Nitriding method and tungsten carbide-based cemented carbide

Abstract

A tungsten carbide-based cemented carbide alloy according to the present invention comprises tungsten carbide and an additive, has a nitride surface, and has a domain in which nitrogen and tungsten carbide coexist. According to the present invention, the hardness of a cemented carbide alloy which has been subjected to a nitriding treatment can be improved.

Description

[Technical Field]

[0001] The present invention relates to a nitriding process. [Background technology]

[0002] There are materials known as cemented carbide. A typical example is a WC-Co cemented carbide, which is made by adding cobalt to tungsten carbide (WC). In order to improve the wear resistance of cemented carbide, it has been proposed to increase the hardness by subjecting the cemented carbide to a nitriding treatment (Patent Document 1, etc.). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-210525 Summary of the Invention [Problem to be solved by the invention]

[0004] Conventional gas nitriding has not always been able to achieve the required hardness. The object of the present invention is to improve the hardness of cemented carbide by atomic nitriding. [Means for solving the problem]

[0005] In one aspect, the present invention provides a tungsten carbide-based cemented carbide comprising tungsten carbide and an additive, having a nitrided surface and having domains in which nitrogen and tungsten carbide coexist. The tungsten carbide-based cemented carbide has a domain where nitrogen and tungsten carbide coexist, and the number of nitrogen atoms present per unit volume in a first layer existing from the surface to a predetermined depth is 3 to 5 times the number of nitrogen atoms present per unit volume in a domain where nitrogen and the additive coexist in a second layer existing in a region deeper than the first layer. In another aspect, the present invention is a material comprising tungsten carbide and an additive, having a nitrided surface, having a domain in which nitrogen and tungsten carbide coexist, and having a Vickers hardness of 2000 HV to 3000 HV measured with a measuring load of 10 g. In a preferred embodiment, the tungsten carbide-based cemented carbide has a Vickers hardness of 1700 HV to 2500 HV measured under a measuring load of 25 g. In another aspect, the present invention provides a tungsten carbide-based cemented carbide comprising tungsten carbide and an additive, having a nitrided surface, having domains in which nitrogen and tungsten carbide coexist, and exhibiting a Vickers hardness measured under a load of 10 g that is at least 30% higher than that of a non-nitrided tungsten carbide. In a preferred embodiment, the tungsten carbide-based cemented carbide is characterized in that the Vickers hardness measured under a measuring load of 25 g is improved by at least 10% compared to a case where the tungsten carbide-based cemented carbide is not nitrided. In a preferred embodiment, the tungsten carbide-based cemented carbide has a Co content of 0 to 25 wt %. In another aspect, the present invention provides a method for producing a nitrided tungsten carbide-based cemented carbide by treating a tungsten carbide-based cemented carbide with electron beam excited nitrogen plasma at a temperature of 550°C to 800°C for 12 hours to 120 hours. [Effects of the Invention]

[0006] According to the present invention, the hardness of the cemented carbide that has been subjected to nitriding treatment is improved. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram showing an outline of an atomic nitriding treatment apparatus 1. [Figure 2A] FIG. 2 is a graph showing the measurement results of hardness of Sample 1 before nitriding treatment. [Figure 2B] FIG. 10 is a graph showing the measurement results of the hardness of Sample 1 after nitriding treatment. [Figure 3A]FIG. 10 is a graph showing the measurement results of hardness of Sample 2 before nitriding treatment. [Figure 3B] FIG. 10 is a graph showing the measurement results of the hardness of Sample 2 after nitriding treatment. [Figure 4] Graph showing the measurement results of an EPMA (electron probe microanalyzer). [Figure 5] FIG. 1 shows the measurement results of EPMA. [Figure 6] FIG. 1 shows the measurement results of EPMA. [Figure 7] FIG. 1 shows the measurement results of EPMA. [Figure 8] FIG. 1 shows the measurement results of EPMA. DETAILED DESCRIPTION OF THE INVENTION

[0008] First, the gist of the present invention will be described. The present invention provides a tungsten carbide-based cemented carbide comprising tungsten carbide and an additive, having a nitrided surface and having domains in which nitrogen and tungsten carbide coexist.

