Abstract:
An austenitic alloy having improved ductility/processability and improved pitting and crevice corrosion resistance comprising, in % by weight, about: 25-30% Ni; 19-23% Cr; 6-8% Mo; 0.3-0.5% N; 0.5% Mn; 0-1.5% Cu; 0-0.2% C; 0-1% Al; 0-0.01% S; 0-1% Ti; 0-1% Si; up to trace amounts of Mg, Ca, and Ce; and balance Fe plus incidental impurities.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to nickel-iron-chromium alloys containing molybdenum for the purpose of providing resistance to pitting and crevice corrosion.  
           [0003]    2. Discussion of the Related Art  
           [0004]    Certain ferrous alloys including INCOLOY® alloy 25-6MO (hereinafter referred to as “alloy 25-6MO”) are particularly useful for their exceptional resistance to many corrosive environments. INCOLOY® is a trademark of the Special Metals group of companies. Alloy 25-6MO nominally contains by weight percent 25 nickel, 20 chromium, and 6 molybdenum. Examples of such corrosion resistant alloys are disclosed in U.S. Pat. No. 4,545,826 as containing by weight percent  20 - 40  nickel, 14-21 chromium, 6-12 molybdenum, maximum of 2 manganese, and 0.15-0.30 nitrogen. These alloys are annealed at relatively high temperatures, namely, over 2100° F. (1149° C.), typically about 2200° F. (1204° C.).  
           [0005]    These nickel-cromium-molybdenum alloys are particularly suited for use in chemical and food processing, pulp and paper bleaching plants, marine and offshore platforms, salt plant evaporators, air pollution control systems, and various equipment for the power industry. These are aggressive aqueous environments which contain halides. Accordingly, the alloys formed into components of such systems must have good resistance to pitting and crevice corrosion. In addition, the alloys must have good processability since they are fabricated into a variety of intricate forms. Processability includes well-known hot forming techniques such as forging and rolling or other forming operations such as drawing and bending to mention a few. However, it is difficult to produce a nickel-chromium-molybdenum alloy with good processability because high concentrations of Mo, Cr and N which provide pitting resistance are also known to be detrimental to the processability of the alloy.  
           [0006]    Accordingly, a need remains for a nickel-chromium-molybdenurm alloy having improved corrosion resistance as well as improved processability.  
         SUMMARY OF THE INVENTION  
         [0007]    This need is met by the nickel-iron-chromium alloy of the present invention which most preferably includes about the following ranges by weight percent:  
                                                   ELEMENT   WEIGHT PERCENT (wt. %)                           Ni   26-29           Cr   20-22           Mo   6.5-7.5           Mn   0-5           Cu   0-1           N   0.3-0.5           Fe   Balance                      
 
           [0008]    Heats of the alloys of the present invention containing nitrogen in the amount of about slightly greater than 0.3 wt. % to 0.4 wt. % exhibit significantly improved pitting resistance and exhibit improved crevice corrosion resistance over prior Ni—Cr—Mo alloys. Presently preferred lower limits for N are 0.31 wt. % and 0.33 wt. %. The alloys of the present invention also provide additional improved properties, such as: (1) at least 100° F. (38° C.) lower sigma solvus temperatures so as to decrease the propensity to form sigma phases during processing, (2) higher yield strength and good ductility, (3) allows the use of relatively low temperature annealing steps, namely, less than 2100° F. (1149° C.), and, hence, improved processability for forming various shaped components. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a graph of sigma solvus temperature contour lines at 22 wt. % nickel;  
         [0010]    [0010]FIG. 2 is a graph of sigma solvus temperature contour lines at 25 wt. % nickel;  
         [0011]    [0011]FIG. 3 is a graph of sigma solvus temperature contour lines at 27 wt. % nickel;  
         [0012]    [0012]FIG. 4 is a graph of PREN contour lines at 22-27 wt. % nickel;  
         [0013]    [0013]FIG. 5 is a graph of the comparison of the effects of molybdenum and nitrogen on both sigma solvus temperature and PREN calculations; and  
         [0014]    [0014]FIG. 6 is a comparison of PREN and sigma solvus temperatures for a composition of the present invention and prior art alloys. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    The present invention is an improvement over INCOLOYO alloy 25-6MO which exhibits improved pitting and crevice corrosion resistance as compared to prior Ni—Cr—Mo alloys. These improvements are believed to be the result of the inclusion of about 6.5-7.5 wt. % Mo and about 0.33-0.40 wt. % N to a corrosion resistant alloy such as INCOLOY® alloy 25-6MO.  
