Metallography

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D. Etching:

Etching is the final process of specimen preparation and it is done to bring out the structure of the polished material for microscopic examination by attacking the grain boundaries of the material leading to a non-similar reflection of light into the objective lens leading to the generation of contrast between the various micro structural features.

The specimen must first be washed and dried thoroughly. Then we etch the surface of the specimen with 3% nitric acid and 97% ethyl alcohol until a change in the specimen is observed. The specimen is washed again with water and brushing with soft brush then dried rapidly by washing with alcohol then left to dry.

Now specimen is ready for observation.

[pic 4]

[pic 5]

E. Microscope procedure: [pic 6]

The specimen is placed on the stage placed and the microscope and the light path selector are turned on. We adjust the brightness, contrast and magnification and we observe the following:

III. Results:

Aluminum microstructure:

[pic 7][pic 8][pic 9]

[pic 10][pic 11][pic 12]

Medium carbon heterogeneous steel alloy microstructure:

[pic 13][pic 14][pic 15]

[pic 16][pic 17][pic 18]

Low carbon steel alloy microstructure:[pic 19][pic 20]

[pic 21]

[pic 22][pic 23][pic 24]

IV.Discussion:

From the experiment that has been done, we can understand the preparation of metallographic specimen. To prepare the sample, it must go through several processes as we discussed which are Mounting, Grinding, Polishing and Etching. All of these are essential for the best result.

The cap on the mount was broken and had to be replaced my warehouse made cap thus the cap was not exactly as the original cap so there were some faults in heat loss to the surroundings.

The mounting and grinding devices both used the same water source , instead each device should have its own source for more accuracy as not to mix the surface removals in the grinding with the mounting device.

During grinding gloves should be worn as not to hurt the user from the removals or heat and also a grill should be placed on the grinder so the specimen does not fall into it.

The specimen was polished on the Struers LaboPol 5 but instead we should have used an electro-polishing device in order to obtain more accurate results.

References:

https://en.wikipedia.org/wiki/Metallography

http://www.metkon.com/en/application_grinding-polishing_12.html