Abstract:
An age hardenable martensitic steel alloy having a unique combination of very high strength and good toughness consists essentially of, in weight percent, about 
     
       ______________________________________ 
     
        C           0.21-0.34   Mn          0.20 max.   Si          0.10 max.   P           0.008 max.   S           0.003 max.   Cr          1.5-2.80   Mo          0.90-1.80   Ni         10-13   Co         14.0-22.0   Al          0.1 max.   Ti          0.05 max.   Ce          0.030 max.   La          0.010 max.______________________________________ 
     the balance essentially iron. In addition, cerium and sulfur are balanced so that the ratio Ce/S is at least about 2 and not more than about 15. A small but effective amount of calcium can be present in place of some or all of the cerium and lanthanum.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an age hardenable martensitic steel alloy, and in particular, to such an alloy which provides a unique combination of very high strength with an acceptable level of fracture toughness. 
     BACKGROUND OF THE INVENTION 
     A variety of applications require the use of an alloy having a combination of high strength and high toughness. For example, ballistic tolerant applications require an alloy which maintains a balance of strength and toughness such that spalling and shattering are suppressed when the alloy is impacted by a projectile, such as a .50 caliber armor piercing bullet. Other possible uses for such alloys include structural components for aircraft, such as landing gear or main shafts of jet engines, and tooling components. 
     Heretofore, a ballistic tolerant alloy steel has been described having the following composition in weight percent: 
     
         ______________________________________   C   0.38-0.43   Mn  0.60-0.80   Si  0.20-0.35   Cr  0.70-0.90   Mo  0.20-0.30   Ni  1.65-2.00   Fe  Balance______________________________________ 
    
     The alloy is treated by oil quenching from 843° C. (1550° F.) followed by tempering. Tempering to a hardness of HRC 57 provides the best ballistic performance as measured by the V 50  velocity. The V 50  velocity is the velocity of a projectile at which there is a 50% probability that the projectile will penetrate the armor. However, when tempered to a hardness of HRC 57, the alloy is prone to cracking, shattering, and petal formation and the multiple hit performance of the alloy is severely degraded. To obtain the best combination of V 50  performance and freedom from cracking, shattering, and petal formation, the alloy is tempered to a hardness of HRC 53. However, in order to provide effective anti-projectile performance at the lower hardness, thicker sections of the alloy must be used. The use of thicker sections is not practical for many applications, such as aircraft, because of the increased weight in the manufactured component. 
     Another alloy, with better resistance to shattering, cracking, and petal formation, has also been described. The alloy has the following composition in weight percent: 
     
         ______________________________________   C   0.12-0.17   Cr  1.8-3.2   Mo   0.9-1.35   Ni   9.5-10.5   Co  11.5-14.5   Fe  Balance______________________________________ 
    
     Although that alloy is resistant to cracking and shattering when penetrated by a high velocity projectile because of its good impact toughness, the alloy leaves much to be desired as an armor material since it has a peak aged hardness of HRC 52. Therefore, in order to provide effective anti-projectile performance, undesirably thick sections of the alloy must be used. As described above, the use of thick sections is impractical for aircraft. 
     In addition, an alloy has been described having the following composition, in weight percent: 
     
         ______________________________________   C           0.40-0.46   Mn          0.65-0.90   Si          1.45-1.80   Cr          0.70-0.95   Mo          0.30-0.45   Ni          1.65-2.00   V           0.05 min.   Fe          Balance______________________________________ 
    
     The alloy is capable of providing a tensile strength in the range of 1931-2068 MPa (280-300 ksi) and a fracture toughness, as represented by a stress intensity factor, K Ic , of about 60.4-65.9 MPa√m (55-60 ksi√in.). 
     High strength, high fracture toughness, age hardenable martensitic alloys have been described having the following compositions in weight percent: 
     
         ______________________________________    Alloy I    Alloy II______________________________________C          0.2-0.33     0.2-0.33Mn         0.2 max.     0.20 max.Si         0.1 max.     0.1 max.P          0.008 max.   0.008 max.S          0.004 max.   0.0040 max.Cr         2-4          2-4Mo         0.75-1.75    0.75-1.75Ni         10.5-15      10.5-15Co         8-17         8-17Al         0.01 max.    0.01 max.Ti         0.01 max.    0.02 max.Ce         Trace-0.001  Small but effective                   amount up to 0.030La         Trace-0.001  Small but effective                   amount up to 0.01Fe         Balance      Balance______________________________________ 
    
     Those alloys are capable of providing a fracture toughness as represented by a stress intensity factor, K Ic , of ≧109.9 MPa√m (≧100 ksi√in.) and a strength as represented by an ultimate tensile strength, UTS, of about 1931-2068 MPa (280-300 ksi). 
     However, a need has arisen for an alloy having an even higher strength than the known alloys to provide improved ballistic performance and stronger structural components. It is known that fracture toughness is inversely related to yield strength and ultimate tensile strength. Therefore, the alloy should also provide a sufficient level of fracture toughness for adequate reliability in components and to permit non-destructive inspection of structural components for flaws which can result in catastrophic failure. 
     SUMMARY OF THE INVENTION 
     The alloy according to the present invention is an age hardenable martensitic steel that provides significantly higher strength while maintaining an acceptable level of fracture toughness relative to the known alloys. In particular, the alloy of the present invention is capable of providing an ultimate tensile strength (UTS) of at least about 2068 MPa (300 ksi) and a K Ic  fracture toughness of at least about 71.4 MPa√m (65 ksi√in.) in the longitudinal direction. The alloy of the present invention is also capable of providing a UTS of at least about 2137 MPa (310 ksi) and a K Ic  fracture toughness of at least about 65.9 MPa√m (60 ksi√in.) in the longitudinal direction. 
     The broad and preferred compositional ranges of the age-hardenable, martensitic steel of the present invention are as follows, in weight percent: 
     
         ______________________________________      Broad       Preferred______________________________________C            0.21-0.34     0.22-0.30Mn           0.20 max.     0.05 max.Si           0.10 max.     0.10 max.P            0.008 max.    0.006 max.S            0.003 max.    0.002 max.Cr           1.5-2.80      1.80-2.80Mo           0.90-1.80     1.10-1.70Ni           10-13         10.5-11.5Co           14.0-22.0     14.0-20.0Al           0.1 max.      0.01 max.Ti           0.05 max.     0.02 max.Ce           0.030 max.    0.01 max.La           0.010 max.    0.005 max.______________________________________ 
    
