Abstract:
The present invention relates to a method of making ultrafine WC—Co alloys from a well dispersed mixture of fine and non-agglomerated WC and Co powders, wherein the Co powders have a narrow grain size distribution wherein at least 80% of the grains have sizes in the interval x+0.2x with the interval of variation of 0.4x is not smaller than 0.1 um, and a carbon content of approximately the amount necessary to provide eta phase formation.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of making ultrafine WC—Co alloys from a well dispersed mixture of fine and non-agglomerated WC and Co powders, optimised grain growth refiner additions and carbon content using low temperature sinter/sinter-HIP conditions. 
     BACKGROUND OF THE INVENTION 
     It is well known that decreasing the WC grain size confers performance advantages to cemented carbide in many applications e.g. PCB (Printed Circuit Board) machining, wood machining, metal cutting. Maintaining a submicron WC grain size requires the use of grain growth refiners such as VC, Cr 3 C 2 , TaC etc. The finer the WC grain the greater the necessary addition of said refiners. In some applications e.g. metal cutting fineness of the WC grain size should not greatly reduce toughness, otherwise edge life will suffer. Grain refiners may reduce toughness if used in excessive amounts. 
     In other applications e.g. PCB machining, fineness of WC grain size is of paramount importance with toughness demand being secondary. Commercially available ultra fine cemented carbide grades already use a grain size of about 0.4 μm. But to reduce the WC grain sizes to below 0.4 μm requires novel raw material and processing technique. 
     DE 40 00 223 (Mitsubishi) discloses a cemented carbide based on WC with 6-14 wt-% binder phase containing vanadium and chromium whereby the ratio Cr/(Cr+V) is &lt;0.95 and &gt;0.50. U.S. Pat. No. 4,539,041 discloses the making of metallic powders by a process for reducing oxides, hydroxides or metal salts with the aid of polyols. Particularly, when starting with cobalt hydroxide it is possible to obtain powders of metallic cobalt as essentially spherical, non-agglomerated particles. Such Co powder is herein referred as polyol cobalt. 
     U.S. Pat. No. 5,441,693 discloses a method of making cemented carbide with an extremely uniform structure by using Co-powder produced according to the above mentioned polyol method and with submicron grain size. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method of making cemented carbide with WC grain size less than 0.8 μm and with a low content of grain refiners. 
     In one aspect, the present invention provides a method of making a cemented carbide having a submicron WC grain size, the method comprising the steps of: (i) forming a powder mixture comprising deagglomerated submicron WC produced by carbothermal reaction, Co powder having deagglomerated spherical grains of submicron size with a narrow grain size distribution wherein at least 80% of the grains have sizes in the interval x+0.2x with the interval of variation of 0.4x is not smaller than 0.1 μm, a carbon content of approximately the amount necessary to provide eta phase formation, and &lt;1 wt. % grain growth inhibitor; (ii) milling the powder mixture; and (iii) sintering. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     According to the method of the present invention cemented carbide compositions with extremely fine microstructure, average grain size &lt;0.8 μm, essentially no grains larger than 1.5 μm, suitable for toughness demanding machining operations are made by milling deagglomerated submicron WC powder produced by carbothermal reaction with a cobalt powder having deagglomerated spherical grains of about 0.4 μm average grain size and with a narrow grain size distribution wherein at least 80% of the particles have sizes in the interval x±0.2x provided that the interval of variation (that is 0.4x) is not smaller than 0.1 μm. Preferably the cobalt powder is polyol cobalt. If the carbon content of the powder mixture to be sintered is held close to etaphase formation no, or a relatively low amount (&lt;1 wt-%) of grain growth refiners such as VC and Cr 3 C 2  need to be added. Sintering with HIP takes place at relatively low temperatures (i.e., &lt;1400° C.). The sintered cemented carbide has a Co-content of 70-85% in terms of cobalt magnetic measurements assuming pure cobalt. 
     In a preferred method the average WC grain size is further reduced to below 0.4 μm by using an optimum VC+Cr 3 C 2  addition in which the ratio of VC/Cr 3 C 2  in wt-% is 0.33-1.0, preferably 0.5-0.9, most preferably 0.7-0.8 for PCB-applications and 0-0.5 for metal cutting, preferably 0-0.2 for non ferrous machining and 0 for ferrous machining. Preferably sintering is performed using gas pressure sintering also referred to as sinter-HIP. 
     In a first embodiment particularly useful for finish and general machining of non-ferrous materials the cemented carbide has 6-10% Co, 0.0-0.3 VC, 0.3-0.75 Cr 3 C 2  and rest WC&lt;0.8 μm. 
     In a second embodiment particularly useful for rough machining in demanding work materials (e.g. austenitic stainless steels) the cemented carbide has 10-16% Co, 0.5-1.2 Cr 3 C 2  and rest WC&lt;0.8 μm. 
     In a third embodiment particularly useful for very tough machining operations, or those with very low cutting speed, (e.g. broaching) the cemented carbide has 16-20% Co, 0.8-1.8 Cr 3 C 2  and rest WC&lt;0.8 μm. 
     In a fourth embodiment particularly useful for PCB and non-metallic routing and slot drilling, the cemented carbide has 5-8% Co, 0.1-0.6 VC, 0.25-0.6 Cr 3 C 2  and rest WC&lt;0.4 μm. 
     In a fifth embodiment particularly useful for PCB micro drilling the cemented carbide has 8-12% Co, 0.2-0.9 VC, 0.4-0.9 Cr 3 C 2  and rest WC&lt;0.4 μm. 
     In a sixth embodiment particularly useful for wood-machining of solid wood or fibreboard the cemented carbide has 2-5% Co, 0.05-0.2% VC, 0.1-0.25% Cr 3 C 2  and rest WC&lt;0.4 μm. 
    
