Nickel-chromium high strength casting
The subject invention is directed to articles and parts cast from nickel-chromium alloys, and particularly to such articles and parts that are subject in use to stress at high temperatures under very corrosive conditions. 
     In U.S. Pat. No. 3,617,263 certain nickel-base alloys are described containing from 27 to 31% chromium, 10 to 25% cobalt, 0.2 to 2% niobium, both titanium and aluminum in a total amount from 2.25 to 4.5% with the ratio therebetween being from 1:1 to 4:1, 0.02 to 0.1% carbon, 0.002 to 0.01% boron and certain optional constituents. 
     It was found that as the combined content of niobium, titanium, aluminum and chromium increased, there was a great decrease in the resistance of these alloys, in the wrought form, to embrittlement, as measured by the impact resistance after prolonged exposure to service temperatures. In addition, there was little or no improvement in the stress-rupture lives of the wrought alloys. For such reasons, these elements were further restricted so that 
     
         5 (% Nb) + 4 (% Ti + % Al) + 2/3 (% Cr) 
    
     Did not exceed 40%. 
     The present invention is based on the discoveries that with further increase in the combined contents of niobium, titanium and aluminum the stress-rupture lives of these alloys in the cast form are increased to a most marked extent while still retaining adequate resistance to embrittlement. Furthermore, even in the substantial absence of boron, which is an essential constituent of the above discussed alloys, the instant alloys not only afford very high stress-rupture lives, but also exhibit surprisingly good weldability under conditions of severe restraint. 
     Generally speaking, the present invention contemplates articles and parts cast from alloys containing (by weight) from 27 to 31% chromium, from 10 to 25% cobalt, from 0 to 1.9% niobium, both titanium and aluminium in a total amount from 3.5 to 5.5%, with the provisos that the ratio of titanium to aluminum is from 1:1 to 4:1 and that the sum of 
     
         5 (% Nb) + 6 (% Ti + % Al) + 2/3 (% Cr) 
    
     Is from 49 to 54, from 0.02 to 0.2% carbon, from 0 to 0.6% zirconium, and from 0 to 1.2% hafnium, with the value of 
     
         % Zr + 0.5 (% Hf) 
    
     Being not more than 0.6%, from 0 to 0.02% boron, and from 0 to 0.2% in total of yttrium and/or lanthanum, the balance, apart from impurities, being nickel. 
     In striving for a superior combination of properties, the chromium content of the alloys is advantageously from 27.5 to 30%, the cobalt from 16 to 22%, to niobium from 0.2 to 1.8%, and the carbon from 0.03 to 0.15%. The best combination of stress-rupture life at 9 tons/square inch (139 MN/m 2 ) at 870° C. and room temperature impact strength after heating for 1000 hours at 850° C. is exhibited by alloys containing from 28 to 29% chromium, 19 to 21% cobalt, 0.4 to 1.7% niobium, 0.04 to 0.08% carbon, with a Ti:Al ratio of from 1.5:1 to 2.5:1. Most advantageously, the niobium content is at least 0.75%. For the highest stress-rupture lives the zirconium content is preferably from 0.02 to 0.2% and the boron content from 0.003 to 0.01%. 
     However, if the castings are to be welded the zirconium level should not exceed 0.025% and the percentage of boron should not exceed 0.002%. To achieve the best impact strength in these substantially zirconium- and boron-free alloys, the niobium content is most preferably at least 1%. The optimum combination of properties including weldability is obtained in alloys containing from 28 to 29% chromium, from 19 to 21% cobalt, from 1.5 to 1.7% niobium, from 0.04 to 0.08% carbon, from 3.75 to 4.1% titanium plus aluminum, with Ti:Al ratio of 1.5:1 to 2.5:1, no more than 0.025% zirconium and no more than 0.002% boron, the balance being nickel. 
    
    
     In carrying the invention into practice and to develop the full stress-rupture properties of the alloys, they should be subjected to a heat-treatment comprising solutiong heating and subsequent aging. The solution treatment may consist of heating from 1 to 8 hours in the temperature range of 1050° C. to 1200° C. and the alloys may then be aged by heating for from 1 to 24 hours in the temperature range of b 600° to 950° C. An intermediate aging treatment consisting of heating for from 1 to 16 hours at 800° to 1050° C. may be interposed between the solution treatment and the final aging stages. The alloys may be cooled at any convenient rate after each heat treatment, e.g., by air cooling (generally to room temperature) or by direct transfer from a furnace at one temperature to one at a lower temperature. A particularly suitable heat treatment consists of solution heating for 4 hours at 1150° C., followed by air cooling and aging for 16 hours at 850° C. and finally again air cooling. Very useful properties are, however, exhibited by the alloys in the as-cast condition. 
     The importance of correlating the contents of niobium, titanium, aluminum and chromium so that the value of the &#34;correlated factor&#34; 
     
         5 (% Nb) + 6 (% Ti + Al) + 2/3 (% Cr) lies between 49 and 54 is shown by the results of numerous tests set forth in Table I. 
    
