82 BULiLETEN 184, UNITED STATES NATIOGSTAl, MUSEiUM 



less to further depress the gamma-alpha transformation, already 

 lowered by the presence of nickel. Inasmuch as dense or eutectoid 

 plessite represents a gamma-alpha transformation structure, pro- 

 duced at low temperatures where diffusion is retarded, any admixture 

 of phosphorus would depress it still further, and thus prolong trans- 

 formation to a point where the resulting structure would be still 

 finer. 



Phosphorus thus tends to promote eutectoid structures in artifi- 

 cial irons containing carbon. A very small percentage of phos- 

 phorus gives rise to small segregations of an iron-phosphide or iron- 

 phosphide-carbon eutectic — the latter known in artificial irons as 

 steadite. According to Stead, such a eutectic can be detected in 

 cast iron containing only 0.03 percent of phosphorus. 



It is to be noted that the tendency of phosphorus to produce 

 eutectoid structures in artificial irons increases with the nickel con- 

 tent. Thus with pure iron 0.80 percent of carbon is required to 

 make a completely pearlitic (eutectoid) steel; but with 8.7 percent 

 of nickel only 0.50 percent of carbon is required, and with 13.20 

 percent of nickel only 0.10 percent of carbon. 



These facts are of interest in connection with an occurrence of 

 steadite in an inclusion in meteoric iron, referred to in the following 

 chapter. 



Studies on the Cape iron. — Vogel (1927) in experiments upon the 

 Cape of Good Hope iron (Ni 15.67 percent, Ni-Co 16.62 percent) 

 found that the structure designated by him as eutectoid (termed by 

 the author paraeutectoid) is stable up to 700° — that is, it is not 

 altered by heating — but that a sample heated to 1,450° and quenched 

 showed a characteristic gamma granular structure with twinning; 

 that when further heated seven hours at 700° and quenched it showed 

 a structure of polyhedral alpha grains without twinning; and that 

 on further heating for 25 hours at 650° it again became eutectoid. 

 From these facts he deduced that the structure expresses the struc- 

 tural equilibrium of a ternary iron-nickel-phosphorus alloy with 0.2 

 to 0.3 percent of phosphorus, a product of slow cooling, either from 

 the original melt or after reheating above 1,000°. 



It is not necessary, however, to ascribe the structure either to the 

 action of phosphorus or to reheating, because the diagram (fig. 5) 

 indicates that the changes thus produced in the Cape iron should be 

 approximately the same even if no phosphorus were present. In 

 fact, the latest analysis of that iron, by Fahrenhorst in 1900, gave it 

 only 0.09 percent of phosphorus. Even in the absence of phos- 

 phorus, in an iron-nickel alloy with as much as 15 percent nickel 

 the transformation would take place below 400°, at which tempera- 

 ture diffusion has practically ceased. Therefore, such a structure 



