SINTER CARBIDE EMPLOYMENT AND METHOD TO ITS PRODUCTION.

The present invention relates to a cemented carbide tool for metal cutting or metal forming made via sintering techniques. More specifically, the invention pertains to a cemented carbide tool that is made via sintering techniques wherein there are two distinct microstructural zones having complementary properties.

In cemented carbides the grain size, as well as the binder phase (e.g., cobalt) content each has an influence on the performance of the composite. For example, a smaller or finer grain size of the tungsten carbide results in a more wear resistant material. An increase in cobalt content typically leads to an increase in toughness.

Cemented carbides having a fine grain size are produced through the incorporation of grain refiners in the initial powder blend. Such cemented carbide has a fine grain size throughout its microstructure. Cemented carbide with a coarse grain size is produced via sintering without the incorporation of any grain refiners since the tendency of a cemented carbide like a WC-Co composite is for the WC grains to coarsen during sintering. Such cemented carbide has a coarse grain size throughout its microstructure. As can be appreciated these hard bodies have a uniform microstructure throughout.

Cemented carbide products are widely used in tools for metal machining as well as for different coldforming operations of materials like steels, copper alloys, composite materials, etc. Examples of the latter type of tools are wire drawing dies, which are a cemented carbide nib usually fit into a steel or metal holder. Such tools should have a hard and wear resistant surface zone which also should have the following additional properties: good thermal conductivity; low coefficient of friction, i.e., it may be self lubricating or assist lubrication with a coolant; good corrosion resistance; resistance to microcracking; and high toughness.

Cemented carbide bodies having at least two distinct microstructural zones are known in the art. For example drills having a core of a tough cemented carbide grade and a cover of a more wear resistant grade are disclosed in EP-A-951576.

EP-A-194018 relates to a wire drawing die made from a central layer with coarse grained tungsten carbide particles and a peripheral layer with finer grained tungsten carbide particles. Initially, the layers have the same content of cobalt. After sintering, the coarse grained layer in the center is reduced in cobalt content.

EP-A-257869 discloses a rock bit button made with a wear resistant tip portion and a tough core. The tip portion is made from a powder with low Co-content and a fine WC grain size and the core portion is made from a powder with high Co content and coarse WC grains. Nothing is disclosed about the Co-content in the two portions after sintering. However, also in this case the Co-content in the coarse grained portion will be reduced at the benefit of the Co-content in the fine grained layer. A similar disclosure is found in US 4,359,335.

An alternative approach is disclosed in US 4,843,039 which discloses cemented carbide bodies preferably for cutting tool inserts for metal machining. The bodies comprise a core of cemented carbide containing eta-phase surrounded by a surface zone of cemented carbide free of eta-phase and having a low content of cobalt in the surface and a higher content of cobalt next to the eta-phase zone. US 4,743,515 is similar, but it relates to rock drilling and mineral cutting.

US 5,623,723 discloses a method of making a cemented carbide body with a wear resistant surface zone. The method includes the following steps: providing a compact of cemented carbide; placing a powder of grain refiner on at least one portion of the exposed surface of the compact; and heat treating the compact and grain refiner powder so as to diffuse the grain refiner toward the center of the green compact thereby forming a surface zone inwardly from the exposed surface in which the grain refiner was placed, and forming an interior zone. As a result, a cemented carbide body is obtained with a surface zone having a grain size that is smaller but with a Co-content that is higher than that of the interior portion of the body. This means that the increased wear resistance that is obtained as a result of the smaller WC grain size is to a certain extent lost by the increase in Co-content.

It is therefore an object of the present invention to provide a cemented carbide tool with a surface zone with low binder phase content and fine WC grain size and thus high wear resistance and a method of making the same.

It has now surprisingly been found that it is possible from a single mixture of tungsten carbide and binder phase to obtain a cemented carbide body with a surface portion with a smaller grain size and lower cobalt content than those in the interior portion.

