Abstract:
Cast iron alloys have application limits with regard to temperature. By means of the use of cobalt an optimal ferritic structure can be achieved such that with an alloy containing silicon 2.0-4.5 wt. %, cobalt 0.5-5 wt. %, carbon 2.5-4 wt. %, molybdenum≦1 wt. %, manganese≦0.25 wt. %, nickel≦0.3 wt. %, the remainder iron where the proportion of silicon cobalt and molybdenum is less than 7.5 wt. % the application limits are shifted to higher temperatures.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2007/050057, filed Jan. 3, 2007 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 06000851.3 EP filed Jan. 16, 2006, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to an alloy, a cast iron comprising cobalt and a component thereof. 
       BACKGROUND OF INVENTION 
       [0003]    The known cast iron alloys now employed (so-called GJS spherocast alloys) primarily use silicon and molybdenum to increase the creep strength, scaling resistance and endurance strength. Over time, however, these elements lead to a significant decrease in the ductility. 
         [0004]    Molybdenum furthermore exhibits a very high susceptibility to segregation. 
       SUMMARY OF INVENTION 
       [0005]    It is therefore an object of the invention to provide 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 an independent claim and e.g. by a component as claimed in a further independent claim. 
         [0007]    Further advantageous measures are listed in the dependent claims, and these may advantageously be combined with one another in any desired way. 
         [0008]    The invention consists in cobalt partially or fully replacing molybdenum. The working limitations presented by the previous GJS alloy can therefore be overcome. The 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 wt %): 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 silicon 
                 2.0%-4.5% 
               
               
                   
                 cobalt 
                 0.5%-5% 
               
               
                   
                 carbon 
                 2.0%-4.5%, in particular 2.5%-4%, 
               
               
                   
                 molybdenum 
                 &lt;1.5%, in particular ≦1.0%, 
               
               
                   
                 manganese 
                 &lt;0.5%, in particular ≦0.25%, 
               
               
                   
                 nickel 
                 &lt;0.5%, in particular ≦0.3%, 
               
               
                   
                 remainder iron. 
               
               
                   
                   
               
             
          
         
       
     
         [0009]    Advantageously, the proportion of silicon, cobalt and molybdenum is less than 7.5 wt %. 
         [0010]    Preferably, the proportion of cobalt in the alloy lies between 0.5 and 1.5 wt % cobalt. 
         [0011]    Advantageous mechanical values are achieved for the alloy respectively when the cobalt content is 0.5 wt %, with 1 wt % cobalt, with 1.5 wt % cobalt and 2.0 wt % cobalt. 
         [0012]    The alloy may contain further elements. Preferably, however, the alloy consists of iron, silicon, cobalt and carbon. 
         [0013]    Particular advantages are also achieved when the alloy consists of iron, silicon, cobalt, carbon and manganese. 
         [0014]    Further advantages are obtained with an alloy which consists of iron, silicon, cobalt, carbon and optionally admixtures of molybdenum, manganese and/or nickel. 
         [0015]    The alloy may optionally contain undesired impurities of at most 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 phosphorus 
                 0.007 wt % 
               
               
                   
                 sulfur 
                 0.008 wt % 
               
               
                   
                 magnesium 
                 0.049 wt %. 
               
               
                   
                   
               
             
          
         
       
     
         [0016]    Furthermore, there is preferably no chromium (Cr) in the alloy except for the usual impurities. 
         [0017]    Likewise, there is preferably no magnesium (Mg) in the alloy except for the usual impurities. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Embodiments of the invention will be explained in more detail with the aid of the following figures, in which: 
       
    
    
       [0019]      
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 FIG. 1 
                 shows a micrograph, 
               
               
                   
                 FIG. 2 
                 shows mechanical characteristics, 
               
               
                   
                 FIG. 3 
                 shows a steam turbine, 
               
               
                   
                 FIG. 4 
                 shows a gas turbine. 
               
               
                   
                   
               
             
          
         
       
     
       DETAILED DESCRIPTION OF INVENTION 
       [0020]      FIG. 1  shows an almost optimal ferritic structure (etched) with spherical graphite made of an alloy with about 2 wt % cobalt: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 carbon 
                 3.67 wt %, 
               
               
                   
                 molybdenum 
                 2.41 wt %, 
               
               
                   
                 manganese 
                 0.029 wt %,  
               
               
                   
                 nickel 
                 1.94 wt %, 
               
               
                   
                 iron 
                 remainder. 
               
