PETROGRAPHY AND MET/tLLOGRAPHY. 165 



5. Metallography of Steel. — Steel is practically an 

 alloy of ferric carbide and iron, and the phenomena attend- 

 ing its cooling from a high temperature are essentially 

 the same as those we have just considered. The com- 

 pound corresponding to the eutectic alloy contains 12% 

 FejC, or 0.8% of pure carbon. This substance is known 

 as pearlyte, and steel of the right composition on gradual 

 cooling from a fused condition becomes entirely con- 

 verted into it at a temperature of 670° C. If more car- 

 bon be present, ferric carbide is separated before this 

 temperature be reached; if iron, that metal is segregated. 

 The carbide is known to microscopists as cementite, the 

 pure iron as ferrite. Very low carbon steels are made 

 up of ferrite, with here and there particles of pearlyte. 

 The proportion of pearlyte increases with an increase of 

 carbon up to 0.8%, after that point being mixed with 

 cementite. 



If steel be suddenly cooled from a high temperature, 

 above that at which segregation begins, a new com- 

 pound is formed known as martensite, and to this sub- 

 stance is due the intense hardness of steel thus quenched. 

 Below the point at which ferrite or cementite begins to 

 separate, sudden cooling produces a mixture of these 

 bodies with martensite. 



If a smooth surface of steel be examined under the micro- 

 scope the presence and the general proportions of these 

 various compounds may be readily made out. The 

 specimen, of some- convenient size, say from half an inch 

 to an inch square and half an inch thick, is first very 

 carefully polished upon one tace with emery and with 



