Abstract:
The repair of cracks according to the state of the art comprises the fact that defects located on the inside must be worked outward in order to be able to be welded closed. The method according to the invention proposes not to work out cracks located on the inside, but to melt the crack located on the inside utilizing a flux agent.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2007/051368, filed Feb. 13, 2007 and claims the benefit thereof, which is incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to the welded repair of defects lying on the inside of components. 
       BACKGROUND OF THE INVENTION 
       [0003]    Components are tested in various ways after production or after use, sometimes then finding defects lying on the inside that often have to be repaired. 
         [0004]    According to the prior art, material is removed from the crack not only if the crack occurs on the surface but also if it is lying on the inside, i.e. does not go through to the outer surface. Such cracks from which material is removed are then closed by being brazed or welded. This removal of material is time-consuming, since it represents a further working step. In particular, it must be ensured that all the material is removed from the crack. 
       SUMMARY OF THE INVENTION 
       [0005]    It is therefore the object of the invention to provide a method that simplifies the welded repair of defects lying on the inside. 
         [0006]    The object is achieved by a method for the welded repair of defects lying on the inside according to the claims. 
         [0007]    This method uses a flux which is applied to the surface of the component near the crack and is then welded. 
         [0008]    Further advantageous measures, which can be combined with one another as desired to achieve further advantages, are listed in the dependant claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention is described in detail with reference to the drawings, in which: 
           [0010]      FIGS. 1 ,  2  and  3  show the procedure involved in the method, 
           [0011]      FIG. 4  shows a gas turbine, 
           [0012]      FIG. 5  shows a perspective view of a turbine blade, 
           [0013]      FIG. 6  shows a perspective view of a combustion chamber and 
           [0014]      FIG. 7  shows a list of superalloys. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1  shows a component  1 ,  120 ,  130 ,  155 , which has in the substrate  7  a crack  4  that does not extend as far as the outer surface  10 , that is to say is completely enclosed in the substrate  7  of the component  1 ,  120 ,  130 ,  155 . 
         [0016]    In particular in the case of turbine components, the material of the substrate  7  is preferably a nickel- or cobalt-based alloy. Examples of nickel- or cobalt-based alloys that are used can be found in  FIG. 7 . Similarly, the method can be used for steels (for stainless high-grade steels, e.g. SS410) and for parts of a compressor or for guiding grooves. 
         [0017]    An arrangement for repairing the crack  4  as shown in  FIG. 1  is schematically shown in  FIG. 2 . Cracks  4  lying on the inside are detected, preferably by means of an eddy current method  5 . 
         [0018]    Then an “active” flux  13 , known from the prior art, is applied to the surface  10  in the region above the crack  4 . The flux  13  is preferably in the form of a powder. The flux  13  preferably consists of powder. 
         [0019]    The flux  13  may preferably also be applied by being brushed or sprayed on as a suspension, or as an alcohol- or water-based slurry, with or without a binder. Similarly preferably, the powder may be in a pressed form, that is to say as a strip or sheet. Fine-grained powder (+1 μm/−45 μm) is preferably used for the filler. 
         [0020]    For welding, fusion welding methods and/or plasma welding methods are preferably used:
   manual arc welding   metal arc welding with a flux-cored wire electrode and no shielding gas;   submerged-arc welding with a wire electrode,   submerged-arc welding with a flux-cored wire electrode,   metal inert gas welding (MIG welding),   metal active gas welding (MAG welding),   tungsten inert gas welding (TIG welding),   gas welding with an oxyacetylene flame.   
 
         [0029]    TIG welding is used with preference. Similarly, other methods such as laser welding or electron-beam welding may also be used. 
         [0030]    By means of a welding device  16  (TIG welding device, laser welding device, etc.), which produces a plasma  19  or emits laser beams  19 , the flux  13  is melted and the region around the crack  4  is completely encompassed, with the result that around the crack  4  there forms a melting zone  22 , which may be up to  8  mm deep (=depth of the molten pool), as shown in  FIG. 3 . A person skilled in the art can set laser parameters (duration, intensity, power, . . . ) material-specifically and component-specifically, in order to encompass a crack  4  completely in a melting zone  22 . 
