Chromium-containing cemented tungsten carbide coated cutting insert

The invention pertains to a chromium-containing cemented tungsten carbide body such as a cutting insert. While applicants contemplate other metalcutting applications, these cutting inserts are suitable for the machining (e.g., milling) of workpieces such as, for example, gray cast iron alloys.

Among the metalcutting processes, milling places the most demands on the cutting insert. The cutting insert repeatedly enters, cuts and then exits the workpiece, and thus sustains repeated mechanical and thermal shocks. Thermal shocks and mechanical shocks can each result in microchipping of the cutting edge of the cutting insert.

While earlier coated cutting insert have satisfactory performance, it would be desirable to provide a coated cutting insert that has improved ability to able to withstand the mechanical shocks and thermal shocks of a machining application such milling. Although these coated cutting inserts may have application to metalcutting applications in general, they would have specific application to the milling of gray cast iron alloys.

In one form, the invention is a coated cutting insert that comprises a tungsten carbide-based substrate that has a rake surface and a flank surface, the rake surface and the flank surface intersect to form a substrate cutting edge. The substrate comprises between about 5.7 weight percent and about 6.4 weight percent cobalt, between about 0.2 weight percent and about 0.8 weight percent chromium, tungsten and carbon. There is a coating on the substrate wherein the coating includes a layer of alumina applied by chemical vapor deposition (CVD). Preferably, the substrate comprises at least 70 weight percent, and more preferably, at least 90 weight percent tungsten and carbon.

The following is a brief description of the drawings that form a part of this patent application:

FIG. 1 is an isometric view of a specific embodiment of a cutting insert;

FIG. 2 is a cross-sectional view of the cutting insert of FIG. 1 taken along section 2 — 2 of FIG. 1 that illustrates a coating scheme in which there is a base coating layer, a mediate coating layer and an outer coating layer; and

FIG. 3 is a cross-sectional view of a second embodiment of a cutting insert that shows a single coating layer.

Referring to the drawings, FIGS. 1 and 2 illustrate a first specific embodiment of a cutting insert generally designated as 10 . The cutting insert is made by typical powder metallurgical techniques. One exemplary process comprises the steps of ball milling (or blending) the powder components into a powder mixture, pressing the powder mixture into a green compact, and sintering the green compact so as to form an as-sintered substrate.

In the present embodiments the typical components of the starting powders comprise tungsten carbide, cobalt, and chromium carbide. As one option, carbon may be a component of the starting powder mixture to adjust the overall carbon content.

Cutting insert 10 has a rake face 12 and a flank face 14 . The rake face 12 and the flank face 14 intersect to form a cutting edge 16 . Cutting insert 10 further includes a substrate 18 that has a rake surface 20 and a flank surface 22 . The rake surface 20 and the flank surface 22 of the substrate 18 intersect to form a substrate cutting edge 24 .

Referring to the composition of the substrate, in one range the substrate may comprise between about 5.7 weight percent to about 6.4 weight percent cobalt, between about 0.2 weight percent to about 0.8 weight percent chromium, and at least 70 weight percent tungsten and carbon. In another range the substrate may comprise between about 5.9 weight percent to about 6.1 weight percent cobalt, between about 0.3 weight percent to about 0.7 weight percent chromium, and the balance comprising tungsten, and carbon. Optionally, titanium, tantalum, niobium, zirconium, hafnium and vanadium may also be present in the substrate.

Specific embodiments of the substrate of FIGS. 1 and 2 have a composition that comprises about 6.0 weight percent cobalt, about 0.4 or about 0.6 weight percent chromium and about 93.6 or 93.4 weight percent tungsten and carbon along with minor amounts of impurities. These specific embodiments of the substrate of FIG. 1 have the following physical properties: a hardness of about 91.7-92.6 Rockwell A, a coercive force (H c ) of about 195-245 oersteds (Oe), a magnetic saturation of about 133-149 gauss cubic centimeter per gram cobalt (gauss-cm 3 /gm).

Cutting insert 10 has a coating scheme. The coating scheme includes a base coating layer 30 applied to the surfaces of the substrate 18 , a mediate coating layer 32 applied to the base coating layer 30 , and an outer coating layer 34 applied to the mediate coating layer 32 . In the embodiment of the cutting insert of FIGS. 1 and 2, the base coating layer 30 comprises a layer of titanium carbonitride applied by conventional CVD and the mediate coating layer 32 comprises a layer of titanium carbide applied by conventional CVD so that the combined thickness of the base layer 30 and the mediate layer 32 equals 2.0 micrometers. The outer coating layer 34 comprises alumina applied by conventional CVD to a thickness of about 2.3 micrometers.

