Abstract:
Precipitation hardening (PH) stainless steels heat treatable to yield strength levels in the range of 200 ksi with exceptionally high fracture toughness are achieved in alloys consisting essentially of 12.25-13.25% chromium, 7.5-8.5% nickel, 2.0-2.5% molybdenum, 0.8-1.35% aluminum, not over 0.05% carbon, not over 0.10% silicon, not over 0.10% manganese, not over 0.010% phosphorus and with especially critical amounts of not over 0.0020% (20 ppm) nitrogen, not over 0.0020% (20 ppm) sulfur, not over 0.0026% (26 ppm) nitrogen plus sulfur; not over 0.04% titanium, and remainder essentially Fe.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to stainless steels and in particular to 13-8Mo steels having significantly improved fracture toughness (K IC ) over conventional 13-8Mo steels. 
     It is well known to those skilled in the art that fracture toughness is a measure of a material&#39;s resistance to crack propagation and catastrophic failure and is an important characteristic in the design of certain critical components. Generally for metallic alloys, toughness is inversely related to strength, i.e. the higher the strength, the lower the toughness. Within this general relationship, individual alloys and families of alloys display distinctive relationships between strength and toughness. These characteristics can be clearly seen in FIG. 1. Precipitation hardening (PH) stainless steels, as a group, tend to be found in the less desirable, low strength, low toughness portion of this figure. 
     It is generally well known that small amounts of certain elements or impurities, including metallics, metalloids or non-metallics, can dramatically alter the properties of all alloys. The specific elements or impurities and the amounts which result in harmful effects vary widely, depending upon the alloy, the condition and the properties of interest. For example, in 13-8Mo steels as described in U.S. Pat. No. 3,556,776 to Clarke et al. which is hereby incorporated in its entirety by reference, critically low levels of manganese, silicon, phosphorus, sulfur and nitrogen resulted in good ductility in combination with great strength. 
     SUMMARY OF THE INVENTION 
     In this invention we have discovered that with precipitation hardening stainless steels of the type known commercially as 13-8Mo, the toughness can be raised to exceptionally high values if the nitrogen and sulfur content is controlled to very low levels. Additionally, it is preferred that the titanium content be controlled to within a desired range. In particular, we have discovered that exceptionally high values of toughness are achieved if the sulfur does not exceed 0.0025% (25 ppm), nitrogen does not exceed 0.0020% (20 ppm) and titanium, if present, is less than 0.05% and preferably does not exceed 0.04%. Furthermore, the combined amount of sulfur plus nitrogen should not exceed 0.0030% (30 ppm). 
     We have further discovered that at or below these critical limits of N 2 , S and Ti, the rate of improvement with decreasing amounts of these elements is significantly increased over that which would occur at higher concentrations that are more typical of levels seen in commercial practice. This effect is clearly shown by the change in slope of curves 2 through 6. 
     The precipitation hardening stainless steels to which the present invention applies may be described as consisting essentially of about 12.25% to 13.25% chromium, about 7.5% to 8.5% nickel, about 2.0% to 2.5% molybdenum, about 0.8% to 1.35% aluminum, not exceeding 0.05% carbon, not exceeding 0.10% silicon, not exceeding 0.10% manganese, not exceeding 0.10% phosphorus, not exceeding 0.0025% sulfur, not exceeding 0.0020% nitrogen, and remainder essentially iron, and wherein the combined amount of sulfur plus nitrogen does not exceed 0.0030%. Preferably, titanium, if present, is less than 0.050%, and more preferably does not exceed 0.04%. In a more specific aspect, the combined sulfur plus nitrogen content should not exceed 0.0020% (20 ppm) and Ti should not exceed 0.02%. 
     Steels of this invention show fracture toughnesses at yield strength levels of up to about 200 ksi of greater than 200 ksi-in 1/2 , which far exceeds those of a wide variety of contemporary, commercial high strength steels, as well as the PH steels, as shown in FIG. 1. 
     The levels of impurity elements required to achieve the noted improvements are significantly lower than those obtained in normal commercial practice for alloys of this type and can only be achieved with careful selection of raw materials with low nitrogen content and special melting practices such as vacuum induction melting and vacuum arc remelting. 
