Abstract:
A cutting insert which comprises a rake face and a flank face wherein there is a cutting edge at the juncture of the rake face and the flank face. The cutting insert has a coating and a substrate wherein the coating is adherently bonded to the substrate. The substrate is a tungsten carbide-based cemented carbide wherein there is a zone of non-stratified cobalt enrichment beginning near and extending inwardly from a peripheral surface of the substrate. The bulk substrate has a porosity of greater than C00 and less than or equal to C04.

Description:
BACKGROUND 
     The invention concerns a coated cemented carbide cutting insert that has a substrate with a porosity (per the ASTM Designation B 276-86, entitled “Standard Test Method for Apparent Porosity in Cemented Carbides”) of greater than C00 and less than or equal to C04 wherein there is a zone of non-stratified, i.e., generally homogeneous, binder enrichment beginning near and extending inwardly from a peripheral surface of the substrate. 
     Heretofore, there has been the Kennametal KC850® coated cutting insert (KC850 is a registered trademark of Kennametal Inc., of Latrobe, Penn. 15650, USA, for cutting inserts) which has a C03/C05 porosity substrate which has a zone of surface binder enrichment. This binder enrichment is a stratified type of binder enrichment meaning that the binder enrichment forms in distinct layers of binder metal. The Nemeth et al. article, “The Microstructural Features and Cutting Performance of the High Edge Strength Kennametal Grade KC850,” Proceedings of Tenth Plansee Seminar, Reutte, Tyrol, Austria, Metalwerke Plansee A.G. (1981), pp. 613-627, describes the “Kennametal KC850®” coated cutting tool (or insert). The “Kennametal KC850®” coated cutting insert has a tri-phase coating of TiC—TiCN—TiN, according to U.S. Pat. No. 4,035,541, to Smith et al., entitled “Sintered Cemented Carbide Body Coated with Three Layers.” 
     SUMMARY 
     The invention is a cutting insert which comprises a rake face and a flank face wherein there is a cutting edge at the juncture of the rake face and the flank face. The cutting insert has a coating and a substrate wherein the coating is adherently bonded to the substrate. The substrate is a tungsten carbide-based cemented carbide which has a bulk composition of between about 3 to about 12 weight percent cobalt, up to about 12 weight percent tantalum, up to about 6 weight percent niobium, up to about 10 weight percent titanium, and the balance tungsten and carbon. There is a zone of non-stratified cobalt enrichment beginning near and extending inwardly from a peripheral surface of the substrate. The zone of non-stratified enrichment has A porosity. The bulk substrate has a porosity of greater than C00 and less than or equal to C04. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following is a brief description of the drawings which form a part of this patent application: 
     FIG. 1 is an isometric view of a specific embodiment of an SPGN 432 style of cutting insert; 
     FIG. 2 is a cross-sectional view of the cutting insert illustrated in FIG. 1 taken along section line  2 — 2 ; 
     FIG. 3 is an isometric view of a specific embodiment of an SNG 433 style of cutting insert; and 
     FIG. 4 is a cross-sectional view of the cutting insert illustrated in FIG. 3 taken along section line  4 — 4 . 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing figures, FIG. 1 illustrates a specific embodiment of the present invention as an indexable cutting insert generally designated as  10 . Cutting insert  10  has cutting edges  12  at the junction of the rake face  14  with the flank faces  16 . Although the cutting insert  10  shown in FIG. 1 is an SPGN 432 style with a honed cutting edge, applicant contemplates that the present invention includes other styles of cutting inserts with or without honed cutting edges. 
     FIG. 2 shows a cross section at the cutting edge  12  of cutting insert  10  taken along section  2 — 2  of FIG.  1 . The substrate generally designated as  18  has a non-binder enriched zone  20 , i.e., a zone comprising the central portion (or bulk region) of the substrate, and an outer (or peripheral) binder enriched zone  22  near the peripheral boundaries  24  and  26  of the substrate. The outer binder enriched zone  22  exhibits a non-stratified type of binder enrichment. In other words, the binder enriched zone  22  is generally homogeneous in nature. This is in distinction to a zone of stratified binder enrichment in which the binder forms as layers one on top of the other such as discussed in Kobori et al., entitled “Binder Enriched Layer Formed Near the Surface of Cemented Carbide,” Powder and Powder Metallurgy, Vol. 34, No. 3, pp. 129-133 (April 1987). 
     In a preferred embodiment, the substrate  18  is a tungsten carbide based cemented carbide substrate containing at least 70 weight percent tungsten carbide, and more preferably, at least 80 weight percent tungsten carbide. The binder is preferably cobalt or a cobalt alloy and, preferably, has a bulk concentration of 3 to 12 weight percent. The more preferable bulk cobalt content is between about 5 to about 8 weight percent. Even more preferably, the bulk cobalt content is between about 5.6 to about 7.5 weight percent. 
     The substrate  18  also contains solid solution carbide and/or carbonitride forming elements such as titanium, hafnium, zirconium, niobium, tantalum and vanadium, with these elements being preferably selected from titanium, niobium and tantalum, either alone or in combination with each other or tungsten. These elements preferably may be added to the mix as a carbide, nitride and/or carbonitride, and more preferably as a nitride, and most preferably, as tantalum (niobium) carbide and titanium nitride. Preferably, the concentration of these elements is within the following ranges: up to 12 weight percent tantalum, up to 10 weight percent titanium, and up to 4 weight percent niobium. More preferably, the sum of the tantalum content and the niobium content is between about 3 and about 7 weight percent and the titanium content is between about 0.5 and about 5 weight percent. Most preferably, the sum of the tantalum content and the niobium content is between about 5.0 and about 5.9 weight percent, and the titanium content is between about 1.7 and about 2.3 weight percent. 
     In the bulk region  20  of the substrate  18 , these elements (i.e., titanium, hafnium, zirconium, niobium, tantalum and vanadium) form, at least to some extent and preferably for the most part, solid solution carbides and/or solid solution carbonitrides with the tungsten carbide in the substrate. In the enriched zone  22 , the solid solution carbides and/or carbonitrides have been wholly, or partially, depleted so that tungsten carbide and cobalt comprise the majority of the composition of the binder enriched zone  22 . 
