Composite carbide powder used for cemented carbide and method of producing the same

Abstract: In order to provide cemented carbide, provision is made about tungsten carbide powder which has a grain size not smaller than 1 µm and which is mixed with carbon powder and chromium powder to form raw powder. The tungsten carbide powder is formed by fine primary crystal particles of tungsten carbide and satisfies an inequality given by: Y > 0.61 - 0.33 log(X), where Y denotes a half-value width of (211) crystal planes in tungsten carbide (JCPDS-card 25-1047, d = 0.9020) measured by a X-ray diffraction method and where X denotes a grain size measured by the FSSS method. The raw powder is heated to a temperature between 1200 and 1700 °C to diffuse the chromium powder into the tungsten carbide powder and is pressed into the cemented carbide which has a high transverse rupture strength and a high hardness.

Description: The present invention relates to a composite carbide powder which serves as a raw material of cemented carbide, namely, hard metal and, more particularly, to the composite carbide powder of a tungsten carbide (WC) series. In addition, the present invention further relates to a method of producing the composite carbide powder. Heretofore, a composite carbide powder is used to produce a wide variety of tools, such as an end mill, a reamer, a shear blade, and the like, to shear or shape an object. As such a composite carbide powder, WC-Co based cemented carbide has been disclosed in Japanese Unexamined Patent Publications Nos. Hei 3-208811 (namely, 208811/1991) and Hei 5-147916 (namely, 147916/1993) both of which will be referred to as first and second references, respectively. As mentioned in both the first and the second references, WC powder which has a mean grain size of 0.5 µm or less is at first prepared as a raw material. To the WC powder, 0.1 to 2.0 % of V, 0.1 to 2 % of Cr, and 0.2 to 3 % of Ta are added by weight as an inhibiter of grain growth. In addition, the cemented carbide further comprises, by weight, 5 to 30 % of Co or Ni and impurities and has a structure specified by a WC phase of 0.8 µm or less. Recent demand has been strongly directed to an automatic shear process or an automatic shaping process. In such an automatic shear or shaping process, tools of cemented carbide have been exposed to very strict conditions. In order to endure such strict conditions, it is thought that the tools should be formed by fine-grain cemented carbide. However, most of fine grain WC powders are practically liable to be easily carburized at a low temperature of 1300 °C or less and are therefore unstable. As a result, grain growth readily occurs during a heat treatment of the cemented carbide. This means that a metallographic structure of a WC phase in the cemented carbide is adversely affected by wide distribution of grain sizes. Therefore, large-size particles have a defect which results in a reduction of strength in the cemented carbide. This makes it difficult to fulfill the requirements for the tools. In order to produce WC powder with a grain size of 1 µm or less, W powder should also have a fine grain size of 1 µm or less. Such W powder of a fine grain size is very expensive in comparison with W powder of 1 µm or more and makes the cemented carbide expensive. US 5,061,661 discloses a method for producing tungsten carbide and cemented tungsten carbide article therefrom having a uniform microstructure. This reference is silent as regards the size of the primary crystals. It is an object of the present invention to provide a composite carbide powder which is capable of making fine grain-cemented carbide without using fine grain tungsten (W) powder. It is another object of the present invention to provide a tool which is formed by the composite carbide powder. It is still another object of the present invention to provide a method of producing the composite carbide powder above-mentioned. According to this invention, there is provided a composite carbide powder comprising, as a main component, tungsten carbide powder which consists essentially of fine primary crystal particles of tungsten carbide, and a chromium powder, wherein the tungsten carbide powder satisfies an inequality given by: Y > 0.61 - 0.33 log(X), where Y denotes a half-value width of (211) crystal planes in the tungsten carbide (JCPDS-card 25-1047, d = 0.9020) measured by an X-ray diffraction method and where X denotes a grain size measured by a FSSS method, said grain size x being between 1.0 and 7.0 µm. According to this invention, there is also provided a method of producing a composite carbide powder having tungsten carbide powder as a main element, the method comprising the steps of preparing tungsten powder which has a mean grain size between 1 and 7 µm, mixing the tungsten powder with carbon powder and chromium containing powder into mixture; and heating the mixture in an atmosphere selected from one of a hydrogen atmosphere, a vacuum atmosphere, and an inert gas atmosphere at a heating rate between 3 and 100 °C/min to a temperature between 1200 and 1700°C, where it is held for 10 to 30 minutes, to form fine primary crystal particle carbide as the composite carbide powder, said chromium containing powder being added in the form of a selected one of metal chromium, organic chromium compound and inorganic chromium compound, such as chromium oxide and chromium carbide. According to this invention, there is also a method of producing a composite carbide powder the method comprising the steps of: preparing tungsten powder, which has a mean grain size between 1 and 7 