OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 mechanical properties. The amount and size of graphite, morphology and distribution of graphite fl akes are critical in determining the mechanical behavior [26–34]. Lamellar graphite of the Gf1 type in our studies has a random orientation. As shown in fi gure 2, b, the morphology of vermicular graphite type Gf5 was observed using optical and electron microscopy. Figure 4 shows a large amount of vermicular graphite with uneven distribution. The ends of vermicular graphite are relatively smooth, round and blunt in shape, while the outer edge has a wavy, uneven shape. It is obvious that the graphite structures are thin and dispersed. Vermicular graphite makes up approximately 50 % of the volume, and several other graphite phases can be seen throughout the fi eld of view. Vermicular graphite inside the eutectic cluster (fi gure 4, b) is a continuous structure with a hemispherical end. The ends of vermicular graphite between the eutectic clusters are not nested into each other and represent full-fl edged and independent particles of the eutectic cluster. Figure 4, b shows metallographic photographs of the core of a gray cast iron casting. The graphite morphology is fi ne vermicular graphite about 100–200 μm in length and only a small amount of spherical graphite. According to the requirements of regulatory documents, it is necessary to calculate each graphite morphology over the cross section of the specimen. The percentage of vermicular graphite on the surface and core of the casting is 93 % and 51 %, respectively. The matrix structure of the casting is pearlitic; a small amount of ferrite precipitates around the graphite. It was found that specially shaped graphite existed in the matrix in addition to vermicular graphite and spherical graphite, as shown in fi gure 4, b. This type of graphite morphology is presented in the form of spherical graphite with a small tail, which was called tadpole graphite (distorted graphite) in [18]. Most of the distorted graphite head has an irregular spherical shape, with a diameter of about 20–50 μm, and a tail length of about 30–120 μm. Interestingly, the graphite tail in some areas is separated from the parent body of spherical graphite. The morphology of distorted graphite is between spheroidal graphite and vermicular graphite, which is not yet fully developed. When analyzing the results (Table 2, 3), it is clear that the combined modifi er demonstrates good modifying properties. The modifi ed specimens showed higher mechanical properties compared to the witness sample. Previously, in [7, 32, 33], we compared modifi ers consisting of silicon dioxide with the standard modifi er FS75, which showed an increase in the positive eff ect on the structure and properties. From the theory and practice of foundry production it is known that the eff ectiveness of modifi cation in the smelting of gray cast iron is checked when processing cast iron with a low carbon equivalent. This study shows that the combined modifi er has a positive eff ect on the mechanical properties of gray cast iron. On specimens without modifi cation (fi gure 3), we see that graphite has a morphological shape in the form of plates. Vermicular graphite (fi gure 4) is a transitional form between fl ake graphite and spherical graphite [8–19], and its roundness factor (RSF) ranges from 0.3 to 0.6. The roundness coeffi cient was calculated according to the formulas [23, 25]. The morphology of graphite plays an important role in the mechanical properties of gray cast irons. According to the theory of cast iron crystallization, the fi nal shape of graphite is uncontrollable at the nucleation stage and depends on the growth stage. The diff erences in graphite morphology are due to diff erent growth rates in all directions. The direction of growth depends mainly on the chemical composition [17–21]. The diff erences in the growth behavior of lamellar, spherical and vermicular graphite depend mainly on the exclusion of selective adsorption of surface-active atoms on the graphite surface [18]. During eutectic cluster growth, low melting point and low content compounds such as sulfur and phosphorus are typically thrown to the grain boundaries, and austenite does not surround the vermicular graphite during growth. As graphite solidifi es, it is able to change the direction of its growth at the solid-liquid interface. During the solidifi cation process of cast iron, the mode of graphite growth and the fi nal morphology depend on the thermodynamic conditions and chemical composition of the molten cast iron. According to works [8–19, 26–39], the mechanism of formation of graphite morphology in cast iron is as follows. When the molten iron is suffi ciently pure and free of surfactants (O, S or other impurities), the main growth direction of graphite is the normal of the basal plane (0001) (c direction), and the graphite will preferentially develop through spiral growth into a spherical shape, since it can occur with minimal activation energy [20, 21]. However, molten iron inevitably contains surfactants such as S and O, which have been found
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