Abstract:
Known cast iron alloys have use limits in respect of the temperature. An alloy (in a weight percentage) including silicon 2.0%-4.5%, carbon 2.9%-4.0%, niobium 0.05%-0.7%, molybdenum 0.3%-1.5%, optionally cobalt 0.1%-2.0%, manganese≦0.3%, nickel≦0.5%, magnesium≦0.07%, phosphorus≦0.05%, sulphur≦0.012%, chromium≦0.1%, antimony≦0.004%, and, iron, is provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2012/054941 filed Mar. 21, 2012 and claims benefit thereof, the entire content of which is hereby incorporated herein by reference. The International Application claims priority to the European Patent Office application No. 11162635.4 EP filed Apr. 15, 2011, the entire contents of which is hereby incorporated herein by reference. 
       FIELD OF INVENTION 
       [0002]    The invention relates to a cast iron containing niobium as claimed in the claims and to a component as claimed in the claims. 
       BACKGROUND OF INVENTION 
       [0003]    The known cast iron alloys now employed (so-called GJS alloys: nodular cast iron) primarily use silicon and molybdenum to increase the creep strength, scaling resistance and LCF behavior. Over time, however, these elements lead to a significant decrease in the toughness. 
         [0004]    Molybdenum furthermore exhibits a very high susceptibility to segregation. 
       SUMMARY OF INVENTION 
       [0005]    It is therefore an object of the invention to specify an alloy and a component, which overcome the aforementioned disadvantages and have better mechanical strengths over the service life. 
         [0006]    The object is achieved by an alloy as claimed in the claims and a component as claimed in the claims. 
         [0007]    The dependent claims list further advantageous measures which are advantageously combined with one another in any desired way. 
         [0008]    The invention consists in the fact that cobalt and/or niobium can partially replace molybdenum. The working limitations presented by the previous GJS alloy can therefore be overcome. 
         [0009]    The iron-based alloy according to the invention has high elongations for the application field in the temperature range of 450° C.-550° C., and has the following composition (in % by weight): 
         [0000]    
       
         
               
               
             
           
               
                   
               
             
             
               
                 silicon (Si) 
                 2.0%-4.5%, in particular 2.3%-3.9%, 
               
               
                 carbon (C) 
                 2.9%-4.0%, in particular 3.2%-3.7%, 
               
               
                 niobium (Nb) 
                 0.05%-0.7%, in particular 0.05%-0.6%, 
               
               
                 very particularly 
                 0.1% to 0.7%, 
               
               
                 molybdenum (Mo) 
                 0.3%-1.5%, in particular 0.4%-1.0%, 
               
               
                 very particularly 
                 0.5%, 
               
               
                 optionally 
               
               
                 cobalt (Co) 
                 0.1%-2.0%, in particular 0.1%-1.0%, 
               
               
                 manganese (Mn) 
                 ≦0.3%, in particular 0.15-0.30%, 
               
               
                 nickel (Ni) 
                 ≦0.5%, in particular ≦0.3%, 
               
               
                 magnesium (Mg) 
                 ≦0.07%, in particular at least 0.03%, 
               
               
                 very particularly 
                 0.03%-0.06%, 
               
               
                 phosphorus (P) 
                 ≦0.05%, in particular 0.02%-0.035%, 
               
               
                 sulfur (S) 
                 ≦0.012%, in particular ≦0.005%, 
               
               
                 very particularly 
                 between 0.003% and 0.012%, 
               
               
                 chromium (Cr) 
                 ≦0.1%, in particular ≦0.05%, 
               
               
                 antimony (Sb) 
                 ≦0.004%, in particular ≦0.003%, 
               
               
                 iron (Fe), 
               
               
                 in particular remainder iron. 
               
