Powder metallurgy high speed tool steel article and method of manufacture
BACKGROUND OF THE INVENTION 
     High speed tool steel articles including intermediate articles of rod and bar and finished articles such as tool bits and the like, must be characterized by good wear resistance for high speed cutting applications as well as good tool life. Wear resistance in high speed tool steels is a function generally of a dispersion of hard, wear resistant material, typically carbides of carbide forming elements such as vanadium, tungsten and molybdenum. Nitrides may also be present for this purpose. The higher the content of the dispersion of hard, wear resistant material the better will be the wear resistance of the article made therefrom. As the dispersion is increased, however, it tends to cause embrittlement of the article, which impairs the tool life. Specifically, after repeated use in high speed cutting applications and the like the article will fail as by cracking. By the use of powder metallurgy techniques to produce high speed tool steel articles, such as by hot isostatic compacting prealloyed powders thereof, combinations of high density and fine, uniform carbide dispersions have been obtained to achieve improved combinations of tool life and wear resistance during high speed cutting applications. Nevertheless, at extremely high concentrations of the hard, wear resistant material, such as carbides, tool life is impaired. 
     SUMMARY OF THE INVENTION 
     It is accordingly a primary object of the present invention to provide a powder metallurgy produced high speed tool steel article and method for manufacturing the same wherein dispersions of hard, wear resistant material may be provided to achieve heretofore unobtainable combinations of wear resistance and tool life. 
     Broadly, in accordance with the method of the invention, a powder metallurgy produced high speed tool steel article having an improved combination of tool life and wear resistance is produced by first providing a particle charge of high speed tool steel particles with the charge constituting a mixture of coated particles and uncoated particles. The coated particles are coated with a hard, wear resistant material, which may be carbides, nitrides or combinations thereof. The particle charge is hot isostatic compacted to essentially full density to produce the article. The coated particles may be present in an amount effective to improve tool life and wear resistance of the article. Specifically, the coated particles may be present in an amount of over 10 to 90%, or alternately 15 to 85% or about 50%. After hot isostatic compacting the article may be hot worked, which includes forging. The resulting article comprises a mixture of the coated prealloyed high speed tool steel particles and uncoated particles wherein the hard, wear resistant material of the coated particles is at boundaries of the coated particles and contained in a continuous matrix of the high speed tool steel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and B are photomicrographs of articles produced in accordance with the invention at a magnification of 30×; 
     FIGS. 2A, B and C are photomicrographs of forged articles produced in accordance with the invention at a magnification of 65×; 
     FIGS. 3A, B and C are photomicrographs of the articles of FIG. 2 but at a magnification of 500×; and 
     FIG. 4 is a cruve relating tool life to the percent of coated prealloyed powder in the mixture constituting the compacted article. 
    
    
     These drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In demonstrating the method and article of the invention gas atomized, prealloyed powder of the high speed tool steel composition designated as T15 was used. The experiments involved the use of different mesh size powders and different weight fractions of coated and uncoated powder particles. The coating constituting the hard, wear resistant material was a dual coating of titanium nitride on titanium carbide applied by chemical vapor deposition. The composition of the T15 high speed tool steel prealloyed powder was, in percent by weight, carbon 1.56, chromium 4.08, vanadium 4.57, tungsten 11.40, molybdenum 0.38, cobalt 5.0, nitrogen 0.032, titanium 0.02 and balance iron. The prealloyed powder particles were produced from the T15 composition by atomizing a molten stream of the alloy with nitrogen to form the discrete particles which were thereafter cooled to solidification and collected. The atomization was performed in an inert atmosphere to protect the particles from contamination, as by oxidation. 
     In the chemical vapor deposition (CVD) process, the coating produced is a product of gas reactions occurring at elevated temperatures inside a stainless steel retort chamber. The powder to be coated was spread to a depth of approximately 1/4 inch over previously coated graphite shelves having a 1/2 inch high retaining lip around their outer edges. The shelves with the particles so positioned thereon were lowered into the retort. The retort was sealed, evacuated, filled with an inert atmosphere and heated to a temperature of approximately 1750° to 2000° F. in about 3 hours. The chamber was held at temperature for another 3 hours while the reaction gases were continuously introduced to the chamber. The gases used include argon which is introduced during the initial heating period and ammonia, nitrogen, methane, propane, hydrogen and titanium tetrachloride depending upon the composition of the coating desired. The resulting coating is chemically bonded to the surfaces of the powder particles. After coating the chamber is allowed to cool before removal of the coated powder. During the coating process, the powder is lightly bonded into a solid layer on the shelf. When the layer is removed it is mechanically broken-up to free the individual powder particles for subsequent use. Powder particles so coated were blended with uncoated T15 powder from the same heat and produced in the identical manner by inert gas atomization. Various powder samples of different portions of coated and uncoated particles were loaded in steel containers. The containers were vacuum outgassed, sealed and hot compacted by hot isostatic pressing in a gas pressure vessel employing nitrogen as the gaseous pressure medium at a pressure of approximately 12,500 psi. After hot compacting to essentially fully density, the compacts were forged to various size bars. Standard 1/2 inch square tool life test specimens were machined from the forged bars and heat treated in the manner conventional for T15 high speed tool steels. The resulting specimens were tested in continuous-cut tests on H13 alloy workpieces. 
     To illustrate the unique microstructure obtained by the practice of the invention, FIG. 1 shows the microstructure of hot compacted material wherein the coated particles are embedded in a continuous matrix of the high speed tool steel composition. After hot working as by forging the coated particles are dispersed further throughout the high speed tool steel matrix, as shown in FIGS. 2 and 3. 
     