               
                   
                   
               
             
          
         
       
     
         [0021]      FIG. 2  shows the influence of cobalt on the mechanical properties of the alloys, which are listed in the following table (data in wt %). 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 cobalt 
                 0 
                 0.54 
                 1.04 
                 1.94 
               
               
                   
                 carbon 
                 3.63 
                 3.61 
                 3.68 
                 3.67 
               
               
                   
                 silicon 
                 2.45 
                 2.44 
                 2.47 
                 2.41 
               
               
                   
                 manganese 
                 0.067 
                 0.036 
                 0.03 
                 0.029 
               
               
                   
                 phosphorus 
                 0.007 
                 0.006 
                 0.007 
                 0.007 
               
               
                   
                 Sulfur 
                 0.009 
                 0.006 
                 0.008 
                 0.008 
               
               
                   
                 Magnesium 
                 0.044 
                 0.04 
                 0.05 
                 0.049 
               
               
                   
                   
               
             
          
         
       
     
         [0022]    The elongation at break R p02  increases from 271 N/mm 2  to 284 N/mm 2 . 
         [0000]    The tensile strength Rm increases from 403 N/mm 2  to 412 N/mm 2 .
 
The elongation at break A 5  increases from 15.5% to 21.9%.
 
Likewise, the necking at fracture Z increases from 13.8% to 29.5%.
 
         [0023]    Even small proportions of cobalt (0.5 wt % to 1.0 wt % or 1.0 wt % to 1.5 wt %) improve the mechanical characteristics. 
         [0024]      FIG. 3  shows a steam turbine  300 ,  303  having a turbine shaft  309  extending along a rotation axis  306 . 
         [0025]    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. 
         [0026]    A bearing  318  is arranged along the rotation axis  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 represented in detail), this high-pressure rotor blading  357  constitutes a first blading region  360 . 
         [0027]    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. 
         [0028]    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  and  309   b  comprises a cooling line  372  formed as a central bore  372   a  along the rotation axis  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) 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  333  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 . 
         [0029]      FIG. 4  shows a gas turbine  100  by way of example in a partial longitudinal section. 
         [0030]    The gas turbine  100  internally comprises a rotor  103 , which will also be referred to as the turbine rotor, mounted so as to rotate about a rotation axis  102  and having a shaft  101 . 
         [0031]    Successively along the rotor  103 , there are an intake manifold  104 , a compressor  105 , an e.g. toroidal combustion chamber  110 , in particular a ring combustion chamber, having a plurality of burners  107  arranged coaxially, a turbine  108  and the exhaust manifold  109 . 
         [0032]    The ring combustion chamber  110  communicates with an e.g. annular hot gas channel  111 . There, for example, four successively connected turbine stages  112  form the turbine  108 . 
         [0033]    Each turbine stage  112  is formed for example by two blade rings. As seen in the flow direction of a working medium  113 , a guide vane row  115  is followed in the hot gas channel  111  by a row  125  formed by rotor blades  120 . 
         [0034]    The guide vanes  130  are fastened on an inner housing  138  of a stator  143  while the rotor blades  120  of a row  125  are fastened on the rotor  103 , for example by means of a turbine disk  133 . 
         [0035]    Coupled to the rotor  103 , there is a generator or a work engine (not shown). 
         [0036]    During operation of the gas turbine  100 , air  135  is taken in and compressed by the compressor  105  through the intake manifold  104 . The compressed air provided at the end of the compressor  105  on the turbine side is delivered to the burners  107  and mixed there with a fuel. The mixture is then burnt to form the working medium  113  in the combustion chamber  110 . From there, the working medium  113  flows along the hot gas channel  111  past the guide vanes  130  and the rotor blades  120 . At the rotor blades  120 , the working medium  113  expands by imparting momentum, so that the rotor blades  120  drive the rotor  103  and the work engine coupled to it. 
         [0037]    During operation of the gas turbine  100 , the components exposed to the hot working medium  113  experience thermal loads. Apart from the heat shield elements lining the ring combustion chamber  110 , the guide vanes  130  and rotor blades  120  of the first turbine stage  112 , as seen in the flow direction of the working medium  113 , are heated the most. 
         [0038]    In order to withstand the temperatures prevailing there, they may be cooled by means of a coolant. 
         [0039]    Substrates of the components may likewise comprise a directional structure, i.e. they are monocrystalline (SX structure) or comprise only longitudinally directed grains (DS structure). 
         [0040]    Iron-, nickel- or cobalt-based superalloys are for example used as material for the components, in particular for the turbine blades  120 ,  130  and components of the combustion chamber  110 . 
         [0041]    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; with respect to the chemical composition of the alloys, these documents are part of the disclosure. 
         [0042]    The blades  120 ,  130  may likewise have coatings against corrosion (MCrAlX; M is at least one element from the group 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). 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, with respect to the chemical composition, are intended to be part of this disclosure. 
         [0043]    On the MCrAlX, there may furthermore be a thermal barrier layer which consists for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. it is not stabilized or is partially or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
         [0044]    Rod-shaped grains are produced in the thermal barrier layer by suitable coating methods, for example electron beam deposition (EB-PVD). 
         [0045]    The guide vane  130  comprises a guide vane root (not shown here) facing the inner housing  138  of the turbine  108 , and a guide vane head lying opposite the guide vane root. The guide vane head faces the rotor  103  and is fixed on a fastening ring  140  of the stator  143 .