     Within the binder enriched zone  22 , the binder (e.g., cobalt) content should reach a maximum value which is between about 125 to about 300 percent. A more preferable range of binder enrichment is between about 150 and about 300 percent of the bulk binder content. The most preferable range of binder enrichment is between about 200 and about 300 percent of the bulk cobalt concentration in the substrate. 
     The binder enriched zone  22  preferably extends to the substrate peripheral surfaces  24  and  26 . In the alternative, there may be a thin layer adjacent to these peripheral boundaries ( 24 ,  26 ) in which cobalt content has been reduced due to evaporation during substrate sintering so that the zone of binder (e.g., cobalt) enrichment  22  extends to near the peripheral surface ( 24 ,  26 ) of the substrate  18 . The thickness of the binder enriched zone is preferably up to about 50 micrometers (μm). 
     Bonded onto the peripheral boundaries  24  and  26  of the substrate  18  is a hard coating, designated by brackets as  29 , preferably having one or more layers applied by chemical vapor deposition (CVD) or a combination of CVD and physical vapor deposition (PVD) techniques. MTCVD (medium temperature CVD) techniques may be used to apply a layer, such as a titanium carbonitride layer. These layers may comprise a base layer  30 , an intermediate layer  32 , and an outer layer  34 . Although FIG. 2 illustrates the layers as having different thicknesses, it should be appreciated that is for illustrative purposes only. The thickness of each layer ( 30 ,  32 ,  34 ) depends upon the specific application for the cutting insert. 
     The base layer  30  is deposited directly onto the surface ( 24 ,  26 ) of the substrate  18 . The thickness of the base layer  30  preferably varies between about 3 micrometers (μm) and about 6 μm. While the composition of the base layer can vary, preferred compositions may include, for example, titanium carbide, titanium carbonitride, and titanium nitride. The intermediate layer  32  is deposited directly onto the surface of the base layer  30 . The thickness of the intermediate layer  32  varies between about 2 μm and about 5 μm. While the compositions of the intermediate layer(s) can vary, preferred compositions may include titanium carbonitride, titanium nitride, titanium carbide, alumina, titanium aluminum nitride and their combinations. The outer layer  34  is deposited directly onto the surface of the intermediate layer  32 . The thickness of the outer layer  34  varies between about 1.5 μm and about 4 μm. While the composition of the outer layer can vary, preferred compositions may include titanium nitride, titanium carbonitride, titanium aluminum nitride, and alumina. 
     While the above description mentions suitable candidates for the coating layers, the preferred coating scheme uses a base coating of titanium carbide, an intermediate coating of titanium carbonitride, and an outer coating of titanium nitride. 
     U.S. Pat. No. 4,035,541, to Smith et al., discloses a three layer coating that is applicable to the cutting insert illustrated in FIG.  2 . In addition, the coating scheme may be applied by a combination of CVD and PVD, such as those processes described in U.S. Pat. No. 5,250,367, to Santhanam et al., for a “Binder Enriched CVD and PVD Coated Cutting Insert,” and U.S. Pat. No. 5,266,388, to Santhanam et al., for a “Binder Enriched Coated Cutting Insert.” Applicant hereby incorporates U.S. Pat. No. 4,035,541, to Smith et al., U.S. Pat. No. 5,250,367, to Santhanam et al., and U.S. Pat. No. 5,266,388, to Santhanam et al., by reference herein. 
     As shown in FIG. 2, for a cutting insert used in milling applications, it is preferred that the binder enriched zone  22  be present underneath peripheral boundaries which lie parallel to the rake face  14  and flank faces  16  of the cutting insert  10 . In other applications such as, for example, turning, it is contemplated that the enriched zone would be present under only the rake face with the zone of enrichment having been removed (e.g., by grinding) from the other faces. In this regard, the cutting insert  40  depicted in FIGS. 3 and 4, which is an SNG 433 style of cutting insert, presents a microstructure in which the enriched zone is present only under the rake faces. 
     Referring to FIGS. 3 and 4, cutting insert  40  has four flank faces  42  which intersect with one rake face  44  and another rake face (not illustrated) opposite from the one rake face  44  so as to form eight cutting edges  48 . Cutting insert  40  has a substrate generally designated as  49  with peripheral boundary  52  at the rake face and a peripheral boundary  54  at the flank face. The substrate  49  has a bulk portion  50  which comprises the majority of the substrate  49 , and a layer of binder enrichment  56  near the peripheral boundary  52  at the rake face. Binder enrichment is absent from the bulk portion  49  including the volume near the peripheral boundary  54 . 
     The substrate  49  for cutting insert  40  is of essentially the same composition as that for cutting insert  10 . The levels of binder enrichment are also essentially the same for cutting insert  40  as those for cutting insert  10 . The basic coating scheme (shown in brackets as  59 ) is also essentially the same for cutting insert  40  as for cutting insert  10 . In this regard, cutting insert  40  has a base coating layer  60 , an intermediate coating layer  62 , and an outer coating layer  64 . 
     The present invention is further described by the following example which is provided solely for the purpose of description, and is not intended to limit the scope of the invention. Inventive Example No. 1 is set forth in conjunction with Comparative Examples Nos. 1 through 3. 
     For the inventive and the comparative examples, the substrate powders contained about 5.8 weight percent cobalt, about 5.2 weight percent tantalum, about 2.0 weight percent titanium, and the balance was tungsten and carbon. The titanium was added in the form of titanium nitride. The tantalum was added in the form of tantalum carbide. The tungsten was added as tungsten carbide and tungsten and the carbon was added in the form of tungsten metal and carbon black. The mixes were charged to various levels of carbon as set forth in Table I below. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Levels of Charged Carbon in the Examples 
               
