ME2151 2 Metallography lab 12/oct/2015

OJBECTIVE 

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

Refer to last page. DISCUSSION AND ANALYSIS

1. Fusion Zone The fusion zone can generally be characterized by the presence of two important microstructural features; olumnar grains and !idmanstatten structures.

"nder the microscope# the grains appear elongated. This is due to the weld being sub$ected 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 olumnar grains.

%n the other hand# the large austenite grains being put through a moderately fast cooling rate results in the formation of !idmanstatten structure.

&.'rain 'rowth Zone The grain growth zone can be identi(ed as the demarcation between grain and growth 1

zone and weld metal zone is generally more distinct compared to other zones.

"nder the microscope# the grains appear larger and rounder as compared to other zones. )t this part of the material# the temperature was still *ery high but it was still lower than its melting point. )s a result# this exposure leads to the growth of austenite grains. %n cooling to room temperature after the weld# this leads to the formation of course ferrite grains and !idmanstatten structure of ferrite and pearlite occurs.

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+.'rain Re(nement Zone The parent metal in this zone is being heated to austenite phase during the weld. ,n comparison to the temperature of the 'rain 'rowth Zone# the lower temperature in this zone causes the austenite grains to nucleate at many points to form smaller austenite grains. "pon cooling# this results in the formation of (ne ferrite and pearlite grains.

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"nder the microscope# the grains in this zone are obser*ed to be much smaller compared to the other regions.

-.Transition Zone uring welding# the parent metal in this zone is heated to a temperature /less than at the fusion zone0 where ferrite and austenite coexist. Therefore# mostly only perlite grains were being transformed into small austenite grains. "pon cooling of the material after the weld# the formation of *ery (ne pearlite grains with ragged looing boundaries occur among the mostly untransformed original ferrite grains.

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2. "naected Zone "nder the microscope# the grains are obser*ed to ha*e no structural change. This is because the parent metal was not heated beyond the eutectoid temperature of 3&34 in this region.

CONCLUSION

This experiment allowed us to experience the process of preparing a specimen for microscopic obser*ations. Through proper grinding and polishing# the specimen is rid of all scratches and dirt. This results in a scratch free surface ready for etching. ) well prepared specimen is

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essential for the examination of its microstructure. From the examination of the specimen# it became apparent that dierent microstructures can be formed depending on the region with respect to the welded $oint. !e can obser*e that the metal exhibits dierent grain structures due to the change in temperature and corresponding cooling rate# aecting the nucleation and redistribution of the grains. The study of metallography allows us to study the $oint strength of dierent welding types with dierent metals# enabling us to better understand the structural properties of materials and the alteration of microstructures that occur after 6

processing. This allows us to mae better choices in the selection of dierent materials for the appropriate applications.

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