

194 DR. S. W. J. SMITH AND ME. J. GUILD: A THERMOMAGNETIC STUDY OF 



The loops significant of the pronounced effects of concentration gradients are now 

 absent. 



12. Possible Evidence of the Effects of Pressure. 



The effects of alternation begun just above the temperature at which the eutectoid 

 previously made its appearance are noteworthy. 



It will be observed that this alternation is sufficient to induce practically the whole 

 of the eutectoid change at a temperature higher than that at which, in its absence, 

 the transformation appeared to begin. 



It seems to us most natural to explain this phenomenon by supposing that the 

 transformation of the eutectoid is retarded by the existence of internal pressure. 



On this view the eutectoid would form at a higher temperature than that at 

 which it ordinarily appears were it not for the expansion which accompanies the 

 transformation and causes pressure to be exerted by the surrounding envelope of iron 

 immediately after it begins. This pressure cannot increase without limit. The 

 separation of the eutectoid in bulk begins when the increase in pressure, required 

 to prevent further transformation as the temperature falls, cannot be supplied. The 

 effective transition point will thus depend upon the elastic properties of the 

 enveloping iron. 



If, as is likely, the capacity to resist strain is reduced when the temperature is 

 raised, the transition should simultaneously be facilitated. Fig. 8 shows that what 

 happens is as if a very slight elevation of temperature at the critical stage induces 

 that amount of breakdown in the envelope which is necessary before the eutectoid 

 can appear in bulk. 



1 3. Evidence of the Existence of other Effects. 



It is apparent that effects of pressure of the kind just contemplated would decrease 

 along with the amount of free iron present beforehand, and would therefore become 

 less important as the percentage of carbon in the steel rose towards that contained 

 by the eutectoid. They might therefore explain the observed gradual rise in the 

 temperature of reappearance of the eutectoid as the percentage of carbon in the steel 

 rises towards 0'9 ; but they would not account for the whole of the lag. In particular, 

 they would not explain the fact that in the hyper-eutectoid steels the temperature 

 of reappearance seems to be practically constant and higher than in any of the 

 others. 



It is, in fact, obvious that surface effects, considered in conjunction with the 

 conditions of equilibrium, must play an important part in determining the temperature 

 at which the eutectoid appears. 



We proceed to consider important cases from this point of view. 