         [0031]    In  FIG. 3 , the melting zone does not extend through the entire thickness of the component  7 . Similarly, particularly in the case of hollow or thin-walled components, the melting zone  22  may preferably extend over the entire thickness. Further welding is not necessary. The worked location is preferably also reworked. 
         [0032]    A flux from the prior art may be chosen for the flux  13 . The flux  13  is preferably a powder. Similarly preferably, the flux  13  may be a powder mixture. A composition of SiO 2  or Fe 2 O 3  may preferably be used. Similarly preferably, a composition of titanium oxide (0-60% by weight), nickel oxide (0-40% by weight) and magnesium silicide (0-10% by weight) can be used. 
         [0033]    The combinations:
   titanium oxide   nickel oxide   manganese silicide   titanium oxide/nickel oxide   titanium oxide/manganese silicide   nickel oxide/manganese silicide   titanium oxide/nickel oxide/manganese silicide   are respectively preferred exemplary embodiments.   
 
         [0042]    The much improved depth of the molten pool when cracks are laser welded and remelted with such an “active” flux  13  or surface-active suspensions, as they are known, is influenced by the following factors: 
         [0043]    a: Marangoni effect: the energy and heat transfer during the welding/remelting takes place by movement of liquid metal in the molten pool. If the movements on the surface of the molten pool are outwardly directed, the molten pool becomes wide and shallow. An inwardly directed movement makes the molten pool become narrow and deep. The kind of movement in the molten pool is influenced by the surface tension of the liquid metal. By deliberately adding surface-active substances (filler), it is possible to change the surface tension in the molten pool in such a way that the movement of the liquid metal is inwardly directed, and consequently a much improved depth of the molten pool is achieved. 
         [0044]    b: Improved absorption of the welding beam: the surface-active substance is remelted in the course of the welding process and forms a very thin layer (slag) on the surface of the molten pool. This thin layer not only influences the surface tension of the molten pool but also at the same time has much improved properties in terms of absorption of the welding beam in comparison with the liquid metal of the base material. It is consequently possible to produce an improved energy transfer and depth of the molten pool, which under certain circumstances can reach up to 4-8 mm. 
         [0045]    The improved energy transfer in the case of TIG welding/remelting with surface-active substances is achieved by an enrichment of the arc with electrons. This enrichment has the effect of constricting the arc, and consequently producing a much higher energy density and improved depth of the molten pool. 
         [0046]      FIG. 4  shows by way of example a gas turbine  100  in a longitudinal partial section. 
         [0047]    The gas turbine  100  has in the interior a rotor  103  with a shaft  101 , which is rotatably mounted about an axis of rotation  102  and is also referred to as a turbine runner. Following one another along the rotor  103  are an intake housing  104 , a compressor  105 , a combustion chamber  110 , for example of a toroidal form, in particular an annular combustion chamber, with a number of coaxially arranged burners  107 , a turbine  108  and the exhaust housing  109 . 
         [0048]    The annular combustion chamber  110  communicates with a hot gas duct  111 , for example of an annular form. There, the turbine  108  is formed for example by four successive turbine stages  112 . 
         [0049]    Each turbine stage  112  is formed for example by two blade rings. As seen in the direction of flow of a working medium  113 , a row of stationary blades  115  is followed in the hot gas duct  111  by a row  125  formed by moving blades  120 . 
         [0050]    The stationary blades  130  are in this case fastened to an inner housing  138  of a stator  143 , whereas the moving blades  120  of a row  125  are attached to the rotor  103 , for example by means of a turbine disk  133 . Coupled to the rotor  103  is a generator or a machine (not presented). 
         [0051]    During the operation of the gas turbine  100 , air  135  is sucked in by the compressor  105  through the intake housing  104  and compressed. The compressed air provided at the end of the compressor  105  on the turbine side is passed to the burners  107  and mixed there with a fuel. The mixture is then burned in the combustion chamber  110  to form the working medium  113 . From there, the working medium  113  flows along the hot gas duct  111  past the stationary blades  130  and the moving blades  120 . At the moving blades  120 , the working medium  113  expands, transferring momentum, so that the moving blades  120  drive the rotor  103  and the latter drives the machine coupled to it. 