         [0016]    In particular, the alloy of the present invention contains the elements set forth in Table 1 by weight percent of the alloy in about the following ranges:  
                                                                   TABLE 1                                       WEIGHT PERCENT            ELEMENT   Broad   Medium   Narrow   Nominal                    Ni   25-30   26-29   26-28   27       Cr   19-23   20-22     20-21.5   21       Mo   6-8   6.5-7.5   6.6-7.5   7       N   0.3-0.5   0.31-0.35   0.33-0.4     0.35       Mn   0-5   0-5    .5-1.5   1.0       Cu     0-1.5   0-1   0.7-1.0   0.8       C     0-0.2   0.01-0.2    0.01-0.02   &lt;0.02       Al   0-1   0.01-0.15   0.05-0.15   0.1       S     0-0.01      0-0.002   &lt;0.002   &lt;0.001       Ti   0-1   0-1     0-0.03   &lt;0.03       Si   0-1   0.1   0.5   &lt;0.5       Mg   &lt;0.1       Ca   &lt;0.1       Ce   &lt;0.1       Fe   Balance   Balance   Balance   Balance       and Incidental       Impurities                  
 
         [0017]    The alloy of the present invention may further contain up to 0.5 wt. % V.  
         [0018]    A particularly preferred alloy of the present invention includes by weight percent about 27 Ni,  21  Cr, 7.2 Mo, 1.0 Mn, 0.8 Cu, and 0.33 N. The present invention is a result of both theoretical calculations and physical testing of alloys containing molybdenum for corrosive environments.  
         [0019]    Certain theoretical calculations are known techniques for evaluating a potential alloy. These calculations include sigma solvus temperature and pitting resistance equivalent number (PREN) which is a numerical estimate of the pitting resistance based on the alloy composition where PREN equals % Cr+3.3 (% Mo)+30(% N). A high sigma solvus temperature in 6MO alloys (alloys containing about 6 wt. % molybdenum) has been known to result in poor metallurgical stability and excessive processing problems. One goal during development of the present invention was to define an alloy composition having the best possible combination of a high PREN for improved pitting resistance as well as a low sigma solvus temperature for stability and improved processing of the alloy. Calculations of sigma solvus temperatures and PREN numbers were made for a factorial design encompassing Ni at 22, 25 and 27 weight percent, Mo at 6.0, 6.5 and 7.0 weight percent, and N at 0.20, 0.28 and 0.35 weight percent with 20.5 Cr and the balance Fe. The calculated effect of Mo and N content on the sigma solvus temperatures in 22 Ni, 25 Ni and 27 Ni compositions are shown in FIGS.  1 - 3 . The contour lines in FIGS.  1 - 3  are drawn to show various sigma solvus temperature levels. FIGS.  1 - 3  demonstrate that the higher contents of nickel and nitrogen decrease the sigma solvus temperature whereas increases in the amount of molybdenum increase the sigma solvus temperature. FIG. 4 presents contour lines for PREN values over a range of 6-7 wt. % Mo and 0.2-0.35 wt. % N in an alloy with 22-27 wt. % Ni and 20.5 wt. % Cr. FIG. 4 demonstrates that higher molybdenum and nitrogen levels lead to higher PREN numbers. Based on these calculated PREN values, the higher the molybdenum and nitrogen levels, the greater the resistance to pitting is expected. However, nitrogen was already shown in FIGS.  1 - 3  to decrease the sigma solvus temperature, whereas molybdenum increases the sigma solvus temperature.  
