Abstract:
A powder-metallurgy produced high-speed steel article having a combination of high hardness and wear resistance, particularly at elevated temperatures. This combination of properties is achieved by the combination of W, Mo, V, and Co. The article is particularly suitable for use in the manufacture of gear cutting tools, such as hobs, and surface coatings.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a powder-metallurgy produced high-speed steel article characterized by high hardness and wear resistance, particularly at elevated temperatures, suitable for use in the manufacture of gear cutting tools, such as hobs and other tooling applications requiring very high wear resistance. 
     In tooling applications requiring high hardness and wear resistance where the tool during use is subjected to elevated temperatures exceeding about 1000° F. and up to for example 1200° F., it is typical to employ carbide material for the manufacture of these tools. Carbide material, however, has the significant disadvantage of being difficult to machine to the desired tooling configurations, particularly intricate cutting surfaces, and is characterized by relatively poor toughness, which renders the tool made therefrom susceptible to cracking and chipping during use. In these applications, it is desirable to employ high speed steels, rather than carbide materials, because high speed steels are easier to machine to the desired tooling configuration and exhibit much higher toughness than carbide materials. High speed steels have not been used in these applications, however, because they do not exhibit the necessary hardness, and thus wear resistance, at the elevated temperatures in which conventional carbide tools are employed. 
     It is accordingly an object of the present invention to provide a powder metallurgy produced high-speed steel article useful for the production of gear cutting tools, such as hobs and other tooling applications requiring high wear resistance. The material shall be capable of attaining and maintaining high hardness at the elevated temperatures anticipated in carbide cutting tool applications and yet have the benefit of high-speed steels from the standpoint of toughness and machinability. 
     SUMMARY OF THE INVENTION 
     The invention relates generally to a powder metallurgy produced high-speed steel article of compacted high-speed steel powder particles. The steel consists essentially of, in weight percent, 2.4 to 3.9 carbon, up to 0.8 manganese, up to 0.8 silicon, 3.75 to 4.75 chromium, 9.0 to 11.5 tungsten, 4.75 to 10.75 molybdenum, 4.0 to 10.0 vanadium, and 8.5 to 16.0 cobalt, with 2.0 to 4.0 niobium being selectively present, and the balance iron and incidental impurities. 
     The following are preferred and more preferred high-speed steel compositions, in weight percent, in accordance with the invention: 
     
         __________________________________________________________________________Alloy No. 1       Alloy No. 2 Alloy No. 3       More        More        More  Composition Preferred Preferred Preferred Preferred Preferred Preferred__________________________________________________________________________C     2.60-3.50       3.00-3.30             2.40-3.20                   2.90-3.10                         2.90-3.90                               3.20-3.60  Mn Max. 0.8 Max. 0.5 Max. 0.8 Max. 0.5 Max. 0.8 Max. 0.5  Si Max. 0.8 Max. 0.5 Max. 0.8 Max. 0.5 Max. 0.8 Max. 0.5  Cr 3.75-4.75 4.2-4.6 3.75-4.50 3.90-4.20 3.75-4.50 3.90-4.20  W  9.0-11.5 10.5-11    9.75-10.75   10-10.5  9.50-11.00 10.00-10.50                                Mo  9.50-10.75 10.00-10.50 6.75-8.25                               7.25-7.75 4.75-6.00 5.00-5.50                                V 4.0-6.0   5-5.5 5.0-7.0   6-6.5                               8.50-10.00 9.00-9.50  Nb 2.0-4.0 2.8-3.2 -- -- -- --  Co 14.00-16.00 14.50-15.00 13.00-15.00   14-14.5  8.50-10.00 9.00-9.50__________________________________________________________________________ 
    
     The article in accordance with the invention may have a minimum hardness of 70 R c  in the as-quenched and tempered condition and preferably a minimum hardness of 61 R c  after tempering at 1200° F. Preferably, the minimum hardness in the as-quenched and tempered condition may be 72 R c . Preferably, the hardness after tempering at 1200° F. may be 63 R c . 
     The article in accordance with the invention may be in the form a gear cutting tool, such as a hob, or a surface coating on a substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing the tempering response of alloys in accordance with the invention compared to conventional powder-metallurgy produced alloys; and 
     FIG. 2 is a graph showing the hot hardness of alloys in accordance with the invention compared to conventional powder-metallurgy produced alloys. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     By way of demonstration of the invention, powder metallurgy produced articles for testing were produced with the alloy compositions, in weight percent, set forth in Table 1. 
     