     Thus, in a further aspect, the present invention provides a method for improving the fracture toughness of stainless steels of the type which have an iron base with 12.25% to 13.25% chromium, 7.5% to 8.5% nickel, 2.0% to 2.5% molybdenum, and 0.8% to 1.35% aluminum. The method comprises melting selected raw materials under controlled conditions to achieve in the stainless steel a sulfur content not exceeding 0.0025%, a nitrogen content not exceeding 0.0020%, a titanium content of less than 0.05%, and a combined amount of sulfur plus nitrogen not exceeding 0.0030%. 
     The present invention further provides a method for producing a stainless steel article of high fracture toughness, wherein a stainless steel is produced which consists essentially of an iron base with 12.25% to 13.25% chromium, 7.5% to 8.5% nickel, 2.0% to 2.5% molybdenum, 0.8% to 1.35% aluminum, not exceeding 0.05% carbon, not exceeding 0.10% silicon, not exceeding 0.10% manganese, not exceeding 0.10% phosphorus, not exceeding 0.0025% sulfur, not exceeding 0.0020% nitrogen, and not exceeding 0.04% titanium; and the stainless steel is heat treated to produce a precipitation hardened stainless steel article having a fracture toughness at yield strength levels below 200 ksi of greater than 200 ksi-in 1/2 . Standard industry heat treatment processes are employed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Some of the features and advantages of the invention having been stated, others will become apparent from the detailed description which follows, and from the accompanying drawings, in which: 
     FIG. 1 is a graph showing the fracture toughness of various steels as a function of yield strength; 
     FIG. 2 is a graph showing the effect of nitrogen content on fracture toughness of precipitation hardening 13Cr-8Ni-2Mo precipitation hardening steel at different sulfur levels; 
     FIG. 3 is a graph showing the effect of nitrogen content on Charpy impact energy of precipitation hardening 13Cr-8Ni-2Mo steel at -22° F. for different sulfur levels; 
     FIG. 4 is a graph showing the effect of combined nitrogen and sulfur content on fracture toughness of 13Cr-8Ni-2Mo steel; 
     FIG. 5 is a graph showing the effect of titanium content on subsize fracture toughness of 13Cr-8Ni-2Mo steel at different impurity levels of nitrogen and sulfur; and 
     FIG. 6 is a graph showing the effect of titanium content on Charpy impact energy of 13Cr-8Ni-2Mo steel at -22° F. at different impurity levels of nitrogen and sulfur. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     To determine the effects of certain elements on fracture toughness, a number of experimental heats were made. The only variables were aluminum, titanium, sulfur and nitrogen. All other elements were held constant and were well within normal analytical variation (Table 1). All heats weighed 150 lbs and were produced by vacuum induction melting followed by vacuum arc remelting to 5.5 inch diameter ingots. Ingots were forged to three inch square from 2000° F., then subsequently rolled to 1&#34;×3.5&#34; flat bar from 1800° F. Test samples were cut from this bar in both longitudinal and transverse orientations and heat treated to the industry standard conditions, i.e. 1700° F. solution plus 1000° F. (H1000) or 1050° F. (H1050) age. Standard ASTM E23 impact specimens were machined and tested. Because of the extremely high toughness of this material, subsize fracture toughness testing based on J-integral concept was performed, as described in ASTM STP514, P.1-39, 1972, leading to toughness value K IJ  which is equivalent to K IC   
     Fracture toughness and impact results for steels prepared for this study are presented in Tables 2 and 3, respectively, along with the varying chemical elements (Al, Ti, S and N 2 ) and corresponding tensile properties. Because toughness varies so dramatically with yield strength, it is necessary to examine the effects of any given variable at a constant strength level which equates to a reasonably narrow aluminum content and a constant aging temperature. Thus the effect of nitrogen and sulfur contents on fracture toughness is presented in FIG. 2 for steels with 1.02-1.07% Al and yield strengths of 202-208 ksi. 
     From this figure it is apparent that N 2  does not exert a significant influence on fracture toughness at levels of about 30 to 100 ppm which corresponds to the range most often seen in commercial practice and which is reasonably consistent with U.S. Pat. No. 3,556,776. However, at N 2  levels of less than about 26 ppm, a dramatic, upward change in the slope of the fracture toughness vs. nitrogen content curve occurs and toughness doubles at 9 ppm nitrogen for the lowest sulfur content materials (&lt;10 ppm S). Although the same general trend occurs for higher sulfur content materials, the level of toughness improvement at the lowest nitrogen contents is depressed somewhat or conversely the improvement in toughness with decreasing N 2  for steels of the present invention is greatest at the lowest possible sulfur contents. Almost identical results were observed for transverse Charpy Impact Toughness values measured at -22° F., as seen in FIG. 3. 
