Abstract:
A welding additive is provided. A component including a welding additive is also provided. The welding additive improves the weldability of a few nickel-based superalloys and includes the following contents (in wt %): 10.0%-20.0% chromium, 5.0%-15.0% cobalt, 0.0%-10.0% molybdenum, 0.5-3.5% tantalum, 0.0%-5.0% titanium, 1.5%-5.0% aluminum, 0.3%-0.6% boron, remainder nickel.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2009/061727, filed Sep. 10, 2009 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 08019282.6 EP filed Nov. 4, 2008. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a weld filler as described in the claims, to the use thereof as claimed in the claims and to a component as claimed in the claims. 
       BACKGROUND OF INVENTION 
       [0003]    Of all high-temperature materials, nickel-based superalloys have the most favorable combination of mechanical properties, resistance to corrosion and processability for gas turbine construction for aircraft and power plants. The considerable increase in strength is made possible in particular by the particle hardening with very high proportions by volume of the coherent γ′ phase Ni 3 (Al—Ti, Ta, Nb). However, in general alloys with a higher γ′ content can only be considered weldable to a limited extent. This poor weldability is caused by: 
         [0000]    a) Nickel alloys generally have a relatively low thermal conductivity and a relatively high coefficient of thermal expansion, similar to the values of austenitic steels and Co alloys. The welding heat which is introduced is therefore dissipated comparatively slowly, and the inhomogeneous heating leads to high thermal stresses, causing thermal fatigue which can only be dealt with at considerable effort.
 
b) Nickel alloys are very sensitive to hot cracks in the event of a rapid change in the temperature cycles within the high temperature range. The cause is grain boundary fusion resulting from fluctuations in the chemical composition (segregations) or the formation of low-melting phases, such as sulfides or borides.
 
c) Nickel alloys generally have a high proportion of the γ′ phase in a γ matrix. In the case of nickel-based superalloys for turbine components, the γ′ phase amounts to greater than 40 vol %. This achieves a high strength but also leads to a low ductility of the material, in particular at low temperatures and in the range of the temperature field in which the γ/γ′ precipitation phenomenon may occur (“ductility-dip temperature range”, also known as the “subsolidus ductility dip”, approximately 700° C. to 1100° C., depending on the alloy). Consequently, stresses which occur can less readily be absorbed through plastic flow, which generally increases the risk of crack formation.
 
d) Nickel alloys exhibit the phenomenon of post-weld heat treatment cracks, also known as strain-age cracking. In this case, cracks are produced in a characteristic way in the first heat treatment following the weld as a result of γ/γ′ precipitation phenomena in the heat-affected zone or—if the weld filler can form the γ′ phase—also in the weld metal. This is caused by local stresses which form during the precipitation of the γ′ phase as a result of the contraction of the surrounding matrix. The susceptibility to strain-age cracking increases with an increasing level of γ′-forming alloy constituents, such as Al and Ti, since this also increases the proportion of γ′ phase in the microstructure.
 
         [0004]    If welds in which the base metal and the filler are identical are attempted at room temperature using conventional welding processes, for many industrial Ni-based superalloys for turbine laser vanes (e.g. IN 738 LC, Rene 80, IN 939), it is not currently possible to avoid the formation of cracks in the heat-affected zone and in the weld metal. 
         [0005]    At present, a number of processes and process steps are known to improve the weldability of nickel-based superalloys: 
         [0000]    a) Welding with Preheating:
 
One way of avoiding cracks when welding nickel-based superalloys using high-strength fillers (likewise nickel-based superalloys) is to reduce the temperature difference and therefore the stress gradient between weld joint and the remainder of the component. This is achieved by preheating the component during the welding. One example is manual TIG welding in a shield and gas box, with the weld joint being preheated inductively (by means of induction coils) to temperatures of greater than 900° C. However, this makes the welding process significantly more complicated and expensive. Moreover, on account of inaccessibility, this cannot be implemented for all regions which are to be welded.
 
b) Welding with Extremely Little Introduction of Heat:
 
This involves the use of welding processes which ensure that very little heat is introduced into the base metal. These processes include laser welding and electron beam welding. Both processes are very expensive. Moreover, they require outlay on programming and automation, which may be uneconomical for repair welds, with frequently fluctuating damage patterns and locations.
 