     The balance of the alloy is essentially iron except for the usual impurities found in commercial grades of such steels and minor amounts of additional elements which may vary from a few thousandths of a percent up to larger amounts that do not objectionably detract from the desired combination of properties provided by this alloy. 
     The alloy of the present invention is critically balanced to consistently provide a superior combination of strength and fracture toughness compared to the known alloys. To that end, carbon and cobalt are balanced so that the ratio Co/C is at least about 43, preferably at least about 52, and not more than about 100, preferably not more than about 75. 
     In one embodiment, the alloy contains up to about 0.030% cerium and up to about 0.010% lanthanum. Effective amounts of cerium and lanthanum are present when the ratio of cerium to sulfur (Ce/S) is at least about 2 and not more than about 15. Preferably, the Ce/S ratio is not more than about 10. 
     In another embodiment, a small but effective amount of calcium and/or other sulfur-gettering element is present in the alloy in place of some or all of the cerium and lanthanum. For best results, at least about 10 ppm calcium or sulfur-gettering element other than calcium is present in the alloy. 
     The foregoing tabulation is provided as a convenient summary and is not intended thereby to restrict the lower and upper values of the ranges of the individual elements of the alloy of this invention for use in combination with each other, or to restrict the ranges of the elements for use solely in combination with each other. Thus, one or more of the element ranges of the broad composition can be used with one or more of the other ranges for the remaining elements in the preferred composition. In addition, a minimum or maximum for an element of one preferred embodiment can be used with the maximum or minimum for that element from another preferred embodiment. Throughout this application, unless otherwise indicated, percent (%) means percent by weight. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The alloy according to the present invention contains at least about 0.21% and preferably at least about 0.22% carbon. Carbon contributes to the good strength and hardness capability of the alloy primarily by combining with other elements, such as chromium and molybdenum, to form M 2  C carbides during an aging heat treatment. However, too much carbon adversely affects fracture toughness, room temperature Charpy V-notch (CVN) impact toughness, and stress corrosion cracking resistance. Accordingly, carbon is limited to not more than about 0.34% and preferably to not more than about 0.30%. 
     Cobalt contributes to the very high strength of this alloy and benefits the age hardening of the alloy by promoting heterogeneous nucleation sites for the M 2  C carbides. In addition, we have observed that the addition of cobalt to promote strength is less detrimental to the toughness of the alloy than the addition of carbon. Accordingly, the alloy contains at least about 14.0% cobalt. For example, at least about 14.3%, 14.4%, or 14.5% cobalt is present in the alloy. Preferably at least about 15.0% cobalt is present in the alloy. However, for applications requiring a particularly high strength alloy, at least about 16.0% cobalt may be present in the alloy. Because cobalt is an expensive element, the benefit obtained from cobalt does not justify using unlimited amounts of it in this alloy. Therefore, cobalt is restricted to not more than about 22.0% and preferably to not more than about 20.0%. 
     Carbon and cobalt are controlled in the alloy of the present invention to benefit the superior combination of very high strength and high toughness. We have observed that increasing the ratio of cobalt to carbon (Co/C) promotes increased toughness and a better combination of strength and toughness in this alloy. Further, increasing the Co/C ratio benefits the notch toughness of the alloy. Accordingly, cobalt and carbon are controlled in the present alloy such that the ratio Co/C is at least about 43 and preferably at least about 52. However, the benefits from a high Co/C ratio are offset by the high cost of producing an alloy having a Co/C ratio that is too high. Therefore, the Co/C ratio is restricted to not more than about 100 and preferably to not more than about 75. 
     Chromium contributes to the good strength and hardness capability of this alloy by combining with carbon to form M 2  C carbides during the aging process. Therefore, at least about 1.5% and preferably at least about 1.80% chromium is present in the alloy. However, excessive chromium increases the sensitivity of the alloy to averaging. In addition, too much chromium results in increased precipitation of carbide at the grain boundaries, which adversely affects the alloy&#39;s toughness and ductility. Accordingly, chromium is limited to not more than about 2.80% and preferably to not more than about 2.60%. 
     Molybdenum, like chromium, is present in this alloy because it contributes to the good strength and hardness capability of this alloy by combining with carbon to form M 2  C carbides during the aging process. Additionally, molybdenum reduces the sensitivity of the alloy to averaging and benefits stress corrosion cracking resistance. Therefore, at least about 0.90% and preferably at least about 1.10% molybdenum is present in the alloy. However, too much molybdenum increases the risk of undesirable grain boundary carbide precipitation, which would result in reduced toughness and ductility. Therefore, molybdenum is restricted to not more than about 1.80% and preferably to not more than about 1.70%. 
     At least about 10% and preferably at least about 10.5% nickel is present in the alloy because it benefits hardenability and reduces the alloy&#39;s sensitivity to quenching rate, such that acceptable CVN toughness is readily obtainable. Nickel also benefits the stress corrosion cracking resistance, the K Ic  fracture toughness and Q-value (defined as  (HRC-35) 3  ×(CVN)÷1000!, where CVN is measured in ft-lbs) measured at -54° C. (-65° F.). However, excessive nickel promotes an increased sensitivity to averaging. Therefore, nickel is restricted in the alloy to not more than about 13% and preferably to not more than about 11.5%. 
     Other elements can be present in the alloy in amounts which do not detract from the desired properties. Not more than about 0.20% and better yet not more than about 0.10% manganese is present because manganese adversely affects the fracture toughness of the alloy. Preferably, manganese is restricted to not more than about 0.05%. Also, up to about 0.10% silicon, up to about 0.1% aluminum, and up to about 0.05% titanium can be present as residuals from small deoxidation additions. Preferably, the aluminum is restricted to not more than about 0.01% and titanium is restricted to not more than about 0.02%. 
     Small but effective amounts of elements that provide sulfide shape control are present in the alloy to benefit the fracture toughness by combining with sulfur to form sulfide inclusions that do not adversely affect fracture toughness. A similar effect is described in U.S. Pat. No. 5,268,044, which is incorporated herein by reference. In one embodiment of the present invention, the alloy contains up to about 0.030% cerium and up to about 0.010% lanthanum. The preferred method of providing cerium and lanthanum in this alloy is through the addition of mischmetal during the melting process in an amount sufficient to recover effective amounts of cerium and lanthanum in the as-cast VAR ingot. Effective amounts of cerium and lanthanum are present when the ratio of cerium to sulfur (Ce/S) is at least about 2. When the Ce/S ratio is more than about 15, the hot workability and tensile ductility of the alloy are adversely affected. Preferably, the Ce/S ratio is not more than about 10. To ensure good hot workability, for example, when the alloy is to be press forged as opposed to rotary forged, the alloy contains not more than about 0.01% cerium and not more than about 0.005% lanthanum. In another embodiment of this alloy, a small but effective amount of calcium and/or other sulfur-gettering elements, such as magnesium or yttrium, is present in the alloy in place of some or all of the cerium and lanthanum to provide the beneficial sulfide shape control. For best results, at least about 10 ppm calcium or sulfur-gettering element other than calcium is present in the alloy. Preferably, the calcium is balanced so that the ratio Ca/S is at least about 2. 
     The balance of the alloy is essentially iron except for the usual impurities found in commercial grades of alloys intended for similar service or use. The levels of such elements must be controlled to avoid adversely affecting the desired properties. For example, phosphorous is restricted to not more than about 0.008% and preferably to not more than about 0.006% because of its embrittling effect on the alloy. Sulfur, although inevitably present, is restricted to not more than about 0.003%, preferably to not more than about 0.002%, and better still to not more than about 0.001% because sulfur adversely affects the fracture toughness of the alloy. 
     The alloy of the present invention is readily melted using conventional vacuum melting techniques. For best results, a multiple melting practice is preferred. The preferred practice is to melt a heat in a vacuum induction furnace (VIM) and cast the heat in the form of an electrode. The alloying addition for sulfide shape control referred to above is preferably made before the molten VIM heat is cast. The electrode is then vacuum arc remelted (VAR) and recast into one or more ingots. Prior to VAR, the electrode ingots are preferably stress relieved at about 677° C. (1250° F.) for 4-16 hours and air cooled. After VAR, the ingot is preferably homogenized at about 1177°-1232° C. (2150°-2250° F.) for 6-24 hours. 
     The alloy can be hot worked from about 1232° C. (2250° F.) to about 816° C. (1500° F.). The preferred hot working practice is to forge an ingot from about 1177°-1232° C. (2150°-2250° F.) to obtain at least about a 30% reduction in cross-sectional area. The ingot is then reheated to about 982° C. (1800° F.) and further forged to obtain at least about another 30% reduction in cross-sectional area. 
     Heat treating to obtain the desired combination of properties proceeds as follows. The alloy is austenitized by heating it at about 843°-982° C. (1550°-1800° F.) for about 1 hour plus about 5 minutes per inch of thickness and then quenching. The quench rate is preferably rapid enough to cool the alloy from the austenizing temperature to about 66° C. (150° F.) in not more than about 2 hours. The preferred quenching technique will depend on the cross-section of the manufactured part. However, the hardenability of this alloy is good enough to permit air cooling, vermiculite cooling, or inert gas quenching in a vacuum furnace, as well as oil quenching. After the austenitizing and quenching treatment, the alloy is preferably cold treated as by deep chilling at about -73° C. (-100° F.) for about 0.5-1 hour and then warmed in air. 
     Age hardening of this alloy is preferably conducted by heating the alloy at about 454°-510° C. (850°-950° F.) for about 5 hours followed by cooling in air. 
     The alloy of the present invention is useful in a wide range of applications. The very high strength and good fracture toughness of the alloy makes it useful for ballistic tolerant applications. In addition, the alloy is suitable for other uses such as structural components for aircraft and tooling components. 
    
