Abstract:
The present invention relates to a method of making a cemented carbide comprising WC, 6-12 wt. % Co and 0.1-0.7 wt. % Cr, wherein the WC-grains are coated with Cr prior to mixing and no milling takes place during the mixing step. As a result a cemented carbide with improved properties is obtained.

Description:
CROSS- REFERENCE TO RELATED APPLICATIONS    
       The present application is a reissue of U.S. Pat. No.  6 , 214 , 287  B 1 , filed Apr.  6 ,  2000 , which claims the benefit of priority to Swedish Application No.  9901207   -   2  filed Apr.  6 ,  1999 .   
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a cemented carbide cutting tool insert, particularly useful for turning, milling and drilling in steels and stainless steels. 
     Conventional cemented carbide inserts are produced by powder metallurgical methods including milling of a powder mixture forming the hard constituents and the binder phase, pressing and sintering. The milling operation is an intensive milling in mills of different sizes and with the aid of milling bodies. The milling time is of the order of several hours up to several days. Such processing is believed to be necessary in order to obtain a uniform distribution of the binder phase in the milled mixture. It is further believed that the intensive milling causes reactivity of the mixture which further promotes the formation of a dense structure. However, milling has its disadvantages. During the long milling time the milling bodies are worn and contaminate the milled mixture. Furthermore even after an extended milling a random rather than an ideal homogeneous mixture may be obtained. Thus, the properties of the sintered cemented carbide containing two or more components depend heavily on how the starting materials are mixed. 
     There exist alternative technologies to intensive milling for production of cemented carbide. For example, particles can be coated with binder phase metal. The coating methods include fluidized bed methods, solgel techniques, electrolytic coating, PVD coating or other methods such as disclosed in e.g. GB 346,473, U.S. Pat. Nos. 5,529,804 or 5,505,902. Coated carbide particles can be mixed with additional amounts of cobalt and other carbide powders to obtain the desired final material composition, pressed and sintered to form a dense structure. U.S. Pat. No. 5,993,730 discloses a method of coating carbide particles with V, Cr, Ti, Ta or Nb. 
     During metal cutting operations like turning, milling and drilling the general properties of the material such as hardness, resistance against plastic deformation, and resistance against formation of thermal fatigue cracks are to a great extent related to the volume fraction of the hard phases and the binder phase in the sintered cemented carbide body. It is well known that increasing the amount of the binder phase reduces the resistance to plastic deformation. Different cutting conditions require different properties of the cutting insert. When cutting in steels with raw surface zones (e.g. rolled, forged or cast) a coated cemented carbide insert must consist of tough cemented carbide and have a very good coating adhesion as well. When turning, milling or drilling in low alloyed steels or stainless steels the adhesive wear is generally the dominating wear type. 
     Measures can be taken to improve the cutting performance with respect to a specific wear type. However, such action will often have a negative effect on other wear properties. 
     SUMMARY OF THE INVENTION 
     It has now surprisingly been found that cemented carbide inserts made from powder mixtures with Cr-coated submicron hard constituents and manufactured without conventional milling have excellent toughness performance for machining of steels and stainless steels. 
     The present invention provides a method of manufacturing a cemented carbide powder, comprising the steps of: coating a hard constituent powder with a coating selected from the group of Cr and Cr+Co to form a coated hard constituent powder, wet-mixing without milling the coated hard constituent powder and with binder metal and pressing agent, to form a wet-mixed powder, and drying said wet-mixed powder to form a dried cemented carbide powder. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     According to the invention there is now provided cemented carbide inserts with excellent toughness properties for machining of steels and stainless steels made from a dried powder of WC and 6-12 wt. % Co, preferably 8-11 wt. % Co, most preferably 9.5-10.5 wt. % Co and 0.1-0.7 wt. % Cr, preferably 0.2-0.5 wt. % Cr. The WC-grains preferably have an average grain size in the range 0.2-1.0 μm, more preferably 0.6-0.9 μm. 
     The microstructure of cemented carbide according to the invention is preferably further characterized by a grain size distribution of WC in the range 0-1.5 μm. 
     The amount of W dissolved in binder phase is controlled by adjustment of the carbon content by small additions of carbon black or pure tungsten powder. The W-content in the binder phase can be expressed as the “CW-ratio” defined as
 
CW-ratio=M s /(wt. % Co * 0.0161)
 
where M s  is the measured saturation magnetization of the sintered cemented carbide body in kA/m  hAm 2   /kg  and wt. % Co is the weight percentage of Co in the cemented carbide. The CW-ratio in inserts according to the invention should preferably be 0.80-1.0, more preferably 0.8-0.90.
 