[0009] In the present invention, the hardness of the cemented carbide is believed to be improved by forming a domain in which nitrogen and tungsten carbide coexist, which was previously unattainable. Specifically, the invention described in the above-mentioned Patent Document 1 states in paragraph 0004 that "when this cemented carbide is subjected to nitriding or nitrocarbide softening, nitrogen atoms and other atoms diffuse and penetrate from the surface of the alloy, nitriding impurity components and exerting a hardening effect, thereby strengthening the bonding strength between the WC-Co alloy." The inventors believed that this method still did not provide sufficient hardness, and discovered that good hardness could be achieved by allowing nitrogen atoms to coexist with tungsten carbide itself (e.g., by diffusing nitrogen atoms from the surface so that nitrogen is present in the tungsten carbide particles).

[0010] In the present invention, the term "hard alloy" broadly refers to a composite material in which carbides of metals from Groups IVa, Va, and VIa of the periodic table are sintered with iron-based metals such as Fe, Co, and Ni. The alloys with the best mechanical properties are those in which tungsten carbide (WC) is sintered with iron-based metals (additives) such as Co and Ni. More specifically, WC-Co alloys in which Co is sintered with tungsten carbide (WC). WC-Co alloys are sometimes referred to as "hard alloys" in the narrow sense. These alloys are characterized by excellent low-temperature and high-temperature hardness, high strength, and stable physical properties. Hard alloys are produced by a process known as powder metallurgy. Hard alloys are produced by powder metallurgy because their main component, tungsten carbide (WC), has a high melting point of 2900°C, making them difficult to melt and machine.

[0011] The tungsten carbide (WC)-based cemented carbide according to the present invention typically includes WC-Co alloys, WC-TiC-Co alloys, WC-TaC-Co alloys, WC-TiC-TaC-Co alloys, WC-Ni alloys, WC-Ni-Cr alloys, WC-Ni alloys, and WC-Ni-Cr alloys, and refers to metals primarily composed of tungsten carbide (hereinafter, tungsten carbide-based cemented carbide may be simply referred to as "alloy"). For example, this tungsten carbide-based cemented carbide contains 0 to 25 wt% of additives. Furthermore, the tungsten carbide (WC)-based cemented carbide according to the present invention may contain impurities (e.g., Fe, Mo) in addition to tungsten carbide and additives.

[0012] In the present invention, the tungsten carbide-based cemented carbide is composed of tungsten carbide and an additive. The additive is not particularly limited, but typically functions as a binder to bind the tungsten carbide. Examples of such additives include Co, TiC, TaC, Ni, and Cr. Considering practicality in use in tools, Co is preferred. The content of such additives is not particularly limited, but is typically 0% by weight or more and 25% by weight or less, with the total weight of the tungsten carbide superalloy taken as 100% by weight. The preferred content varies depending on the application of the tungsten carbide-based cemented carbide (cutting tools, wear-resistant and impact-resistant tools, civil engineering and mining tools, earthwork tools, non-magnetic materials), but is, for example, 3% by weight or more and 15% by weight or less for cutting tools, 3% by weight or more and 25% by weight or less for wear-resistant and impact-resistant tools, 5% by weight or more and 20% by weight or less for civil engineering and mining tools, 1% by weight or more and 5% by weight or less for earthwork tools, and 5% by weight or more and 20% by weight or less for non-magnetic materials.

[0013] Furthermore, the tungsten carbide-based cemented carbide of the present invention has a nitrided surface and has domains in which nitrogen and tungsten carbide coexist. Here, "nitrided surface" refers to a state in which nitrogen is distributed or present in a predetermined depth region from the surface (outermost surface) of the tungsten carbide-based cemented carbide. To create such a nitrogen distribution or presence state, a method of diffusing nitrogen from the surface to the interior of the tungsten carbide-based cemented carbide using the atom nitriding method described below may be used.

[0014] The surface refers to the surface of the alloy, and when nitriding the alloy with other substances (in an argon and nitrogen gas atmosphere), it is the interface separating the alloy from the mixed gas phase. When nitrogen atoms adhere to this interface at a certain temperature, they are incorporated into the alloy (metal crystal).