1. Figure 1 is a graph showing hardness (HV3) and cobalt content versus distance from the edge in an insert according to the invention.
2. Figure 2 is a graph showing chromium content versus distance from the edge in an insert according to the invention.
3. Figure 3 is a micrograph showing the microstructure at a distance of 100 µm from the edge (FEG-SEM, 20000X, BSE mode) in an insert according to the invention.
4. Figure 4 is a micrograph showing the microstructure at a distance of 3 mm from the edge (FEG-SEM, 20000X, BSE mode) in an insert according to the invention.
5. Figure 5 is a graph showing the cobalt content versus the distance to the previously Cr₃C₂-covered surface and also the chromium content versus the distance to the previously Cr₃C₂-covered surface in another embodiment of the invention.
6. Figure 6 is a micrograph showing the microstructure at a distance of 100 µm from the surface where the Cr₃C₂-powder was placed (FEG-SEM, 15000X, BSE mode).
7. Figure 7 is a micrograph showing the microstructure at a distance of 3 mm from the surface where the Cr₃C₂-powder was placed (FEG-SEM, 15000X, BSE mode.)

According to the present invention there is provided a cemented carbide tool for metal cutting or metal forming comprising hard constituents in a binder phase of Co and/or Ni comprising at least one surface portion, 0-2000 µm thick, preferably 5-1200 µm, more preferably 10-800 µm, most preferably 10-300 µm and an interior portion in which surface portion the grain size is smaller than in the interior portion and the binder phase content is lower than that in the interior portion and the Cr-content is higher than that in the interior portion. More particularly the binder phase content of the surface portion is <1, preferably <0.92, more preferably <0.85, of the binder phase content in the interior portion and the WC grain size of the surface portion is <1, preferably <0.9, more preferably <0.8, of the WC grain size in the interior portion. Preferably the surface portion contains Cr such that the ratio between the parameter A=((wt-% Cr/wt-% binder phase)+0.01) in the surface portion and the parameter B=((wt-% Cr/wt-% binder phase)+0.01) taken at the part of the body that is characterized by the lowest Cr content is A/B >1.5, preferably A/B>3.0.

In a first embodiment the WC grain size of the surface zone is submicron.

In a second embodiment the WC grain size of the interior portion is 1-3 µm.

In a third embodiment the composition of the cemented carbide is WC+Co with a binder phase content >1.5 wt-%, preferably >5 wt-%, but <25 wt-%, preferably <15 wt-%.

In a fourth embodiment the cemented carbide in addition contains 0-30, preferably 0.2-16, most preferably 0.4-9 vol-% γ-phase.

In a fifth embodiment the cemented carbide tool is a cutting tool insert for metal machining. It is obvious to the man skilled in the art that the invention can be applied to other cemented carbide cutting tools, such as endmills and drills.

In a sixth embodiment the cemented carbide tool is a coldforming tool. Other examples of use of cemented carbide in forming applications are from such variable fields as the forming of beverage cans, bolts, nails and other applications known to the person skilled in the art.

All these embodiments can contain grain refiners such as VC and Cr₃C₂.

The cemented carbide tool may further be provided with a wear resistant coating as known in the art, preferably 1-40 µm thick, more preferably 1-15 µm thick.

The present invention also relates to a method of making a cemented carbide body for metal cutting or metal forming, such as a cutting tool insert for chip forming machining or a coldforming tool, with a wear resistant surface zone, including the following steps:

• providing a compact of cemented carbide made from one single powder comprising powders forming hard constituents, possibly grain refiners such as VC and Cr₃C₂, and binder phase of Co and/or Ni;
• placing a powder of grain refiner on at least one portion of the exposed surface of the compact by dipping, spraying painting, applying a thin tape or in any other way; the grain refiner, preferably being any chromium carbide (Cr₃C₂, Cr₂₃C₆ and Cr₇C₃ or mixtures of these) or a mixture of chromium and carbon or other compounds containing chromium and carbon and/or nitrogen.
• sintering the compact and grain refiner powder so as to diffuse the grain refiner away from the surface(s) of grain refiner application thereby forming a gradient zone characterized of lower cobalt content, higher chromium content and lower WC grain size compared to the interior portion;
• possibly adding an isostatic gas pressure during the final stage of sintering to obtain a dense body;
• possibly reducing the thickness of the surface portion using grinding or any other mechanical method;
• possibly removing undesired carbides and graphite from the surface using grinding or any other mechanical method;
• possibly depositing a wear resistant coating as known in the art;
• in case of the tool being a cutting insert, possibly performing an edge treatment as known in the art.

