Metallography

Metallurgical Microscope.

The metallurgical microscope differs from other microscopes mainly in being specially designed for the observation of opaque objects requiring, in some cases, the highest available resolving and magnifying powers. The two typical methods of illumination employed in the examination of metals are indicated in the diagrams in fig. 1. Light from the source or lamp falls upon a reflector placed within the microscope tube, close above the objective. This reflector is, in fig. I (a) a small totally reflecting prism or mirror so placed as to cover one-half of the objective; in fig. I (b), the reflector is a thin slip of flat glass placed at an angle of approximately 45 degrees above the objective. Either type of reflector sends a portion of the incident light down through the objective on to the surface of the specimen, which reflects some of this light back into the objective. A part is inevitably returned to the lamp, but a part passes upwards through the transparent reflector or through the uncovered half of the objective to form the image at the upper end of the microscope tube. It will be seen that where the surface of the specimen is truly horizontal the incident light is fully reflected back into the microscope, and that portion of the sur face will appear bright in the resulting image. Where the surface is roughened, however—for instance by the formation on it of numerous minute facets inclined to the horizontal—the light will be reflected to some extent away from the object lens of the microscope and such roughened areas will appear more or less dark in the resulting image. Where the distance between ob jective lens and specimen is sufficiently great—as it is with the lenses used for more moderate magnifications—it is possible to vary the mode of illumination by throwing a strong beam of light obliquely upon the surface from one or more sides. This completely alters the character of the image observed, parts previously dark now appearing light and vice versa. When a suitably etched specimen of a pure metal is thus obliquely illuminated and is then slowly rotated, a remarkable effect, known as that of the "oriented lustre" of the crystals, is observed. The same effect can be seen with the unaided eye where specimens of metal having a very coarse crystalline structure are available. An example of this kind, showing the radiating crystals of a piece of cast lead, is illustrated in fig. 6, Plate 1.

A typical example of a metallurgical microscope designed for prolonged visual work as well as for photography, is illustrated in fig. 2, which shows the Rosenhain metallurgical microscope.

The main feature of an instrument of this kind is that the tube carrying the optical system is fixed and that all focussing, both coarse and fine, is done by movements of the stage. In recent years, however, the inverted or Le Chatelier type of microscope, in which the specimen is placed face downwards on the stage, has become popular owing to its convenience for photomicro graphic work. A disadvantage is that while it facilitates photog raphy it renders visual observa tion difficult and tiring and work ers take photographs rather than study their specimens in detail.

The magnifications are limited only by the resolving power of the microscope. Classical optical

theory places this limit at not much more than i,000 diameters, but recent American workers have employed much higher mag nifications with some measure of success. The attempt is also being made to utilise ultra-violet light of very short wave-length and therefore of correspondingly higher resolving power.

Much depends on the speci men to be examined. It is only in materials of great uniformity that any section cut at random will give a structure typical of the whole. In the early days of metallography doubt was widely felt whether the examina tion of so small an area as that which can be seen under the microscope could furnish reliable information regarding masses of metal weighing perhaps many tons. It is now, however, rec ognised that sections cut from properly chosen parts of a mass of metal furnish most valuable information. It is advisable to cut and examine sections taken in at least two directions in each place and often taken from a number of places in the same piece of metal before a true picture can be formed.

"Macroscopic" Examination and Sulphur Printing.— Considerable guidance can be obtained by what is known as "macroscopic" examination. For this purpose a relatively large area of the metal—usually a complete cross-section—is roughly polished, leaving the surface covered with fairly fine emery scratches. This surface is then exposed to the action of a fairly vigorous solvent. In the case of iron and steel, a solution con taining slightly acid copper chloride is often used, but there are a number of special reagents for this purpose. These are allowed to act for a much longer time than is required for microscopic etching, and they produce a deep attack on the surface, generally roughening and darkening it. None the less the surface thus attacked shows a pattern which is known as the "macro-structure'' which indicates the general arrange ment of the crystals. Where the process is applied to a casting, or to an ingot which has not undergone much deformation, the arrangement of the original crystals formed during solidification can generally be clearly seen. In a forging it is possible, - as a rule, to trace the lines of flow of the metal. This is particularly the case in regard to iron and steel, where the presence of non metallic impurities and the persistent segregation of phosphorus make the outlines particularly distinct.

A

method applicable mainly to iron and steel is known as "sulphur printing." It was originated by Baumann in Germany. A roughly polished surface is again prepared but instead of etching it, a piece of photographic silver bromide paper soaked in a 10 per cent solution of sulphuric acid is brought into contact with the surface. Wherever sulphide enclosures are present they are attacked by the acid and sulphuretted hydrogen is evolved. This immediately forms a spot of silver sulphide on the paper and a print, showing traces representative of all the sul phide enclosures present, is obtained. This is an excellent means of showing the presence of segregation in steel. An example taken from a segregated steel rail, is shown in fig. 7, Plate 1.