34 BFLLETTN IS 4, UNITED STATES NATTONiAl, MUSEIUM 



until at 0.80 percent carbon, where the A3 and Ai Hnes coincide, 

 the alloy passes directly from the gamma to the alpha phase plus 

 FcaC as the eutectoid pearlite. This is eutectoid steel (pi. A, fig. 4). 



At the point E on the diagram the line EC begins, marking the 

 limit of solubility of cementite in austenite. It is called the Acm 

 line. At this line the austenite holds in solution all the cementite 

 that it can, and on crossing the line in cooling it begins to give up 

 free cementite — that is, whatever cementite there may be in excess 

 of the eutectoid ratio corresponding with 0.80 percent of carbon. 



Thus an iron containing more than 0.80 percent of carbon on 

 passing the line EC is no longer a groundmass of ferrite grains with 

 segregations of pearlite, but a groundmass of pearlite with segrega- 

 tions of cementite. It is termed a hypereutectoid steel (pi. B, fig. 1). 



When the carbon content exceeds about 2.5 percent the metal 

 is no longer a steel but a cast iron, in which free carbon (graphite) 

 may appear. If the carbon is all in combined form it is a white 

 cast iron, consisting of cementite and pearlite (pi. B, figs. 2, 3). If 

 part of the carbon is in the form of flakes of graphite it is a gray cast 

 iron (pi. B, fig. 4). Whether the cast iron is white or gray does not 

 depend wholly on its carbon content, but largely on its rate of cool- 

 ing, although the carbon content is generally higher in gray irons. 

 The development of graphite is greatly influenced by the presence 

 of a small proportion of silicon, which is commonly present in com- 

 mercial cast iron. 



At the point B, with a carbon content of 4.3 percent, the eutectic 

 ratio, the melt solidifies at 1,130°, the lowest point for an iron-carbon 

 alloy. It solidifies directly, without passing through any transi- 

 tional range in which the solid and liquid phases are mingled. The 

 product is eutectic cast iron (pi. C, fig. 1), consisting of particles of 

 pearlite in a matrix of cementite. 



The iron-carbon diagram, as shown in figure 3, generally is not 

 carried beyond 5 percent of carbon, but at temperatures above 1,130° 

 iron may hold considerably more than that percentage of carbon (in 

 the form of carbide) in solution. Therefore, to the right of the point 

 B, if the diagram were extended, the line AB would rise to a tempera- 

 ture around 2,200° where iron may contain as much as 9.6 percent of 

 carbon, the maximum that it can hold in solution in any conditions. 



In an iron in which the content is less than about 0.01 percent of 

 carbon at room temperature (0.0088 percent) the changes above 

 described do not take place. It behaves as if it were carbon-free, 

 and the final product on cooling is a homogeneous solid solution of 

 carbon in ferrite, indistinguishable from pure iron. 



