Abstract:
An austenitic alloy comprising (in weight %):
       C: 0.01-0.05   Si: 0.05-0.80   Mn: 1.5-2   Cr: 26-34.5   Ni: 30-35   Mo: 3-4   Cu: 0.5-1.5   N: 0.05-0.15   V: &lt;0.15
 
the balance being Fe and unavoidable impurities, wherein 40&lt;% Ni+100*% N&lt;50.

Description:
RELATED APPLICATION DATA 
     This application is a §371 National Stage Application of PCT International Application No. PCT/EP2013/050723 filed Jan. 16, 2013 claiming priority of EP Application No. 12151566.2, filed Jan. 18, 2012. 
     TECHNICAL FIELD 
     The present invention relates to an austenitic alloy according to the preamble of claim  1 . The invention also relates to a component for a combustion plant comprising the inventive austenitic alloy. 
     BACKGROUND 
     Power generation based on the combustion of biomass is regarded both sustainable and carbon neutral and is becoming an increasingly important source of energy. 
     A problem in biomass combustion is that the combustion products of the wide range of biomass fuels that are used are corrosive and may cause depositions on components in the biomass power plant. Especially exposed are superheaters, re-heaters and evaporators in biomass power plants, as well as in conventional steam boilers. A further problem in biomass power plants is that the materials in the components start to creep due to the high temperatures and the high pressures in the power plant. Today, biomass plants operate at a pressure of 150-200 bar and at a temperature of 500-550° C. In the future, biomass power plants temperatures are expected to be even higher than today, 600-650° C. This will put even higher demands on the hot corrosion resistance and the creep strength of the structural parts of the power plant. 
     Attempts have been made to increase corrosion resistance in steels. For example U.S. Pat. No. 4,876,065 and WO0190432 describe steels that are designed for use in corrosive environments in the oil- and gas industry. 
     Studies have further shown that austenitic stainless steel with high Mo content shows good resistance to high temperature corrosion: James R. Keisler, Oak ridge National laboratory, NACE Corrosion 2010, No 10081. 
     However, these steel do not exhibit the necessary creep strength to be suitable in biomass power plants. 
     Therefore, it is an object of the present invention to achieve an austenitic alloy which exhibits high corrosion resistance and high creep strength. It is also an object of the present invention to achieve a component for a steam boiler plant that comprises the inventive alloy. 
     SUMMARY OF THE INVENTION 
     According to the invention, this object is achieved by an austenitic alloy comprising (in weight %):
         C: 0.01-0.05   Si: 0.05-0.80   Mn: 1.5-2   Cr: 26-34.5   Ni: 30-35   Mo: 3-4   Cu: 0.5-1.5   N: 0.05-0.15   V: ≦0.15
 
the balance Fe and unavoidable impurities, characterized in that 40≦% Ni+100*% N≦50
       

