OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 4 2024 by the splashing of lubricant during operation, lead to increased wear of the moving package of piston rings located in the upper part of the cylinder [1]. Compression rings and oil rings are part of the CPG. Oil rings are used in four-stroke engines with a lubricant spray system to remove excess oil from the bottom of the cylinder and regulate its fl ow to the top of the cylinder. Compression rings have two functions: sealing and heat dissipation, as well as helping to distribute oil over the cylinder walls during operation. During operation, the compression rings make several types of movement. Forward-backward (radial) movement of the rings within the piston groove (ring grooves on the cylindrical surface of the piston) creates deformations perpendicular to the formations and contribute to wear of both the rings and the bottom surface of the piston groove. This leads to deterioration of the sealing eff ect of compression rings, and afterwards to radial vibration and ring breakage, most often in the middle part, opposite the lock. Axial movement is due to the diff erence in gas pressure above and below the ring, the gravity of the ring itself and the friction force between the ring and the piston groove surface. Rotational movement of the rings is caused by the engine shaft revolutions. The operating conditions of the upper and lower compression rings are diff erent. The pressure behind the upper compression ring is 0.75Pr, behind the lower compression ring is 0.20Pr (Pr is the operating pressure) and is one of the components of the force pressing the rings to the cylinder, and also creates radial deformations of the ring material. This causes increased wear of the upper compression ring and the lower surface of the piston groove on which it is seated. The service life of the CPG depends is highly dependent on the wear rate of the compression rings. In order to increase the service life of piston rings, various methods of hardening of mating surfaces have been developed: plastic deformation, hardening by high frequency currents, creation of adhesive surfaces, placement of wear-resistant inserts in the friction zone of the ring at the top dead center, porous chromium plating, grinding of grooves for fi lling with tin, application of hardening coatings of molybdenum and other materials that increase tribotechnical properties [2, 3]. High strength and elasticity, wear resistance, low coeffi cient of friction are the main design requirements for rings. The uniform distribution of radial pressure around the circumference of the ring is of great importance [4]. Piston rings are made of grey alloyed cast iron with lamellar graphite or high-strength cast iron with spheroidal graphite. Along with high casting properties grey cast iron has good damping ability, high antifriction properties, lower tendency to thermal deformations compared to steel. Diff erences of phase states of grey cast irons, diff usion of elements, inhomogeneity of linear and volume expansion coeffi cients of ferrite, cementite and graphite lead to anisotropy of stress state. This is a source of nucleation and development of defects called dislocations. It seems expedient to evaluate the change of cast iron stress state manifestations under the infl uence of various factors in order to predict the conditions of compression rings fracture in the process of operation. The technology of compression rings manufacturing is determined by reliability requirements and is described in the relevant standards for each type of rings. Quality control of rings is carried out by various methods of assessing the structure of castings, taking into account the technology of hardening, normalisation, heat treatment and machining [1–5]. The use of grey cast iron for the manufacture of compression rings is due to its good fl uidity and low shrinkage. Mechanical properties of cast iron are determined by the quantitative ratio of structural components, mainly ferrite, pearlite and graphite. Cast iron with perlite base is the strongest and most wearresistant. Ferrite reduces mechanical properties of cast iron. Large graphite inclusions reduce strength, but provide high cyclic toughness and low sensitivity to external notches. The material structures of the upper and lower rings undergo signifi cant changes during operation. Rings made of grey alloyed cast iron with lamellar graphite should have a certain microstructure: the metallic base consists of medium and fi ne lamellar or sorbitic pearlite. Perlite corresponds to high hardness, wear resistance and good machinability by cutting. The presence of ferrite in the form of individual small inclusions should not exceed 5 % of the section area. Ferrite indicates a decrease in mechanical properties and wear resistance of cast iron.
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