ABSTRACT

The experiment was conducted to understand the procedure metallographic sample preparation and basic function of optical microscope and its operation. Furthermore, the experiment was conducted to perform analysis from the microstructures observation and lastly to understand theoretical background of sample preparation and metallographic specimens. There are six process was conducted during the experiment which are cutting, moulding, grinding, polishing, etching and metallographic observation. The result can be obtained during metallographic observation which is the grain structure of ferrite and pearlite.

INTRODUCTION

It is often to analyse the phase exist and the grain size in the structure in the study of the metallic materials. The details of the structure if metals are very difficult and not readily visible through human naked eyes but the grain structures can be seen with the aid of optical microscope. By studying the micrograph, metal characteristics such as grain sizes, effect of the heat treatment and carbon content of steels can be determined. For this purpose, the metal used in the metallurgical examination must be prepared and polished carefully before a good microscopic image can be seen. It is very important to ensure that the surface is totally flat and smooth before microstructure observation. Any irregularity will appear as a dark image which may cause confusions during analysis of the structure. In order to obtain smooth and clean flat surface, there are five process that are required which are cutting, moulding, grinding, polishing and etching. Then, the specimen can be analysed under the optical microscope.

CuttingCutting process is carried out in order to get the desired size of the sample so that it can be easily moulded / handle.

MouldingMoulding process is carried out to make the sample easier to hold throughout the polishing procedure. There are two types of moulding process which are hot moulding and cold moulding.

GrindingGrinding process was carried out to remove rough surface. There are two types of grinding process that are rough grinding which purpose to remove rough scale and gross imperfections on the surface of sample and fine grinding which purpose to improve the specimen's surface until it begins to shine and reflect light slightly.

PolishingPolishing process is then carried out to remove any imperfections that cannot be removed during grinding process. There are two types of polishing process that are rough polishing: which purpose to remove the imperfections that grinding has left and fine polishing which purpose to remove all scratches and leave a mirror like on the surface.

EtchingVarious etchants are used to selectively attack the surfaces of metals to reveal grain boundaries, phase, precipitates, inclusion and variations in composition.

Metallographic ObservationWhen a mirror-like metal surface has been obtained during the etching process, the structure can be observed with the aid of optical microscope. The optical microscope magnifies an image by sending a beam of light through the object as seen in the schematic diagram. The condenser lens focuses the light on the sample and the objective lenses (for example 10X, 40X) magnifies the beam, which contains the image, to the projector lens so the image can be viewed by the observer.

THEORY

There are several processes or steps that need to be done in order to get the best result in this experiment. The processes are cutting, moulding, grinding, polishing and etching. These processes need to be done in correct order to avoid any damage on the specimen that is used.

The first process is cutting. Abrasive cutting is the best solution in order to avoid any alteration in the microstructure of the specimen. The process time is faster and the surface of the specimen that had been cut is also smooth. With enough coolant and suitable speed control, the specimen can be cut without causing any damage on it. The abrasive cut-off wheel that is used in the machine is made of the mixture of abrasive grains such as aluminium oxide, rubber and other materials. These mixtures are made into a thin circular wheel. For wet operation, rubber bonded wheel is used while in dry operation, resin bonds is used. Then a specimen is cut, the abrasive grains on the cut-off wheels will become dull. Thus the bond must be destroyed at a correct rate so that new grains will form at the edge. There are several factors that affect the breakdown of the bonds, which are: Hardness of the bond Size and speed of the wheel Amount of pressure applied by the wheel to the specimen Power of the driving motor

The next process that needs to be done is moulding. Moulding is one of the mounting technique used specimens. It can provide better specimen edge retention if compared to the cast mounting resins. Compressible mounting resins are available in different colours and also with various fillers for different type of hardness. Several advantages of moulding are: Easier to hold the specimen Provide protection to the edges Provide proper specimen orientation Enabling to label and store specimen Provide standard format to mount multiple specimensThe primary mounting resins are: Phenolic resins (Black, red and green) Acrylic resins (Clear) Diallyl phthalate resins (Blue and black) Conductive resins (Phenolics with copper or graphite filler)

