Abstract:
A corrosion resistant metal alloy having improved formability and workability is disclosed which alloy contains in weight percent about 25% to 45% nickel, about 12% to 32% chromium, of at least one of 0.1% to 2.0% columbium, 0.2% to 4.0% tantalum, and 0.05% to 1.0% vanadium, up to about 0.20% carbon, about 0.05% to 0.50% nitrogen, about 0.001% to 0.02% boron and the balance being iron plus impurities and wherein the carbon and nitrogen content are controlled so that the amount of free carbon and nitrogen defined as ##EQU1## is greater than 0.14% and less than 0.29%. The alloy may also include in limited amounts one of aluminum, titanium, silicon, manganese, cobalt, molybdenum, tungsten, zirconium, yttrium, cerium and other rare earth metals.

Description:
REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of our U.S. Patent Application Serial No. 154,606, now U.S. Pat. No. 4,853,185. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to metal alloys containing substantial amounts of iron, nickel and chromium and more particularly to a carefully balanced composition suitable for use in aggressive environments at high temperature. 
     DESCRIPTION OF THE PRIOR ART 
     Many people have attempted to develop alloys exhibiting high mechanical strength, low creep rates and good resistance to corrosion at various temperatures. In U.S. Pat. No. 3,627,516 Bellot and Hugo report that it was well known to make alloys having mechanical strength and corrosion resistance by including in the alloy about 30% to 35% nickel, 23% to 27% chromium and relatively low carbon, manganese, silicon, phosphorus and sulfur. Mechanical properties of this type of alloy were improved by adding tungsten and molybdenum. Bellot and Hugo further improved this alloy by adding niobium in a range of from 0.20% to 3.0% by weight. Two years later in U.S. Pat. No. 3,758,294 they taught that high mechanical strength, low creep rate and good corrosion resistance could be obtained in the same type of alloy by including 1.0% to 8.0% niobium, 0.3% to 4.5% tungsten and 0.02% to 0.25% nitrogen by weight. Both patents teach a carbon content of the alloy in the range 0.05% to 0.85%. 
     Bellot and Hugo appear to have no concern about the hot workability and fabricability of their alloys. It is well known that carbon contents in excess of 0.20% greatly impair hot workability and fabricability. Many of the alloys disclosed by Bellot and Hugo have more than 0.20% carbon. The claims of both their patents require about 0.40% carbon. Because of these high carbon levels such alloys are difficult to hot work, fabricate or repair. 
     In U.S. Pat. No. 3,627,516 Bellot and Hugo attempt to avoid the use of expensive alloying elements such as tungsten and molybdenum to improve mechanical properties by adding 0.20% to 3.0% niobium. But in U.S. Pat. No. 3,758,294 they later find that tungsten is required to achieve high weldability and easy resistance to carburization. Thus, the teaching of Bellot and Hugo is that tungsten although expensive is necessary to achieve high weldability in a corrosion resistant alloy. 
     Carbon and tungsten as well as other solid solution strengtheners such as molybdenum are used in alloys of the Ni-Cr-Fe family having generally about 15 to 45% nickel and 15 to 30% chromium to provide strength at high temperatures. The use of substantial amounts of carbon and solid solution strengtheners adversely affect thermal stability, reduce resistance to thermal cycling and usually raise the cost of the product excessively. Precipitation hardening is normally either limited to relatively low temperature strength improvements or has associated thermal stability and fabricability problems. 
     In addition to these strength considerations, prior art alloys of this family have only average corrosion resistance to aggressive high temperature environments such as those containing hydrocarbons, CO, CO 2  and sulfur compounds. 
     SUMMARY OF THE INVENTION 
     The present invention is a Fe-Ni-Cr alloy having improved mechanical properties and improved hot workability through the addition of a carefully controlled amount of nitrogen and the provision of nitrogen, columbium and carbon within a defined relationship. Boron in the range of 0.001% to 0.02% is added to improve creep strength of elevated temperatures. Preferably, columbium is added to comprise up to 1% of the alloy in order to produce complex carbonitride compound particles which form while the alloy is in service, and promote strengthening. Columbium also increases nitrogen solubility in the alloy, which allows for a higher level of nitrogen to be included in the alloy to yield higher strength. The presence of stronger nitride formers, such as aluminum and zirconium is limited to avoid excessive initial coarse nitride formation during alloy manufacture and consequent loss of strength. Chromium is present at levels over 12% to provide for both adequate oxidation resistance and adequate nitrogen solubility. In the presence of columbium, vanadium or tantalum in the alloy, a very small amount of titanium will have beneficial strengthening effects (not over 0.20% Ti). Silicon may be added up to 3.0% to optimize oxidation resistance, however, strength drops off markedly over about 1% Si. So two classes of alloy are possible: up to 1% Si has excellent strength and 1%-3% Si has lower strength but better oxidation resistance. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present alloy is a Fe-Ni-Cr alloy preferably having 25%-45% nickel and 12% to 32% chromium. More particularly the composition should fall within these ranges: 
     