    
     EXAMPLES 
     Twenty laboratory VIM heats were prepared and cast into VAR electrode-ingots. Prior to casting each of the electrode-ingots, mischmetal or calcium was added to the respective VIM heats. The amount of each addition was selected to result in a desired retained amount of cerium, lanthanum, and calcium after refining. In addition, high purity electrolytic iron was used as the charge material to provide better control of the sulfur content in the VAR product. 
     The electrode-ingots were cooled in air, stress relieved at 677° C. (1250° F.) for 16 hours, and then cooled in air. The electrode-ingots were refined by VAR and vermiculite cooled. The VAR ingots were annealed at 677° C. (1250° F.) for 16 hours and air cooled. The compositions of the VAR ingots are set forth in weight percent in Tables 1 and 2 below. Heats 1-16 are examples of the present invention and Heats A-D are comparative alloys. 
     
                                           TABLE 1__________________________________________________________________________Heat No.1.sup.1  2.sup.2      3.sup.3          4.sup.4              5.sup.2                  6.sup.3                      7.sup.4                          8.sup.4                              9.sup.4                                  10.sup.2__________________________________________________________________________C  .249  .312      .311          .297              .296                  .256                      .258                          .294                              .341                                  .239Mn &lt;.01  &lt;.01      &lt;.01          &lt;.01              &lt;.01                  &lt;.01                      &lt;.01                          &lt;.01                              &lt;.01                                  &lt;.01Si &lt;.01  &lt;.01      &lt;.01          &lt;.01              &lt;.01                  &lt;.01                      &lt;.01                          &lt;.01                              &lt;.01                                  &lt;.01P  &lt;.005  &lt;.005      &lt;.005          &lt;.005              &lt;.005                  &lt;.005                      &lt;.005                          &lt;.005                              &lt;.005                                  &lt;.005S  &lt;.0005  &lt;.0005      &lt;.0005          &lt;.0005              &lt;.0005                  &lt;.0005                      &lt;.0005                          &lt;.0005                              &lt;.0005                                  &lt;.0005Cr 2.45  2.41      2.40          2.43              2.43                  1.45                      1.95                          2.43                              2.43                                  2.44Mo 1.41  1.40      1.46          1.60              1.70                  1.44                      1.44                          1.46                              1.45                                  1.48Ni 11.10  10.95      10.93          10.93              10.93                  10.95                      10.97                          10.94                              10.98                                  11.07Co 15.01  16.05      17.05          15.05              15.07                  15.02                      15.03                          15.03                              15.07                                  15.05Al &lt;.01  .004      .004          .004              .004                  .003                      .004                          .003                              .003                                  .004Ti .01 .009      .010          .010              .009                  .010                      .009                          .009                              .008                                  .007Ce .004  .002      .003          .003              .003                  .003                      .004                          .003                              .004                                  .004La .001  .001      .001          .001              .001                  .001                      .001                          .001                              .001                                  &lt;.001Ca --  --  --  --  --  --  --  --  --  --Ce/S.sup.5   10  5   8   8   8   8   10  8   10  10Co/C   60.3  51.4      54.8          50.7              50.9                  58.7                      58.2                          51.1                              44.2                                  63.0Fe Bal.  Bal.      Bal.          Bal.              Bal.                  Bal.                      Bal.                          Bal.                              Bal.                                  Bal.__________________________________________________________________________ .sup.1 Also contains &lt;0.01 Cu, &lt;5 ppm N, and 8 ppm O. .sup.2 Also contains &lt;5 ppm O and 5-8 ppm N. .sup.3 Also contains &lt;5 ppm O and &lt;5 ppm N. .sup.4 Also contains 5-7 ppm O and &lt;5 ppm N. .sup.5 When S is reported to be &lt;0.0005, the S content is assumed to be 0.0004 for calculation of the Ce/S ratio. 
    
