Microscopic Formal SIAP

7/30/2019 Microscopic Formal SIAP

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Objectives

1. To study the microscopic structures of material2. To observe the purpose of etching process

Equipments and tools

1) Abrasive cutting machine To cut PCS and aluminum rod to small 10mm diameter

2) Hot mounting press machine Mount the specimen with Phenolic resin under temperature and pressure

3) Sand paper To grind the specimen until smooth surface is obtained

4) Polisher machine To polish the specimen

5) Ultrasonic bath machine To remove micro dirt and particle stuck on the specimen

6) Hand dryer To dry the specimen quickly

7) Optical microscope To observe the surface structure for 20x and 50x magnification

8) Inverted microscope To observe the surface structure for 100x magnification


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Theory

The properties of metals 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

microstructure. 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 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. In order to establish a scale for grain size,

ASTM E112 shows charts with outline grain structures at various dimensions. This has led to a

universally accepted standard by which grain sized range from 1 (very coarse) to 10 (very fine).

A material's grain size is important as it affects its mechanical properties. In most materials, a

refined grain structure gives enhanced toughness properties and alloying elements are

deliberately added during the steel-making process to assist in grain refinement. Grain size is

determined from a polished and etched specimen using inverted microscopy at a magnification

of 100X.


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1. Mounting process

Mounting of specimens is usually necessary to allow the specimen to be handled easily.

It also minimizes the amount of damage likely to be caused to the specimen itself. Specimenscan be hot mounted (about 200C) using a mounting press either in a thermosetting plastic, e.g.

phenolic resin or a thermosoftening plastic e.g. acrylic resin. If hot mounting will alter the

structure of the specimen, a cold setting resin can be used, e.g. epoxy, acrylic or polyester resin.

Porous materials must be impregnated by resin before mounting or polishing, to prevent grit,

polishing media or etchant being trapped in the pores and to preserve the open structure of the

material

2. Grinding process

Surface layers damaged by cutting must be removed by grinding. Mounted specimens are

ground with rotating discs of abrasive paper, for example wet silicon carbide paper. The

coarseness of the paper is indicated by a number: the number of grains of silicon carbide per

square inch. For example, 180 grit paper is coarser than 1200. Typical grit sequence: 60, 120,

180, 240, 320, 400, 600, 1200 grit


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3. Polishing processPolishing discs are covered with soft cloth impregnated with abrasive diamond particles and

an oily lubricant. Particles of two different grades are used (i) a coarser polish- typically with

diamond particles 6 microns in diameter which should remove the scratches produced from the

finest grinding stage and (ii) a finer polish- typically with diamond particles 1 micron in

diameter, to produce a smooth surface. Before using a finer polishing wheel the specimen

should be washed thoroughly with warm soapy water followed by alcohol to prevent

contamination of the disc. Polish the specimen on a rotating cloth which is covered with

extremely fine abrasive usually applied as a liquid suspension (Al2O3-water) by using automatic

polishing machine

4. EtchingEtching is used to reveal the microstructure of the metal through selective chemical attack.

In alloys with more than one phase etching creates contrast between different regions through

differences in topography or the reflectivity of the different phases. The rate of etching is

affected by crystallographic orientation, so contrast is formed between grains, for example in

pure metals. The reagent will also preferentially etch high energy sites such as grain boundaries.

This results in a surface relief that enables different crystal orientations, grain boundaries,

phases and precipitates to be easily distinguished. The specimen is etched using a reagent. For

example, for etching stainless steel or copper and its alloy, a saturated aqueous solution of

ferric chloride, containing a few drops of hydrochloric acid is used. This is applied using a cotton

bud, wiped over the surface a few times. Titanium alloy will use Krolls reagent (92% Distilled

water,6%Nitric acid,2%Hydrofluoric acid). The specimen should then immediately be washed in

alcohol and dried.


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5. Cleaning processImmerse the specimen in the ultrasonic water bath and dry the specimen by using hand dryer.

Procedures

1. Specimen preparationa. Two pairs of small sizes specimen were cut 10mm (plain carbon steel and aluminium)

using abrasive cutting machine.

b. The surface of the specimen were made sure to be smooth.


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2. Mountinga. Flat, smooth faces were selected for mounting by using hot mounting press

machine.

b. The suitable resin (phenolic resin) was selected before putting the specimen inthe mounting die.

c. The specimens were placed in the die, desired face downward, resting on thepolished face of the base. The specimens were centered as well as possible.

d. 20gm of resin were used for mounting processese. The mounting die cover was closed tidily and the pressure, time and temperature

were set according to the specification given.

