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Metal Casting Technologies : Dec 2009
44 www.metals.rala.com.au BacktoBASICS etallography is the science and art of preparing a metal sample by polishing and etching so as to reveal the microstructural constituents. After preparation, the sample can be analysed with the aid of a metallurgical microscope. Microscopic examination using the optical microscope is an invaluable tool for the study of cast microstructure and metallurgical defects. Most microstructural constituents can be observed and identified with the optical microscope based on their characteristics. Identification can be aided by the use of polarised light and the use of selective reagents. Accurate interpretation of what is observed under the microscope depends very much on specimen preparation. Sampling An ideal specimen size for microscopic examination is approximately 1cm2 of polishable area. The height should be a minimum that is convenient to hold, the greater the height to area ratio, the more difficult the specimen is to hold flat. Specimens for examination should be excised by cold sawing in order to avoid overheating damage. If the hardness of the material precludes cold sawing, then an abrasive wheel can be used, however, copious amounts of coolant and low cutting speeds should be employed to minimise specimen damage. Initial preparation The initial preparation of a cut surface may be by linishing, filing, milling or grinding in order to achieve a generally flat surface suitable for subsequently finer abrasive operations. If sampling has been performed by abrasive wheel or thermally cut, care should be taken to ensure the heat-affected zone has been completely removed. Mounting If the specimen is too small to be held by hand, or when edge retention is of importance, the specimen should be mounted prior to fine grinding. Figure 2. The most common mounting materials are cold set resins or thermosetting resin powders. Liquid cold set resins are mixed with a catalyst and poured into a mould containing the specimen. After the resin hardens, the mount is stripped from the mould. In the case of thermosetting resins, a heated mounting press is required to harden the mount. Abrasive dressing Abrasive dressing refers to the progressively improved surface finish obtained by abrasion on successively finer grades of waterproof silicon carbide paper. A suitable sequence of grit sizes is 100, 240, 600 and 1200 grit. The appropriate sheet of paper is placed on a flat surface such as a sheet of plate glass or motorised disc and the specimen carefully abraded against it with a steady flow of water for lubrication. Moderate pressure should be applied and the specimen abraded until all indications from the previous treatment have been removed. The specimen should be rinsed with water between steps to avoid contamination of the finer grit paper. The direction of abrading should be at 90o to the marks left by the previous paper. Basics of Metallography M J. F. Meredith, Casting Solutions Pty Ltd Fig 1. The metallurgical microscope Fig 2. Mounted specimen
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