     
                                           TABLE 2__________________________________________________________________________Heat No.11.sup.1  12.sup.1      13.sup.1          14.sup.1              15.sup.1                  16.sup.1                      A.sup.3                          B.sup.1                              C   D.sup.1__________________________________________________________________________C  .247  .243      .240          2.42              .247                  .250                      .236                          .238                              .252                                  .244Mn &lt;.01  &lt;.01      &lt;.01          &lt;.01              &lt;.01                  &lt;.01                      &lt;.01                          &lt;.01                              &lt;.01                                  &lt;.01Si .01 &lt;.01      &lt;.01          &lt;.01              &lt;.01                  &lt;.01                      &lt;.01                          &lt;.01                              &lt;.01                                  &lt;.01P  .001  .001      .001          .001              .001                  .001                      &lt;.005                          .001                              &lt;.005                                  .001S  &lt;.0005  &lt;.0005      &lt;.0005          .0006              &lt;.0005                  .0005                      &lt;.0005                          &lt;.0005                              &lt;.0005                                  &lt;.0009Cr 2.46  2.43      2.46          2.45              2.46                  2.44                      3.10                          2.43                              2.44                                  2.46Mo 1.46  1.47      1.46          1.47              1.48                  1.47                      1.16                          1.46                              1.48                                  1.48Ni 10.98  11.04      11.04          11.06              11.00                  11.06                      11.14                          11.02                              10.99                                  11.06Co 15.04  15.07      15.08          15.05              15.04                  125.06                      13.49                          15.05                              15.04                                  15.10Al .003  .006      .005          .003              .003                  .004                      .004                          .004                              &lt;.01                                  .003Ti .011  .010      .011          .010              .011                  .010                      .010                          .010                              .010                                  .011Ce .001  .001      .002          .001              .001                  .001                      .004                          &lt;.001                              .013                                  .001La .001  .001      .001          &lt;.001              &lt;.001                  &lt;.001                      &lt;.001                          &lt;.001                              .003                                  &lt;.001Ca &lt;.0005  &lt;.0005      &lt;.0005          &lt;.0005              .0010                  .0014                      --  &lt;.0005                              &lt;.0005                                  .0033Ce/S.sup.4   3   3   5   1.7 3   2.0 10  &lt;1.1                              33  1.1Co/C   60.9  62.0      62.8          62.2              60.9                  60.2                      57.2                          63.2                              59.7                                  61.9Fe Bal.  Bal.      Bal.          Bal.              Bal.                  Bal.                      Bal.                          Bal.                              Bal.                                  Bal.__________________________________________________________________________ .sup.1 The values reported are the average of a measurement taken at each end of the bar. .sup.2 The Ce/S ratio from measurements taken on the VIM dip samples is &lt;1.1. Since VAR is known to remove Ce, the product Ce/S ratio is assumed to be &lt;1.1. .sup.3 Also contains &lt;5 ppm O and &lt;5 ppm N. .sup.4 When S is reported to be &lt;0.0005, the S content is assumed to be 0.0004 for calculation of the Ce/S ratio. 
    
     I. Example 1 
     The VAR ingot of Example 1 was homogenized at 1232° C. (2250° F.) for 6 hours, prior to forging. The ingot was then press forged from the temperature of 1232° C. (2250° F.) to a 7.6 cm (3 in.) high by 12.7 cm (5 in.) wide bar. The bar was reheated to 982° C. (1800° F.), press forged to a 3.8 cm (1.5 in.) high by 10.2 cm (4 in.) wide bar, and then air cooled. The bar was normalized at 968° C. (1775° F.) for 1 hour and then cooled in air. The bar was then annealed at 677° C. (1250° F.) for 16 hours and air cooled. 
     Standard longitudinal and transverse tensile specimens (ASTM A 370-95a, 6.4 mm (0.252 in.) diameter by 2.54 cm (1 in.) gage length), CVN test specimens (ASTM E 23-96), and compact tension blocks for fracture toughness testing (ASTM E399) were machined from the annealed bar. The specimens were austenitized in salt for 1 hour at 913° C. (1675° F.) The tensile specimens and CVN test specimens were vermiculite cooled. Because of their thicker cross-section, the compact tension blocks were air cooled to insure that they experience the same effective cooling rate as the tensile and CVN specimens. All of the specimens were deep chilled at -73° C. (-100° F.) for 1 hour, then warmed in air. The specimens were age hardened at 482° C. (900° F.) for 6 hours and then air cooled. 
     The results of room temperature tensile tests on the longitudinal and transverse specimens of Example 1 are shown in Table 3 including the 0.2% offset yield strength (YS), the ultimate tensile strength (UTS), as well as the percent elongation (Elong) and percent reduction in area (RA). In addition, the results of room temperature fracture toughness testing on the compact tension specimens in accordance with ASTM Standard Test E 399 (K Ic ) are shown in the table. The longitudinal measurements were made on duplicate samples from three separately heat treated lots. The transverse measurements, however, were made on duplicate samples from two separately heat treated lots. 
     
