Abstract:
Process for preparing turbine blades or vanes for a subsequent treatment, for example the application of a coating and/or a material-removing operation, wherein the turbine blade or vane has an airfoil, which is delimited at at least one end by an endplate with peripheral surfaces, wherein at least one of the peripheral surfaces is at least partially covered prior to the treatment of the turbine blade or vane. In the process a covering strip is fitted onto at least one peripheral surface to form a plug connection.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of European application No. 06000401.7 EP filed Jan. 10, 2006, which is incorporated by reference herein in its entirety. 
     FIELD OF INVENTION 
     The invention relates to a process for preparing turbine blades or vanes for a subsequent treatment, for example the application of a coating and/or a material-removing operation, wherein the turbine blade or vane has an airfoil, which is delimited at at least one end by an endplate with peripheral surfaces, wherein at least one of the peripheral surfaces is at least partially covered prior to the treatment of the turbine blade or vane. The invention also relates to a turbine blade or vane having an airfoil, which is delimited at at least one end by an endplate that has a hot-gas side facing the airfoil and adjoining peripheral surfaces. 
     BACKGROUND OF INVENTION 
     To improve their temperature and/or abrasion resistance, turbine blades or vanes, such as guide vanes and rotor blades which are intended for gas turbines, are coated with suitable metals, metal alloys or ceramics. The coating is done by means of a spray coating apparatus in which the turbine blade or vane is spray-coated. Examples of spray-coating processes include atmospheric plasma spraying (APS) and high-velocity oxyfuel spraying (HVOF) (cf. Ullmanns Encyclopedia of Industrial Chemistry, 2003, Vol 21, pages 573 and 575). 
     In the context of turbine blades or vanes, a distinction is drawn between guide vanes and rotor blades. Both have an airfoil which is exposed to the hot gas and at one end merges into a root body which serves to secure the turbine blade or vane either to a rotor (in the case of a rotor blade) or to a holder (in the case of a guide vane). At the other end of the airfoil, a guide vane additionally has a head body which, like the root body, is intended for securing to a holder. At the transition to the airfoil, root and head bodies form endplates in the form of root plates or head plates, which have a hot-gas side facing the airfoil and adjoining peripheral surfaces. 
     The coating described above is carried out only on those surfaces which are exposed to the hot gas, i.e. the airfoil and the hot-gas sides of the root plate and if present also head plate. The peripheral surfaces of these endplates and also the remaining parts of the root body and if present head body, according to the specification, must remain free of coating, since they have already been machined to their final dimensions. Therefore, in the spray coating apparatus the root body and if present also the head body are covered as far as possible, apart from the respective hot-gas side. However, it is virtually inevitable that coating material will also reach those parts of the peripheral surfaces of the endplates which are adjacent to the hot-gas side, i.e. that some overspray, as it is known, will occur. This requires the coating to be removed by grinding (overspray grinding) in a subsequent process step. This presents the risk of uncoated parts of the peripheral surfaces also being ground, with the result that their final dimensions change. 
     SUMMARY OF INVENTION 
     An object of the invention is to provide a process which can be used to protect the peripheral surfaces of the endplates of turbine plates or vanes both during the coating operation and during the subsequent overspray grinding. A further object is to design a turbine blade or vane which is suitable for this purpose. 
     According to the invention, the first part of the object is achieved by a process in which a covering strip is fitted to at least one peripheral surface to form a plug connection. Therefore, it is a basic concept of the invention for the peripheral surfaces of the endplate(s) to be protected with the aid of covering strips, which are each held on the turbine blade or vane by means of a plug connection. The plug connection allows simple yet secure fixing of the covering strip in the region of the peripheral surfaces. The covering strip protects the peripheral surfaces from being sprayed with coating material during the coating operation. Should overspray, i.e. coating of part of the peripheral surfaces, occur, this overspray can be removed following the coating operation by grinding with the covering strip still attached, in which case the covering strip then protects the uncoated part of the peripheral surfaces. This significantly reduces the risk of the size dropping below the required final dimensions. 