         [0006]    US 2004/0115086 A1 has disclosed a nickel alloy with various additions. 
       SUMMARY OF INVENTION 
       [0007]    Therefore, it is an object of the invention to provide a weld filler, a use of the weld filler, a welding process and a component which overcome the problems of the prior art. 
         [0008]    The object is achieved by a weld filler as claimed in the claims, by the use of the weld filler as claimed in the claims and a component as claimed in the claims. 
         [0009]    The subclaims give advantageous configurations which can advantageously be combined with one another as desired. 
         [0010]    The invention proposes a weld filler and a use thereof which allows the repair welding of gas turbine blades or vanes and other hot-gas components made from nickel-based superalloys by manual or automated welding at room temperature. The weld filler is likewise a γ′-hardened nickel-based superalloy, but differs in particular from the material of a substrate of a component that is to be prepared. The welding repair allows a low cycle fatigue (LCF) corresponding to approximately 50% or more of the properties of the base metal (the weld withstands 50% of the LCF cycles of the base metal). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention is explained in more detail below. In the drawing: 
           [0012]      FIG. 1  shows a list of the composition of materials which can be welded using the filler according to the invention, 
           [0013]      FIG. 2  shows a gas turbine, 
           [0014]      FIG. 3  shows a perspective view of a turbine blade or vane, and 
           [0015]      FIG. 4  shows a perspective view of a combustion chamber element. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0016]    The invention proposes an increase in the boron fraction as an alloy element. The fraction of boron in the alloy should lie between 0.3 wt % and 0.6 wt %. Said increase is intended to improve the hot cracking resistance with fillers of the same and similar type. The filler, which allows repair welding of gas turbine blades and other hot-gas components made from Ni-based superalloys by manual or automated welding at room temperature. 
         [0017]    Mechanism for Improving Resistance to Hot Cracking: 
         [0018]    Hot cracks occur if, in the temperature range, the so-called BTR (Brittleness Temperature Range), the local deformability is too low to absorb the expansion caused by the welding. In relation to conventional Ni-based superalloys in which the boron fraction is limited to a maximum of 0.03 wt %, the boron eutectic completely encompasses the grain boundaries and, during solidification, acts as a type of “damper” or “buffer” and absorbs the deformation forces generated. Here, the melting temperature of the eutectic should be lower than the temperature Tu—temperature beyond which the deformation capability P increases again. 
         [0019]    The base materials should not be overheated. Therefore, and use the welding parameters with low current intensity and fillers with small diameters are recommended. 
         [0020]    Furthermore, the increased fraction of boron in the alloy increases the creep rupture strength and heat resistance with a parallel increase in resistance to oxidation in the aggressive media. 
         [0021]    The invention proposes a welding process for welding components such as hot-gas components  138 ,  155  ( FIG. 3 ,  4 ) and turbine blades or vanes  120 ,  130  ( FIG. 2 ) made from nickel-based superalloys, which preferably includes the following characteristics:
       Heat treatment prior to the welding with a view to coarsening γ′ phase in the base metal made from nickel-based superalloy (cf. EP 1 428 897 A1). This heat treatment, also known as overageing, increases the ductility and therefore the weldability of the base metal.   Welding without preheating (at room temperature) using conventional manual welding processes, such as TIG or plasma powder welding, or alternatively welding using automated processes, such as laser powder welding or automated plasma powder welding, likewise at room temperature.   Use of closed shielding gas or vacuum boxes, into which the entire component is introduced during welding, in order to protect it from oxidation, is not required. There is also no need for through-flow boxes, in which the component is protected during welding by a correspondingly large flow of shielding gas.   For base metals which are extremely prone to hot cracking and/or oxidation during welding, it is recommended to using shielding gas which contains nitrogen to suppress the hot cracking and/or hydrogen to reduce the oxidation (see EP 04011321.9).   Heat treatment after welding to homogenize base metal and weld filler: solution annealing. The solution annealing temperature should be adapted to the base metal. The solution annealing temperature must be higher than the solution annealing temperature but lower than the solidus temperature of the weld filler. The single-stage or multi-stage age hardening to set the desired γ′ morphology (size, shape, distribution) can take place immediately afterwards or at a later stage during the processing of the hot-gas components.       
 
