OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 the influence of the structure on the formation of cast iron’s mechanical properties it is necessary to control its structural features including the uneven distribution of graphite inclusions in the volume of the material, the presence of chill zones which are the cause of embrittlement of cast iron, etc. Improving the structure of cast irons, reducing the amount of structural defects and increasing mechanical properties is facilitated by the alloyage with nickel, molybdenum, phosphorus, vanadium, aluminum, boron, etc. The role of alloying elements and modifying additives in the formation of the structure and complex of mechanical properties of cast irons is reflected in the works [1–10]. The shape, size and volume fraction of graphite inclusions as well as its distribution in the bulk of the material are the most significant structural features determining the level of mechanical properties of gray cast irons [2, 9, 10]. Graphite plates distributed in cast iron, on the one hand, can be considered as natural concentrators of mechanical stresses that contribute to the formation of cracks and destruction of the material, and on the other hand, graphite plates can be “pockets” where microvolumes of solid lubricant are concentrated, helping to reduce the friction coefficient and as a result increase the service life of friction pairs. Graphite distributed in gray cast iron prevents the seizing of surfaces worked as dry sliding friction units [9–14]. According to the Application standards of the Russian Railways, the main requirements for cast irons used for the manufacture of parts of railway transport, are ensuring the ultimate strength during deformation at a level of at least 350 MPa according to the tensile scheme and hardness in the range of 250–350 HB. Typical parts made from these materials should provide at least 160,000 km of mileage of railway transport. The search for technical solutions that provide the possibility of improving the mechanical properties of cast irons is an urgent task of applied importance. One of these solutions is related to the alloying of gray cast iron. Previous studies [1–14] indicate a significant effect of nickel, molybdenum and vanadium on the physical and mechanical properties of gray cast irons. The previously developed cast iron grade ChMN-35M [12] produced in correspondence with tech spec 0812-001-10036140-2014 does not fully meet the requirements for parts operating under frictional conditions. First of all we are talking about the harsh operating modes of the equipment (dry sliding friction with a high level of contact loads). It has been experimentally established that seizure centers appear under such conditions on the surfaces of parts made of ChMN-35M cast iron, the development of which results in an increase in wear intensity. The aim of this work is to study the effect of alloying elements (nickel, molybdenum, vanadium) on the structure and mechanical properties of gray cast iron intended for the manufacture of structural components operating under conditions of dry sliding friction. The level of tensile fracture resistance was the main parameter controlled during the study. Its value was not less than 450 MPa (with hardness in the range of 250–350 HB). The limiting requirement was the cost of the material provided that the minimum required level of tensile strength was guaranteed. Research methods SCh35 gray cast iron was chosen as the base material for the research. Smelting was carried out in an induction melting furnace with a crucible volume of 150 kg. Scrap 4A GOST 2787–75 weighing 100 kg was used as a charge. Samples were taken to assess the chemical composition after charge melting and carburizing of the material. Cast iron was alloyed with nickel, molybdenum and vanadium, the concentration of which was varied in the range from 0.1 to 0.8 wt. % in order to increase the strength properties. Alloying was carried out by addition the calculated amount of ferroalloys of nickel, vanadium and molybdenum directly into the SCh35 gray cast iron melt. The temperature of the melt before draining from the furnace was 1,425–1,440 °C. The mold filling time did not exceed 5 minutes [11]. An optical emission spectrometer GNR Solaris CCD Plus was used to determine the chemical composition of the studied materials. Tensile tests of the samples were performed on a universal electromechanical testing system Instron 3360 according to GOST 27208–87. Sample preparation was carried out in obedience to ISO 185–88.
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