             
          
           
               
                   
                 Comparative 
                 Comparative 
                 Comparative 
                 Inventive 
               
               
                   
                 Example 
                 Example 
                 Example 
                 Example 
               
               
                 Example 
                 No. 1 
                 No. 2 
                 No. 3 
                 No. 1 
               
               
                   
               
               
                 Charged 
                 5.92 
                 5.98 
                 6.01 
                 5.95 
               
               
                 Carbon (wt. %) 
               
               
                   
               
             
          
         
       
     
     The 5 kilograms (kg) of the mix charge for each example was added to a 7.5 inch inside diameter by 9 inch steel mill jar along with 21 kg of ⅜ inch diameter cemented carbide cycloids and heptane to the top of the jar. The mix was rotated for 40 hours at 52 revolutions per minute (rpm) at ambient temperature. The slurry from each charge was dried, paraffin added as a fugitive binder, and the powders were granulated so as to provide for adequate flow properties. The granulated powders were pressed into SNG433 style cutting insert blanks and sintered at 2650° F. (1456° C.) for about 30 minutes under a vacuum. These cutting insert substrates were then allowed to furnace cool. 
     The rake faces were then ground and the cutting insert blanks reheated at 2650° F. (1456° C.) for about 60 minutes under a vacuum followed by a controlled cool down of 100° F. (56° C.)/hour until reaching 2100° F. (1149° C.). Table II below presents properties of the resultant substrates after reheating. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 Compositions and Physical Properties of Comparative 
               
