Colour Metallography of Ferrous Alloys and its Applications to iron and Steel Making

Abstract

Reagents used to show the structure of metallographie samples may give reaction products which dissolve in the reagent; or, on the contrary , products may be insoluble and remain in the place, forming thin films, more or less adherent. Both kinds of reagents attack the various const-ituents,at least in the initial stage, at different rates ; namely, they are selective with respect to: 1. The different phases in the sample. 2. Heterogeneity of solid solutions. 3. Crystallographic orientation of the phases. 4. Internal stress in the lattices. In boundaries between crystal grains of a, metal aggregate, these different heterogeneities accumulate in large measure, and, therefore, boundaries are gen-erally etched by reagents more quickly than the core of the grains, consequently, reagents of the first kind, at the first steps of attack, develop on the sur-face of the sample a network of grooves, more or less deep, which shows the granular pattern of the aggregate. Therefore, the surface of the sample, which was mirrorlike before etching, becomes uneven and, at the microroscope, through the contrast of lights and shadows, reveals the structure of the metal. With the second kind of reagents, on the other hand, the products of reaction form, at least in the first steps of attack, films which are more or less epitassic, and whose thickness is a function of etching rate, and, therefore, of the above mentioned heterogeneities. When etching goes on for a longer time, the first layers formed protect to some extent the underlaying metal, and the rate of subsequent etching is influenced also by other factors, as for instance, diffusion rate of the reagent through the first layers formed.