Patent Publication Number: US-2012039718-A1

Title: Casting apparatus for producing a turbine rotor blade of a gas turbine and turbine rotor blade

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2010/054930, filed Apr. 15, 2009 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 09005533.6 EP filed Apr. 20, 2009. All of the applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF INVENTION 
     The invention relates to a casting apparatus for producing a turbine rotor blade of a gas turbine according to the claims. Furthermore, the invention relates to a turbine rotor blade according to the claims. 
     BACKGROUND OF INVENTION 
     Casting apparatuses for producing a turbine rotor blade of a gas turbine have been known from the prior art for a very long time. As is known from U.S. Pat. No. 5,465,780, for example, a casting apparatus comprises a plurality of mold shells arranged in a cluster for simultaneously casting a plurality of turbine rotor blades. Each mold shell is hollow in form here, with the cavity representing the negative mold of the turbine rotor blade to be produced. Since turbine rotor blades, in particular the rotor blades of front turbine stages, generally have to be cooled, they are likewise hollow in form. A cooling medium, usually cooling air, can be conducted through the cavities of the turbine rotor blade during operation, and therefore the turbine rotor blades have a particularly long service life and do not sustain premature, thermally induced damage on account of the hot-gas flow which flows past them. Cooling air is supplied here via openings arranged in the blade root, which are connected in terms of flow to the cavity or the cavities of the rotor blade. The mold shell for producing such a turbine rotor blade therefore usually comprises one or more casting cores, which are arranged in the cavity of the casting apparatus. What the casting cores leave behind in the cast turbine rotor blade after they have been removed are the cavities through which the coolant flows during operation of the gas turbine. 
     It is also known for the casting apparatus to have at least one inlet channel, usually called a feeder, through which the casting material can be fed into the cavity of the mold shell during casting of the rotor blade. Consequently, the inlet channel issues by way of its inlet opening into that surface which delimits the cavity of the mold shell. 
     In the case of turbine rotor blades, it has emerged that the walls of those portions of the cooling channels which are arranged in the blade root tend to undergo cracking and crack propagation during operation. These cracks can impair, and possibly shorten, the service life of the turbine rotor blade. 
     SUMMARY OF INVENTION 
     It is therefore an object of the invention to provide a turbine rotor blade with an increased service life and to provide a casting apparatus for producing such a turbine rotor blade. 
     The object directed to the casting apparatus is achieved by a casting apparatus designed according to the features of the claims. The object relating to the turbine rotor blade is achieved by a turbine rotor blade designed as claimed in the claims. 
     The invention is based on the realization that the formation of cracks in the walls of the cooling channels in the region of the turbine rotor blade on the blade root side during solidification of the melt is production-related. In the prior art, turbine rotor blades are cast vertically as standard, with the cavity in the mold shell being formed in such a manner that the negative mold of the main blade part of the turbine rotor blade is formed at the bottom, and the platform and the blade root are formed thereabove. The terms “top” and “bottom” refer here to the horizontal plane. The inlet for the molten casting material is usually likewise located at the top, since it has been found to be advantageous for turbine rotor blades to be produced in a top-cast method in which the location of the last solidification of the casting material is at the top and therefore at the higher-mass blade root. In casting apparatuses known from the prior art, the inlet channel runs transversely to the longitudinal axis of the turbine rotor blade and therefore approximately parallel to the horizontal plane, in order to specify a casting apparatus of lesser height. By virtue of this transverse feeding-in of the molten casting material, the latter flows into the cavity of the mold shell, after it has emerged from the inlet opening, and then falls onto the base of the mold shell, where the negative of the main blade part tip is formed. The continued supply of molten casting material completely fills the cavities for the main blade part, the platform and the blade root of the turbine rotor blade with liquid, hot casting material. Since the blade root usually has a symmetrical form in the form of a hammer or in the form of a fir tree, and the cooling channels are usually positioned centrally in the blade root, the conventional casting apparatuses always encountered situations in which the liquid casting material flowing into the