Applicants contemplate that an alternate multi-layer coating scheme for the specific embodiment of FIGS. 1 and 2 may comprise a base layer of titanium nitride applied to the surface of the substrate by conventional CVD to a thickness of 1.0 micrometers. A mediate layer of titanium carbonitride applied to the base layer by moderate temperature chemical vapor deposition (MTCVD) to a thickness of 2.0 micrometers. An outer layer of alumina applied to the mediate layer by conventional CVD to a thickness of about 2.0 micrometers.

FIG. 3 illustrates a cross-sectional view of a second specific embodiment of a cutting insert generally designated as 40 . Cutting insert 40 comprises a substrate 42 that has a rake surface 44 and a flank surface 46 . The rake surface 44 and the flank surface 46 intersect to form a substrate cutting edge 48 . The composition of the substrate of the second specific embodiment of the cutting insert is the same as the composition of the substrate of the first specific embodiment of the cutting insert.

Cutting insert 40 has a single layer coating scheme comprising a layer 50 of titanium aluminum nitride applied to the surface of the substrate by physical vapor deposition (PVD). The coating layer 50 is of a thickness equal to about 3.5 micrometers.

In alternate embodiments, applicants contemplate that the base coating layer may comprise any one of the nitrides, carbides and carbonitrides of titanium, hafnium and zirconium and additional coating layers may comprise one or more of alumina and the borides, carbides, nitrides, and carbonitrides of titanium, hafnium and zirconium. These coating layers may be applied by any one or combination of CVD, physical vapor deposition (PVD) [e.g., titanium nitride, titanium carbonitride, titanium diboride, and/or titanium aluminum nitride], or moderate temperature chemical vapor deposition (MTCVD) [e.g., titanium carbonitride]. U.S. Pat. No. 5,272,014 to Leyendecker et al. and U.S. Pat. No. 4,448,802 to Behl et al. disclose PVD techniques. Each one of U.S. Pat. No. 4,028,142 to Bitzer et al. and U.S. Pat. No. 4,196,233 to Bitzer et al. discloses MTCVD techniques, which typically occur at a temperature between 500-900 degrees Centigrade.

The inventors believe that essentially all of the chromium is in the binder and that preferably during the CVD coating operation, chromium from the substrate diffuses into the base coating layer. The base coating layer is preferably one of the carbides, nitrides, or carbonitrides of titanium, hafnium, or zirconium. When during the CVD coating operation cobalt also diffuses into the base coating layer, the ratio of chromium to cobalt in atomic percent (Cr/Co ratio) in the base coating layer is greater than the Cr/Co ratio in the substrate. The inventors believe that diffusion of chromium during CVD coating (>900° C.) into the base layer coating from the substrate enhances coating adhesion during metalcutting and forms a chromium solid solution with the base layer material (e.g., a titanium chromium carbonitride or titanium tungsten chromium carbonitride) having improved wear resistance and adhesion.

Applicants' assignee is the assignee of co-pending United States patent application entitled CHROMIUM-CONTAINING CEMENTED CARBIDE BODY, and filed on the same day as this patent application (Kennametal Inc., U.S. Ser. No. 09/638,048. This co-pending patent application pertains to a chromium-containing cemented carbide body (e.g., tungsten carbide-based cemented carbide body) that has a surface zone of binder alloy enrichment.

Applicants' assignee is also the assignee of co-pending U.S. patent application entitled CHROMIUM-CONTAINING CEMENTED TUNGSTEN CARBIDE BODY, and filed on the same day as this patent application (Kennametal Inc., U.S. Ser. No. 09/637,280. This co-pending patent application pertains to a chromium-containing cemented carbide body (e.g., tungsten carbide-based cemented carbide body) that has a substrate that comprises between about 10.4 weight percent and about 12.7 weight percent cobalt, between about 0.2 weight percent and about 1.2 weight percent chromium, tungsten and carbon. There is a coating on the substrate.

Five milling tests (i.e., Milling Tests Nos. 1 through 5) were conducted to determine the performance of cutting inserts of the present invention as compared to other cutting inserts. A matrix of cutting inserts presenting fifteen different combinations of substrate compositions and coating compositions was tested in the milling of gray cast iron by Milling Tests Nos. 1 through 5. Table 1 below sets forth the compositions of the substrates that consist of Inventive Substrates Nos. 1 and 2, and Comparative Substrates A through C.

The coating schemes comprise a first coating scheme, a second coating scheme and a TiAlN coating scheme.

The first coating scheme comprises a base layer of titanium carbonitride applied by conventional CVD to the surface of the substrate and a mediate layer of titanium carbide applied by conventional CVD to the base layer so that the combined thicknesses of the base layer and the mediate layer equals 2.3 micrometers. The C994M coating scheme further includes an outer layer of alumina applied to the mediate layer by conventional CVD to a thickness of 2.3 micrometers.