                       TABLE 3______________________________________   Heat     YS      UTS   Elong                               RA   K.sub.ICOrientation   Treat Lot            (MPa)   (MPa) (%)  (%)  (MPam)______________________________________Long.   1        1902    2208  14.3 64.5 --            1928    2176  14.1 65.4 --   2        1877    2161  14.6 62.7 77.0            1924    2204  14.1 63.2 72.8   3        1901    2191  14.4 65.3 74.0            1895    2186  14.5 63.0 70.8   Average  1904    2188  14.3 64.0 73.6Trans.  1        1919    2195  13.9 59.4 68.7            1906    2183  27.1.sup.1                               57.5 67.9   2        1891    2180  14.2 60.5 72.7            1906    2187  13.5 58.9 64.0   Average  1905    2186  13.9 59.1 68.3______________________________________ .sup.1 Value not included in the average. 
    
     The data in Table 3 clearly show that Example 1 provides a combination of very high strength and good fracture toughness relative to the alloys discussed in the background section above. 
     II. Examples 2-10 
     For Examples 2-10, the VAR ingots were homogenized at 1232° C. (2250° F.) for 16 hours, prior to forging. The ingots were then press forged from the temperature of 1232° C. (2250° F.) to 8.9 cm (3.5 in.) high by 12.7 cm (5 in.) wide bars. The bars were reheated to 982° C. (1800° F.), press forged to 3.8 cm (1.5 in.) high by 11.4 cm (4.5 in.) wide bars, and then air cooled. The bars of each example were normalized at 954° C. (1750° F.) for 1 hour and then cooled in air. The bars were annealed at 677° C. (1250° F.) for 16 hours and then cooled in air. 
     Standard transverse tensile specimens, CVN specimens, and compact tensile blocks were machined, austenitized, quenched, and deep chilled similarly to Example 1. In addition, notched tensile specimens were processed similarly to the transverse tensile and CVN specimens. The samples were age hardened according to the conditions given in Table 4. The conditions in Table 4 were selected to provide a room temperature ultimate tensile strength of at least about 2034 MPa (295 ksi). 
     
                       TABLE 4______________________________________Heat No.  Age Hardening Treatment______________________________________2         496° C. (925° F.) for 7 hours then air cooled3         496° C. (925° F.) for 8 hours then air cooled4         496° C. (925° F.) for 5 hours then air cooled5         496° C. (925° F.) for 4.75 hours then air cooled6         482° C. (900° F.) for 2 hours then air cooled7         482° C. (900° F.) for 4.5 hours then air cooled8         496° C. (925° F.) for 5 hours then air cooled9         496° C. (925° F.) for 7 hours then air cooled10        482° C. (900° F.) for 6 hours then air______________________________________     cooled 
    
     The notched tensile specimens were machined such that each specimen was cylindrical having a length of 7.6 cm (3.00 in.) and a diameter of 0.952 cm (0.375 in.). A 3.18 cm (1.25 in.) length section at the center of each specimen was reduced to a diameter of 0.640 cm (0.252 in.) with a 0.476 cm (0.1875 in.) minimum radius connecting the center section to each end section of the specimen. A notch was provided around the center of each notched tensile specimen. The specimen diameter was 0.452 cm (0.178 in.) at the base of the notch; the notch root radius was 0.0025 cm (0.0010 in.) to produce a stress concentration factor (K t ) of 10. 
     The results of room temperature tensile tests on the transverse specimens of Examples 2-10 normalized at 954° C. (1750° F.) are shown in Table 5 including the 0.2% offset yield strength (YS), the ultimate tensile strength (UTS), and the notched UTS in MPa, as well as the percent elongation (Elong) and percent reduction in area (RA). The results of room temperature Charpy V-notch impact tests (CVN) and the results of room temperature fracture toughness (K Ic ) testing are also given in Table 5. 
     
                       TABLE 5______________________________________Ht.  YS      UTS     Elong                     RA   CVN  K.sub.IC                                      NotchedNo.  (MPa)   (MPa)   (%)  (%)  (J)  (MPa√m)                                      UTS (MPa)______________________________________2    1804    2120    10.7 47.3 23.0 50.6   25481843    2195    11.9 53.5 22.4 50.3   23663    1757    1974    11.8 51.7 20.3 47.5   22201925    2215    11.8 52.2 18.3 45.2   24554    1882    2260    12.9 57.2 23.0 53.4   25931872    2207    11.4 45.4 29.8 54.1   26455    1871    2200    12.9 57.8 22.4 54.1   27101900    2240    12.6 55.6 29.8 51.6   25686    1922    2294    10.5 46.5 33.2 43.7   24501859    2235    11.5 47.5 25.1 43.8   25597    1873    2158    12.2 52.1 33.2 47.1   27541871    2155    12.2 50.4 32.5 49.7   27578    1626    1844    15.1 65.1 31.2 56.3   28061891    2206    11.9 54.1 27.1 59.7   27839    1780    2057    8.3  62.3 24.4 44.5   24191884    2240    11.4 48.9 26.4 46.8   257010   2060    2468    9.5  39.8 37.3 66.2   28901882    2206    13.1 59.7 33.9 65.2   2854______________________________________ 
    
     The data in Table 5 show that Examples 2-10 provide a combination of high ultimate tensile strength and acceptable K Ic  fracture toughness in the transverse direction. Since properties measured in the transverse direction are expected to be worse than the same properties measured in the longitudinal direction, Examples 2-10 are also expected to provide the desired combination of properties in the longitudinal direction. 
     Additional testing of Examples 2, 4, 5, 9, and 10 was conducted on test specimens taken from bars processed as described above, except that a normalization temperature of 899° C. (1650° F.) was used. The results are given in Table 6. 
     
                       TABLE 6______________________________________Ht.  YS       UTS      Elong RA    CVN   K.sub.ICNo.  (MPa)    (MPa)    (%)   (%)   (J)   (MPam)______________________________________2    1955     2213     11.1  50.9  25.8  52.11941     2215     10.8  46.0  15.6  55.64    1944     2264     10.5  44.4  22.4  51.41956     2260     10.6  47.1  19.0  50.95    1929     2244     11.1  50.5  25.8  54.71953     2250     11.2  50.1  23.0  54.69    1922     2236     11.6  51.6  24.4  45.91917     2240     10.8  46.5  24.4  46.510   1888     2200     13.2  59.0  40.0  64.61885     2195     13.3  59.4  35.9  68.9______________________________________ 
    
     The data in Table 6 for a normalization temperature of 899° C. (1650° F.), when considered together with the data in Table 5 for a normalization temperature of 954° C. (1750° F.), show that the high strength and K Ic  fracture toughness of Examples 2, 4, 5, 9, and 10 can be achieved at normalization temperatures ranging from at least 899° C. (1650° F.) to 954° C. (1750° F.). 
     Room temperature (RT) and -54° C. (-65° F.) tensile tests were conducted on the specimens of Examples 2-5 and 8-10. Transverse specimens were prepared as described above using a normalization temperature of 954° C. (1750° F.) and the age hardening conditions given in Table 7. The conditions of Table 7 were selected to provide a room temperature ultimate tensile strength of at least about 2275 MPa (330 ksi). 
     
