrite that is characterised by needle shaped crystallites 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.[1]
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.
See also
References
- ^ Bhadeshia, Harshad Kumar Dharamshi Hansraj; Honeycombe, Robert William Kerr (2006), Steels: microstructure and properties (3rd ed.), Butterworth-Heinemann, p. 155, ISBN 9780750680844.
External links