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Metal Casting Technologies : December 2006
is placed upon manganese alone, it would be impossible to obtain a killed steel. Manganese has a higher affinity for oxygen than iron, but in amounts normally present in cast steels it cannot lower the FeO content sufficiently to prevent reaction with carbon during freezing. The theoretical amount of iron oxide in equilibrium with 0.50 percent manganese at 1,590°C is about 0.2 percent. Aluminum is the strongest of the common deoxidizers. A slight excess of aluminum will reduce the FeO content to 0.002 percent. A greater excess will lower the FeO to less than 0.001 percent. With the FeO at such a low value, a wide margin of safety is gained against possible bubble formation. Aluminum in quantities of 0.05 percent (1/2kg. per tonne of steel) is usually sufficient to prevent pinhole formation; however, larger quantities of aluminum are often used to prevent the occurrence of low-ductility properties. It has been reported that aluminum up to 0.10-0.15% gave adequate protection against pinhole porosity if a vigorous boil is used in the melting procedure. It is the generally accepted practice in industry to make aluminum additions of 1 kg. per tonne of steel melt. Note: The quantities of aluminum given are residual quantities in the steel. Residual quantities of 0.05 percent aluminum and 0.10-0.15 percent aluminum have beneficial effects; but residual quantities of between 0.05% and 0.10% have a deleterious effect on the mechanical properties of the steel. Since aluminum has a grain refining action, the amount to be added could be determined by experimentation on the foundry's melting practice; break a sample and look at the fracture's grain fineness. Measurement of tapping temperature Spoon Test Method. The spoon test method can be used in lieu of an optical pyrometer. Molten metal is taken from the furnace in a slag-coated spoon, and the time is recorded in seconds (mentally count one-thousand-one, one-thousand- two, one-thousand-three, etc., each is equivalent to one second) that it takes the steel to film over. The number of seconds required for any particular heat are correlated with the number of molds to be poured. Also, the number of seconds can be correlated with actual pyrometer measurements of the heat. When thus calibrated, there would be less need for the pyrometer later. Steel Bar Method. Plunge a 5/8" steel bar into the bath, leave it there for seven seconds, then withdraw it quickly. If there is a clean cut, the steel is ready for tapping. If the end of the bar is tapered, the metal temperature is still too low. At first this method should be correlated with pyrometer readings for calibration. Then, there would be less need for the pyrometer. Porisity test A test bar of about 15 mm in diameter and about 150 mm in length is cast vertically in a green sand mold with a sample of steel from the furnace. If the test solidifies with a definite shrink, the bar is broken and the fracture examined for pinhole porosity. If there is a tendency for the metal to rise in the mold, additions of ferro-silicon and sometimes ferro-manganese are made. BRONZE FOUNDRY PRACTICE Measuring temperature of a bronze melt Color Scale for Temperatures. Molten bronze has a range of temperature in the crucible of 1000-1250°C. The following color scale permits a rough approximation of such high temperatures: COLOR OF STEEL TEMPERATURE 0C Lowest visible red to dark red 475 to 650 Dark red to cherry red 650 to 750 Cherry red to bright cherry red 750 to 815 Bright cherry red to orange 815 to 900 Orange to yellow 900 to 1090 Yellow to light yellow 1090 to 1315 Light yellow to white 1315 to 1540 White to dazzling white 1540 to higher The entries in bold italics are the ranges that can be used in determining molten bronze metal temperatures. This can be done by plunging a 1/2-inch steel bar into the melt, holding it there for about five to seven seconds, then looking at the color of the steel bar after withdrawal. Correlate this with pyrometer readings. Standardize the practice and, thus calibrated, there would thence be less need for the pyrometer. Melting Manganese bronze and determining tapping temperature In melting manganese bronze, quick melting is desirable with the furnace atmosphere being oxidizing (a slightly yellow flame). Pack the crucible as much as possible and place a furnace cover. If more metal is needed, add this as soon as some of the charge has melted and cover the furnace as before. No flux or charcoal is needed. When the metal starts to fume, shut down the burner and skim the metal well. Hold until, on passing the skimmer gently over the surface of the metal, only a slight incipient flare of zinc oxide is noted. Then, and not until then, tap and pour the metal into the molds in a manner to insure as quiet an entry as possible, taking care that the pouring spout is close to the sprue head. No deoxidizer is required. Melting manganese bronze does not require a pyrometer or any other method of temperature measurement; the molten bath itself has a built-in temperature gauge. ● METAL Casting Technologies December 2006 51