374 



NATURE 



[December 3, 1914 



unfortunate Allies in the attempt to revive their astro- 

 nomical journal. It will be issued once a month 

 unless circumstances should enable it to appear more 

 frequently. Subscriptions and correspondence should 

 be sent to Felix de Roy, Hon. Sec, 29 Stamford 

 Street, London, S.E. 



THE HARDENING OF METALS. 



A VISITOR at the recent meeting of the Faraday 

 Society could scarcely have failed to be struck 

 by the fact that although the society was supposed 

 to be discussing the hardening of metals, in reality 

 the discussion centred almost entirely round the 

 various theories of the hardening of steels. The reason 

 for this was probably twofold. First, in spite of, or 

 perhaps on account of, the considerable amount of 

 research work that has been published on this problem 

 it still remains the most keenly debated topic in 

 metallurgical circles, and one on which widely different 

 opinions have been held. Secondly, from a practical 

 point of view it may be broadly stated that, in regard 

 to hardening, steels are the only alloys that really 

 matter. 



• Yet there can be no doubt that the council of the 

 Faraday Society made a wise choice in the title of the 

 topic for discussion. Even the simplest pure iron 

 carbon steel is a complex material. Its complexity 

 is due to the facts that (i) iron exists in at least two 

 well-defined allotropic forms ; (2) above 780° C. 

 it is non-magnetic ; below it, magnetic ; and (3) carbon 

 has a remarkable tendency to form compounds of a 

 high degree of molecular complexity. While consider- 

 able attention has been directed to the first two aspects 

 of the matter, the third has not yet received the atten- 

 tion that it deserves, and will require before a com- 

 plete solution of the problem can be reached. The 

 position, therefore, is that while steels may be the 

 most interesting alloys to investigate it does not 

 follow that they are most suitable. What is first of 

 all required is a fundamental investigation of the 

 theory of the hardening of metals from which the 

 foregoing disturbing causes are absent. When the 

 foundations of this have been securely laid the precise 

 effect of the above " variables " will no doubt be 

 elucidated by suitably chosen experiments. 



This aspect of the matter was clearly seen many 

 years ago by Dr. G. T. Beilby, who has investigated 

 the physical mechanism of the hardening of metals 

 such as gold, silver, and copper, which can be ob- 

 tained in a high state of purity, and from which 

 magnetic and chemical complications are absent. 

 Until quite recently it would have been possible to 

 say that allotropic complications were also absent, 

 and so far as present knowledge goes they were 

 absent in the case of gold and silver. Recent investi- 

 gations by Prof. Ernst Cohen (Utrecht), however, 

 have led him to conclude that the metals cadmium, 

 lead, bismuth, copper, zinc, and antimony ordinarily 

 occur as metastable systems consisting of two or 

 more allotropic forms, so that this consideration must 

 be kept in view. Prof. Cohen was unfortunately 

 unable to be present at the meeting, but an important 

 summary of his results was available and is worthy 

 of very close study. Ordinarily the transformations 

 from one allotropic form to another are subject to 

 strongly marked retardations, and it is only by em- 

 ploying certain devices such as the addition of an 

 electrolyte and the use of the metal in a finely divided 

 state that the transformation velocity can be in- 

 creased to such an extent that the change from the 

 metastable to the stable form occurs within a short 

 time. These transformations are frequently accom- 

 panied by marked volume changes leading to com- 

 plete disintegration of the metal, 



NO. 2353, VOL. 94] 



These recent developments do not, however, alter 

 the fact that Dr. Beilby chose for his investigations 

 metals of the most suitable kind from the point of 

 view of arriving at a physical conception of the 

 mechanism of hardening by mechanical deformation, 

 and they enabled him to prove the existence of a 

 thermally stable crystalline and a mechanically stable 

 amorphous vitreous phase in each metal investigated. 

 How far-reaching and fundamental these conclusions 

 have proved to be is evidenced by the fact that all the 

 papers dealing with the hardening of steels presented 

 at the meeting incorporated and made more or less 

 use of them. According to Mr. Humfrey, "The hard 

 structure which can be produced in carbon steels by 

 quenching and in certain alloy steels by normal cool- 

 ing is due to the presence of a hard, amorphous solu- 

 tion of a iron and iron carbide." Mr. McCance's 

 view is that the hardening is due to " interstrained " 

 a iron, and the suppression of the carbide change. 

 The theory of Profs. Edwards and Carpenter is that 

 the hardness is caused by the complete suppression of 

 the carbide change, together with the presence of 

 amorphous layers existing at the surfaces of slip 

 upon which copious twinning occurs when carbon 

 steels are quenched. On this view the final cause of 

 hardening by quenching is exactly the same as that 

 of hardening by cold working, viz., the internal 

 deformation of the crystals. 



When these theories are carefully examined it is 

 significant to note how much they have in common. 

 All of them agree that the carbide (Ar i) change is 

 suppressed, a fact of very fundamental importance. 

 The differences centre round the precise condition of 

 the iron, and arise chiefly from differences in conclu- 

 sions drawn from the magnetic condition of the alloy. 

 They will not be adjusted until it has been settled 

 whether iron can be magnetic in other than the 

 a condition. Mr. McCance apparently denies that 

 the iron in hardened steels is amorphous. It is not 

 yet clear precisely what he means by the term "inter- 

 strain," and how it differs, if at all, from the term 

 " internal tension," suggested many years ago by 

 Metcalf and Langley. 



Most significant of all Is the fact that none of the 

 above theories make use of /3 iron. This means that 

 the controversy has essentially changed. It Is no 

 longer between the allotropists who laid chief stress 

 on the postulated existence of a hard, stable, crystal- 

 line ^ iron, which was held to be primarily respons- 

 ible for the hardness of quenched steel, and the 

 carbonists, who denied this altogether, and ascribed 

 the hardness to the action of carbon without, however, 

 being able to explain It. The complete solution of 

 the problem now appears to be bound up with the 

 acquisition of a more intimate knowledge of the 

 molecular combinations between iron and carbon in 

 hardened steels and their variations in the hardening 

 range of temperature. Stimulus to Investigations of 

 this character will no doubt be given by the sub- 

 stantial prize offered by Sir Robert Hadfield, the pre- 

 sident of the Faraday Society, for the best research 

 dealing with the combinations between iron and 

 carbon. He has rendered an Important service in 

 directing attention to this difficult but neglected side 

 of the subject. H. C. H. Carpenter. 



AIR, CLIMATE, AND TUBERCULOSIS.^ 



IN October, 1891, Thomas George Hodgklns, of 

 Setauket, New York, made a donation to the 

 Smithsonian Institution the income from a part of 

 which was to be devoted to "the increase and diffusion 

 of more exact knowledge in regard to the nature and 



1 Smithsonian Miscellaneous Collections. Vol. Ixiii., No. i. Hodgkins 

 Fund. "Atmospheric Air in Relation to Tuberculosis." By Dr. G. 

 Hinsdale. Pp. x + 136. (Washington : Smithsonian Institution, 1914.) 



