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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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