     The inventive austenitic alloy has good resistance to high temperature corrosion, in particular good fire side corrosion. By balancing the additions of nickel and nitrogen in the alloy so that the condition 40≦% Ni+100*% N≦50 is fulfilled, a high creep strength and high ductility are further achieved in the alloy. The good resistance to high temperature corrosion in combination with high creep strength makes the inventive austenitic alloy very suitable as a material for structural parts in steam boilers. The inventive alloy is particularly useful in biomass power plants which operate under corrosive conditions at high temperatures and pressures. 
     Preferably, said austenitic alloy fulfils the requirement: 40≦% Ni+100*% N≦45. The alloy then exhibits very good creep strength and high ductility. This is advantageous when the material is used in steam boilers since it allows for high thermoplastic expansion and contraction of the material during start and shutdown of the boiler. Thus, the material can be subjected to cyclic heating and cooling without cracking. 
     Preferably the content of silica (Si) in the austenitic alloy is 0.3-0.55 wt %. Very high creep strength is thereby achieved in the alloy due to minimal formation of brittle sigma phase and minimal formation of oxygen containing inclusions. 
     Preferably, the content of carbon (C) in said austenitic alloy is 0.01-0.018 wt % in order to optimize the resistance to corrosion. 
     The invention also relates to a component for a combustion plant, preferably a biomass power plant or a biomass steam boiler that comprises the inventive austenitic alloy. 
     Said component may for example be a superheater or a reheater or an evaporator, preferably a tube of such a superheater, reheater or evaporator, and wherein the component is subjected to flue gases and elevated heat when in its operative position. The invention may thus, as an alternative, be defined as a combustion plant, preferably a biomass power plant, comprising a boiler, preferably a biomass steam boiler, comprising a component, preferably a superheater tube, a reheater tube or an evaporator tube, arranged in the boiler and subjected to flue gases and heat generated by said boiler during operation thereof, wherein said component comprises the alloy according to the invention. 
     DESCRIPTION OF THE INVENTION 
     The inventive austenitic alloy comprises the following alloy elements: 
     Carbon (C) 
     Carbon is an austenite stabilizing element and should therefore be included in the inventive alloy in an amount of at least 0.01 wt % Carbon is further important for increasing the creep strength of the material by the formation of carbonitrides. However, in the presence of chromium carbon forms chromium carbides which increases the risk of intergranular-corrosion. High carbon contents further reduces weldability. To minimize the formation of chromium carbides and to ensure good weldability the carbon content should not exceed 0.05 wt %. To inhibit the formation of chromium carbides even further, the content of carbon should preferably be in the range of 0.01-0.018 wt %. 
     Silicon (Si) 
     Silicon is used as a deoxidising element in the production of steel. However a high content of silicon is detrimental to weldability. In order to ensure low oxygen content in the steel and thereby few inclusions, the content of silicon should be at least 0.05 wt %. The content of silicon should however not exceed 0.80 wt % in order to ensure weldability of the steel. It has been found that when the content of silicon is in the range of 0.30-0.55 wt % very high creep strength is achieved in the inventive alloy. It is believed that the formation of sigma phase increases when the silicon level exceeds 0.55 wt %. The sigma phase reduces the ductility of the inventive alloy and therefore also the creep strength. Below 0.30 wt % the creep strength is reduced due to increased formation of oxygen-containing inclusions. 
     Manganese (Mn) 
     Manganese, like Si, is a deoxidising element, and it is also effective to improve the hot workability. The maximum content of manganese needs to be limited to control the ductility and toughness of the inventive alloy at room temperature. Therefore, the content of manganese should be in the range of 1.50-2.0 wt %. 
     Chromium (Cr) 
     Chromium is an effective element to improve the fire side corrosion resistance and steam oxidation resistance. In order to achieve a sufficient hot corrosion resistance for use as e.g. boiler tubes in biomass combustion power plants, a chromium content of at least 26% is needed. However, if the chromium is higher than 34.5%, the nickel content must be further increased since a higher Cr content can increase the risk of formation of intermetallic phases such as sigma phase. The chromium content should therefore be in the interval of 26.0 wt %-34.5 wt %. In the case of the present invention, very good material properties have been obtained with chromium contents in the range of 26.0-29.0 wt %, which is therefore to be regarded as a preferred range or at least an even more limited range within which the technical effect of the invention is achieved. 
     Nickel (Ni) 
