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Metal Casting Technologies : June 2006
40 METAL Casting Technologies June 2006 Back to the Compiled and Edited by John Hermes D. Bautista, Technical Consultant, PMAI Gray Cast Irons for Enameling ENAMELED GRAY CAST IRONS1 itreous enameled castings, especially those used for sanitary wares, have not received recognition commensurate with their high resistance to service hazards. The advantage of iron castings starts in the enameling furnace where their design is such as to make them much less subject to warpage and distortion than wrought steel sheets or welded assemblies. The final enameled casting, because of its resistance to flexure and its high damping capacity, is also much more resistant to disastrous flaking of the enamel than thinner wrought steel parts. Many applications in the chemical industry have been found, including rolls, piping, processing and containing vessels of all sorts. Development of new enamels and improved enameling procedure should expand the potential usefulness of enameled gray irons to industry. Gray irons in a wide range of composition can be enameled. Good foundry practice and a surface free from pinholes is as important as analysis. In general, it is advisable to keep the combined carbon fairly low (about 0.40%) and avoid alloys that affect carbide stabilization. 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. THE FREEZING MECHANISM2 Solidification of Gray Cast Iron. The presence of silicon in the alloy is the most important single composition factor promoting graphitization in gray cast irons. The effect of silicon may be visualized with the aid of the vertical sections of the ternary alloy system of Fe-C-Si. Consider the freezing processes for an Fe-C-Si alloy with 2.00% Si and about 3.50% C. Under equilibrium freezing conditions primary austenite dendrites are formed in the temperature range from the liquidus curve to the curve indicating the beginning of eutectic freezing; about 1260° to 1125° C (2300° to 2060° F). Solidification of the eutectic austenite plus graphite completes the freezing process. The eutectic freezing occurs in a temperature range of about 1125° to about 1100° C (2060° - 2010° F). When the solidification is complete in the alloy under consideration, the microstructure consists of about 20% primary austenite dendrites and 80% austenite-graphite eutectic. At the solidus temperature, austenite is saturated with carbon. Further decrease in temperature is accompanied by rejection of carbon from the austenite as graphite and its precipitation on the graphite flakes in the eutectic. Carbon precipitation continues until the eutectoid temperature range is reached at about 840° to 720° C (about 1550° to 1333° F), depending on the silicon content. At the eutectoid temperature, the 2.00% Si austenite contains about 0.60% C. Equilibrium cooling through the range results in the transformation of austenite to ferrite and precipitation of the remaining carbon on the graphite flakes. The final microstructure then consists of isolated areas of ferrite originating in the primary austenite dendrites and other areas of mixed ferrite and flake graphite having their origin in the austenite-graphite eutectic. The microstructural changes described above are those occurring in a ternary alloy of Fe-C-Si. Similar processes in commercial cast irons are much more complex since many other elements are present and a number of other factors are introduced. However, the simple alloy considered does point out the three important phases of graphitization, which are: 1. Graphitization during solidification. 2. Graphitization by carbon precipitation from austenite (solid state). 3. Graphitization during the eutectoid transformation (solid state). Some graphitization also occurs below the transformation range down to about 540° C (1000° F), although this is of lesser importance unless the time spent at that temperature is very long. These phases of graphitization and their effects on microstructure and properties should be clearly understood for the proper control of combined carbon in gray cast irons. Graphitization in the Solid-State. At the end of the freezing process, a gray iron of 3.60% C and 2.10% Si will contain about 2.0% graphitic carbon and 1.50% of carbon dissolved in austenite. Slow cooling, as, for example, in a sand mold, permits the carbon to be rejected from the austenite as graphite as temperature drops to the eutectoid temperature of about 785° C (1450° F). This carbon rejection is a process of solid-state graphitization and proceeds until about 0.60% C remains in the austenite. Very slow cooling through the eutectoid transformation range permits a large portion of the 0.60% C that remains in the austenite to be rejected as graphite and is accompanied by austenite transformation to ferrite. The iron is then completely graphitized, as discussed in the previous section. However, commercial practice generally is aimed at retaining a pearlitic structure or some proportion of pearlite. The proper balance of manganese and sulfur assists in retaining pearlite V