[0015] The tungsten carbide-based cemented carbide according to the present invention has a domain where nitrogen and tungsten carbide coexist. The term "domain (phase) where nitrogen and tungsten carbide coexist" refers to the presence of a region where nitrogen and tungsten carbide coexist. Whether or not a domain where nitrogen and tungsten carbide coexist can be determined by appropriate analysis. For example, this can be determined by determining whether or not there is a region where both tungsten and nitrogen atoms are detected in an EPMA (Electron Probe Micro Analyzer) image. More specifically, elemental mapping (e.g., color mapping) of tungsten and nitrogen atoms is performed on a cross section (cross section including the surface) of the tungsten carbide-based cemented carbide using an EPMA, and the two mapping images are compared. If both elements are detected in the same region, this region is determined to be a domain where nitrogen and tungsten carbide coexist.

[0016] In the present invention, the number of nitrogen atoms per unit volume present in a first layer (nitride layer) existing from the surface to a predetermined depth and having a domain where nitrogen and tungsten carbide coexist is preferably 3 to 5 times the number of nitrogen atoms per unit volume present in a domain (phase) where nitrogen and additives coexist in a second layer (diffusion layer) existing deeper than the first layer. Here, the predetermined depth is a depth of 2 μm to 10 μm from the surface, preferably a depth of 2 μm to 4 μm from the surface.

[0017] The relationship between the distance (depth) from the surface and the distribution of each element can be evaluated using an EPMA (electron probe microanalyzer) as described above. In this invention, the following analytical conditions were used: electron beam diameter 1 μm, acceleration voltage 10 kV to 20 kV, current 2 × 10 A, magnification 3000 times (40 μm × 30 μm), and resolution 256 × 192 pixels. The detailed analysis results will be described later.

[0018] As described above, the tungsten carbide (WC)-based cemented carbide according to the present invention has improved hardness because nitrogen atoms penetrate into tungsten carbide to form domains in which nitrogen and tungsten carbide coexist. Specifically, the tungsten carbide (WC)-based cemented carbide according to the present invention has a Vickers hardness of 2000 HV to 3000 HV measured under a load of 10 g, and a Vickers hardness of 1700 HV to 2500 HV measured under a load of 25 g.

[0019] Furthermore, the tungsten carbide (WC)-based cemented carbide according to the present invention has a Vickers hardness measured under a load of 10 g that is at least 30% higher than that of a non-nitrided tungsten carbide, and a Vickers hardness measured under a load of 25 g that is at least 10% higher. Here, the Vickers hardness of the "non-nitrided" tungsten carbide may be measured by measuring the Vickers hardness of the surface before nitriding, or a method may be used in which the non-nitrided surface of the tungsten carbide (WC)-based cemented carbide according to the present invention is used and the Vickers hardness of this surface is measured.

[0020] The tungsten carbide (WC)-based cemented carbide according to the present invention is produced by treating a tungsten carbide-based cemented carbide with electron beam excited nitrogen plasma at a temperature of 550° C. to 800° C. for 12 to 120 hours. By employing this production method, it becomes easier to obtain a tungsten carbide (WC)-based cemented carbide having a domain in which nitrogen and tungsten carbide coexist. Preferably, for continuous operation, the nitriding temperature is 650°C ± 20°C and the nitriding time is 96 hours or longer. For intermittent operation, the nitriding temperature is 670°C to 730°C and the nitriding time is 24 hours or longer. Furthermore, further investigations have shown that continuous nitriding at temperatures above 670°C tends to increase the formation of a brittle Co3W layer on the WC-Co surface, resulting in increased surface roughness. Furthermore, while the surface hardness and wear resistance improve to a certain extent, significant improvements are not yet observed. However, even under these conditions, optimizing the nitriding conditions makes it possible to form domains in which nitrogen and tungsten carbide coexist. Intermittent operation (i.e., temperature control with an appropriate incubation time between nitriding treatments) has been shown to suppress the formation of a Co3W layer and reduce the increase in surface roughness, making it easier to achieve increased surface hardness and improved wear resistance. Intermittent operation is also preferable because it allows the use of a high nitriding temperature range, thereby shortening the treatment time.

[0021] The present invention will be described in detail below with reference to the drawings. However, the following is merely one preferred embodiment of the present invention, and the present invention is not limited to the following embodiment.