         [0052]    The components that are exposed to the hot working medium  113  are subjected to thermal loads during the operation of the gas turbine  100 . The stationary blades  130  and moving blades  120  of the first turbine stage  112 , as seen in the direction of flow of the working medium  113 , are thermally loaded the most, along with the heat shielding elements lining the annular combustion chamber  110 . 
         [0053]    In order to withstand the temperatures prevailing there, these may be cooled by means of a coolant. Similarly, substrates of the components may have a directional structure, i.e. they are monocrystalline (SX structure) or only have longitudinally directed grains (DS structure). 
         [0054]    Iron-, nickel- or cobalt-based superalloys are used for example as the material for the components, in particular for the turbine blade  120 ,  130  and components of the combustion chamber  110 . 
         [0055]    Such superalloys 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 constitute part of the disclosure with respect to the chemical composition of the alloys. 
         [0056]    The stationary blade  130  has a stationary blade root (not represented here), facing the inner housing  138  of the turbine  108 , and a stationary blade head, at the opposite end from the stationary blade root. The stationary blade head faces the rotor  103  and is fixed to a fastening ring  140  of the stator  143 . 
         [0057]      FIG. 5  shows in a perspective view a moving blade  120  or stationary blade  130  of a turbomachine, which extends along a longitudinal axis  121 . 
         [0058]    The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
         [0059]    The blade  120 ,  130  has, following one after the other along the longitudinal axis  121 , a fastening region  400 , an adjoining blade platform  403  and also a blade airfoil  406  and a blade tip  415 . 
         [0060]    As a stationary blade  130 , the blade  130  may have a further platform at its blade tip  415  (not represented). 
         [0061]    In the fastening region  400  there is formed a blade root  183 , which serves for the fastening of the moving blade  120  to a shaft or a disk (not represented). 
         [0062]    The blade root  183  is designed for example as a hammer head. Other designs as a firtree or dovetail root are possible. 
         [0063]    The blade  120 ,  130  has for a medium which flows past the blade airfoil  406  a leading edge  409  and a trailing edge  412 . 
         [0064]    In the case of conventional blades  120 ,  130 , solid metallic materials, in particular superalloys, are used for example in all the regions  400 ,  403 ,  406  of the blade  120 ,  130 . 
         [0065]    Such superalloys 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 constitute part of the disclosure with respect to the chemical composition of the alloy. 
         [0066]    The blade  120 ,  130  may in this case be produced by a casting method, also by means of directional solidification, by a forging method, by a milling method or combinations of these. 
         [0067]    Workpieces with a monocrystalline structure or structures are used as components for machines which are exposed to high mechanical, thermal and/or chemical loads during operation. The production of monocrystalline workpieces of this type takes place for example by directional solidification from the melt. This involves casting methods in which the liquid metallic alloy solidifies to form the monocrystalline structure, i.e. to form the monocrystalline workpiece, or in a directional manner. Dentritic crystals are thereby oriented along the thermal flow and form either a columnar grain structure (i.e. grains which extend over the entire length of the workpiece and are commonly referred to here as directionally solidified) or a monocrystalline structure, i.e. the entire workpiece comprises a single crystal. In these methods, the transition to globulitic (polycrystalline) solidification must be avoided, since undirected growth necessarily causes the formation of transversal and longitudinal grain boundaries, which nullify the good properties of the directionally solidified or monocrystalline component. 
         [0068]    While reference is being made generally to solidified structures, this is intended to mean both monocrystals, which have no grain boundaries or at most small-angle grain boundaries, and columnar crystal structures, which indeed have grain boundaries extending in the longitudinal direction but no transversal grain boundaries. These second-mentioned crystalline structures are also referred to as directionally solidified structures. 
         [0069]    Such methods are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1; these documents constitute part of the disclosure with respect to the solidification method. 
         [0070]    Similarly, the blades  120 ,  130  may have coatings against corrosion or oxidation (MCrAlX; M is at least one element of the group comprising iron (Fe), cobalt (Co) and nickel (Ni), X is an active element and represents yttrium (Y) and/or silicon and/or at least one element of the rare earths, or hafnium (HD). Such alloys are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1, which are intended to constitute part of this disclosure with respect to the chemical composition of the alloy. 
