Laboratories la-cSpecimen Preparation and Use of the Optical MicroscopePolishing ProcedureDo's and Don'tsProceduresMatSE 471Laboratories la-cSpecimen Preparation and Use of the Optical MicroscopeObjectivesThe objectives in laboratories 1a-c are to prepare polished and etched samples for examination in the optical microscope and to record and print the images. This examination of polished and etched surfaces is called METALLOGRAPHY.BackgroundGeneralA number of methods are available for analyzing metals and alloy systems. These include thermalanalysis, dilatometry, x-ray analysis, electrical conductivity, microscopy, etc. Metallography is one of the most important tools in the field of physical metallurgy. It deals chiefly with the internal structure of the solid and the relation of this structure to the properties of the material and any problems (such as fracture)that may have been encountered in its service. Magnifications involved are usually in the range of 10 to 2000X; thus a microscope is employed, and the structures are usually called microstructures.Metallography is essentially an art, and various persons may have or develop their own individualtechniques, but the methods to be used here are of a general nature and will produce satisfactory results. Certain variations may be required for different materials. In general, the procedure for preparation of metallographic specimens consists of the following steps:1. Obtaining a convenient specimen for handling by sawing, cutting, mounting, etc. 2. Preparation of a flat surface by means of files, belt sanders, etc. 3. Sanding with a series of abrasive papers of increasing grit fineness 4. Polishing on rotating wheels covered with suitable surfaces and abrasives Considerable latitude exists as to specific preparation procedures. The objective of these four steps is to produce a scratch-free, undistorted mirror-like surface. Care and cleanliness cannot be overemphasized.5. Following preparation, the specimens must be etched with the proper reagent before examination under the microscope. Since the metallurgical microscope forms an image from light reflected from the surface of the specimen instead of light transmitted through the specimen, as in biological microscopes, examination of an as-polished surface will reveal nonmetallic particles, but show little detail otherwise. Etching is a selective attack which produces a variation in the reflectivity of the surface; thus, the structure can be observed.Microscopes and metallographs are quality instruments and must be used carefully and properly. Be sure you understand the details of operation of the various models available.Preparation of Samples for Metallographic StudySample Mount PreparationIn certain instances, it is necessary to embed the sample in a mounting medium. Two types of mount are used, hot and cold. Unless the sample is going to be etched in highly corrosive chemicals, it is easier to use cold mounting.For cold mounting; place the sample in a cold mount mold. Mix the cold mount powder with the.hardener in the ratio of 2 parts of powder to one part of solution (by volume). Pour the resultant paste into the mold and allow it to harden (30 minutes at room temperature). It can then be removed from the mold.Polishing ProcedureFirst, it is necessary to ensure that the specimen surface is accurately flat. This is accomplished bywet grinding on The Belt. This process also removes any dirt adhered to the surface. (Note: Belt grinding is not always required).The specimen is then ground on grinding papers. There are four types of papers: 240 grit, 320 grit, 400 grit, and 600 grit. 240 grit is used for rough grinding, the other papers for successively finer grindingwith 600 grit paper being the finest grit. Polishing is performed using uniform long strokes up & down thelength of the paper. Wash hands & specimen between papers. Rotate specimen 90° before grinding on thenext finer paper.Final polishing is performed on rotating wheels. Here again, there are coarse and fine stages. In the coarse stage, 1.0 m and 0.3 m alumina powders are used; in the fine stage, 0.05 m alumina powder is used. The wheels and the polishing cloths on the wheels must be cleaned thoroughly in water prior to use. Then, the appropriate powder is mixed in water in the ratio of one large teaspoon of powders to 200 cc of water. Some metallographers prefer a thicker slurry. This solution is sprinkled onto the cloth and used for polishing the samples.Etching is the final step in the preparation of a specimen. The specimen is etched either by swabbing the surface with the etchant or by immersing the specimen in the etchant. This is decided on the basis of the etchant used. The surface of the specimen is then rinsed thoroughly with an appropriate solvent (water, alcohol, sometimes both) and dried. In etching steels, it is usually best to avoid rinsing with water.The length of time the etchant is in contact with the etchant is decided by trial and error.Do's and Don'ts1. Make sure the water is on when using grinding belts.2. The belt has to be adjusted to be in the center. 3. Round off all sides of the sample, since a sharp edge can tear the belt or even cut your hand.4. Clean grinding paper before use; always turn water on before grinding.5. If the paper needs replacing, do it. But clean the surface of the block before replacing paper.6. Water is essential while grinding. Otherwise, the specimen might leave particles that will scratch the surface in subsequent polishing.7. Wash specimen and hands between grinding papers.8. Clean the polishing cloth well before using it.9. Use the correct type of powder at each stage of wheel polishing.10. Never allow any two polishing powders to mix.11. Clean hands and sample after every stage of polishing.12. KEEP WORK AREA CLEAN!!!!!!!13. Slowly rotate samples clockwise on the disks while polishing.14. After polishing, flood the cloth with water to remove polishing powder and debris.15. Dispose of the etchants after use as directed by the Teaching Assistant.16. Turn the water on while discarding used etchants, if they can be flushed down the sink.17. If you make up an etchant and store it in a bottle, write the name of etchant, your name, and date made up on the bottle.Examination of Optical SamplesPrior to etching, it is always worthwhile to examine the polished surface both by eye and on the light microscope. The former will often permit the detection