420 



NATURE 



\_March 6, 1890 



TH]^ RELATION BETWEEN THE ATOMIC 

 VOLUMES OF ELEMENTS PRESENT IN 

 IRON AND THEIR INFLUENCE ON ITS 

 MOLECULAR STRUCTURE. 



IN a lecture on the Hardening and Tempering of Steel, 

 published in November last (Nature, vol. xli. pp. 1 1, 

 32), an attempt vi^as 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 investi- 

 gations, Osmond argues that there are two kinds of iron, 

 one [hard] /3 iron, and the other [soft] a iron. The 

 molecular change from ^ 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 tempera- 

 tures. The second evolution of heat only occurs in car- 

 burized iron or steel, and marks the point at which carbon 

 itself changes from the dissolved or ' hardening-car- 

 bon,' 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 refer- 

 ence 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 a iron passes to /3 iron when sub- 

 mitted to any stress which produces permanent deforma- 

 tion 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 pro- 

 perty of the metal would be most affected by the added 

 matter. It appeared safe, however, to point to the pos- 

 sibility 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. 



I am glad that so eminent an authority and admirable 

 experimenter as M. Osmond has satisfied himself as to 

 the probable accuracy of this view. In two recent papers 

 communicated to the Acad(^mie des Sciences, the results 

 of his experiments are given, and the following is a 

 translation of the later of these {Cojtiptes rendus, vol. ex. 

 p. 346) :— 



" Within the last few years and quite recently {Comptes 

 rendus, Stances des 26 octobre et 6 decembre 1886, 4 

 avril 1887, et 3 fdvrier 1890), I have had the honour to 

 submit to the Academy facts relating to the allotropic 

 modifications of iron, and to the part played in such 

 changes by foreign bodies alloyed with the mass. Prof. 

 Roberts- Austen, by studying the effect produced on the 

 mechanical properties of gold by the addition of the same 

 weight (about 02 per cent.) of seventeen foreign metals, 

 has discovered a curious relation between the results ob- 



tained and the position occupied by the added metals in 

 the periodic classification (Phil. Trans. Roy. Soc, vol. 

 clxxix. p. 339, 1888). Prof Roberts-Austen has deduced 

 from this that an analogous relation should exist for iron, 

 but the irons and steels of commerce are such complex 

 products, and the same metal may assume such different 

 aspects, that the relation in question is not readily apparent 

 from a study of their mechanical properties. 



" In reviewing my former experiments with these new 

 ideas as guides, it appeared to me that the law of Roberts- 

 Austen was well based, and new experiments undertaken 

 to verify it have only confirmed my first view. 



" The foreign elements whose action on the critical points 

 of iron I have studied experimentally with more or less 

 completeness, are ranged as follows in two columns in the 

 order of their atomic volumes : — 



" The elements in column I., whose atomic volumes are 

 smaller than that of iron (7 '2), delay during cooling, 

 ccBteris paribus, the change of iS [hard] iron to a [soft] iron, 

 as well as that of 'hardening-carbon' {carbone de trempe) 

 into * carbide-carbon ' {carbone de reeuit). For these 

 two reasons they tend to increase, with equal rates of 

 cooling, the proportion of /3 iron that is present in the 

 cooled iron or steel, and consequently the hardness of the 

 metal. Indeed, their presence is equivalent to a more or 

 less energetic hardening.^ 



" On the other hand, the elements of column II., whose 

 atomic volumes are greater than that of iron, tend to 

 raise or at least to maintain near its normal position, 

 during cooling, the temperature at which the change of 

 /3 to a iron takes place ; further, they render the inverse 

 change during heating more or less incomplete, and 

 usually hasten the change of ' hardening-carbon ' to 

 ' carbide-carbon.' ^ 



" Thus they maintain the iron in the a [soft] state at high 

 temperatures, and must therefore have the same effect in 

 the cooled metal. In this way they would act on iron as 

 annealing does, rendering it soft and malleable, did not 

 th -ir individual properties, or those of their compounds, 

 often intervene and partially mask this natural conse- 

 quence of their presence. 



"The essential part, therefore, played by foreign elements 

 alloyed with iron, is either to hasten or delay the passage 

 of iron, during cooling, to an allotropic state, and to 

 render the change more or less incomplete in one sense 

 or the other, according to whether the atomic volume of 

 the added impurity is greater or less than that of iron. 

 In other words, foreign elements of low atomic volume 

 tend to make iron itself assume or retain the particular 

 molecular form that posses es the lowest atomic volume, 

 whilst elements with large atomic volume produce the 

 inverse effect. 



" It should be noted that carbon, whilst obeying the 

 general law, possesses on its own account the property of 

 undergoing, at a certain critical temperature, a change the 

 nature of which is still disputable, although its existence is 

 acknowledged. It is this property which gives carbon a 

 place by itself in the metallurgy of iron." 



M. Osmond has shown me the curves which represent 

 the results of his experiments, and these will doubtless 



' To the elements of column I. hydrogen may be added. As is well 

 known, this element renders electro-deposited iron hard and brittle ; perhaps 

 it would be better to say with Graham hydrogenimtt , for hydrogen gas does 

 not appear to have a marked influence on the critical temperature. 



^ I'ungsten alone presents certain anomalies. 