         [0027]    This weld filler has relatively good welding properties at room temperature. To achieve this, the levels of Al and Ti in the alloy were selected in such a way as to achieve a very low susceptibility to strain-age cracking. The chromium content is selected such that the alloy forms a corrosion-resistant Cr 2 O 3  covering layer and contains a sufficient reservoir for regeneration of this layer under operating conditions. 
         [0028]    Iron: Iron is preferably limited to at most 1.5 wt %, in order to improve the resistance of the alloy to oxidation and to reduce the risk of embrittling TCP phases (TCP=topologically closed packed) being formed. 
         [0029]    Silicon: Silicon is preferably limited to at most 0.5 wt %, in order to minimize hot cracking. 
         [0030]    Moreover, limits were determined for the optional constituents C, Fe, Mn, S, P, Hf, La, Si or Zr, in which an optimum between negative and positive influence is given. 
         [0031]    When producing the component and during welding, oxides and in particular sulfides may form at the grain boundaries. These thin, intercrystalline eutectics containing sulfur and oxygen on the one hand embrittle the grain boundaries. On the other hand, they have a low melting temperature, which leads to a high susceptibility to grain boundary cracking as a result of local fusion of the grain boundaries. 
         [0032]    The oxygen embrittlement is counteracted in particular by a local change in the chemical composition of the grain boundaries brought about by the addition of Hf, which segregates at the grain boundary and thereby makes grain boundary diffusion on the part of the oxygen more difficult, thus impeding grain boundary embrittlement, which is caused by oxygen. Moreover, hafnium is incorporated in the γ′ phase, increasing its strength. 
         [0033]    The following table represents the invention (details in wt %). 
         [0000]    
       
         
               
               
             
               
               
               
             
           
               
                   
               
               
                 Element 
                 Effect 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Cr 
                 10.0-20.0 
                 Corrosion resistance, increases the 
               
               
                   
                   
                 resistance to sulfidation, solid solution 
               
               
                   
                   
                 hardening 
               
               
                 Co 
                  5.0-15.0 
                 Reduces the stacking fault energy, 
               
               
                   
                   
                 resulting in increased creep strength, 
               
               
                   
                   
                 improves the solution annealing properties 
               
               
                 Mo 
                  0.0-10.0 
                 Solid solution hardening, increases the 
               
               
                   
                   
                 modulus of elasticity, reduces the 
               
               
                   
                   
                 diffusion coefficient 
               
               
                 Ta 
                 0.0-3.5 
               
               
                 Ti 
                 0.0-5.0 
                 Substitutes Al in γ′, increases the γ′ 
               
               
                   
                   
                 volume proportion 
               
               
                 Al 
                 1.5-5.0 
                 γ′ formation, only effective long-term 
               
               
                   
                   
                 protection against oxidation at &gt; approx. 
               
               
                   
                   
                 950° C., strong solid solution hardening 
               
               
                 Fe 
                 max 1.5 
                 Promotes the formation of TCP phases, 
               
               
                   
                   
                 has an adverse effect on resistance to 
               
               
                   
                   
                 oxidation 
               
               
                 Mn 
                 max 0.1 
               
               
                 Si 
                 max 0.5 
                 Promotes the formation of TCP phases, 
               
               
                   
                   
                 increases hot cracking 
               
               
                 C 
                  max 0.25 
                 Carbide formation 
               
               
                 B 
                 0.3-0.6 
                 Element with grain boundary activity 
               
               
                   
                   
                 (large atom), increases the grain boundary 
               
               
                   
                   
                 cohesion, reduces the risk of incipient 
               
               
                   
                   
                 cracking, increases the ductility and creep 
               
               
                   
                   
                 rupture strength, prevents the formation of 
               
               
                   
                   
                 carbide films on grain boundaries, reduces 
               
               
                   
                   
                 the risk of oxidation 
               
               
                 Zr 
                 0.0-0.1 
                 Bonds S and C, increases resistance to hot 
               
               
                   
