Abstract:
A process including local heating of a brazing point in order to remove an integrally brazed component from a structural part is provided. The brazing point joins a metal sheet in the interior of a cavity to a structural part. The process makes the removal of the metal sheet from the cavity much easier compared to the existing mechanical removal. A plasma source or an induction source may be used for heating the filler metal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Office application No. 09012637.6 EP filed Oct. 6, 2009, which is incorporated by reference herein in its entirety. 
       FIELD OF INVENTION 
       [0002]    The invention relates to the removal of brazed metal sheets from a structural part. 
       BACKGROUND OF INVENTION 
       [0003]    Brazing is a joining technique for bonding components to structural parts. This is the case for metal sheets for turbine blades or vanes, which are brazed on in the root and also in the head (if present). 
         [0004]    During refurbishment, i.e. after the turbine blades or vanes have been used, these metal sheets have to be removed. This takes place in a lengthy process by mechanical removal of the metal sheet on a machining machine. 
       SUMMARY OF INVENTION 
       [0005]    It is therefore an object of the invention to solve the problem mentioned above. 
         [0006]    The object is achieved by a process as claimed in the claims. 
         [0007]    The dependent claims list further advantageous measures, which can be combined with one another to obtain further advantages. 
         [0008]    During the process, the brazing point is heated locally. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIGS. 1 ,  2 ,  3  show a structural part with a brazing point, 
           [0010]      FIG. 4  shows an arrangement for removing the brazed structural part, and 
           [0011]      FIG. 5  shows a turbine blade or vane. 
       
    
    
       [0012]    The figures and the description represent only exemplary embodiments of the invention. 
       DETAILED DESCRIPTION OF INVENTION 
       [0013]      FIG. 1  shows a structural part  1 , in particular a turbine blade or vane  120 ,  130 , having a cavity  13 , which  13  is delimited by walls  22 . 
         [0014]    Above the cavity  13 , a component  4 , in particular a metal sheet  4 , is brazed onto the wall  22  or onto the end faces  29  of the wall  22  of a structural part  1 ,  120 ,  130 . 
         [0015]    The brazed joint is preferably a continuous brazed seam  11 , such that the metal sheet  4  is sealed off in an airtight mariner with respect to the cavity  13  in the region of the brazed seam  11 . 
         [0016]    A brazed seam  11  is not punctiform and its length is preferably at least five times its width. 
         [0017]      FIG. 2  shows a plan view of  FIG. 1 , and in  FIG. 2  the contour profile of the cavity  13 , which can run in any desired way, is indicated by dashed lines. 
         [0018]    This is preferably a plan view of an underside  29  of a platform  403  of a guide vane  130 . 
         [0019]      FIG. 2  shows that the metal sheet  4  rests on the upper end face  29  of the wall  22 . 
         [0020]    A filler metal  10 ,  10 ′, preferably a filler metal which is completely circulatory in the form of a brazed seam  11 , is present between the profile indicated by dashed lines and the outer profile of the metal sheet  4 . 
         [0021]    It may likewise be possible for the metal sheet  4  to be fixed, in particular welded, at some points before it is brazed on (see X in  FIG. 3 ), so that the metal sheet  4  is stabilized during brazing. 
         [0022]    The metal sheet  4  is preferably brazed on by firstly spot-welding the metal sheet  4  at points X, then preferably applying a brazing paste around the outer edge of the metal sheet  4  and then heating it in a furnace and by drawing the filler metal into the gap between the metal sheet  4  and the structural part  1 ,  120 ,  130  by capillary action. 
         [0023]      FIG. 4  schematically shows how the brazing point  10  is heated locally, i.e. by means of a heating source. 
         [0024]    Locally means that not all of the structural part  1 ,  120 ,  130  is heated, such that the structural part  120 ,  130  is merely heated locally at the brazing point  10 ,  10 ′ or locally at the brazed seam  11 . 
         [0025]    The heating source  19  used can preferably be a plasma, an induction source or a laser. 
         [0026]    The filler metal is not directly accessible from above. 
         [0027]    The detachment of the metal sheet  4  can be assisted by the action of manual force or by removal of the molten filler metal by suction, in particular if welding points (x) are present. 
         [0028]    The filler metal  10 ,  11  is gradually heated, and the bond between the component  4  and the structural part  1 ,  120 ,  130  is gradually released until it is completely removed. 
