Unhardened steels never look like this. Their appearance is shown in photomicrographs Nos. 3b, 5, 22 and 24a.
No. 80. Martensite, the Constituent of Hardened Steel
(Magnification 400 Diameters)
In unhardened steels having less than .90% of carbon we find two constituents.
“Ferrite” is the name which has been given to one, the soft and ductile constituent, pure iron. With ordinary etching the ferrite usually shows as light-colored or white grains bounded by black lines, which, if the patch is large enough, give a fish-net appearance. It is soft and ductile like copper, for pure iron and pure copper are not so greatly different in malleability and ductility as one might suppose.
The darker and more or less triangular patches at the corners of the ferrite grains are “pearlite,” a name originating because of their “pearly” appearance under the microscope. How this pearly appearance comes about will be readily understood from photomicrograph No. 23e which was taken at a magnification of 400 diameters. It is seen that it results from alternate black and white layers.
Again we must give up the idea of any finality in the things we learn or think we have learned. We just learned, for instance, that ferrite usually was light or white in color. Well, in pearlite, as shown in photomicrograph No. 23e, every other plate is of ferrite but they are not the white but the black ones.
No. 23e. Pearlite at Magnification of 400 Diameters
You may not have understood before that color as shown under the metallographic microscope depends not so much upon actual color of the material itself as upon its ability to reflect light. For metallographic observations it is necessary to have very strong illumination. Usually the powerful beam from an electric arc is concentrated by means of condensing lenses upon a thin disc of glass called an oblique reflector which directs the beam upon the polished and etched specimen beneath the objective of the microscope. Often a prism is used. The rays of light returning from this highly illuminated “field” under observation return up through the tube and eye piece of the microscope and can be focused upon a small screen convenient for observation or upon the ground glass of the attached camera by means of which the pictures are taken. Unless the surface of the specimen being examined is perfectly plain and level, not all of the vertical rays thrown down upon it will be reflected back up through the tube and eye piece. Those portions of the field which are absolutely at right angles to the vertical rays appear at the eye piece or upon the screen as white or light-colored portions, while those which, during the polishing or etching have been dug or eaten away reflect the light imperfectly or in directions other than up the tube of the microscope, wherefore such portions show as darker or black sections.