                                           TABLE 1__________________________________________________________________________Alloy    C  Mn Si Cr W  Mo V  Nb Co Ti Al P  S  O  N__________________________________________________________________________Rex 76    1.52  0.32     0.32        3.79           9.72              5.31                 3.14                    -- 8.22                          -- -- 0.015                                   0.059                                      0.009                                         0.031  Rex 25 1.78 0.33 0.43 3.93 12.6 6.52 5.1 0.02 0.34 0.004 -- 0.017 0.062                                         --  0.046  M25a 1.93 0.33 0.43 3.94 12.6 6.52 5.1 0.02 0.34 0.004 -- 0.017 0.062                                         -- 0.046  M25b 2.03 0.33 0.43 3.94 12.6 6.52 5.1 0.02 0.34 0.004 -- 0.017 0.062                                         -- 0.046  M2511a 1.89 0.26 0.76 4.2 11.91 10.95 5.01 -- --  -- -- --  -- 0.005                                         0.03  M2511b 2.19 0.26 0.76 4.2 11.91 10.95 5.01 -- -- -- -- -- -- 0.005 0.03  M2511c 2.34 0.26 0.76 4.2 11.91 10.95 5.01 -- -- -- -- -- -- 0.005 0.03  M2511d 2.44 0.26 0.76 4.2 11.91 10.95 5.01 -- -- -- -- -- -- 0.005 0.03  M766a 2.23 0.47 0.38 3.88 10.01 5.1 6.07 -- 9.11 -- -- 0.01 0.006 0.029                                         0.05  M766b 2.33 0.47 0.38 3.88 10.01 5.1 6.07 -- 9.11 -- -- 0.01 0.006 0.029                                         0.05  M766c 2.53 0.47 0.38 3.88 10.01 5.1 6.07 -- 9.11 -- -- 0.01 0.006 0.029                                         0.05  M769a 2.97 0.47 0.35 3.94 10.19 5.2 9.12 -- 9.17 -- -- 0.01 0.005 0.011                                         0.039  M769b 3.12 0.47 0.35 3.94 10.19 5.2 9.12 -- 9.17 -- -- 0.01 0.005 0.011                                         0.039  E1a 2.24 0.42 0.50 3.96 12.15 6.75 5.04 2.59 5.99 -- -- 0.01 0.004                                         0.009 0.041  E1b 2.39 0.42 0.50 3.96 12.15 6.75 5.04 2.59 5.99 -- -- 0.01 0.004                                         0.009 0.041  E2a 1.80 0.42 0.51 4.04 6.11 9.86 3.07 1.97 11.96 -- 0.52 0.01 0.006                                         0.009 0.021  E2b 1.95 0.42 0.51 4.04 6.11 9.86 3.07 1.97 11.96 -- 0.52 0.01 0.006                                         0.009 0.021  E3a 2.19 0.42 0.51 3.98 4.96 10.10 4.90 2.53 7.83 -- -- 0.01 0.005                                         0.008 0.042  E3b 2.34 0.42 0.51 3.98 4.96 10.10 4.90 2.53 7.83 -- -- 0.01 0.005                                         0.008 0.042  E4a 2.34 0.42 0.50 4.00 5.00 10.22 4.01 2.45 7.85 0.51  0.71 0.01 0.005                                         0.009 0.044  E4b 2.39 0.42 0.50 4.00 5.00 10.22 4.01 2.45 7.85 0.51  0.71 0.01 0.005                                         0.009 0.044  E6a 3.04 0.58 0.67 4.00 10.04 6.00 9.98 -- 17.81 -- -- 0.01 0.011 0.01                                         0.035  E6b 3.54 0.58 0.67 4.00 10.04 6.00 9.98 -- 17.81 -- -- 0.01 0.011 0.01                                         0.035  E7 2.46 0.56 0.56 4.04 9.06 10.11 4.47 2.50 14.69 -- -- 0.01 0.013                                         0.008 0.017  A1a 2.66 0.56 0.56 4.04 9.06 10.11 4.47 2.50 14.69 -- -- 0.01 0.013                                         0.008 0.017  A1b 2.96 0.56 0.56 4.04 9.06 10.11 4.47 2.50 14.69 -- -- 0.01 0.013                                         0.008 0.017  A1c 3.02 0.44 0.44 4.41 10.99 10.2 5.22 3.08 14.96 -- -- 0.016 0.014                                         0.01 0.021  A1d 3.27 0.44 0.44 4.41 10.99 10.2 5.22 3.08 14.96 -- -- 0.016 0.014                                         0.01 0.021  A2a 2.44 0.58 0.54 3.90 10.05 7.59 5.31 -- 13.97 -- -- 0.01 0.011 0.009                                         0.017  A2b 2.59 0.58 0.54 3.90 10.05 7.59 5.31 -- 13.97 -- -- 0.01 0.011 0.009                                         0.017  A2c 2.74 0.58 0.54 3.90 10.05 7.59 5.31 -- 13.97 -- -- 0.01 0.011 0.009                                         0.017  A2d 2.82 0.43 0.42 3.98 10.43 7.44 6.35 -- 14.15 -- -- 0.008 0.012                                         0.011 0.024  A2e 3.07 0.43 0.42 3.98 10.43 7.44 6.35 -- 14.15 -- -- 0.008 0.012                                         0.011 0.024  A3a 3.37 0.47 0.35 3.94 10.19 5.2 9.12 -- 9.17 -- -- 0.01 0.005 0.011                                         0.039  A3b 3.47 0.47 0.35 3.94 10.19 5.2 9.12 -- 9.17 -- -- 0.01 0.005 0.011                                         0.039  A3c 3.57 0.47 0.35 3.94 10.19 5.2 9.12 -- 9.17 -- -- 0.01 0.005 0.011                                         0.039__________________________________________________________________________ 
    
