Lab 2- Sample Preparation PDF

AIN SHAMS UNIVERSITY FACULTY OF ENGINEERING Structure and Properties of Materials MDP 151 Lab 2- Metallographic Examination OBJECTIVE 1- Learn to prepare a specimen for metallographic examination...

AIN SHAMS UNIVERSITY FACULTY OF ENGINEERING Structure and Properties of Materials MDP 151 Lab 2- Metallographic Examination OBJECTIVE 1- Learn to prepare a specimen for metallographic examination. 2- Learn to use an optical microscope. 3- Learn to identify structure features. 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. Macro investigation Is carried out with naked eye or at magnification up to 5 in which the grain shape and size, the cavities, non- metallic inclusions such as slag and oxides are observed. Micro investigation At higher magnifications, other details of the structure like the different phases, their distributions, micro- segregation…. etc. can be observed. 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 1 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. Low speed cutting saw 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 2  to 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 (as shown), 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. 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  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 1 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. 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 2. 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. 4  Figure 2 Figure 1 Figure 3 shows photomicrographs of a specimen after two stages of grinding. Figure 3 5  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 4) 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 4 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 6 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 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. Figure 5 shows the effect of etching the polished surface of a metal on the microstructure observed in the optical microscope. 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. 7 Equipment: Metallurgical microscope - emery papers of different grade (180, 320, 400, and 600) Polishing wheel - polishing power or paste-alcohol – suitable 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. Answer will be in Tutorial session 8 Example: Austenitic stainless steel, colour etched Porosity in an Al-10%Cu alloy A color mixture of austenite, ferrite and Austenite phases are present as white island in sigma phases in duplex stainless steel blue sea of ferrite in duplex stainless steel Al – SiC composite, as polished Gray iron with lamellar graphite, etched in Nital. 9 Types of Cast Iron Gray iron graphite flakes weak & brittle under tension stronger under compression excellent vibrational dampening wear resistant Ductile iron add Mg or Ce graphite in nodules not flakes matrix often pearlite - better ductility Malleable iron heat treat at 800-900ºC graphite in rosettes more ductile Your results 10 11