µm; mixing the tungsten powder with carbon powder and chromium containing powder together with additional powder formed by oxide or carbide of at least one component selected from the group consisting of Ta, Mo, Nb and Zr to form a mixture; and heating the mixture in an atmosphere selected from one of a hydrogen atmosphere, a vacuum atmosphere, and an inert gas atmosphere at a heating rate between 3 and 100 °C/min to a temperature between 1200 and 1700°C, where it is held for 10 to 30 minutes, to form fine primary crystal particle carbide as the composite carbide powder, to form fine primary crystal particle of tungsten carbide as the composite carbide powder; said chromium containing powder being added in the form of a selected one of metal chromium, organic chromium compound and inorganic chromium compound, such as chromium oxide and chromium carbide. Description will be made as regards an embodiment of the present invention. In the present invention, provision is made about W powder and carbon (C) powder. In this event, it is to be noted in the embodiment of the present invention that the W powder has a mean grain size between 1.0 and 7.0 µm and does not include a fine grain size less than 1 µm. In addition, a chromium containing material or powder (will be called Cr powder) is also prepared which is selected from a group consisting of metal chromium, chromium oxide, organic chromium compound, inorganic chromium compound, and chromium carbide. In the example, vanadium powder which may be selected from vanadium oxide, vanadium metal, and vanadium carbide is also prepared together with additional powder formed by oxide, metal or carbide of at least one component of Ta, Mo, Nb, and Zr. Then, the W powder, the C powder, the Cr powder, the vanadium powder and the additional powder are thus prepared as a raw material or raw powder and mixed together to form a mixture. The mixture is introduced into an atmosphere selected from a hydrogen atmosphere, a vacuum atmosphere, and an inert gas atmosphere, such as nitrogen (N 2 ) and argon (Ar) and heated to obtain fine primary crystal particles of tungsten carbide. Specifically, the mixture is heated at a heating rate between 3 and 100 °C/min to a temperature between 1200 and 1700 °C to be kept at the temperature for 10 to 300 minutes (preferably, 200 minutes) in the above-mentioned atmosphere. During the heat treatment, the mixture is carburized into a composite carbide powder which includes the fine primary crystal particles of the tungsten carbide (WC). It has been confirmed that the fine primary crystal particles thus obtained satisfy an inequality of Y > 0.61 - 0.33 log X , where X denotes a grain size measured by the FSSS method and Y denotes a half-value width of (211) crystal planes in tungsten carbide (JCPDS card 25-1047, d = 0.9020) measured by an X-ray diffraction method. In addition, it has been found out that the obtained composite carbide powder is formed by fine cemented carbide grains with a homogeneous mean grain size. This shows that the fine cemented carbide grains can be attained without fine WC powder which has a grain size of 1 µm when it is measured by FSSS method. This shows that the fine cemented carbide grains can be cheaply obtained with this method. Herein, consideration will be made about the reasons of limitations. In the present invention, a mean grain size of the W powder is restricted to a range between 1.0 and 7.0 µm. When the grain size is smaller than 1.0 µm, a grain size of W is too small to diffuse Cr into a W grain to produce a fine primary crystal of WC. On the other hand, when the mean grain size of the W powder is greater than 7.0 µm, Cr can not be diffused into a center of each W particle, which gives rise to an inhomogeneous metallographic structure in the cemented carbide. As mentioned above, the heating rate is restricted to the range between 3 and 100 °C/minutes. In the meanwhile, the Cr carbide and the WC can not form a solid solution, as known in the art. Under the circumstances, when the heating rate exceeds 100 °C/minutes, a reaction between the W powder and the C powder quickly proceeds. Such a quick reaction makes growth of the fine primary crystal WC particles insufficient. On the other hand, when the heating rate is less than 3 °C/minute, a growth of W particles objectionably occurs. In addition, an amount of chromium carbide is restricted to a range between 0.2 and 2.5% by weight. When the amount is less than 0.2%, the fine primary particles of WC are not sufficiently formed. Herein, it is to be noted that the fine primary particles of WC are liable to occur as the amount of chromium carbide increases. However, the amount of chromium carbide which is more than 2.5% by weight exceeds a limit of solid solution in the cemented carbide and gives rise to deposition of a phase which is weak in strength and which makes the alloy fragile. Furthermore, a total amount of vanadium carbide, tantalum carbide, molybdenum carbide, niobium carbide, and zirconium carbide which may be referred to as the additional powder, as mentioned above falls within a range between 0.1 and 3.0% by weight. When the total amount is less than 0.1%, it is difficult to suppress particle growth which might occur during the heat treatment of the cemented carbide or hard metal. Consequently, the fine grains can not be obtained. When the total amount of the additional powder is more than 3.0% by weight, another phase is deposited in the cemented carbide because the limit of the solid solution is exceeded. Such a phase is weak in strength and is reduced in toughness. The half-value width