[0022] FIG. 1 shows an outline of the atom nitriding treatment apparatus 1 used in the present invention. A WC-Co cemented carbide (i.e., a tungsten carbide alloy with cobalt (Co) additive) is placed in a chamber, which is then evacuated. Argon gas is introduced into the plasma source on the left side of the figure to generate argon plasma. An electron beam is extracted from the argon plasma to excite nitrogen gas, generating a high concentration of nitrogen atoms (electron-beam-excited nitrogen plasma). By exposing the WC-Co cemented carbide sample to the electron-beam-excited nitrogen plasma, nitrogen atoms are diffused from the surface to the interior of the sample. A heater is installed in the container in which the sample is placed, and the sample is maintained at a constant temperature during the nitriding process. Specifically, the sample temperature is maintained at 550°C to 800°C, preferably 650°C to 700°C. The treatment time is 12 to 120 hours, preferably 96 hours.

[0023] Regarding the temperature, if it is higher than 800°C, damage to the sample surface tends to increase, while if it is lower than 550°C, the temperature is too low and diffusion tends to be difficult. Regarding the treatment time, as will be described later, if it is too short, diffusion of nitrogen atoms is difficult to proceed, and even if the treatment time is increased, the effect of improving hardness tends to plateau. As a result, there may be multiple combinations of temperature and treatment time that are preferable as production conditions for obtaining the tungsten carbide-based cemented carbide according to the present invention, and the above conditions can be optimized and set depending on the tungsten carbide and additive materials used.

[0024] In what is generally called WC-Co cemented carbide, the cobalt content varies, typically from 0% to 25% by weight. One advantage of the nitriding method (manufacturing method) according to the present invention is that it is easy to obtain a tungsten carbide cemented carbide with good hardness, regardless of the cobalt content, even when the cobalt content varies greatly from 0% to 25% by weight. In the following examples, two representative samples were used: one containing 87% by weight of WC and 13% by weight of cobalt (Sample 1) and one containing 92% by weight of WC and 8% by weight of cobalt (Sample 2).

[0025] Using the above equipment, nitriding treatment was carried out on WC-Co cemented carbide samples. Figures 2A, 2B, 3A, and 3B show the hardness measurement results. Figures 2A and 2B show the measurement results at five different positions on one sample (Sample 1) before treatment and the results at five different positions on the same sample after nitriding, respectively. For Sample 1, the atomic nitriding temperature was 650°C and the nitriding time was 96 hours. Similarly, Figures 3A and 3B show the measurement results at five different positions on Sample 2 before treatment and the results at five different positions on the same sample after nitriding, respectively. For Sample 2, the atomic nitriding temperature was 70°C and the nitriding time was 96 hours. 0℃The nitriding treatment time was set to 30 hours. Vickers hardness (unit: HV) of each sample was measured using a measuring instrument (model HM-125) manufactured by Mitutoyo Corporation.

[0026] Here, when measuring the hardness of a sample, the measurement load setting is important. This is because hardness is expected to be depth-dependent, and the measurement load determines which part of the sample's thickness is being measured. Specifically, a measurement load of 10 g corresponds to measuring physical properties up to a depth of approximately 5 μm from the surface. A measurement load of 25 g corresponds to measuring physical properties up to a depth of 9 μm from the surface. In Figure 2 (before nitriding), there is not much difference between the shallow part from the surface (up to a depth of approximately 5 μm from the surface) and the deep part from the surface (up to a depth of 9 μm from the surface). However, as can be seen from Figures 2B and 3B, the shallower part from the surface is harder in the nitrided sample.

[0027] As shown in Figure 2B, the Vickers hardness of Sample 1 measured with a 10g load ranged from 1623HV to 2500HV, with the average of the three intermediate values excluding the maximum and minimum values being 2110HV. Three measurements exceeded 2000HV. Compared to the sample without nitriding, the Vickers hardness measured with a 10g load was improved by approximately 39% (+597HV) on average of the three intermediate values excluding the maximum and minimum values. On the other hand, the Vickers hardness measured with a measuring load of 25g was 1496HV to 1781HV, and the average of the three intermediate values excluding the maximum and minimum values was 1694HV. There were also three measured values exceeding 1700HV. Compared to the case without the nitriding treatment, the Vickers hardness measured with a measuring load of 25g was improved by approximately 15% (+219HV).

