Cavitation erosion investigations on thermal spray coatings VASILE COJOCARU, DOINA FRUNZAVERDE, CONSTANTIN VIOREL CAMPIAN, GABRIELA MARGINEAN, RELU CIUBOTARIU, ANA-MARIA PITTNER Faculty of Engineering “Eftimie Murgu” University of Resita No. 1-4, P-ta Traian Vuia, 320085, Resita ROMANIA [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], http://www.uem.ro Abstract: The cavitation erosion is a major problem for the hydro turbine components. This phenomenon which consists in the progressive loss of material from a solid surface, affects the runner blades and adjacent areas of the runner. Usually, the repairs of affected areas are made by welding. In order to increase the cavitation resistance, new repair methods are tested. The goal of this study was to investigate the cavitation resistance of thermal spray coatings. Two laboratory samples were realized by thermal spraying of wolfram carbide powder onto martensitic stainless steel substrates. The cavitation investigations were made using the ultrasonic method. Metallographic investigations and hardness results are also presented. Key-Words: Cavitation, Erosion, Thermal Spray Coatings, Wolfram Carbide, Martensitic Stainless Steel 1 Introduction The cavitation phenomenon consists in formation and collapse of vapor bubbles in a fluid due to decreasing of local pressure under the equilibrium vapor pressure of water [ 1 ]. The effects of cavitation are: noises, vibrations and cavitation erosion of runner blades and adjacent areas. Cavitation erosion affects the components of mechanical systems which work in liquid environment. The biggest economic impact is found in hydraulic turbines, were runner blades and adjacent areas of the runner are submitted to cavitation. Up to now the best results regarding repair techniques applied to turbine runner blades were obtained by overlay welding of cold hardening austenitic stainless steels [ 2,3,5 ]. The main problems in case of repair welding in situ are connected to the residual stresses and to the important structural modifications of the base and filler materials, which are appearing during the welding process. These effects, especially when extensive welding is applied, can lead to the damage of the repaired component during following operation. In order to enhance the lifetime of the turbine, the interval between two welding repairs must be extended. To increase the cavitation erosion resistance, new repair methods are tested [ 4 ]: reinforced epoxy coatings and thermal sprayed coatings. Thermal spraying is a process by which finely divided metallic or nonmetallic materials, in molten or semi-molten conditions are deposited to a surface. Different thermal spray processes are applied: combustion powder flame spray, combustion wire flame spray, wire arc spray, plasma spray, high velocity oxyfuel (HVOF). 2 The research program The research [ 6 ] was carried out on two samples (table 2) realized by HVOF thermal sparying of WC-CoCr powder on a martensitic stainless steel base material (0.03%C, 12.6%Cr, 3.63%Ni). The martensitic stainless steles are used in the manufacturing of the turbines rotor blades. WC- CoCr-based high velocity oxy fuel (HVOF) coatings are used for components which are exposed to severe erosion or abrasion A liquid penetrant inspection was applied on the base material, before thermal spray. After this inspection, made in accordance with ISO 4987, the samples did not present any cracks. Table 1 Sample Base material HVOF coatings 1 Martensitic stainless steel type 1.4313 WC-CoCr micropowder 2 WC-CoCr nanopowder Latest Trends on Engineering Mechanics, Structures, Engineering Geology ISSN: 1792-4294 177 ISBN: 978-960-474-203-5
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Cavitation erosion investigations on thermal spray coatings
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Cavitation erosion investigations on thermal spray coatings
VASILE COJOCARU, DOINA FRUNZAVERDE, CONSTANTIN VIOREL CAMPIAN,