    
     EXAMPLE 1 
     PCB drill blanks were produced from submicron WC made by carbothermal reaction and milled deagglomerated with cobalt powder having special deagglomerated grains of about 0.4 μm average grainsize and with a narrow grain size distribution and VC+Cr 3 C 2 . The following compositions were made containing in addition to WC: 
     A. 8 wt-% Co, 0.3 wt-% VC and 0.4 wt-% Cr 3 C 2  with a carbon content according to the invention. For comparison blanks with the same composition but with carbon content according to prior art. 
     B. 9 wt-% Co and 0.35 wt-% VC and 0.45 wt-% Cr 3 C 2  with a carbon content according to the invention. For comparison blanks with the same composition as prior art from A were made. 
     C. 7 wt-% Co and 0.26 wt-% VC and 0.35 wt-% Cr 3 C 2  with a carbon content according to the invention. For comparison blanks with the composition (in wt-%) 6.5 Co, 0.6 VC and 0.32 Cr 3 C 2  according to prior art were made. 
     The blanks were pressed and sintered with HIP at 1340° C. Magnetic cobalt content, (CoM), coercive force, Hc, were measured and performance was tested in a microdrilling and a routing test. 
     The microdrilling test was performed under the following conditions: 
     Drill diameter 0.3 mm 
     Speed 80000 to 120000 rpm 
     Feed 15 μm/rev increasing at every 500 hits by 5 μm/rev until failure 
     Material tested three stacked PCB copper lined FR4 resin 
     The routing test was performed under the following conditions: 
     Diameter 2.4 mm 
     Measurement of wear levels after 50 m routing at speeds ranging from 30000 to 42000 rpm (8 μm tooth at 2.4 mm diameter) 
     Material three stacked PCB copper lined FR4 resin. 
     The following results were obtained 
     
       
         
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                   
                 Tool life ratio 
               
               
                   
                   
                   
                   
                 of invention 
               
               
                   
                   
                   
                   
                 with respect 
               
               
                   
                 examples: 
                 CoM 
                 Hc 
                 to prior art 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Performance: PCB microdrilling 
               
             
          
           
               
                   
                 A. 
                 invention 
                 5.80 
                 38.3 
                 1.27 
               
               
                   
                   
                 prior art 
                 7.34 
                 37.0 
                 1 
               
               
                   
                 B. 
                 invention 
                 7.33 
                 40.5 
                 1.59 
               
               
                   
                   
                 prior art 
                 7.34 
                 37.0 
                 1 
               
             
          
           
               
                   
                 Performance: PCB Routing 
               
             
          
           
               
                   
                 C. 
                 invention 
                 6.04 
                 40.7 
                 1.1 
               
               
                   
                   
                 prior art 
                 5.11 
                 41.6 
                 1 
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE 2 
     Endmill blanks were produced from submicron WC made by carbothermal reaction and milled deagglomerated with cobalt powder having special deagglomerated grains of about 0.4 μm average grainsize and with a narrow grain size distribution and Cr 3 C 2 . The following compositions were made containing, in addition to WC: 
     D. 10 wt-% Co and 0.5 wt-% Cr 3 C 2  with a carbon content according to the invention. CoM was 8.3 and Hc24 kA/m. 
     E. 12 wt-% Co and 0.6 wt-% Cr 3 C 2  with a carbon content according to the invention. CoM was 10.6 and Hc 21.5 kA/m. 
     F. For comparison blanks with 10% Co, 0.5%, Cr 3 C 2  but with carbon content according to prior art and with 0.8 μm grain size. 
     The blanks were pressed and sintered at 1360° C. Magnetic cobalt content (CoM) coercive force (Hc) were measured. The blanks were ground to 8 mm diameter endmills PVD coated with TiCN. Performance was tested in an end milling and slotting operation. An edge milling test was performed under the following conditions: 
     Work piece material: Inconel 718 age hardened 
     Speed 20 m/min 
     Feed 0.021 mm/tooth 
     Depth of cut: 8 mm 
     Radial depth of cut: 4 mm 
     Flood coolant 
     Result 
     D(Invention) reached 0.9 m cutting distance tool life and the reference F achieved 0.42 m cutting distance. 
     A slot milling test was performed under the following conditions: 
     Work piece material: 316 austenitic stainless steel 
     Speed 50 m/min 
     Feed 0.042 mm/tooth 
     Depth of cut: 4 mm 
     Width of cut: 8 mm 
     Flood coolant 
     Result 
     E(Invention) reached 8.5 m cutting distance tool life and the reference F achieved 5 m cutting distance. 
     The principles, preferred embodiments and mode of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the invention.