         [0020]    Hence, in the present invention, a balance was struck between these two desired goals. The desired lower sigma solvus temperatures dictate using a higher nitrogen content and lower molybdenum content while the desired PREN values suggest using higher molybdenum and nitrogen levels. This is shown graphically in FIG. 5 where the PREN contour lines are superimposed on the sigma solvus contour lines produced in FIGS.  1 - 4  for 27 Ni-20.5 Cr compositions. It was determined that the best combination of a relatively low sigma solvus temperature of about 1900° F. and an acceptable PREN level of about 54 was obtainable at a nitrogen level of about 0.35%. This is noted by the data point with an “*” for a 27-7 composition (27Ni, 20.5Cr, 7Mo and 0.35N). This 27Ni-20.5Cr-7Mo-0.35N composition was shown to have significantly higher PREN and lower sigma solvus  5  temperatures than commercially available alloys. The common compositions of several commercial 6MO type alloys are set forth in Table 2.  
                                                                   TABLE 2                           Composition of Common 6MO Type Alloys                Element   25-6MO   6XN   254SMO   654SMO                            Ni   25   24   18   22           Cr   20.5   21   20   24           Mo   6.7   6.5   6.1   7.3           N   0.20   .22   .22   0.5           Mn   0.7       0.6   3           Cu   0.9       0.7   0.5           σ Solvus   2036° F.   2079° F.   2102° F.   2179° F.               (1113° C.)   (1137° C.)   (1149° C.)   (1193° C.)           PREN   48.0   49.0   46.4   63.1                      
 
         [0021]    Theoretical calculations show that 27Ni-20.5Cr-7Mo-0.3SN composition has significantly lower sigma solvus temperature and higher PREN number than most of the conventional alloys, FIG. 6. Although alloy 654SMO has a very high PREN number, it also has a very high sigma solvus temperature, which corresponds with more difficult processing and product limitations and, hence, is less acceptable than the alloy of the present invention. The experimental sigma solvus temperature for a 27Ni-20.5Cr-7Mo-0.35N composition was marginally higher than the theoretical prediction.  
         [0022]    It is believed that the molybdenum content can be about 6.5-7.5 wt. % and the nitrogen content can be about 0.33-0.40 wt. % to exhibit the desired balance of properties. Accordingly, the present invention lies in the use of about 6.5-7.5 wt. % Mo and about 0.33-0.40 wt. % N in a nickel-chromium alloy. Although the invention has been described generally above, the following particular examples give additional illustrations of the product and process steps typical of the present invention.  
       EXAMPLE 1  
       [0023]    Laboratory sized heats (50 lbs.) were produced by both air and vacuum melting. The amount of deoxidizing elements, other residuals and the hot rolling practice were varied as set forth in Table 3.  
         [0024]    Ingots were rolled to 2.25 inch square, 0.250 inch flat, 0.125 inch strip and/or {fraction (5/8)} inch rod. Chemical analyses were conducted on ladle samples and/or final products. Critical pitting temperature and crevice corrosion temperature (the lowest temperatures at which attack occurs) were both conducted according to ASTM G48, Practices C and D on armealed specimens with a 120 grit ground surface.  
                                                                                                                                                           TABLE 3                           Experimental Compositions            Heat   C   Mn   S   Si   Cu   Ni   Cr   Al   Ti   Mo   Mg   Ca   Ce   N   Sample   Process                    HV9240*   .010   .59   .005   .45   .05   26.00   20.61   .01   .02   6.59   &lt;.001   &lt;.001   .001   .36   Ld Sq   Ho       HV9242A   .010   .60   .005   .47   .05   27.76   21.19   .01   .02   7.23   .013   .003   .002   .35   Ld Plate   Ho, Ma       HV9244A   .012   .59   .003   .45   .05   25.91   20.73   .01   .01   6.64   .010   .002   .001   .34   Ld Plate   Ho, Ma       HV9438   .018   .55   .004   .43   .73   28.22   21.06   .05   .02   7.28   .016   &lt;.001   .005   .35   Ld Plate   Ho, Ma       HV9439   .015   1.94   .005   .42   .15   27.96   21.03   .05   .02   7.48   .012   &lt;.001   .006   .35   Ld Plate   Ho, Ma       HV9440   .016   .64   .004   .44   .21   27.11   21.05   .01   .02   7.17   .002   .002   .003   .33   Ld Plate   Ho, Ma       HV9441   .018   .57   .004   .44   .21   27.11   21.07   .01   .02   7.17   .003   .003   .002   .34   Ld Plate   Ho, Ma       HV9117A**   .048   1.93   .002   .34   .85   25.93   20.01   .11   .00   6.79   —   —   —   .20   Plate                                          
 
       COMPARATIVE EXAMPLE  
       [0025]    A 50 lb. laboratory heat of an alloy having less nitrogen than that of the present invention was produced and also appears in Table 3 as Heat HV9117A.  