cavity of the mold shell impinged transversely on the casting cores positioned upstream of the inlet opening. In detail, the molten casting material came into contact with the root region of the centrally positioned casting core. This had the effect that the casting cores experienced greater heating at the point of impingement of the hot casting material than in other regions. These hotter regions of the casting cores are also referred to as hot spots. The other regions of the casting cores, by contrast, were not heated to such a great extent. During cooling of the casting material and the resulting solidification, delayed solidification of the casting material occurred in those regions of the casting material which adjoined the locally hotter regions of the casting cores, as compared with cooler regions of the casting cores. The delayed solidification of the casting material in the corresponding regions led to disturbances in the microstructure of the solidified material which, during operation, promoted the formation of cracks and crack propagation. On account of this realization, the invention proposes that hot, liquid casting material has to be fed into the cavity of the mold shell, during casting of the turbine rotor blade, such that it does not impinge directly on casting cores. According to the invention, the intention is for the casting material to flow into the cavity freely and without disturbances and to impinge on the base of the mold shell, which finals the blade tip. Since the inlet is usually arranged centrally in the region of the blade root at the end face, this requires the casting cores to be arranged eccentrically, with respect to the longitudinal axis of the blade root. This leads to a casting apparatus in which that part of the cavity into which an imaginary extension of the inlet channel protrudes is free of casting cores at least on the inlet opening side. 
     A casting apparatus according to the invention therefore prevents the hot casting material flowing in from impinging transversely on casting cores as it is being introduced into the cavity of the mold shell, and avoids the resultant creation of hot casting core regions, so-called hot spots. Locally delayed solidification of the casting material also no longer occurs during cooling by virtue of hotter casting core regions being avoided. The solidification of the casting material is thus made more uniform overall, and therefore imperfections in the microstructure of the turbine rotor blade material can be avoided. By avoiding the imperfections, the formation of cracks and propagation of cracks in the material of turbine rotor blades which surrounds the cooling channel portions on the blade root side during operation are effectively avoided. This reduces the amount of rejects and increases the service life of turbine rotor blades. 
     Since the casting cores are generally formed like a bar at least in the portion of the blade root, the eccentric positioning thereof in the mold shell has the effect that the openings of cooling channels in the blade root of the turbine rotor blade are likewise arranged eccentrically, with respect to the, generally symmetrical, outer contour of the blade root. Here, the symmetry relates to the blade root contour, which is in the form of a fir tree or in the form of a hammer in cross section. 
     The surface of the mold shell has a contour for the blade root of the turbine rotor blade which is minor-symmetrical along a blade root center. The contour here is in the form of a fir tree or in the form of a hammer. Here, it is also the case that the inlet channel is arranged centrally and one of the casting cores is arranged eccentrically at least in the region of the inlet opening—both with respect to a blade root center which, by definition, lies centrally between the lateral, undulating surfaces or contours of the blade root. On account of the eccentric arrangement of the casting cores and since the blade root needs to be kept compact, it is necessary for the cross-sectional area of the previous casting core to be divided into two casting cores. By dividing the previous, centrally positioned cooling channels into in each case two cooling channels positioned eccentrically in parallel, it is possible to further maintain the cross-sectional area required for the cooling air, but with the previous cross-sectional area then being distributed over the in each case two cooling channels, which then each have half the previous cross-sectional area. Consequently, a cooling channel inlet present from the prior art is divided into two cooling channel inlets in the case of a turbine rotor blade according to the invention. 
     This has the effect that, in the case of a turbine rotor blade, two openings are arranged on the underside, but on both sides of the blade root center, the openings representing supply openings for coolant for the turbine rotor blade. Each opening thus forms an end of a cooling channel of the turbine rotor blade. 
     Advantageous configurations of the casting apparatus and of the turbine rotor blade are given in the dependent claims. 