               
                   
               
             
          
         
       
     
         [0010]    Advantageously, the proportion of silicon, cobalt, niobium and molybdenum is ≦7.5% by weight, in particular ≦6.5% by weight. 
         [0011]    Even small proportions of cobalt and/or niobium and molybdenum improve the mechanical characteristics. 
         [0012]    Niobium improves the endurance strength with a constantly high LCF strength and good toughness. 
         [0013]    By the precipitation of finely distributed Nb carbides, niobium brings about a higher high-temperature strength, as a result of which the working limitations are shifted to high temperatures. 
         [0014]    Cobalt brings about a solid solution solidification, which has a positive effect on the properties of the alloy at high temperatures and given low stresses. 
         [0015]    The addition of molybdenum to the alloy (preferably 0.4%-1.0%) has a positive influence on the high-temperature strength (Rp0.2 and Rm in the elevated temperature range) and the endurance behavior (creep strength). 
         [0016]    Preferably, the proportion of cobalt in the alloy lies between 0.5% by weight and 1.5% by weight. 
         [0017]    Advantageous mechanical values are achieved for the alloy respectively when the cobalt content is 0.1% by weight to 1.0% by weight cobalt. 
         [0018]    Magnesium obtains the nodular formation of the graphite and magnesium is preferably present in an amount of at least 0.03% by weight, at most 0.07% by weight. 
         [0019]    Depending on the application, chromium (Cr) is preferably present in an amount of at least 0.01% by weight, but at most 0.05% by weight, and this increases the oxidation resistance. 
         [0020]    The alloy may comprise further elements. 
         [0021]    The alloy optionally contains small minimum admixtures of 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 phosphorus (P) 
                  0.05% by weight, 
               
               
                   
                 sulfur (S) 
                 0.001% by weight, 
               
               
                   
                 magnesium (Mg) 
                  0.01% by weight, 
               
               
                   
                 antimony (Sb), 
               
               
                   
                 cerium (Ce), 
               
               
                   
                   
               
             
          
         
       
     
         [0022]    which have a positive influence on the castability and/or the formation of the nodular graphite, but also must not be excessively high since otherwise the negative influences prevail. 
         [0023]    Furthermore, there is preferably no chromium (Cr) in the alloy. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    Exemplary embodiments of the invention will be explained in more detail with reference to the following figures, in which: 
           [0025]      FIG. 1  shows a steam turbine, 
           [0026]      FIG. 2  shows a gas turbine. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0027]    The component with the alloy has an optimal ferritic microstructure with nodular graphite. 
         [0028]    The table shows exemplary alloys according to the invention which have improved mechanical properties. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 C 
                 Si 
                 Mo 
                 Co 
                 Nb 
                 Mg 
                 Mn 
                 P 
                 S 
                 Sb 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 3.2 
                 3.5 
                 0.5 
                 0 
                 0.5 
                 0.04 
                 0.2 
                 0.03 
                 0.005 
                 0.0009 
               
               
                 2 
                 3.3 
                 3.6 
                 0.5 
                 0 
                 0.1 
                 0.05 
                 0.2 
                 0.03 
                 0.005 
                 0.0003 
               
               
                 3 
                 3.7 
                 2.7 
                 1.0 
                 0.9 
                 0.4 
                 0.05 
                 0.2 
                 0.03 
                 0.005 
                 0.0004 
               
               
                 4 
                 3.5 
                 2.4 
                 1.0 
                 0 
                 0.5 
                 0.06 
                 0.3 
                 0.03 
                 0.004 
                 0.0002 
               
               
                 5 
                 2.3 
                 3.9 
                 0.5 
                 0 
                 0.4 
                 0.03 
                 0.3 
                 0.03 
                 0.007 
                 0.0030 
               
               
                 6 
                 3.3 
                 3.4 
                 0.5 
                 1.0 
                 0.5 
                 0.04 
                 0.2 
                 0.02 
                 0.005 
                 0.0030 
               
               
                 7 
                 3.3 
                 3.4 
                 0.5 
                 0.5 
                 0.5 
                 0.04 
                 0.2 
                 0.02 
                 0.005 
                 0.0039 
               
               
                 8 
                 3.0 
                 3.3 
                 0.4 
                 0 
                 0.2 
                 0.05 
                 0.2 
                 0.03 
                 0.004 
                 0.0014 
               
               
                   
               
             
          
         
       
     
         [0029]    The alloy preferably contains no vanadium (V) and/or titanium (Ti) and/or tantalum (Ta) and/or copper (Cu). 
         [0030]    The ratio of C and Si should give an almost-eutectic composition, i.e. should correspond to a carbon equivalent CE of between 4.1% and 4.4%, 
         [0000]    
       
         
           
             
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                 CE 
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                     by 
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                      
                     
                         
                     