of the X-ray diffraction method and the grain size measured by the FSSS method are limited as follows. Herein, it is assumed that, in the X-ray diffraction of the tungsten carbide, the half-value width of (211) crystal planes in tungsten carbide (JCPDS card 25-1047, d = 0.9020) is represented by Y and the grain size measured by the FSSS method is represented by X. Taking a relationship between X and Y into consideration, an inequality of Y > 0.61 - 0.33 log(X) holds between X and Y. When the inequality does not hold, it has been found out that the half-value width related to the grain size measured by the FSSS method becomes too small to produce the cemented carbide with a homogeneous grain size. Next, description will be made as regards first through thirteenth samples according to the embodiment of the present invention. In the samples, the composite carbide powder is assumed to be used for powder metallurgy and contains fine primary crystal particles of tungsten carbide after it is subjected to a process according to the present invention. Characteristics of carbide Characteristic of 10% Co cemented carbide Sample No. Mean Grain Size of WC (µm) Total carbon (%) Half-value width (° ) Fe (%) Mean Grain Size of WC (µm) Transverse Rupture Strength (kg/mm 2 ) Hardness (HRA) 1 1.6 6.19 0.70 0.009 0.7 410 92.5 2 1.8 6.12 0.68 0.007 0.8 390 92.4 3 1.6 6.15 0.67 0.007 0.6 368 92.9 4 2.8 6.08 0.70 0.005 0.6 392 92.9 5 5.4 6.11 0.67 0.006 0.4 405 93.3 6 7.0 6.42 0.55 0.004 0.7 340 92.7 7 1.8 6.25 0.72 0.006 0.4 405 93.5 8 1.4 5.78 0.63 0.013 0.6 355 93.0 9 2.9 6.16 0.64 0.008 0.6 398 92.9 10 1.0 6.09 0.68 0.009 0.6 363 93.1 11 1.7 6.20 0.59 0.006 0.6 400 93.0 12 1.9 6.21 0.56 0.007 0.7 372 92.6 13 1.2 6.18 0.59 0.009 0.6 393 92.9 In Table 1, each of the samples are prepared as raw powder and includes a mixture of the tungsten(W) powder, the carbon (C) powder, the chromium (Cr) powder, and the additional powder selected from the group consisting of Ta, V, Mo, Nb, and Zr. As shown in Table 1, the tungsten (W) powder has a grain size between 1.0 and 6.4 µm and is not smaller than 1.0 µm before it is mixed with the other components. The chromium powder may be added in the form of metal chromium, chromium oxide, inorganic chromium compound, organic chromium compound, or chromium carbide. The mixture is mixed with a Henschell mixer (product name) for 30 minutes. Under the conditions shown in Table 1, the samples are carburized to obtain the WC powder including fine primary grains of WC and all exhibit characteristics as shown in Table 2. As readily understood from Table 2, the grain size of the WC powder processed in the above-mentioned method falls within a range between 1.0 and 7.0 µm and is somewhat different from the grain size of the tungsten (W) powder shown in Table 1. In this case, 10% of Co powder by weight is mixed with each WC powder. In addition, C powder is also added so as to adjust a carbon content to an optimum two-phase region. Under the circumstances, the mixture mentioned above is completely mixed for 10 hours in a wet atmosphere by the use of an Attoritor (product name). Each of the mixtures is dried and pressed at a pressure of 1 ton/cm 2 to form a compact block. Thereafter, each compact block is sintered at 1400 °C for 60 minutes and subjected to a hot-isostatic-pressing (HIP) process at a temperature of 1350 °C in an Ar atmosphere kept at 1000 atm. to form an HIP material. Each of the metallographic structures of HIP materials is observed about a grain size of the WC phase by the use of a scanning electron microscope (SEM) at a magnification of 10000. Furthermore, characteristics of transverse rupture strength and hardness are measured in connection with each HIP material. The results of the measurement are also shown in Table 2. As shown in Tables 1 and 2, first through thirteenth ones of the samples have the transverse rupture strength and the hardness (HRA) which are not lower than those of tungsten carbide materials produced by the use of tungsten carbide of fine grain sizes less than 1 µm. The characteristics of HIP materials of the samples of the present invention are shown in Table 2. From Table 2, it is understood that each composite carbide powder according to the first through the thirteenth samples is specified by the inequality of: Y > 0.61 - 0.33 log(X), wherein Y denotes a half-value width of (211) crystal planes in tungsten carbide which are measured by an X-ray diffraction method and X denotes a grain size measured by the FSSS method. In any event, the composite carbide powders have fine primary WC crystal particles or grains which diffuse Cr, V, Ta, Mo, Nb, or Zr components into WC particles. On the other hand, it is confirmed that each of conventional samples that has been produced by the use of a fine grain has a small half-value width less than 0.50° even when the fine grain size of the WC is used. As a result, the inequality mentioned above can not be satisfied in the conventional samples. This shows that the conventional samples are represented by Y < 0.61 - 0.33 log(X). As mentioned above, each of the composite carbide powders of the present invention is specified by the relationship between the half-value width Y and the grain size X measured by the FSSS method and is specifically given by the inequality of Y > 0.61 - 0.33 log(X). From this fact, it is concluded that each of the cemented carbide made of the composite carbide powder has a homogeneous grain size, a high hardness, and a high strength in comparison with the conventional samples produced by tungsten carbide of fine grain sizes less than 1 µm.

