Abstract:
A turbine blade is provided, having a fastening section, a platform, and a blade, which follow one another along a longitudinal axis of the turbine blade, wherein the platform, in radial relation to the longitudinal axis, has an inner platform part and an outer platform part, wherein the outer platform part is designed as a closed platform frame that encloses the outer edge of the inner platform part. An associated method for producing a turbine blade is also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2013/056594 filed Mar. 27, 2013, and claims the benefit thereof. The International Application claims the benefit of European Application No. EP12162103 filed Mar. 29, 2012. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a turbine blade with a fastening portion, with a platform for delimiting a flow duct and with a blade airfoil, which succeed one another along a longitudinal axis of the turbine blade. The invention relates, furthermore, to a method for producing a turbine blade, comprising the step: production of a monolithic trunk blade comprising a blade airfoil, a platform for delimiting a flow duct and a fastening portion. 
       BACKGROUND OF INVENTION 
       [0003]    Turbine blades and methods for producing turbine blades are known in many different forms from the abundant prior art. For example, turbine blades for gas turbines are often produced by a casting method. During casting, the blade foot, platform and blade airfoil are formed simultaneously from the casting material, and therefore turbine blades of this type are in one piece. The surfaces, that it to say the platform and blade airfoil, which are exposed to the hot gas of the turbine are subsequently also provided with a corrosion protection layer and with a heat protection layer, in order to increase the service life of the turbine blade. The cast turbine blades are mostly also of hollow form, so that a means for cooling the blade material can flow inside. Turbine blades of steam engines are mostly milled from solid or forged. 
         [0004]    The turbine blades used in stationary turbomachines are subjected, when the turbomachine is in operation, to a multiplicity of loads which cause the turbine blades to age and to become worn both in a predictable and in an unpredictable way. 
         [0005]    In detail, both low-cyclic and higher-cyclic fatigue loads and also thermal mechanical loads occur. Turbine blades must also be protected against oxidation and against creep. The abovementioned loads relate particularly to those surfaces and components of the turbine blades which are exposed directly to the hot gas or hot steam. Moreover, turbine blades are also exposed on the fastening side to what are known as “bearing loads” and “frictional loads”. In view of these different loads and requirements, the material of one-piece turbine blades has to be selected such that, as far as possible, a multiplicity, if not even all, of the loads are absorbed by the material, without premature aging or a premature end of the service life of the turbine blade being reached. With regard to the thermal load and to the corrosion load, it is known, for example, to equip turbine blades of gas turbines with a layer system which protects their material both against corrosion and against an excessive introduction of heat. 
         [0006]    Nevertheless, in various regions of the turbine blades, wear phenomena, such as cracks, may occur, which put the operation of the turbomachine at risk. For this reason, it is known to check turbine blades for defects of this kind after a predetermined period of use and, if one of these is discovered, to exchange the affected turbine blades or treat them again. 
         [0007]    It is also known for turbine blades to have a modular configuration and for the corresponding modules to be produced from those materials which are tailored to their local requirements. However, a permanent and reliable connection is often absent here. 
         [0008]    In this respect, for example, U.S. Pat. No. 4,650,399 A1 discloses a moving blade with two platform halves which are arranged on both sides of a blade airfoil. In order to prevent the platform halves from being displaced along the blade airfoil, a positive connection is provided, the two platform halves being pinned to one another. However, the fastening of the platforms is classed as unsafe. EP 1 905 950 A1 and US 4,019,832 A1 disclose likewise dividing platforms into two halves. 
         [0009]    Instead of the platform halves of U.S. Pat. No. 4,650,399 A1, WO 2000/057032 A1 discloses a single platform element between two directly adjacent moving blades of a moving blade ring. The mounting of the components on the rotor seems to be a disadvantage here. 
         [0010]    Further variants of built-up turbine blades are shown in U.S. Pat. No. 3,451,654 A1 and U.S. Pat. No. 7,762,781 B1. In both publications, it is proposed to use a one-piece platform for turbine blades which has a recess conforming to the profile of the blade airfoil. To complete the turbine blades, in each case the platform-free blade airfoil is inserted through the platform and is fastened by means of intermediate pieces. 
