Surface Hardening of 40KH Steel by Electric-Spark Alloying
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SURFACE HARDENING OF 40KH STEEL BY ELECTRIC-SPARK ALLOYING S. І. Kryshtopa,1 D. Yu. Petryna,1 I. М. Bogatchuk,1 I. B. Prun’ko,1, 2 and V. М. Меl’nyk1
UDC 621.9.048:621.92
We substantiate the possibility of application of the method of electric-spark alloying of the surface layers of 40Kh steel by electrodes made of Т15К6 and VK8 hard alloys for the purposes of hardening of the working surfaces of elements of the drilling equipment. It is shown that, as a result of local electric discharges between the cathode and the anode, the surface layers can be alloyed with Co, W, and Ti down to a depth of 10–20 µm. It is established that, in order to guarantee the serviceability of surfacehardened elements under severe operating conditions, it is necessary to get a sufficiently high microhardness of the surface layer, optimize the sizes and density of oxides in this layer, and to minimize the residual tensile stresses responsible for cracking. To get the best combinations of these characteristics, we optimize the technological parameters of electric-spark alloying. Keywords: electric-spark alloying, surface hardening, structure, microhardness, residual stresses.
The problem of prolongation of the service life of the rods of boring pumps is one of the most urgent tasks in the extractive industry. Under the operating conditions, they are subjected to the action of alternating loads and corrosive media, and their serviceability depends on resistance to fatigue, corrosion, and abrasive wear. It is worth noting that just the properties of surface layers of the metal exert a significant influence on these characteristics. The rods of boring pumps are made either of 12KhN4, 20KhNZА, and 34KhN1М alloyed steels with the use of carburization for their surface hardening or of 40Kh, 40KhN, and 38KhNYuА steels subjected to quenching with heating by high-frequency currents [1]. In addition, it is also customary to use the following kinds of treatment: diamond vibration smoothening [2], chromium plating [3] or carburization, plastic deformation by hydro-shot-blasting treatment [4], and plasma spraying with powders [5, 6]. As disadvantages of these methods, we can mention their technological complexity, ecological hazards, etc. Various technologies of the surface hardening of rods with different efficiencies are used by all leading firms specialized in the production of oil-extracting equipment [7]. Thus, in particular, the “National,” “Oilwell,” “Harrisburg,” “Reed American,” and “ТRW Mission” firms use the procedure of gas-thermal spraying of rods with a chromium-boron-nickel alloy (colmonoy-spray technology) with melting of the coating and subsequent grinding; this guarantees an HRC hardness of 60–62. At the same time, the “Super Service” firm applies induction heating down to a depth of at most 1.5 mm with quenching to HRC = 59 [8]. Hence, despite the fact that foreign firms produce rods of high-quality alloyed steels, they additionally harden their surfaces to guarantee higher resistance to abrasive wear and corrosion. However, all kno
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