     Nickel is an essential element for the purpose of ensuring a stable austenitic structure in the inventive alloy so that the formation of inter-metallic phases like sigma phase is suppressed. Sigma-phase is a hard and brittle intermetallic phase with chromium and molybdenum and is formed at elevated temperatures. Sigma phase has a negative impact of the ductility and elongation of the steel. By stabilizing the austenitic phase in the alloy, the formation of sigma phase is minimized. Nickel is therefore important for ensuring sufficient ductility and elongation of the steel. Nickel has also a positive effect on the corrosion resistance of the inventive alloy since it promotes the formation of a passive Cr-oxide film that suppresses further oxide growth, s c. scaling. The content of nickel should be at least 30 wt % in the inventive alloy in order to ensure structure stability, corrosion resistance and ductility. However, nickel is a relatively expensive alloy element and in order to maintain low production costs the content of nickel should be limited. Nickel further decreases the solubility of nitrogen in the alloy and therefore the content of nickel should not exceed 35 wt %. 
     Molybdenum (Mo) 
     Molybdenum is included in the inventive alloy in order to improve the hot corrosion resistance on the fire side of boiler tubes. Addition of Mo further improves the general-corrosion resistance of the inventive alloy. However, Mo is an expensive element and promotes precipitation of sigma-phase and thus invites deterioration of toughness of the steel. In order to ensure good hot corrosion resistance in the steel the content of molybdenum should be at least 3 wt %. The upper limit of molybdenum is 4 wt % to avoid precipitation of sigma phase. 
     Copper (Cu) 
     Addition of copper can improve both the creep strength by precipitation of copper rich phase, finely and uniformly precipitated in the matrix. However, an excessive amount of copper results in decreased workability. A high amount of copper can also lead to a decrease of ductility and toughness. Therefore the content of copper in the inventive alloy should be in the interval of 0.5-1.5 wt %. In the case of the present invention, particularly good results have been obtained with a copper content in the range of 0.8-1.2 wt %, which is therefore, at least for that reason, to be regarded as a preferred range or at least a more limited range within which the technical effect of the invention is achieved. 
     Nitrogen (N) 
     Nitrogen has a strong stabilizing effect on the austenitic structure and reduces therefore the formation of sigma-phase. This has a positive effect on the ductility of the steel. In the inventive alloy the main effect of nitrogen is that it, together with carbon, forms precipitations in the form of carbonitrides. The small carbonitride particles are generally precipitated at the grain boundaries of the steel and stop dislocations from propagating within the crystal grains of the steel. This greatly increases the creep resistance of the steel. The content of nitrogen should be at least 0.05 wt % in the inventive alloy in order to ensure a stable austenitic structure and that a sufficient amount of carbonitrides are formed. However, if nitrogen is present in high amounts large primary precipitations of nitrides could appear which reduce the ductility and toughness of the inventive alloy. Therefore, the content of nitrogen in the inventive alloy should be limited to 0.15 wt %. 
     Vanadium (V) 
     Addition of vanadium, titanium or niobium contributes to improve the creep rupture strength through the precipitation of MX phase. However, the excessive amount of vanadium can decrease the weldability and hot workability. Vanadium could therefore be allowed in the inventive alloy in an amount of ≦0.15 wt %. 
     Phosphorus (P) and Sulphur (S) 
     Phosphorus and sulphur are typically included as impurities in the raw materials for the inventive alloy and could cause weld cracking in high amounts. Therefore phosphorus should not exceed 0.035%. Sulphur should not exceed 0.005%. 
     Requirement: 40≦% Ni+100*% N≦50 
     In the inventive alloy, the content of nickel and the content of nitrogen should be balanced to fulfil the requirement: 40≦% Ni+100*% N≦50. It has shown that within this interval very good creep strength and ductility is achieved. It is believed that the good creep strength is the result of a synergistic effect from nickel and nitrogen. Preferably, the content of nickel and the content of nitrogen should be balanced to fulfil the requirement: 40≦% Ni+100*% N≦45. 
     As stated above, nitrogen forms carbonitrides which promotes the creep strength by increasing the creep strain in the alloy. However, creep strength is affected negatively by any brittle phases, such as sigma phase. The addition of both nickel and nitrogen suppresses the formation of sigma-phase in the steel and increases thereby rupture elongation or the ductility of the alloy. This will reduce stress concentration and possible crack initiation and propagation. Consequently, this leads to an increase of the creep strength. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1 : A table over alloy compositions 
         FIG. 2 : A diagram showing results from creep tests at 600° C. of inventive alloys and comparative alloys. 
         FIG. 3 : A diagram showing results from creep tests at 650° C. of inventive alloys and comparative alloys. 
     