     In a refinement of the invention, the covering strip used, or the required number of covering strips used, in the plug-on state covers that region of the respective peripheral surface which extends on the side of the plug connection remote from the airfoil. In this case, overspray can only occur in the part which is left uncovered by the covering strip. 
     According to the invention, it is also proposed that peripheral surface(s) and covering strip(s) are matched to one another in such a manner that their plug connection produces a tongue-and-groove connection. This can be done in such a way that a groove is formed into at least one of the peripheral surfaces, and the covering strip used has a plug-in limb which is plugged into the groove. The covering strip can in this case be formed as angle profiled section with a protective limb adjoining the plug-in limb, in which case it is expedient for the plug-in limb and protective limb to be at right angles to one another. 
     It has proven expedient to use a covering strip with a length that is greater than the depth of the airfoil, so that the covering strip has a projecting portion. 
     According to the invention, a step is also formed into the peripheral surfaces at least in the region of the plug connection. It may project outward on the side of the plug connection remote from the airfoil, but also on the side facing the airfoil. 
     According to the invention, the second part of the object is achieved by a turbine blade or vane in which at least one of the peripheral surfaces in each case has at least one plug-connection recess and/or a plug-connection projection. If a plug-connection recess is provided, it may, for example, be formed as a groove. 
     As has already been mentioned above, the turbine blade or vane may also be formed in such a way that the peripheral surfaces, in the region of the plug-connection recess or the plug-connection projection, have a step which projects either on the side of the plug-connection recess or plug-connection projection remote from the airfoil or on the side facing the airfoil. 
     Furthermore, according to the basic concept of the process according to the invention, it is also provided that a covering strip is fitted onto at least one peripheral surface, so as to form a plug connection to the plug-connection recess or the plug-connection projection, this covering strip advantageously covering that region of the peripheral surface which extends on the side of the plug connection remote from the airfoil. The plug connection should preferably be formed as a tongue-and-groove connection. This can be realized in such a way that the covering strip has a plug-in limb which fits into the at least one plug-connection recess. 
     The covering strip is advantageously formed as an angle profiled section having a protective limb which adjoins the plug-in limb at an angle. The limbs may form a right angle with one another. The length of the covering strip should be greater than the depth of the airfoil. 
     The covering strip may consist of a suitable material which is able to withstand overspray grinding. 
     It should expediently consist of a metal, such as steel or the like. 
     If the turbine blade or vane is in the form of a guide vane, the airfoil of which is delimited by endplates at both ends, a covering strip should be fitted onto in each case at least one of the peripheral surfaces of both endplates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail in the drawing and with reference to exemplary embodiments. In the drawing: 
         FIG. 1  shows a perspective view of a guide vane for a gas turbine; 
         FIG. 2  shows a partial cross section through the root plate of the guide vane shown in  FIG. 1 ; 
         FIG. 3  shows a partial cross section through a second variant of a root plate, and 
         FIG. 4  shows a partial cross section through a third variant of a root plate. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The guide vane  1  illustrated in  FIG. 1  is intended for a turbomachine onward. This may be a gas turbine for an aircraft or a power plant for power generation, a steam turbine or a compressor. 
     The guide vane  1  has, in succession along its extent a securing region  2 , an adjoining root plate  3 , an airfoil  4  and a head part  5  which adjoins the vane tip and has a head plate  6  adjacent to the airfoil  4 . The head part  5  is not present if the turbine blade or vane is configured as a rotor blade. 
     In the securing region  2  there is a blade root  7 , which is used to secure the guide vane  1  to a disk (not shown). The vane root  7  is in this case designed as dovetail root. Other configurations, such as a fir-tree or hammerhead root, are also possible. The airfoil  4  has a leading edge  8  and a trailing edge  9  for a medium which passes through the turbomachine, flowing past the airfoil  4 . 
     Conventional vanes consist, for example, of solid metallic materials, in particular superalloys. Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949; these documents form part of the disclosure with regard to the chemical composition of the alloy. The turbine blade may in this case be produced by a casting process, also by means of directional solidification, by a forging process, by a milling process or combinations thereof. 
