OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 Ta b l e 2 Mechanical properties of SCh35, ChMN-35M and SChKM-45 cast irons Cast iron grade Ultimate tensile strength, MPa, at least Brinell hardness, HB from to SCh35 345–365 272 288 ChMN-35M 362–395 277 319 SChKM-45 470–505 268 321 Cast iron SCh35 is characterized by the formation of graphite inclusions with a length of ~ 10–200 µm. The shape of graphite is lamellar, partially swirling (Fig. 4, a) [28, 29]. The structure of ChMN-35M cast iron is characterized by a uniform distribution of graphite inclusions with the size of ~ 10–150 µm (Fig. 4, b) [11, 27]. Simultaneous alloying of cast iron with molybdenum and vanadium contributes to the formation of the corresponding solid solutions mainly in the α-phase, which contributes to a higher degree of graphitization; at the same time the melt volumes enriched in molybdenum and vanadium are characterized by an increased number of crystallization centers [28]. Thereby with an increase in the degree of alloying, the size of graphite inclusions decreases. The length of graphite inclusions observed in SChKM-45 cast iron (10–110 μm) is approximately two times less than in SCh35 gray cast iron. The shape of the inclusions in SChKM-45 cast iron is lamellar, partially swirling (Fig. 4, c). The main structural component of the metal matrices of all three cast iron grades is lamellar perlite. Its content ranges from 92 vol. % in ChMN-35 cast iron up to 100 vol. % in SCh35 cast iron. The pearlite fraction in the SChKM-45 alloy is ~ 86 vol. %. Thereby the result of alloying elements addition into cast irons is an increase of the fraction of the ferrite in the structure [1, 2, 10, 31]. The effect of molybdenum on the volume fraction of ferrite is more significant since it is more soluble in the α-phase compared to vanadium. It is also noted that thermodynamic stability of ferrite can be increased by alloying with molybdenum and vanadium [31, 32]. Structural features of the metal matrix analyzed by light microscopy are shown in Fig. 5. Ferrite precipitated in ChMN-35 and SChKM-45 cast irons is mainly localized near graphite inclusions. The reason for this phenomenon is the presence of nickel and molybdenum in alloys, the complex effect of which leads to the same effect as a decrease in the cooling rate of the melt. Thus a ferrite rim is formed along the edges of graphite inclusions in these cast irons [32, 33]. Ferrite observed in ChMN-35 cast iron is predominantly Ta b l e 3 Structural parameters of SCh35, ChMN-35M and SChKM-45 cast irons Characteristic of microstructure according ISO 945–75 Cast iron grade SCh35 ChMN-35M SChKM-45 Shape of graphite inclusions, scale 1A, ×100 vermicular vermicular vermicular Length of graphite inclusions, scale 1Б, ×100 60–120 µm 30–60 µm 30–60 µm Distribution of graphite inclusions, scale 1В, ×100 reticular equilibrium equilibrium Type of the structure of the metal base of cast iron, scale 5, ×500 lamellar pearlite lamellar pearlite lamellar pearlite Number of graphite inclusions, scale 1Г, ×100 ≤ 3 % 3–5 % 5–8 % The content of pearlite and ferrite in the structure of cast iron. % scale 6А, row 1, ×100 perlite ≥ 98 % ferrite ≤ 2 % perlite 90–94 % ferrite 6–10 % perlite 80–90 % ferrite 10–20 % Perlite dispersion distance between cementite plates ≥ 1.6 µm distance between cementite plates 1.3–1.6 µm distance between cementite plates 0.8–1.3 µm
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