    
    
     EXAMPLE 
     Following the inventive alloy will be described with reference to a concrete example. 
     Ten steel heats were prepared by conventional steel making methods. The composition of respective steel heat is shown in table 1. The conventional metallurgical process according to which the heats were prepared was as follows: Melting by AOD method-hot rolling-extruding-cold pilgring (cold deformation)-solution annealing-water quenching. The hollow bar material after the hot extruding was then cold pilgred with a cold deformation between 40 to 80%, followed by a solution annealing at a temperature between 1050 to 1180° C. depending on the dimension. The following table shows the details. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Cold 
                   
                   
               
               
                   
                   
                 deformation 
                   
                   
               
               
                 Alloy 
                 Heat 
                 (%) 
                 Annealing 
                 Cooling 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 763554 
                 40-80 
                 1050-1180° C./5-25  
                 water 
               
               
                   
                   
                   
                 minutes 
                 quenching 
               
               
                 2 
                 462269 
                 40-80 
                 1050-1180° C./5-25 
                 water 
               
               
                   
                   
                   
                 minutes 
                 quenching 
               
               
                 3 
                 477353  
                 40-80 
                 1050-1180° C./5-25 
                 water 
               
               
                   
                   
                   
                 minutes 
                 quenching 
               
               
                 4 
                 469837 
                 40-80 
                 1050-1180° C./5-25 
                 water 
               
               
                   
                   
                   
                 minutes 
                 quenching 
               
               
                 5 
                 471988 
                 40-80 
                 1050-1180° C./5-25 
                 water 
               
               
                   
                   
                   
                 minutes 
                 quenching 
               
               
                 6 
                 469718 
                 40-80 
                 1050-1180° C./5-25 
                 water 
               
               
                   
                   
                   
                 minutes 
                 quenching 
               
               
                 7 
                 477217 
                 40-80 
                 1050-1180° C./5-25 
                 water 
               
               
                   
                   
                   
                 minutes 
                 quenching 
               
               
                 8 
                 477203 
                 40-80 
                 1050-1180° C./5-25 
                 water 
               
               
                   
                   
                   
                 minutes 
                 quenching 
               
               
                 9 
                 460335 
                 40-80 
                 1050-1180° C./5-25 
                 water 
               
               
                   
                   
                   
                 minutes 
                 quenching 
               
               
                 10 
                 463024 
                 40-80 
                 1050-1180° C./5-25 
                 water 
               
               
                   
                   
                   
                 minutes 
                 quenching 
               
               
                   
               
             
          
         
       
     
     Alloys 1, 7-9 are comparative samples and contain relatively low concentrations of nitrogen. Alloys 2, 3 and 10 are comparative samples and contain comparatively high nitrogen concentrations. Alloys 4-6 are inventive samples which fulfil the requirement 40≦% Ni+100*% N≦50. Alloys 1 and 10 are low in silicon content. 
     Test samples of each steel heat were prepared. The samples were subjected to creep testing in order to determine their creep properties. Creep testing was performed at two different temperatures: 600° C. and 650° C., by applying a constant stress on each sample and determining the time to rupture and rupture elongation of each sample. Rupture elongation is the length increase until rupture expressed as percentage of nominal length for each sample. The applied stress equals the creep rupture strength of the alloy. The creep rupture strength is defined as the stress which, at a given temperature, will cause a material to rupture in a given time. 
     The creep tests were performed according to conventional testing methods and conventional mathematic models were used for extrapolating the results. 
       FIG. 2  shows the creep strength at 600° C. for inventive alloys 4-6 in comparison to the creep strengths of comparative alloys 1, 7 and 9.  FIG. 3  shows the creep strength at 650° C. for inventive alloys 4-6 in comparison to comparative alloys 1, 8, 9. From  FIGS. 1 and 2  it is clear that the inventive alloys, for a given creep stress, shows a longer time to rupture than the comparative alloys. 
     Some other results from the creep testing are shown in tables 2 and 3. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Creep testing at 600° C. 
               
             
          
           
               
                   
                   
                 Time to rupture 
                   
                 Rupture 
               
               
                 Alloy 
                 Heat 
                 (hours) 
                 Stress (MPa) 
                 elongation (%) 
               
               
                   
               
             
          
           
               
                 1 
                 763554 
                 32621 
                 150 
                 55 
               
               
                 2 
                 462269 
                 49738 
                 170 
                 71 
               
               
                 3 
                 477353 
                 50986 
                 170 
                 72 
               
               
                 4 
                 469837 
                 117561 
                 160 
                 71 
               
               
                 5 
                 471988 
                 67644 
                 160 
                 79 
               
               
                 6 
                 469718 
                 102321 
                 160 
                 90 
               
               
                 7 
                 477217 
                 104958 
                 150 
                 38 
               
               
                 8 
                 477203 
                 105889 
                 150 
                 46 
               
               
                 9 
                 460335 
                 85940 
                 140 
                 63 
               
