Ferrite Growth During Cooling Through the Ferrite-Austenite-Graphite Field in SGI
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SPHEROIDAL graphite cast iron (SGI) refers to an alloy mainly composed of Fe, C and Si, in which graphite adopts a spheroidal shape. At room temperature, its microstructure consists of graphite nodules dispersed in a metallic matrix. Figure 1 shows a schematic binary cut of the Fe-C-Si system for a constant Si content. According to the stable equilibrium, the metal matrix should be formed by ferrite. Nevertheless, in most cases, even when relatively slow cooling conditions from the austenitic state prevail, the conditions to transform the austenite according to the stable equilibrium cannot be fulfilled. As cast irons are cooled down from the austenite-graphite two-phase field into the ferrite-austenite-graphite field, stable equilibrium dictates that ferrite must form and most of the C dissolved in the austenite must precipitate at the existing graphite spheroids. To complete this transformation, ferrite should nucleate and grow below the upper limit of the three-phase field (Ta ), until all the austenite ideally disappears as the lower limit of the three-phase LAURA NOEL GARCI´A and FERNANDO DIEGO CARAZO are with CONICET, Godoy Cruz 2290, C1425FQB Buenos Aires, Argentina and also with the Instituto de Meca´nica Aplicada, Universidad Nacional de San Juan, Av. Libertador Gral. San Martı´ n 1109 (Oeste), J5400 San Juan, Argentina. Contact e-mail: [email protected] ROBERTO ENRIQUE BOERI is with CONICET and also with the Instituto de Investigaciones en Ciencia y Tecnologı´ a de Materiales, Universidad Nacional de Mar del PlataCONICET, B7608FDQ Mar del Plata, Argentina. Manuscript submitted July 3, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
field (Ta ) is reached. However, depending on the chemical composition of the cast iron, the cooling rate and the dispersion of graphite, the remaining austenite often becomes enough undercooled to transform according to the metastable diagram, leading to the precipitation of pearlite. Ferrite first precipitates at the graphite/austenite interface, often enveloping the nodules into a nearly spherical halo. This gives rise to the microstructure shown in Figure 2, where ferrite is usually referred to as precipitated in bull’s eye morphology. The mechanism governing the precipitation of ferrite and pearlite in SGI is currently under discussion.[1–6] Different industrial applications call for SGI of ferritic, pearlitic or ferritic-pearlitic matrices. Fully pearlitic matrices are often reached through the use of alloying elements such as Mn and Cu. Both, together with Si, redistribute between solid and liquid during solidification; Si and Cu—graphitizing elements—tend to segregate toward the first-to-freeze (FTF) zones; Mn—a carbide-stabilizing element—tends to concentrate in the residual liquid, i.e., in the last-to-freeze (LTF) zones.[7] Under continuous cooling, no diffusion of the substitutional elements is expected during the eutectoid transformation, as recently reaffirmed by Freulon et al.[8] and Garcı´ a et al.[9] As a result, the final structure inherits the segregatio
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