Considerations for Fatigue Abatement of 4340 AISI in Inner-Diameter HVOF spraying of WC-Co-Cr Coatings

Miniature HVOF systems are currently being introduced to replace hard-chrome plating with sprayed carbide coatings in space restricted inner-diameters in aerospace and industrial application. The global interest in this technology is largely driven by regulatory restrictions for the use of hexavalent chrome, such as REACH. Fatigue abatement is one of the most important design criteria for the industries and is affected by both the substrate temperature history during the spray process and by the residual stresses in the coating, which are inherently different in small inner-diameter coated cylinders than in conventional HVOF setups. The short spray distances and limited cooling option in small ID cylinders can impose high thermal loads, while the smaller feedstocks impose lower coating compressive stresses. This presentation discusses the fatigue abatement of flat 4340 AISI coupons, being part of the inner wall of 4.8 inch diameter cylinders, by the application of WC-Co-Cr coatings using the Spraywerx Mini-Nova ID-HVOF in terms of residual stresses, peening stress relaxation and martensite over-tempering, as modeled through isothermal testing. Operating windows and tool path options are delineated to attain a debit below 30% for >150 μm coating, as compared to uncoated flat specimen in ASTM E466 axial fatigue testing.