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Transactions. 



ther changes after the iron has cooled to a temperature at which 

 it is no longer capable of retaining dissolved carbon as such. 

 This explains the existence of somewhat similar curves in the 

 lower portion of fig. 5. 



Fig. 5 



In this solid solution, with changes of temperature actual 

 changes of structure take place, and on cooling crystallizing- 

 out of its constituents occurs, accompanied with character- 

 istic retardation of fall of temperature. The loci for vary- 

 ing proportions of carbon of such retardations, known as the 

 Ar 3 point, the Ar 2 point, and the A^ point, are shown by the 

 lower curves on the figure. 



Taking a steel of, say, - 15 per cent, carbon at a tempera- 

 ture of 1,000° C, such steel at that temperature will be in the 

 form of a solid solution of carbon in iron ; allowing it to slowly 

 cool, on its temperature falling to about 850° C. pure iron will 

 separate out in the form of ferrite-crystals — in much the same 

 way that the ice-crystals formed in the water-and-salt solu- 

 tion. This action will go on until the temperature has dropped 

 to about 760° C, when a second point of retardation marks 

 a magnetic change in the iron. 



Crystallization out of the ferrite still continues until at a 

 temperature of about 680° eutectic proportions are reached. 

 At this the critical point, Ar l5 the carbon, in the form of the 



