Effect of regular thicknesses on the microstructural and quantitative analysis for a hypo-eutectic ductile iron alloyed with Ni and V

Abstract

Ductile iron contains free graphite nodules inside the metallic matrix, which generally consists of ferrite and pearlite in the as-cast condition. The casting thicknesses greatly influence on the size, shape, and quantity of the microconstituents of the metallic matrix and the graphite nodules and, therefore, on the mechanical properties. This investigation studied the cooling rate (imposed by the casting thicknesses) on the metallic matrix and the nodular characteristics of a ductile iron low alloyed with 0.8 %Ni and 0.15 %V. The ductile iron was manufactured by the sandwich technique with ladle inoculation. Six plates of different thicknesses, from 4.3 mm to 25.4 mm, were fabricated in a green sand mold. The microstructural characterization was performed by optical microscopy (OM), scanning electron microscopy (SEM), and the image J software by applying different quantification methods. The area method to obtain the average nodule size and nodularity provided more reliable results than the perimeter and total particle count methods. The hardness test on the Rockwell C scale carried out the mechanical characterization. The low content of vanadium added to the ductile cast iron had a negligible effect on the solidification pattern, mainly due to the graphitizing impact of the nickel and silicon addition. Thus, the microstructural features results are primarily due to the cooling rate imposed by the casting thickness. The thinnest casting section significantly improves the nodule count (414 Nod/mm2), sphericity (0.96), and nodularity (96.21 %). In contrast, the thickest casting plate obtained the highest volume fraction of graphite (10.85 %) and the lowest volume fraction of undesirable particles (0.36 %). The high cooling rate in the thinnest casting plate obtained the highest hardness of 31.56 HRC because of the higher contents of the volume fraction of pearlite (33.7 %) and carbides (4.5 %).