         ______________________________________Ni               25% to 45%Cr               12% to 32%Cb               0.10 to 2.0%            (min. 9 × carbon content)Ti               Up to 0.20% maxSi               Up to 3% maxN                0.05 to 0.50%C                0.02 to 0.20%Mn               Up to 2.0% maxAl               Up to 1.0% maxMo/W             Up to 5% maxB                0.001% to 0.02% maxZr               Up to 0.2 maxCo               Up to 5 maxY, La, Ce, REM   Up to 0.1% maxand the balance iron and typical impurities______________________________________ 
    
     The nitrogen in this alloy acts as a solid solution strengthener and also precipitates as nitrides in service as a further strengthening mechanism. The prior art involves alloys with generally less than enough nickel to provide a stable austenitic matrix when subjected to long term thermal aging in service at elevated temperature. Nitrogen acts to stabilize austenitic structure, but if nickel is less than 25%, once nitrides are precipitated during service exposure at greater than 1000° F., the matrix is depleted in nitrogen, and alloys are prone to embrittlement from sigma phase precipitation. To avoid this, our alloys contain greater than 25% Ni, and preferably greater than 30% Ni. 
     It is known that titanium in the presence of nitrogen in an iron-base alloy will form undesirable, coarse titanium nitride particles. These nitrides form during alloy manufacture and contribute little towards elevated temperature strength in service. The exclusion of titanium from this type of alloy avoids depletion of nitrogen from the solid solution by the manner described, but does not provide optimum strengthening. We have found that in the presence of columbium, vanadium or tantalum in the alloy, a very small amount of titanium will have beneficial strenghtening effects as long as there is not more than 0.20% Ti. Consequently, we provide up to 0.20% titanium in our alloy. As those skilled in the art will recognize, columbium, vanadium or tantalum, which have a somewhat greater affinity for carbon than for nitrogen, can be added to this type of alloy to increase nitrogen solubility without depleting the majority of the nitrogen as coarse primary nitride or nitrogen-rich carbonitride particles. In excess of 2.0% columbium is undesirable because of a tendency to form deleterious phases such as Fe 2  Cb laves phase or Ni 3  Cb orthorhombic phase. For this reason, we provide a columbium to carbon ratio of at least 9 to 1 but generally less than 2.0%. Without columbium or an equivalent amount of vanadium or tantalum, the addition of nitrogen would not provide as much strength. To achieve similar results, half the weight in vanadium or double the weight in tantalum should be used whenever they are substituted for columbium. 
     Silicon may be added up to 3.0% to optimize oxidation resistance. However, strength drops off markedly over about 1% Si. Thus, one can use up to 1% Si for excellent strength or provide 1%-3% Si to obtain lower strength but better oxidation resistance. Strong nitride formers, such as aluminum and zirconium, are limited to avoid excessive coarse nitride formation during alloy manufacture, and consequent loss of strength in service. Chromium is present at levels over 12% to provide for both adequate oxidation resistance and adequate nitrogen solubility. 
     EXAMPLE I 
     To determine the influence of columbium in this alloy, we prepared an alloy having a nominal composition of 33% Ni, 21% Cr, 0.7% Mn, 0.5% Si, 0.3% Al, plus carbon, nitrogen, titanium and columbium as set forth in Table I and the balance iron. These alloys were tested to determine the time required for one percent creep under three temperature and stress conditions. The results of that test are set forth in Table 1. 
     This data indicates that Ti ties up N in preference to carbon, forming TiN with possibly some Ti (C, N). Cb ties up C in preference to N, so as long as C/Cb ratio stays relatively constant, N is available to form strengthening Cr 2  N and CbN precipitates, or to provide solid solution strengthening. So the strength levels exhibited by alloys C, D and E are nearly the same. Note that adding nitrogen to replace carbon by more than 2:1 without Cb does little to improve strength, as evidenced by alloys A and F versus alloy E. Also, simply adding Cb to alloy containing Ti does not significantly improve strength, as evidenced by comparing alloy G to alloy A. Finally, the alloys with titanium levels at 0.40 and 0.45 performed poorly suggesting that such high titanium levels are detrimental. 
     