               
                 Examples and Examples of the Present Invention 
               
             
          
           
               
                   
                 Comparative 
                 Comparative 
                 Comparative 
                 Inventive 
                 Kennametal 
               
               
                 Property/Example 
                 Example No. 1 
                 Example No. 2 
                 Example No. 3 
                 Example No. 1 
                 KC850 Grade 
               
               
                   
               
             
          
           
               
                 Mag. Sat. (gauss- 
                 155 
                 158 
                 158 
                 158 
                 158 
               
               
                 cm 3 /g cobalt) 
               
               
                 H c  (oersteds) 
                 146 
                 142 
                 148 
                 149 
                 160 
               
               
                 Hardness 
                 91.5 
                 91.3 
                 91.4 
                 91.3 
                 91.6 
               
               
                 (Rockwell A) 
               
               
                 Depth of Binder 
                 32 
                 40 
                 42 
                 45 
                 20 
               
               
                 Enrichment (μm) 
               
               
                   
               
             
          
         
       
     
     The cutting insert blanks were then peripheral ground and honed so that in the resulting substrate there was cobalt enrichment on the rake faces and the flank faces did not have cobalt enrichment. The cutting insert blanks were then coated with a tri-phase coating according to U.S. Pat. No. 4,035,541. The base layer was titanium carbide applied via CVD to a thickness of 4.5 micrometers (μm). The intermediate layer was titanium carbonitride applied via CVD to a thickness of 3.5 μm. The top layer was titanium nitride applied via CVD to a thickness of 3.0 μm. 
     The turning performance for the comparative examples and the inventive example was done according to the following test procedure: 
     Workpiece Material: AISI 4340 Steel (300 BHN) 
     Turning conditions: 
     450 surface feet per minute (sfm) [137.2 surface meters per minute] or 550 sfm [167.8 surface meters per minute], feed of 0.020 inch per revolution (ipr) [0.0508 centimeters per revolution] and 0.1 inch (0.254 centimeter) depth of cut (doc) 
     Coolant: TrimSol Regular (20%) 
     Insert Style SNG-433 with radius hone (0.003 inches) [0.0076 centimeters] edge preparation. 
     Insert Life Criteria: 
     Maximum Flank Wear=0.030 inches (0.076 centimeters) 
     Uniform Flank Wear=0.015 inches (0.038 centimeters) 
     Chip=0.030 inches (0.076 centimeters) 
     Crater Wear (depth)=0.004 inches (0.010 centimeters) 
     Nose Wear=0.030 inches (0.076 centimeters) 
     Depth of Cut Notching=0.030 inches (0.076 centimeters) 
     The turning performance of the comparative examples and the inventive example was also done according to the following procedure: 
     Workpiece Material: AISI 1045 Steel (210 BHN) 
     Turning conditions: 
     750 sfm (228.8 surface meters per minute) 0.020 ipr (0.0508 centimeters per revolution) 0.1 inch (0.254 centimeter) depth of cut (doc) 
     Coolant: TrimSol Regular (20%) 
     Insert Style SNG-433 with radius honed (0.003 inches) [0.0076 centimeters] edge preparation. 
     Insert Life Criteria: 
     Maximum Flank Wear=0.030 inches (0.076 centimeters) 
     Uniform Flank Wear=0.015 inches (0.038 centimeters) 
     Chip=0.030 inches (0.076 centimeters) 
     Crater Wear (depth)=0.004 inches (0.010 centimeters) 
     Nose Wear=0.030 inches (0.076 centimeters) 
     Depth of Cut Notching=0.030 inches (0.076 centimeters) 
     The impact strength of the comparative examples and the inventive example was done according to the following slotted bar (41L50 steel) turning test procedure: 
     Speed: 350 sfm (106.8 surface meters per minute) 
     Depth of Cut=0.1 inches (0.254 centimeters) 
     Feed=the starting feed was 0.015 inches per revolution (0.038 centimeters per revolution) with the feed increased 0.005 inches per revolution (0.0127 centimeters per revolution) every 100 impacts until the test reached 800 impacts which was a feed of 0.050 inches per revolution (0.127 centimeters per revolution) or until breakage, whichever occurred first. 
     Table III below sets forth the test results for testing of Comparative Examples Nos. 1 through 4 and the Inventive Example No. 1. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE III 
               
