Patent Application
20220267882
The present invention relates to a nanoscale or ultrafine cemented carbide including tungsten carbide, an additional metal carbide phase that is in a cubic crystal structure, and a binder metal phase, to a process for the preparation thereof, and to the use thereof for preparing tools and wear parts. Further, the present invention relates to a component prepared from the cemented carbide described.
Cemented carbides are metal matrix composite materials in which hard materials in the form of small particles are cemented together by a matrix of metal. Cemented carbides are employed predominantly in applications in which materials having a high wear resistance and hardness while showing a high strength are required. Thus, cemented carbides are used, for example, as a cutting material for tools (such as turning tools, drills, and milling tools) and as wear-resistant matrices, e.g., in reforming or punching tools. However, conventional cemented carbides have the drawback of having a very low fracture toughness, which significantly limits their applicability. Conventionally, increasing the fracture toughness is possible by increasing the content of binder metal, which results in a decrease of hardness, however. Ideally, a tool made of a cemented carbide should have a high hardness while also having a high fracture toughness.
U.S. Pat. No. 5,593,474 describes a sintered body of a composite material comprising a plurality of regions of a first metal carbide; and a plurality of regions of a second metal carbide, the first metal carbide having a larger particle size than the second metal carbide.
DE 10 2004 051 288 addressed the object or providing a polycrystalline powder of hard material with improved hardness while the toughness is maintained. This object is achieved by a polycrystalline powder of hard material consisting of polycrystalline grains of hard material, which are made of crystals of carbides, nitrides and/or carbonitrides of the transition metals of groups 4, 5 and 6 (titanium, vanadium and chromium groups) of the Periodic Table.
WO 2017/186468 relates to a cemented carbide comprising a phase of hard material grains and a phase of a heterogeneously distributed binder metal, wherein said hard material grains have an average grain size within a range of from 1 nm to 1000 nm, and said heterogeneously distributed binder metal is present in the form of binder islands within the cemented carbide that have an average size 0.1 μm to 10 μm, and an average distance between neighboring binder islands of from 1 μm to 7 μm.
EP 1 526 189 describes a cemented carbide comprising WC, a binder phase based on Co, Ni or Fe, and a gamma phase in which said gamma phase has an average particle size of less than 1 μm. Said gamma phase is prepared from presynthesized mixed carbides in the form (Me,W)C.
CN 103540823 describes a cemented carbide composition comprising from 40 to 50% by weight of WC, from 5 to 10% by weight of vanadium carbide, from 3 to 8% by weight of chromium carbide, from 5 to 9% by weight of titanium carbide, from 6 to 11% by weight of tantalum carbide, from 2 to 5% by weight of niobium carbide, and from 12 to 18% by weight of cobalt. The particle size of said WC is within a range of from 0.1 to 0.8 μm.
EP 1 557 230 relates to a cemented carbide body comprising from 10 to 12% by weight of cobalt, less than 3% by weight of tantalum carbide, from 1 to 5.5% by weight of niobium carbide, and from 3 to 5% by weight of titanium carbide, the balance being WC. Said WC has a particle size of from 0.4 to 1.5 μm, especially from 0.8 to 1.5 μm.
U.S. Pat. No. 4,698,266 discloses a cutting tool comprising a maximum of 70% by weight of WC, and from 5 to 10% by weight of a cobalt binder phase, the rest of the composition being formed by metal carbides selected from the group consisting of TiC, TaC, NbC, HfC, and mixtures thereof. The average grain size of said WC is from 0.9 to 1.3 μm.
Even if some solution approaches are already offered in the prior art, there is still no commercial solution for cemented carbides that have both a high hardness and wear resistance, and a high fracture toughness.
Therefore, it is the object of the present invention to provide a cemented carbide that pas an improved combination of hardness and fracture toughness, and preferably is accessible in a simple way.
Surprisingly, it has been found that this object is achieved by providing a nanoscale or ultrafine cemented carbide based on tungsten carbide, which further includes a metal carbide phase that is in a cubic crystal structure at room temperature.
Therefore, the present invention first relates to a cemented carbide, comprising
wherein said additional metal carbide phase is in a cubic crystal structure at room temperature, and wherein the proportion of said additional metal carbide phase in said cemented carbide is at least 4% by volume, based on the total volume of the cemented carbide, and wherein the average grain size was determined by the linear-intercept technique according to ISO 4499-2.
The conversion from percent by volume to percent by weight, or the conversion from percent by weight to percent by volume, is effected according to the following formulas:
wherein mi represents the mass proportion, vi represents the volume proportion, and ρi represents the density of the respective component.
The cemented carbide according to the invention is a nanoscale or ultrafine cemented carbide, whose classification is effected in accordance with ISO 4499-2.