       SUMMARY OF INVENTION 
       [0011]    An object of the invention is to provide a turbine blade with a fastening portion, with a platform for delimiting a flow duct and with a blade airfoil, which succeed one another along a longitudinal axis of the turbine blade, which turbine blade can be regenerated at especially low outlay. A further object of the invention is to provide a method for producing turbine blades. 
         [0012]    The object aimed at the turbine blade is achieved by means of a turbine blade according to the features of the claims. The object aimed at the method for producing a turbine blade is achieved by means of the method steps according to the claims. Advantageous refinements are specified in the respective subclaims. In this case, the features of the respective subclaims can be readily combined with the features of other subclaims. 
         [0013]    Aspects of the invention are based on the recognition that, particularly in the case of operationally stressed turbine blades, defects caused by oxidation may occur even at the outer margin of the platform. These oxidation problems arise, in particular, when the heat insulation layer often also provided there flakes off locally. Such consequences may lead in the turbine blade to an increased operating risk, and therefore turbine blades of this type are exchanged or treated. The retreatment of the turbine blade has hitherto been comparatively complicated. At the same time, the regeneration rate, that is to say the fraction of retreated blades which, after retreatment, actually still qualify for use in the turbomachine, may be rather small. In order to counteract these effects, it is proposed by the invention that the platform comprises radially with respect to a central longitudinal axis an inner platform part and an outer platform part, the outer platform part being designed as a continuous platform frame surrounding the outer margin of the inner platform part. The platform frame is thus continuously peripheral and in one piece and could also be designated as closed. 
         [0014]    In this case, the invention does not attempt to provide a design for modular turbine blades, and therefore that surface of the inner platform part over which the hot gas can flow is preferably substantially larger than that surface of the platform frame over which the hot gas can flow. 
         [0015]    Insofar as an operationally stressed turbine blade is to be regenerated in the region of the platform margin, the defects specified further above in the vicinity of the platform margin must be removed by the margin of the platform being set back, for example, by milling or grinding. According to aspects of the invention, there is in this case provision whereby not only the local defect is removed when the platform margin is set back. 
         [0016]    On the contrary, the platform margin is set back along the entire periphery in order thereby to produce a monolithic trunk blade, the platform of which, as an inner platform part of the turbine blade to be produced, provides a bearing surface for a platform frame. After a continuous platform frame has been attached to the inner platform part, the turbine blade thus produced then has a platform, the dimensions of which correspond to the original turbine blade. 
         [0017]    Admittedly, in this method, unimpaired or undamaged blade material is also removed. However, this has the advantage that retreatment does not have to be carried out individually, that is to say as a function of the defect, but can take place in an automated way. This reduces the outlay for retreatment and the reject rate. 
         [0018]    Of course, new components, that is to say turbine blades which are not operationally stressed, can also be produced according to the method, in that, first, what is known as a trunk blade comprising a blade airfoil, a platform and a fastening portion is provided in a monolithic embodiment. The monolithic trunk blade can be produced conventionally by the casting method and, for example, can also be solidified in a monocrystalline or directional manner. After the production of the trunk blade, the platform margin of the trunk blade has to be brought to the predetermined exact dimension, if appropriate, also along the entire periphery, by minor grinding or milling, in order to provide the inner platform part of the turbine blade with a dimensionally accurate bearing surface for the platform frame. Before, during or after this, the platform frame must be produced as a mostly rectangular structure. After the attachment or mounting of the platform frame on the dimensionally accurate margin of the inner platform part, the turbine blade is then produced as a new component. The platform frame may have various forms in cross section. However, those forms are preferred which bring about a positive connection to the margin of the inner platform part. For example, the cost-sectional form may be diamond-shaped or C-shaped. The margin of the inner platform part is in this case always designed to match with the cross-sectional form. 
         [0019]    A particular advantage of the turbine blade according to aspects of the invention and also of the method is that, in particular, even two different materials can be used for the trunk blade and for the platform frame. Thus, the various local loads can additionally be taken into account, thereby leading, where appropriate, to a prolonged service life of the turbine blade. 