     The articles for testing, the compositions of which are set forth in Table 1, were produced by conventional powder metallurgy practices including the production of prealloyed powder by nitrogen gas atomization followed by consolidation to full density by hot isostatic compacting. 
     The samples of Table 1 were austenitized, quenched in oil, and tempered four times, each time for two hours, at the temperatures shown in Table 2. They were then tested to measure hardness after tempering at these temperatures. Wear resistance was determined, as reported in Table 3, by pin abrasion testing and cross-cylinder testing. Bend fracture strength and Charpy C-notch impact toughness were determined on longitudinal and transverse specimens after heat treatment using the hardening and tempering temperatures given in Table 3. 
     
                                           TABLE 2__________________________________________________________________________Tempering Response Potential Alloys for Ultra High Hardness Application    Tempering Response* - Hardness RcAlloy    Aust. T. ° F.     950° F.         1000° F.              1025° F.                   1050° F.                        1100° F.                             1150° F.                                  1200° F.__________________________________________________________________________Rex 76    2200  66.9         68.9 --   66.5 65.9 --   57.0  Rex 25 2250 67.8 67.8 -- 66.1 64.4 -- 55.7  M25a 2225 68.4 68.5 -- 66.7 65.2 -- 56.6  M25b 2225 67.4 68.4 -- 67.8 65.7 -- 57.2  M2511a 2250 69.1 68.8 68.1 -- -- 63.2 --  M2511b 2250 66.7 69.2 69.7 -- -- 66.4 --  M2511c 2225 65.7 68.6 69.2 -- -- 66.6 --  M2511d 2225 64.2 67.5 68.7 -- -- 65.3 --  M766a 2200 70.0 70.2 -- 68.7 66.8 -- 57.1  M766b 2200 69.7 70.1 -- 69.2 67.5 -- 58.2  M766c 2175 69.3 69.8 -- -- -- -- --  M769a 2200 70.2 69.8 -- 67.9 66.4 -- 56.2  M769b 2175 70.2 70.0 -- -- -- -- --  E1a 2200 69.3 68.2 -- 67.2 62.2 -- 52.4  E1b 2200 69.3 69.4 -- 67.4 62.9 -- 55.8  E2b 2200 70.4 69.8 -- 68.1 63.9 -- 55.6  E3a 2200 68.9 67.5 -- 65.4 61.4 -- 53.9  E3b 2200 69.2 68.2 -- 66.4 64.9 -- 53.9  E4a 2200 69.1 68.9 -- 67.6 62.2 -- 54.9  E4b 2200 69.0 69.9 -- 67.2 63.9 -- 55.0  E6a 2225 70.1 68.9 -- 67.8 66.1 -- 60.6  E6b 2225 71.7 70.7 -- 69.5 67.1 -- 59.3  E7 2225 72.2 70.3 -- 70.4 67.6 -- 57.5  A1a 2240 71.7 72.3 -- 70.8 68.9 -- 62.5  A1b 2225 68.9 71.3 -- 71.1 70.0 -- 63.8  A1c 2200 70.3 72.6 -- 72.2 70.9 -- 63.1  A1d 2200 70 72.3 -- 72.6 70.9 -- 63.8  A2a 2225 71.8 71.0 -- 70.8 68.5 -- 60.9  A2b 2200 69.5 71.4 -- 71.0 68.8 -- 60.3  A2c 2200 67.5 70.9 -- 70.6 68.8 -- 60.3  A2d 2200 69.2 71.6 -- 70.8 69.9 -- 62.3  A2e 2200 69.4 71.4 -- 71.4 69.3 -- 62.6  A3a 2240 67.7 71.2 -- 69.6 68.5 -- 62.5  A3b 2240 66.2 69.2 -- 70.2 68.9 -- 62.5  A3c 2240 68.7 70.2 -- 70.0 68.1 -- 62.6__________________________________________________________________________ *Hardness after tempering 4 × 2 hours at the given temperature. 
    
