Method of forming turbine blade root

Abstract

The present invention relates to a process for forming the root of a turbine blade using the EDM method. EDM machining of the root is done in a manner that any re-cast layer left behind is less than 1 micron. After EDM machining the turbine blade is subjected to MPI to check for depth or presence of surface cracks. A liquid tracer coating is then applied to the root, which allows the ability of the next process to be verified as complete. The root is then subjected to glass beading to remove any re-cast and insure the surface finish prior to shot peening is consistent and contains no scratch marks or machine marks. The root is then subjected to shot peening to reduce residual tensile stresses by imparting to the surface small indentations or dimples and produce a compressed surface which resists further surface cracks.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:  
     
         1 . A process for forming the root of a turbine blade comprising the following steps: 
 (a) EDM machining of the root into the desired shape in a manner that any re-cast layer left behind is less than 4 micron or greater if acceptable;    (b) After EDM machining, the root is then subjected to glass beading to remove any re-cast and insure the surface finish is consistent and contains no scratch marks or machine marks; and    (c) Then the root is subjected to shot peening to reduce residual tensile stresses by impart to the surface small indentations or dimples and produce a compressed surface which resists further surface cracks.    
     
     
         2 . A process according to  claim 1  wherein the process forms the straight root of a turbine blade.  
     
     
         3 . A process according to  claim 1  wherein the process forms the straight root of a turbine blade.  
     
     
         4 . A process according to  claim 2  wherein additional steps after EDM machining and before glass beading are provided, namely: after EDM machining the turbine blade is subjected to MPI to check for depth or presence of surface cracks; and then a liquid tracer coating is applied to the root.  
     
     
         5 . A process according to  claim 3  wherein additional steps after EDM machining and before glass beading are provided, namely: after EDM machining the turbine blade is subjected to MPI to check for depth or presence of surface cracks; and then a liquid tracer coating is applied to the root.  
     
     
         6 . A process of forming the radial root of a turbine blade comprising of the following steps: 
 a. Calculation of the incremental angle at which tangents are drawn to produce a radial root.    b. Inputting data corresponding to the radial portion of the root blade into an EDM apparatus.    c. EDM machining of the root into the desired shape in a manner that any re-cast layer left behind is less than 4 micron or greater if acceptable;    d. Calculation of the radial drop in order to inspect the radial drop on the root.    e. Glass beading of the root to remove any re-cast and insure the surface finish is consistent and contains no scratch marks or machine marks; and    f. Shot peening of the machined portion of the root to reduce residual tensile stresses.    
     
     
         7 . A process of forming the radial root of a turbine blade according to  claim 6  wherein the calculation in step (a) is for a female root  
     
     
         8 . A process of forming the radial root of a turbine blade according to  claim 6  wherein the calculation in step (a) is for a male root.  
     
     
         9 . A process according to  claim 6  wherein additional steps after EDM machining and before glass beading are provided, namely: after EDM machining the turbine blade is subjected to MPI to check for depth or presence of surface cracks; and then a liquid tracer coating is applied to the root.  
     
     
         10 . A process according to  claim 7  wherein additional steps after EDM machining and before glass beading are provided, namely: after EDM machining the turbine blade is subjected to MPI to check for depth or presence of surface cracks; and then a liquid tracer coating is applied to the root.  
     
     
         11 . A process according to  claim 8  wherein additional steps after EDM machining and before glass beading are provided, namely: after EDM machining the turbine blade is subjected to MPI to check for depth or presence of surface cracks; and then a liquid tracer coating is applied to the root.  
     
     
         12 . A process according to  claim 1  wherein the re-cast layer is less than 1 micron.  
     
     
         13 . A process according to  claim 6  wherein the re-cast layer is less than 1 micron.  
     
     
         14 . A turbine blade having a root formed by the process of any of  claims 1  to  13 .