     The turbine blades are provided with a coating protecting against corrosion or oxidation, e.g. MCrAlX (M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and represents yttrium (Y) and/or silicon and/or at least one rare earth element, or hafnium (Hf). Alloys of this type are known from EP 0 486 489 B1, EP 0 786,017 B1, EP 0 412 297 B1 or EP 1 306 454 A1, which are intended to form part of the present disclosure with regard to the chemical composition of the alloy. 
     It is also possible for a thermal barrier coating, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, to be present on the MCrAlX coating. Columnar grains are produced in the thermal barrier coating by means of suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD), or grains which are porous, have microcracks and have macrocracks are produced in the thermal barrier coating by atmospheric plasma spraying (APS). 
     Refurbishment means that after they have been used, protected layers may have to be removed from components (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the component are also repaired. This is followed by recoating and reuse of the turbine vane. 
     The guide vane  1  is hollow in form. On the pressure side, visible in  FIG. 1 , of the airfoil  4  it has, in the region of the trailing edge  9 , a row of film-cooling holes—denoted for example by  10 —via which cooling air introduced into the airfoil  4  can flow out, thereby cooling the trailing edge  9 . 
     The root plate  3  and the head plate  6  each have a hot-gas side  11 ,  12  facing the airfoil  4 . The hot-gas sides  11 ,  12  are in each case adjoined by four peripheral surfaces  13 ,  14 ,  15 ,  16  and  17 ,  18 ,  19 ,  20 , respectively, which are each provided with a step  21 ,  22 , as can also be seen from  FIG. 2 . The steps  21 ,  22  project outward with respect to that region of the peripheral surfaces  13  to  20  which is in each case adjacent to the airfoil  4 . The steps  21 ,  22  merge into a peripheral groove  23 . Grooves  23  are formed both into the peripheral surfaces  13  to  16  of the root plate  3  and into the peripheral surfaces  17  to  20  of the head plate  6 . 
     Covering strips  24 ,  25  are plugged into the grooves  23 . The covering strips  24 ,  25  extend over the entire width of the peripheral surface  13  or  17 . They are formed as angle profiled sections, each having a plug-in limb  26  and a protective limb  27 ,  28  in each case running at right angles to the plug-in limb. The plug-in limbs  26  are seated in the grooves  23  and in this way form a plug connection to the root plate  3  or the head plate  6 . The covering strips  24 ,  25  are fitted into the grooves  23  in such a way that their protective limbs  27 ,  28  are directed away from the respective hot-gas side  11  or  12  and cover those regions of the peripheral surfaces  13  or  17  which—as seen from the airfoil  4 —lie behind the grooves  23 . 
     The guide vane  1  having the covering strips  24 ,  25  is provided with the coating described above in a spray coating apparatus, specifically in such a manner that the airfoil  4  and the hot-gas sides  11 ,  12  of the root plate  3  and head plate  6  are coated. The coating operation also produces an overspray coating  29 ,  30  on those regions of the peripheral surfaces  13 ,  17  which are not covered by the covering strips  24 ,  25 . The overspray coatings  29 ,  30  are manually ground away after the coating operation, with the covering strips  24 ,  25  remaining on the root plate  3  and head plate  6 , thereby providing protection against slipping on the part of the grinding unit. In this way, the grinding operation remains restricted to the region which has the overspray coating  29 ,  30 . The covering strips  24 ,  25  are only removed again after the grinding operation, and can then be reused. 
       FIGS. 3 and 4  show forms of peripheral surfaces  31 ,  32  of root plates  33 ,  34  which differ from the embodiment shown in  FIGS. 1 and 2 . In an embodiment shown in  FIG. 3 , a step  35  is formed by that region of the peripheral surface  31  which is adjacent to the airfoil  4  projecting outward, an overspray coating  36  having formed on this part. The step  35  merges into a groove  37 , into which the covering strip  24  is plug-fitted by means of a plug-in limb  26 . 
     There is no step present in the exemplary embodiment shown in  FIG. 4 . A groove  38  has been formed into the peripheral surface  32 , and the covering strip  24  has been plug-fitted into this groove  38  in such a manner that its protective limb  27  extends in the direction away from the airfoil  4 . That region of the peripheral surface  32  which is adjacent to the airfoil  4  has been provided with an overspray coating  39 .