Physical Properties of Pure Compact Iron

The properties of iron are affected to such a remarkable and unique extent by the presence of small quantities of alloying elements, chief amongst which is carbon, that these phenomena are an important study in themselves. It is not intended in this section, therefore, to deal with the physical properties of any commercial iron other than the chemically pure and compact metal. For a discussion of the physical and metallurgical properties of various types of commercial iron and its alloys. Pure iron is a white metal which can be readily machined in a lathe, and even cut with a knife. It crystallises according to the cubic system, but crystals are rare, the metal being usually massive. Dendritic crystals may be obtained artificially with branches parallel to the cubic axes. Shock apparently assists or induces crystallisation in iron.

Pure iron, prepared by reducing ferrous chloride with hydrogen at temperatures up to about 800° C., usually separates in small hexahedra, although it sometimes yields rhombic dodecahedra and tetrakishexahedra. The mechanical properties of the individual crystals of iron vary with the crystallographic orientation. For example, the metal is brittle in the direction of the planes of cleavage, but exhibits considerable plasticity in other directions. In compact iron the crystals are separated from each other by an amorphous cement, which acts as a binding agent. The metal is ductile and malleable. It possesses considerable tenacity, a wire 2 mm. in diameter being capable of supporting 250 kgms. This value is greatly influenced by the presence of alloying elements, particularly carbon.

At a low temperature, such as that of liquid air, pure iron is very brittle; indeed most alloys of iron, with the exception of those containing nickel, lose in ductility as the temperature falls.

The density of the metal varies somewhat according to its mechanical history, the usual values obtained for the pure metal ranging from 7.85 to 7.88.

As a rule the density of a metal decreases with cold working, and iron appears to be no exception. Iron filings are less dense than the compact metal from which they are obtained, as indicated by the following data: -

	Density
Pure compact iron	7.8558
Filings from ditto	7.8172
Decrease in density	0.0886

The effect of cold-drawing of wire upon the density of the metal is clearly shown by the following data, obtained with iron piano-wire. The density has been calculated for a vacuum at 4° C.

	Mean Density.
Piano-wire annealed	7.7970
Piano-wire cold drawn	7.7772
Decrease in density	0.0198

Upon annealing the density tends to return to its original value.

The density of solid iron near its melting-point is 6.95, whilst that of the liquid metal a few degrees higher in temperature is 6.88.

Addition of carbon effects a reduction in the density of the metal. The melting-point of iron has frequently been determined; the most reliable results are between 1505 and 1533.

According to Knocke, the volatilisation of iron in vacuo is sufficiently great to be detected at 755° C, The metal boils at 2450° C. ±50° C. at 36 mm. pressure.

Moissan succeeded in volatilising iron in his electric furnace with a current of 350 amperes at 70 volts. In a few minutes a sublimate or distillate of iron was obtained on a water-cooled tube as a grey powder mixed with some brilliant and malleable scales, and possessed of the same chemical properties as the finely divided metal. The distillation of iron in this manner is difficult on account of the violent frothing caused by the boiling metal evolving occluded gases.

The specific heat of iron rises with the temperature.

The following values for the specific heat of a sample of pure iron (Fe 99.87 per cent.) for temperatures ranging from 0° to 100° C. are given by Griffiths, the extreme temperature ranges being 1.4° C.: -

Temperature.°C.	Specific Heat.
0	0.1045
10	0.1059
20.5	0.1078
50.3	0.1105
97.5	0.1137

For intermediate temperatures, the specific heat (S$) may be calculated from the equation

S_t = 0.1045 (1 + 0.001520t — 0.00000617t²).

Other recent results are: -

Temperature Interval. °C.	Specific Heat.
17 to 100	0.10983
17 to 100	0.1098
-188 to + 20	0.0859
-185 to + 20	0.095

There is marked discontinuity above 900° C.

The specific heat of iron appears to be very slightly increased by cold working.

The coefficient of linear expansion of iron with rise of temperature has been determined for a pure iron containing

Carbon	0.057 %
Manganese	0.13 %
Silicon	0.05 %

The results are as follow: -

Temperature Interval.°C.	Total Expansion per Unit Length.	Coefficient of Expansion per 1° C.
0 to 100	0.0011	0.000011
0 to 200	0.0023	0.0000115
0 to 300	0.0036	0.000012
0 to 400	0.0050	0.0000125
0 to 500	0.0065	0.000013
0 to 600	0.0081	0.0000135
0 to 700	0.00975	0.000014
0 to 800	0.01125	0.000014

When heated to 950° C. in an inert atmosphere, iron is disintegrated, emitting particles at right angles to its surface and assuming an etched appearance.

The refractive index of iron, for sodium light, is 1.85.

The most intense lines in the spectrum of iron are as follow: -

Arc: 3020.75, 3021.19, 3047.72, 3440.77, 3441.13, 3466.02, 3490.73, 3565.54, 3570.24, 3570.29, 3581.34, 3581.38, 3609.01, 3618.91, 3631.60, 3648.00, 3705.74, 3708.06, 3709.39, 3720.09, 3722.73, 3735.02, 3737.30, 3745.70, 3748.40, 3749.62, 3763.92, 3763.99, 3816.00, 3820.61, 3824.60, 3826.07, 3828.00, 3834.40, 3860.03, 3886.45, 4045 .99, 4063.74, 4063.77, 4071.92, 4260.68, 4271.95, 4308.09, 4325.97, 4383.71, 4404.95, 4415.31, 5167.67, 5233.15, 5269.70, 5324.38, 5365.00, 5367.61, 5370.13, 5383.58, 5404.34, 5411.15, 5415.40, 5424.30, 5429.94, 5445.28, 5447.15, 5455.81, 5573.09, 5586.98, 5615.89, 6400.25, 6495.25.

Spark: 2599.50, 2739.63, 2749.41, 2755.80, 4045.99, 4308.10, 4325.97, 4383.73, 4404.95.