Method of making a cemented carbide body with increased wear resistance

Abstract

The present invention relates to a method of making a cemented carbide body with a bimodal grain size distribution by powder metallurgical methods including wet mixing, without milling, of WC-powders with different grain size distributions with binder metal and pressing agent, drying, pressing and sintering. The grains of the WC-powders are classified in at least two groups, a group of smaller grains and a group of larger grains. According to the method of the present invention, the grains of the group of smaller grains are precoated with a growth inhibitor with or without binder metal.

1. A method of making a cemented carbide body with a bimodal grain size distribution comprising the steps of:
 (i) wet mixing, without milling, WC-powders with a binder metal and a pressing agent, the WC powders comprising smaller grains precoated with a grain growth inhibitor, and larger grains;  
 (ii) drying the mixture of step (i);  
 (iii) pressing the dried mixture to form a pressed body; and  
 (iv) sintering the pressed body.  
 
     
     
       2. The method of  claim 1 , wherein the smaller grains have a maximum size a max , and the larger grains have a minimum size b min  and wherein b min -a max >0.5 μm. 
     
     
       3. The method of  claim 2 , wherein the variation in grain size within each group of smaller and larger grains is at least 1 μm. 
     
     
       4. The method of  claim 1 , wherein the smaller grains comprise at least 10% of the total amount of WC grains, and the larger grains comprise at least 10% of the total amount of WC grains. 
     
     
       5. The method of  claim 1 , wherein the grain growth inhibitor is at least one of V and Cr. 
     
     
       6. The method according to  claim 1 , wherein the group of larger grains are precoated with binder metal. 
     
     
       7. The method according to  claim 1 , wherein the composition of the mixture of step (i) comprises WC and 4-20 wt-% Co and <30 wt-%, cubic carbide comprising TiC, TaC, NbC or mixtures or solid solutions thereof including WC. 
     
     
       8. The method according to  claim 1 , wherein in the WC grains being classified in two groups with a weight ratio of fine WC grains having a size of 0-1.5 μm to coarse WC particles having a size of 2.5-6.0 μm is in the range of 0.25-4.0. 
     
     
       9. The method according to  claim 6 , wherein the smaller grain size ranges from 0-1.5 μm and the larger grain size ranges from 2.5-6.0 μm. 
     
     
       10. The method according to  claim 1 , wherein the body is a cutting tool insert. 
     
     
       11. The method according to  claim 10  wherein the insert body is provided with a thin wear resistant coating. 
     
     
       12. The method according to  claim 11  wherein the coating comprises TiC x N v O z   TiC x   N   y   O   z  with columnar grains followed by a layer of α-Al 2 O 3 , κ-Al 2 O 3  or a mixture of α- and κ-Al 2 O 3 . 
     
     
       13. The method according to  claim 1 , wherein the W-content in the Co binder phase 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 body in κA/m  hAm 2   /kg  and wt-% Co is the weight percentage of Co in the cemented carbide is 0.82-1.0.  
 
     
     
       14. The method of  claim 1 , wherein step (ii) includes spray drying. 
     
     
       15. The method of  claim 1 , wherein the precoating of the smaller grains of step (i) comprises binder metal. 
     
     
       16. The method of  claim 7 , wherein the composition of the mixture of step (i) comprises WC and 5-12.5 wt. % Co and <15 wt. % of the cubic carbides. 
     
     
       17. The method of  claim 8 , wherein the weight ratio is in the range of 0.5-2.0. 
     
     
       18. The method of  claim 1 , wherein only the smaller grains are precoated with the grain growth inhibitor. 
     
     
       19. A method of making a cemented carbide body comprising the steps of:
 (i) providing a WC powder, the WC powder comprises a group of fine WC grains and a group of course  coarse WC grains;  
 (ii) precoating the fine WC grains with a grain growth inhibitor;  
 (iii) precoating the course  coarse WC grains with a binder metal;  
 (iv) wet mixing, without milling, the precoated fine WC grains, the precoated course  coarse WC grains, additional binder metal and a pressing agent;  
 (v) drying the mixture of step (iv);  
 (vi) pressing the dried mixture to form a pressed body; and  
 (vii) sintering the pressed body.  
 
     
     
       20. The method of  claim 19 , wherein steps (iv) and (vii) are performed such that no change in grain size or grain size distribution are produced. 
     
     
       21. The method of  claim 19 , wherein the binder metal comprises Co. 
     
     
       22. The method of  claim 19 , wherein the fine WC grains have a maximum size a max , the coarse WC grains have a minimum size b min , and b min -a max <0.5 μm  b min - a   max   > 0 . 5  μm.    
     
     
       23. The method of  claim 19 , wherein the fine grains comprise at least 10% of the total amount of WC grains, and the course grains comprise at least 10% of the total amount of WC grains. 
     
     
       24. The method of  claim 19 , wherein the grain growth inhibitor comprises at least one of V and Cr. 
     
     
       25. The method of  claim 19 , wherein the fine grains have a size of 0-1.5 μm and the coarse grains have a size of 2.5-6.0 μm. 
     
     
       26. The method of  claim 25 , wherein a weight ratio of fine WC grains to coarse WC grains is 0.25-4.0. 
     
     
       27. The method of  claim 26 , wherein the ratio is 0.5-2.0. 
     
     
       28. The method of  claim 19 , wherein the cemented carbide body has a CW- ratio of  0 . 82 - 1 . 0 .   
     
     
       29. The method of  claim 28 , wherein the CW- ratio is  0 . 86 - 0 . 96 .   
     
     
       30. The method of  claim 13 , wherein the CW- ratio is  0 . 86 - 0 . 96 .