Abstract:
The invention relates to a weld filler and to a use of a weld filler which significantly improves the weldability of some nickel-based superalloys and includes the following constituents (in wt %): 17.5%-20.0% chromium (Cr) 10.0%-12.0% cobalt (Co) 9.0%-10.5% molybdenum (Mo) 0.1%-3.3% titanium, in particular 3.0%-3.3% titanium (Ti), 1.4%-1.8% aluminum (Al), 0.04%-0.12% carbon, 0.003%-0.01% boron (B), remainder nickel.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefits of European Patent application No. 05023192.7 filed Oct. 24, 2005. All of the applications are incorporated by reference herein in their entirety.  
       FIELD OF INVENTION  
       [0002]     The invention relates to a weld filler, to the use thereof and to a welding process as claimed in the claims.  
       BACKGROUND OF THE 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: 
        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.        
 
         [0008]     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. IN738LC, Rene80, IN939), it is not currently possible to avoid the formation of cracks in the heat-affected zone and in the weld metal.  
         [0009]     At present, a number of processes and process steps are known to improve the weldability of nickel-based superalloys: 
        a) Welding with preheating:        
 
         [0011]     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:        
 
         [0013]     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.  
         [0014]     US 2004/0115086 A1 has disclosed a nickel alloy with various additions.  
       SUMMARY OF INVENTION  
       [0015]     Therefore, it is an object of the invention to provide a weld filler, a use of the weld filler and a welding process which overcome the problems of the prior art.  
         [0016]     The object is achieved by the weld filler, by the use of the weld filler and by the welding process as claimed in the claims.  
         [0017]     The subclaims give advantageous configurations which can advantageously be combined with one another as desired.  
         [0018]     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  
       [0019]     The invention is explained in more detail below. In the drawing:  
         [0020]      FIG. 1  shows a list of the composition of materials which can be welded using the filler according to the invention,  
         [0021]      FIG. 2  shows a perspective view of a turbine blade or vane,  
         [0022]      FIG. 3  shows a perspective view of a combustion chamber element, and  
         [0023]      FIG. 4  shows a gas turbine. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0024]     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 (the shielding gas disclosed in EP 04011321.9 and the composition of the shielding gas form part of the present disclosure).     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 (approx. 1315° C. for SC 52). 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.        
 
