Abstract:
A novel ferritic martensitic alloy is provided. The ferritic martensitic alloy enables the use temperature to be increased from 500° C. to 550° C., where the strength is maintained or is even maximized and the toughness, especially for low temperatures, is maintained compared to the known iron-based alloys. Tunsten is preferably not used.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2009/003640, filed May 22, 2009 and claims the benefit thereof. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a ferritic-martensitic alloy, to a component and to a process. 
       BACKGROUND OF INVENTION 
       [0003]    Iron-based alloys are inexpensive alloys compared to nickel-based superalloys, but the strengths and toughnesses thereof are lower compared to the nickel-based superalloys. 
         [0004]    Similarly, EP 1 466 993 B1 is known, in which use is made of tungsten. 
       SUMMARY OF INVENTION 
       [0005]    It is therefore an object of the invention to propose an alloy by means of which the use temperature can be increased, and at the same time the strength is maximized and the toughness is retained especially for lower temperatures. 
         [0006]    The object is achieved by an alloy as claimed in the claims, by a component as claimed in the claims and by a process as claimed in the claims. 
         [0007]    The dependent claims list further advantageous measures which can be combined with one another, as desired, to achieve further advantages. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIGS. 1 ,  2 ,  3  show exemplary embodiments, and 
           [0009]      FIG. 4  shows a steam turbine. 
       
    
    
       [0010]    The figures and the description represent merely exemplary embodiments of the invention. 
       DETAILED DESCRIPTION OF INVENTION 
       [0011]    The prior art is formed by iron-based alloys, known from EP 0 867 523, in which use is made of tungsten. 
         [0012]    With preference, the new ferritic-martensitic alloy dispenses with the addition of tungsten (W) except for the customary impurities, which lie considerably below 0.1% by weight, in particular below 0.01% by weight. 
         [0013]    The tables in  FIGS. 1 to 3  show some exemplary embodiments of the invention. 
         [0014]    The iron-based alloy comprises, in an inconclusive list (in % by weight):
   carbon (C): 0.13-0.22,   chromium (Cr): 9.0-9.8,   molybdenum (Mo): 1.0-2.0, in particular 1.4-1.6,   nickel (Ni): 0.3-0.8, in particular 0.3-0.7,   vanadium (V): 0.25-0.35, in particular 0.25-0.3,   aluminum (Al): 0.005-0.01,   niobium (Nb): 0.04-0.06,   boron (B): 20 ppm-70 ppm, in particular 35 ppm-55 ppm,   nitrogen (N): 150 ppm-500 ppm,   cobalt (Co): 0-1.5, in particular up to 1.3,   manganese (Mn): 0-0.15,   silicon (Si): 0-0.1,   phosphorus (P): 0-0.005,   sulfur (S): 0-0.003,   arsenic (As): max. 0.015,   tin (Sn): max. 0.015,   antimony (Sb): max. 0.015,   copper (Cu): max. 0.1,   iron (Fe).   
 
         [0034]    The alloy preferably consists of these elements. 
         [0035]    The boron content gives rise to a very good long-term stability at elevated temperatures. Here, the boron content is optimized with the required nitrogen content, in order to avoid the formation of boron nitrides. This gives rise to a good balance of strength and toughness. 
         [0036]    Boron stabilizes the microstructure by incorporation in chromium-based M23C6 carbides and reduces the growth of the M23C6 carbides, as a result of which a high microstructure stabilization and consequently creep rupture strength are achieved. 
         [0037]    It has been established that tungsten must not be used in order to achieve a high long-term stability with good long-term toughness. As a result, the toughnesses do not change depending on the temperature and time. 
         [0038]    Tungsten is preferably not added since, although tungsten acts as a solid solution hardener, in the long term tungsten is precipitated as a Laves phase and then also coarsens quicker than other particles and therefore no longer participates in the stabilization of the particles of the microstructure. 
         [0039]    In addition, the long-term toughness can be impaired by tungsten at temperatures &lt;550° C. 
         [0040]    The nickel content gives rise to a good forgeability. 
         [0041]    The nickel content is lowered owing to the fact that the creep rupture strength is improved by reducing the diffusion coefficients in the microstructure. 
         [0042]    The changed ability to achieve full hardening is compensated for by the addition of carbon (C) and cobalt (Co). 
         [0043]    The content of carbon (C) is lowered owing to the balance with other elements for achieving a martensite microstructure with a high toughness. The lowered carbon content makes it possible for the austenite to be completely converted upon cooling to room temperature (no residual austenite), as a result of which a high microstructure homogeneity, a good martensite lath structure, a high toughness and a fine carbide formation of M23C6 are achieved, and therefore a good creep rupture strength is achieved. Carbon is required for the formation of M23C6. It is advantageous to use carbon contents &gt;0.13% by weight. 
         [0044]    Nitrogen forms MX particles (VN, VCC, N) Nb(C, N) for hardening the particles of the martensite microstructure based on (V, Nb)N, as a result of which the creep rupture strength is increased (MX stands for precipitations of the form VN, V(C, N) Nb(C, N). 
         [0045]    It is advantageous to use nitrogen contents &gt;150 ppm. 
         [0046]    The silicon content is lowered since this improves the long-term toughness and reduces the nucleation for Laves phase precipitation (see under tungsten). 
         [0047]    The manganese content is lowered owing to the positive effect on the increase in the creep rupture strength by increasing the Ac1 temperature, as a result of which it is possible to achieve a higher use temperature without influencing the microstructure or without a ferrite/martensite-austenite microstructure conversion: 
         [0048]    Ac1 is the conversion temperature from ferrite to austenite; in the time/temperature conversion graph, “Ac1” is the first conversion point when heating the material. It denotes the start of the alpha-gamma conversion (start of the austenite formation). 
         [0049]    The proportions of phosphorus, sulfur and copper are lowered in order to improve the initial toughness of the microstructure and to ensure a high long-term toughness. 
         [0050]    It is preferable not to use titanium, since otherwise nitrogen would become bonded as TiN and therefore the MX particles of the form (V,Nb)N which are required for the creep rupture strength would be absent. 
         [0051]    The use temperature for components is increased by this alloy, with the toughness/ductility being retained at relatively low temperatures. 
         [0052]    The minimum contents in the claims are in each case preferably
   0.1% by weight for cobalt (Co),   0.01% by weight for silicon (Si),   0.001% by weight for phosphorus (P),   0.05% by weight for manganese (Mn),   0.01% by weight for copper (Cu);
 
