How to Prepare a Specimen for Microscopic Examination.

Introduction : 


Metallographic consists of the microscopic study of the structural characteristics of a metal or an alloy. It is possible to determine size, shape and distribution of various phases and inclusions, which have a great effect on the mechanical properties of the metal. The microstructure will reveal the mechanical and thermal treatment of metal, and it may be possible to predict its expected behavior under a given set of conditions. Experience has indicated that success in microscopic study depend largely upon the care taken in the preparation of the specimen. The most expensive microscope will not reveal the structure of a specimen that has been poorly prepared. The procedure to be followed in the preparation of a specimen is comparatively simple and involves a technique, which is developed only after constant practice. The ultimate objective is to produce a flat, scratch-free, mirror-like surface.

Tool and Instruments : 


1. Abrasive cut off machine or hacksaw.

2. Belt sander or abrasive grinding m/c.

3. Mounting press.

4. Polishing machine.

5. Hot air drier.

Materials : 


1. Specimens to be prepared.

2. Synthetic plastic material e.g. Bakelite, Lucite, aerylic, polystyrene

3. Emery papers of grade 100,200,300,400,500,600 and 800. (Silicon carbide)

4. Etchant e.g. nitric acid (nital), picric acid (picral) etc

5. Cotton plugs.

Experimental Procedure :


1. Sampling:

The choice of a sample for microscope study may be very important, If a failure is to be investigated, the sample should be chosen as a close as possible to the area of failure and should be compared with one taken from the normal section.

The sample should be taken from the longitudinal section, if the effect of rolling extrusion or wire drawing process to be studied. For observation of case hardened steels, edges of the sample should be carefully preserved. Sectioning methods include fracturing, shearing, sawing, abrasives cutting and electric discharge machining. Fracturing is recommended for extremely brittle materials. Blows of hammer carry it out. Less brittle materials can be cooled in liquid nitrogen before breaking to obtain a flatter surface. Low carbon steel sheets and other soft materials can be cut to size by shearing. Sawing is widely used for sectioning. It may be manual or power saw.

Fig: Sample cutting

To cut larger pieces, power saw is used. Manual sawing is used for less hard materials. Abrasive cutting method is used for very hard material such as hardened steels. For this, consumable or non-consumable cut-off wheels are used. Silicon carbide impregnated wheels are used for the nonferrous and nonmetals. Allumina is recommended for ferrous metals. Coarse grain wheels are used to cut heavier sections while fine grain wheels are recommended for delicate materials, e.g. thin wall tubes. For metallographic purpose 60 to 120 grit sizes are used. The coolant used contains water-soluble oils with rust inhibitor additives and foaming agents. Wire saws are used to cut very thin sections. The sectioning of metallographic specimens can also be performed on electric discharge machine.

2. Rough grinding:

Whenever possible, the specimen should be of a size that is convenient to handle. A soft material may be made flat by slowly moving it up and back across the surface of a flat smooth file.

                               

Fig: Rough filing and grinding

The soft or hard specimen may be rough ground on a belt sander, with the specimen kept cool by frequent dipping into water during grinding operation. In all grinding and polishing operations the specimen should be moved perpendicular to the existing scratches; this will facilitate recognition of the stage when the deeper scratches have been replaced by shallower ones characteristics of the finer abrasive. The rough grinding is continued until the surface is flat and free of nicks, burrs, etc. and all scratches due to the hacksaw of cutoff wheel are no longer visible.

3. Mounting:

  Small and odd shaped components are mounted to facilitate handling during preparation. Sharp edges and corners are eliminated to avoid damage to polish papers and cloth. Mounting provides uniform sized and shaped specimens which are convenient to prepare, view and store. Standard mountings are usually of 25mm diameter and 12mm thickness. Proper thickness of mount is necessary because thin mounts are difficult to handle while very thick mounts are difficult to hold flat during polishing. Mechanical mounting devices are used to prepare transverse or longitudinal sheet surfaces. For this purpose, specially designed metal clamps are used. Plastic mounting materials can also be effectively used. Some of the plastic materials require heat and pressure while others are cast able at room temperature. Mounting is completed with help of specimen mounting press. This is done in temperature range 135-170’c and pressure range 2500-4200 psi within 5 to 12 minutes.

4. Intermediate polishing:

After mounting, the specimen is polished on a series of emery papers containing successively finer abrasives. The first paper is usually 100, then 220, 320, 400, 500, 600 and finally 800. The intermediate polishing operations using emery

Papers are usually done dry; however in certain cases such as preparation of soft materials, silicon carbide abrasive may be used.

5. Fine polishing:

Fine polishing is done with single or double disc polishing machine. The time consumed and the success of the fine polishing depends largely upon the care that was exercised during the previous polishing steps.

Fig: Two disc polishing machine

The final approximation to flat scratch-free surface is obtained by use of a wet rotating wheel covered with a special cloth that is charged with carefully sized abrasive particles. The gamma form of aluminum oxide for ferrous and copper-based materials, cerium oxide for aluminum, magnesium and their alloys is used for fine polishing. Other final polishing abrasives often used are diamond paste, chromium oxide and magnesium oxide. The cloths used for polishing are silk, broad cloth, billiard cloth, canvas duck, velvet etc. A properly polished sample will show only the non-metallic inclusions and will be of scratch-free. 

6. Etching:

This is accomplished by use of appropriate reagent, which subjects the polished surface to chemical action. The purpose of etching is to make visible the many structural characteristics of the metal or alloy.

Fig: Etching solutions

Etching is carried out by immersion method. Swabbing is generally carried out for certain metals in which gas evolution is observed due to reaction with etchant. It consists of keeping swabbing the polished surface of the sample is thoroughly raised under water and dried with hot air drier. In immersion method, the sample is dipped in etchant for specified time. Then is washed and dried. Etching is normally continued and the mirror finish of sample becomes dull.

In alloys composed of two or more phases, the components are revealed during etching by a preferential attack of one or more of these constituents by the reagent, because of difference in chemical composition of the phases. In uniform single- phase alloys or pure metals, contrast is obtained and grain boundaries are made visible because of difference in the rate at which various grains are attacked by the reagent. This difference in the rate of attack is mainly associated with the angle of different grain sections to the plane of the polishing surface. Because of the chemical attack by the etching reagent, the grain boundaries will appear as valley in the polishing surface. Light from the microscope hitting the side of these valleys will be reflected out of the microscope, making the grain boundaries appear as dark lines.

 

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