Within the scope of the present invention, “cemented carbide” describes a sintered composite material. Said additional metal carbide phase, which is in a cubic crystal structure at room temperature, i.e., at 25° C. within the scope of the present invention, is hereinafter referred to interchangeably as a “cubic metal carbide”.
The cemented carbide according to the invention has a high hardness and a high fracture toughness. The problem that the fracture toughness decreases as the hardness of the cemented carbide increases, i.e., that the material becomes brittle and friable, which occurs in conventional cemented carbides, was not observed in the case of the cemented carbide according to the invention. Without being bound by theory, it is considered that the positive properties of the cemented carbide according to the invention can be attributed, in particular, to the combination of the small grain size of the tungsten carbide and the presence of the cubic metal carbide phase. Therefore, the tungsten carbide used in the cemented carbide according to the invention has an average grain size of from 0.05 to 0.5 μm, preferably from 0.05 to 0.23 μm, more preferably from 0.05 to 0.09 μm, as determined by the linear-intercept technique according to ISO 4499-2.
In a further preferred embodiment, the metal carbide phase that is in a cubic crystal structure at room temperature is selected from the group consisting of titanium carbide, tantalum carbide, niobium carbide, hafnium carbide, zirconium carbide, mixtures thereof, and mixed carbides of these compounds.
Preferably, the metal carbide phase used in the cemented carbide according to the invention has an average grain size of from 0.3 to 4.0 μm, preferably from 0.5 to 1.5 μm, as determined by the linear-intercept technique according to ISO 4499-2.
Surprisingly, it has been found that a particularly advantageous relationship of hardness and fracture toughness could be achieved if the metal carbide phases present in the cemented carbide according to the invention are homogeneously distributed. Therefore, an embodiment is preferred in which the metal carbide phase contained in the cemented carbide is in a periodically repeated distribution with an average distance of from 0.5 to 10 μm, preferably from 1 to 3 μm. Said average distance can be determined by linear analysis (linear-intercept technique) on electron micrographs of sections, and relates to the distance from grain center to grain center. Without being bound by theory, the particularly homogeneous distribution of the metal carbide phase in the cemented carbide according to the invention is attributed, inter alia, to the use of a tungsten carbide powder having the above mentioned grain sizes.
It has been found advantageous if powders having a particular particle size are used as starting materials for the tungsten carbide and the cubic metal carbide, wherein a “starting material” within the scope of the present invention means an unsintered powder. Therefore, in a preferred embodiment, a tungsten carbide powder having an average particle size dBET of from 0.05 to 0.30 μm, preferably from 0.05 to 0.25 μm, more preferably from 0.05 to 0.2 μm, is used as a starting material. The average particle size dBET is calculated from the specific surface area of the starting material as determined according to BET (BET surface area) by converting it according to the formula dBET=6/(BET surface area*density). The specific surface area can be determined by the BET method according to DIN ISO 9277. The density corresponds to the physical density of the pure solid and can be extracted from the literature, the density of tungsten carbide usually being stated as 15.7 g/cm3.
As the starting material for the cubic metal carbide, there is preferably employed a cubic metal carbide powder having an average particle size dBET of from 0.3 to 5 μm, more preferably from 0.4 to 1 μm, as determined according to the BET surface area of the starting material, and by conversion according to the formula dBET=6/(BET surface area*density). The physical density of the respective cubic carbide is to be used as said density. The values can be extracted from the literature.
In a preferred embodiment, the binder metal is a compound selected from the group consisting of cobalt, iron, nickel, and mixtures thereof. More preferably, the binder metal is cobalt. In a further preferred embodiment, the binder metal is a mixture consisting of iron, cobalt, and nickel, in which the proportion of the respective metals in the mixture is more than 1% by mass.
Surprisingly, it has been found that the sole addition of the cubic metal carbides as described above has no influence on grain growth during the production process, so that grain growth inhibitors may be optionally added to the cemented carbide according to the invention for reducing grain growth during the production process thereof. Therefore, an embodiment is preferred in which the cemented carbide further includes grain growth inhibitors, preferably those selected from the group consisting of vanadium carbide, chromium carbide, mixtures thereof, and mixed carbides of such compounds. The proportion of grain growth inhibitor in the cemented carbide is preferably from 0.05 to 6% by volume, based on the total volume of the cemented carbide.
Within the scope of the present invention, it has been found advantageous if the proportion of tungsten carbide in the cemented carbide according to the invention does not exceed a proportion of 95% by volume. Therefore, an embodiment is preferred in which the proportion of tungsten carbide in the cemented carbide according to the invention is from 40 to 80% by volume, based on the total volume of the cemented carbide. In this way, a sufficient hardness and fracture toughness of the cemented carbide can be ensured.
Further, it has been found advantageous to limit the proportion of binder metal in the cemented carbide. Therefore, an embodiment is preferred in which the proportion of binder metal in the cemented carbide according to the invention is not more than 40% by volume, preferably from 10 to 32% by volume, respectively based on the total volume of the cemented carbide.
