Abstract:
Superalloy solidified in a directional manner often cannot be subjected to heat treatment because the heat treatment leads to recrystallization. As a result of the temperature profile during a heat treatment according to the invention which oscillates in the manner of a pendulum, a recrystallization during heat treatment can be avoided because mechanical stresses are reduced thanks to the recurring succession of dissolutions and precipitations of the precipitate. The method can be applied to a Ni-based superalloy with γ-precipitates. After the cyclic heat treatment, the temperature can be adjusted to and maintained at a temperature which is the same as or higher than the complete dissolution temperature. An oscillating movement can also take place above the complete dissolution temperature.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2007/052461, filed Mar. 15, 2007 and claims the benefit thereof. The International Application claims the benefits of European application No. 06008688.1, filed Apr. 26, 2006, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a heat treatment method for a material which has a precipitate. 
       BACKGROUND OF THE INVENTION 
       [0003]    Nickel-based superalloys which are used particularly for gas turbine components, such as turbine blades or combustion chamber inserts, have a γ-phase which, within the framework of a repair, that is to say during refurbishment, is subjected to what is known as γ-solution annealing in order to restore the original material properties. 
         [0004]    This is not possible without difficulty in components having directionally solidified nickel-based superalloys. γ-solution annealing leads in the case of a mechanically deformed surface, such as, for example, in the region of the moving blade feet, to a recrystallization of the γ-phase on the component surface. Since, in contrast to conventional nickel-based superalloys, directionally solidified nickel-based superalloys have no or only few elements consolidating the grain boundaries, the grain reformation, caused by recrystallization, on the component surface is an unacceptable material weakening. 
       SUMMARY OF THE INVENTION 
       [0005]    The object of the invention, therefore, is to overcome the abovementioned problem. 
         [0006]    The object is achieved by a heat treatment method according to the independent claim, in which, by dissolving the precipitate, precipitating the precipitate and, once again, dissolving and precipitation, the mechanical stresses are reduced, so that no recrystallization can occur. 
         [0007]    The dependant claims list further advantageous measures which may advantageously be combined with one another in any desired way. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    In the drawing: 
           [0009]      FIGS. 1-12  show exemplary embodiments of the temperature profile of heat treatment methods according to the invention, 
           [0010]      FIG. 13  shows a list of superalloys, 
           [0011]      FIG. 14  shows a gas turbine, 
           [0012]      FIG. 15  shows a turbine blade in perspective, and 
           [0013]      FIG. 16  shows a combustion chamber in perspective. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The heat treatment according to the invention is carried out, in particular, for nickel-based superalloys. Such DX or SX nickel-based superalloys ( FIG. 13 ) are used, in particular, for turbine blades  120 ,  130  ( FIG. 14 ,  15 ) and combustion chamber elements  155  ( FIG. 16 ) for turbines, in particular for gas turbines  100  ( FIG. 14 ). 
         [0015]    The heat treatments may also be carried out with aircraft turbine components (in particular, blades). 
         [0016]    By way of example, the method of heat treatment of nickel superalloys, which have the γ-phase, is explained, that is to say γ-solution annealing. 
         [0017]    Before heat treatment, fluoride ion cleaning (FIC) may also be carried out, which may be utilized, on the one hand, in order to clean oxides from cracks, but also in order, in particular, to deplete the component surface of metallic elements of the material of the substrate, in particular of aluminum and/or titanium, such as superalloys, since these two elements are γ-formers. A depletion of the γ-phase of superalloys in the region of the component surface lowers the inherent stresses which have occurred in the surface due to mechanical load. By this stress being lowered, the motive force for grain reformation (recrystallization) is reduced. 
         [0018]    The FIC cleaning required for this purpose is preferably carried out at temperatures of around 1000° C. by means of HF/H 2  mixtures. 
         [0019]    The γ-solution annealing for the complete dissolving of the precipitate (here γ) according to the prior art has for superalloys a γ-full solution annealing temperature T LG  which is calculated according to the following formula: 
         [0000]      T LG =1229.315+3.987 W−3.624 Ta+2.424 Ru+0.958 Re−6.362 Cr−4.943 Ti−2.602 Al−2.415 Co−2.224 Mo. 
         [0020]    The solution annealing temperature profile in time T(t) is dealt with below. 
         [0021]    In the figures, the temperature profile T(t) is plotted against the time t, the temperature T LG  representing the full solution annealing temperature described above, and the dissolution temperature T SOLV  representing a material-specific temperature beyond which the precipitate can first be dissolved, but a complete dissolution of the precipitates lasts too long. 
