Abstract. Metallographic analysis is applied to the study of the chunky graphite morphology in heavy-section castings of spheroidal graphite cast irons. Three castings with different Si and Ni contents were prepared. Three positions in casting from the edge to the centre, with different cooling rates, were chosen for microstructure observation. The effect of the Si and Ni content on the graphite morphology and mechanical properties of heavy-section spheroidal graphite cast iron parts was investigated. Cerium containing commercial inoculant was used for in-stream inoculation. Chunky graphite area was estimated in micro-and macrostructure. Mechanical properties were determined on tensile test bars taken from the centre of the casting. Macro-and microstructure examination showed that the castings with high Si-content and Ni addition had chunky graphite present, while the castings produced by use of low Si and Ni containing charge had no chunky graphite. High Si-content is strong chunky graphite promoter, especially in castings with slow cooling rate. Ni addition also promotes chunky graphite formation, but only in thermal centre of the casting (where the cooling rate is the lowest). The elongation is severely lowered when chunky graphite appears in the microstructure.
The study presents the results of investigations of the thermal effects that take place during the heating and cooling of samples of the spheroidal graphite cast iron with different amounts of chunky graphite (CHG) in the microstructure. In order to obtain a different amount of CHG, cone blocks F 300 mm × 350 mm were casted. Two different test melts, EN-GJS-400-18 with 0.7 % of the mass fraction of Ni and EN-GJS-400-15 with 3.5 % of the mass fraction of Si were used. From each block three samples, with different cooling rates, were prepared. For studies of phase transformations, the technique of differential scanning calorimetry (DSC) was used. The microstructure was examined before and after DSC by optical microscopy. The obtained values of the enthalpy of melting and solidification differ and depend on the initial microstructure of the examined samples. The remelting of samples results in magnesium oxidation and significant changes in the eutectic morphology, which, at room temperature, consists of fine and/or coarse undercooled types of graphite with a lamellar morphology.
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