     The combined effect of N 2  +S on toughness for steels of varying strength levels is shown in FIG. 4. From this figure it is also apparent there is a very abrupt change in the response of toughness to the combined effects of N 2  +S. Between 30 or 40 ppm and 130 ppm N 2  +S, there is little effect on toughness. Below this level, however, the slope of the curves again increase dramatically with toughness, more than doubling at the lowest N 2  +S contents for steels of both strength ranges shown. The critical N 2  +S contents for this abrupt change in toughness occur at a lower level for steels of the higher yield strengths. 
     Titanium is frequently added to steels of this type, as described in U.S. Pat. No. 3,556,776, at levels of 0.05 to 0.50%. Like N 2 , it has been discovered in accordance with the present invention that restricting Ti to levels much lower than normally employed is essential to achieving significantly improved toughness. The dramatic toughness improvements noted above for ultra low N 2  +S levels can only be obtained with levels of Ti substantially less than 0.05%. This is seen clearly from FIGS. 5 and 6. With Ti levels of 0.05% to 0.10%, there is almost no change in toughness. Below 0.05% Ti, the slope of both fracture toughness and Charpy Impact curves increase dramatically, nearly doubling at 0.02% Ti, but only for the low N 2  heats. For the higher N 2  and higher S heats, there is no consistent effect of Ti content within the range investigated. For purposes of the present invention, the titanium content should be less than 0.05% and preferably should not exceed 0.04%, and most desirably should not exceed 0.02%. 
     Fracture toughness of steels that comprise this invention is plotted as a function of yield strength in FIG. 1. While the curve appears to be quite steep, similar to other commercial steels HP 9-4-20 and HP 9-4-30, toughnesses at levels of below about 200 ksi Y.S. are outstanding (&gt;260 ksi-in 1/2 ) and are significantly higher than other commercial high strength alloys, especially other PH steels. 
     Those skilled in the art will recognize that the steel of the present invention can be employed in all of the applications where conventional precipitation hardening 13-8Mo steel has been employed, and its dramatically enhanced toughness opens the possibility for uses in additional applications where high toughness is important. It will also be understood that all references herein to percentages and to parts per million (ppm) are calculated on a weight/weight basis. 
     The present invention is not limited to the specific examples given above, which are intended to be illustrative of the present invention but not restrictive. 
     
                                           TABLE 1__________________________________________________________________________Chemistry of Test SteelsTest Chemistry (wt. %)            PPMSteelC  Si Mn Cr  Ni Mo Ti Al P   S  N__________________________________________________________________________G999-1.035   0.04      0.01         12.44             8.26                2.19                   0.02                      0.77                         &lt;.003                             22 7WA06-1.035   0.01      0.01         12.58             8.39                2.20                   0.02                      0.77                         &lt;.003                             5  9WB-18.036   0.01      0.01         12.38             8.25                2.20                   0.03                      0.81                         &lt;.003                             6  38WA01-1.033   0.01      0.01         12.51             8.31                2.22                   0.02                      1.06                         &lt;.003                             22 4WD13 .037   0.01      0.01         12.46             8.34                2.24                   0.01                      1.04                         .003                             48 26WA02 .033   0.01      0.01         12.49             8.31                2.22                   0.05                      1.07                         &lt;.003                             20 13WA01-2.033   0.01      0.01         12.51             8.36                2.22                   0.09                      1.06                         &lt;.003                             22 10WA09-1.034   0.01      0.01         12.52             8.34                2.21                   0.02                      1.06                         &lt;.003                             33 97WA10 .034   0.01      0.01         12.51             8.28                2.20                   0.05                      1.05                         &lt;.003                             31 57WA09-2.034   0.01      0.01         12.49             8.31                2.21                   0.09                      1.06                         &lt;.003                             32 82WA06-2.034   0.01      0.01         12.47             8.31                2.20                   0.02                      1.03                         &lt;.003                             6  9WD15 .035   0.01      0.01         12.51             8.32                2.22                   0.05                      1.06                         .003                             6  7WD16 .036   0.01      0.01         12.49             8.30                2.21                   0.09                      1.02                         .003                             7  9WD17 .034   0.01      0.01         12.54             8.38                2.24                   0.01                      1.03                         .003                             6  27WD14 .035   0.01      0.01         12.49             8.30                2.23                   0.01                      1.07                         .003                             10 40WD19 .034   0.01      0.01         12.57             8.29                2.22                   0.01                      1.05                         &lt;.003                             6  72WD22-1.032   0.01      0.01         12.56             8.31                2.22                   0.01                      1.02                         &lt;.003                             6  43WB-19.036   0.01      