         [0071]    The density is preferably 95% of the theoretical density. A protective aluminum oxide layer (TGO=thermal grown oxide layer) forms on the MCrAlX layer (as an intermediate layer or as the outermost layer). 
         [0072]    The composition of the layer preferably comprises Co-30Ni-28Cr-8Al-0.6Y-0.7Si or Co-28Ni-24Cr-10Al-0.6Y. Apart from these cobalt-based protective coatings, nickel-based protective coatings are also preferably used, 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. 
         [0073]    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. is unstabilized, partially stabilized or completely stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, may also be present on the MCrAlX. 
         [0074]    The thermal barrier coating covers the entire MCrAlX layer. Columnar grains are produced in the thermal barrier coating by suitable coating methods, such as for example electron-beam physical vapor deposition (EB-PVD). 
         [0075]    Other coating methods are conceivable, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may have grains which are porous, are provided with microcracks or are provided with macrocracks for better thermal shock resistance. The thermal barrier coating is therefore preferably more porous than the MCrAlX layer. 
         [0076]    The blade  120 ,  130  may be hollow or be of a solid form. If the blade  120 ,  130  is to be cooled, it is hollow and may also have film cooling holes  418  (indicated by dashed lines). 
         [0077]      FIG. 6  shows a combustion chamber  110  of the gas turbine  100 . The combustion chamber  110  is designed for example as what is known as an annular combustion chamber, in which a multiplicity of burners  107 , which produce flames  156  and are arranged in the circumferential direction around an axis of rotation  102 , open out into a common combustion chamber space  154 . For this purpose, the combustion chamber  110  is designed as a whole as an annular structure, which is positioned around the axis of rotation  102 . 
         [0078]    To achieve a comparatively high efficiency, the combustion chamber  110  is designed for a comparatively high temperature of the working medium M of approximately 1000° C. to 1600° C. To permit a comparatively long operating time even with these operating parameters that are unfavorable for the materials, the combustion chamber wall  153  is provided on its side facing the working medium M with an inner lining formed by heat shielding elements  155 . 
         [0079]    On account of the high temperatures in the interior of the combustion chamber  110 , a cooling system may also be provided for the heat shielding elements  155  or for their holding elements. The heat shielding elements  155  are for example hollow and, if need be, also have cooling holes (not represented) opening out into the combustion chamber space  154 . 
         [0080]    Each heat shielding element  155  of an alloy is provided on the working medium side with a particularly heat-resistant protective layer (MCrAlX layer and/or ceramic coating) or is produced from material that is resistant to high temperature (solid ceramic bricks). 
         [0081]    These protective layers may be similar to the turbine blades, meaning for example MCrAlX: M is at least one element of the group comprising iron (Fe), cobalt (Co) and nickel (Ni), X is an active element and represents yttrium (Y) and/or silicon and/or at least one element of the rare earths, or hafnium (Hf). Such alloys are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1, which are intended to constitute part of this disclosure with respect to the chemical composition of the alloy. 
         [0082]    A thermal barrier coating which is for example a ceramic thermal barrier coating and consists for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. is unstabilized, partially stabilized or completely stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, may also be present on the MCrAlX. 
         [0083]    Columnar grains are produced in the thermal barrier coating by suitable coating methods, such as for example electron-beam physical vapor deposition (EB-PVD). 
         [0084]    Other coating methods are conceivable, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may have grains which are porous, are provided with microcracks or are provided with macrocracks for better thermal shock resistance. 
         [0085]    Refurbishment means that turbine blades  120 ,  130  and heat shielding elements  155  may have to be freed of protective layers after use (for example by sandblasting). This is followed by removal of the corrosion and/or oxidation layers or products. If need be, cracks in the turbine blade  120 ,  130  or the heat shielding element  155  are then also repaired. This is followed by recoating of the turbine blades  120 ,  130  or heat shielding elements  155  and renewed use of the turbine blades  120 ,  130  or the heat shielding elements  155 .