     All the alloys were vacuum melted and test pieces were machined from cast blanks of the given composition which had been heat treated by solution heating for 4 hours at 1150° C. and air cooling, followed by aging for 16 hours at 850° C. and again air cooling. For each alloy, the stress-rupture life and elongation obtained in stress-rupture tests under a stress of 9 t.s.i. at 870° C. and the impact strength at room temperature after heating for 1000 hours at 850° C. are given in the last three columns. The impact strength was determined by the Charpy method using un-notched test pieces of 0.45 inch diameter. The values are in Joules. 
     It will be seen from Table I that Alloys 1 to 6, which were in accordance with the invention and for which the value of the &#34;Correlated Factor&#34; was between 49 and 54, exhibited much superior combinations of stress-rupture and impact properties than Alloys A to K inclusive, which were not in accordance with the invention and had values outside the 49 to 54 range. Generally speaking, the boron and zirconium-containing alloys in accordance with the invention had stress-rupture lives at 9 t.s.i. and 870° C of at least about 800 hours in combination with an impact strength of at least about 27 Joules. The stress-rupture test piece of Alloy No. 2, which broke after it has been subjected to a stress of 9 t.s.i. at 870° C for 1282 hours was examined microscopically and found to be free from sigma phase. 
     
                                           TABLE I                                 
__________________________________________________________________________
                                               Stress-rupture             
                                                       Impact             
Composition (% by weight)*                     9 t.s.i./870°       
                                                       strength after     
Alloy                                    Correlated                       
                                               Life                       
                                                   El. 1000h/850°  
                                                       C.,                
No. C   Cr  Co  Nb  Ti  Al  Ti = Al                                       
                                 Zr  B   Factor                           
                                               hrs.                       
                                                   %   Joules             
__________________________________________________________________________
A   0.057                                                                 
        28.4                                                              
            20.4                                                          
                2.1 2.6 1.55                                              
                            4.15 .065                                     
                                     0.004                                
                                         54.3  1.255                      
                                                   8.2 19                 
B   0.056                                                                 
        28.4                                                              
            20.3                                                          
                3.1 2.35                                                  
                        1.40                                              
                            3.75 0.070                                    
                                     0.004                                
                                         56.9  295 1.7 9.5                
C   0.054                                                                 
        28.4                                                              
            20.3                                                          
                3.1 2.6 1.60                                              
                            4.20 0.070                                    
                                     0.004                                
                                         59.6  184 2.5 4                  
D   0.059                                                                 
        28.5                                                              
            20.4                                                          
                1.65                                                      
                    2.1 1.10                                              
                            3.20 0.065                                    
                                     0.004                                
                                         46.5  447 14.4                   
                                                       --                 
E   0.059                                                                 
        28.5                                                              
            20.4                                                          
                2.15                                                      
                    2.0 1.15                                              
                            3.15 0.065                                    
                                     0.004                                
                                         48.7  380 6.5 45                 
F   0.049                                                                 
        28.5                                                              
            20.1                                                          
                0.53                                                      
                    2.65                                                  
                        1.42                                              
                            4.07 0.065                                    
                                     0.003                                
                                         46.1  578 6.8 147                
G   0.050                                                                 
        28.2                                                              
            20.1                                                          
                1.10                                                      
                    2.40                                                  
                        1.43                                              
                            3.83 0.070                                    
                                     0.003                                
                                         47.3  581 19.2                   
                                                       162                
H   0.056                                                                 
        28.3                                                              
            20.1                                                          
                1.15                                                      
                    3.20                                                  
                        1.77                                              
                            4.97 0.070                                    
                                     0.004                                
                                         54.5  843 4.8 8                  
1   0.058                                                                 
        28.3                                                              
            20.3                                                          
                1.60                                                      
                    2.30                                                  
                        1.44                                              
                            3.74 0.065                                    
                                     0.004                                
                                         49.3  851 17.0                   
                                                       139                
2   0.058                                                                 
        28.3                                                              
            20.3                                                          
                1.60                                                      
                    2.65                                                  
                        1.67                                              
                            4.32 0.065                                    
                                     0.004                                
                                         52.8  1,282                      
                                                   10.3                   
                                                       53                 
3   0.051                                                                 
        28.1                                                              
            20.6                                                          
                0.94                                                      
                    2.82                                                  
                        1.60                                              
                            4.42 0.060                                    
                                     0.003                                
                                         49.9  1,379                      
                                                   15.3                   
                                                       --                 
4   0.060                                                                 
        28.1                                                              
            19.6                                                          
                0.52                                                      
                    3.1 1.75                                              
                            4.85 0.10                                     
                                     0.005                                
                                         50.4  1,034                      
                                                   4.6 136                
5   0.060                                                                 
        28.1                                                              
            19.6                                                          
                0.52                                                      
                    3.3 1.95                                              
                            5.25 0.095                                    
                                     0.005                                
                                         52.8  1,279                      
                                                   3.6 30                 
6   0.042                                                                 
        28.5                                                              
            20.1                                                          
                --  3.3 1.7 5.0  0.075                                    
                                     0.004                                
                                         49.0  947 6.5 152                
I   0.057                                                                 
        28.4                                                              
            20.4                                                          
                2.1 2.35                                                  
                        1.40                                              
                            3.75 0.065                                    
                                     0.004                                
                                         51.9  793 4.8 31                 
J   0.053                                                                 
        28.5                                                              
            20.4                                                          
                1.5 3.15                                                  
                        1.80                                              
                            4.95 0.065                                    
                                     0.004                                
                                         56.2  740 7.4 --                 
K   0.051                                                                 
        28.2                                                              
            19.6                                                          
                0.75                                                      
                    2.5 1.25                                              
                            3.75 0.06                                     
                                     0.004                                
                                         46.0  478 12.8                   
                                                       145                
__________________________________________________________________________
 *balance nickel and impurities                                           
 
