Abstract:
Improved nickel-base alloys of enhanced strength and corrosion resistance, produced by atomization of an alloy melt under an inert gas atmosphere and of composition 0-20Fe, 10-30Cr, 2-12Mo, 6 max. Nb, 0.05-3 V, 0.08 max. Mn, 0.5 max. Si, less than 0.01 each of Al and Ti, less than 0.05 each of P and S, 0.01-0.08C, less than 0.2N, 0.1 max. 0, bal. Ni.

Description:
CONTRACTUAL ORIGIN OF THE INVENTION 
     The United States Government has rights in this invention pursuant to Contract No. DE-AC07-94ID13223 between Lockheed Idaho Technologies Company and The United States Department of Energy. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to improved high strength, corrosion-resistant nickel base alloys of retained ductility and containing vanadium and/or niobium and having restricted contents of aluminum and titanium. 
     2. Prior Art 
     Efforts have been made over the past thirty years or so to develop alloys having high strength and resistant to many corrosive environments. Prominent among such alloys are nickel-base alloys, including the so-called &#34;superalloys.&#34; Representative of such alloys is Alloy 625, a Ni--Cr--Mo--Nb alloy developed by the International Nickel Co. (INCO). The same efforts resulted in Alloy 718, a Ni--Fe--Cr--Mo--Nb alloy with relatively high Al and Ti contents. &#34;The Invention and Definition of Alloy 625, H. L. Eiselstein and D. J. Tillack, Inco Alloys International, Inc., P.O. Box 1958, Huntington, W. Va. &#34;Superalloys 718, 625 and Various Derivatives,&#34; E. A. Loria, ed., The Metals Society, Warrendale, Pa. (1991), pages 1-14 . 
     At least some of these alloys, such as Alloy 625, have been produced by powder metallurgy techniques for controlled strengthening by conventional heat treatments. F. J. Rizzo and J. Radavich, Microstructural Characterization of PM 625-Type Materials, Crucible Compaction Metals, McKee and Robb Hill Roads, Oakdale, Pa. 15071 and Purdue University, West Lafayette, Ind. 47906. 
     Modifications have been made to at least some of these alloys, such as 625 Plus Alloy, nominally 21Cr-8Mo-3.4Nb-1.3Ti-0.2Al-5Fe-bal.Ni, to achieve still higher strength with corrosion resistance at least comparable to the unmodified alloy. Custom Age 625 Plus Alloy--A Higher Strength Alternative to Alloy 625, R. B. Frank, Carpenter Technology Corporation, P.O. Box. 14662, Reading, Pa. 19612. 
     The trademarked Hastelloys, nickel-based alloys, have been commercially available for some time for high strength performance in corrosive environment applications. 
     Still further improvement is needed in strength properties with good corrosion resistance and retained ductility for many high performance applications. 
     SUMMARY OF THE INVENTION 
     The invention relates to a new class of nickel-base alloys produced by atomization of an alloy melt under cover of an inert gas. These alloys include a corrosion-resistant superalloy of nominal composition 25Cr-15Fe-5.5Nb-3Mo-0.07C-bal.Ni which obtains its strength largely from precipitation hardening by the intermetallic phase gamma&#34; (Ni 3  Nb), and an alloy comprising 25Cr-10Mo-4Fe-0.5V-0.5Nb-0.6C-0.15N-bal.Ni competitive with the corrosion resistant Hastelloys. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1C are graphs relating percent corrosion weight loss and time for an alloy of the invention and comparison commercial alloys pursuant to supercritical water oxidation tests. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A number of experimental and comparison alloys were prepared, the compositions of which are shown in Table 1. 
     