[0028] Similarly, for Sample 2, as shown in Figure 3B, the Vickers hardness measured with a 10g load ranged from 2770HV to 3929HV, with the average of the three intermediate values excluding the maximum and minimum values being 3007HV. Compared to the sample without nitriding, the Vickers hardness measured with a 10g load was improved by approximately 48% on average, excluding the maximum and minimum values. On the other hand, the Vickers hardness measured with a 25g load ranged from 2421HV to 2564HV, with the average of the three intermediate values excluding the maximum and minimum values being 2530HV. Compared to the sample without nitriding, the Vickers hardness measured with a 10g load was improved by approximately 27% on average, excluding the maximum and minimum values.

[0029] Thus, it was confirmed that the hardness improved after the nitriding treatment at any measured load in Samples 1 and 2. Furthermore, at least in the depth region of approximately 5 μm to 9 μm, the rate of hardness improvement was greater in shallower regions (closer to the surface).

[0030] The depth dependence of the hardness improvement rate is presumed to be due to the depth dependence of the degree of nitrogen atom distribution. Therefore, the element distribution in the sample at each depth was measured using an electron probe microanalyzer (EPMA) (JEOL, model number JXA8530F).

[0031] The measurement results are shown in Figures 4 to 8. These are diagrams of the cross section (thickness direction) of the treated sample. Figures 4, 5, and 6 show the distribution of tungsten atoms (tungsten atoms in tungsten carbide), cobalt atoms, and nitrogen atoms, respectively. Figure 8 is a chart showing the relationship between the color of the image data in Figures 4 to 6 and the atomic density. In all of Figures 4 to 6, the vertical axis represents the distance (depth) from the sample surface. The brightness represents the atomic density at that position (the density gradually decreases in the order of red → vermilion → yellow → yellow-green → green → light blue → blue → black; see Figure 8). Figure 7 shows the results of measuring the distribution of nitrogen atoms in a cross section taken along line A in Figure 6.

[0032] As shown in Figure 4, tungsten (i.e., tungsten carbide) is distributed almost uniformly in the thickness direction. As shown in Figure 5, the distribution of cobalt atoms also varies in the thickness direction, but no regularity is observed in the depth dependence.

[0033] As shown in Figure 6, it is clear that the distribution of nitrogen atoms is diffused from the surface to a depth of at least 30 μm. In particular, as shown in Figure 7, there is a peak near the surface (near a depth of 2 μm in the figure), and the density is roughly constant at depths deeper than about 4 μm, although there is some variation. The nitrogen atom density at the peak value is about four times the density of nitrogen atoms at depths deeper than 4 μm.

[0034] In Figure 7, the peak of nitrogen atom density appears to be located inward from the surface. However, considering that the resolution of the electron beam is approximately 1 μm, the peak of nitrogen atom density is likely to be in a region less than 4 μm deep (shallow) from the surface, as can be seen from the fact that the surface in Figure 6 appears the brightest. The nitrogen atoms distributed in this region nitride the tungsten carbide, so this region forms a domain where nitrogen and tungsten carbide coexist, and can be considered a nitride layer. Of particular note is the depth of approximately 1 μm from the surface in Figure 6. The blue color here is lighter in a horizontal line than the surrounding area, indicating a higher nitrogen density. Meanwhile, the presence of tungsten (tungsten carbide) in this region is clear from Figure 4. When comparing the depth of about 1 μm from the surface in FIG. 5 with the depth of about 1 μm from the surface in FIG. 4, a region where tungsten and nitrogen coexist can be observed. This region is a domain where nitrogen and tungsten carbide coexist, and it is believed that the presence of this domain improves hardness.