                      
                     C 
                   
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                          
                         
                             
                         
                          
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                         P 
                       
                     
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         [0031]      FIG. 1  shows a steam turbine  300 ,  303  having a turbine shaft  309  extending along an axis of rotation  306 . 
         [0032]    The steam turbine comprises a high-pressure turbine part  300  and a medium-pressure turbine part  303 , each with an inner housing  312  and an outer housing  315  enclosing the latter. The high-pressure turbine part  300  is, for example, configured in pot design. The medium-pressure turbine part  303  is, for example, configured to be twin-streamed. It is likewise possible for the medium-pressure turbine part  303  to be configured to be single-streamed. 
         [0033]    A bearing  318  is arranged along the axis of rotation  306  between the high-pressure turbine part  300  and the medium-pressure turbine part  303 , the turbine shaft  309  comprising a bearing region  321  in the bearing  318 . The turbine shaft  309  is mounted on a further bearing  324  beside the high-pressure turbine part  300 . In the region of this bearing  324 , the high-pressure turbine part  300  comprises a shaft seal  345 . The turbine shaft  309  is sealed relative to the outer housing  315  of the medium-pressure turbine part  303  by two further shaft seals  345 . Between a high-pressure steam intake region  348  and a steam outlet region  351 , the turbine shaft  309  in the high-pressure turbine part  300  comprises the high-pressure rotor blading  357 . With the associated rotor blades (not shown in more detail), this high-pressure rotor blading  357  constitutes a first blading region  360 . 
         [0034]    The medium-pressure turbine part  303  comprises a central steam intake region  333 . Associated with the steam intake region  333 , the turbine shaft  309  comprises a radially symmetric shaft shield  363 , a cover plate, on the one hand to divide the steam flow into the two streams of the medium-pressure turbine part  303  and also to prevent direct contact of the hot steam with the turbine shaft  309 . In the medium-pressure turbine part  303 , the turbine shaft  309  comprises a second blading region  366  with the medium-pressure rotor blades  354 . The hot steam flowing through the second blading region  366  flows from the medium-pressure turbine part  303  out of a discharge port  369  to a low-pressure turbine part (not shown) connected downstream in terms of flow technology. 
         [0035]    The turbine shaft  309  is composed for example of two turbine shaft parts  309   a  and  309   b,  which are connected firmly to one another in the region of the bearing  318 . Each turbine shaft part  309   a,    309   b  comprises a cooling line  372  formed as a central bore  372   a  along the axis of rotation  306 . The cooling line  372  is connected to the steam outlet region  351  via a feed line  375  comprising a radial bore  375   a.  In the medium-pressure turbine part  303 , the coolant line  372  is connected to a cavity (not shown in more detail) below the shaft shield. The feed lines  375  are configured as a radial bore  375   a,  so that “cold” steam from the high-pressure turbine part  300  can flow into the central bore  372   a . Via the discharge line  372  also formed in particular as a radially directed bore  375   a,  the steam passes through the bearing region  321  into the medium-pressure turbine part  303  and there onto the lateral surface  330  of the turbine shaft  309  in the steam intake region  333 . The steam flowing through the cooling line is at a much lower temperature than the temporarily superheated steam flowing into the steam intake region  333 , so as to ensure effective cooling of the first rotor blade row  342  of the medium-pressure turbine part  303  and the lateral surface  330  in the region of this rotor blade row  342 . 
         [0036]      FIG. 2  shows, by way of example, a partial longitudinal section through a gas turbine  100 . 
         [0037]    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. 
         [0038]    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 . 
         [0039]    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 . 
         [0040]    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 . 
         [0041]    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 . 
         [0042]    A generator or a working machine (not shown) is coupled to the rotor  103 . 
         [0043]    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 burnt 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. 
         [0044]    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. 
         [0045]    To be able to withstand the temperatures which prevail there, they may be cooled by means of a coolant. 
         [0046]    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). 
         [0047]    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 . 
         [0048]    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. 
         [0049]    The blades or vanes  120 ,  130  may likewise have coatings protecting against corrosion (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon, scandium (Sc) and/or at least one rare earth element, or hafnium). 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. 
         [0050]    It is also possible for a thermal barrier coating to be present on the MCrAlX, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. it is unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
         [0051]    Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0052]    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 .