High strength nickel-chromium-iron austenitic alloy
GOVERNMENT CONTRACT 
     This invention was conceived during the performance of work under Contract EY-76-C-14-2170 for the Department of Energy. 
    
    
     BACKGROUND OF THE INVENTION 
     There is, of course, a need for alloys for use at temperatures over 650° C. which must have high tensile, yield and creep-rupture strengths at elevated temperatures. One such alloy is described in U.S. Pat. No. 2,994,605; and while very broad ranges of composition are given in that patent, the only specific examples given have the following range of composition: about 50 to 70% nickel, about 14% chromium, about 2% niobium and/or tantalum, about 2.75 to 3.5% molybdenum and/or tungsten, less than 0.1% titanium, about 1% aluminum, about 0.35% manganese, about 0.5 to 0.75% silicon, about 0.03% carbon and the remainder iron. Such an alloy is described as having an ultimate tensile strength of 115,000 p.s.i. and a 0.2% yield strength of 46,750 p.s.i. at room temperature. 
     SUMMARY OF THE INVENTION 
     The present invention resides in the discovery that a high temperature Ni-Cr-Fe alloy having exceptionally good strength characteristics can be derived with lower amounts of nickel and chromium than used in prior art alloys of this type, higher amounts of niobium than the prior art alloys and with the addition of about 1% vanadium. Additionally, the alloy contains up to 0.06% zirconium, 0.1 to 0.3% titanium, 0.1 to 0.3% aluminum, 0.02 to 0.05% carbon, the remainder being essentially all iron. 
    
    
     The above and other objects and features of the invention will become apparent from the following detailed description describing an exemplary embodiment of the invention. 
     The alloys of the invention have the following broad range and nominal composition: 
     
                       TABLE I                                                     
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          Broad Range    Nominal                                          
          weight %       weight %                                         
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Nickel      42-48            45                                           
Chromium    11-13            12                                           
Niobium     2.6-3.4          3                                            
Silicon      .2-1.2          1                                            
Vanadium     .5-1.5          1                                            
Molybdenum  2.6-3.4          3                                            
Aluminum     .1-.3           .2                                           
Titanium     .1-.3           .2                                           
Carbon      .02-.05          .03                                          
Boron       .002-.015        .01                                          
Zirconium     0-.06          .03                                          
Iron        Bal              Bal                                          
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     The molybdenum and niobium contents are particularly critical. To illustrate the effect of niobium and molybdenum, the alloys identified as D16 and D17 in the following Table II were vacuum-induction melted and cast as 100-pound ingots: 
     
                       TABLE II                                                    
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Alloy    Fe     Ni     Cr   Mo   Nb  V   Si  Zr                           
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D16      Bal    45.0   12.0 1.5  1.0 1.0 1.0 0.03                         
D17      Bal    45.0   12.0 3.0  3.0 1.0 1.0 0.03                         
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Alloy    Ti    Al    C    B                                               
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D16      0.2   0.2   0.03 0.01                                            
D17      0.2   0.2   0.03 0.01                                            
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     Following surface conditioning, the alloys were charged into a furnace, heated to 1093° C. and then soaked for 2 hours prior to hot rolling to 21/2 by 21/2 inch square billets. The billets were then rolled to 1/2 inch thick plate which was annealed at 1038° C. and surface-ground. Sheet, 0.03 inch thick, was then produced using cold-reductions of 50% and process anneals at 1038° C. 
     The mechanical properties of the 0.03 inch sheet were then evaluated for two heat treatments, namely anneal for 1 hour at 1038° C. followed by an air-cool and an anneal for 1 hour at 1038° C. followed by an air-cool plus 30% cold-work. The tensile and stress rupture properties determined for these treatments are given in the following Tables III and IV: 
     
                       TABLE III                                                   
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                   Test                                                   
     Thermo-       Temper-                                                
Al-  mechanical    ature    0.2% YS                                       
                                   UTS   E1                               
loy  Treatment     (°C.)                                           
                            (MPa)  (MPa) (MPa)                            
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D16  1038° C./1 hr                                                 
                   RT       367    613   28.5                             
                   550      263    483   40.0                             
                   600      238    459   28.5                             
                   650      230    403   27.5                             
D16  1038° C./1 hr + 30%                                           
                   RT         --.sup.(a)                                  
                                   --    --                               
     cold-work     550      649    694   3.0                              
                   600      592    645   2.0                              
                   650      474    730   5.5                              
D17  1038° C./1 hr                                                 
                   RT       384    738   23.5                             
                   550      360    663   19.6                             
                   600      306    581   36.5                             
                   650      307    513   36.0                             
D17  1038° C./1 hr + 30%                                           
                   RT       --     --    --                               
     cold-work     550      787    860   5.0                              
                   600      678    766   6.0                              
                   650      552    661   9.5                              
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 .sup.(a) No RT testing was done in the coldworked condition.             
 
    
     
                       TABLE IV                                                    
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                Test                                                      
Thermo-         Temper-                                                   
mechanical      ature    Rupture Strength (MPa)                           
Alloy Treatment     (°C.)                                          
                             100 hr Est. 1000 hr                          
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D16   1038° C./1 hr                                                
                    550      386    331                                   
                    600      272    234                                   
                    650      200    172                                   
D16   1038° C./1 hr                                                
                    550      483    400                                   
                    600      359    290                                   
                    650      283    234                                   
D17   1038° C./1 hr                                                
                    550      510    448                                   
                    600      441    414                                   
                    650      290    255                                   
D17   1038° C./1 hr                                                
                    550      690    648                                   
                    600      538    483                                   
                    650      400    317                                   
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     Note that Alloy D17 containing 3% niobium and 3% molybdenum has better tensile properties than Alloy D16 containing only 1.5% molybdenum and 1% niobium. Thus, after annealing at 1038° C. for 1 hour, Alloy D17, at a test temperature of 650° C., has a 0.2% yield strength of 307 MPa, an ultimate tensile strength of 513 MPa and a percent elongation of 36. This is contrasted with Alloy D16 which, under the same circumstances, has a 0.2% yield strength of 230 MPa, an ultimate tensile strength of 403 MPa and a percent elongation of 27.5. For that matter, it will be observed that all of the properties of Alloy D17 are superior to those of Alloy D16 under all circumstances. Thirty percent cold-work after solution annealing gives further improved results as shown in Table III. 
     Table IV shows the stress rupture properties of Alloys D16 and D17. Here, again, the properties of Alloy D17 are superior to those of Alloy D16. For example, the rupture strength of Alloy D16 at 650° C. after 100 hours is in the range of 200 to 283 MPa whereas the rupture strength of Alloy D17 under the same circumstances is in the range of 290 to 400 MPa. It is estimated that the rupture strength of Alloy D17 at 1000 hours will be in the range of 255 to 317 MPa. 
     Although the invention has been shown in connection with certain specific embodiments, it will be readily apparent to those skilled in the art that various changes in compositional limits can be made to suit requirements without departing from the spirit and scope of the invention.