[0035] Furthermore, as can be seen by comparing Figures 5 and 6, the distribution of high nitrogen atom density roughly coincides with the distribution of cobalt atoms from the surface to a depth of at least approximately 30 μm. This is thought to be because nitrogen atoms that diffused through the nitride layer and into the interior rapidly diffused through the cobalt domains to deeper regions. On the other hand, due to the slower diffusion rate in the tungsten (tungsten carbide) domains, nitrogen atoms are thought to penetrate only to a depth of approximately 4 μm. Figure 6 shows that nitrogen atoms diffuse almost uniformly throughout the entire surface of the alloy from the surface to a depth of approximately 4 μm. This contrasts with the non-uniform distribution of cobalt density in this depth region. Furthermore, in comparison with Figure 3, the depth region where hardness improves (i.e., the nitride layer from the surface to a depth of approximately 4 μm) corresponds to this nitrogen atom distribution. In other words, the greater the nitrogen diffusion in the region, the greater the strength. This is consistent with the result in Figure 3 that the rate of improvement in hardness after nitriding is higher in the shallower region, from the surface to 5 μm, than in the region from the surface to 9 μm. In other words, it is thought that while nitrogen atoms present in the tungsten (tungsten carbide) domains in the nitrided layer contribute to improving hardness, nitrogen atoms present in the cobalt atom domains do not contribute much to improving hardness.

[0036] It is believed that the achievement of this distribution state, in which nitrogen atoms are diffused almost uniformly across the entire surface of the alloy to a depth of approximately 4 μm from the surface, is due to the long treatment time of 96 hours. Conventionally, treatment times for general nitriding processes have generally been on the order of several hours. For cemented carbide, no quantitative evaluation has been conducted on the depth dependence of hardness and nitrogen atom distribution. By significantly extending the treatment time, the present invention has made it possible to form domains in which nitrogen and tungsten carbide coexist, which was previously unattainable, and has succeeded in producing cemented carbide with a good hardness of 2000 HV or more in a region several micrometers deep from the surface. The formation of domains in which nitrogen and tungsten carbide coexist is influenced not only by the treatment time, but also by the treatment temperature and the nitriding method used. Therefore, by appropriately combining the nitriding method, nitriding temperature, and treatment time, it is possible to appropriately form domains in which nitrogen and tungsten carbide coexist.

[0037] Here, several microns is roughly the same level as the thickness of a hard film coated on the surface of cutting tools, etc. That is, for the wear resistance of parts such as various tools and dies, it is sufficient if the hardness is improved in a thickness region of several microns, and the hardness in deeper regions is not a major issue. That is, the nitriding method of the present invention is particularly effective in improving the wear resistance of such cemented carbide by increasing the surface hardness. [Explanation of symbols]

[0038] 1. Atom nitriding treatment device

Claims

1. A tungsten carbide-based cemented carbide that has been atomically nitrided by treating it with electron beam excited nitrogen plasma at a temperature of 550°C to 800°C for 12 hours to 120 hours, It consists of tungsten carbide and an additive, The surface is nitrided, and has a domain in which nitrogen and tungsten carbide coexist, which exists from the surface to a depth of at least 1 μm.

1. A tungsten carbide-based cemented carbide.

2. The number of nitrogen atoms per unit volume present in a first layer existing from the surface to a predetermined depth and having a domain where nitrogen and tungsten carbide coexist is 3 to 5 times the number of nitrogen atoms per unit volume present in a domain where nitrogen and the additive coexist in a second layer existing in a region deeper than the first layer.

2. The tungsten carbide-based cemented carbide according to claim 1.

3. It consists of tungsten carbide and an additive, The surface is nitrided and has domains in which nitrogen and tungsten carbide coexist; The Vickers hardness measured with a 10g load is 2000HV to 3000HV. Characterized by The tungsten carbide-based cemented carbide according to claim 1.

4. It consists of tungsten carbide and an additive, The surface is nitrided and has domains in which nitrogen and tungsten carbide coexist; The Vickers hardness measured with a 25g load is 1700HV to 2500HV. Characterized by The tungsten carbide-based cemented carbide according to claim 1.

5. It consists of tungsten carbide and an additive, The surface is nitrided and has domains in which nitrogen and tungsten carbide coexist; The Vickers hardness measured at a load of 10 g is improved by at least 30% compared to when the nitrided material is not used. Characterized by The tungsten carbide-based cemented carbide according to claim 1.

6. It consists of tungsten carbide and an additive, The surface is nitrided and has domains in which nitrogen and tungsten carbide coexist; The Vickers hardness measured with a measuring load of 25 g is improved by at least 10% compared to when the nitrided material is not used. Characterized by The tungsten carbide-based cemented carbide according to claim 1.

7. The tungsten carbide-based cemented carbide has a Co content of 0 to 25 wt %. Characterized by The tungsten carbide-based cemented carbide according to any one of claims 1 to 6.

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