Hot mounting press machine

Specimen

ResinPressure

(bar)Time (s)

Temperature

(oC)

Type Amount (gm)

Plain carbon steel Phenolic

resin

20 6 720 200

Aluminum alloy Phenolic

resin

20 6 720 200

f. Unscrew the cover die and press the lift button so that the die moves up and themounted specimens loosen up.

g. The mounted specimens were taken from the mounting machine.


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3. Grinding and polishing processa. The surfaces of the mounted specimen were grinded using sand paper. Start the

process by using coarser grade followed up to finer grade(240,400,600 and

1000).

b. Make sure to apply water on the surfaces, while grinding make sure apply smallpressure and grind with one direction.

c. The specimens were grinded until the core line disappear and rotate 90 fromtime to time.

d. The specimens were polished using a polishing machine. Start with high-gradecloth and followed by low grade cloth (from 6 to grade 3).

e.

While polishing the specimen, make sure the water tab is running and liquidsuspension (Al2O3-water) aluminium oxide and water is applied.


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4. Cleaning processa. The specimens were immersed in the ultrasonic water bath, the time,

temperature and sonic frequency were set according to the specification.

b. The specimens were dried using a hand dryer.

5. Etching process- The specimen was immerse in the etching liquid.

Specimen 1: nital Specimen 2: kellers

6. Microstructure examination (for non-etched specimens)a. After the specimens were dried, the specimens were placed on the mechanical

stage microscope.

b. Select the lenses to 20x, 50, and 100x magnification

c. The images were recorded


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7. Microstructure examination (for etched specimens)a. The specimen were immerse on the etchantb. The specimens were removed when a bloom appeared.c. The specimen were thoroughly cleaned by water after etchantd. The specimen were dried using a hand dryer

e. After dried, the specimens were placed on the mechanical stage microscope.f. Select the lenses to 20x, 50, and 100x magnification

g. The images were recorded.


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RESULTS

SPECIMEN

FIGURE

20X 50X 100X

PLAIN

CARBON

STEEL

ETCH

PLAIN

CARBON

STEEL

N-ETCH

ALUMINIUM

ETCH

ALUMINIUMN-ETCH


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Discussion

Explain each particular microstructure for both etched specimens and non-etched specimen

which observed in the laboratory session

Microstructure examination of etched/non-etched specimen for 100x magnification

Specimens Etched Non-etched

Aluminum alloy

After polishing process was completed and the specimen were put under the

microscope, we did not get a same image for both of the specimen. For the etched aluminium

alloy, we get lesser black spot compared to the non-etched image. Theoretically, we will get

more black spot on the image of an etched specimen due to the chemical reaction of etching

agent on it, but for our experiment, we got it vice-versa. This was maybe because we did not do

the etching process correctly or there are some problems with the microscope.

Hence, the results for the etched and non-etched were not accurate with the theoretical

results. The expected result was that, the surface of a non-etched was clearer, with fine visible

lines. Whereas the etched specimen, the surface would be rougher and the grain boundaries

could be seen.


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Specimens Etched Non-etched

Plain carbon steel

When polishing was completed, it was determined that the specimen had some scratches

that made it hard to do accurate observations. Due to the scratches, some lines appear when

viewed using the microscope and the image we get is not as accurate as the actual image of the

surface.

From the results, no grain boundaries can be seen clearly from non etched image. In the

other hand, the grain boundaries can be clearly seen from the etchant plain carbon steel image. The

large black spot on etchant plain carbon steel is due to etching process which the surface of the

specimen were chemically reacted with selective reagent that reveals the microstructure details

on the specimen surface under microscopic lenses view.


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Conclusion

From the experiment, we found that to obtain accurate result, the grinding and

polishing process must be carried out slowly. To achieve an accurate result, the process need both

patience and use right grinding techniques. A specimen that has been poorly grinded will either be

unrecognizable or it will affect the image of grain boundaries when viewed using a metallographic

microscope. It also proved by etching process that makes the cell boundaries easier to distinguish

under the microscope due to the chemical reaction on the surface of the specimens.

Microscope is a tool used to enlarge images of small objects that are hard to study with bare

eyes. The compound light microscope, is an instrument with two lenses and various knobs to focusthe image. To be specific,while handling the microscope we need to used our two hands. Where

one holding the arm, and another holding the base. Also, when putting down the microscope, it

should be placed gently. The specimen must be centered in the field of view on low power before

going to high power because if the specimen is observed on high power from the beginning, it gets

very hard to find the specimen. When low-power objective is used from the beginning, we are can

start the observation with the entire image of the specimen. Then, it gets much easier to observe

the specimen when magnified.

Reference

- Foundation of Material Science and Engineering by William F. Smith and Javed Hashemi- Lectures notes

Microscopic Formal SIAP

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