1919] on The Hardening of Steel 507 



sider anything like the whole of the evidence which may be brought 

 to bear on the problem of the cause of the hardeniug of steel by 

 quenching. In particular, that which is furnished by the behaviour 

 of alloy steels, specially those containing nickel or manganese, or 

 lioth, has enabled the conditions intermediate between austenite on 

 the one hand, and pearlite on the other, to be studied in a more 

 systematic and comprehensive way than is possible with pure carbon 

 steels. ]Moreover, the discovery and widespread use of the so-called 

 high-speed cutting tools, all of them alloy steels, has widened the 

 scope of the problem to an extent which could not have been reaUzed 

 as recently as twenty years ago. It will be oljserved that the three 

 modern theories, of which some account has been given, present con- 

 siderable resemblance to one another, in spite of certain differences 

 which are obvious on the surface. The theories of Humfrey and 

 Edwards and myself attribute hardening to the existence of a hard 

 amorphous constituent in the steel. McCance does not go so far as 

 this, but regards interstrain as the cause of hardness, a condition 

 which I think it is fair to describe as occupying an intermediate 

 position between crystalline and amorphous materials. Perhaps at no 

 very distant date McCance will develop a theory of interstrain action. 

 The acute differences which divided sharply the allotropists and 

 the carbonists find no counterpart in the schools of thought of to-day. 

 Indeed it is interesting to reflect for one moment on the position of 

 the allotropic theory of Osmond, in the light of the most modern 

 views on the subject. According to these, M. Osmond was correct 

 in attributino- hardness to somethins^ — stated in its most o-eneral 

 terms — intermediate between crystalline y and crystalline a iron. 

 The view that this intermediate something is a crystalline /5 iron 

 such as he supoosed has not been completely abandoned, but is not 

 held by many workers to-day. This conception has given place to the 

 interstrained iron of McCance and the amorphous iron and iron- 

 carbide complex of Humfrey, Edwards, and myself. The important 

 influence exerted by carbon, insisted upon by the carbonists, is fully 

 recognized in the modern theories described. Speaking with the 

 ampler knowledge of to-day one may summarize the position by saying 

 that the older theories, tested and to some extent corrected by this, 

 have contributed to a broader view of the matter, in whicJi their 

 apparent differences have been put into their proper proportion, and 

 the elements of truth they contained incorporated into other theories 

 which,, for the time being, are more in accord with such knowledge. 

 That these in their turn are likely to be modified and amphfied by 

 the advance of knowledge I should be the first to admit. 



[H. C. H. C] 



