The Relation Between the Atomic Volumes of Elements Present in Iron and their Influence on its Molecular Structure

Abstract

IN a lecture on the Hardening and Tempering of Steel, published in November last (NATURE, vol. xli. pp. 11, 32), an attempt was made to set forth the prominent facts developed in recent researches, more especially those of M. Osmond, which tend to prove that iron, like many other elements, can pass from the normal state to an allotropic one. It was shown that as a mass of iron or steel cools down, there are at least two distinct evolutions of heat, one occurring at a variable temperature not higher than 855° C., the other at a more constant temperature, near 650° C. From a long series of most patient investigations, Osmond argues that there are two kinds of iron, one [hard] β iron, and the other [soft] α iron. The molecular change from β to a iron is indicated by the first evolution of heat in the cooling mass of iron or steel, and at this point the cooling mass of iron regains the magnetic properties which it loses at higher temperatures. The second evolution of heat only occurs in carburized iron or steel, and marks the point at which carbon itself changes from the dissolved or ‘hardening-carbon,’ to the state of combined or ‘carbide-carbon.’ In highly carburized steel, the two points at which heat is evolved coincide, and experimental evidence has been given (loc. cit. p. 34) as to the abnormal molecular weakness which is exhibited when a very hot bar of such steel cools down to about 660° C. In a recent communication to NATURE (February 20, p. 369), Prof. Carl Barus, of Washington, has pointed out, with reference to this molecular weakness, “that when iron passes through the temperature of recalescence its molecular condition is almost chaotic ”; whilst with regard to Osmond's view that α iron passes to β iron when submitted to any stress which produces permanent deformation of the mass, Prof. Barus says that “there is reason to be urged even in favour of the extreme view” that such molecular change may be produced in most metals. In the lecture at Newcastle, I expressed the belief (NATURE, loc. cit.) that it would be shown that the influence of small quantities of other elements on masses of iron would be found not to be at variance with the periodic law. I had already given experimental evidence to show that the action of small quantities of impurity on the tenacity of gold was closely in accordance with that law, but in the case of iron it was difficult to say what property of the metal would be most affected by the added matter. It appeared safe, however, to point to the possibility that the direct connection with the periodic law would “be traced by the effect of a given element in retarding or promoting the passage of ordinary iron to the allotropic state,” a point of much importance, as the mechanical properties of the metal must depend on the atomic arrangement in the molecules.