    
     The superior weldability of alloys substantially free from zirconium and boron is shown by tests carried out on other alloys according to the invention, Alloys Nos. 7 and 8 having the nominal composition: 
     
         __________________________________________________________________________
Alloy                                                                     
    Composition (% by wt)                                                 
No. C  Cr Co Nb Ti Al Ti+Al                                               
                           Zr  B   Ni                                     
__________________________________________________________________________
7   0.06                                                                  
       28.5                                                               
          20.0                                                            
             1.6                                                          
                2.5                                                       
                   1.4                                                    
                      3.9  &lt;0.02                                          
                               &lt;0.002                                     
                                   bal                                    
8   0.06                                                                  
       28.5                                                               
          20.0                                                            
             0.50                                                         
                3.3                                                       
                   1.7                                                    
                      5.0  &lt;0.02                                          
                               &lt;0.002                                     
                                   bal                                    
__________________________________________________________________________
 
    
     Apart from the boron and zirconium, the compositions of Alloy No. 7 is very similar to that of Alloy No. 1 and the composition of Alloy No. 8 is very similar to Alloy Nos. 4 and 5. In butt-welding tests using matching filler rods under severely restrained conditions such that test pieces of Alloy Nos. 1, 4 and 5 would crack, test pieces of Alloy Nos. 7 and 8 remained crack-free. 
     The stress-rupture and impact properties of Alloy Nos. 7 and 8 under the same conditions as those for the alloys in Table I are set forth in Table II. 
     
                       TABLE II                                                    
______________________________________                                    
Stress-rupture        Impact Strength                                     
139 MN/m.sup.2 /870° C                                             
                      after 1000h/850° C                           
Alloy   Life       El                                                     
No.     (h)        (%)        (J)                                         
______________________________________                                    
7       807        9.0        99                                          
8       882        2.4        11                                          
______________________________________                                    
 
    
     It will be seen that under these tests conditions Alloy No. 7 had a stress-rupture life of 807 hours, which is only slightly less than the life of 852 hours for Alloy No. 1 under the same conditions. Alloy No. 8, which contained only 0.5% niobium, had a stress-rupture life of 882 hours, which, although somewhat inferior to those of Alloy Nos. 4 and 5, is still very good, but had an impact strength of 11 Joules which is inferior to that of Alloy Nos. 4 and 5 but still acceptable. 
     Further results for the boron- and zirconium-free Alloy No. 7 confirm the excellent stress-rupture properties of this alloy under various test conditions. 
     
                       TABLE III                                                   
______________________________________                                    
Stress (MN/m.sup.2)                                                       
            Temp(° C)                                              
                        Life (h)  Elongation                              
______________________________________                                    
170         870          183      8.6                                     
124         870         1277      5.6                                     
263         815          213      6.6                                     
193         815         1713      7.6                                     
______________________________________                                    
 
    
     The alloys of the invention can be air-melted, but to ensure the best stress-rupture properties they are preferably melted and cast under vacuum. 
     In addition to their excellent combination of stress-rupture and impact properties, the alloys of the invention also exhibit high resistance to corrosion when exposed to a molten mixture of 25% by weight of sodium chloride and 75% sodium sulphate at 900° C. 
     Although the alloys are primarily intended for use in the cast form as blades and other components of gas turbine engines, particularly stator blades, they are suitable for use in other applications where a combination of good stress-rupture strength and resistance to crorosion is required, particularly for articles and parts that are subject in use to stress at high temperatures while exposed to the combustion products of impure hydrocarbon fuels or to salt or both. 
     Although the present invention has been described in conjuction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.