Claims: A composite carbide powder comprising, as a main component, tungsten carbide powder, which consists essentially of fine primary crystal particles of tungsten carbide, and a chromium containing powder, wherein said tungsten carbide powder satisfies an inequality given by: Y > 0.61 - 0.33 log (x) where Y denotes a half-value width of (211) crystal planes in the tungsten carbide (JCPDS-card 25-1047, d = 0.9020) measured by X-ray diffraction method where x denotes a grain size measured by a FSSS method, said grain size x being between 1.0 and 7.0 µm. A composite carbide powder as claimed in claim 1, wherein said fine primary crystal particles contain, by weight, 0.2 to 2.5% of chromium carbide. A composite carbide powder as claimed in claims 1 or 2, wherein said fine primary crystal particles contain, by weight, 0.2 to 2.5% of chromium carbide and 0.1 to 3.0 % of at least one carbide component selected from the group consisting of vanadium carbide, tantalum carbide, molybdenum carbide, niobium carbide and zirconium carbide. A method of producing a composite carbide powder having tungsten carbide as a main element, according to one of claims 1 to 3, the method comprising the steps of: preparing tungsten powder, which has a mean grain size between 1 and 7 µm; mixing the tungsten powder with carbon powder and chromium containing powder into mixture; and heating the mixture in an atmosphere selected from one of a hydrogen atmosphere, a vacuum atmosphere, and an inert gas atmosphere at a heating rate between 3 and 100 °C/min to a temperature between 1200 and 1700°C, where it is held for 10 to 300 minutes, to form fine primary crystal particle carbide as the composite carbide powder, said chromium containing powder being added in the form of a selected one of metal chromium, organic chromium compound and inorganic chromium compound, such as chromium oxide and chromium carbide. A method as claimed in claim 4, wherein the atmosphere selected is the inert gas atmosphere which is either one of an argon gas atmosphere and a nitrogen atmosphere. A method as claimed in claim 4, wherein the composite carbide powder consists essentially of fine primary crystal particles of tungsten carbide, which satisfies an inequality given by: Y > 0.61 - 0.33 log (x) where Y denotes a half-value width of (211) crystal planes in the tungsten carbide (JCPDS-card 25-1047, d = 0.9020) measured by X-ray diffraction method where x denotes a grain size measured by a FSSS method. A method as claimed in claim 4, the method comprising the steps of: preparing tungsten powder, which has a mean grain size between 1 and 7 µm; mixing the tungsten powder with carbon powder and chromium containing powder together with additional powder formed by oxide or carbide of at least one component selected from the group consisting of Ta, Mo, Nb and Zr to form a mixture; and heating the mixture in an atmosphere selected from one of a hydrogen atmosphere, a vacuum atmosphere, and an inert gas atmosphere at a heating rate between 3 and 100 °C/min to a temperature between 1200 and 1700°C, where it is held for 10 to 30 minutes, to form fine primary crystal particle carbide as the composite carbide powder, to form fine primary crystal particle of tungsten carbide as the composite carbide powder; said chromium containing powder being added in the form of a selected one of metal chromium, organic chromium compound and inorganic chromium compound, such as chromium oxide and chromium carbide.

Inventors: ASADA NOBUAKI, DOI YOSHIHIKO, YAMAMOTO YOSHIHARU, ASADA, NOBUAKI, DOI, YOSHIHIKO, YAMAMOTO, YOSHIHARU, Asada, Nobuaki, c/o Tokyo Tungsten Co. Ltd., Doi, Yoshihiko, c/o Tokyo Tungsten Co. Ltd., Yamamoto, Yoshiharu, c/o Tokyo Tungsten Co. Ltd.
Applicants: TOKYO TUNGSTEN KK, TOKYO TUNGSTEN CO., LTD.
Application year: 1997
Main classification: C22C29
IPC codes: B23B27, B22F1, C01B31