             
             
               
                   
               
               
                 Insert Life and Edge Strength Test Results 
               
               
                 for Comparative Examples No. 1 Through 3 
               
               
                 and the Inventive Example No. 1 
               
             
          
           
               
                   
                   
                 Edge Strength 
                 1045 Steel 
                 4340 Steel 
                 4340 Steel 
               
               
                 Example/Property 
                 Porosity Rating 
                 (# of Impacts) 
                 750 sfm (minutes) 
                 450 sfm (minutes) 
                 550 sfm (minutes) 
               
               
                   
               
             
          
           
               
                 Comp. Ex. No. 1 
                 CO0 
                 635 
                 13.7 
                 24.1 
                 10.6 
               
               
                 Comp. Ex. No. 2 
                 CO3 
                 800 
                 10.7 
                 20.7 
                 9.5 
               
               
                 Comp. Ex. No. 3 
                 CO4 
                 800 
                 5.6 
                 17.6 
                 7.1 
               
               
                 “Kennametal 
                 CO3/CO5 
                 800 
                 5.3 
                 18.75 
                 7.2 
               
               
                 KC850 ® Coated 
               
               
                 Cutting Insert 
               
               
                 Inventive Ex. No. 1 
                 CO2 
                 800 
                 13.1 
                 24.1 
                 10.5 
               
               
                   
               
             
          
         
       
     
     The porosity rating for Table III is done according to the ASTM Designation B 276-86, entitled “Standard Test Method for Apparent Porosity in Cemented Carbides.” The depth of the binder enrichment was determined by optical examination of a cross-section of the specimen via a metallograph at a magnification of 1500×. 
     The edge strength sets forth the number of impacts until either breakage or the test was terminated at 800 impacts via the slotted bar test described above. The turning test results reflect the inserts tool life in minutes from the test procedures described above. 
     The data from Table III shows very clearly that the Inventive Example No. 1 has excellent slotted bar edge strength (800 impacts). It also demonstrated excellent tool life in the turning of 1045 and 4340 steels. The overall metalcutting properties of the Inventive Example No. 1 are superior to all of the other examples shown (i.e., Comparative Examples Nos. 1 through 3 and the “Kennametal KC850®” coated cutting insert). 
     More specifically, the edge strength of the Inventive Example No. 1 is equivalent to the edge strength of the higher carbon Comparative Examples Nos. 2 and 3, and superior to the edge strength of the lower carbon Comparative Example No. 1. Inventive Example No. 1 also has an edge strength that is equivalent to that of the higher carbon alloy “Kennametal KC850®” coated cutting insert. 
     Along with the excellent edge strength, the Inventive Example No. 1 also demonstrated superior 1045 steel tool life in comparison to the other high carbon examples. Inventive Example No. 1 had a tool life of 13.1 minutes in comparison with 10.7 minutes for Comparative Example No. 2, 5.6 minutes for Comparative Example No. 3, and 5.3 minutes for the “Kennametal KC850®” coated cutting insert. The 4340 steel tool life of the Inventive Example No. 1 is also superior to the tool life of the other (800 impact) edge strength higher carbon examples (e.g., Comparative Examples Nos. 2 and 3, and the “Kennametal KC850®” coated cutting insert). Although the 4340 and 1045 steel tool life was only equivalent to, or slightly lower than, the lower carbon Comparative Example No. 1, the Inventive Example No. 1 has superior edge strength in that it sustained 800 impacts verses 635 impacts for Comparative Example No. 1. 
     It is very apparent that the present invention presents a cutting insert with improved characteristics over the Comparative Examples Nos. 1 through 3, as well as the “Kennametal KC850®” coated cutting insert. These improved characteristics are especially apparent in conjunction with the impact strength and wear resistance demonstrated in the interrupted and continuous turning of steel as shown above. 
     All patents and other documents identified in this application are hereby incorporated by reference herein. 
     Other embodiments of the invention will be apparent to those skilled in the art from a consideration of the specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as illustrative only, with the true scope and spirit of the invention being indicated by the following claims.