         [0022]    The time duration t 1 , preferably at least 1 h, is the time from when the temperature T SOLV  is first overshot to the time point t 3  from which the temperature T dwells, preferably constant, at the full solution annealing temperature T LG . The dwell duration at the full solution annealing temperature preferably amounts to at least 1 hour (1 h). 
         [0023]    In  FIG. 1 , the oscillating movement of the temperature T commences even below the temperature T SOLV  and then rises continuously (see the dashed ascending line) and in an oscillating manner to the temperature T LG . 
         [0024]    After overshooting the temperature T SOLV , the temperature T SOLV  can be undershot due to the oscillating movement (not the case in  FIG. 1 ). 
         [0025]    Preferably from a specific time t 3 , the temperature T dwells, constant, at the full solution annealing temperature T LG  at which it dwells preferably for at least 1 h. 
         [0026]    In  FIG. 1 , four local maxima of the temperature profile can be seen, that is to say four oscillating movements are present. However, even five or more oscillating movements may be generated. 
         [0027]    In  FIG. 2 , the temperature profile is similar to that in  FIG. 1 , but the oscillating movement commences only above the temperature T SOLV . The temperature T SOLV  is preferably not undershot due to the oscillating movement. 
         [0028]    Preferably from a specific time t 3 , the temperature T dwells, constant, at the full solution annealing temperature T SOLV  at which it dwells preferably for at least 1 h. 
         [0029]    In  FIG. 2 , three local maxima can be seen, so that, here, three oscillating movements are present. 
         [0030]    In  FIG. 3 , the temperature T rises (not in an oscillating manner) above the temperature T SOLV  and here falls again, for example once, below the temperature T SOLV  and then rises in an oscillating manner up to the temperature T LG . 
         [0031]    Preferably from a specific time t 3 , the temperature T dwells, constant, at the full solution annealing temperature T LG  at which it dwells preferably for at least 1 h. 
         [0032]    Three local maxima can be seen in  FIG. 3 , so that, here, three oscillating movements are present. 
         [0033]    In the continuously rising oscillating movement (see the dashed lines) of the temperature T according to  FIGS. 1 ,  2  and  3 , the temperature may oscillate once or more than once from a temperature above T SOLV  to below the temperature T SOLV . 
         [0034]    In  FIG. 4 , the temperature T rises (not in an oscillating manner) above the temperature T SOLV  to the solution annealing temperature T LG  and oscillates to and fro between these two temperatures T LG , T SOLV . 
         [0035]    The oscillating temperature profile T(t) then preferably runs uniformly, as can be seen from the dashed line running horizontally. 
         [0036]    Preferably from a specific time t 3 , the temperature T dwells, constant, at the full solution annealing temperature T LG  at which it preferably dwells for at least 1 h. 
         [0037]      FIG. 4  illustrates two oscillating movements. However, three or more oscillating movements may be carried out. 
         [0038]    In  FIG. 5 , the temperature T also rises (not in an oscillating manner) to the full solution annealing temperature T LG  and then falls, although the temperature T SOLV  is not reached (difference ΔT&gt;0). 
         [0039]    The oscillating temperature profile T(t) then preferably runs uniformly, as can be seen from the dashed line running horizontally. 
         [0040]    Preferably from a specific time t 3 , the temperature T dwells, constant, at the full solution annealing temperature T LG  at which it dwells preferably for at least 1 h. 
         [0041]    Three local maxima can be seen in  FIG. 5 , and therefore, here, three oscillating movements are present. 
         [0042]    In  FIG. 6 , the temperature T rises (not in an oscillating manner) beyond the temperature T SOLV  to a temperature below the temperature T LG  and then oscillates to and fro between these two values. The oscillating temperature profile T(t) then preferably runs uniformly, as can be seen from the dashed line running horizontally. 
         [0043]    Preferably from a specific time t 3 , the temperature T dwells, constant, at the full solution annealing temperature T LG  at which it dwells preferably for at least 1 h. 
         [0044]      FIG. 6  illustrates two oscillating movements. However, even three or more oscillating movements may be carried out. 
         [0045]    In  FIG. 7 , the temperature T rises (not in an oscillating manner) beyond the temperature T SOLV  to a temperature below the temperature T LG  and oscillates to and fro between this temperature below T LG  and a temperature above T SOLV . The oscillating temperature profile T(t) then preferably runs uniformly, as can be seen from the dashed line running horizontally. 
         [0046]    Preferably from a specific time t 3 , the temperature T dwells, constant, at the full solution annealing temperature T LG  at which it dwells preferably for at least 1 h. 