these lie considerably above the detection limits for these elements and the degree of impurity thereof.
   
 
         [0058]      FIG. 2  illustrates a steam turbine  300 ,  303  with a turbine shaft  309  extending along an axis of rotation  306 . 
         [0059]    The steam turbine has a high-pressure part-turbine  300  and a medium-pressure part-turbine  303 , each having an inner housing  312  and an outer housing  315  surrounding the inner housing. The high-pressure part-turbine  300  is, for example, of pot-like design. The medium-pressure part-turbine  303  is, for example, of two-flow design. It is also possible for the medium-pressure part-turbine  303  to be of single-flow design. 
         [0060]    Along the axis of rotation  306 , a bearing  318  is arranged between the high-pressure part-turbine  300  and the medium-pressure part-turbine  303 , the turbine shaft  309  having a bearing region  321  in the bearing  318 . The turbine shaft  309  is mounted on a further bearing  324  next to the high-pressure part-turbine  300 . In the region of this bearing  324 , the high-pressure part-turbine  300  has a shaft seal  345 . The turbine shaft  309  is sealed with respect to the outer housing  315  of the medium-pressure part-turbine  303  by two further shaft seals  345 . Between a high-pressure steam inflow region  348  and a steam outlet region  351 , the turbine shaft  309  in the high-pressure part-turbine  300  has the high-pressure rotor blading  357 . This high-pressure rotor blading  357 , together with the associated rotor blades (not shown in more detail), constitutes a first blading region  360 . 
         [0061]    The medium-pressure part-turbine  303  has a central steam inflow region  333 . Assigned to the steam inflow region  333 , the turbine shaft  309  has a radially symmetrical shaft shield  363 , a cover plate, on the one hand for dividing the flow of steam between the two flows of the medium-pressure part-turbine  303  and also for preventing direct contact between the hot steam and the turbine shaft  309 . In the medium-pressure part-turbine  303 , the turbine shaft  309  has a second blading region  366  having the medium-pressure rotor blades  354 . The hot steam flowing through the second blading region  366  flows out of the medium-pressure part-turbine  303  from an outflow connection piece  369  to a low-pressure part-turbine (not shown) which is connected downstream in terms of flow. 
         [0062]    The turbine shaft  309  is composed, for example, of two turbine part-shafts  309   a  and  309   b,  which are fixedly connected to one another in the region of the bearing  318 . Each turbine part-shaft  309   a,    309   b  has a cooling line  372  fanned 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 an inflow line  375 , which has a radial bore  375   a.  In the medium-pressure part-turbine  303 , the coolant line  372  is connected to a cavity (not shown in more detail) beneath the shaft shield. The inflow lines  375  are designed as a radial bore  375   a,  with the result that “cold” steam from the high-pressure part-turbine  300  can flow into the central bore  372   a.  Via the outflow line  372 , which is in particular also designed as a radially oriented bore  375   a,  the steam passes through the bearing region  321  into the medium-pressure part-turbine  303 , where it then passes onto the lateral surface  330  of the turbine shaft  309  in the steam inflow region  333 . The steam flowing through the cooling line is at a significantly lower temperature than the reheated steam flowing into the steam inflow region  333 , so that effective cooling of the first rotor blade rows  342  of the medium-pressure part-turbine  303  and of the lateral surface  330  in the region of these rotor blade rows  342  is ensured.