Abstract

Water turbine parts damaged by cavitation erosion (CE) and/or slurry erosion (SE) may cause excessive operational costs for plants worldwide. The damages can be reduced by choosing more resistant materials and right technology in the first-production or at repair and overhaul. Thermal spray technologies have a great potential in the field of repairing works. Thick multilayered coatings deposited by wire electric arc spraying (WAS) has been developed and applied as CE and SE protection at the repair of stationary Francis turbine blades. Repair technology by WAS was performed on large eroded areas (1-3 m3) of preguide blades of Francis turbine: 1) local damaged depths 30-35 mm maximum were repaired by sprayed materials, 2) subsequently wire arc spraying of functional coating was applied. Three types of functional coatings with total thickness 10 mm a) duplex high - Cr stainless steel with NiAl bond coat, b) graded NiAl - Cr stainless steel coatings, and c) multilayered graded NiAl - Cr stainless steel coatings were compared by means of stress measurements and structural analysis. The coating structure influences very strongly the residual stress level and adhesive-cohesive strength. Multilayered graded NiAl - Cr stainless steel coatings showed the best results and were sprayed on water turbine blades in 4 Czech water power station during regular cut-off repair periods. After 30 - 36 months' continuous operation, Francis turbine blades repaired by WAS technology show better behaviour in comparison with original material from the point of wear resistance, reliability, cost-effect and life-time.

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