The term metallography has several definitions. In the strictest sense, metallography is the study of the structure of metals and metal alloys, typically using magnification by optical or scanning electron electron microscopy. A second widely used definition of metallography is the technique and process of preparing metal samples to reveal and display their internal structure, or, microstructure. For large components a smaller piece, or sample, is cut from the component for preparation. Selecting the appropriate area of the larger part for sampling can be critical relative to the objective of the evaluation since many components do not exhibit a uniform microstructure. The sample is then encapsulated in thermosetting plastic or cold cure epoxy, called a mount or micro, that is typically between 1 inch and 2 inch in diameter. The type of mounting material used depends on the characteristics of the sample configuration and on what aspects of the sample are of interest.
The mount holds the sample in the desired orientation and makes it easier to handle for the next steps in the process – grinding and polishing. Grinding is done using progressively finer grits of abrasives, and must be done carefully to avoid smearing of the metal which obscures (or distorts) its internal structure. Following grinding, the mounted sample is polished to a mirror finish, typically with fine diamond particles suspended in a light oil, and then with even finer aluminum oxide particles (alumina) suspended in filtered water. A great deal of information can be learned by examining the mount in this condition, and photomicrographs are often recorded to document the sample in this “as-polished” condition. However, this process is usually followed by etching of the sample with an acid. Etching is done using a wide variety of acids or combinations of acids depending on the material that is being etched. Etching reveals a vast amount of information about the sample’s microstructure, and through interpretation of that microstructure, the “history” of the material, such as how it was heat treated; what temperatures it was exposed to in service; whether or not it was properly forged, machined, or plated; whether it was exposed to corrosive environments; and other valuable information. This “history,” as revealed by examination on an optical metallurgical microscope (see metallograph), and/or a scanning electron microscope and photographically documented.
