OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 1 2022 Introduction The high-effective machinery parts operating in contact with high-speed liquid media (for example, turbine blades of hydroelectric power plants, pump impellers, ship propellers) are subjected to cavitation erosion [1–5]. During the cavitation process high-pressure shock waves (1,500 MPa [6, 7]) are initiated, and the velocity of emerging liquid microjets can exceed 120 m/s [8–10]. On the surface subjected to cavitation, local plastic deformation occurs, followed by the destruction starting from the surface layers [11, 12]. As a result, defects that appear in this case (micro pits or cavities) lead to reduction of the equipment effi ciency and increasing repair costs [13]. Fig. 1 represents a typical example of cavitation erosion damage occurring in pump impeller blade made of AISI 316L austenitic stainless steel used in power plant-cooling systems. As reported, steel AISI 316 cavitation erosion resistance is not high enough [14]. Fig. 1. Cavitation wear of water pump impeller Surface treatment is promising way for reducing the cavitation damage [15, 16]. Another way to increase the resistance of parts against cavitation erosion is the deposition of coatings by arc surfacing [17–19] and thermal spraying [5, 20, 21]. Arc surfacing is widely used due to low cost and the possibility of formation dense coatings [22]. Particularly, austenitic electrodes/wire of the E308L-17 type (Russian analogue 03Cr19Ni10) has become widespread due to good weldability and adequate cavitation resistance [23, 24]. Metastable austenitic steels (MAS) are potentially promising alternatives to more expensive alloys based on Co and Ni. When the external load is applied to MAS, a phase transformation from austenite to martensite (γ → ʹ) takes place accompanied by synergistic effects. First, an increase in the proportion of the martensite phase leads to an increase in hardness. Second, the energy of the external load, applied to the surface, is dissipated due to the strain induced nucleation of martensite. Also, due to the phase transformation (γ → ʹ), compressive stresses arise in the surface layer of the part, preventing the occurrence of microcracks [25]. As a result, wear resistance is improved under various conditions (for example, abrasive, hydro- and gas-abrasive, erosive, cavitation, adhesive, and fatigue loads) [26, 27]. For 50Ni9Cr5 MAS, it is shown that the phase transformation (γ → ʹ) occurs at a threshold level of external load from 1,000 to 2,500 MPa with an increase in the initial amount of martensite from 15 to 75 %. At strains exceeding the threshold value, the amount of deformation martensite increases linearly with increasing strains [28]. The authors obtained similar results for 50Cr18 MAS deposited coatings under the action of highly dynamic impact loads [29] and for 60Cr8TiAl MAS coatings under abrasive action [30]. The presented external loads correspond to cavitation loads, which exceed 1,500 MPa, as shown above [6-10]. This suggests the possibility of (γ → ʹ) phase transformation in 60Cr8TiAl MAS coating during the cavitation. The purpose of this study is to evaluate the cavitation erosion resistance and analyze structural changes in the deposited coating of steel 60Cr8TiAl in comparison with austenitic steels 316L (bulk workpiece) and E308L-17 (deposited layer).
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