         [0026]    Results of the critical pitting temperature (CPT) and critical crevice corrosion temperatures (CCT) tests conducted on plate samples of certain of the alloys set forth in Table 3 are reproduced in Tables 4 and 5.  
         [0027]    The data set forth in Tables 4 and 5 demonstrates that both pitting resistance and crevice corrosion resistance improve with increasing amounts of Mo and N. The typical CPT and CCT temperatures for conventional 25-6MO alloys are 158° CF (70° C.) and 95° F. (35° C.), respectively. Upon slightly increasing the Mo, as was done in Heat HV9117A of the Comparative Example, the CPT and CCT values were increased to 176° CF (80° C.) and 104° F. (40° C.), respectively. However, further increases in the amount of Mo and N in Heat HV9242A (an alloy of the present invention) increased the CPT and CCT values to 195° F. (85° C.) and 140° F. (60° C.), respectively. Hence, higher Mo and N levels are believed to be beneficial.  
         [0028]    An autogenous gas tungsten arc welding (GTAW) test was conducted in the flat position on 0.062″ thick sheet rolled from Heat HV9438 and others to evaluate tungsten deterioration and molten metal fluid flow. Visual examination of the tungsten after welding did not illustrate excessive deterioration or spalling. Weld-bead profile and geometry were not adversely affected by the 0.35 percent addition of nitrogen. In addition, the fluidity and wetting characteristics of the molten metal were not significantly degraded by the nitrogen additions.  
         [0029]    The mechanical properties of the alloys of the present invention were also tested. The effect of annealing on room temperature tensile properties was tested for Heat HV9242A. INCOLOYS alloy 25-6MO generally is required to have a minimum 0.2% yield strength of 43 Ksi and a minimum elongation of 40%. To obtain these properties, it has been previously necessary to use a relatively high annealing temperature of 2200° F. (1204° C.) to obtain the desired ductility. Nevertheless, the strength at this ductility is often only marginally better than 43 Ksi. Table 6 presents the impact on room temperature properties of annealing temperatures from 2050° F. to 2150° F. on 0.125″ strip formed from heat HV9242A after cold rolling to 50%. Table 7 presents the results of testing the same heat HV9242A as 0.1501″ strip after cold rolling to 50% when annealed at temperatures of 1800° F. to 2200° F. as compared to commercial heat of 25-6MO.  
         [0030]    The data shows that higher yield strength and elongation are obtained for the new alloy as compared to 25-6MO throughout the annealing temperature range. It is believed that the higher nickel or lower sigma solvus temperature contributes to the improved ductility whereas the higher molybdenum and nitrogen content provide the high strength for the alloy. Alloy 25-6MO has a high sigma solvus temperature that requires a high annealing temperature of 2200° F. (1204° C.). The alloy of the present invention may be annealed at reduced temperatures compared to conventional alloy 25-6MO which also results in increased strength.  
         [0031]    Thus, the alloy according to the present invention, with the combination of both a high PREN number (“pitting resistance equivalent number”) and a low sigma solvus temperature, provides superior corrosion resistance with the added advantage of easier processing. A low sigma solvus temperature allows hot rolling or forming operations with less danger of precipitating deleterious sigma phase. Also, final annealing can be performed at a lower temperature than materials which are more prone to sigma phase and require a higher solution annealing temperature to remove unwanted precipitation. Lower processing and annealing temperatures reduce unwanted oxidation, lower energy costs and provide a higher strength, fine grain size final product.  