     
                       TABLE 3______________________________________Charpy C-Notch Impact Energy, Bend Fracture Strength and Wear  Resistance of Selected Alloys for Ultra High Hardness Application               C-Notch           Pin                                        Heat Treat. Energy  Abra-                                        Aust./Temp. (ft. lbs.) BFS                                      (ksi) sion Cr. Cyl.Alloy (° F./° F.)           Long.  Trans.                        Long.                             Trans.                                   (mg) 10.sup.10 psi______________________________________REX 76 2175/1025 11     6.5   576  390   38.3 42  REX 25 2250/1025 9.5  531  E6a 2250/1025 4.7 3.7 360 300  E6b 2240/1025 2.7 2.2 253 228 9.3 104  E7 2225/1025 3.8 3.5 321 154 15 71  A1c 2200/1025 1.7 1.6 196 158.0 2.2 73  A2a 2200/1025 2.6 2.6 294 218 4.9 77  A2d 2200/1025 2.0 1.7 219 163 2.9 81  A3a 2225/1025 3.8 3.3 292 231 2.1 102______________________________________ 
    
     Alloys A1a through A1d, A2a through A2e, and A3a through A3c are alloy compositions in accordance with the invention. As may be seen from the tempering response data set forth in Table 2 and graphically presented in FIG. 1, alloys of the series A1, A2, and A3 in accordance with the invention exhibited superior hardness at tempering temperatures up to 1200° F. relative to the existing commercial alloys. Likewise, as shown in Table 3, samples A1c, A2a, A2d, and A3a in accordance with the invention also exhibited excellent wear resistance as determined by the pin abrasion and cross-cylinder test results. Of these invention alloys, alloys A1 exhibited optimum combination of the tempering response and wear resistance. Alloys A2 exhibited slightly lower hardness after tempering at 1200° F., but somewhat improved toughness and bend fracture strength than alloys A1. All of the invention alloys, however, as shown in Table 3 and FIG. 1, exhibited improved combinations of tempering response, toughness and wear resistance over the existing commercial alloys. 
     
                       TABLE 4______________________________________Hot Hardness (HRC) of CPM Rex 76 and New Alloys Test Temperature (° F.)Alloy 75     950     1000 1050 1100  1150 1200 1300______________________________________REX 76 67.5   60      59.5 59   58    52.5 46.5 --  A1c 73.5 -- 64.5 -- 63 -- 57.5 39  A2d 72 -- 63 -- 60 -- 56 38.5  A2e 72 -- 62.5 -- 60 -- 56 39  A3a 71.5 -- 61 -- 58.5 -- 53 33.5______________________________________ 
    
     Table 4 and FIG. 2 indicate the hot hardness values for alloys A1c, A2d, A2c, and A3a, in accordance with the invention, compared to the existing commercial alloy (REX 76). As may be seen from this data, all of the alloys in accordance with the invention exhibited improved hot hardness at elevated temperatures up to 1300° F., compared to the existing commercial alloy. 
     All compositions set forth in the specification are in weight percent, unless otherwise indicated.