engr230 lab 2e m

AIN SHAMS UNIVERSITY

FACULTY OF ENGINEERING

NEW PROGRAMS IN ENERGY & MANUFACTURING

Structure and Properties of Materials ENGR 230 1/7

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LAB 2

METALLOGRAPHIC EXAMINATION

OBJECTIVE

To study the microscopic structures of metals.

BACKGROUNG

The properties of materials highly depend on their structures. The internal structures

determine how materials perform under a given application. The effects of most

industrial processes applied to metals to control their properties can be explained by

studying their rnicrostructures. The branch of materials science dealing with

microscopic examination of metals is called Metallography.

The most common method used to examine the structures of materials is optical

technique. A specimen about 20-mm on an edge is cut from the metal to be

examined. In some cases where the subject is small or unhandy like razor blade, it is

embedded in a plastic case. A mirror polish is produced on one face of the specimen

by grinding on successively fine emery (sand) papers and polishing on revolving cloth

wheels with fine abrasives such as diamond or alumina powder. To reveal the

structural details such as grain boundaries, phases and inclusions this polished surface

is etched with chemical solutions. The etchant attacks various parts of the specimen

at different rates and reveals the structure. A metallographic microscope is used to

examine the microstructure.

PREPARATION OF SPECIMENS FOR MICROSCOPIC

EXAMINATION

Metallography is basically the study of the structures and constitution of metals and

alloys, using metallurgical microscopes and magnifications from 100X to 1500X, so

that the physical and mechanical properties of an alloy can be related to its observed

microstructure.

1- SECTIONING

A truly representative sample should be selected carefully so as to provide the

maximum information over the smallest area. The area for metallographic

examination should be relatively small and usually does not exceed 15 mm x 15 mm.

More specimens must be prepared in order to reveal all microstructures of interest.

Care must be taken to avoid alteration of the microstructure by excessive heating or

work hardening as a result of the cutting action throughout the cutting operation.





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2- MOUNTING

Small samples are usually mounted in a matrix of thermosetting or thermoplastics

polymers. The most common mounting methods employed in metallurgical

laboratories are hot mounting and cold mounting.

The importance of mounting is: ●for convenience in handling

●protecting the extreme edges and enabling

micrographs of these areas to be taken.

●the polymeric matrix does not affect etching

2-1 Hot mounting

Hot mounting takes place under pressure in a

mounting press, where the specimen is placed in a

cylinder together with the appropriate mounting

resin. A temperature of up to 200 º C and a

pressure of up to 50-kN are then applied during the

embedding of the specimen. Figure 1 illustrates the

hand press.





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2-2 Cold mounting

When heat and/or pressure can cause damage to the

metallurgical specimen or change its microstructure,

the cold mounting method is recommended.

Cold mounting or embedding is when a resin is mixed

with a hardener (or accelerator) to provide the

mounting compound, and then the polymerization

process take place to form the block. In some cases,

this process gives-off heat. However this heat

generation can be controlled by the use of ice or cool

air blow setting. Cold mounting compounds are

preferred for specimens that are sensitive to the heat or

pressure, which applies during the hot mounting

process.

3- GRINDING

A grinding sequence involving a series of emery papers with grit sizes of 240, 320,

400, and 600, is recommended (remember that the larger the number of the grade,

the finer the particles). Figure 2 illustrates a typical bench unit for hand grinding

metallographic specimens. This step is aimed at gradually reducing the thickness of

the distorted layer that results from sectioning.

The best grinding is done wet and by hand on a flat surface. A stream of water is

necessary to cool down the metal sample and to wash away loosened grit, thus

preventing both scratching of the surface being ground as well as clogging of the

emery paper.





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You must make sure that the entire surface of the sample is being ground and that the

surface remains almost normal to the axis of the mount. The removal of striations

from coarser abrasive is achieved more readily if the specimen is rotated 90° during

transfer to successively finer abrasive papers as shown in Figure 3.

It is very important that you wash the specimen and your hands after each

grinding step to ensure the absolute cleanliness of the sample before going to the

next finer grade of emery paper.

Figure 2 Figure 3

Figure 4 shows photomicrographs of a specimen after two stages of grinding.

Figure 4





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4- POLISHING

This is the most important operation in the

procedure for preparing metallographic

specimens. It is aimed at removing from the

surface to be examined the fine scratches

that would result from the preceding

grinding operation, thus producing a highly

polished surface.

Final polishing is usually conducted on a

revolving wheel covered with a napped

cloth (like synthetic rayon or velvet) onto

which a suspension of aluminum oxide

(alumina) having a micron particle size is

charged. Heavier hand pressure should be

used in this operation, though pressure may

be reduced near the end. Also, the operating

time should be kept to an absolute

minimum.

Suspensions of aluminum oxide having particle sizes of 15, 6 (or 3), and 1 micron are

used. Again you are reminded to thoroughly wash both the specimen and your hands

between steps.

In order to ensure uniform surface removal

throughout the entire surface, the specimen

should be moved in a clockwise direction (as

shown in Figure 5) since polishing wheels

normally rotate counterclockwise.

After the final polishing operation is completed,

you should wash your sample under running

water, rub it lightly with a cotton swab, flush its

polished surface with alcohol, and finally dry it

under a blast of hot air. Now your sample that

has a scratch-free surface with a mirror-like

finish is ready for metallographic

examination.

Figure 5

The metallurgical microscope is used at a magnification of usually l00X in order to

reveal structural features like the presence of porosity, hair (micro) cracks, and/or

nonmetallic inclusions. The microstructure cannot be observed yet while the

specimen is still in its as-polished condition, except for certain nonferrous alloys and

only when polarized light is used. You must always remember that a polished





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sample should be stored in a dessicator and that you should not touch or scratch

the polished surface. You must, therefore, not carry a polished metallographic

specimen in your pocket, nor allow any hands or clothes to come in contact with the

polished surface.

5- ETCHING

In order to make visible the many structural characteristics of the metal that are not

revealed by the microscopic examination of the as-polished specimen (such as the

grain boundaries, plastic flow of crystallites, twinning, etc.), the polished surface must

be briefly etched. The etching operation involves subjecting the surface to the

chemical action of an appropriate reagent under carefully controlled conditions.

Etching is conducted by either swabbing the polished surface lightly with cotton

gauze saturated with the reagent, or by immersing it into a small vessel partly filled

with the reagent.

When the bright metallic shine of the polished surface disappears, the specimen is

removed from the etchant and quickly rinsed with a stream of running water. The

surface of the specimen is then flushed with methyl alcohol to remove water droplets

and subsequently dried under a blast of warm air.

Experiment

Equipment:

Metallurgical microscope - emery papers of different grade (180, 320, 400, and

600) - polishing wheel - polishing power or paste-alcohol - etchant-some

specimen to be examined.

Procedure:

Follow the systematic sequence that previously explained to prepare and examine

the given specimen.

Required:

You are requested to sketch the structure you have got and try to explain it.

Comment on your experiment indicating the difficulties encountered in

sample preparation and the surface obtained.





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Example:

Austenitic stainless steel, colour etched Porosity in an Al-10%Cu alloy

A color mixture of austenite, ferrite and

sigma phases in duplex stainless steel

Austenite phases are present as white

island in blue sea of ferrite in duplex

stainless steel

Al – SiC composite, as polished Gray iron with lamellar graphite, etched in

Nital.

engr230 lab 2e m

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