PSI - Issue 65

A.V. Byzov et al. / Procedia Structural Integrity 65 (2024) 48–55 A.V. Byzov, A.E. Konygin, D.G. Ksenofontov, O.N. Vasilenko / Structural Integrity Procedia 00 (2024) 000–000

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Zenin and Ovechkin (2008) note that the main methods of obtaining the surface hardened state are heat treatment, chemical-thermal treatment and mechanical treatment. Thermal treatment for surface hardening consists of localized heating of the product surface to the Ac3 temperature and abrupt subsequent cooling in hardening media, usually water or oil, and it can be carried out by, for example, flame quenching (Shevtsov et al. (2014)), laser quenching (Wang et al. (2022)), high- and industrial- frequency current quenching (Nikolaev and Korotin (2017); Kahrobaee and Kashefi Torbati (2011)). Chemical heat treatment is the saturation of the workpiece surface to be hardened with atoms of a chemical element through diffusion processes. The most common types of such treatments are cementation (Kahrobaee and Kashefi Torbati (2011b)), nitriding, cyanidation, boriding, alitizing, etc. Machining consists of modifying the surface of an article by plastic deformation and surface riveting. Methods of such machining include plastic deformation by rolling, centrifugal machining, shot blasting, and diamond smoothing (Vorob’ev and Malykhin (2019); Kotelnikov et al. (2014)). Surface hardening results in an increase in the hardness of the surface layers of the workpiece with an increase in abrasion resistance and endurance. The unifying step for all hardening processes is the heating of the surface layer of the workpiece to the hardening temperature followed by rapid cooling. These methods differ in how the parts are heated. The thickness of the hardened layer in surface hardening is determined by the heating depth. The most common is induction hardening with heating of products with high-frequency currents, as well as gas-flame hardening (Skobelev (2017)). In general, researchers find a correlation between the physical parameter of the product (in this case it is hardness) and the measured parameter of the eddy current system. After that, by regression analysis, the corresponding equations are determined, which the authors of the works discussed below propose to use as calibration equations. For example, Efimov et al. (2016) focus on finding a relationship between the ECP signal and the hardness of the product surface. Hardness in that study is an indicator of pipe quality since localized areas of surface hardening occur during pipe manufacturing where hardness differs from that in the rest of the pipe. In the work by Zhang et al. (2019), a surface parametric ECP is used. The coefficients of the second-order regression equation relating the hardness of the product hardened at different temperatures to the normalized impedance or normalized inductive resistance of the ECP coil were determined. Similarly, in Wang et al. (2022) using regression analysis, a linear relationship between the change in the impedance of the probe and the hardness of 45 and 30Cr steel products hardened by laser quenching is sought. In the paper by Kahrobaee and Kashefi Torbati (2011), the authors demonstrated a developed technique to reconstruct the hardness profile of a cylindrical surface-hardened product using an encircling eddy current probe. The experiment involved nine cylindrical rods of 30 mm diameter and 150 mm length, hardened to different depths by induction hardening. Then, the frequencies for eddy current penetration to the given depths were calculated. Also, Vickers microhardness profiles were determined for each specimen. A relationship between hardness and the measured parameters during eddy current inspection was revealed since they are influenced by the microstructure of the material. The main parameters obtained from the measurements to construct the hardness profile were the normalized impedance and the primary and secondary coil voltages. In another work by the same authors, published in the same year (Kahrobaee and Kashefi Torbati (2011)), cylindrical specimens that had been previously subjected to cementation were used as research objects. As a result, a linear relationship between the normalized impedance of the coil of the encircling probe and the values of the total and effective cementation depths was noticed. 1.2. Eddy current testing

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