     It is preferable for the inlet channel to issue into that part of the surface of the cavity of the mold shell which forms the negative of the end face of the blade root of the turbine rotor blade. It is thereby possible to form a sufficiently large inlet channel. At the same time, a top-cast method for turbine rotor blades with the blade root arranged at the top makes it possible to cast turbine rotor blades of which the region having the greatest volume, specifically the blade root, solidifies last. Shrinkage of the casting material, which possibly occurs during solidification, can be compensated for by the afterflow of molten casting material from the gate region. In addition, a compact casting apparatus can therefore be specified. 
     It is preferable in the case of a casting apparatus for that part of the cavity into which an imaginary extension of the inlet channel protrudes to be completely free of casting cores. Therefore, not only is the cavity of the mold shell free of casting cores on the inlet opening side, but also that region of the cavity which is located opposite the inlet opening is free of casting cores. 
     Depending on the configuration of the turbine rotor blade, of the casting apparatus and of the process parameters set for casting the turbine rotor blade, it suffices for not all of the casting cores to be arranged eccentrically, but merely those which are arranged particularly close to the inlet opening. In other words: the casting cores which are located furthest away from the inlet opening and the portions of which are arranged in the rotor blade region of the turbine rotor blade can also lie in the imaginary extension of the inlet channel, if the casting material flowing into the cavity does not reach them. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in further detail with reference to the exemplary embodiment shown in the drawing. Expedient configurations result here by the advantageous combination of features of the apparatuses described hereinbelow. 
         FIG. 1  is a perspective illustration showing a casting apparatus with casting cores arranged according to the invention therein, and 
         FIG. 2  is a perspective illustration showing a turbine rotor blade according to the invention for a gas turbine. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  is a perspective, diagrammatic illustration showing part of a casting apparatus  10  for producing a turbine rotor blade of a gas turbine. The casting apparatus  10  comprises at least one mold shell  12  with a cavity  14 . The cavity  14  is delimited by a surface  16 , which represents the negative mold of the turbine rotor blade to be produced. A total of six casting cores  18  are arranged in the cavity  14 . The casting cores  18  here are always arranged in pairs. There are a total of three pairs of casting cores. It goes without saying that a larger or a smaller number of (pairs of) casting cores can also be present in the mold shell  12 . Furthermore, an inlet channel  20  is provided in the mold shell  12  for the introduction of the liquid casting material. Here, the inlet opening  22  of said inlet channel issues into the surface  16  which delimits the cavity  14 . The cavity  14  is formed in the mold shell  12  in such a manner that the negative mold of the main blade part tip of the turbine rotor blade is arranged right at the bottom. The part of the surface arranged thereabove forms the negative of the main rotor blade part. Above the latter, in turn, the part of the surface is contoured such that the negative mold of the platform of the turbine rotor blade is formed. Adjoining the latter, and therefore arranged right at the top with respect to the horizontal plane, the rest of the surface  16  forms the contour of the blade root. 
     By way of its inlet opening  22 , the inlet channel  20  issues into that part of the surface which specifies an end face of the blade root. Here, the inlet channel  20  has a rectilinear longitudinal extent immediately upstream of the inlet opening  22  in the casting apparatus shown. Here, the longitudinal extent of the inlet channel  20  runs approximately parallel or at a slight inclination with respect to the horizontal plane. 
     The casting cores  18  are not shown in their entirety in  FIG. 1 .  FIG. 1  merely shows those portions of the casting cores  18  which are arranged in the uppermost part of the cavity  14 , which specifies the negative mold of the blade root. The form, contour and nature of the casting cores  18  in the region on the platform side or in the region on the side of the main blade part are not of further interest for the invention and can therefore be designed as desired, for example in meandering form, rectilinearly or else with only a slight curvature. In this case, the respective cooling channels can also be brought together again in part. 