                                           TABLE 1__________________________________________________________________________Alloy  Nominal Composition, Weight PercentC   N  Ni Fe Cr Mo Nb V  W  Co Mn Ti Al__________________________________________________________________________ABD2   Bal.     15 25 3  5.5                 -- -- -- 0.5                             -- --0.07    0.01625    Bal.     3  22 9  3.4                 -- -- -- 0.05                             0.3                                0.20.02    --718    Bal.     19 19 3  5.1                 -- -- -- 0.3                             1.0                                0.60.04    --ABD5   Bal.     4  25 10 0.5                 0.5                    -- -- 0.5                             -- --0.06    0.15C22.sup.a  Bal.     3  22 13 -- 0.5                    3  2.5                          0.5                             -- --0.02    --C276.sup.a  Bal.     5  16 15.4              -- 0.1                    3.3                       1.4                          0.5                             -- --0.003    --__________________________________________________________________________ .sup.a Hastelloy alloys 
    
     For the experimental alloys ABD2 and ABD5, powders for each alloy were prepared by induction melting of 15 pound ingots. Alloy ABD2 was melted under argon, whereas Alloy ABD5 was melted under nitrogen. The temperature of the melts prior to gas atomization was about 1700° C., representing a superheat of about 250° C. Gas atomization involved the use of helium (Alloy ABD2) or nitrogen (Alloy ABD5). The rapidly solidified powders (RSP) from each run were consolidated into bars by hot extrusion involving an extrusion ratio of 10 to 1. The extruded powders exhibited full densification with no evidence of porosity or prior particle boundaries. Ingot material for each experimental alloy was also extruded for comparison with the consolidated powders. 
     Conventionallly processed counterparts (CPC) of the gas-atomized alloys of the invention were prepared by conventional ingot metallurgical practice, and some of the commercial alloys were prepared also in the form of gas atomized powders followed by consolidation by extrusion. 
     The size and temperature stability of alloy grains is important for obtaining and retaining alloy strength at elevated temperatures. One hour heat treatments, between 1000° to 1300° C., followed by a water quench, were performed on the two experimental consolidated alloy powders and also on the conventionally processed counterparts (CPC) of those alloys. Microstructural examinations, including grain size measurements on polished and etched metallographic specimens, were performed after the 1 hour heat treatments. The average grain sizes are shown in Table 2. 
     
                       TABLE 2______________________________________Grain Size, mmAlloy      1000° C.               1100° C.                          1200° C.                                 1300° C.______________________________________ABD2-RSP.sup.a      0.005    0.011      0.016  --.sup.bABD2-CPC.sup.c      0.020    0.034      0.099  --.sup.bABD5-RSP.sup.a      0.005    0.006      0.009  0.036ABD5-CPC.sup.c      0.022    0.030      0.042  0.106______________________________________ .sup.a Rapidly solidified powder, according to the invention. .sup.b Not determined .sup.c Conventionally processed counterpart. 
    
     The results of such tests show that the grain sizes for the rapidly solidified experimental alloys are considerably smaller and more stable at high temperatures than their conventionally processed counterparts. 
     The superalloy, ABD2, was given further, time-at-temperature aging heat treatments for precipitation of the intermetallic, gamma&#34; phase (Ni 3  Nb). Maximum hardness was observed after a 675° C., 50 hour aging treatment. 
     Tensile properties for the experimental alloys, after testing at room temperature, 600° C. and 800° C., are shown in Table 3. 
     