                   
                 cracking 
               
               
                 Hf 
                 0.0-0.5 
                 Reduces hot cracking capability during 
               
               
                   
                   
                 casting, 
               
               
                   
                   
                 is incorporated in γ′, increases the strength 
               
               
                   
                   
                 thereof, improves resistance to oxidation 
               
               
                 La 
                 0.0-0.1 
                 Bonds S, increases hot cracking capability 
               
               
                 S 
                  max 0.015 
                 Metallurgical impurity, increases hot 
               
               
                   
                   
                 cracking 
               
               
                 P 
                  max 0.03 
                 Metallurgical impurity, has an adverse 
               
               
                   
                   
                 effect on weldability 
               
               
                 Ni 
                 Remainder 
               
               
                   
               
             
          
         
       
     
         [0034]    One application example is the welding of the alloy Rene80, in particular when subject to operational stresses, by means of manual TIG welding and plasma-arc powder surfacing. Further welding processes and repair applications are not ruled out. The weld repair joints have properties which allow “structural” repairs in the airfoil/platform transition radius or in the airfoil of a turbine blade or vane. 
         [0035]    Other nickel-based fillers can be selected according to the level of the γ′ phase, specifically for preference greater than or equal to 35 vol %, with a preferred maximum upper limit of 75 vol %. 
         [0036]    The materials IN 738, IN 738 LC, IN 939, PWA 1483 SX or IN 6203 DS can preferably be welded using the weld filler according to the invention. 
         [0037]      FIG. 2  shows, by way of example, a partial longitudinal section through a gas turbine  100 . 
         [0038]    In the interior, the gas turbine  100  has a rotor  103  with a shaft  101  which is mounted such that it can rotate about an axis of rotation  102  and is also referred to as the turbine rotor. 
         [0039]    An intake housing  104 , a compressor  105 , a, for example, toroidal combustion chamber  110 , in particular an annular combustion chamber, with a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust-gas housing  109  follow one another along the rotor  103 . 
         [0040]    The annular combustion chamber  110  is in communication with a, for example, annular hot-gas passage  111 , where, by way of example, four successive turbine stages  112  form the turbine  108 . 
         [0041]    Each turbine stage  112  is formed, for example, from two blade or vane rings. As seen in the direction of flow of a working medium  113 , in the hot-gas passage  111  a row of guide vanes  115  is followed by a row  125  formed from rotor blades  120 . 
         [0042]    The guide vanes  130  are secured to an inner housing  138  of a stator  143 , whereas the rotor blades  120  of a row  125  are fitted to the rotor  103  for example by means of a turbine disk  133 . 
         [0043]    A generator (not shown) is coupled to the rotor  103 . 
         [0044]    While the gas turbine  100  is operating, the compressor  105  sucks in air  135  through the intake housing  104  and compresses it. The compressed air provided at the turbine-side end of the compressor  105  is passed to the burners  107 , where it is mixed with a fuel. The mix is then burned in the combustion chamber  110 , forming the working medium  113 . From there, the working medium  113  flows along the hot-gas passage  111  past the guide vanes  130  and the rotor blades  120 . The working medium  113  is expanded at the rotor blades  120 , transferring its momentum, so that the rotor blades  120  drive the rotor  103  and the latter in turn drives the generator coupled to it. 
         [0045]    While the gas turbine  100  is operating, the components which are exposed to the hot working medium  113  are subject to thermal stresses. The guide vanes  130  and rotor blades  120  of the first turbine stage  112 , as seen in the direction of flow of the working medium  113 , together with the heat shield elements which line the annular combustion chamber  110 , are subject to the highest thermal stresses. 
         [0046]    To be able to withstand the temperatures which prevail there, they have to be cooled by means of a coolant. 
         [0047]    Substrates of the components may likewise have a directional structure, i.e. they are in single-crystal form (SX structure) or have only longitudinally oriented grains (DS structure). 
         [0048]    By way of example, iron-based, nickel-based or cobalt-based superalloys are used as material for the components, in particular for the turbine blade or vane  120 ,  130  and components of the combustion chamber  110 . 
         [0049]    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. 
         [0050]    The guide vane  130  has a guide vane root (not shown here), which faces the inner housing  138  of the turbine  108 , and a guide vane head which is at the opposite end from the guide vane root. The guide vane head faces the rotor  103  and is fixed to a securing ring  140  of the stator  143 . 
