An investigation was carried out to examine the influence of as-cast heterogeneities on the resulting microstructure and mechanical properties of an austempered ductile cast iron (ADI). A commercial hypereutectic ductile iron alloy with 3.59% C, 2.68% Si, 0.24% Mn and 0.46% Cu (in wt. %) was produced in a coreless induction furnace. The specimens were obtained by casting the alloy into Y-block molds. The austempering heat treatments consisted of pre-heating at 500°C, followed by austenitizing step at 900°C and 930°C during 60 minutes. Austempering was carried out in molten metal baths at temperatures of 300°C and 370°C for 30 minutes. Microstructural characterization was carried out by light optical microscopy (LOM) with image analysis, scanning electron microscopy (SEM-FEG) and X-ray diffraction with Rietveld refinement. The mechanical properties were evaluated by tensile and Vickers hardness tests. The results of this investigation indicate that as-cast heterogeneities have a detrimental effect on the transformed microstructure and mechanical properties of ADI. During austempering, non uniform distribution of graphite nodules and segregation of alloying elements at cell boundaries resulted in transformation gradients throughout the metallic matrix. Intercellular regions displayed inhomogeneous microstructures containing eutectic carbides and martensite. Yield strength, tensile strength and hardness decrease for increasing austempering temperature. Fracture surfaces showed a rapid transition from ductile to cleavage mode at intercellular regions, containing solidification carbides.
ResumoO defeito de olho de peixe é geralmente observado em ferros fundidos nodulares. O mecanismo de formação desses defeitos não é totalmente compreendido e muito pouco tem sido publicado a esse respeito. Esse trabalho investiga o efeito de diferentes luvas na formação do defeito olho de peixe. Luvas exotérmicas e isolantes, com diferentes concentrações de flúor, foram trituradas e misturadas com areia a verde. A mistura foi usada como areia de faceamento dos moldes de fundição. A caracterização morfológica e química do defeito foi realizada por microscopia óptica (MO) e microscopia eletrônica de varredura com microanálise química por dispersão de raios X (MEV/EDS). Os resultados de EDS mostraram elevadas concentrações de alumínio e flúor na região do defeito. Palavras
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