                       TABLE I                                                     
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Continuous Cut Tool Life Test Results                                     
from Mixtures of Uncoated and Coated T15 Powder Tools                     
                       Time to Failure                                    
                       (Minutes).sup.2                                    
                                         Grade                            
                                   Test  Aver-                            
Grade          Bar    Tool   HRC.sup.1                                    
                                   Values                                 
                                         age                              
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15% coated -16 mesh in                                                    
               84-4   1      68    25  38  38                             
85% uncoated -16 mesh 2      68    30  40                                 
                      3      68    50  45                                 
30% coated -16 mesh in                                                    
               84-5   1      68.5  40  25  35                             
70% uncoated -16 mesh 2      68.5  35  36                                 
                      3      68.5  45  28                                 
15% coated -120 mesh in                                                   
               84-6   1      68    50  25  44                             
85% uncoated -16 mesh 2      68    60  60                                 
                      3      68    44  25                                 
15% coated -16 mesh in                                                    
               84-7   1      68    70  --  43                             
85% uncoated -16 mesh 2      68    35  35                                 
                      3      68    35  40                                 
30% coated -16 mesh in                                                    
               84-8   1      68.5  20  28  30                             
70% uncoated -120 mesh                                                    
                      2      68.5  25  30                                 
                      3      68.5  50  24                                 
10% coated -16 mesh in                                                    
               84-9   1      68    20  28  27                             
90% uncoated -120 mesh                                                    
                      2      68    25  22                                 
                      3      68    42  22                                 
30% coated -120 mesh in                                                   
               83-12  H      68.5  30  20  29                             
70% uncoated -30 mesh J      68.5  30  35                                 
CPM T15, 100% uncoated                                                    
               81-33  Z4     67.5  39  20  27                             
                      Z5     67.5  30  15                                 
                      Z6     67.5  25  25                                 
               81-51  C      67.5  29  30                                 
               74-47  1      68    32  25                                 
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 .sup.1 All tools hardened:                                               
 2250° F., 5 minutes, oil quenched                                 
 1025° F., 2+2+2 hours                                             
 .sup.2 Cutting Conditions:                                               
 Speed  45 sfpm                                                           
 Feed  .0055 in./rev.                                                     
 Depth  .0625 in.                                                         
 No lubrication                                                           
 Workpiece  H13 at HRC 40                                                 
 
    
     Table I shows the results of tool life tests with various mixtures of uncoated and coated powders constituting the charge from which the samples were produced for testing. As shown in Table I, in continuous-cut testing on H13 alloy workpieces the tools tested from bars 84-6 and 84-7 exhibited approximately 60% improvement in tool life over conventional uncoated powder metallurgy produced tools designated as CPM T15. This material was obtained from standard commercial bar stock. Tools from bar 84-4 exhibited a 40% improvement and tools from bar 84-5 a 28% improvement over this conventional material. Tools from bars 84-8, 84-9 and 83-12 performed only comparably to the conventional CPM T15 product. 
     
