IRON AND STEEL, Metallography of. As the study of the constitution of rocks, based, in part at least, upon microscopical examina tion, is known as petrography, so the study, by similar methods, of the constitution of metals and alloys has been called by analogy metallog raphy, for it was discovered that alloys and im pure industrial metals were made up of con stituents bearing close resemblance, in many respects, to the minerals of nature. This new department of metallurgy has been actively de veloped for the last 20 years in all metallurgical countries and notable progress made.
In order to examine the structure of metals through the microscope, it is necessary to pre pare surfaces almost absolutely free from the minutest scratches. To accomplish this, it is quite evident that the sample must be rubbed successively over various abrasive substances of increasing fineness. Supposing the surface to be examined has been filed with a smooth file or ground on an emery wheel, the marks left by the tool or wheel could not be removed and a perfectly specular condition imparted to the surface in a single operation. The transforma tion must be gradual. The file marks must be effaced by rubbing the sample over a properly selected polishing substance, and replaced by finer markings. These, in turn, must be re moved by a second rubbing with a finer abrasive agent, being replaced by still finer marks, and so on, until finally, the last operation removes the very minute marks from the previous treat ment and leaves the surface quite, if not abso lutely, free from scratches. Emery powder of various degrees of fineness is the abrasive sub stance which naturally suggests itself, at least for the first treatments. The powders may be used in the shape of emery wheels, emery cloths or papers, or even spread loose over a suitable support, in which case they are kept wet during the rubbing. Emery may, of course, be re placed by carborundum for these operations. The polishing powder known as ((jeweler's* or !gold') rouge suggests itself for the ing. It is generally spread over a piece of
wash leather or some other cloth having a soft and smooth texture, which in turn is fastened to a block of wood or to some other suitable support. Washed alumina is also widely used. The powder is generally kept wet during the rubbing. The details of the manipu lation vary greatly with different operators, but it may be said that they all use emery (or car borundum) in some form for the rough ing and jeweler's rouge or washed alumina for the final treatment. Tripoli powder may be used with advantage immediately before the lastpolishing.
The polishing may be done entirely by hand, or it may be hastened by the use of some sim ple power-driven machine consisting of revolv ing emery wheels and discs upon which the vari ous powders are spread.
Many treatments have been tried to develop the structure of polished samples of iron and steel, i.e., to make their structure apparent when examined under the microscope. To do this, it is necessary to impart unlike appearances to the various constituents. The most successful meth ods consist in etching the polished surfaces with highly diluted aqueous solutions of nitric or picric acid or preferably with alcoholic solu tion of these acids. Absolute alcohol contain ing some 10 per cent of nitric acid or 5 per cent picric acid are the etching reagents most widely used.
Microstructure of Pure a properly prepared sample of a pure metal is examined under the microscope it is generally found to be made up of irregular polyhedric grains as shown in Fig. 1, which represents a drawing of the structure of pure gold under a magnification of 50 diameters.
It will be noticed that many of the grains are hexagonal, which strongly suggests that gold crystallizes in the regular cubic system, these hexagonal grains being probably due to inter fering cubes and octahedra.
Many pure metals, including iron, crystal lize in the cubic system, and exhibit, therefor% a structure similar to that of gold.