         [0030]     The weld filler is divided into a base alloy SC 52 and variants of this alloy SC 52+.  
         [0031]     SC 52  
         [0032]     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 Al content was selected to be less than 4% and the Cr content was selected to be 17-20%, so 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.  
         [0033]     SC 52+ 
         [0034]     The changes described below can be implemented by comparison with SC 52.  
         [0035]     Titanium: The titanium content is preferably reduced to at most 1.5 wt %, thereby eliminating the risk of the embrittling, incoherent η phase Ni 3 Ti being formed. The η phase is formed in the event of high titanium to aluminum contents, for example in the Ni-based superalloy IN939 containing approx. 3.7 wt % Ti and approx. 1.9 wt % Al).  
         [0036]     Tantalum: It is preferable to add up to 2.5 wt % tantalum to the alloy, in order to compensate for the loss of γ′-forming titanium.  
         [0037]     (Titanium+tantalum): The level of (Ti+Ta) is preferably limited to ≦3.5 wt %, in order to suppress the risk of strain-age cracking. The minimum content is in particular 3 wt %.  
         [0038]     Iron: Iron is preferably limited to at most 0.35 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.  
         [0039]     Silicon: Silicon is preferably limited to at most 0.1 wt %, in order to minimize hot cracking.  
         [0040]     Zirconium: Zirconium is preferably added in an amount of 0.01 to 0.1 wt %. It bonds with sulfur and carbon and thereby, in the proportions added, reduces hot cracking.  
         [0041]     Lanthanum: Lanthanum is preferably added in an amount of 0.05 wt % to 0.1 wt %, since, like zirconium, it bonds with sulfur and reduces hot cracking.  
         [0042]     Sulfur: Sulfur is preferably limited to at most 0.0075 wt %, in order to suppress hot cracking.  
         [0043]     Hafnium: Hafnium is preferably added to the alloy in an amount of 0.25 wt % to 0.5 wt %. It bonds with sulfur, reduces the hot cracking and is incorporated in γ′, thereby increasing the strength of the latter.  
         [0044]     These changes minimize the risk of embrittling TCP phases (topologically closed packed) being formed, and in particular the formation of the η phase Ni 3 Ti. At the same time, the level of harmful impurities, such as Fe, Mn, S, Si and P, is limited, since these components have a detrimental effect on the weldability and the properties of the alloy of the component.  
         [0045]     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.  
         [0046]     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.  
         [0047]     Zirconium, lanthanum and also hafnium bond with sulfur and thereby allow a significant improvement to be made to the resistance to hot cracking.  
         [0048]     The following table summarizes two exemplary embodiments (details in wt %).  
                                                   Variant           Element   SC 52   SC 52+   Effect                   Cr   17.5-20.0    17.5-20.0   Corrosion resistance, increases                   the resistance to sulfidation,                   solid solution hardening       Co   10.0-12.0    10.0-12.0   Reduces the stacking fault energy,                   resulting in increased creep                   strength, improves the solution                   annealing properties       Mo    9.0-10.5    9.0-10.5   Solid solution hardening, increases                   the modulus of elasticity, reduces                   the diffusion coefficient       Ta   0   0.1 to 2.5   Substitutes Al in γ′, increases                   the γ′ solution temperature, delays                   γ′ coarsening       Ti   3.0-3.3   0.1 to 1.5   Substitute Al in γ′, increases                   the γ′ volume proportions       Ti + Ta   —   3 = (Ti +               Ta) = 3.5       Al   1.4-1.8    1.4-1.8   γ′ formation, only effective                   long-term protection against                   oxidation at &gt; approx. 950° C.,                   strong solid solution hardening       Fe   max 5   max 0.35   Promotes the formation of TCP                   phases, has an adverse effect on                   resistance to oxidation       Mn   max 0.1   max 0.5       Si   max 0.5   max 0.1   Promotes the formation of TCP                   phases, increases hot cracking       C   0.04-0.12    0.04-0.12   Carbide formation       B   0.003-0.01    0.003-0.01   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   0.01-0.1   Bonds with S and C, increases the                   resistance to hot cracking       Hf   0   0.25-0.5   Reduces the hot cracking during                   casting, is incorporated in γ′,                   increasing its strength, improves                   the resistance to oxidation       La   0   0.05-0.1   Bonds with S, increases the                   resistance to hot cracking       S   max 0.015   max 0.0075       P   max 0.03   max 0.015       Ni   Remainder   Remainder                  
 
         [0049]     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.  
         [0050]     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 %.  
         [0051]     The materials IN738, IN939, PWA1483SX or IN6203DS can preferably be welded using the weld filler according to the invention.  
         [0052]      FIG. 2  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbo machine, which extends along a longitudinal axis  121 .  
         [0053]     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.  
         [0054]     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 . 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 . 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. 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 .  
         [0056]     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 .  
         [0057]     Superalloys of this type are known, for example, from EP 1 204 776 B 1, 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 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.  
         [0058]     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.  
         [0059]     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.  
         [0060]     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.  
         [0061]     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). Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1; these documents form part of the disclosure.  
         [0062]     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 haffnium (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, which are intended to form part of the present disclosure with regard to the chemical composition of the alloy.  
         [0063]     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. 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).  
         [0064]     Refurbishment means that after they have been used, protective layers may have to be removed from components  120 ,  130  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the component  120 ,  130  are also repaired, in which context it is possible to use the weld filler according to the invention. This is followed by recoating of the component  120 ,  130 , after which the component  120 ,  130  can be reused.  
         [0065]     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).  
         [0066]      FIG. 3  shows a combustion chamber  110  of a gas turbine  100  ( FIG. 4 ). 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 .  
         [0067]     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 .  
         [0068]     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). 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 B 1, EP 0 786 017 B1, EP 0 412 397 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.  
         [0069]     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.  
         [0070]     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).  
         [0071]     Refurbishment means that after they have been used, protective layers may have to be removed from heat shield elements  155  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the heat shield element  155  are also repaired using the weld filler according to the invention. This is followed by recoating of the heat shield elements  155 , after which the heat shield elements  155  can be reused.  
         [0072]     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 film-cooling holes (not shown) opening out into the combustion chamber space  154 .  
         [0073]      FIG. 4  shows, by way of example, a partial longitudinal section through a gas turbine  100 .  
         [0074]     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. 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 . 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 .  
         [0075]     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 .  
         [0076]     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 .  
         [0077]     A generator (not shown) is coupled to the rotor  103 .  
         [0078]     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.  
         [0079]     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. To be able to withstand the temperatures which prevail there, they have to be cooled by means of a coolant.  
         [0080]     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). 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 . 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 alloys.  
         [0081]     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 .