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621.762The properties of hard alloys based on fine-grained hard-alloy mixtures, obtained by coreduction of tungsten oxide with the oxides CoO and Co304, are discussed. A linear Depending on the conditions under which it is used, the cutting capacity of a hard-alloy tool is determined by the mechanical and physicochemical properties of the initial hard alloys [1]. Coarse-grained hard alloys that are sufficiently strong, "lose" considerably in hardness and cannot be used to make a small tool [2]. Fine-grained hard alloys are obtained in one of two ways: either by mixing highly disperse tungsten carbide and cobalt powders or by the coreduction and carbidization of a mixture of tungsten and cobalt oxides [3][4][5]. A uniform distribution of the binder is the primary condition for increasing the strength of a fine-grained hard alloy. In the present study we compare the mechanical characteristics of hard alloys, sintered from hard-alloy systems containing 6 and 8 mass% Co.A hard-alloy composition was obtained by coreducing and then carbidizing a mixture of tungsten and cobalt oxides. The physicochemical properties of various grades of tungsten and cobalt oxides are given in Table 1.Using the results of thermogravimetric analyses, we chose coreduction and carbidization conditions that enabled us to obtain reduced powder with a specific surface of 0.5-0.7 m2/g and after mixing and grinding with carbon black to increase the specific surface to 5.9 m2/g. Hard-alloy mixtures with the composition and physicochemical properties indicated in Table 2, depending on the initial amount of carbon black, were obtained by carbidization in pure argon and carbidized media. The intensity of the rt-phase (Co3W3C) weakens when the carbidization temperature is raised and the carbon content in the initial mixture is increased. Parallel carbidization was carried out in a methane-hydrogen gas mixture. Hard-alloy mixtures of stoichiometric composition with a specific surface of 1-2 m2/g were obtained.The hard-alloy mixtures were vacuum-sintered in two ways: under ordinary conditions at 1400, 1450, and 1500°C as well as with long homogenizing preannealing in hydrogen at 1070°C and in a vacuum at 1380°C (alloy VK6) and 1440°C (alloy VK8).Metallographic sections of vacuum-sintered specimens of VK8 alloy (Fig. 1) indicate that a nonuniform structure forms inside agglomerates as a result of intensive sintering: fairly large particles (larger than 5 #m) are observed in addition to small particles (smaller than 0.5 ~m). Especially in specimens obtained without homogenizing preannealing, pores are distributed nonuniformly over the field of the metallurgical section and their number varies from 0.2 to 0.8%: carbon accumulation is observed. Those factors lower the strength characteristics of hard alloys obtained by ordinary-sintering, without allowance for the high dispersion of the hard-alloy mixtures.The variation of the strength characteristics of hard alloys with the sintering temperature is illustrated in Figs. 2 and 3. The strength depends on t...
621.762The properties of hard alloys based on fine-grained hard-alloy mixtures, obtained by coreduction of tungsten oxide with the oxides CoO and Co304, are discussed. A linear Depending on the conditions under which it is used, the cutting capacity of a hard-alloy tool is determined by the mechanical and physicochemical properties of the initial hard alloys [1]. Coarse-grained hard alloys that are sufficiently strong, "lose" considerably in hardness and cannot be used to make a small tool [2]. Fine-grained hard alloys are obtained in one of two ways: either by mixing highly disperse tungsten carbide and cobalt powders or by the coreduction and carbidization of a mixture of tungsten and cobalt oxides [3][4][5]. A uniform distribution of the binder is the primary condition for increasing the strength of a fine-grained hard alloy. In the present study we compare the mechanical characteristics of hard alloys, sintered from hard-alloy systems containing 6 and 8 mass% Co.A hard-alloy composition was obtained by coreducing and then carbidizing a mixture of tungsten and cobalt oxides. The physicochemical properties of various grades of tungsten and cobalt oxides are given in Table 1.Using the results of thermogravimetric analyses, we chose coreduction and carbidization conditions that enabled us to obtain reduced powder with a specific surface of 0.5-0.7 m2/g and after mixing and grinding with carbon black to increase the specific surface to 5.9 m2/g. Hard-alloy mixtures with the composition and physicochemical properties indicated in Table 2, depending on the initial amount of carbon black, were obtained by carbidization in pure argon and carbidized media. The intensity of the rt-phase (Co3W3C) weakens when the carbidization temperature is raised and the carbon content in the initial mixture is increased. Parallel carbidization was carried out in a methane-hydrogen gas mixture. Hard-alloy mixtures of stoichiometric composition with a specific surface of 1-2 m2/g were obtained.The hard-alloy mixtures were vacuum-sintered in two ways: under ordinary conditions at 1400, 1450, and 1500°C as well as with long homogenizing preannealing in hydrogen at 1070°C and in a vacuum at 1380°C (alloy VK6) and 1440°C (alloy VK8).Metallographic sections of vacuum-sintered specimens of VK8 alloy (Fig. 1) indicate that a nonuniform structure forms inside agglomerates as a result of intensive sintering: fairly large particles (larger than 5 #m) are observed in addition to small particles (smaller than 0.5 ~m). Especially in specimens obtained without homogenizing preannealing, pores are distributed nonuniformly over the field of the metallurgical section and their number varies from 0.2 to 0.8%: carbon accumulation is observed. Those factors lower the strength characteristics of hard alloys obtained by ordinary-sintering, without allowance for the high dispersion of the hard-alloy mixtures.The variation of the strength characteristics of hard alloys with the sintering temperature is illustrated in Figs. 2 and 3. The strength depends on t...
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