The second coating scheme comprises a base layer of titanium nitride applied by conventional CVD to the surface of the substrate to a thickness of 1.0 micrometers, a mediate layer of titanium carbonitride applied to the base layer by moderate temperature chemical vapor deposition (MTCVD) to a thickness of 2.0 micrometers, and a outer layer of alumina applied to the mediate layer by conventional CVD to a thickness of 2.0 micrometers.

The TiAlN coating scheme comprises a single layer of titanium aluminum nitride applied to the surface of the substrate by PVD to a thickness of about 3.5 micrometers.

Flycut Milling Test No. 1 was performed on gray cast iron at the following parameters: a speed equal to about 900 surface feet per minute (sfm); a feed equal to 0.010 inches per tooth (ipt); an axial depth of cut (a.doc) equal to 0.1 inches and a radial depth of cut (r.doc) equal to 3 inches wherein the tool life criteria were 0.015 inches uniform flank wear (UFW) and 0.030 inches maximum flank wear (FW). The milling was done dry without a coolant. The cutting inserts were a SPG433 style of cutting insert with a 30 degree lead angle.

Table 2 presents the test results for Flycut Milling Tests No. 1 in the form of tool life in minutes, the standard deviation as a percentage of tool life, and the relative tool life as measured against Comparative Substrate A.

Flycut Milling Test No. 2 was performed on gray cast iron at the following parameters: a speed equal to about 900 surface feet per minute (sfm); a feed equal to 0.010 inches per tooth (ipt); and an axial depth of cut (a.doc) equal to 0.1 inches and a radial depth of cut (r.doc) equal to 3 inches wherein the tool life criteria were 0.015 inches uniform flank wear (UFW) and 0.030 inches maximum flank wear (FW). The milling was with flood coolant. The cutting inserts were a SPG433 style of cutting insert with a 30 degree lead angle.

Table 3 presents the test results for Flycut Milling Tests No. 2 in the form of tool life in minutes, the standard deviation as a percentage of the tool life, and the relative tool life as measured against Comparative Substrate A.

Flycut Milling Test No. 3 was performed on gray cast iron at the following parameters: a speed equal to about 1200 surface feet per minute (sfm); a feed equal to 0.010 inches per tooth (ipt); and an axial depth of cut (a.doc) equal to 0.1 inches and a radial depth of cut (r.doc) equal to 3 inches wherein the tool life criteria were 0.015 inches uniform flank wear (UFW) and 0.030 inches maximum flank wear (FW). The milling was done dry without a coolant. The cutting inserts were a SPG433 style of cutting insert with a 30 degree lead angle.

Table 4 presents the test results for Flycut Milling Tests No. 3 in the form of tool life in minutes, the standard deviation as a percentage of the tool life, and the relative tool life as measured against Comparative Substrate A.

Flycut Milling Test No. 4 was performed on gray cast iron at the following parameters: a speed equal to about 900 surface feet per minute (sfm); a feed equal to 0.010 inches per tooth (ipt); and an axial depth of cut (a.doc) equal to 0.1 inches and a radial depth of cut (r.doc) equal to 3.5 inches wherein the tool life criteria were 0.015 inches uniform flank wear (UFW) and 0.030 inches maximum flank wear (FW). The milling was done dry without a coolant. The cutting inserts were a SPG433 style of cutting insert with a 30 degree lead angle.

Table 5 presents the test results for Flycut Milling Tests No. 4 in the form of tool life in minutes, the standard deviation as a percentage of the tool life, and the relative tool life as measured against Comparative Substrate A.

Flycut Milling Test No. 5 was performed at the following parameters: a speed equal to about 900 surface feet per minute (sfm); a feed equal to 0.010 inches per tooth (ipt); and an axial depth of cut (a.doc) equal to 0.1 inches and a radial depth of cut (r.doc) equal to 3.5 inches wherein the tool life criteria were 0.015 inches uniform flank wear (UFW) and 0.030 inches maximum flank wear (FW). The milling was done with flood coolant. The cutting inserts were a SPG433 style of cutting insert with a 30 degree lead angle.

Table 6 presents the test results for Flycut Milling Tests No. 5 in the form of tool life in minutes, the standard deviation as a percentage of the tool life, and the relative tool life as measured against Comparative Substrate A.

The patents and other documents identified herein 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 a practice of the invention disclosed herein. It is intended that the specification and examples are illustrative only and are not intended to be limiting on the scope of the invention. The true scope and spirit of the invention is indicated by the following claims.