         [0047]    Three local maxima can be seen in  FIG. 7 , and therefore, here, three oscillating movements are present. 
         [0048]    In contrast to  FIGS. 4 and 6 , the temperature T in  FIGS. 8 and 9  also oscillates below the temperature T SOLV . 
         [0049]    In  FIG. 8  the temperature always reaches a maximum value of the full solution annealing temperature T LG , whereas, in  FIG. 9 , the maximum value of the temperature profile reaches a temperature above T SOLV,  but below the full solution annealing temperature T LG . 
         [0050]    Preferably from a specific time t 3 , the temperature T in  FIGS. 8 and 9  dwells, constant, at the full solution annealing temperature T LG  at which it dwells preferably for at least 1 h. 
         [0051]      FIG. 8  illustrates two oscillating movements. However, even three or more oscillating movements may be carried out. 
         [0052]      FIG. 9  illustrates two oscillating movements. However, even three or more oscillating movements may be carried out. 
         [0053]    In  FIG. 10 , the temperature T rises (not in an oscillating manner) above the temperature T SOLV  and oscillates to and fro between this value and a lower value (≧T SOLV ). The oscillating temperature profile T(t) then preferably runs uniformly, as can be seen from the dashed line running horizontally. 
         [0054]    Thereafter, after a specific time t 2 , the temperature rises, in particular in an oscillating manner, to the full solution annealing temperature T LG . 
         [0055]    Preferably from a specific time t 3 , the temperature T dwells, constant, at the full solution annealing temperature T LG  at which it dwells preferably for at least 1 h. 
         [0056]    In  FIG. 10 , four local maxima are present, and therefore four oscillating movements occur. However, even five or more oscillating movements may be carried out. 
         [0057]      FIG. 12  illustrates a further exemplary embodiment of the oscillating temperature profile T(t) according to the invention. 
         [0058]    The average value of the temperature T about which the temperature fluctuates is increased in steps here until, from a time t 3 , the temperature is set, constant, at a temperature T LG . 
         [0059]    Initially, the temperature T oscillates about the temperature T SOLV , then rises to a higher temperature, so that the temperature T SOLV  is preferably no longer undershot, oscillates and rises further again in a third or in further steps, the maximum temperature T LG  being reached here or a clearance with respect to the temperature T LG  being present. 
         [0060]    Preferably from a specific time t 3 , the temperature T dwells, constant, at the full solution annealing temperature T LG  at which it dwells preferably for at least 1 h. 
         [0061]      FIGS. 1 to 12  illustrate the oscillating movements only preferably in wavy or sinusoidal form, but they may also be formed triangularly ( FIG. 11 ), rectangularly (not illustrated) or otherwise. 
         [0062]    Likewise, in the oscillating movements, the temperature T LG  may also be reached or overshot once or more than once by means of the oscillating movement. 
         [0063]    After the end of the oscillating movement, the temperature can be set at a temperature equal to or higher than the full solution annealing temperature T LG  and be held there, in particular for at least one hour. 
         [0064]    If a temperature higher than the full solution annealing temperature T LG  is set at the end in a specific time t 3 , an oscillating movement above the full solution annealing temperature T LG  may preferably take place. 
         [0065]    It is also advantageous if the full solution annealing temperature is not overshot, apart from an unwanted overshooting when the temperature is being set to the full solution annealing temperature. 
         [0066]    It is also advantageous that the temperature rises in an oscillating manner. The oscillating rise of the temperature T in  FIGS. 1 ,  2 ,  3  and  10  takes place at least intermittently, in particular at least during the overshooting of the temperature T SOLV . 
         [0067]    In particular, the oscillating rise in the temperature T is followed by a holding time at a temperature ≧ of the full solution annealing temperature T LG . 
         [0068]    The oscillating rise in the temperature can be seen from the dashed line which rises, the temperature of a maximum of the oscillating movement being increased in relation to the maximum of the preceding maximum. Correspondingly, the minima, that is to say the valleys of the oscillating movement, are not identical, but rise with the time t. 
         [0069]      FIG. 13  shows a list of nickel-based DS or SX superalloys which can be treated by means of the method according to the invention. 
         [0070]    For the material IN 6203 DS, the temperature T SOLV  amounts to 1100° C. and the temperature T LG  to 1150° C. 
         [0071]    For the material IN 792 DS, the temperature T SOLV  amounts to 1140° C. and the temperature T LG  to 1230° C. 
         [0072]    The material PWA 1483 SX has a temperature T SOLV  of 1150° C. and a temperature T LG  of 1250° C. 
         [0073]      FIG. 14  shows a gas turbine  100  by way of example in a longitudinal part section. 
