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VII. A Thermomagnetic Study of the Eutectoid Transition Point of 



Carbon Steels. 



By S. W. J. SMITH, M.A., D.Sc., F.R.S., Assistant Professor of Physics, and 

 J. GUILD, A.R.C.S., D.I.C., Assistant Demonstrator of Physics, Imperial 

 College, South Kensington. 



Eeceived December 29, 1913, Bead February 12, 1914. Keviscd copy received February 15, 1915. 



1 . Introduction. 



THE manner in which the ferromagnetism of nearly pure iron varies with temperature 

 has been the subject of many investigations ; but the corresponding and even more 

 interesting study for steel has been much less complete. Knowledge of the con- 

 stituents of iron-carbon alloys, acquired in different ways in recent years, makes 

 it certain that, in the earlier papers upon the change of permeability witli 

 temperature, salient features of the thermomagnetic curves have escaped notice. 

 This has happened because of the discontinuous character of the observations upon 

 which the published curves have been based. An example of the way in which a 

 striking variation may remain undiscovered was given in a paper published by the 

 Physical Society in 1912.* 



The experiments described in the present paper were made upon a series of steels 

 containing percentages of carbon varying between 0'15 and 1'53, and our discussion 

 of them is purposely confined to phenomena observed in the neighbourhood of 700 C. 

 It is in this region that one of the most important events in the thermal history of 

 steel occurs. Here steel containing about 0'9 per cent, of carbon changes during 

 cooling from an apparently homogeneous material into a heterogeneous mixture of 

 two different substances. One is apparently pure iron and the other is the carbide 

 Fe 3 C. This mixture is the eutectoid. 



The same change takes place in steels containing other percentages of carbon ; but 

 it is preceded by the separation, at higher temperatures, of the carbide or of iron 

 according as the steel contains more or less than the eutectoid percentage of carbon. 

 In the former the hyper-eutectoid steels the eutectoid therefore co-exists below 

 700 C. with excess of carbide, whilst in the latter the hypo-eutectoid steels there 



* ' Proc. Phys. Soc.,' XXV., pp. 77-81. 

 VOL. CCXV. A 529. 2 A [Published April 30, 1915. 



