Chemical elements
  Iron
    History of Iron
    Mineralogy
    Isotopes
    Energy
    Production
    Application
    Physical Properties
      Allotropy
      Occlusion of Gases
      Absorption of Nascent Hydrogen
      Permeability to Gases
      Passivity of Iron
      Iron Powder
      Iron sponge
      Iron amalgam
      Colloidal Iron
      Pyrophoric Iron
      Catalyst
      Iron Ions
      Atomic Weight
    Chemical Properties
    Corrosion
    Iron Salts
    PDB 101m-1aeb
    PDB 1aed-1awd
    PDB 1awp-1beq
    PDB 1bes-1c53
    PDB 1c6o-1ci6
    PDB 1cie-1cry
    PDB 1csu-1dfx
    PDB 1dgb-1dry
    PDB 1ds1-1e08
    PDB 1e0z-1ehj
    PDB 1ehk-1f5o
    PDB 1f5p-1fnp
    PDB 1fnq-1fzi
    PDB 1g08-1gnl
    PDB 1gnt-1h43
    PDB 1h44-1hdb
    PDB 1hds-1i5u
    PDB 1i6d-1iwh
    PDB 1iwi-1jgx
    PDB 1jgy-1k2o
    PDB 1k2r-1kw6
    PDB 1kw8-1lj0
    PDB 1lj1-1m2m
    PDB 1m34-1mko
    PDB 1mkq-1mun
    PDB 1muy-1n9x
    PDB 1naz-1nx4
    PDB 1nx7-1ofe
    PDB 1off-1p3t
    PDB 1p3u-1pmb
    PDB 1po3-1qmq
    PDB 1qn0-1ra0
    PDB 1ra5-1rxg
    PDB 1ry5-1smi
    PDB 1smj-1t71
    PDB 1t85-1u8v
    PDB 1u9m-1uyu
    PDB 1uzr-1vxf
    PDB 1vxg-1wri
    PDB 1wtf-1xlq
    PDB 1xm8-1y4r
    PDB 1y4t-1ygd
    PDB 1yge-1z01
    PDB 1z02-2a9e
    PDB 2aa1-2azq
    PDB 2b0z-2boz
    PDB 2bpb-2ca3
    PDB 2ca4-2cz7
    PDB 2czs-2dyr
    PDB 2dys-2ewk
    PDB 2ewu-2fwl
    PDB 2fwt-2gl3
    PDB 2gln-2hhb
    PDB 2hhd-2ibn
    PDB 2ibz-2jb8
    PDB 2jbl-2mgh
    PDB 2mgi-2o01
    PDB 2o08-2ozy
    PDB 2p0b-2q0i
    PDB 2q0j-2r1h
    PDB 2r1k-2spm
    PDB 2spn-2vbd
    PDB 2vbp-2vzb
    PDB 2vzm-2wiv
    PDB 2wiy-2xj5
    PDB 2xj6-2ylj
    PDB 2yrs-2zon
    PDB 2zoo-3a17
    PDB 3a18-3aes
    PDB 3aet-3bnd
    PDB 3bne-3cir
    PDB 3ciu-3dax
    PDB 3dbg-3e1p
    PDB 3e1q-3eh4
    PDB 3eh5-3fll
    PDB 3fm1-3gas
    PDB 3gb4-3h57
    PDB 3h58-3hrw
    PDB 3hsn-3ir6
    PDB 3ir7-3k9y
    PDB 3k9z-3l4p
    PDB 3l61-3lxi
    PDB 3lyq-3mm8
    PDB 3mm9-3n62
    PDB 3n63-3nlo
    PDB 3nlp-3o0f
    PDB 3o0r-3p6o
    PDB 3p6p-3prq
    PDB 3prr-3sel
    PDB 3sik-3una
    PDB 3unc-4blc
    PDB 4cat-4erg
    PDB 4erm-4nse
    PDB 4pah-8cat
    PDB 8cpp-9nse