                       TABLE II                                                    
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Crossed Cylinder Wear Test Results                                        
from Mixtures of Uncoated and Coated T15 Powder Materials                 
                              Wear                                        
                              Resistance                                  
                              (10.sup.10 psi)                             
               Bar            Test     Aver-                              
Powder Mixtures                                                           
               No.    HRC.sup.1                                           
                              Values   age                                
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10% coated -120 mesh in                                                   
               83-11  68.5    123, 102, 77,                               
                                       94                                 
90% uncoated -30 mesh         79, 88                                      
30% coated -120 mesh in                                                   
               83-12  69.5    132, 124, 135,                              
                                       137                                
70% uncoated -30 mesh         143, 151                                    
15% coated -16 mesh in                                                    
               84-4   67.5    75, 96, 78, 79                              
                                       82                                 
85% uncoated -16 mesh                                                     
30% coated -16 mesh in                                                    
               84-5   68      97, 79, 77, 87                              
                                       85                                 
70% uncoated -16 mesh                                                     
15% coated -120 mesh in                                                   
               84-6   67.5    79, 84, 68, 87                              
                                       80                                 
85% uncoated -16 mesh                                                     
15% coated -16 mesh in                                                    
               84-7   67.5    79, 99, 101, 91                             
                                       93                                 
85% uncoated -16 mesh                                                     
30% coated -16 mesh in                                                    
               84-8   68.5    65, 76, 67, 58                              
                                       67                                 
70% uncoated - 120 mesh                                                   
10% coated -16 mesh in                                                    
               84-9   67.5    62, 84, 60, 81                              
                                       72                                 
70% uncoated -120 mesh                                                    
CPM T15, 100% uncoated                                                    
                      67               55-60                              
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 .sup.1 All speciments hardened:                                          
 2250° F., 5 minutes, oil quenched                                 
 1050° F., 2+2+2 hours                                             
 
    
     Table II provides the results of cross-cylinder wear tests with various coated and uncoated powder mixtures compared to a conventional CPM T15 material which contains only uncoated particles. As may be seen from Table II all the coated powder mixture materials in accordance with the invention exhibited superior wear resistance compared to the standard material. 
     
                       TABLE III                                                   
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Tool Life Test Results                                                    
with Uncoated and Coated Powder T15                                       
                 Time to Failure,                                         
                 Continuous Cut                                           
                 (Minutes)                                                
Powder Mixtures                                                           
               HRC     Test 1  Test 2 Average                             
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CPM, T15 100% uncoated.sup.1                                              
               67      10      10     9.5                                 
               67      9       9                                          
15% coated.sup.1                                                          
               67      17      13                                         
               67      13      10     13.3                                
50% coated.sup.1                                                          
               68      24.5    18                                         
               68      20      20     20.6                                
100% coated.sup.2                                                         
               68      8       7                                          
               68      6       8      7.3                                 
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 .sup.1 2240° F., 5 min, oil quenched                              
 1025° F., 2+2+2 hrs                                               
 .sup.2 2225° F., 5 min, oil quenched                              
 1025° F., 2+2+2 hrs                                               
 Test Conditions:                                                         
 50 sfm                                                                   
 0.0055 in/rev.                                                           
 0.0625 in depth                                                          
 No lubrication                                                           
 Workpiece  H13 at HRC C40                                                
 
    
     To determine the effect of varying additions of coated particles in increased amounts in the mixture, samples were produced containing 50% coated and 50% uncoated T15 powder particles as well as 100% coated mixtures. The material was processed in a manner identical to that described with reference to the test reported in Table I. The test results are shown in Table III and FIG. 4 of the drawings. As may be seen, the optimum performance with respect to tool life was obtained with the tools made from 50% coated and 50% uncoated mixtures of powder particles. Over a 100% improvement in tool life was found for the 50% coated and 50% uncoated material when compared to the standard CPM T15. The 100% coated particle sample tool showed a tool life of less than that obtained for the standard CPM T15 tool, which contained only uncoated particles. 
     Although the invention has been demonstrated with respect to prealloyed powder particles of T15 high speed tool steel, it is to be understood that the invention is applicable to any cutting tool alloy where it is desired to increase the dispersion of the hard, wear resistant phase, particularly a carbide phase distribution. The invention is amenable to use of any of the well known carbide forming elements and carbides therefrom which typically are used in cutting tool alloys for the purpose of providing the required hard, wear resistant dispersion. This may include vanadium, molybdenum and tungsten carbides which may be used singly, but conventionally in most cases are combined in a specific high speed tool steel composition used in cutting tool applications. The invention may be used to produce by hot compacting, and specifically hot isostatic compacting, either intermediate products in the form of billets, bar or rod or final pressed-to-shape articles, such as tool bits.