A new unidirectional solidification method to study gray cast iron
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I. INTRODUCTION FLAKE gray-cast iron has been extensively studied by unidirectional solidification (US) (References 1, 2, etc.). Much lesser works were performed using US techniques on nodular and vermicular graphite varieties.[3,4,5] In none of them, microstructures showing oriented vermicular graphite were obtained. In two of them,[3,5] equiaxial nucleation does not seem to be completely avoided. So, the different microstructures observed at different times only reveal the fading effect of the modifiers but not really an evolution of directionally solidified microstructure during the morphological modification process. The most widely used US technique is the Bridgman type, schematically shown in Figure 1. In this technique, the thermal gradient (G) and the solidification rate (R) are externally controlled. However, it is restricted to the use of a small inner diameter (1 to 2 mm) and very thin, wallwidth crucibles. Another used US technique is known as “directional casting,” schematically illustrated in Figure 2. In this case, the only magnitude externally controlled is the direction of the heat flux. The G and the R vary during the process. But, no restrictions exist in relation to shape and size of the crucibles. To avoid the equiaxial nucleation in front of the solid/ liquid (S/L) interface, it is necessary to control the imposed G, as the Bridgman technique permits. But, in order to change the chemistry of the melt to modify the graphite morphology during the solidification process, larger dimensions of the crucible are necessary, as in the case of the directional casting technique. So, in order to obtain US gray-cast iron with modified graphite and maintain the growth parameter, G/R, a new method that combines both features must be developed. In addition, the following aims were also searched for: (1) constant S/L interface position with respect to a laboratory frame, A.N. ROVIGLIONE, Professor, is with the Departamento de Ingenierı´a Meca´nica y Naval, Facultad de Ingenierı´a, Universidad de Buenos Aires (UBA), Paseo Col n 850, CP:1063 Buenos Aires, Argentina. Contact email: [email protected] J.D. HERMIDA, Head, is with the Departamento de Materiales, Centro Ato´mico Constituyentes, Comisio´n Nacional de Energia Ato´mica, Av. General Paz 1499, CP:1650 San Martı´n, Argentina. Manuscript submitted May 18, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
(2) temperature measurement of the melt in front of the S/L interface, (3) quenching facility of the melt in front of the S/L interface, and (4) protective atmosphere to avoid the melt oxidation. II. EQUIPMENT SET DESCRIPTION In Figure 3, the equipment set is schematically shown. The parts of the equipment set will now be described in detail. (1) Induction furnace mark Radyne 10 kHz radio frequency tube generated. The power transference is accomplished by magnetic coupling of two coaxial coils. One of them stays put, while the other, of smaller section, is mechanically shifted along the axis to produce the variation of the concatenated electromagnetic flux. A
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