                       TABLE 1______________________________________Cb vs TiNominal (%): Fe-33% Ni-21% Cr-0.7% Mn-0.5% Si-0.3% Al         Time to 1% Creep         (Hours for Two Samples)% Other Elements           1400° F./                     1500° F./                              1600° F./Alloy C      N     Ti  Cb   13 ksi  10 ksi 7 ksi______________________________________A     .07    .01   .40 .05  1, 1    1, 1   1, 2B     .06    .20   .31 .05  4, 5    --     --C     .05    .20   +   .46  12, 18   9, 10 34, 55D     .09    .19   +   1.00 13, 15  7, 8   34, 41E     .02    .19   +   .26   7, 14   9, 11 32, 32F     .01    .19   +   .05  2, 4    1, 2    8, 10G     .08    .04   .45 .48  --      1, 2   2, 5______________________________________ + means less than 0.01% 
    
     EXAMPLE II 
     The effect of nitrogen and carbon is revealed in tests of several alloys having the same nickel, chromium, manganese, silicon and aluminum content as the iron-base alloys of Example I and carbon, nitrogen, titanium and columbium content set forth in Table 2 and Table 2A. 
     The data in Table 2 demonstrates that strength goes up with increasing (C+N). Greater than 0.14% &#34;free&#34; (C+N) is necessary for good high temperature strength. At a columbium level of 0.20%, a carbon level of 0.05% and a nitrogen content of 0.02% (the minimum values taught by Bellot and Hugo), the &#34;free&#34; (C+N) =0.05% which is not adequate for good strength. To obtain the needed minimum of 0.14% &#34;free&#34; (C+N) with carbon at 0.05% at least 0.11% nitrogen is required. At a columbium level of 0.50% and carbon level of 0.05%, nitrogen greater than 0.15% is required to obtain &#34;free&#34; (C+N) above 0.14%. If carbon is increased to 0.10% with the same columbium content, then more than 0.10% nitrogen is still required to obtain the desired level of &#34;free&#34; (C+N). Finally, at a third level of columbium of 1.0% we still see a relationship between carbon and nitrogen. With carbon at 0.05%, nitrogen greater than .20% is required for free (C+N) to be above 0.14%. At C =0.10% then N greater than 0.15% is required. And, at C =0.15% then N greater than 0.10% is required. Consequently, to achieve acceptable strength levels (C+N) must be greater than 0.14% ##EQU2## 
     Table 2A shows that thermal of high (C+N) level compositions can be poor. In order to maintain adequate stability, &#34;free&#34; (C+N) should be less than 0.29%. Therefore, (C+N) must be less than 0.29% ##EQU3## Thus, the critical ranges of (C+N) at four levels of Cb are as follows: 
     
         ______________________________________Cb (%)    (C + N) min. (%)                   (C + N) max. (%)______________________________________0.25      0.17          0.320.50      0.20          0.350.75      0.22          0.371.00      0.25          0.40______________________________________ 
    
     
                       TABLE 2______________________________________Effect of (C + N) &amp; &#34;Free&#34; (C + N) on Strength                                     Hours to 1%                            Free     CreepHeat  C      N     Cb  Ti  C + N (C + N)* 1600° F./7______________________________________                                     ksi7984-1 .08    .08   .47 .07 .16   .09      1220883 .04    .12   .48 +   .16   .10       821283 .09    .14   .98 +   .23   .12       97483  .08    .14   .51 .17 .22   .11      195785  .08    .14   .51 .07 .22   .14      255485  .06    .18   .52 .08 .24   .16      338784  .07    .16   .49 .05 .23   .16      408284  .08    .16   .48 .02 .24   .18      358884  .09    .27   .51 .07 .36   .28      888984  .09    .40   .50 .05 .49   .42      94______________________________________ + less than 0.01% ##STR1## 
    
