The term metallography originally covered the microscopic study of metals under substantial magnification and the recording of microstructural details by photography. Initially all the work was done with an ordinary visible-light microscope, and photographs made of the details observed at various magnifications were called photomicrographs. In about 1960, the electron microscope was put to use in examination of metallic structures. In recent years, variations of the electron microscope have developed, such as the transmission electron microscope (TEM), and the scanning
electron microscope (SEM), scanning transmission electron microscopy (STEM), and ion microscopy. Associated with these advances in microscopy were developments in chemical analysis of microstructural constituents, using an electron-probe microanalyzer,
ion-probe microanalyzer, and Auger electron spectroscopy among other late 20th century analytical developments.
The following information is confined to the practical knowledge of the structure of metals as obtained using the optical metallograph. The optical metallograph is a special microscope with an inverted stage that allows a flat specimen to be placed face-down on it so that portions of interest on the specimen can be scanned. The metallograph usually has an integral camera (often a Polaroid camera), and can have a number of features for changing specimen illumination and for measuring details observed on the specimen.
Specimen Preparation. Metallographic examination requires a small metal specimen, usually not over 25 mm (1 in.) diameter or square, that is cut to provide a flat surface. The flat surface is ground and polished by a specific procedure until it is as scratch-free as possible. A complete procedure for preparing metallographic specimens is found in ASTM Standard E3, Standard Methods for Preparation of Metallographic Specimens. A polished specimen surface, when examined with the metallograph at a magnification in the range of 100 to 500X, is uniformly reflective and featureless unless there are cracks, porosity, or nonmetallic inclusions in the metal.
It is necessary to etch the polished surface of the metallographic specimen to reveal the microstructure. Etching can be accomplished in a number of different
ways, depending on the metal or alloy, and conditions such as whether the metal is cast, wrought, or weld metal. Many ferrous specimens can be etched by merely dipping or swabbing for a few seconds in a solution of 1 to 5% nitric acid in alcohol (commonly
called 2% nital). Metals and alloys that are resistant to acid attack, such as nickel or stainless steel, can be electrolytically etched. Details for etching various metals and alloys can be found in ASTM E407, Standard Methods for Microetching Metals and Alloys.
Specimens containing a weld often present a challenge to the metallographer because of the marked difference in etching rates between the base metal, heat-affected zone, and weld metal, especially when working with welded joints of dissimilar metals.
Grain Size. The first feature noted by the metallographer during the examination of the microstructure of a polished and etched metal is its grain size. The size of the grains exerts a profound effect on the properties of a metal, especially its mechanical properties. In most metals and alloys, both grain growth or grain size reduction can be accomplished by either mechanical working or heat treatment or both.
Standardized methods of measuring grain size to permit evaluation of metal properties, specifications, and control have evolved, and are described in ASTM El 12, Standard Methods for Determining Average Grain Size.
Because weldments may benefit or may suffer from grain growth in cold-worked metal, the mechanics of recrystallization must be considered. During cold working the grains in a metal are severely deformed and heavy reductions produce very elongated grains, but grain boundaries persist despite severe grain deformation. When the temperature is raised, the grains distorted by cold-working recrystallize to undistorted
equiaxed grains. The temperature at which the distorted grains are replaced by equiaxed grains is called the recrystallization temperature. Metal that is heated above the recrystallization temperature and held for long periods of time will experience the growth of abnormally large grains. The temperature at which grain growth becomes significant depends a great deal on the metal and alloy.
Where the weld is made in a single pass, the grain size and grain growth in the weld zone are largely dependent on the travel speed of the pass. Welds made at slow travel speed tend to be relatively coarse grained while welds made at fast travel speeds tend to be relatively fine-grained. Welds of the latter type tend to have solidified last at the centerline and are susceptible to centerline hot cracking. Welds made at moderate travel speed are more typical of commercial practice and a fine grain structure is developed in the weld zone.
In the case of multiple pass welds, the first pass is reheated during the making of the second weld pass so that the first weld pass is tempered and the grain structure is refined. Each successive pass tempers and refines grain of the previous passes. This produces a weld microstructure that is desirable since a substantial portion of the weld has been grain-refined and tempered by subsequent weld passes. These multipass
welds usually have excellent mechanical properties and are usually much tougher than a single pass weld.
For a specific type of steel and strength level, fine- grained steels have superior mechanical properties compared to the coarse-grained steels, especially strength, ductility, and notch toughness. For elevated temperature service, coarse-grain steels have superior performance since fine-grain steels will exhibit lower strength. Obviously, grain size is a feature of the microstructure that deserves close scrutiny in the examination of metal structures.
Austenitic Grain Structure. The austenitic grain size of a steel depends on the austenitizing temperature. Grain refinement occurs when a steel that will transform is heated to a temperature slightly above its A3 temperature and is then cooled to room temperature. A fine grain size is desirable for improved toughness and ductility. Steel forgings and castings frequently are normalized specifically to produce grain refinement. At higher austenitizing temperatures (over 1000°C [1800oF]), steels usually develop a coarse austeniticgrain structure. Coarse-grained steels usually are inferior to fine-grained steels in strength, ductility, and toughness.
Microstructure of Metals. Much of the practical knowledge of the structure of metals has been obtained using the optical metallograph. This knowledge was obtained by examination of polished and etched metallurgical specimens at magnifications from 50 to 1500X. Steel and other iron alloys have been more extensively studied than other metals and alloys because of their wide commercial usage. This knowledge has been applied to the weldments of iron and steel to insure that the metallurgical structures in the weldments are suitable for the service conditions expected of the structure. Microstructures in steel weld metal are markedly different from those of either cast
or wrought base metals. The microstructure of weld metal is controlled principally by composition and cooling rate.