Surprisingly, it has been found that the hardness of the cemented carbide can be increased while the fracture toughness is maintained, if the volume proportion of the additional metal carbide phase in the cemented carbide according to the invention comprises at least 4% by weight. Therefore, an embodiment is preferred in which the proportion of the additional metal carbide phase is from 4 to 30% by volume, preferably from 10 to 20% by volume, alternatively from 25 to 37% by volume, respectively based on the total volume of the cemented carbide.
In a particularly preferred embodiment, the cemented carbide according to the invention has the following composition:
balance: additional metal carbide phase;
wherein the proportion of the additional metal carbide phase is at least 4% by volume, based on the total volume of the cemented carbide, and wherein said percent by volume are respectively based on the total volume of the cemented carbide and sum up to 100% by volume, optionally considering further components, such as grain growth inhibitors.
Conventional cemented carbides have the disadvantage that although the hardness is increased by reducing the content of binder metal, the fracture toughness decreases. At the same time, an undesirable increase of thermal conductivity may occur. Surprisingly, it has been found that the cemented carbide according to the invention has an advantageous thermal conductivity. In a preferred embodiment, the cemented carbide according to the invention has a thermal conductivity of less than 50 W/m*K, preferably less than 40 W/m*K, as determined by the laser flash technique at 40° C.
In addition to an advantageous thermal conductivity, the cemented carbide according to the invention is further characterized by an improved fracture toughness. Therefore, an embodiment is preferred in which the cemented carbide according to the invention has a fracture toughness of more than 8.0 MPa*m1/2, as determined from Vickers hardness impressions according to the Palmquist method as described in Shetty et al., Journal of Materials Science 20 (1985), pp. 1873 to 1882.
The present invention further relates to a process for the preparation of a cemented carbide according to the invention, comprising:
The average particle size dBET is determined as described above from the BET surface area and conversion according to the formula dBET=6/(BET surface area*density).
The proportion of said additional cubic metal carbide powder in the powder mixture is selected for the cemented carbide obtained to have a proportion of at least 4% by volume of the cubic metal carbide phase, based on the total volume of the cemented carbide.
The binder metal powders as stated above are preferably used.
In a preferred embodiment, said forming and sintering of the mixture is performed to obtain a cemented carbide body. Said cemented carbide body may be, for example, a component.
In a preferred embodiment, the sintering within the scope of the process according to the invention is effected at a temperature of from 1150 to 1550° C. In this way, the cemented carbide according to the invention is accessible by a process that can be realized simply in industry.
Within the scope of the present invention, it has been surprisingly found that, for preparing the cemented carbide according to the invention, it is not necessary to use presynthesized mixed carbides of the form (Me,W)C, as described in the prior art.
Rather, the cemented carbide according to the invention can be prepared from the pure metal carbides or mixtures thereof.
The cemented carbide according to the invention is suitable, in particular, for use in fields of application in which a high hardness and at the same time a good fracture toughness are required. Therefore, the present invention further relates to the use of the cemented carbide according to the invention for the production of tools. Preferably, the tools are tools with defined and undefined cutting edges, and tools for the machining of all kinds of materials.
The present invention further relates to a component obtained by forming the cemented carbide according to the invention. Preferably, the component is selected from the group consisting of drills, solid carbide cutters, indexable inserts, saw teeth, forming dies, sealing rings, extrusion punches, press dies, and wear parts.
The present invention is further explained by means of the following Examples, which are by no means to be understood as limiting the idea of the invention.
As the starting powder, a WC powder having a dBET value of 90 nm, a cobalt metal powder having a dBET value of 205 nm, a TiC powder having a dBET value of 610 nm, a TaC powder having a dBET value of 370 nm, a Cr3C2 powder having a dBET value of 430 nm, and a VC powder having a dBET value of 350 nm were used. A 200 g mixture of 62.7% by volume (77% by weight) WC, 15.9% by volume (11% by weight) Co, 12.9% by volume (5% by weight) TiC, 4.4% by volume (5% by weight) TaC, 1.9% by volume (1% by weight) Cr3C2, and 2.2% by volume (1% by weight) VC was ground in n-heptane in a ball mill for 48 hours. The dispersion of cemented carbide obtained was dried and pressed uniaxially with a pressing power of 300 MPa into rectangular test specimens with a green density of >50% of the density to be expected for a solid body (theoretical density). The test specimens were compacted under vacuum at a temperature of 1450° C. and with a holding time of 30 min to above 95% of the theoretical density, followed by a final compaction under an argon atmosphere at the same temperature (Sinter-HIP technology). The test specimens proved to be completely dense under an optical microscope. The porosity according to ISO 4505 corresponded to >A02, B00, C00. The Vickers hardness was determined to be 1770 HV10, and the fracture toughness (K1C) was calculated by measuring the crack lengths and using the formula of Shetty (Shetty 1985—Indentation fracture of WC-Co cermets, see reference above) to be 9.5 MPa*mJ1/2. The thermal conductivity (TC) was determined to be 29 W/m*K (measurement at 40° C. by the laser flash technique).