     
                       TABLE 2A______________________________________Effect of (C + N) &amp; &#34;Free&#34; (C + N) on Thermal Stability                                  Exposure at                                  1400° F./1000 Hrs.                     C +  Free    Residual RTHeat  C     N     Cb  Ti  N    (C + N)*                                  Tensile El (%)______________________________________22584 .08   .04   .48 .45 .12  .00     40984-2 .05   .07   .48 .20 .12  .01     387984-1 .08   .08   .47 .07 .16  .09     347483  .08   .14   .51 .17 .22  .11     295785  .08   .14   .51 .07 .22  .14     325485  .06   .18   .52 .08 .24  .16     328784  .07   .16   .49 .05 .23  .16     248284  .08   .16   .48 .02 .24  .18     248884  .09   .27   .51 .07 .36  .28     255885  .08   .29   .49 .08 .37  .29     118984  .09   .40   .50 .05 .49  .42     14______________________________________ ##STR2## 
    
     EXAMPLE III 
     The criticality of titanium can be seen from creep data for alloys I, K, L and M which have similar base materials as the other alloys tested. The creep data for those alloys tested at 1400° F. and 13 ksi are shown in Table 3. In that table the alloys are listed in order of increasing titanium content. This data indicates that any titanium is beneficial. However, the data from Table I indicates an upper titanium limit of not more than 0.40%. 
     
                       TABLE 3______________________________________Ti CriticalityNominal (%): Fe-33% Ni-21% Cr-0.7% Mn-0.5%Si-0.3% Al-005% B              Average Hours to 1%% Other Elements   Creep at 1400° F./13ksiAlloy C       N     Ti     Cb  (Hours)______________________________________K     .08     .18   Nil    .49 35L     .08     .16   .02    .48 47I     .08     .14   .07    .51 92M     .08     .14   .17    .51 59______________________________________ 
    
     EXAMPLE IV 
     Silicon is an important component of the alloy. Its influence is shown in Table 4. The data in that table indicates that silicon must be carefully controlled to achieve optimum properties. Low levels of silicon are fine. However, when silicon levels reach and exceed about 2% performance drops sharply. This is apparently caused by silicon nitride which has formed in increasing amounts as the silicon level increases. 
     
                       TABLE 4______________________________________Si CriticalityNominal (%): Fe-33% Ni-21% Cr-0.7% Mn-0.5%Si-0.3% Al-0.005% B        Time to 1% Creep (Hours)          1400° F./                    1600° F./                              1800° F./% Other Elements          13 ksi    7 ksi     2.5 ksiAlloy C     N     Ti  Si   1%   R    1%   R    1%   R______________________________________I     .08   .14   .07 .57  81   951  23   179   43  160                      104  948  27   214  160  402N     .07   .12   .02 1.40 61   592  25   321  216  672                      40   640  10   227O     .08   .15   .06 1.96 3     73   3    58  112  315                      4     79   4    56  206  547P     .08   .14   .08 2.41 4     55   2    47  138  470                      2     49   2    48  137  512______________________________________ 
    
     EXAMPLE V 
     The data shown in Table 5 reveals that the presence of zirconium at 0.02% dramatically reduces creep time. Also, as aluminum content approaches 1.0% it produces a similar result. 
     
                       TABLE 5______________________________________Adverse Effects of Al &amp; ZrNominal (%): Fe-33% Ni-21% Cr-0.5% Cb-0.7% Mn-005% B               Average Hours to 1%% Other Elements    Creep at 1400° F./13 ksiAlloy C      N     Si    Al  Zr   (Hours)______________________________________Q     .08    .14   .60   .24 Nil  59R     .08    .14   .61   .86 Nil  13S     .07    .12   1.40  .28 Nil  49T     .07    .21   1.48  .28 .02   7______________________________________ 
    
     Based upon the data from Tables 1 through 5, we selected alloys I and two other alloys, U and V, and provide creep data in Table 6. 
     Alloys I and V compare favorably to prior art alloys in mechanical properties as shown in Tables 7, 8 and 9. 
     
