The pearlite, then, is made up of little plates of soft ferrite alternating with others of a very much harder constituent. The harder plates are much less affected during polishing and etching than are those of the softer ferrite, hence they stand out in relief and reflect abundant rays of light, whereas the “dug-out” ferrite plates reflect the light imperfectly or not at all and therefore appear as dark lines.
These white, hard plates of the pearlite contain all of the carbon of the low carbon alloys. They are this other constituent, “cementite,” so named because it was first discovered in steel made by the “cementation” process. It is a very hard and brittle substance, hard enough to scratch glass. It is the chemical compound (Fe3C), unvarying in composition as chemical compounds always are. It consists of just three atoms of iron (93.4% by weight) and one of carbon (6.6%).
Pearlite, therefore, is a sort of mechanical mixture of two separate constituents, ferrite or pure iron, and this chemical compound, carbide of iron, which is called cementite. Pearlite is common to all unhardened steels whether of low, medium or high carbon content and may be considered characteristic.
That we may understand clearly the structures of the annealed steels, let us start with pure iron and gradually change it into higher and higher carbon steels by gradual addition of carbon. Pages [328] and [329] show such a series.
Photomicrograph No. 99b is open-hearth iron which is entirely made up of free ferrite. In No. 3b there is considerable pearlite, here appearing black, though the sample of steel yet contains but .10% of carbon. In No. 5, which is of a steel containing .30% of carbon, we have more pearlite and in No. 22c with .50% carbon we have yet more. Manifestly at this rate the comparative pearlite areas are growing so that there will soon be room for no ferrite at all. In No. 23g this has occurred. This, the photomicrograph of a steel containing .86% of carbon is one of the steels in which we found that the point of recalescence, loss of magnetism, decrease in electrical conductivity and rate of expansion take place all at the one point.
No. 99b. Carbonless Iron
No. 3b. Steel with .1 Per Cent Carbon
