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 cooled hot gas path components.
• Inserts are required in air and steam cooled nozzles to provide impingement cooling to airfoil walls and ribs. The prior art methods of production of inserts use sheet metal forming operations, including welding and brazing, and electro discharge machining of the holes. Such a method is for instance disclosed in EP 0 974 735. The problem with the prior art methods is that the large complicated inserts combined with strict dimensional tolerances result in 3D structures that have a very low yield relative to design specifications. This problem is solved by the metod defined in claim 1. Preferred embodiments are defined in dependent claims.
• The process casts the inserts to tight tolerance using any ceramic core casting process such as, for example, pressure casting, centrifugal casting, squeeze casting or vacuum casting (also known as counter gravity casting). Counter gravity casting as employed by Hitchiner Manufacturing Co., Inc. of Milford, New Hampshire uses metal dies with a vacuum pour to help fill thin sections of the casting and eliminate porosity. After casting has been completed, the core material is then used as a backer during subsequent drilling operations. Finally, the core is removed by acid leaching and the cast parts are machined, if necessary, to finish specifications.
• With the manufacturing method certain commercial and performance needs are met. For example, the method facilitates the production of complicated 3D insert geometrics in a Ni-base superalloy with thin walls and tight tolerances. The method also provides for accurate and precise drilling of the cast inserts with improved processing times and fixtures.
• The method also solves the problems in the prior art by using the ceramic 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, with reference 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 insert holes but with the ceramic core still in place; and
• Figure 3 shows the cast and drilled insert immersed in an acid bath for removal of the ceramic core.
• The method uses, for example, Hitchiner's thin wall casting process to produce the insert geometry from a wax model. The Hitchiner process substitutes counter gravity casting for more conventional casting by, for example, gravity pouring.
• In the basic investment casting process, wax replicas of the desired castings are produced by injection molding. Depending on the size of the castings, multiple wax replicas may be attached to a central waxed stick, termed a sprue, to thereby form a casting assembly. Thereafter, a ceramic shell is formed around the casting or the casting assembly made up of the multiple wax replicas of the desired castings. Next, the ceramic is dried and the wax is melted out creating a negative impression of the casting assembly within the ceramic shell.
• If using the basic casting process, the casting shell is filled with molten metal by gravity pouring. Conversely, in the Hitchiner counter gravity process the ceramic casting shell is placed within a vacuum and dipped into a hot metal melt 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 is released and residual metal flows back in to the melt. Figure 1 schematically shows a cast insert 10 with ceramic core 12 shown in place by dotted lines. Also shown is laser drilling apparatus 14 prior to the drilling process.
• Preferably, the cast inserts are made from IN625 (Ni-base superalloy). After the insert are cast into the desired geometry, the ceramic core is not immediately leached out. Instead the casting is put in a fixture for laser drilling holes with the ceramic core still in place. The casting tolerances are such 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 backer stops backwall strikes and will act as a breakthrough detector. Figure 2 schematically shows insert 10 after holes 16 have been drilled by laser drilling apparatus 14.
• After the large number of holes (-300 holes/insert) are drilled with laser precision, the ceramic core is leached out with suitable caustics. Figure 3 schematically shows ceramic 12 being removed by immersing cast insert 10 in acid bath 18.
• Another advantage of the inventive method is the reduction in the number of heat treatments that the part goes through relative to current state of the art processing. Reducing heat treatments reduces the amount of distortion caused by residual stresses and results in higher quality. The invention will produce inserts and baffles for hot gas path hardware at a greater level of performance and yield.
• While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood 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 path components 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) without removing 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 casting comprises pressure casting.
4. A method as claimed in claim 1 or 2, wherein said casting comprises centrifugal casting.
5. A method as claimed in claim 1 or 2, wherein said casting comprises squeeze casting.
6. A method as claimed in claim 1 or 2, wherein said casting comprises counter gravity casting.
7. A method as claimed in any preceding claim 1 or 2, wherein the inserts (10) are made of a Ni-base superalloy.

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