The carbon content of the cemented carbide compact shall be determined out of consideration for the carbon contribution from the applied chromium carbide. In the case of γ-phase containing cemented carbide, the chromium solubility in the γ-phase has to be compensated for. Also compacts that would result in an eta-phase containing microstructure can be used. The sintering shall be performed for optimal time to obtain the desired structure and a body with closed porosity, preferably a dense body. This time depends on the grain size of WC and the composition of the cemented carbide and can therefore not be closer defined. It is within the purview of the person skilled in the art to determine whether the requisite structure has been obtained and to modify the sintering conditions in accordance with the present specification. If necessary the body can be post-HIP-ed at a lower HIP-temperature compared to the sintering temperature and at a pressure of 1-100 MPa.

Alternatively, the grain refiner powder is placed on a sintered body which is subsequently heat treated to obtain the desired structure at a temperature higher than that for presintering.

Cemented carbide pressed compacts in the style B-SNGN120408 were made according to the following: Green compacts were pressed from a powder with the composition of 90 weight-% WC and 10 weight-% Co. The WC raw material was fine-grained with an average grain size of 0.25 µm (FSSS). The rake faces were covered with a Cr₃C₂ containing thin layer (0.02 g Cr₃C₂/cm²). Thereafter the compacts were sintered at 1370°C for 30 minutes whereafter the outer 1 mm deep portion was removed by grinding. A cross-section of a sintered and ground blank was examined. Figure 1 shows a graph of hardness and cobalt content versus the distance from the edge. The cobalt content is lowest close to the edge and increases with increasing distance while the hardness is highest close to the edge and decreases with the distance. Figure 2 shows a graph of chromium content versus the distance from the edge. The chromium content is highest close to the edge and decreases with the distance. Cobalt and chromium contents were measured using EPMA (electron probe microanalyser). Figure 3 is a micrograph showing the microstructure at a distance of 100 µm from the edge (FEG-SEM, 20000X, BSE mode). Figure 4 is a micrograph showing the microstructure at a distance of 3 mm from the edge (FEG-SEM, 20000X, BSE mode). The WC-grain size 100 µm from the edge and 3 mm from the edge was measured as 0.28 µm and 0.36 µm respectively (arithmetic mean of linear intercept values).

Cemented carbide pressed compacts in the style B-SNGN120408 were made according to the following: Green compacts were pressed from a powder with the composition of 94 weight-% WC and 6 weight-% Co. The WC raw material was relatively fine-grained with an average grain size of 0.25 µm (FSSS). The rake faces were covered with 0.007 g/cm² Cr₃C₂.

The pressed compacts with Cr₃C₂-layers were sintered at 1350°C for 30 minutes and post-HIP-ed at 1300°C and 6 MPa for 30 minutes. A cross-section of a sintered blank was examined. No Cr₃C₂ was observed on the surface. The following table shows HV3, cobalt-content, chromium-content and WC grain size for this example:

Cemented carbide pressed compacts in the style B-SNGN120408 were made according to the following: Green compacts were pressed from a powder with the composition of 90 weight-% WC and 10 weight-% Co. The rake faces were covered with a Cr₃C₂ containing thin layer (0.01 g Cr₃C₂/cm²). Thereafter the compacts were sintered at 1370°C for 30 minutes. A cross-section of a sintered blank was examined. No Cr₃C₂ was observed on the surface. The following table shows HV3, cobalt-content, chromium-content and WC grain size for this example:

Cemented carbide pressed compacts in the style B-SNGN120408 were made according to the following: Green compacts were pressed from a powder with the composition of 90 weight-% WC and 10 weight-% Co. The WC raw material was fine-grained with an average grain size of 0.25 µm (FSSS). The rake faces were covered with a Cr₃C₂ containing thin layer (0.018 g Cr₃C₂/cm²). Thereafter the compacts were sintered at 1410°C for 60 minutes. A cross-section of a sintered blank was examined. No Cr₃C₂ was observed on the surface. The following table shows HV3, cobalt-content, chromium-content and WC grain size for this example:

Cemented carbide pressed compacts in the style B-SNGN120408 were made according to the following: Green compacts were pressed from a powder with the composition of 94 weight-% WC and 6 weight-% Co. The WC raw material was submicron. The pressed compacts were sintered at 1370°C. The sintered blanks were ground into style SNKN1204 EN and covered with a thin Cr₃C₂-containing tape (0.01 g/cm²) on the clearance face and resintered at a sintering temperature of 1390°C for 15 minutes. A cross-section of a sintered blank was examined. No Cr₃C₂ was observed on the surface. The following table shows HV3, cobalt-content, chromium-content and WC grain size for this example:

Cemented carbide pressed compacts in the style B-SNGN120408 were made according to the following: Green compacts were pressed from a powder with the composition of 77 weight-% WC, 6 weight-% TaC, 2 weight-% NbC, 4 weight-% TiC and 11 weight-% Co. The compacts were covered with a thin Cr₃C₂-containing tape (0.02 g/cm²) and sintered at a sintering temperature of 1370°C for 30 minutes and thereafter HIP-ed at 1200°C and 100 MPa for 60 minutes. A cross-section of a sintered blank was examined. The cobalt content and the WC grain size was significantly lower close to the edge compared to the interior as was verified through the following HV3 values.

Inserts were produced according to the following:

1. Composition: 91.6 weight-% WC + 0.23 weight-% TaC + 0.16 weight-% NbC + 8.0 weight-% Co
2. Style: CNMG120408-QM
3. Sintering temperature: 1370°C

The inserts were given a rounded cutting edge and thereafter split into two variants. Variant A was covered with Cr₃C₂ on the rake face according to the invention using a painting technique (0.01 g/cm²). Variant B was not covered with Cr₃C₂.

For the rest of the manufacturing the two variants were dealt with together and in the same way involving resintering at 1390°C for 15 minutes, blasting, cleaning and coating with a 4 µm thick TiAlN PVD-layer. Cross-sections of each variant were examined. No Cr₃C₂ was observed between the TiAlN-layer and the cemented carbide material on Variant A. The following table shows cobalt-content, chromium-content and WC grain size for this example:

The inserts were tested in a specially designed facing operation in order to compare the resistance to plastic deformation according to the following:

1. Workpiece: Inconel 718
2. Cutting depth: 1 mm
3. Feed: 0.25 mm/rev
4. Cutting speed: 80-140 m/min
5. Result (maximum cutting speed for keeping plastic deformation below 0.25 mm):
   1. Variant A: 120 m/min
   2. Variant B: 100 m/min

Cemented carbide pressed compacts were made according to the following: A cylindrical green compact were pressed from a powder with the composition of 96.7 weight-% WC and 3.3 weight-% Co and 0.2% VC. The WC raw material was relative fine-grained with an average grain size of 0.8 µm (FSSS). One surface was covered with a Cr₃C₂ containing thin layer (0.02 g Cr₃C₂/cm²). Thereafter the compacts were sintered at 1370°C for 30 minutes. A cross-section of the sintered body was examined. Cobalt and chromium contents were measured using EPMA (electron probe microanalyser). Figure 5 is a graph showing cobalt content versus the distance from the previously Cr₃C₂-covered surface. The cobalt content is lowest close to the surface and increases with increasing distance showing a tendency to formation of a Co richer zone between the surface and the bulk. Figure 5 also shows the chromium content versus the distance from the previously Cr₃C₂-covered surface. The chromium content is highest close to the surface and decreases with the distance. Figure 6 is a micrograph showing the microstructure at a distance of 100 µm from the surface where the Cr₃C₂ powder was placed (FEG-SEM, 15000X, BSE mode). Figure 7 is a micrograph showing the microstructure at a distance of 3 mm from the surface where the Cr₃C₂ powder was placed (FEG-SEM, 15000X, BSE mode.) The following values show cobalt-content, chromium-content and WC grain size. The WC-grain size measured as arithmetic mean of intercept values.