About Equilibrium Mode Ruling Ferritic Transformation in Low-Alloy SGI
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Ferrite precipitating around the graphite nodules shaping the typical bull’s-eye microstructure could grow under negligible partitioning local equilibrium or under paraequilibrium conditions, as both imply that ferrite inherits the composition of the parent austenite. The first mechanism has been rejected by other researchers by means of simple calculations of the silicon spike width necessary for local equilibrium conditions to take place. Nevertheless, experimental analyses are necessary to verify this conclusion. In this study, transmission electron microscopy has been used to assess the presence of a silicon spike in front of the growing ferrite interface. The outcome allowed the authors to confirm that a paraequilibrium mode governs the transformation, supporting the conclusions of previous calculations. In addition, some issues about ferrite growth modeling are discussed. https://doi.org/10.1007/s11661-019-05485-6 Ó The Minerals, Metals & Materials Society and ASM International 2019
I.
INTRODUCTION
SPHEROIDAL graphite cast iron (SGI) is a Fe-C-Si alloy in which free graphite precipitates in spheroidal or nodular shape. Under regular cooling conditions, its metal matrix microstructure consists of graphite nodules enveloped by ferrite halos and variable amounts of pearlite. This microstructure, known as bull’s-eye ferrite (Figure 1), results from the solid-state transformations experimented by the austenite during its cooling at rates greater than 1.2 °C/min.[1,2] The growth of ferrite is driven by C diffusion from austenite to graphite through the ferrite halo. At these cooling rates, no diffusion of substitutional elements has been reported to take place during austenite transformation[1] and the final microstructure, either ferrite or pearlite, inherits the microsegregation profiles developed during solidification. Lacaze et al.[1] described the ferrite precipitation by means of a stable Fe-C-Si isopleth section calculated for a nominal content of Si, as shown in Figure 2.
LAURA NOEL GARCI´A and FERNANDO DIEGO CARAZO are with the 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 the 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 19, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
According to Lacaze et al.[1] once the temperature of the alloy drops below the upper limit of the ferrite/ austenite/graphite (a + c + G) field Toa , ferrite nucleates at the G/c interface but cannot grow. Later on, it grows as spherical shells by diffusion of C from the austenite to the graphite through the ferrite halo. Consequently, ferrite growth can proceed only at temperatures below the lower limit of the three-phase field Ta , when the C
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