                       TABLE 7______________________________________Heat No.   Age Hardening Treatment______________________________________2          482° C. (900° F.) for 8 hours then air cooled3          482° C. (900° F.) for 10 hours then air cooled4          482° C. (900° F.) for 4 hours then air cooled5          482° C. (900° F.) for 4 hours then air cooled8          482° C. (900° F.) for 4 hours then air cooled9          482° C. (900° F.) for 8 hours then air cooled10         482° C. (900° F.) for 6 hours then air______________________________________      cooled 
    
     The test results are shown in Table 8 including the 0.2% offset yield strength (YS), the ultimate tensile strength (UTS), and the notched UTS in MPa, as well as the percent elongation (Elong.) and percent reduction in area (RA). The results of room temperature and -54° C. (-65° F.) Charpy V-notch impact tests (CVN) are also given in Table 8. In addition, the results of room temperature and -54° C. (-65° F.) fracture toughness testing on the compact tension specimens in accordance with ASTM Standard Test E399 (K Ic ) are shown in the table. 
     
                                           TABLE 8__________________________________________________________________________Ht.   Test  YS  UTS Elong             RA CVN K.sub.IC                         NotchedNo.   Temp.  (MPa)      (MPa)          (%)             (%)                (J) (MPa√m)                         UTS (MPa)__________________________________________________________________________2  RT.sup.1  2035      2318          10.4             44.3                14.9                    38.3 2667  2037      2324          11.6             40.7                20.3                    38.4 2796   -54° C.  2175      2486          7.1             30 14.9                    29.2 2137  2063      2458          8.5             35.6                16.3                    --   --3  RT.sup.1  2024      2270          10.7             50.8                23.0                    41.0 2804  2108      2341          10.0             46.8                19.0                    41.0 2654   -54° C.  2159      2417          10.4             43.8                15.6                    30.1 2378  2228      2479          9.1             40.9                13.6                    29.4 21354  RT.sup.1  2003      2334          8.0             33.5                14.2                    39.3 2677  2036      2345          9.6             43.2                17.6                    36.0 2627   -54° C.  2167      2521          8.2             35.4                10.2                    29.4 2375  2412      2522          7.6             32.4                9.5 30.2 25465  RT.sup.1  2050      2358          10.6             46.3                13.6                    38.1 2565  2028      2343          9.8             42.0                14.2                    --   2452   -54° C.  2184      2508          9.4             40.7                11.5                    27.5 2045  2190      2525          8.6             36.3                12.9                    27.6 22888  RT.sup.1  2043      2345          10.6             46.1                16.3                    43.0 2272  2035      2354          10.6             44.6                23.7                    45.2 19039  RT.sup.1  2010      2332          10.6             44.8                21.7                    37.6 2763  2018      2332          9.8             42.7                20.3                    38.9 3232   -54° C.  2115      2488          8.2             35.7                13.6                    28.6 2314  2090      2486          9.2             39.8                14.9                    27.9 191810 RT.sup.1  1886      2270          12.6             54.7                30.5                    --   --  1838      2268          12.8             53.6                27.1                    --   --__________________________________________________________________________ .sup.1 &#34;RT&#34; denotes room temperature. 
    
     The data in Table 8 show that Examples 2-5 and 8-10 provide very high ultimate tensile strength, both at room temperature and at -54° C. (-65° F.). Further, the K Ic  fracture toughness values are significantly higher than would be expected from the known alloys when treated to provide the same level of ultimate tensile strength. 
     III. Examples 1-16 and Comparative Heats B-D 
     For Examples 11-16 and Comparative Heats B-D, the VAR ingots were homogenized at 1232° C. (2250° F.) for 16 hours. The ingots were then press forged from the temperature of 1232° C. (2250° F.) to 8.9 cm (3.5 in.) high by 12.7 cm (5 in.) wide bars. The bars were annealed at 677° C. (1250° F.) for 16 hours and then cooled in air. A 1.9 cm (0.75 in.) slice was removed from each end of the bars. A 30.5 cm (12 in.) long section was then removed from the bottom end of each bar. The 30.5 cm (12 in.) sections were heated to 1010° C. (1850° F.) and then forged to 3.8 cm (1.5 in.) by 10.8 cm (4.25 in.) by 91.4 cm (36 in.) bars and then air cooled. The bars were normalized at 899° C. (1650° F.) for 1 hour and air cooled. The bars were then annealed at 677° C. (1250° F.) for 16 hours and air cooled. 
     Standard longitudinal and transverse tensile specimens, CVN test specimens, and compact tension blocks were machined from the annealed bars. The specimens were austenitized in salt for 1 hour at 899° C. (1650° F.). The tensile specimens and CVN test specimens were vermiculite cooled, whereas the compact tension blocks were air cooled. All of the specimens were deep chilled at -73° C. (-100° F.) for 1 hour, warmed in air, age hardened at 482° C. (900° F.) for 5 hours, and then cooled in air. 
     The results of room temperature tensile tests on the longitudinal (Long.) and transverse (Trans.) specimens are shown in Table 9, including the 0.2% offset yield strength (YS) and the ultimate tensile strength (UTS) in MPa, as well as the percent elongation (Elong) and percent reduction in area (RA). The results of room temperature Charpy V-notch impact tests (CVN) and the results of room temperature fracture toughness testing on the compact tension specimens in accordance with ASTM Standard Test E399 (K Ic ) are shown in Table 9. 
     