         [0051]      FIG. 3  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbo machine, which extends along a longitudinal axis  121 . 
         [0052]    The turbo machine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
         [0053]    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 . 
         [0054]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0055]    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 . 
         [0056]    The blade or vane root  183  is designed, for example, in hammerhead for Other configurations, such as a fir-tree or dovetail root, are possible. 
         [0057]    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 . 
         [0058]    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 . 
         [0059]    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. 
         [0060]    The blade or vane  120 ,  130  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. 
         [0061]    Work pieces with a single-crystal structure or structures are used as components for machines which, in operation, are exposed to high mechanical, thermal and/or chemical stresses. 
         [0062]    Single-crystal work pieces 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 work piece, or solidifies directionally. 
         [0063]    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 work piece and are referred to here, in accordance with the language customarily used, as directionally solidified) or a single-crystal structure, i.e. the entire work piece 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 component. 
         [0064]    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). 
         [0065]    Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1. 
         [0066]    The blades or vanes  120 ,  130  may likewise have coatings protecting against corrosion or oxidation, for example (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 0 412 397 B1 or EP 1 306 454 A1. 
         [0067]    The density is preferably 95% of the theoretical density. 
         [0068]    A protective aluminum oxide layer (TGO=thermally grown oxide layer) forms on the MCrAlX layer (as intermediate layer or as outermost layer). 
         [0069]    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. 
         [0070]    The thermal barrier coating covers the entire MCrAlX layer. 
         [0071]    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). 
         [0072]    Other coating processes are conceivable, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may have porous, microcrack-containing or macrocrack-containing grains for better thermal shock resistance. The thermal barrier coating is therefore preferably more porous than the MCrAlX layer. 
         [0073]    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). 
         [0074]      FIG. 4  shows a combustion chamber  110  of the gas turbine  100 . The combustion chamber  110  is configured, for example, as what is known as an annular combustion chamber, in which a multiplicity of burners  107  arranged circumferentially around the axis of rotation  102  open out into a common combustion chamber space  154  and which generate flames  156 . For this purpose, the combustion chamber  110  overall is of annular configuration positioned around the axis of rotation  102 . 
         [0075]    To achieve a relatively high efficiency, the combustion chamber  110  is designed for a relatively high temperature of the working medium M of approximately 1000° C. to 1600° C. To allow a relatively long service life even with these operating parameters, which are unfavorable for the materials, the combustion chamber wall  153  is provided, on its side which faces the working medium M, with an inner lining formed from heat shield elements  155 . 
         [0076]    On account of the high temperatures in the interior of the combustion chamber  110 , it is also possible for a cooling system to be provided for the heat shield elements  155  and/or for their holding elements. The heat shield elements  155  are in this case for example hollow and may also have cooling holes (not shown) opening out into the combustion chamber space  154 . 
         [0077]    On the working medium side, each heat shield element  155  is equipped with a particularly heat-resistance protective layer (McrAlX layer and/or ceramic coating) or is made from material that is able to withstand high temperatures (solid ceramic bricks). 
         [0078]    These protective layers may be similar to the turbine blades or vanes, i.e. for example 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 0 412 397 B1 or EP 1 306 454 A1. 
         [0079]    It is also possible, for example, for a ceramic thermal barrier coating to be present on the McrAlX, 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. 
         [0080]    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). 
         [0081]    Other coating processes are conceivable, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may have porous, microcrack-containing or macrocrack-containing grains for better thermal shock resistance. 
         [0082]    Refurbishment means that after they have been used, protective layers may have to be removed from turbine blades or vanes  120 ,  130 , heat shield elements  155  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. 
         [0083]    If appropriate, cracks in the turbine blade or vane  120 ,  130  or the heat shield element  155  are also repaired using the weld filler according to the invention. This is followed by recoating of the turbine blades or vanes  120 ,  130 , heat shield elements  155 , after which the turbine blades or vanes  120 ,  130  or the heat shield elements  155  can be reused.