Manchester Memoirs, Vol. Iv. (191 1), No. 34. 1 1 



the best refining temperature is held to be that which 

 produces the best mechanical tests. 



In the experiments of Osmond and Cartaud, alluded 

 to in the beginning of this paper, the investigators were 

 able to prove that the three modifications of iron* 



* Adopting the nomenclature of Osmond (vide Traiisfoiiiiations du Fer 

 et du Carbolic dans Ics Fers, les Aciers et les F'ontes blanches ; Baudoin, Paris, 

 1888), as was done by Sir W. Roberts-Austen in the Fifth Report of the 

 Alloys Research Committee, 1899, to the modificaiions of iron that take 

 place at certain temperature ranges, the term "gamma" iron is applied to 

 iron above its highest critical point, the term "beta" iron to iron between 

 this point and its magnetic change point, and the term "alpha" iron to iron 

 lietween this point and the temperature of recalescence. The points observed 

 on heating, Osmond marked " c " {chauffage), and the points on cooling " r " 

 ( I'efroidisseincnt) . 



The following figures are recent estimations by Professor Arnold, of 

 Sheffield University (vide Lecture "On a Fourth Recalescence in Steel,'' 

 1910) :— 



Critical Points in Cooling Curves. 



A heating curve with '20 carbon steel showed the following ranges : — 

 718^ — 729'C. Ac^, transformation of pearlite inlo hardenite (cement into 

 hardening carbon). 

 735^C. End of "alpha" range of temperature. 

 738" — 753°C. Ac.i, magnetic change point. 



756°C. Beginning of "beta" range of temperature. 

 840°C. INIaximum of Ac;j. 



S5o°C. Beginning of "gamma" range of temperature. 

 The recalescence data of a pure saturated carbon steel (c.c. = '89%) 

 were determined by Arnold and McWilliam as follows (vide Journal oj 

 Iron and Steel Institute, 1905, II.) : — 



On heating Ac^, „, «, maximum at 720°C. Range 7io''C. to 730°C. 



On cooling Ar^, .^, 3, maximum at 675°C. Range 69o"C. to 66o°C. 



Acj, Ar^ have to do with the carbon change points, Ac.^, Ar., and Acj, 

 Ar. with the iron change points, but, as seen above, the carbon is the 

 dominating influence, and with increasing carbon contents all these change 

 points gradually merge into the one corresponding to the carbon change 

 point only. 



