THE METALLOGRAPHY OF METEORIC IRON 35 



This phase, which might be shown as an extremely narrow zone at 

 the left side of the diagram, is omitted. 



Martensite. — Three iron-carbon structures in artificial irons should 

 be noted, because of their alleged limited development in certain 

 meteoric irons, and also because of analogous iron-nickel structures 

 which are often observed. 



Pearlite is formed under equilibrium conditions of cooling, a per- 

 fect lamellar separation of ferrite and cementite. Martensite, 

 troostite, and sorbite, however, are nonequUibrium structures, the 

 products of cooling conditions which do not permit the normal for- 

 mation of pearlite. 



When a steel in the austenitic (gamma) range is cooled so quickly 

 that the austenite cannot transform normally, it does not change 

 to pearlite below the critical range, but to an intermediate or tran- 

 sitional phase termed martensite, appearing typically as plates in 

 low-carbon steels (pi. C) and as needles in hypereutectoid steels. 

 Their orientation, suggesting the Widmanstatten structure, is related 

 to the octahedral planes of the gamma phase. Some writers refer 

 to it as a supersaturated solution of carbon in alpha iron. Author- 

 ities are not m full agreement as to the exact nature and process of 

 formation of this constituent. 



True martensite should not be confused with the acicular or banded 

 iron-nickel structure which resembles it, which is found in many 

 meteorites (see Chapter XI). The latter may conveniently be re- 

 ferred to as martensitic in structure, because its origin appears to 

 be analogous to that of the true iron-carbon martensite in artificial 

 irons. 



Needles of martensite are reported by Vogel (1927) in certain 

 meteoric irons, attributed by him to local carbon enrichment, but 

 the author considers them to be the iron-nickel transformation 

 structure referred to. 



Troostite ; sorbite. — With slower cooling, but still too fast to permit 

 formation of normal pearlite, a black structure is produced which 

 is unresolved at 1,000 diameters but at very high magnification is 

 shown to be actually pearlite in a very fine dispersion (pi. C). This 

 has been called troostite, but by the more recent writers it is desig- 

 nated as fine pearlite. In artificial irons it occurs in irregidar nodular 

 forms with an obscure radiating structure. Minute inclusions of 

 similar appearance observed in some meteoric u'ons have been iden- 

 tified with troostite, but the author is of the opinion that they are 

 not an iron-carbon but an iron-nickel structure produced in a some- 

 what analogous manner. 



Troostite forms at around 550° to 600°. With still slower cooling. 



