OBRABOTKAMETALLOV Vol. 23 No. 3 2021 MATERIAL SCIENCE EQUIPMENT. INSTRUMENTS 6 4 With a small gap, the risk of prolonged double arcing during the operation stage or short circuit to the discharge chamber when arcing increases, which can lead to catastrophic failure of the working elements (Figure 2, c, d). One of the reasons for the plasma torch failure can be the melting of holes in the swirl ring, leading to a sharp increase in temperature in the cavity between the electrode and the nozzle due to the lack of heat removal by the gas flow (Figure 2, e, f). As a result, the metal of the nozzle and electrode melts sharply and the nozzle hole is blocked. The process of gradual wear of the nozzle and electrode material is mainly associated with hightemperature erosion during the interaction of copper with the plasma and gas flow (Figure 2, g–l). This process can be further complicated by the operation of the starting arc during cutting (Figure 2, g, h) or by inaccuracy in the manufacture of plasma torch elements (Figure 2, i, k). With an average operating time per failure of more than 250–300 starts of consumable elements (nozzle and electrode) in the process of cutting hot-rolled plate (up to 100 mm) products, untimely starting arcing off or inaccuracies in manufacturing can reduce this parameter to 100–150 starts. The processes of catastrophic failure of plasma torch lead to its abrupt failure even after one start. The reason is mainly insufficient clearance between nozzle and electrode. The temperature in the zone of the discharge chamber rises so high in a short time that the metal begins to melt and boil, leaving a characteristic structure on the surface in the form of a melting zone with a large number of pores (2 in Figure 3). Moreover, there are practically no traces of oxides or erosion products on the electrode surface, and the thin melting zone 2 passes into the base metal 1 (Figure 3, a, c–e). The inner surface of the nozzle has traces of oxidation, metal boiling and erosion (Figure 3, b, f–h). The metal entering the nozzle channel quickly crystallizes and plugs it. The Fig. 3. The structure of plasma torch elements after a catastrophic failure of the nozzle and electrode with insufficient clearance between it: a, b – macrostructure of the electrode and nozzle, c–e – microstructure of individual sections of the electrode; f–h – microstructure of nozzle sections
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