26 BTJLLETm 18 4, UNITED STATE'S NATIONAL MUSEIUM 



cause it is necessary to use the methods and terminology of the 

 technical metallographers. 



Iron and its alloys. — Absolutely pure iron is perhaps unknown, but 

 iron prepared by the electrolytic process approaches chemical purity, 

 and the purest u'on produced commerciallj'" contains only negligible 

 amounts of other substances. Iron (substantially free from ad- 

 mixture) as a structural component is known in metallography as 

 ferrite, and its structm'e under the microscope is a network of irreg- 

 ular polyhedral grains (pi. A). Its melting point is about 1,530° C.^ 



Most artificial irons (like all natural ones) are alloys. The most 

 important added constituents are carbon (in steels and cast irons) 

 and the various metals used in alloy steels, of which nickel is the 

 only one that is important in meteoric irons. Nickel is close to 

 iron in its chemical and physical properties. Its melting point is 

 about 100 degrees lower than that of iron, and it is soluble with iron 

 in aU proportions. 



Carbon occurs in two forms — as free carbon (graphite) and as 

 iron carbide (FcsC), which is termed cementite. It is in the latter 

 form that carbon appears in steels, which contain less than about 

 2.5 percent of carbon, and in white cast iron, which contains more 

 than that percentage. In gray cast irons, which often contain a 

 still higher percentage, the carbon may be partly free and partly 

 combined, or it may appear chiefly as graphite in irregular leaflike 

 flakes. See plate B. 



At its melting point iron can dissolve about 5 percent of carbon; 

 but about 4.5 percent is the maximum carbon content actually pro- 

 duced in cast iron — unless some component be present (such as 

 manganese) that increases the solubility of carbon. 



Phases of pure iron. — Iron (and other metals) in cooling pass 

 through certain allotropic transformations known as phases, in 

 which lattice structure, crystalline habit, and magnetic properties 

 undergo changes. These transformations, with their modifications 

 by reason of thermal conditions and added components, produce 

 the great variety of structures and properties in natural and artificial 

 iron alloys. 



The metal begins to solidify at about 1,530°, forming first dendritic 

 crystals and then grains. This is the uppermost or delta phase of 

 the solid state. Delta iron is cubic in crystallization, with a body- 

 centered space lattice. This phase continues, as the metal cools, 

 to 1,400°, when the grains of delta iron change to gamma iron, with 

 a face-centered cubic lattice. At 910° another transformation takes 

 place, when the grains of face-centered gamma u-on become body- 

 centered cubic alpha iron. This, like all allotropic changes, is accom- 



» All temperatures are centigrade unless otherwise stated. 



