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Metal Casting Technologies : June 2006
METAL Casting Technologies June 2006 41 even when castings are cooled in sand molds. In the latter case, the transformation of the eutectoidal austenite forms pearlite. The iron is not completely graphitized and about 0.60% combined carbon remains. Rapid solid-state cooling and the presence of carbide-forming elements can cause a substantially greater percentage of combined carbon to be retained. Fine graphite flake size, regardless of type, promotes solid- state graphitization with the flakes serving as precipitation centers for the carbon. Type D cellular graphite is often intermixed with ferrite because of the ease with which graphitization may occur. Gray Iron Type for Cooking Utensils and Grates.3 The recommended gray iron type for these uses is Class 20 that is a soft gray iron, commonly melted in the cupola. It has very good fluidity and castability and is very easily machinable. It is particularly suited in medium- to thin-sectioned castings, and generally advisable where only moderate strength is required and it is desirable to avoid high casting stresses even in complicated sections. Recommended Target Chemical Composition:3 TCGCCCSiMnPSAlloysCE 3.40 3.00 0.40 2.40 0.50 <0.60 <0.10 None 4.30 Recommended Casting Practice:4 Avoid alloys in the charge, especially carbide formers Cr, V, Mo, >0.60% Mn, >0.25% Ti, and >0.50% Al. However, <2.00% Cu or Ni, inherent in the charge, may be tolerated. Use inoculant during tapping into ladle. Use very slow cooling in the mold before shakeout. The length of time used in cooling in the mold might be determined by trial-and-error. The target is about 0.40% combined carbon. STRESS RELIEVING OF IRON CASTINGS Residual Stresses5 Residual stresses are present in all castings in the as-cast condition and these are caused by: 1. Non-uniform cooling of the surface of the casting in comparison with the center of any given cross-section. 2. The difference in cooling rates between sections of the same casting, because of different cross-sections or locations in the mold. 3. Resistance of the mold to contraction of the casting during cooling. Castings having a variety of section thicknesses or very complex designs can have especially high residual stresses. Effect of Shakeout Practice The shakeout practice used by the foundry may be influential in establishing the pattern of residual stresses in the castings, since the basic cause of most residual stresses is the different cooling rates of light and heavy sections, which cause the casting to contract differently in these sections. This differential contraction may cause the casting to rupture in the mold ("hot crack") or, if the iron has adequate ductility at high temperature, plastic flow nay occur without hot cracking. However, a casting that has undergone localized plastic flow because of the temperature variation, is likely to have some residual stresses when it reaches room temperature. Areas that were hottest in the mold will generally be too short in relation to those that were colder, and the casting may be distorted or left with high residual stress. It is not always possible to help this condition by a long delay before dumping the mold, though this is beneficial in many instances. Each casting must be analyzed to determine the influence of different practices on decreasing the temperature difference between sections. In addition to its effect on residual stress, shakeout practice may influence the microstructure and hardness of the gray iron castings. If the iron is austenitic at the time the mold is dumped, higher hardness may result. Many types of microstructure may be obtained, since the austenite in thin sections may transform in the mold, whereas in heavier sections that cool more slowly the transformation may be delayed until the air cooling after shakeout. In general, the effect of shakeout practice on hardness is negligible in unalloyed irons. Relieving Residual Stresses6 The heat treatment that relieves residual stresses is commonly called "aging", "normalizing" or "mild annealing"; the term "stress relieving" is more accurately descriptive. This treatment can be accomplished by heating the cast iron to between 426º and 593º C (800º and 1100º F), holding at temperature from 30 minutes to 5 hours, and cooling slowly in the furnace. This treatment will cause only a slight decrease in hardness, very little decomposition of the cementite, and only a slight change in the strength of the metal at room temperature. The amount of stress relief in gray irons1 below 400º C (750º F) is small, but increases rapidly at higher temperatures. It appears that if substantial stress relief is necessary, the temperature of at least 510º C (950º F) is advisable.* In rare cases where almost complete stress relief (over 80%)