OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 The hardness of cast irons was determined according to GOST 24648–90 on a hardness testing machine ITRB-3000. Microstructural studies were carried out on a metallographic microscope Carl Zeiss Axio Observer Z1m in accordance with ISO 945–75 using the «Thixomet Pro» software [15–26]. The microhardness of the samples was evaluated according to ISO 6507–1:2005 on a microhardness testing machine MN-6 at a load of 0.2452 N. An array of indents from diamond pyramid in the amount of 15×15 was applied to the surface of each of the weakly etched samples. Indents located on the ferrite/ pearlite, ferrite/graphite boundaries as well as on the graphite plates were not taken into account. The evaluation of tribotechnical properties under dry friction conditions was carried out according to the “shaft-block” scheme on a friction machine 2168 UMT. Friction pairs “the material under study – steel 30CrMnSiA (0.28–0.34 % C, 0.8–1.1 % Cr, 0.8–1.1 % Mn, 0.9–1.2 % Si) / 20MnL (0.15–0.25 % C, 1.2–1.6 % Mn, 0.2–0.4 % Si) / 09Mn2Si (< 0.12 % C, 1.3–1.7 % Mn, 0.5–0.8 % Si)” were studied. Impact bending tests were carried out on a pendulum impact tester Metrocom in accordance with ISO 83–76. Samples with a U-shaped stress concentrator 2 mm deep cut on a Sodick AG400L electric spark wire machine were used for testing. SCh35 cast iron and its closest analogue ChMN-35M cast iron were used as a reference material for mechanical characteristics [12]. Results and discussion Experiments for choosing optimal concentration of alloying additives were carried out to develop the chemical composition of SChKM-45 cast iron with increased complex of mechanical properties. In accordance with the results of the studies, concentration of nickel providing the required level of hardness (at least 250 HB) is 0.4–0.7 wt. %. In this case the ultimate strength exceeds 450 MPa (Fig. 1) [11, 27]. A similar conclusion can be drawn regarding the amount of molybdenum. The addition of more than 0.7 wt. % of molybdenum is not rational due to a significant increase in the level of hardness (more than 350 HB) and material embrittlement. In this case, the ultimate strength increases to a lesser extent (Fig. 2) [11, 27]. Concentration of vanadium which ensures the requirements for the level of hardness and ultimate strength is in the range from 0.2 to 0.4 wt. %. With the addition of this element in an amount of less than 0.2 wt. % level of ultimate strength does not reach 450 MPa. The excess of the vanadium content of more than 0.4 wt. % is accompanied by chilling of cast iron and the appearance of islands of skeletal eutectic. It should be emphasized that an increase in the vanadium content does not lead to an increase in the ultimate strength of the material (Fig. 3), but the cost of the material increases. The noted circumstance is one of the factors that significantly limit the efficiency of the alloyed alloy [11, 27]. a b Fig. 1. Effect of nickel concentration in SChKM-45 gray cast iron containing 0.45 wt.% of molybdenum and 0.34 wt.% of vanadium: a – on hardness; b – on ultimate strength
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