It was thought for many years that wrought iron actually had a fibrous structure, and in deed, the number of persons who still hold this view is surprisingly large. Many valuable properties were attributed to puddled iron on account of its "fibrous structure" which were denied to steel because of the "crystalline struc ture" of that metal. The miscroscope has sum marily disposed of this erroneous belief in showing that the ferrite which constitutes the hulk of wrought iron is in no way different from the ferrite forming the bulk of low carbon steel. Both are equally crystalline.
The presence of carbon in wrought iron re sults in the occurrence of another constituent (pearlite) to be described later.
Transverse Section.—The microstructure of the transverse section of a wrought iron bar is shown in Fig. 4. Like the structure of a longitudinal section (Fig. 3), it consists in a mass of crystalline grains of ferrite. The slag, however, which in the latter section occurred as fibres running in a direction parallel to the rolling, here assumes the shape of irregular, dark areas, which correspond to the cross-sec tions of the slag fibres.
The Microstructure of Low Carbon Steel. — If a small amount of carbon he introduced into molten iron and the resulting product cast, the metal is converted into what is known as low carbon steel. From the fact that it was ob tained in a molten condition it will follow that, unlike wrought iron, it is quite, if not alto gether, free from slag. The absence or pres ence of slag constitutes the most distinct dif ference between wrought iron and low carbon steel, for these two metals may otherwise have identical chemical composition and very similar, if not identical, physical properties.
The microstructure of very low carbon steel is illustrated in Fig. 5. It will be noted that it consists in a mass of ferrite; that is, of car bonless iron, made tip of polyhedric crystalline grains very similar to the ferrite constituting the bulk of wrought iron. At the junction lines of many ferrite grains, however, some dark areas will be seen, which indicate thepres ence in the metal of another constituent. Since ferrite does not contain any carbon, it is evi dent that all the carbon present in the steel has segregated into these small dark masses, and thus we arrive at the interesting conclusion that in low carbon steel the carbon is not dis seminated all through the mass, as might rea sonably have been expected, but that it gathers in small particles embedded in a mass of iron.
Pearlite.—Speculating as to the nature of this new constituent, we know that it cannot consist of pure carbon ; for it is known that the carbon present in steel does not exist in the free state, but is combined with some iron, forming the carbide or iron FeirC. This iron carbide must necessarily be located in the dark areas, hut are these made up exclusive of this carbide? To assist us in determining the char acter of this constituent, let us examine it under a high magnification. We then find that each dark particle breaks up into two components (Fig. 6), which occur as small parallel plates al ternately bright and dark As to the nature of these components, it is evident that one of them is the carbide Fe,C and the other must neces sarily be iron or ferrite; for these are the only two constitutents which, to the best of our knowledge, are pres,nt in unhardened steel.
Dr. Sorby, who was the first- observer to describe the appearance of this interesting con stituent, proposed for it the name of interesting constituent,A because it frequently exhibits a display of color very suggestive of mother of pearl, especially when viewed by oblique illu mination. Later Professor Howe suggested the name of pearlite, which has been universally adopted.
The carbide FeaC has been called cementite, also by Professor Howe, be cause it occurs abundantly in cemented steel, merely owing to the fact that this steel is gen erally a high carbon steel. Pearlite then is a mechanical mixture of ferrite and cementite after the fashion described in the preceding paragraphs.
It should be stated here that pearlite does not always assume such a _distinctly laminated structure. In many instances its structure re per cent of carbon. The pearlite occupies here over one-half of the total area. It will be noted that the ferrite areas are not resolved into polyhedric grains, apparently because the fer rite now occurs in particles often too small to he made up of several crystalline grains. The FIG. Structure of Pearlitc Highly Magnified (Osmond). mains ill-defined or assumes a granular rather than a lamellar appearance, this being due chiefly to the treatment to which the steel has been subjected.