0.01         12.35             8.27                2.21                   0.03                      1.04                         &lt;.003                             6  37WD18 .034   0.01      0.01         12.56             8.31                2.23                   0.05                      0.99                         .033                             6  35WA07-2.035   0.01      0.01         12.45             8.33                2.20                   0.10                      1.04                         &lt;.003                             6  41WD20 .034   0.01      0.01         12.64             8.44                2.24                   0.01                      1.31                         .003                             5  8AMS  .05   .10      .10         12.25/             7.5/                2.00/                   /  0.90/                         0.01                             80 1005629 max   max      max         13.25             8.5                2.50  1.35                         max max                                max__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________Tensile Properties and Toughness of 13Cr--8Ni--2Mo Steels(1&#34; Thick Flat Bar Heat Treated 1700° F. × 1 Hr, AC to&lt;300° F.,IWQ + 1050° F. × 4 Hrs, AC to &lt;100° F., IWQ for 30min.)Chemistry        Tensile      S  N.sub.2            0.2% YS                 UTS        K.sub.IJ (Ksi-in.sup.1/2)Heat No.Al %   Ti %      ppm         ppm            Ksi  Ksi                    % EI                        % RA                            Longitudinal                                  Transverse__________________________________________________________________________Steels of this invention:WA06-21.03   0.02      6  9  203.0                 212.2                    16.9                        68.2                            242.0 221.7            201.7                 212.7                    16.8                        67.3                            238.6 220.5WA01-11.06   0.02      22 4  204.3                 213.6                    17.3                        69.5                            --    178.6            204.6                 213.4                    16.8                        69.1                            180.5 180.9G999-10.77   0.02      22 7  182.4                 192.3                    15.5                        61.8                            330.0 299.7            189.4                 196.7                    16.7                        62.1                            327.8 327.2WA06-10.77   0.02      5  9  186.7                 196.9                    18.9                        73.4                            416.6 361.0            184.9                 193.2                    17.9                        73.8                            402.2 379.8WD20 1.31   0.01      5  8  221.1                 228.8                    13.7                        61.6                            94.5  91.2            220.5                 227.6                    13.3                        61.6                            95.7  84.8Steels not of this invention:WD13 1.04   0.01      48 26 206.1                 212.0                    14.1                        60.9                            118.6 114.9            208.4                 214.8                    13.6                        62.5                            121.3 111.1WD17 1.03   0.01      6  27 205.5                 210.9                    14.4                        66.2                            123.1 117.4            207.5                 212.3                    13.5                        64.5                            121.9 122.6WD22-11.02   0.01      6  43 208.3                 213.1                    14.0                        65.9                            118.5 124.9            202.1                 206.6                    14.6                        67.3                            119.8 123.6WD14 1.07   0.01      10 40 207.8                 214.3                    13.8                        64.0                            138.1 126.9            203.3                 207.5                    13.3                        65.4                            129.7 125.6WD19 1.05   0.01      6  72 211.9                 217.5                    14.0                        62.5                            105.5 96.1            204.9                 210.2                    13.2                        63.0                            99.0  102.2WA09-11.06   0.02      33 97 202.3                 213.1                    15.1                        58.2                            120.0 65.1            199.5                 210.3                    14.9                        56.6                            99.5  71.0WB18 0.81   0.02      6  38 187.7                 195.8                    17.8                        73.0                            133.5 115.4            191.2                 199.5                    18.6                        71.7                            192.2 126.6WD08-10.81   0.02      38 88 188.3                 197.1                    17.9                        73.6                            101.7 78.5            186.8                 195.1                    18.5                        73.0                            102.7 76.5WB19 1.04   0.03      6  37 204.1                 213.6                    17.0                        67.7                            95.7  100.1            203.3                 212.8                    16.5                        69.3                            102.0 82.8WD15 1.06   0.05      6  7  211.5                 217.7                    17.1                        71.7                            122.0 111.7            215.2                 220.9                    16.3                        71.9                            121.8 113.1WD16 1.02   0.09      7  9  212.6                 219.8                    15.1                        70.4                            121.4 111.9            210.6                 217.9                    14.5                        72.3                            117.5 112.8WA02 1.07   0.05      20 13 210.9                 220.9                    16.4                        69.6                            119.7 93.4            