         [0029]      FIG. 5  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
         [0030]    The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
         [0031]    The blade or vane  120 ,  130  has, in succession along the longitudinal axis  121 , a securing region  400 , an adjoining blade or vane platform  403  and a main blade or vane part  406  and a blade or vane tip  415 . 
         [0032]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0033]    A blade or vane root  183 , which is used to secure the rotor blades  120 ,  130  to a shaft or a disk (not shown), is formed in the securing region  400 . 
         [0034]    The blade or vane root  183  is designed, for example, in hammerhead form. Other configurations, such as a fir-tree or dovetail root, are possible. 
         [0035]    The blade or vane  120 ,  130  has a leading edge  409  and a trailing edge  412  for a medium which flows past the main blade or vane part  406 . 
         [0036]    In the case of conventional blades or vanes  120 ,  130 , by way of example solid metallic materials, in particular superalloys, are used in all regions  400 ,  403 ,  406  of the blade or vane  120 ,  130 . 
         [0037]    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. 
         [0038]    The blade or vane  120 ,  130  may in this case be produced by a casting process, by means of directional solidification, by a forging process, by a milling process or combinations thereof. 
         [0039]    Workpieces with a single-crystal structure or structures are used as structural parts for machines which, in operation, are exposed to high mechanical, thermal and/or chemical stresses. 
         [0040]    Single-crystal workpieces of this type are produced, for example, by directional solidification from the melt. This involves casting processes in which the liquid metallic alloy solidifies to form the single-crystal structure, i.e. the single-crystal workpiece, or solidifies directionally. 
         [0041]    In this case, dendritic crystals are oriented along the direction of heat flow and form either a columnar crystalline grain structure (i.e. grains which run over the entire length of the workpiece and are referred to here, in accordance with the language customarily used, as directionally solidified) or a single-crystal structure, i.e. the entire workpiece consists of one single crystal. In these processes, a transition to globular (polycrystalline) solidification needs to be avoided, since non-directional growth inevitably forms transverse and longitudinal grain boundaries, which negate the favorable properties of the directionally solidified or single-crystal structural part. 
         [0042]    Where the text refers in general terms to directionally solidified microstructures, this is to be understood as meaning both single crystals, which do not have any grain boundaries or at most have small-angle grain boundaries, and columnar crystal structures, which do have grain boundaries running in the longitudinal direction but do not have any transverse grain boundaries. This second form of crystalline structures is also described as directionally solidified microstructures (directionally solidified structures). 
         [0043]    Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1. 
         [0044]    The blades or vanes  120 ,  130  may likewise have coatings 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 stands for 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 0 412 397 B1 or EP 1 306 454 A1. 
         [0045]    The density is preferably 95% of the theoretical density. 
         [0046]    A protective aluminum oxide layer (TGO=thermally grown oxide layer) is formed on the MCrAlX layer (as an intermediate layer or as the outermost layer). 
         [0047]    The layer preferably has a composition Co-30Ni-28Cr-8Al-0.6Y-0.7Si or Co-28Ni-24Cr-10Al-0.6Y. In addition to these cobalt-based protective coatings, it is also preferable to use nickel-based protective layers, such as Ni-10Cr-12Al-0.6Y-3Re or Ni-12Co-21Cr-11Al-0.4Y-2Re or Ni-25Co-17Cr-10Al-0.4Y-1.5Re. 
         [0048]    It is also possible for a thermal barrier coating, which is preferably the outermost layer and consists 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. 
         [0049]    The thermal barrier coating covers the entire MCrAlX layer. 
         [0050]    Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0051]    Other coating processes are possible, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include grains that are porous or have micro-cracks or macro-cracks, in order to improve the resistance to thermal shocks. The thermal barrier coating is therefore preferably more porous than the MCrAlX layer. 
         [0052]    Refurbishment means that after they have been used, protective layers may have to be removed from structural parts  120 ,  130  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the structural part  120 ,  130  are also repaired. This is followed by recoating of the structural part  120 ,  130 , after which the structural part  120 ,  130  can be reused. 
         [0053]    The blade or vane  120 ,  130  may be hollow or solid in form. If the blade or vane  120 ,  130  is to be cooled, it is hollow and may also have film-cooling holes  418  (indicated by dashed lines).