| Abstract: |
Many researches in the materials selection and surface engineering have the purpose to reduce the cavitation damage and residual tensions after the depositionin hydraulic turbines. In this work a cobalt stainless steel was applied by arc thermal spraying. The influence of air pressure deposition and plasma remelting on the microstructure, oxide volumetric fraction, porosity, microhardness and cavitation resistance were studied. Microstructures and properties of the AS895HY cobalt stainless steel coatings were investigated by X-ray diffraction, optical and electronic microscopy, microhardness and ultrasonic cavitation test, ASTM G32-93. The increase in air pressure, 280 to 550kPa, modified the oxide fraction from 10,9% to 24,8%, in the samples. The mass loss in vibration-induced cavitation were 13,8, 19,2 and 15,0mg/h, respectively, for the samples with 280, 410 and 550kPa of the air pressure. Phase transformations not occurred in the arc thermal spray coatings after cavitation tests. The remelting of the sprayed coatings eliminated the typical thermal spray microstructure. The rise in the thickness of the arc thermal spray coatings and the lowering in the plasma arc current reduce the dilution of the substrate. On the AWS309L substrate austenite formation was observed in the two remelted layer, decrease in microhardness in the first layer was observed, probably because of the AWS309L substrate dilution. The PTA remelting reduces to 0,497mg/h the cavitation rate mass loss. Incubation period was 8,02 hours in the remelted coating. The SEM analysis verified that the mass loss in arc thermal spray coatings occurred because of the interlamellae oxide fracture and the splats delamination, and phase transformations were observed in the remelted coating, but not verified in the arc thermal spray coatings.
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