         [0074]    The gas turbine  100  has inside it a rotor  103  rotary-mounted about an axis of rotation  102  and having a shaft  101 , said rotor also being designated as a turbine rotor. 
         [0075]    An intake casing  104 , a compressor  105 , a, for example, toroidal combustion chamber  110 , in particular annular combustion chamber, with a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust gas casing  109  succeeding one another along the rotor  103 . 
         [0076]    The annular combustion chamber  110  communicates with a, for example, annular hot-gas duct  111 . There, for example, four turbine stages  112  connected one behind the other form the turbine  108 . 
         [0077]    Each turbine stage  112  is formed, for example, from two blade rings. As seen in the direction of flow of a working medium  113 , a guide blade row  115  is followed in the hot-gas duct  111  by a row  125  formed from moving blades  120 . 
         [0078]    The guide blades  130  are in this case fastened to an inner casing  138  of a stator  143 , whereas the moving blades  120  of a row  125  are mounted on the rotor  103 , for example, by means of a turbine disk  133 . A generator or a working machine (not illustrated) is coupled to the rotor  103 . 
         [0079]    While the gas turbine  100  is in operation, air  135  is sucked in by a compressor  105  through the intake casing  104  and is compressed. The compressed air provided at the turbine-side end of the compressor  105  is routed to the burners  107  and is mixed there with a fuel. The mixture is then burnt in the combustion chamber  110  so as to form the working medium  113 . 
         [0080]    The working medium  113  flows from there along the hot-gas duct  111  past the guide blades  130  and the moving blades  120 . At the moving blades  120 , the working medium  113  expands so as to transmit a pulse, so that the moving blades  120  drive the rotor  103  and the latter drives the working machine coupled to it. 
         [0081]    The components exposed to the hot working medium  113  are subject to thermal loads while the gas turbine  100  is in operation. The guide 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 subjected to the highest thermal load, in addition to the heatshield elements lining the annular combustion chamber  110 . 
         [0082]    In order to withstand the temperatures prevailing there, these blades may be cooled by means of a coolant. 
         [0083]    Likewise, substrates of the components may have a directional structure, that is to say they are monocrystalline (SX structure) or have only longitudinally directed grains (DS structure). 
         [0084]    The material used for the components, particularly for the turbine blade  120 ,  130  and components of the combustion chamber  110 , is, for example, iron-, nickel- or cobalt-based superalloys. 
         [0085]    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 publications are part of the disclosure with regard to the chemical composition of the alloys. 
         [0086]    The guide blade  130  has a guide blade foot (not illustrated here) facing the inner casing  138  of the turbine  108  and a guide blade head lying opposite the guide blade foot. The guide blade head faces the rotor  103  and is secured to a fastening ring  140  of the stator  143 . 
         [0087]      FIG. 15  shows a perspective view of a moving blade  120  or guide blade  130  of a turbomachine which extends along a longitudinal axis  121 . 
         [0088]    The turbomachine may be a gas turbine of an aircraft or of a power station for electricity generation, a steam turbine or a compressor. 
         [0089]    The blade  120 ,  130  has successively along the longitudinal axis  121  a fastening region  400 , a blade platform  403  contiguous to the latter and also a blade leaf  406  and a blade tip  415 . 
         [0090]    As a guide blade  130 , the blade  130  may have (not illustrated) a further platform at its blade tip  415 . 
         [0091]    In the fastening region  400 , a blade foot  183  is formed which serves (not illustrated) for fastening the moving blades  120 ,  130  to a shaft or a disk. The blade foot  183  is configured, for example, as a hammer head. Other configurations as a pinetree or dovetail foot are possible. The blade  120 ,  130  has a leading edge  409  and a trailing edge  412  for a medium which flows past the blade leaf  406 . 
         [0092]    In conventional blades  120 ,  130 , for example, solid metallic materials, in particular superalloys, are used in all regions  400 ,  403 ,  406  of the blade  120 ,  130 . 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 publications are part of the disclosure with regard to the chemical composition of the alloy. 
         [0093]    The blade  120 ,  130  may in this case be manufactured by means of a casting method, also by means of a directional solidification, by a forging method, by a milling method or combinations of these. 
         [0094]    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. 
         [0095]    The manufacture of monocrystalline workpieces of this type takes place, for example, by directional solidification from the melt. These are casting methods in which the liquid metallic alloy solidifies into the monocrystalline structure, that is to say into the monocrystalline workpiece, or directionally solidifies. 