     The casting cores  18  which form a respective pair are spaced apart from one another. Here, the distance A between them is sufficiently large that hot, liquid casting material does not impinge directly on the casting cores  18  as the cavity  14  is being filled. In a manner of speaking, the hot casting material fed into the cavity  14  flows through between two directly adjacent casting cores  18 . The intention is therefore to avoid contact between inflowing liquid casting material and the casting core surface in the root region as far as possible. This avoids casting core regions with a locally elevated temperature. The locally elevated casting core temperature was the cause of crack phenomena, occurring in the prior art, on the walls of cooling channels of turbine rotor blades. 
     The imaginary extension of the longitudinal extent of the inlet channel  20  consequently extends into the free region between the two casting cores  18  of a pair of casting cores. 
     According to the configuration shown in  FIG. 1 , the imaginary extension of the inlet channel is completely free of casting cores  18 . As an alternative to this, it is possible for merely only that part of the imaginary extension which is formed on the inlet opening side to be free of casting cores  18 . With respect to the configuration in  FIG. 1 , this means that, by way of example, the pair of casting cores shown in the center and the pair of casting cores shown on the left can each also be replaced by a single casting core, the portions of which which are arranged in the blade root are positioned in the imaginary extension of the inlet channel  20 . However, this assumes that the coverage of the inflowing hot casting material is not so great that the inflowing jet can impinge thereon. 
       FIG. 2  is a perspective view showing a turbine rotor blade  30 , which has been produced using the casting apparatus shown in  FIG. 1 . The turbine rotor blade  30  has a blade root  32 , which is contoured in the form of a fir tree in longitudinal section and on which a platform  34  is arranged. The platform  34  is adjoined by an aerodynamically curved main blade part  36 , which ends at a detached main blade part tip  38 . The turbine rotor blade  30  therefore extends along a longitudinal axis  40  from the blade root  32  to the main blade part tip  38 . The longitudinal axis  40  here is arranged in such a manner that it runs centrally or symmetrically with respect to the contour of the blade root  32  in the form of a fir tree. The face of the blade root  32  which faces away from the main blade part  36  and runs transversely to the longitudinal axis  40 , and is also referred to as the underside  42 , has a plurality of openings  44 , which remain when the casting cores  18  have been removed from the cast turbine rotor blade  30 . Here, the openings  44  are arranged on both sides of the blade root center, which is defined in cross section by the longitudinal axis  40  and also lies centrally between the lateral, undulating surfaces of the blade root. Here, they lie in two rows each with three openings  44 . The openings  44  serve for the introduction of a coolant into the interior of the turbine rotor blade  30 . Each opening  44  here forms an end of a cooling channel of the turbine rotor blade  30 . The course of the openings within the turbine rotor blade  30  is not of further importance for the invention. 
     The invention prevents nonuniform, local overheating of the casting cores  18  in the vicinity of the inlet as the cavity  14  is being filled. At the same time, it is possible for better filling to take place, since casting cores  18  no longer block the inlet opening  22 . A collision between inflowing hot casting material and casting cores  18  is prevented by the use of the invention. In addition, the unhindered afterflow of hot casting material (feed) from the inlets can further improve the solidification, and this reduces residual stress and avoids crack formation. 
     In summary, it can be stated that the invention relates to a casting apparatus  10  for producing a turbine rotor blade  30  of a gas turbine, wherein the mold shell  12 , the inlet thereof and the casting cores  18  arranged therein are oriented with respect to one another in such a way that a casting material flowing into the cavity  14  of the mold shell  12  does not impinge directly on casting cores  18 . So-called hot spots on casting cores  18  are thereby avoided, which until now have had negative effects on the solidification of the casting material. Particularly in the region of the blade root  32  of the turbine rotor blade  30  to be produced, it is therefore possible to obtain improved solidification of the casting material, reducing disturbance in the microstructure of the solidified casting material. On account of the reduction or prevention of the disturbances, the formation of cracks and the propagation of cracks in the region of the cooling channel portions on the blade root side are avoided, increasing the service life of the turbine rotor blade  30 .