                       TABLE 9______________________________________Ht.            YS      UTS   Elong                             RA   CVN  K.sub.ICNo.  Orientation          (MPa)   (MPa) (%)  (%)  (J)  (MPa√m)______________________________________11   Trans.    1928     2194 11.2 48.0 32.5 63.1          1903     2153 12.5 55.5 27.1 56.7          1875     2124 12.2 55.1 28.5 64.0Long.     1915     2120 12.6 57.9 33.9 68.3          1904     2148 11.6 52.1 41.4 73.8          1914     2150 12.3 56.3 35.2 70.912   Trans.    1911     2145 11.9 54.8 36.6 63.3          1934     2152 11.5 54.3 33.2 64.1          1935     2151 12.4 58.8 33.9 59.2Long.     1906     2195 13.7 61.2 32.5 75.6          1928     2178 13.9 62.2 35.2 70.2          1918     2188 13.8 62.2 36.6 65.613   Trans.    1898     2157 11.9 52.0 33.9 63.7          1890     2135 12.4 51.5 38.0 64.1          1882     2132 13.1 55.1 38.0 59.7Long.     1926     2188 13.9 60.5 32.5 65.5          1914     2183 14.7 63.3 35.9 75.9          1897     2155 14.1 63.0 36.6 73.614   Trans.    1913     2146 11.3 50.9 27.1 59.4          1918     2164 11.7 51.3 32.5 59.9          1904     2153 11.8 52.1 36.6 54.2Long.     --       2153 14.3 64.4 33.9 71.0          1911     2176 10.7 62.2 35.9 61.0          1939     2190 13.6 61.9 36.6 63.615   Trans.    1926     2171 12.0 54.5 29.8 59.9          1933     2189 12.4 55.5 31.2 59.9          1920     2177 12.2 55.0 35.2 63.6Long.     1915     2157 14.3 64.0 34.6 72.7          1911     2173 14.1 65.0 35.2 69.8          1924     2171 14.8 65.0 36.6 65.716   Trans.    1947     2200 11.9 56.3 33.9 65.6          1935     2194 13.6 59.3 33.9 54.6          1942     2179 13.3 58.2 36.6 65.6Long.     1951     2190 14.7 63.7 37.3 68.1          1937     2182 14.6 63.5 40.7 71.0          1918     2190 14.4 64.4 41.4 68.9B    Trans.    1900     2120 12.6 57.9 38.0 54.8          1896     2148 11.6 52.1 51.5 57.1          1911     2150 12.3 56.3 30.5 57.4Long.     1931     2170 12.1 60.0 34.6 63.6          1902     2192 14.4 60.4 38.0 57.6          1945     2199 13.7 60.4 35.2 62.0C    Trans.    1884     2130 1.8  8.7  13.6 60.9          1873     2113 3.2  11.9 16.3 61.0          1888     2136 7.2  27.2 16.3 56.6Long.     1876     2141 12.9 53.2 20.3 72.7          1875     2127 13.4 57.8 29.8 70.9          1912     2173 12.3 51.1 30.5 68.4D    Trans.    1931     2171 12.2 54.4 29.8 --          1930     2185 12.1 52.7 31.2 51.3          1924     2182 12.4 50.3 33.9 53.2Long.     1916     2193 14.0 60.3 29.8 54.3          1919     2187 13.8 59.7 36.6 55.0          1913     2174 14.3 62.9 54.2 53.0______________________________________ 
    
     The data in Table 9 show that Examples 11-16 provide the desired combination of properties in accordance with the present invention. The longitudinal specimens of Examples 11-16 all exhibit an average UTS of at least 2137 MPa (310 ksi) and an average K Ic  fracture toughness of at least 65.2 MPa√m (59.3 ksi√in.). In contrast, Comparative Heats B and D exhibit low K Ic  at similar UTS values. In addition, although Comparative Heat C appears to have acceptable longitudinal properties, its % Elong, % RA, and CVN values in the transverse direction are so low as to render it unsuitable. 
     IV. Comparison of Example 10 and Comparative Heat A 
     A comparison of Example 10 and Comparative Heat A was undertaken. The VAR ingots of Example 10 and Comparative Heat A were processed in the same manner as described above for Example 1. 
     Standard transverse tensile specimens (ASTM A 370-95a, 0.64 cm (0.252 in.) diameter by 2.54 cm (1 in.) gage length), CVN test specimens (ASTM E 23-96), and compact tension blocks were machined from the annealed bars. The specimens of each alloy were divided into fifteen groups. Each group was austenitized in salt for 1 hour at the austenizing temperature indicated in Table 10. The tensile specimens and CVN test specimens of all the groups were vermiculite cooled, whereas the compact tension blocks were air cooled. All of the specimens were deep chilled at -73° C. (-100° F.) for 1 hour, and then warmed in air. Each group was then age hardened at 482° C. (900° F.) for the period of time indicated in Table 10 under the column labeled &#34;Aging Time&#34;. Following age hardening, each specimen was cooled in air. 
     The results of the room temperature tensile tests on the transverse specimens are also shown in Table 10, including the 0.2% offset yield strength (YS) and the ultimate tensile strength (UTS) in MPa, as well as the percent elongation (Elong) and percent reduction in area (RA). The results of room temperature Charpy V-notch impact tests (CVN) and Rockwell Hardness C measurements (HRC) are also given in Table 10. 
     
