EP1358958B1 - A way to manufacture inserts for steam cooled hot gas path components - Google Patents

Description

• The invention relates to a way to manufacture inserts for steam cooledhot gas path components.
• Inserts are required in air and steam cooled nozzles to provide impingementcooling to airfoil walls and ribs. The prior art methods of production of insertsuse sheet metal forming operations, including welding and brazing, andelectro discharge machining of the holes.Such a method is for instance disclosed in EP 0 974 735.The problem with the prior artmethods is that the large complicated inserts combined with strict dimensionaltolerances result in 3D structures that have a very low yield relative to designspecifications.This problem is solved by the metod defined inclaim 1.Preferred embodiments are defined in dependent claims.
• The process casts the inserts to tight tolerance using any ceramiccore casting process such as, for example, pressure casting, centrifugalcasting, squeeze casting or vacuum casting (also known as counter gravitycasting). Counter gravity casting as employed by Hitchiner ManufacturingCo., Inc. of Milford, New Hampshire uses metal dies with a vacuum pour tohelp fill thin sections of the casting and eliminate porosity. After casting hasbeen completed, the core material is then used as a backer duringsubsequent drilling operations. Finally, the core is removed by acid leachingand the cast parts are machined, if necessary, to finish specifications.
• With the manufacturing method certain commercial andperformance needs are met. For example, the method facilitates theproduction of complicated 3D insert geometrics in a Ni-base superalloy withthin walls and tight tolerances. The method also provides foraccurate and precise drilling of the cast inserts with improved processingtimes and fixtures.
• The method also solves the problems in the prior art by using theceramic core in the insert casting process to act as a backer for laser drilling.The laser holes then facilitate the use of leachant to quickly remove the core,speeding the manufacturing process.
• The invention will now be described in greater detail, by way of example, withreference to the drawings, in which:-
• Figure 1 show a cast insert prior to laser drilling of the insert holes;
• Figure 2 shows the case insert of Figure 1 after laser drilling of the insertholes but with the ceramic core still in place; and
• Figure 3 shows the cast and drilled insert immersed in an acid bath forremoval of the ceramic core.
• The method uses, for example, Hitchiner's thin wall casting processto produce the insert geometry from a wax model. The Hitchiner processsubstitutes counter gravity casting for more conventional casting by, forexample, gravity pouring.
• In the basic investment casting process, wax replicas of the desired castingsare produced by injection molding. Depending on the size of the castings,multiple wax replicas may be attached to a central waxed stick, termed asprue, to thereby form a casting assembly. Thereafter, a ceramic shell isformed around the casting or the casting assembly made up of the multiplewax replicas of the desired castings. Next, the ceramic is dried and the wax ismelted out creating a negative impression of the casting assembly within theceramic shell.
• If using the basic casting process, the casting shell is filled with molten metalby gravity pouring. Conversely, in the Hitchiner counter gravity process theceramic casting shell is placed within a vacuum and dipped into a hot metalmelt which is then siphoned up around and into the ceramic casting assembly.After the metal is allowed to solidify on the ceramic casting the vacuum isreleased and residual metal flows back in to the melt. Figure 1 schematicallyshows acast insert 10 withceramic core 12 shown in place by dotted lines.Also shown islaser drilling apparatus 14 prior to the drilling process.
• Preferably, the cast inserts are made from IN625 (Ni-base superalloy). Afterthe insert are cast into the desired geometry, the ceramic core is not immediately leached out. Instead the casting is put in a fixture for laserdrilling holes with the ceramic core still in place. The casting tolerances aresuch that a fixture can handle a production run without a lot of rework.
• The insert is then laser drilled with the ceramic core as a backer. The backerstops backwall strikes and will act as a breakthrough detector. Figure 2schematically showsinsert 10 afterholes 16 have been drilled bylaser drillingapparatus 14.
• After the large number of holes (-300 holes/insert) are drilled with laserprecision, the ceramic core is leached out with suitable caustics. Figure 3schematically shows ceramic 12 being removed by immersingcast insert 10inacid bath 18.
• Another advantage of the inventive method is the reduction in the number ofheat treatments that the part goes through relative to current state of the artprocessing. Reducing heat treatments reduces the amount of distortioncaused by residual stresses and results in higher quality. The invention willproduce inserts and baffles for hot gas path hardware at a greater level ofperformance and yield.
• While the invention has been described in connection with what is presentlyconsidered to be the most practical and preferred embodiment, it is to beunderstood that the invention is not to be limited to the disclosed embodiment.

Claims (7)

1. A method of forming inserts (10) for steam cooled hot gas pathcomponents in a turbine, said method comprising:

   casting the geometrical configuration of the insert(10); and

   laser drilling impingement holes (16) in the cast insert (10) withoutremoving a ceramic casting core (12) of the cast insert (10).
2. A method as claimed in claim 1, wherein after the laser drilling, the ceramic casting core (12)is removed by leachant.
3. A method as claimed in claim 1 or 2, wherein said castingcomprises pressure casting.
4. A method as claimed in claim 1 or 2, wherein said castingcomprises centrifugal casting.
5. A method as claimed in claim 1 or 2, wherein said castingcomprises squeeze casting.
6. A method as claimed in claim 1 or 2, wherein said castingcomprises counter gravity casting.
7. A method as claimed in any preceding claim 1 or 2, wherein theinserts (10) are made of a Ni-base superalloy.

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