                                                   TABLE 4                           Critical Pitting Temperature Test Results in ASTM G48, C       0.250° Plate, HR + 2200° F./½ hr. WQ                    Test                   Temperature   Max. Pit       Heat No.   Composition   ° C.   Depth., mils                    HV9117A   26Ni—20Cr—6.8Mo—.20N   70   0       ″   ″   75   0       ″   ″   80   5       ″   ″   85   5       HV9242A   28Ni—21Cr—7.2Mo—.35N   70   0       ″   ″   75   0       ″   ″   80   0       ″   ″   85   11       HV9244A   26Ni—20.7Cr—6.6Mo—.34N   70   0       ″   ″   75   0       ″   ″   80   20       ″   ″   85   5                                                  
 
         [0032]    [0032]                                                           TABLE 5                           Critical Crevice Temperature Test Results in ASTM G48, D       0.250″ Plate, HR + 2200° F./½ hr. WQ                    Test   Max. Crevice Attack   % Crevices       Heat No.   Composition   Temperature, ° C.   Depth, mils   Attacked                    HV9117A   26Ni—20Cr—6.8Mo—.20N   35   0   0       ″   ″   40   3   13       ″   ″   40   0   0       ″   ″   45   35   50       ″   ″   45   23   50       HV9242A   28Ni—21Cr—7.2Mo—.35N   35   0   0       ″   ″   40   0   0       ″   ″   45   0   0       ″   ″   50   0   0       ″   ″   55   0   0       ″   ″   60   85   100       HV9244A   26Ni—20.7Cr—6.6Mo—.34N   40   0   0       ″   ″   40   0   0       ″   ″   45   3   8       ″   ″   45   0   0       ″   ″   50   1   4       ″   ″   50   1   4       ″   ″   55   4   33       ″   ″   55   9   50                                                    
         [0033]    [0033]                                     TABLE 6                           Effect of Annealing Temperature on Room       Temperature Tensile Properties       Heat HV9242A: 28Ni—21Cr—7.2Mo—0.35N       0.125″ Strip, Cr 50% + Anneal            Annealing Temperature   0.2% Yield Strength Ksi   Tensile Strength Ksi   Elongation   Hardness Rb               2050° F. (1121° C.)/1/4 h, WQ   56.9   122.7   47.4   87       2100° F. (1149° C.)/1/4 h, WQ   61.0   120.8   48.3   86       2150° F. (1177° C.)/1/4 h, WQ   54.6   119.7   48.7   90                    
         [0034]    [0034]                                                                                                                 TABLE 7                           Effect of Annealing Temperature on RTT Properties, Strip CR 50% + Anneal as Indicated       Exp. 4022 HV9242A, Strip 0.150″, Test Drawing No. CR-15       Commercial 25-6MO Strip Z9237P, 0.063″ Test Drawing No. T-9A            Annealing Temp./   0.2% Y.S., Ksi   ULT, Ksi   % Elongation   Hardness, HRb            ¼ h, AC   HV9242A   25-6MO   HV9242A   25-6MO   HV9242A   25-6MO   HV9242A   25-6MO                    1800° F. (982° C.)   69.2   65.7   132.7   126.2   32.0   31.69   99   97       1900° F. (1038° C.)   66.0   59.4   129.9   119.3   39.6   37.5   97   94       1925° F. (1052° C.)   66.1   59.0   131.2   118.8   41.9   37.3   96   91       1950° F. (1066° C.)   60.5   55.5   128.5   113.6   44.0   40.0   93   88       1975° F. (1079° C.)   63.8*   46.3   129.0*   102.5   45.2*   45.2    93*    95*       2000° F. (1093° C.)   60.3   47.7   125.5   107.8   48.5   43.8   94   84       2100° F. (1149° C.)   54.2   43.1   122.3   98.8   50.0   42.0   92   84       2200° F. (1204° C.)   57.3   41.3   119.2   96.4   50.4   42.2   87   81                            
         [0035]    While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. The presently preferred embodiments described herein are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.