     The sintered inserts according to the invention are used coated or uncoated, preferably coated with conventional PVD (TiCN+TiN) or PVD (TiN). 
     According to the method of the present invention coated WC-powder with submicron grain size distribution is wet mixed without milling with binder metal and pressing agent, dried preferably by spray drying, pressed to inserts and sintered. 
     WC-powder with grain size distribution according to the invention with coarse grains tails greater than 1.5 μm having been eliminated can be prepared by milling and sieving such as in a jetmill-classifier. It is an important feature of the invention that the milling takes place without milling i.e. there should be no change in grain size or grain size distribution as a result of the mixing. 
     According to the method of the present invention the submicron hard constituents, after careful deagglomeration are coated with a grain growth inhibitor metal such as Cr, V, Mo, W, preferably Cr using methods disclosed in U.S. Pat. No. 5,993,730 and, optionally, an iron group binder metal, preferably Co, using methods disclosed in patent U.S. Pat. No. 5,529,804. In such case the cemented carbide powder obtained from the above method includes Cr-coated, or optionally Cr+Co coated, WC, possibly with further additions of Co-powder in order to obtain the desired final composition. 
     The following examples are given to illustrate various aspects of the invention. 
     EXAMPLE 1 
     Cemented carbide tool inserts of the type N151.2-400-4E, an insert for parting, with a composition having WC, 0.4 wt. % Cr, and 10 wt. % Co, with a grain size of 0.8 μm, were produced according to the invention. Chromium and cobalt coated WC with 0.44 weight % Cr and 2.0 weight % Co, prepared according to U.S. Pat. Nos. 5,993,730 and 5,529,804 was mixed with additional amounts of Co to obtain the desired material composition. The mixing was carried out in ethanol (0.25 fluid per kg cemented carbide powder) for 2 hours in a laboratory mixer and the batch size was 10 kg. Furthermore, 2 wt. % lubricant, was added to the slurry. The carbon content was adjusted with carbon black to a binder phase alloyed with W to obtain a CW-ratio of 0.85. After spray drying, the inserts were pressed and sintered according to standard practice and dense structures with porosity A00 and hardness HV3=1550 were obtained. 
     EXAMPLE 2 
     Cemented carbide tool inserts of the type N151.2-400-4E were produced in the same way as in Example 1 but from chromium and cobalt coated WC having 0.22 weight % Cr, 2.0 weight % Co and with a final powder composition of WC of 0.2 weight % Cr and 10.0 weight % Co. The same physical properties (porosity A00; HV3=1550) as in Example 1 were obtained. 
     EXAMPLE 3 
     Cemented carbide tool inserts of the type N151.2-400-4E were produced in the same way as in Example 1 but from chromium coated WC having 0.44 weight % Cr and with a final powder composition of the WC of 0.4 weight % Cr and 10.0 weight % Co. The same physical properties (porosity A00; HV3=1550) as in Example 1 were obtained. 
     EXAMPLE 4 
     Cemented carbide tool inserts of the type N151.2-400-4E were produced in the same way as in Example 1 but from chromium coated WC having 0.22 weight % Cr and with a final powder composition of WC, 0.2 weight % Cr and 10.0 weight % Co. The same physical properties (porosity A00; HV3=1550) as in Example 1 were obtained. 
     Comparative Example 1 
     Cemented carbide standard tool inserts of the type N151.2-400-4E were produced with the same chemical composition, average grain size of WC and CW ratio as in Example 1 but from powder manufactured with a conventional ball milling technique. The same physical properties (porosity A00; HV3=1550) as in Example 1 were obtained. 
     Comparative Example 2 
     Cemented carbide standard tool inserts of the type N151.2-400-4E were produced with the same chemical composition, average grain size of WC and CW-ratio as in Example 1 but from powder manufactured with the a conventional ball milling technique and with the powder composition WC, 0.2 weight % Cr and 10.0 weight % Co. Initial abnormal grain growth and reduction in hardness compared to Example 1 (porosity A00; HV3=1500) were obtained. 
     EXAMPLE 5 
     Sintered inserts from Examples 1-4 and Comparative Examples 1 and 2 were treated in a standard PVD (TiCN+TiN) coating process with all inserts charged in the same coating batch. 
     Coated inserts according to the invention from Examples 1-4 were compared in toughness behaviour against coated reference inserts from Comparative Examples 1 and 2 in a technological parting test. 
     The test data were: 
     
       
         
               
               
             
               
               
               
             
               
               
             
           
               
                   
               
             
             
               
                 Operation: 
                 Parting off 3 mm thick discs from a bar 
               
               
                 Material: 
                 SS1672, diameter 46 mm 
               
               
                 Cutting data: 
               
               
                 Speed = 150 m/min 
               
             
          
           
               
                 Feed = 0.33 mm/rev 
                 diameter 
                 46-8 mm 
               
               
                 Feed = 0.05 mm/rev 
                 diameter 
                  8-4 mm 
               
               
                 Feed = 0.03 mm/rev 
                 diameter 
                  4-0 mm 
               
               
                 Number of subtests (edges): 
                 3 
               
             
          
           
               
                 Evaluation of toughness: 
                 Number of cuts before fracture 
               
               
                 Results 
               
               
                 Example 
                 No. of cuts 
               
               
                 1 
                 220 
               
               
                 2 
                 270 
               
               
                 3 
                 210 
               
               
                 4 
                 280 
               
               
                 Comp. 1 (prior art) 
                 180 
               
               
                 Comp. 2 (prior art) 
                 160 
               
               
                   
               
             
          
         
       
     
     As clearly demonstrated by the above comparative data, cemented carbide bodies formed consistent with the principles of the present invention possess unexpectedly superior properties when compared to conventional materials. 
     The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by those skilled in the art without departing from the scope of the present invention as defined by the following claims.