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Metal Casting Technologies : December 2006
Back to the QUALITY CONTROL OPERATING TIPS The author has found the following practical tips useful during his years of foundry practice. You may, too. CAST IRON FOUNDRY PRACTICE Chill test for cast irons The chill test is a practical method of determining the properties of molten metal in lieu of metal testing instruments. The cooling speed and chemical composition are used to determine how fast cast iron converts to white iron. As the chill test is fairly reliable, it can be adopted as an alternative to testing instruments. Sprue-surface pattern on cast irons Patterns of solidification on the sprue-surface can frequently be observed and noted to determine the approximate properties of the poured metal, although these cannot be verified numerically. The pattern generally appears at a temperature of about 1,350°C. When melting goes wrong, however, the patterns do not appear because the molten surface is covered by an oxide film. This film does not appear when the temperature is high enough, resulting in a mirror-like surface on the top surface of the sprue. The surface patterns begin to appear with a drop in the temperature. A tortoise-shell pattern indicates a relatively high carbon content; while a bamboo- leaves pattern indicates a relatively low carbon content. Correlation of tests with lab results At the beginning, the various tests described should be correlated with laboratory determinations of chemical compositions a sort of calibration of the test methods. Results of the "calibration" should be plotted. After a while, the tests alone can be read with some accuracy without the need for test instruments. These tests are suitable for cast irons only. STEEL FOUNDRY PRACTICE Deoxidation of steel melts The deoxidation of steel refers, strictly speaking, to the removal of oxygen from the steel. Oxygen is not removed from the steel, however, but is merely fixed in a less active condition. The term "deoxidation" is used in a broader sense and includes all the phenomena in the final preparation of steel that would permit it to solidify free from gas cavities and from dissolved oxygen of all kinds. Silicon is apparently a good deoxidizer and, if present in sufficient quantities, it is quite adequate to prevent the formation of CO. For a medium-carbon steel, the minimum silicon content that will suppress the reaction to form CO gas is about 0.25 percent. For safety, that figure is usually given a margin of ten points, or a total content of 0.35 percent silicon. All silicon additions up to this value rapidly lower the FeO content, but beyond the value the returns are small. Manganese is considered a deoxidizer; but if dependence Fig. 3. Block Chill Test Piece and its measurements CHILL TEST FOR CAST IRON (MEDIUM) The figure shows a block chill test piece for the forced chill test and its measurement. This test is used for cast iron with relatively high carbon content to be determine the effects of forced cooling. The test is conducted by pouring molten metal into the piece which is in contact with a chill block. Classification A B H L G Code (mm) (mm) (mm) (mm) (mm) 1C 6.5 3.5 40 52 0.8 2C 8.0 5.0 40 52 0.8 3C 11.0 8.0 40 52 1.0 4C 13.0 10.0 40 52 1.0 Fig. 2. Wedge Test Piece and Its Measurement. (Nihon Imono Kyokai method) CHILL TEST FOR CAST IRON (HARD) The figure shows a wedge test piece where the width of the chilled portion is measured to determine the properties of the molten metal Classification Code B(mm) H(m) L(mm) W1 6 25 100 W2 12 32 100 W3 20 38 100 W4 32 50 150 Tolerance ±0.8 ±32 Fig. 4. Standard Metal Cylinder Mold and head-shape per CE of iron CHILL TEST FOR CAST IRON (SOFT) The figure shows a metal mold test method which involves pouring the molten metal into a metal cylinder mold and observing the volume expansion phenomenon in the head due to the precipitation of the graphite which results in different head configurations of the cooled metal in the mold. The specific head shape obtained is then classified according to the chart of that figure, and the chill depth is measured by breaking the cooled metal into two pieces and looking at the fracture. Classification 1 2 3 4 5 6 Code Head Shape METAL Casting Technologies December 2006 50