         [0020]    A further advantage of the turbine blade according to aspects of the invention is the higher precision in terms of the outer dimensions of the platform, since these can be implemented more simply during the production of the platform frame than when a purely monolithic turbine blade is being cast. 
         [0021]    Various methods can be employed for connecting the platform frame permanently to the trunk blade. Since the platform frame is configured as a continuous frame, it is preferably appropriate to shrink the platform frame onto the peripheral margin of the inner platform part. Before shrinking on, the platform frame can be heated and/or the trunk blade cooled. After the assembling of the platform frame and trunk blade and subsequent temperature equalization, the platform frame is then seated firmly on the peripheral margin of the inner platform part. Even soldering and welding, in spot form and also along the connecting line of the margin of the inner platform part and platform frame, are possible. 
         [0022]    According to a first advantageous development, the platform frame bears against the inner platform part in a sheet-like manner, the contact area forming at least partially with the longitudinal axis at an angle which is larger than 0° and smaller than 90°. Such an arrangement prevents a parallel displacement of the platform frame along the longitudinal axis, at least in one direction, this being advantageous particularly when the invention is used on turbine moving blades. In this case, the centrifugal force acting upon the platform frame during the operation of the turbomachine is also transferred by positive connection into the platform part because the contact area is inclined with respect to the longitudinal axis. This reliably prevents the loss of the platform frame due to the centrifugal force. 
         [0023]    Preferably, the angle amounts to a size of between 15° and 35°, for example the angle lies at 20°. 
         [0024]    According to a second advantageous development, the platform frame has, on at least one surface pointing laterally outward, a slot for the reception of a sealing element. Such a refinement affords the advantage that, in the event of wear of slots present in the platform margin, because of the sheet-like sealing elements seated in them, the invention which is in this case provided offers a simple and reliable possibility of regenerating even operationally stressed turbine blades of this type. Moreover, such slots can be produced most cost-effectively than in the case of purely monolithic turbine blades. 
         [0025]    The turbine blade may expediently be designed both as a guide blade or as a moving blade. 
         [0026]    So that the turbine blade, with the inner platform part and the outer platform part in the form of the continuous platform frame surrounding the inner platform part, can be used even in high-temperature applications, it is advantageous if the inner platform part and the platform frame are coated in a coating operation. A seamless protective layer can thus be applied to both platform parts. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    Further advantages of features of the invention are specified in the description of the figures in which: 
           [0028]      FIG. 1  shows, in a perspective illustration, the side view of a turbine blade according to an embodiment of the invention, 
           [0029]      FIG. 2  shows, in a perspective illustration, the top view of a platform frame, 
           [0030]      FIG. 3  shows the cross section through a turbine blade according to  FIG. 1 , and 
           [0031]      FIG. 4  shows a detail of the corner of a platform of the turbine blade according to  FIG. 1  during the assembly of the trunk blade and platform frame. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0032]    Identical features are given identical reference numerals in all the figures. 
         [0033]      FIG. 1  shows a perspective illustration of a turbine blade  10 . The turbine blade  10  is designed as a moving blade. However, it may also be designed as a guide blade. The turbine blade  10  comprises, succeeding one another directly along its longitudinal axis  12 , a fastening portion  14 , a platform  16  and a blade airfoil  18 . The fastening portion  14  is contoured in a manner of a pine tree profile in a way typical of a moving blade. Guide blades for turbines mostly have, instead of the pine tree-shaped fastening portion  14 , a plurality of hooks which are pushed into a guide blade carrier, not illustrated in any more detail, of the turbomachine. 
