Effects of surface treatments on the fatigue strength of AISI 4340 aeronautical steel

Internal residual stresses significantly influence the fatigue strength of coated materials. It is well known that chromium plating is the most used e lectrodeposited coating for important industrial applications. However, pre ssure to identify alternatives or to improve the chromium electroplating pr ocess have increased in recent years, related to the reduction in fatigue s trength of the base material and to environmental requirements. The high ef ficiency and fluoride free hard chromium electroplating there called "accel erated") is an improvement to the conventional process. One environmentally safer and cleaner alternative to hard chromium plating is tungsten carbide thermal spray coating applied by the High Velocity Oxy-Fuel (HVOF) process . To increase the fatigue strength of chromium plated materials, coating th ickness and microcracks density are important parameters to be controlled. Techniques as compressive residual stresses induced by shot peening and mul tilayers, are also used. The aim of this study was to analyse the effects o n AISI 4340 steel, in the rotating bending fatigue behaviour, of the: tungs ten carbide thermal spray coating applied by HP/HVOF process; chemical nick el underplate, and shot peening process applied before coating deposition, in comparison to hard chromium electroplatings. Rotating bending fatigue te st results indicate better performance for the conventional hard chromium p lating in relation to the accelerated hard chromium electroplating. Tungste n carbide thermal spray coating and accelerated hard chromium plate over ni ckel resulted in higher fatigue strength when compared to samples conventio nal or accelerated hard chromium plated. Shot peening showed to be an excel lent alternative to increase fatigue strength of AISI 4340 steel hard chrom ium electroplated. (C) 2001 Elsevier Science Ltd. All rights reserved.