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Metal Casting Technologies : September 2006
91 Back to the VITREOUS OR PORCELAIN ENAMELS itreous or porcelain enamels have been used as surface finishes for gray iron for over a hundred years. The excellent adherence of enamels to a sandblasted gray iron surface has been an important factor in their popularity. Enamels differ from organic coatings in that they are completely composed of inorganic material, resembling glass in composition, and must be fused onto the casting at high temperatures. One of the most striking and useful features of vitreous enamel, and one which has led to its wide use on gray iron castings, is its ability to confer almost complete chemical and corrosion resistance. Acid resistant enamels are used which can resist all common acids except hydrofluoric. The resistance to alkalis, though not quite as great, is sufficient for many applications. Of almost equal importance is the extreme hardness and scratch-resistance possessed by the vitreous enamel coating, permitting the use of enameled castings in positions where abrasion would quickly destroy an organic coating. Vitreous enamels possess the further advantage that they are highly attractive finishes ranging from a very high gloss to matte and can be easily produced in a wide variety of colors. Because of their refractory base, they can be used at high temperatures (315°-538°C or 600°-1,000° F) without discoloring or heat cracking, and can successfully withstand rapid changes in temperature without damage. Vitreous enamels consist essentially of: (1) refractories (silica, feldspar); (2 fluxes (borax, soda ash, cryolite, fluorspar, sodium nitrate, zinc oxide, and lithium compounds); and (3) opacifiers (oxide of titanium, antimony, zirconium, and tin) with, if desired, pigments (usually colored oxides of cobalt, iron, and the like). They can be applied by either the wet or the dry process. The wet process involves suspending the basic enamel (or frit) in a solution of electrolytes in water or an organic solvent, and then either dipping or spraying the enamel onto the casting. This process can be used on any size casting, but is usually applied to medium weight and small castings. Although, as noted later, some one-coat enameling is done, it is more usual to apply three coats. The coats are generally thinner and more opaque than in the dry process. Because they are dipped or sprayed on, the process is more flexible than the dry process, particularly where deep pockets, etc. are encountered. Firing temperatures rarely exceed 787°C (1450°F). The dry process consists of dusting dry, finely divided enamel on a previously dipped or spayed ground coat of a casting heated to around 870°C (1600°F), the casting then being returned to the firing furnace. The dry process is generally preferred for large castings, such as bathtubs. In the case of multiple coats, economy in heating can be assured in the dry process because the heavy part can be maintained at an elevated temperature between coats; furthermore, if blistering occurs, the blister can be broken and dusted again with enamel. Firing temperatures are higher (up to 925°C or 1700°F) to assure flowing of the dry enamels. In general, the enamel thickness approximates 0.04-inch and is materially greater than in the wet process. The final dry enamel surface possesses a higher gloss than when the wet process is used. If a colored enamel is to be applied mainly for decorative purposes and a finish of maximum resistance is not needed, a single coat may be used. Such enamels are generally of a dark color and furnish appreciable protection. For the best work, however, two or more coats of enamel are necessary. On cast iron, the main functions of the first or ground-coat are: (1) to prevent chemical reaction between the casting and later cover-coats, (2) to prevent oxidation of the gray iron at high temperature, (3) to seal up or smooth over surface irregularities, and (4) to promote adhesion between the enamel and the base metal. This last function is, perhaps, not as important in the case of gray iron as it is with steel because, as noted previously, a sandblasted gray iron casting gives an ideal mechanical bond so that the manganese and nickel oxides usually added to provide a chemical bond are apparently not essential. The choice of the proper ground-coat depends on a large number of factors. In general, the ground-coat must have good adhesion and be able to withstand repeated firing without failure. Furthermore, if the part is to have high resistance to thermal shock (for example, skillets, casseroles, and stove parts) or must resist chipping on small radii, ground-coats and cover-coats of a high coefficient of expansion should be chosen. The cover or top coat, in most cases, determines the characteristics of the enameled finish. The choice of the cover- coat is, therefore, complicated by the large number of factors which must be taken into consideration: what color is desired, whether a gloss or matte finish is wanted, and what degree of V J. Hermes D. Bautista -- Technical Consultant, PMAI Enameling of Gray Irons Porcelain, (US), n -- A fine translucent ware of superior whiteness, hardness, and sonority; china. adj -- Like china. Vitreous, (Br), adj -- Pertaining to or derived from glass; like glass, as in color, brittleness, luster, etc. WHO'S WHO OF METALS -- ANNUAL 2006/7