OJBECTIVEThe aim of this experiment is to prepare a section of welded steel specimen for microscopic examinations by grinding and polishing and to compare the different microstructures that occur due to the fusion welding.

RESULTSRefer to last page.

DISCUSSION AND ANALYSIS1. Fusion Zone

The fusion zone can generally be characterized by the presence of two important microstructural features; Columnar grains and Widmanstatten structures.

Under the microscope, the grains appear elongated. This is due to the weld being subjected to rapid cooling from its molten state as steel is a

good conducted of heat. This results in chilled casting in the material, bringing about the formation of Columnar grains.

On the other hand, the large austenite grains being put through a moderately fast cooling rate results in the formation of Widmanstatten structure.

2. Grain Growth Zone The grain growth zone can be identified as the demarcation between grain and growth zone and weld metal zone is generally more distinct compared to other zones.

Under the microscope, the grains appear larger and rounder as compared to other zones. At this part of the material, the temperature was still very high but it was still lower

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than its melting point. As a result, this exposure leads to the growth of austenite grains. On cooling to room temperature after the weld, this leads to the formation of course ferrite grains and Widmanstatten structure of ferrite and pearlite occurs.

3. Grain Refinement Zone The parent metal in this zone is being heated to austenite phase during the weld. In comparison to the temperature of the Grain Growth Zone, the lower temperature in this zone causes the austenite grains to nucleate at many points to form smaller austenite grains. Upon

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cooling, this results in the formation of fine ferrite and pearlite grains.

Under the microscope, the grains in this zone are observed to be much smaller compared to the other regions.

4. Transition Zone During welding, the parent metal in this zone is heated to a temperature (less than at the fusion zone) where ferrite and austenite coexist. Therefore, mostly only perlite grains were being transformed into small austenite grains. Upon cooling of the material after the weld, the formation of very fine pearlite grains with ragged looking boundaries occur among the mostly untransformed original ferrite grains.

5. Unaffected Zone

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Under the microscope, the grains are observed to have no structural change. This is because the parent metal was not heated beyond the eutectoid temperature of 727˚C in this region.

CONCLUSIONThis experiment allowed us to experience the process of preparing a specimen for microscopic observations. Through proper grinding and polishing, the specimen is rid of all scratches and dirt. This results in a scratch free surface ready for etching. A well prepared specimen is essential for the examination of its microstructure.

From the examination of the specimen, it became apparent that different microstructures can be formed depending on the region with respect to the welded joint. We can observe that

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the metal exhibits different grain structures due to the change in temperature and corresponding cooling rate, affecting the nucleation and redistribution of the grains.

The study of metallography allows us to study the joint strength of different welding types with different metals, enabling us to better understand the structural properties of materials and the alteration of microstructures that occur after processing. This allows us to make better choices in the selection of different materials for the appropriate applications.

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