PSI - Issue 59

Olha Maksymiv et al. / Procedia Structural Integrity 59 (2024) 378–384 Olha Maksymiv et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Damage of machine parts and machineries working under the combined action of aggressive environments and high contact loads often starts from the surface. Therefore, the study of the mechanism of surface damage and the development of technologies for the improvement of physical and mechanical properties is an actual issue. Nowadays, in addition to the different methods of surface modification studied by Shatskyi et al. (2016), Stupnyts’kyi et al. (2016) , Student et al. (2018, 2023), Mazur et al. (2019), Hutsaylyuk et al. (2021), Posuvailo et al. (2022), Kovalchuk et al. (2023) and others, significant interest is caused by surface nanocrystallization. It is explained by specific properties of materials with nanocrystalline structure (NCS), e.g. self-lubricating described by Babak et al. (2023) or high modulus of elasticity described by Kyryliv et al. (2023a). The most widely used methods for obtaining surface NCS on metals using severe plastic deformation (SPD) are described by Yurkova et al. (2006), Olugbade and Lu (2020), Khomenko (2020), Vasiliev et al. (2021), Gurey et al. (2023) and others. One of the methods for fabricating surface NCS is MPT (Nykyforchyn et al. (2016, 2019, 2021), Kyryliv et al. (2016, 2023a,b)). It is based on using SPD generated by high-speed friction (Kalichak et al. (1989), Nykyforchyn et al. (1998)). It provides enhancement of the wear resistance, corrosion-fatigue resistance and working capacity of machine components demonstrated by Nykyforchyn et al. (1998, 2016, 2019, 2021) and Kyryliv et al. (2016) due to the improvement of the corrosion stability and a decrease in hydrogen permeation in the surface nanocrystalline layer studied by Nykyforchyn et al. (2015a, 2015b, 2016, 2019). Mineral oil, as well as different aqueous and oil-based liquids, are used as coolants and technological fluids (TF) during MPT. All mineral TF consists of hydrogen, oxygen and carbon. Due to thermal and mechanical destruction in the friction contact zone, they decompose to the atomic state and penetrate into the surface layers of NCS localizing mainly on the grain boundaries, and, as considered by Glikman et al. (1972), could be useful (carbon) or harmful (oxygen, hydrogen, and so on) depending on increasing or decreasing interatomic interaction. In this paper, the influence of the surface NCS, obtained on the alloy structural steel by MPT in air, an aqueous solution of emulsol and mineral oil used as TF, on the contact durability in oil and tap water is investigated. 2. Materials and methods Cylindrical specimens of 10 mm in diameter and a length of working part of 50 mm manufactured of the alloy structural steel were investigated. The chemical composition of the steel is as follows, mass. %: 0.2 C; 0.9 Cr; 3 Ni; 0.4 Mn; 0.3 Si; 0.01 S; 0.01 P; Fe: balance. The steel is used for manufacturing legs of drilling bits. A series of specimens were tested: 1) after MPT in different TF (air, aqueous solution of emulsol and mineral oil); 2) after cementation, quenching and tempering at 150°C, which is usually used for such alloy steel; 3) after cementation, quenching and tempering at 150°C and following MPT. Specimens after cementation, quenching and tempering at 150°C had a hardness of НRС 60…62 . Previous cementation and heat treatment were applied since rolling contact fatigue significantly depends on the steel strength, as it was shown by Beynon et al. (1996). After heat treatment, they were ground, and some of them were treated by MPT to obtain NCS. Different types of TF were used during MPT: i) air was used as the reference environment with the lowest content of hydrogen and oxygen; ii) 10% aqueous solution of emulsol Emulgut 800; and iii) low-viscosity industrial base oil ISO VG 100. MPT was performed on a modernized turning machine (model 1K62) using a strengthening tool made of 40X steel (0.4% C;1% Cr) with a width of the working part of 5 mm. The treatment regimes of MPT were as follows: linear, rotational velocity of strengthening tool – 60 m/s, rotational frequency of the specimen – 3.83 s -1 , treatment time – 15 s, cutting depth – 0.4 mm, which corresponds to the pressure in the friction contact zone 0.8 GPa. The phase composition of the surface layer metal after MPT was carried out with diffractometer DRON-3 using CuK  – radiation ( U = 30 kV, I = 20 mA) with a step of 0.05° and the exposition of 4 s. The diffractograms were post-processed using the software CDS developed by Kraus and Nolze (1996). The X-ray diffraction patterns were analyzed after JCPDS-ASTM using Powder Diffraction File Search (1974).