         [0034]    The fastening portion  14  merges into the platform  16 . According to the illustration chosen in  FIG. 1 , the platform  16  has an upwardly pointing platform surface  20  on which the blade airfoil  18  is seated. In the exemplary embodiment illustrated, the platform  16  comprises radially with respect to the longitudinal axis  12  an inner platform part  22  and an outer platform part  24 , the outer platform part  24  being designed as a continuous platform frame  28  surrounding the outer margin  26  of the inner platform part  22 . According to the exemplary embodiment illustrated, both the fastening portion  14 , the inner platform part  22  and the blade airfoil  18  are formed monolithically, that is to say in one piece. This monolithic unit is also designated as a trunk blade  19 . The surfaces  20  of the inner platform part  22  and of the outer platform part  24  which point toward the blade airfoil  18  according to this illustration are offset-free with respect to one another, so that, when the turbine blade  10  is used in a turbomachine, they provide an edge-free and step-free boundary wall for the working medium flowing in the turbomachine. 
         [0035]    In this case, that surface of the inner platform part  22  over which the hot gas can flow is substantially larger than that surface of the platform frame  28  over which the hot gas can flow. 
         [0036]    The turbine blade  10  may be designed to be internally cooled with the aid of a cooling medium in any way. Even film cooling ports and trailing edge ports for coolant may be provided. The turbine blade may, of course, also be uncooled. 
         [0037]      FIG. 2  shows a perspective illustration of the platform frame  28 . The platform frame  28  comprises two longitudinal struts  30  parallel to one another and two transverse struts  32  parallel to one another. The platform frame  28  may be produced from a material other than that of the trunk blade  19  illustrated in  FIG. 1 . However, the platform frame  28  may also be produced from the same material. The platform frame may also be produced by welding the longitudinal struts  30  to the transverse struts  32 . It may also be cast or be milled from solid.  FIG. 3  shows a section through the turbine blade  10  along the longitudinal axis  12 . In contrast to the turbine blade illustrated in  FIG. 1 , the fastening portion according to  FIG. 3  is not of pine tree-shaped form, but instead is dovetail-shaped. Moreover,  FIG. 3  shows the platform frame  28  during mounting on the trunk blade  19 , shortly before the platform frame  28  reaches its ultimate mounting position. According to the exemplary embodiment illustrated, the platform frame  28  is diamond-shaped in cross section. Other forms are also possible. 
         [0038]    Each strut  30 ,  32  of the platform frame  28  has an inwardly directed first bearing surface  34  and a second bearing surface  36 . The outer margin  26  of the inner platform part  22  likewise has a laterally outward-pointing first bearing surface  38  and a second bearing surface  40 . After the mounting of the platform frame  28 , its first bearing surface  34  bears in a sheet-like manner against the first bearing surface  38  of the inner platform part  22  and its second bearing surface  36  bears in a sheet-like manner against the second bearing surface of the inner platform part  22 . The first contact areas and second contact areas thus formed are inclined differently with respect to the longitudinal axis  12 . The first contact area is oriented parallel to the longitudinal axis  12  in cross section. However, the second contact area is inclined in cross section at an acute angle α to the longitudinal axis  12 . This embodiment also prevents, by positive connection, the release of the platform frame  28  from the trunk blade  19  under centrifugal forces acting upon the turbine blade  10 . 
         [0039]      FIG. 4  shows a perspective illustration of a corner of the inner platform part  22  and of the platform frame  28  during mounting. In addition to the features already described,  FIG. 4  also shows, on a laterally outward-pointing surface  42  of the platform frame  28 , a slot  44  for the reception of a sheet-like sealing element. 
         [0040]    Thus, overall, the invention relates to a turbine blade  10  with a fastening portion  14 , with a platform  16  and with a blade airfoil  18 , which directly succeed one another along a longitudinal axis  12  of the turbine blade. In order to provide a turbine blade  10  having an especially long service life, it is proposed that the platform  16  comprises radially with respect to the longitudinal axis  12  an inner platform part  22  and an outer platform part  24 , the outer platform part  24  being designed as a continuous platform frame  28  surrounding the outer margin  26  of the inner platform part  22 . In terms of the method, it is proposed to use a monolithically produced trunk blade  19  or to adapt or set back an already operationally stressed turbine blade  10  along the entire periphery of the platform margin and to produce a continuous platform frame  28  which can subsequently be mounted on the platform margin  26 , in order thereby to produce the original or planned dimensions of the turbine blade  10 .