Repair Process Technology Development and Experience of Frame 7FA+E, Stage 1 Turbine Buckets

The 60 Hz, Frame 7F engine has been in commercial operation for more than two decades now with approximately 600 Frame 7 (F, FA, FA+ and FA+e models) machines existing in North America and a total of over 1100 F-class machines throughout the world. Volatile market dynamics in the electrical power generation field continues to force power companies to identify prudent material cost reductions opportunities in their Operations and Maintenance (O&M) business. Today, there is an industry-recognized need for advanced hot gas path component repair and reconditioning capability for operators of F-Class gas turbines that can be highly cost effective with short cycle times. The first stage buckets of the Frame 7F engine are unshrouded; whereas the next 2 stages are shrouded. Out of these rotating components, the first stage buckets show the worst degradation and thus repair of these components have been a focus point for Power Systems Mfg., LLC (PSM). The technical objective is to develop a comprehensive set of repair schemes for the stage 1 buckets since these components have the highest frequency of replacement. Listed below are some of the special repair processes that have been developed for the first stage bucket: a) Acid stripping of the MCrAlY coating (and internal aluminide coating having endured 48,000 hours of service). b) High speed grinding off of the electron beam (EB) or laser beam (LB) welded tip cover plates. c) High frequency gas tungsten arc (GTA) weld repairs of platform cracks using a new and novel developed high strength yet ductile weld filler metal. d) High frequency gas tungsten arc weld attachment of new tip cover plates. e) Laser metal forming/cladding of new squealer tips. f) Rejuvenation heat treatment for buckets that have reached 48,000 hours. g) Application of new internal aluminide coating. h) Application of upgrade design features, such as platform cooling and platform undercut. i) Application of a superior MCrAlY bond coating to that of the Original Engine Manufacturer (OEM). j) Application of a vertically cracked high density (VCHD) strain tolerant thermal barrier coating (TBC). This technical paper describes the repair development process, the implementation of the different stages of the repair schemes and presents metallurgical and mechanical characteristics of the repaired regions of the component.