The next process is grinding. In this experiment, there are 2 types of grinding that are used, course grinding and fine grinding. The purpose of coarse grinding is mainly to produce initial flat surface for the next grinding (fine) and polishing step. Due to cutting and grinding, the material may be got cold worked to a depth with a resultant transition zone of deformed material pattern on the surface. In this experiment, coarse grinding is done with wet surface using 240 and 320 grit electrically powered disc but it is also must be noted that significant heat must be avoided on the specimen as it may damage its microstructure. It is important that each preparation stages must be carefully performed. The specimen must: Free from scratchers and stains that can mark the surface Retain non-metallic inclusions Free from all traces of disturbed metal Reveal no evidence of chipping due to brittle intermetallic compounds

In fine grinding, the idea is still almost the same as coarse grinding. But in fine grinding, the abrasive used is finer. In fine grinding, the grits are 400 and 1200. Proper grinding involves the rotation of the sample between stages while the grinding angle must be held constant during the grinding at any stage. Wet operation is also used in fine grinding to avoid side effects such as excess heating that cause tempering, transformation and aging. It also provides flushing action for loose particles and keep sharp edges of the grinding medium exposed at all time.

An important factor throughout both of the grindings operation is that the scratchers must be uniform in size and parallel to each other. Rotation of 90 between stages while grinding is required and the angle must be held constant during the operation.

After grinding, the next process is polishing. Polishing involves the use of abrasive, water, on a cloth covered electrically powered wheel. Diamond abrasive provides the best compounds utilized in polishing but it is also very expensive. The powder used in this experiment is called aluminar. The sample must be washed and dried before proceeding to this stage. This is because every other material even the smallest hard dust particles in the air could cause unwanted scratching on the surface on the specimen.

At the first stage of this process, with a 25-micron suspended aluminar on a nylon-cloth, the fine grinding surface resulted from the previous process should be completely removed. During the initial polishing stage, moderate pressure can be applied to the specimen and the entire stage should generally take up to 1 or 2 minutes. After that, wash the specimen carefully and proceed to the 5-micron stage where a separate polishing wheel with 5-micron particles. Light pressure is applied during the stage. The final polishing stage is 1-micron suspended aluminar with a slower speed of rotating wheel. If the specimen had become a mirror-like surface free of scratchers, it indicates that the polishing stages are already completed.

For precision work, the wetness of the cloth for final polishing is very important. If it is too wet, the specimen will show pits on its surface and if it is too dry, smearing will occur. Other than that, extremely fine grades of diamond abrasive may be used for the final polishing stage. Cerium oxide is an excellent example of material that is used for the final polishing of aluminium and other soft metal and alloys.

The final process in this experiment is called etching. Etching is the process of highlighting and identifying microstructural features. This process occurs when the chemical is placed on the surface of the specimen. This process is done by applying the specimen to the chemicals for several seconds. In this experiment, the etchants used is called nital, the mixture of nitric acid and ethanol. The specimen is dipped into the chemical. After 5-10 seconds, the specimen is immediately washed with water, and then rinsed with alcohol. Finally it is dried with an air blast. The surface must not be touched after this process as it can damage the microstructure of the specimen. If touched, the whole process must be redo once again.

After the 5 processes are completed, the specimen will proceed to the microscope for metallographic observation. The important characteristic of the microscope that should be known are the magnification and the resolution. The resolution of the field can be adjusted during the process of observation. The magnification also must be set in increasing order to get the best view of the microstructure of the specimen. Starting with magnification m-5, the microstructure of the specimen can already be examined. After that, the magnification is set to m-10, m-20 and lastly the highest magnification in the experiment, which is m-40. For a higher detailed microstructural study, a view of magnification up to m-100 is used by the microscope in order to observe the microstructure of the specimen.

2. The experiment was conducted in six processes which are start from cutting and followed moulding, grinding, polishing, etching and lastly is the observation under an optical microscope.

