Requirements for specimens under metallographic microscopes
Metallographic samples are objects for observation, research and photography with a metallographic microscope. In order to achieve the purpose of observation and research, the interception (sampling), processing (grinding and polishing), corrosion, etc. of the specimen should meet certain technical requirements. The above-mentioned operations on metallographic samples are collectively referred to as "preparation of metallographic samples". The preparation of specimens is a laboratory technology. Readers may refer to the book "Metallographic Research Methods" written by Comrade Gai Yaonian (China Industrial Press) and other related works. The requirements for metallographic samples are only considered here from the perspective of using a metallographic microscope.
(1) Dimensions of specimen
The size of the sample is preferably a rectangular parallelepiped with a diameter of 12 mm and a height of 10 mm, with a base area of 12x12 mm2 and a height of 10 mm. Some samples (such as hard and brittle white iron, high tin bronze etc.) is not easy to cut and process into the above size, you can use a heavy hammer to crush it, select appropriate pieces, and inlay them on the plastic, so as to obtain an inlaid sample of suitable size and neat appearance.
(2) Samples for observation
After the sample has been processed (including sampling, trimming, precision cutting, and polishing), the surface should be as smooth as a mirror, and no scratches or holes are allowed. For specimens that are only used for microscopic observation, the surface finish can be appropriately reduced. For example, slight wear marks on the surface are allowed, but it shall not affect the observation and research. The surface of the test specimen is not allowed to be touched by hand to avoid scratches. It is contaminated due to printed waves, affecting the observation effect.
During the entire preparation process of heat-treated metallographic specimens, strict cooling measures should be taken to prevent the specimen from generating heat during processing and changing the original structure of the specimen.
The corrosion degree of the sample should be such that it can reveal the structure. The etched test rod must be cleaned immediately, and a hair dryer should be used to dry the liquid or moisture on the surface of the sample to prevent the sample from being immersed and to avoid bringing corrosion and moisture into the microscope.
(3) Samples for photography
The preparation of metallographic test grids for photography must be more careful and of better quality than the preparation of observation specimens. It is often the case that people are satisfied with the sample used for microscopic observation, but it is far from being able to obtain good photos when used for metallographic photography. Especially in some cases where the metallographic structure is very small, such as fine-grained or flake-shaped pearlite sorbite, fine needle-like martensite, fine precipitates dispersed in the alloy sodium solid solution, etc., some part of it will always be lost during photography. some details. Therefore, eliminating grinding traces on the surface of the sample as much as possible and arranging various defects and interferences are important conditions for making the photos more truly reflect the appearance of the metallographic structure.
When the depth of field of the objective lens is small, it is impossible to clearly show the details of the tissue deeper than the surface of the specimen. For this reason, the depth of relief on the ground surface of the specimen should be reduced to a minimum during polishing, and the depth of erosion should not be greater than the depth of field of the objective lens. During erosion, in order to maximize the contrast of the tissue, it is more appropriate to choose erosion punishment. It would be ideal if an etching agent could be used to dye each unit tissue in different colors. This is most beneficial for color film photography.
