METALLURGICAL MICROSCOPE:
4.0 Metallography:
Metallography is the study of the internal structure of metals and alloys under microscope at different magnifications. Useful information on grain size, shape and distribution and on percentage of various micro constituents can be achieved by this. There are two types of metallographic examinations. They are
- Macro examination, where the specimens are observed by naked eye or at low magnification (5X).
- Micro examination, where small areas of specimen are observed at high magnifications. (50-2000X)
4.1 Metallurgical microscope:
It is an important device used for micro examination. It is very helpful in determining the size shape and distribution of phases. No structure property correlation can be made with out the help of a metallurgical microscope.
The construction of this is similar to that of a biological microscope. But it differs in the manner by which the specimen is illuminated. Different types are available, and all of them work on the principle of reflection of light from the specimen surface. Such a microscope consists of two parts, a compound microscope and an electrical illuminating system. A beam of light from the illuminating source enters the tube of microscope from side. The light is reflected down by a prism and pass through the objective lens and falls on the metallic specimen. The surface reflects the light back and it passes through the objective lens again and reaches the eyepiece. A virtual image of the metallic structure is formed by the objective lens, and the eyepiece magnifies this image.
The body of the microscope is fixed on a base. Eyepiece is fixed at the top of a drawtube. This drawtube slides in the upper end of the body tube of the microscope. Body tube is connected to the upper end of the limb by a rack and pinion arrangement. By sliding the drawtube the distance between the eyepiece and objective can be adjusted, thus adjusting the focus. For coarse adjustment the body tube is raised or lowered. Fine adjustment helps the final focusing.
Eyepiece is easily detachable and is available in varying such as 5X, 8X, 10X, and 12Xetc. The objective lens is much more powerful (50X). If the objective gives a magnification of 50, and if an eyepiece of 5X is used, the total magnification will be 50x5=250times. A permanent record of microstructure can be obtained by mounting a photographic camera to the eyepiece.
Magnification depends on the focal length. Lesser the focal length, greater will be the magnification. Due to the curvature of the lens the image formed may be observed as sharply focused at center and a little bit blurred at the edges. To avoid this a special eyepiece may be used. Compensation is thus obtained to get a sharp definition of the whole field of view.
Prior to micro examination, the specimen has to be prepared suitably for the examination. This is done in various steps. Specimen has to be cut with a hacksaw to a suitable size. If the specimen is too small, it has to be fixed on a Bakelite or Lucite base for the ease of handling. The sample is then filed and ground. Then the sample is polished with various grades of emery, on the increasing order of fineness. Next stage is the final polishing carried out with rotating disc covered with sylvet or billiards cloth. This cloth is
soaked in water and sprinkled with slurry of alumina or calciend magnesia. Once a scratch free mirror like surface is obtained, the specimen is taken for etching. Only after etching the internal structure will become apparent under a microscope. This is since the effect of etchant differs from phase to phase. Grain boundaries are attacked more easily and at a faster rate. There fore they look as thin dark lines in the structure. After etching the components are washed in running water and then dried in hot air.
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