         [0096]    In this case, dendritic crystals are oriented along the heat flow and form either a columnar-crystalline grain structure (columnar, that is to say grains which run over the entire length of the workpiece and here, according to general linguistic practice, are designated as being directionally solidified) or a monocrystalline structure, that is to say the entire workpiece consists of a single crystal. These methods must avoid the transition to globulitic (polycrystalline) solidification, since, due to undirected growth, transverse and longitudinal grain boundaries are necessarily formed which nullify the good properties of the directionally solidified or monocrystalline component. 
         [0097]    When directionally solidified structures are referred to in general terms, this means both monocrystals which have no grain boundaries or at most small-angle grain boundaries and columnar-crystal structures which have grain boundaries running in the longitudinal direction, but no transverse grain boundaries. In the case of these second-mentioned crystalline structures, directionally solidified structures are also referred to. 
         [0098]    Such methods are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1; these publications are part of the disclosure with regard to the solidification method. 
         [0099]    The blades  120 ,  130  may likewise have coatings against corrosion or oxidation, for example (MCrAlX; M is at least one element of the group 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 (HO). 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 to be part of the disclosure with regard to the chemical composition of the alloy. 
         [0100]    The density is preferably around 95% of the theoretical density. 
         [0101]    A protective aluminum oxide layer (TGO=thermal grown oxide layer) is formed on the MCrAlX layer (as an intermediate layer or as the outermost layer). 
         [0102]    On the MCrAlX, a heat insulation layer may also be present, which is preferably the outermost layer and consists, for example, of ZrO 2 , Y 2 O 3 —ZrO 2 , that is to say it is not or is partially or is completely stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. The heat insulation layer covers the entire MCrAlX layer. 
         [0103]    By means of suitable coating methods, such as, for example, electron beam evaporation (EB-PVD), columnar grains are generated in the heat insulation layer. 
         [0104]    Other coating methods may be envisaged, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The heat insulation layer may have porous microcrack- or macrocrack-compatible grains for better thermal shock resistance. The heat insulation layer is therefore preferably more porous than the MCrAlX layer. 
         [0105]    The blade  120 ,  130  may be hollow or solid. If the blade  120 ,  130  is to be cooled, it is hollow and, if appropriate, also has film-cooling holes  418  (indicated by dashes). 
         [0106]      FIG. 16  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 around an axis of rotation  102  in the circumferential direction issue into a common combustion chamber space  154  and generate the flames  156 . For this purpose, the combustion chamber  110  is configured in its entirety as an annular structure which is positioned around the axis of rotation  102 . 
         [0107]    To achieve comparatively high efficiency, the combustion chamber  110  is designed for a comparatively high temperature of the working medium M of about 1000° C. to 1600° C. In order to make it possible to have a comparatively long operating time even in the case of these operating parameters which 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 from heatshield elements  155 . 
         [0108]    On account of the high temperatures inside the combustion chamber  110 , moreover, a cooling system may be provided for the heatshield elements  155  or for their holding elements. The heatshield elements  155  are then, for example, hollow and, if appropriate, also have cooling holes (not illustrated) issuing into the combustion chamber space  154 . 
         [0109]    Each heatshield element  155  consisting of an alloy is equipped on the working medium side with a particularly heat-resistant protective layer (MCrAlX layer and/or ceramic coating) or is manufactured from material resistant to high temperature (solid ceramic bricks). 
         [0110]    This protective layer may be similar to those of the turbine blades, that is to say, for example, MCrAlX means: M is at least one element of the group 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). 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 to be part of the disclosure with regard to the chemical composition of the alloy. 
         [0111]    On the MCrAlX, a, for example, ceramic heat insulation layer may also be present and consist, for example, of ZrO 2 , Y 2 O 3 —ZrO 2 , that is to say it is not or is partially or is completely stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
         [0112]    By means of suitable coating methods, such as, for example, electron beam evaporation (EB-PVD), columnar grains are generated in the heat insulation layer. 
         [0113]    Other coating methods may be envisaged, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The heat insulation layer may have porous microcrack- or macrocrack-compatible grains for better thermal shock resistance. 
         [0114]    Refurbishment means that turbine blades  120 ,  130  and heatshield elements  155 , after their use, must, where appropriate, be freed of protective layers (for example, by sandblasting). A removal of the corrosion and/or oxidation layers or products is then carried out. In solution annealing, the method according to the invention is used. If appropriate, cracks in the turbine blade  120 ,  130  or in the heatshield element  155  are also repaired. A recoating of the turbine blades  120 ,  130  and heatshield elements  155  and a renewed use of the turbine blades  120 ,  130  or of the heatshield elements  155  then take place.