Cutting1. The specimen is marked at the point to be cut.2. The specimen is then been clamped under the abrasive cutter.3. The outer shell of the abrasive cutter machine is closed.4. The water pipe is opened and the machine is turned on.5. The cutter is slowly lowered to the specimen manually using hand.6. After the cut is completed, the pipe is closed and the machine is turned off.7. The outer shell of the machine is opened.8. The clamp is unclamped and the specimen that has been cut is collected.

Molding1. The specimen that has been cut is rubbed with a lubricant gel.2. The specimen is placed inside the hollow space located at the middle of the automatic mounting pressure machine with the surface that is intended for observation is faced downward.3. The same hollow space is filled with 10 grams of phenolic powder.4. The hollow space is then closed and locked.5. The machine is set to be 10 minutes for hot and pressure and 8 minutes for cooling.6. After the time is up, unlocked and the specimen is collected.

Grinding1. A sand powder with grit 240 is placed on top of the platform of the grinding machine.2. The water pipe is opened and the grinding machine is turned on.3. As the sand paper is rotating, the molded specimen is applied on top of the sand paper with moderate amount of pressure.4. The specimen is then grinded with different grit of sand paper after a uniform pattern of straight scratches are seen on the grinded surface.5. Process 1 to 4 is repeated using sand paper of grit 320,400 and 1200 as the specimen is rotated 90 degree differ from the previous position of the specimen being hold when changed to the next grit.

Polishing1. The polishing process starts with rough polishing.2. A small amount of Aluminar powder of 9.5 micron was applied on top of polishing cloth of the wheel.3. Distilled water was applied to the Aluminar powder until it gives the suitable surface for the specimen to polish.4. The machine was turn on to let the wheel spin.5. The specimen was hold 90 degree from its horizontal small scratch line and was put carefully to the spinning wheel.6. A minimal force was applied to the specimen during the wheel is spinning.7. Distilled water was applied time to time to keep the surface of polishing cloth smooth for the specimen to polish.8. The specimen and hand was wash and dried before proceed to next stage.9. Step 2 to 8 was repeated for another rough polishing using 5 microns of Aluminar powder.10. Step 2 to 8 was repeated for 2 fine polishing step using 3 microns and 1 microns Aluminar powder.11. The surface of polishing cloth and the wheel was washed with water to remove the powder after finish all polishing step.12. The wheel was covered when it is not in used.

Etching 1. The specimen was placed on the table under the fume hood.2. The fume hood was turned on.3. The suitable etchant which is Nital was placed in a petri dish.4. The specimen was being dipped into the etchant for about 5 to 10 seconds. 5. By using air blower, the surface of the specimen was dried after the etching process.

Observing Under an Optical Microscope1. The specimen was placed on the stage of the microscope under the lens.2. The microscopes light was switched on and the light intensity was altered to number 5 by rotating the light knob.3. The specimen then was observed by using M5 lens.4. By using the coarse and the fine knob, the stage was being increased or decreased until the microstructure of the specimen appear clearly. 5. Then, the magnifying was increased by rotating the turret using M10, M20, M30 and M40 respectively.6. Lastly, the result was recorded by drawing a picture of the microstructure of the specimen.

REFERENCES

1. ASM, Metallography and Microstructures, ASM Metals Hand Book, 9th edition, Metals Park, New York, 1983, pp.228.2. G.Petzow, Metallographic Preparation, Metallographic Etching, 2nd Edition, ASM International, New York, 1999, pp.209.3. George F. Vander Voort, Macroetchants fo Revealing Strain Patterns, Metallography, Principles and Practise, ASM International, New York, 1984, pp.9.4. Henry Thompson, Etching of Specimens, Microscopical Technique in Metallurgy, Pitman, London, 1954, pp.32.5. Norman Emme Woldman, Preparation and Microscopic Examination of a Specimen, Physical Metallurgy, Chapman & Hall, London, 1930, pp.14.