                       TABLE 6______________________________________Cb vs TiNominal (%): Fe-0.5% Cb-0.7% Mn-0.5% Si-0.3% Al-0.005% B          Time to 1% Creep (Hours)% Other Elements 1400° F./                     1600° F./                              1800° F./Alloy Ni      Cr     C   N   13 ksi 7 ksi  2.5 ksi______________________________________I     34.0    20.8   .08 .14 92     25      83U     40.3    20.9   .06 .18 60     33     119V     39.8    30.0   .07 .16 77     40     274______________________________________ 
    
     
                       TABLE 7______________________________________COMPARATIVE PROPERTIES (Sheet)Alloy I     Alloy V  800H   253MA 601  310  316______________________________________YieldStrength (ksi)RT      41      49       35   51    42   321,200° F.   26      27       22   24    38   17   211,400° F.   24      28       20   22    39   15   181,600° F.   20      25       13   16    16   12   111,800° F.   11      10        8   --     9    6    6TensileElongation(%)RT      42      45       46   51    47   46   --1,200° F.   42      50       45   48    50   39   --1,400° F.   45      40       62   44    41   73   --1,600° F.   61      35       56   --    65   69   --1,800° F.   56      66       83   --    86   54   --______________________________________ 
    
     
                       TABLE 8______________________________________COMPARATIVE PROPERTIES (Sheet)       Room Temperature Properties AfterExposure    1,000 Hours at TemperatureTemperature Alloyl I Alloyl V 800H  601   310______________________________________1,200° F.    UTS    98       16     88    127  86    YS     41       57     38    76   37    EL     35       30     38    31   411,400° F.    UTS    94       121    83    106  100    YS     39       62     34    51   41    EL     32       24     41    37   211,600° F.    UTS    90       108    78    91   84    YS     35       48     30    38   35    EL     33       32     39    45   23As Annealed    UTS    99       108    82    95   81    YS     41       49     36    42   32    EL     42       45     46    47   46______________________________________ 
    
     
                                           TABLE 9__________________________________________________________________________COMPARATIVE PROPERTIES (Sheet)         ALLOY I               ALLOY V                     800H                         253MA                             601                                310                                   316__________________________________________________________________________Stress Rupture Life (Hours)1,400° 949/13 ksi               551   104 110 205                                10 951,600° F./7 ksi         196   194   88  40  98 5  --Creep Life (Hours to 1%)1,400° F./13 ksi          92    77   3   18  46 1  --1,600° F./7 ksi          25    40   8   10  29 1  --__________________________________________________________________________ 
    
     From the data discussed above, we have found that an alloy comprised of 25 to 45% nickel, about 12% to 32% chromium, at least one of 0.1% to 2.0% columbium, 0.2% to 4.0% tantalum and 0.05% to 1.0% vanadium, up to about 0.20% carbon, and about 0.05% to 0.50% nitrogen with the balance being iron plus impurities has good hot workability and fabricability characteristics provided (C+N) F  is greater than 0.14% and less than 0.29%. As previously stated ##EQU4## In versions of the alloy wherein vanadium and tantalum are substituted separately or in combination for all or part of the columbium (C+N) F  is defined by ##EQU5## Boron content of 0.001% to 0.2% will improve creep strength, but higher levels will impair weldability markedly. 
     Silicon may be added to the alloy but preferably it does not exceed 3% by weight. Up to 1% silicon has excellent strength while 1% to 3% silicon has lower strength but better oxidation resistance. Titanium may also be added to improve creep resistance. However, not more than 0.20% titanium should be used. Manganese and aluminum may be added basically to enhance environment resistance, but should generally be limited to less than 2.0% and 1.0% respectively. 
     Molybdenum, tungsten and cobalt may be added in moderate amounts to further enhance strength at elevated temperatures. Molybdenum and tungsten will provide additional strength without significant thermal stability debit up to about 5%. Higher levels will produce some measurable loss in thermal stability, but can provide significant further strengthening up to a combined content of about 12%. 
     While we have described certain present preferred embodiments of our invention, it is to be distinctly understood that the invention is not limited thereto but may be variously embodied within the scope of the following claims.