Photomicroyraphy of Iron and Steel. By W. H. Merrett. 



5 



for the thermometer used in the Guthrie experiments, on account of 

 the high temperatures at which the constituents separate. 



In fig. 2 it will be seen that the line GOS marked "iron" 

 corresponds to the " ice " line in fig. 1, and the line S E marked " iron 

 carbide" corresponds to the "salt" line in fig. 1, and the eutectic 

 line P S P' is similar to the eutectic line in the ice-salt curve. 



It will, for the present, be as well to neglect all curves not already 

 mentioned in the description of fig. 2, as they only tend to complicate 

 the case. 



It will be seen from the curve that the point S in fig. 2, indicating 

 • 8 per cent, carbon, is the eutectic point, and that steels containing 

 less than this amount of carbon are made up of iron and eutectic, whilst 



IS 00' 



1 4 CO 



I2CC 



10CO 



I 2 3 4 5% CARBON 



Equilibrium Curve For Carburised Iron 



Fro. 2. 



the steels or white irons containing more than ■ 8 per cent, of carbon 

 are composed of eutectic and iron carbide. 



While the formation of the structures of frozen ice-salt solutions 

 and those of iron and iron carbide are very similar, yet there is one 

 feature in which they differ very much indeed. In the equilibrium 

 curves of saline solutions the temperatures at which the constituents 

 begin to segregate from the mass also indicate the beginning of the 

 solidification of the constituents ; but in the case of the iron-carbon 

 alloys the mass has long been solid when the critical changes occur. 

 In other words, in the iron-carbon alloys the separation takes place 

 from a solid solution some hundreds of degrees below their melting- 

 points. 



