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Metal Casting Technologies : September 2006
92 Back to the resistance to acids, alkalis, weathering, hot water, soil corrosion, scratching and abrasion, thermal shock or high temperature is required. For simplicity, this discussion will cover only the white enamels which have found a more widespread use than any other type. Two kinds of white enamels have been widely applied; these are classified according to the oxide which is used as the opacifier, as antimony, zirconia, or antimony-zirconia formulations. The antimony enamels, the oldest of the three types, can be made acid resistant; but do not easily produce a good finish. Zirconia enamels, while having poorer resistance to acids, are highest of all in alkali resistance and higher than antimony enamels in resistance to thermal shock. Zirconia enamels also provide a better finish and, since they have greater opacity, or hiding power, can be applied in thinner coats. To some degree the advantages of both types can be obtained by formulation. Recently, titania opacified enamels have aroused interest because of their resistance qualities, ability to be applied in thin coats, high scratch resistance and thermal shock resistance. Their application to gray iron using the dry process may be somewhat delayed due to their higher melting point as compared to conventional enamels for gray iron. This same limitation does not exist in the case of the wet process and widespread interest is being shown in developing titania enamels specially suited to gray iron castings. Gray irons in a wide range of composition can be enameled. Good foundry practice and a surface free from pinholes are as important as analysis. In general, it is advisable to keep the combined carbon fairly low (approximately 0.40%) and avoid alloys that affect carbide stabilization. (TC = 2.80-3.40% as per "Batman" specifications.) Sound castings that can be sandblasted to a fairly smooth surface are naturally required. Design of castings intended for enameling is important. Care should be taken to avoid castings with uneven section thickness, as these will not heat up uniformly during firing and thus may cause blistering of the enamel. Corners, both inside and out, should be rounded and the radii of fillets should be generous. Either sandblasting or blasting with a mixture of sand and steel grit is preferable to shot-blasting or grit-blasting alone. If the casting has been coated with oil or grease, has been stored in a damp place and thus adsorbed moisture, or contains chilled spots or chilled skin on certain surfaces, it should be annealed for several minutes at 700°C (1300°F) in the enameling furnace. In fact, some enamelers regularly anneal at 650°C (1200°F) to degrease and stress relieve before sandblasting. Gray iron castings should be enameled immediately after sandblasting. Any large surface cavities or imperfections that may exist should be filled with an enamel paste similar in composition to the ground-coat enamel to be used on the rest of the casting. The first or ground-coat should be thin, porous, and uniformly distributed; this is followed by a thicker coat. As noted above, this ground-coat is applied by spaying or dipping, and later cover-coats by spraying, dipping, or dusting. If the wet process is used, all sprayed or dipped enamel coats must be dried; that is, the water that held the enamel in suspension during application must be evaporated off. This is accomplished in a drier that evaporated the water in such a manner and at such a rate as to avoid tearing of the dried coat. This step is, of course, not necessary in the dry process after initial firing of the ground-coat. After removing the enamel by brushing from the local sections of the casting that are to be machined or otherwise do not require a coating, the remaining enamel is burned or fused onto the base metal. Gray iron enamels are usually fired at from 650-870°C (1200-1600°F) depending on the type of enamel and whether a wet or dry process is used. Careful control is necessary to avoid over-burning or under-burning; almost all enamels will blister at the beginning of the firing; but, if they are not heated too rapidly, such blisters will heal over. Coatings with very unequal section thicknesses sometimes have to be removed from the furnace during the firing to allow thinner sections to cool down so that they will not be over-burned. Decorative patterns may be either applied over or made a part of the fused enamel. Possibilities of improving sales appeal through production of artistic effects are only beginning to be effectively exploited. Vitreous enameled castings, although firmly entrenched in applications such as sanitary ware, have not received recognition commensurate with their high resistance to service hazards. The advantage of casting starts in the enameling furnace where their design is such as to make them much less subject to warpage and distortion than wrought, sheet, or welded assemblies. The final enameled casting, because of its resistance to flexure and its high vibration absorption, is also much more resistant to disastrous flaking of enamel than thinner wrought steel parts. This is emphasized if a heavy object is accidentally dropped on a gray iron bathtub or basin. In such a case, damage is normally limited to a nick in the enamel of the casting as compared to removal of a sizable flake of a wrought enameled object. Gray iron castings finished by vitreous enameling have found wide use in industry and in the home. Perhaps the most familiar examples are gray iron bathtubs, basins, sinks, stove parts, and hospital wear where resistance and sanitation requirements are at a maximum. Many applications in the chemical industry have been found, including rolls, piping, processing and containing vessels of all sorts (popularly called "glass-lined") and miscellaneous equipment. Development of new enamels and improved enameling procedure should expand the potential usefulness of coated gray irons to industry. © Charles F. Walton, ed., Gray Iron Castings Handbook, (Cleveland: Gray Iron Founders' Society, 1958) www.metals.rala.com.au