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ME 2151E Lab Report

Metallography

LIN SHAODUN A0066078X

Lab Group 6

Date 21st Oct 2011

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TABLE OF CONTENTS

OBJECTIVE 2

EXP E RI M E N T RE S UL T S 3

DISCUSSION 5

CONLCUSION 6

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OBJECTIVE

Metallography is the science of interpreting and reporting the microstructure of engineering

materials. Sectioning, mounting, grinding and polishing are the sample preparation steps prior to

the microscopic evaluation.

The study of microstructural detail is important due to its correlation with the ensuring

mechanical properties of the material.

The objective of this experiment includes:

To develop an understanding of the principles of practical metallography.

To observe the various microstructure in a welded mild steel joint.

To learn specimen preparation techniques in metallography.

EXPERIMENT RESULTS

1. Overview of 5 different microstructures in welded steel

The macroscopic examination of welded structure:

5 different microstructures in welded steel:

Weld Metal Zone

Heat Affected Zone

Unaffected Zone

Weld Metal

Zone Grain Grow

Zone Grain refinement

Zone Transition

Zone Unaffected

Zone

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2. Detail Drawing for Weld Metal Zone

3. Detail Drawing for Grain Growth Zone

4. Detail Drawing for Grain Refinement Zone

α-ferrite

Pearlite

Magnification: 200X

α-ferrite

Pearlite

Magnification: 200X

α-ferrite

Pearlite

Magnification: 500X

Martensite

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5. Detail Drawing for Transition Zone

6. Detail Drawing for Unaffected Zone

α-ferrite

Pearlite

Magnification: 200X

α-ferrite

Pearlite

Magnification: 200X

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DISCUSSION

1. Weld Metal Zone

The weld metal zone is generally characterized by the presence of two important microstructural

features, i.e. Columnar grains and Widmanstatten structures.

Weld metal zone is formed as the weld metal solidifies from the molten state. This is a mixture

of parent metal and electrode (or filler metal).

Weld metal zone‘s microstructure reflects the cooling rate in the weld. Depending upon the

chemical composition, a Martensite structure in the weld indicates a very fast cooling rate; fine

pearlite, and coarse pearlite showing comparatively slower rates of cooling respectively.

From the molten weld pool, the first metal to solidify grows epitaxially (with its orientation

controlled by the crystal substrate) upon the solid grains of the unmelted base metal. Depending

upon composition and solidification rates, the weld solidifies in a cellular or dendritic growth

mode.

Both modes cause segregation of alloying elements and consequently, the weld metal is less

homogeneous on the micro level than the base metal and therefore cannot be expected to have

the same properties as the wrought parent metal unless the filler metal has in the as deposited

condition properties equal to the parent metal.

2. Grain Growth Zone

Adjacent to the weld metal zone is the heat-affected zone that is composed of parent metal that

did not melt but was heated to a high enough temperature for a sufficient period that grain

growth occurred. Heat affected zone is that portion of the base metal whose mechanical

properties and microstructure have been altered by the heat of welding.

The heat affected zone is subjected to a complex thermal cycle (sudden heating followed by

rapid cooling) in which all temperatures from the melting range of the steel down to

comparatively much lower temperatures are involved and HAZ therefore consists of a series of

graded structures ringing the weld bead. HAZ, usually contains a variety of microstructures.

Grain growth region is immediately adjacent to the weld metal zone (fusion boundary). In this

zone parent metal has been heated to a temperature well above the austenite phase temperature,

this resulted in grain growth or coarsening of the structure.

The maximum grain size and the extent of this grain growth region increase as the cooling rate

decreases. Within the Grain growth region, most of regions is pearlite with smaller grains of

ferrite.

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3. Grain refinement Zone

Adjacent to the grain growth region is the grain refined zone. The refined zone indicates that in

this region, the parent metal has been heated to just above the austenite phase temperature (912

C ) where grain refinement is completed and the finest grain structure exists. Because of the

relatively lower temperatures, the austenite grains began to nucleate at many points to form

smaller austenite grains which on cooling will result in fine ferrite and pearlite grains.

4. Transition Zone

In the transition zone, a temperature range exists between the eutectoid and austenite phase

transformation temperatures where partial allotropic recrystallization takes place. The structure

of the transition zone shows the ferrite grains have not been altered but the pearlite regions have

been made much finer.

This change was produced by heating into the critical range which transformed the pearlite into

austenite and by subsequent cooling reformed the pearlite.

5. Unaffected Zone

The unaffected zone represents the region of parent metal that was not heated beyond the

eutectoid temperature (727°C) and there is no observable structural change.

Outside the heat affected zone is the parent metal that was not heated sufficiently to change its

microstructure. The typical grain structure of the parent metal [ferrite (white) and pearlite (dark)]

which was welded and whose weld metal zone and HAZ microstructures were discussed and

shown earlier.

CONCLUSION

From this experiment, I have learnt how to prepare a specimen for metallographic inspection by

series of processes such as grinding, polishing, and etching, as well as how to operate the

grinding / polishing machines with correct parameter settings.

I also learnt how to differentiate the microstructures the specimen in different zones, and

correlate the different zones to the Fe-Fe3C phase diagram.

In conclusion, a wealth of knowledge and experience has been achieved by completing this

experiment