Allotropy of Iron






When a bar of pure iron is allowed to cool from its melting-point to 0° C., its time-temperature cooling curve exhibits three breaks, or arrests, designated by the symbols Ar4, Ar3, and Ar2, respectively, which is also known as Allotropy of Iron. These arrests are due to evolution of a small amount of heat consequent upon some internal alteration in the metal, whereby the rate of cooling is retarded. Indeed, the evolution of heat at the Ar3 point is sufficient to raise the temperature of the iron by a very appreciable amount. The phenomenon is termed recalescence. Similarly, on reheating the metal, three arrests, due to heat absorption, are observed at temperatures denoted by Ac2, Ac3, and Ac4. The points occur at approximately the following temperatures: -

Ac2770° CAc3910° CAc41404° C.
Ar2760° CAr3890° CAr41401° C.


The point now known as Ar3 was the first to be discovered, it being observed that, during the cooling of a piece of iron from a high temperature, on reaching " a very dull heat, a sudden accession of temperature occurred, so that it glowed once more with a bright heat."

It will be observed that the Ac points are slightly higher than the corresponding Ar points, but are indicative of the same phenomena. The divergence is explained by assuming that a certain amount of inertia or resistance to change exists, known as lag, which tends to lower the temperature of arrest during cooling, and probably to raise it slightly during heating.

When determinations are made of the variation of other physical properties of iron, with rise or fall of temperature, discontinuities or abrupt changes are usually observed at temperatures approximating to the various A points. The first of these to be observed was the sudden elongation manifested by iron wire, at a temperature not stated, when allowed to cool from red heat to about 15° C. This peculiarity, announced by Gore in 1869, was confirmed by Barrett, who also showed that during the heating of the wire an interruption of the expansion of the metal occurred at approximately the same temperature. The elongation of the metal on cooling was found to occur simultaneously with the recalescent point now known as the Ar3 point.

The electric resistance of pure iron increases from 0° C. to a maximum at 757° C., corresponding to the A2 point, and then falls to a minimum at 894° C. - the A3 point. On cooling the reverse changes occur at practically the same temperatures. The presence of hydrogen under atmospheric pressure does not materially affect the resistance of the metal up to 920° C.

The thermo-electric potential differences between platinum and iron at various temperatures reach a maximum at 850° C. - a temperature closely approximating to the A3 point.

The magnetic susceptibility of iron rapidly increases at 1365° C., when the metal is raised to this temperature from a lower one, and the reverse change takes place at 1310° C. on cooling from a higher temperature. These points evidently correspond to the Ac4 and Ar4 points respectively.

The foregoing results are usually interpreted as indicating that iron is capable of existing in four allotropic modifications, designated respectively as α, β, γ, and δ ferrite, the points A2, A3, and A4 representing their transition temperatures (that is, the temperatures at which the arrests, on cooling or reheating, would be observed were it not for the lagging), also known as Allotropy of Iron. Thus a ferrite is the ordinary variety of pure iron, stable below A2, at which point both a and β ferrite are in equilibrium. Between A2 and A3 the β ferrite constitutes the stable phase; between A3 and A4 γ ferrite is stable, whilst above A4 δ ferrite exists. This may be represented diagrammatically as follows: -



Many metallurgists, however, incline to the view that ferrite is really a solid solution of у in a ferrite; in other words, the assumption is made that when iron cools below A3 the conversion of у ferrite into α ferrite is not quite complete, a few γ molecules remaining dissolved in the a until the A2 point is reached, at which point the transformation is completed, α ferrite alone remaining.

Whether this view be accepted or not, it is convenient to retain the name β ferrite to indicate that particular phase of the metal between the A2 and A3 points.

a, β, and у ferrite crystallise in the cubic system, but present well-marked specific characters indicative of varying internal structures. It has been suggested that δ ferrite is monatomic iron, Fe; γ ferrite is diatomic, Fe2; and β ferrite is triatomic, Fe3. At present, however, very little is known of the structure of the iron molecule.

Carbon readily dissolves in у ferrite, is soluble to a slight extent in ferrite, but is practically insoluble in a ferrite.


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