212.5                 222.2                    16.8                        70.8                            110.5 104.2WA10 1.05   0.05      31 57 203.6                 214.5                    17.0                        66.4                            101.0 104.0            --   -- --  --  97.5  108.0WD18 0.99   0.05      6  35 211.3                 218.1                    13.7                        66.8                            98.2  87.4            210.2                 215.8                    14.3                        68.4                            95.5  89.1WA09-21.06   0.09      32 82 214.6                 220.2                    16.1                        63.0                            103.7 83.2            208.2                 220.2                    16.0                        63.1                            94.9  92.3WA07-21.04   0.10      6  41 212.9                 225.9                    16.2                        66.3                            100.1 93.3            212.4                 224.6                    16.9                        67.4                            103.9 100.7WA01-21.06   0.09      22 10 207.6                 220.0                    16.9                        69.4                            87.1  83.8            208.1                 219.1                    17.6                        68.2                            84.0  78.3__________________________________________________________________________ 
    
     
                                           TABLE 3__________________________________________________________________________Tensile &amp; Impact Properties of 13Cr--8Ni--2Mo Steels(1&#34; Thick Flat Bar Heat Treated 1700° F. × 1 Hr, AC to&lt;300° F.,IWQ + 1050° F. × 4 Hrs, AC to &lt;100° F., IWQ for 30min.)  Chemistry   Tensile         Charpy Impact - ft-lbs        S  N.sub.2              0.2% YS                   UTS        Longitudinal                                      TransverseHeat No.  Al %     Ti %        ppm           ppm              Ksi  Ksi                      % EI                          % RA                              RT -22° F.                                      RT -22° F.__________________________________________________________________________Steels of this invention:WA06-2 1.03     0.02        6  9  181  188                      19  74  146                                 160  145                                         144              181  188                      19  74  173                                 157  153                                         139WA01-1 1.06     0.02        22 4  184  192                      19  73  136                                 133  136                                         133              184  193                      18  74  143                                 135  127                                         --Steels not of this invention:WD13   1.04     0.01        48 26 182  186                      15  67  72 63   55 48              184  180                      16  68  65 63   55 49WD17   1.03     0.01        6  27 176  180                      17  71  104                                 89   78 55              187  190                      16  68  91 55   83 76WD22-1 1.02     0.01        6  43 185  188                      16  71  87 88   73 65              176  180                      17  72  82 76   75 65WD14   1.07     0.01        10 40 184  188                      15  70  86 70   56 54              184  187                      17  71  80 74   60 51WD19   1.05     0.01        6  72 187  191                      16  67  66 52   42 35              183  187                      17  69  60 47   49 35WA09-1 1.06     0.02        33 97 179  186                      16  61  41 45   25 27              181  189                      17  61  47 40   26 23U.S. Pat.  1.0     -- 30 18 188  197                      14  68  120                                 --   -- --No. 3,556,776      185  194                      15  70  102                                 --   -- --WB19   1.04     0.03        6  37 185  193                      19  72  111                                 55   109                                         53              183  191                      18  73  129                                 60   109                                         49WA02   1.07     0.05        20 13 182  188                      19  73  160                                 87   125                                         54              190  197                      18  73  164                                 126  129                                         62WA10   1.05     0.05        31 57 184  191                      18  72  119                                 64   78 53              182  191                      19  70  110                                 72   83 49WD15   1.06     0.05        6  7  195  197                      18.1                          74.6                              156                                 116  128                                         75              186  188                      18.1                          74.4                              168                                 115  102                                         58WD18   0.99     0.05        6  35 184  187                      18  73  99 79   77 36              182  186                      17  74  99 64   68 43WD16   1.02     0.09        7  9  200  205                      17  74  105                                 69   95 47              199  203                      17  74  124                                 80   96 55WA07-1 1.04     0.10        6  41 193  201                      17  70  112                                 73   -- 46              190  197                      18  70  115                                 50   74 45WA01-2 1.06     0.09        22 10 191  199                      19  72  122                                 63   101                                         39              195  204                      18  71  81 53   65 30WA09-2 1.06     0.09        32 82 197  203                      17  66  65 30   49 30              190  198                      17  68  48 30   75 22__________________________________________________________________________