                                           TABLE 3__________________________________________________________________________                        Ductility, % Heat    Test  Stress,                   MPa  Total                            Red.Alloy Treatment         Temp., °C.               Yield                   Ultimate                        Elong.                            Area__________________________________________________________________________ABD2-RS 1100° C./1 hr.         24    500 953  49  53ABD2-CPC 1100° C./1 hr.         24    313 759  57  55ABD2-RS 1000° C./1 hr. +         24    1257                   1450 22  36 aging.sup.aABD2-CPC 1000° C./1 hr. +         24    871 1120 30  36 aging.sup.aABD2-RS 1100° C./1 hr. +         600   968 1153 20  24 aging.sup.aABD2-CPC 1100° C./1 hr. +         600   656 773  10  -- aging.sup.aABD2-RS 1100° C./1 hr. +         800   505 514  17  22 aging.sup.aABD2-CPC 1100° C./1 hr. +         800   394 416  15  16 aging.sup.aABD5-RS 1200° C./1 hr.         24    515 999  46  44ABD5-CPC 1200° C./1 hr.         24    359 787  70  64ABD5-RS 1200° C./1 hr.         600   356 788  39  32ABD5-CPC 1200° C./1 hr.         600   222 597  67  52ABD5-RS 1200° C./1 hr.         800   359 440  35  33ABD5-CPC 1200° C./1 hr.         800   206 371  54  45__________________________________________________________________________ .sup.a Aging was for 675° C. for 50 hours. 
    
     These results clearly show that the rapid solidification processing (RSP) of the alloys produces very significant improvements in strengthening as compared to conventionally processed counterparts. In addition, the strengthening is accompanied by retention of good ductility. The improvements from the rapid solidification processing are attributed, at least in part, to composition homogeneity and fine grain size. The superalloy, ABD2, exhibits the highest level of strengthening, due to the age-hardenability with intermetallic precipitates. 
     Room temperature tensile property comparisons of the experimental alloys and several commercial nickel-base alloys (compositions given in Table 1) are shown in Table 4. 
     
                       TABLE 4______________________________________   Stress, MPa  PercentAlloy     Yield   Ultimate   Total Elong.                                Red. in Area______________________________________718 (CPC) 958     1344       29      27625 (CPC).sup.a     872     1214       30      --625 (P/M).sup.b     770     1152       35      45ABD2 (CPC)     871     1120       30      36ABD2 (RSP)     1257    1450       22      36C22 (CPC).sup.c     310      690       45      --C22 (RSP) 618     1049       46      54ABD5 (CPC)     363      797       61      64ABD5 (RSP)     657     1048       37      40______________________________________ .sup.a Eiselstein and Tillack, &#34;The Invention and Definition of Alloy 625,&#34; Superalloys 718, 625 and Various Derivatives, Ed. E. A. Loria, The Metals Society, Warrendale, Pa. (1991), pp. 1-14. .sup.b Rizzo and Radavich, &#34;Microstructural Characterization of PM 625Typ Materials, Ibid, pp. 297-308. .sup.c Data sheet from VOM NickelTechnologies A6, a company of Krupp Stahl. 
    
     The experimental RSP alloys clearly exhibit superior strengthening while retaining ductility. 
     The new RSP alloys also possess enhanced creep resistance as compared to their conventionally processed counterparts. The stress-to-rupture values for the ABD2-RSP and ABD2-CPC alloys are shown in Table 5. 
     
                       TABLE 5______________________________________Alloy  Test Temp., °C.               Stress, MPa                         Rupture Time, Hrs.______________________________________CPC    650          600       4.7RSP    650          600       42.5CPC    650          500       40.1RSP    650          500       240.3______________________________________ 
    
     It is apparent from the Table 5 data that the rapid solidification processing has improved the creep time-to-rupture lifetime for the ABD2 alloy. Creep testing of the ABD5 alloy, at 600° C. and 400 and 450 MPa (58 and 65 ksi), showed rupture times of 881 and 445 hours, respectively. Thus, despite the substantial absence of Al and Ti, which are included in the 625 alloy for deoxidation and creep resistance, the alloys of the invention show good creep resistance. 
     Corrosion tests were performed on the ABD2-RSP consolidated powder in a very hostile supercritical water oxidation/hydrochloric acid environment, at 240 atm. pressure, and under three different temperature and pH conditions: 650° C. and pH 0.65; 600° C. and pH 0.8, and 350° C. and pH 1.5. In these tests, the performance of the ABD2-RSP alloy was compared to the behavior of several commercial, conventionally processed, corrosion-resistant nickel-base alloys (compsitions of which are given in Table 1 above). As shown in FIGS. 1A-1C, the corrosion resistance of the ABD2-RSP alloy exceeded that of conventionally processed nickel-base alloys C276 nickel-base alloy C22 which is especially intended for corrosion-resistant applications. 
     Most broadly, the alloys of the invention fall within the ranges of elements as shown in Table 6. 
     