                                           TABLE 10__________________________________________________________________________           Example 10            Comparative Heat A    Aging    Austenizing           YS  UTS Elong                      RA CVN     YS  UTS  Elong                                             RA  CVNGroup    Time (h)    Temp. (°C./°F.)           (MPa)               (MPa)                   (%)                      (%)                         (J)                            HRC.sup.1                                 (MPa)                                     (MPa)                                          (%)                                             (%) (J)                                                    HRC.sup.1__________________________________________________________________________1   2    885/1625           1846               2251                   11.6                      47.9                         27.1                            57.0 (0.0)                                 1758                                     2135 13.1                                             52.9                                                 42.0                                                    55.3 (0.3)           1882               2264                   11.4                      46.5                         23.7                            57.0 (0.0)                                 1762                                     2133 13.2                                             54.5                                                 33.9                                                    53.3 (0.3)2   2    899/1650           1862               2263                   12.9                      53.8                         30.5                            57.0 (0.0)                                 1758                                     2146 13.3                                             53.8                                                 36.6                                                    55.0 (0.0)           1848               2262                   11.5                      47.0                         27.8                            57.5 (0.0)                                 1738                                     2147 13.3                                             55.8                                                 40.7                                                    55.5 (0.0)3   2    913/1675           1886               2270                   12.6                      54.7                         29.8                            57.0 (0.0)                                 1765                                     2144 13.8                                             56.3                                                 42.0                                                    55.0 (0.0)           1838               2268                   12.8                      53.6                         29.8                            57.0 (0.0)                                 1771                                     2151 14.6                                             54.0                                                 39.3                                                    55.3 (0.3)4   4    885/1625           1891               2239                   11.2                      45.4                         28.5                            56.2 (0.3)                                 1792                                     2081 13.3                                             57.7                                                 31.9                                                    54.8 (0.3)           1878               2236                   11.5                      48.6                         31.2                            56.3 (0.3)                                 1759                                     2061 13.7                                             60.1                                                 47.4                                                    54.2 (0.3)5   4    899/1650           1882               2226                   11.7                      47.7                         23.7                            56.0 (0.0)                                 1754                                     2088 13.6                                             58.3                                                 42.0                                                    54.2 (0.3)           1872               2236                   10.9                      44.2                         28.5                            56.5 (0.0)                                 1748                                     2086 13.6                                             58.5                                                 38.6                                                    53.8 (0.3)6   4    913/1675           1860               2237                   10.9                      47.0                         29.1                            56.5 (0.5)                                 1803                                     2088 13.3                                             58.7                                                 38.6                                                    44.2 (0.3)           1866               2240                   13.0                      52.4                         29.1                            56.8 (0.3)                                 1771                                     2078 13.8                                             61.3                                                 35.9                                                    55.0 (0.0)7   6    885/1625           1849               2165                   12.0                      50.9                         28.5                            55.7 (0.3)                                 1768                                     2007 13.6                                             60.1                                                 38.6                                                    49.0 (0.0)           1856               2165                   11.5                      49.2                         31.2                            56.0 (0.0)                                 1766                                     1993 13.7                                             59.1                                                 43.4                                                    53.0 (0.0)8   6    899/1650           1833               2194                   12.4                      53.7                         32.5                            56.0 (0.0)                                 1770                                     2008 14.1                                             61.2                                                 43.4                                                    54.0 (0.0)           1852               2185                   12.1                      52.3                         32.5                            56.0 (0.0)                                 1773                                     2017 13.9                                             60.4                                                 40.7                                                    52.7 (0.3)9   6    913/1675           1851               2188                   13.2                      56.4                         30.5                            56.0 (0.0)                                 1774                                     2024 13.8                                             59.0                                                 44.7                                                    53.2 (0.3)           1838               2172                   13.4                      55.7                         27.1                            55.5 (0.5)                                 1771                                     2022 13.4                                             57.7                                                 43.4                                                    53.2 (0.3)10  8    885/1625           1855               2143                   11.2                      46.9                         29.8                            55.0 (0.0)                                 1741                                     1946 13.6                                             58.4                                                 42.0                                                    52.7 (0.3)           1839               2136                   12.4                      54.6                         31.2                            55.5 (0.0)                                 1735                                     1931 13.1                                             57.7                                                 44.7                                                    51.0 (0.5)11  8    899/1650           1851               2142                   13.1                      56.1                         29.1                            55.5 (0.0)                                 1700                                     1895 14.5                                             61.0                                                 44.7                                                    52.8 (0.3)           1855               2149                   12.4                      52.9                         33.9                            55.7 (0.8)                                 1706                                     1911 14.0                                             61.0                                                 31.1                                                    53.2 (0.3)12  8    913/1675           1875               2153                   12.7                      56.5                         29.1                            55.5 (0.0)                                 1707                                     1939 14.1                                             62.2                                                 43.4                                                    52.7 (0.3)           1862               2155                   12.4                      54.6                         32.5                            55.5 (0.0)                                 1733                                     1975 14.0                                             63.3                                                 50.2                                                    52.8 (0.3)13  10   885/1625           1856               2135                   12.4                      53.7                         33.2                            55.3 (0.3)                                 1705                                     1900 13.9                                             61.5                                                 46.1                                                    51.3 (0.8)           1851               2130                   12.2                      52.8                         23.0                            55.0 (0.0)                                 1715                                     1887 14.0                                             60.4                                                 44.7                                                    50.0 (0.5)14  10   899/1650           1839               2134                   13.3                      57.3                         31.9                            55.2 (0.3)                                 1715                                     1905 13.5                                             59.3                                                 44.7                                                    52.5 (0.0)           1869               2162                   11.9                      50.0                         22.4                            55.0 (0.0)                                 1681                                     1879 14.2                                             64.6                                                 42.0                                                    52.0 (0.0)15  10   913/1675           1850               2127                   12.3                      52.9                         34.6                            55.0 (0.0)                                 1697                                     1891 14.8                                             63.5                                                 48.8                                                    50.0 (0.0)           1860               2151                   13.0                      58.4                         33.2                            55.0 (0.0)                                 1685                                     1867 14.6                                             65.8                                                 48.8                                                    48.2__________________________________________________________________________                                                    (0.3) .sup.1 The values reported for HRC are the average of three measurements. The standard deviation is given in parentheses. 
    
     The data of Table 10 clearly show that, over a wide range of austenizing temperatures and aging times, Example 10 of the present invention provides a higher ultimate tensile strength relative to Comparative Heat A. 
     Tensile and compact tension block specimens of Group 9 were tested to compare the ultimate tensile strength and K Ic  fracture toughness. The results are shown in Table 11. 
     
                       TABLE 11______________________________________Ht.    YS        UTS     Elong  RA   K.sub.ICNo.    (MPa)     (MPa)   (%)    (%)  (MPam)______________________________________10     1888      2200    13.2   59.0 64.6  1885      2195    13.3   59.4 68.9A      1744      2023    13.9   59.5 108  1787      2028    14.4   61.6 112______________________________________ 
    
     The data in Table 11 show that the ultimate tensile strength of Example 10 is significantly higher than that of Heat A. Although Heat A appears to have a higher K Ic  fracture toughness than Example 10, if Heat A was treated to increase its UTS to the same level as Example 10, the resulting K Ic  fracture toughness of Heat A would be expected to be significantly less than that measured for Example 10. Accordingly, Example 10 provides a superior combination of strength and K Ic  fracture toughness than Heat A. 
     It will be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. It should therefore be understood that this invention is not limited to the particular embodiments described herein, but is intended to include all changes and modifications that are within the scope and spirit of the invention as set forth in the claims.