               
                 10 
                 463024 
                 7629 
                 165 
                 65 
               
               
                   
               
             
          
         
       
     
     Table 2 shows the time to rupture and the creep strength or applied stress of each alloy at 600° C. Table 2 further shows the rupture elongation i.e. the length increase until rupture expressed as percentage of nominal length for each sample. 
     From the test results it can be concluded that the inventive alloys 4-6 shows the highest time to rupture when the magnitude of the creep strength i.e. applied stress is taken into consideration. Alloy 4 shows a peak value of 117561 hours at an applied stress of 160 MPa. Alloys 4-6 further show very high rupture elongation. 
     The high results on time to rupture in alloys 4-6 are believed to depend on a synergistic effect of addition of both nitrogen and nickel. Addition of nitrogen increases the time to rupture by interstitial solution strengthening and also by precipitation strengthening by the formation of carbonitrides. The dense small carbonitrides that are precipitated in the material effectively block dislocation movement through the grains of the alloy material and hence increase the resistance to deformation. Addition of nickel, and also nitrogen, suppresses the formation of intermetallic phase, such as sigma phase, that affects the ductility negatively and hence improves the ductility of the material. The improved ductility reduces stress concentration, crack initiation and crack propagation. The synergistic effect of these properties results in a very high creep strength. 
     High ductility, which is expressed as rupture elongation in tables 2 and 3, is further advantageous when the material is used in steam boilers since it allows for high thermoplastic expansion and contraction of the material during start and shutdown of the boiler. Thus, the material can be subjected to cyclic heating and cooling without cracking. 
     The comparative alloys 1-3, 9 and 10 have comparatively high rupture elongation, see for example comparative alloys 2 and 3 which exhibit a rupture elongation of 71% and 72% respectively. However, theses alloys exhibit a shorter time to rupture, than the inventive alloys. It is believed that the shorter time to rupture in alloys 1-3, 9 and 10 is due to the fact that these alloys contain relatively small amounts of nitrogen. The low nitrogen content results in that fewer carbonitrides are precipitated in these materials than in the inventive alloys. Since alloys 1-3, 9 and 10 comprise few carbonitrides, dislocations can move more easily through these materials. This causes in turn a higher strain rate in the material, i.e. the material deforms faster. 
     Comparative alloys 7 and 8 exhibits rather high creep resistance, expressed as longer time to rupture at a given applied stress. However, it should be noted that the longer time to rupture for these alloys was determined at a lower stress, i.e. 150 MPa, than the inventive alloys which were evaluated at a stress of 160 MPa. Hence, the time to rupture of the comparative alloys 7 and 8 is lower than the time to rupture of the inventive alloys 4 and 6. The low time to rupture of alloys 7 and 8 is believed to be caused by brittleness induced by intermetallic phase precipitates. As is shown in table 2, alloys 7 and 8 have a rupture elongation of merely 38% and 46% respectively. 
     Table 3 shows the result of creep testing at some applied loads at a temperature of 650° C. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Creep testing at 650° C. 
               
             
          
           
               
                   
                   
                 Time to rupture  
                   
                 Rupture 
               
               
                 Alloy 
                 Heat 
                 (h) 
                 Stress (MPa) 
                 elongation (%) 
               
               
                   
               
             
          
           
               
                 1 
                 763554 
                 32621 
                 95 
                 45 
               
               
                 4 
                 469837 
                 116711 
                 95 
                 70 
               
               
                 5 
                 471988 
                 106165 
                 95 
                 52 
               
               
                 6 
                 469718 
                 95883 
                 105 
                 45 
               
               
                 6 
                 469718 
                 188609 
                 95 
                 31 
               
               
                 8 
                 477203 
                 32665 
                 120 
                 62 
               
               
                 9 
                 460335 
                 44168 
                 105 
                 50 
               
               
                   
               
             
          
         
       
     
     Table 3 shows that inventive alloys 4-6 have better creep properties expressed as time to rupture, creep strength and rupture elongation than the comparative alloys. The ductility for all alloys, i.e. the rupture elongation is lower at 650° C. in comparison to the ductility at 600° C. The reduction in ductility is caused by the fact that more precipitations are formed at higher temperatures and by faster grain growth at higher temperature.