Table 1 shows the characteristics determined as compared to a cemented carbide having a composition without additions of cubic metal carbide, but with an otherwise comparable content of binder metal.
As the starting powder, a WC powder having a dBET value of 90 nm, a cobalt metal powder having a dBET value of 205 nm, a TiC powder having a dBET value of 610 nm, a TaC powder having a dBET value of 370 nm, a Cr3C2 powder having a dBET value of 430 nm, and a VC powder having a dBET value of 350 nm were used. A 200 g mixture of 68.9% by volume (80.6% by weight) WC, 16% by volume (10.6% by weight) Co, 4% by volume (2.6% by weight) TiC, 7% by volume (4.3% by weight) TaC, 1.9% by volume (0.9% by weight) Cr3C2, and 2.2% by volume (1% by weight) VC was ground in n-heptane in a ball mill for 48 hours. The dispersion of cemented carbide obtained was dried and pressed uniaxially with a pressing power of 300 MPa into rectangular test specimens with a green density of >50% of the density to be expected for a solid body (theoretical density). The test specimens were compacted under vacuum at a temperature of 1450° C. and with a holding time of 30 min to above 95% of the theoretical density, followed by a final compaction under an argon atmosphere at the same temperature (Sinter-HIP technology). The test specimens proved to be completely dense under an optical microscope. The porosity according to ISO 4505 corresponded to >A02, B00, C00. The Vickers hardness was determined to be 1690 HV10, and the fracture toughness (K1C) was calculated by measuring the crack lengths and using the formula of Shetty (Shetty 1985—Indentation fracture of WC-Co cermets, see reference above) to be 9.7 MPa*mJ1/2. The thermal conductivity (TC) was determined to be 39 W/m*K (measurement at 40° C. by the laser flash technique).
Table 1 shows the characteristics determined as compared to the characteristics from Example 1.
As the starting powder, a WC powder having a dBET value of 90 nm, a cobalt metal powder having a dBET value of 205 nm, a TiC powder having a dBET value of 610 nm, a TaC powder having a dBET value of 370 nm, a Cr3C2 powder having a dBET value of 430 nm, and a VC powder having a dBET value of 350 nm were used. A 200 g mixture of 68.5% by volume (79.1% by weight) WC, 10% by volume (6.5% by weight) Co, 10.1% by volume (3.7% by weight) TiC, 9% by volume (9.6% by weight) TaC, 1.2% by volume (0.6% by weight) Cr3C2 and 1.2% by volume (0.5% by weight) VC was ground in n-heptane in a ball mill for 44 hours. The dispersion of cemented carbide obtained was dried and pressed uniaxially with a pressing power of 300 MPa into rectangular test specimens with a green density of >50% of the density to be expected for a solid body (theoretical density). The test specimens were compacted under vacuum at a temperature of 1460° C. and with a holding time of 30 min to above 95% of the theoretical density, followed by a final compaction under an argon atmosphere at the same temperature (Sinter-HIP technology). The test specimens proved to be completely dense under an optical microscope. The porosity according to ISO 4505 corresponded to >A02, B00, C00. The Vickers hardness was determined to be 2020 HV10, and the fracture toughness (K1C) was calculated by measuring the crack lengths and using the formula of Shetty (Shetty 1985—Indentation fracture of WC-Co cermets, see reference above) to be 8.5 MPa*m1/2. The thermal conductivity was determined to be 35 W/m*K (measurement at 40° C. by the laser flash technique).
Table 2 shows the characteristics determined as compared to a cemented carbide having a composition without additions of cubic metal carbide, but with an otherwise comparable content of binder metal.
As can be seen from Tables 1 and 2, the cemented carbides of the invention according to the Examples have a fracture toughness that is improved over that of conventional cemented carbides, and a lower thermal conductivity without adversely affecting the Vickers hardness of the cemented carbides according to the invention within the accepted tolerance of ±20 HV10.
1—tungsten carbide phase
2—cubic metal carbide phase
3—binder metal phase
| Number | Date | Country | Kind |
|---|---|---|---|
| 18200028.1 | Oct 2018 | EP | regional |
This application is the national stage entry of International Patent Application No. PCT/EP2019/075352 having a filing date of Sep. 20, 2019, which claims priority to and the benefit of European Patent Application No. 18200028.1 filed in the European Patent Office on Oct. 12, 2018, the entire contents of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/075352 | 9/20/2019 | WO |