                       TABLE 6______________________________________Element          Weight Percent______________________________________iron             0 to 20chromium         10 to 30molybdenum       2 to 12niobium          6 max.vanadium         3.0 max.,            preferably 0.05 to 3.0 and            most preferably 0.5 to 3.0manganese        0.8 max.silicon          0.5 max.aluminum         less than 0.01titanium         less than 0.01phosphorus       less than 0.05sulfur           less than 0.05carbon           0.01 to 0.08nitrogen         less than 0.2oxygen           0.1 max.nickel           balance______________________________________ 
    
     More specifically, within the aforesaid broad range, the permissable range of elements for the ABD2 alloy is given in Table 7, and that for the ABD5 alloy is given in Table 8. 
     
                       TABLE 7______________________________________Element             Weight Percent______________________________________iron                3 to 18chromium            18 to 25molybdenum          8 max.niobium             3.5 to 6.0vanadium            3.0 max.manganese           1 max.silicon             1.0 max.aluminum            0.01 max.titanium            0.01 max.phosphorus          less than 0.05sulfur              less than 0.05carbon              0.01 to 0.08nitrogen            0.01 to 0.5oxygen              0.005 to 0.1nickel              balance______________________________________ 
    
     
                       TABLE 8______________________________________Element             Weight Percent______________________________________iron                4 to 15chromium            18 to 25molybdenum          3 to 12niobium             1 max.vanadium            0.05 to 3.0manganese           1 max.silicon             1 max.aluminum            0.05 max.titanium            0.05 max.phosphorus          less than 0.05sulfur              less than 0.05carbon              0.01 to 0.08nitrogen            0.01 to 0.5oxygen              0.005 to 0.1nickel              balance______________________________________ 
    
     Vanadium is important in certain aspects of the invention, as illustrated by alloy ABD5, to form vanadium carbides and/or nitrides for strengthening in the substantial absence of Al and Ti. In this regard, the substantial absence of Al and Ti is important to avoid formation of nickel aluminide or nickel/titanium aluminide intermetallic precipitates which would interfere with the strengthening action of the interstitials C and 0. In another aspect of the invention, as illustrated by alloy ABD2, such strengthening is replaced with strengthening by the gamma&#34; phase (Ni 3  Nb), so that the presence of niobium is required, although vanadium is not. 
     In addition to rapid solidification processing via atomization, which is important for achieving the fine and stable microstructure and associated improvements in mechanical properties, the preferred alloys of Table 6 are distinguished from commercial alloys 625 and 718 in the essential presence of V and the absence of significant amounts of the elements aluminum and titanium, and from the commercial Hastelloys C22 and C276 in the lower amounts of molybdenum and the absence of tungsten and cobalt and, in the case of C276, a higher carbon content in the inventive alloys. Compositionally, the alloys of Table 7 are distinguished from commercial alloy 625 by a higher Nb content, a lower Mo content, and by the substantial absence of Al and Ti; from alloy 718 by a lower iron content and the substantial absence of Al and Ti; and from the Hastelloys C22 and C276 by lower Mo and by the absence of W and Co. Compositionally, the alloys of Table 8 are distinguished from commercial alloy 625 by a higher iron content and lower Nb content, in the essential presence of the element vanadium and the substantial absence of aluminum and titanium; from alloy 718 by a lower iron content, lower Nb and by the essential presence of V and the substantial absence of Al and Ti; from alloy C22 by higher iron and lower Mo contents, and the absence of W and Co; and from alloy 276 by higher Cr, lower Mo, and the absence of W and Co, as well as a higher C content.