JP2012246537A - Hard mold suitable for energization heating and material therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hard mold suitable for energization heating and a material therefor.SOLUTION: A mold for energization sintering is structured by a composite hard material. The composite hard material is composed of a sintered body of a hard composite material, wherein the hard composite material is prepared by dispersing spherical carbon in a composite material made of conductive ceramic particles and an intermetallic compound of iron and aluminum. The composite hard material has a composite structure in which carbon particles are dispersed in a metallic binder phase of the intermetallic compound composed of iron and aluminum. The conductive ceramic particles contain tungsten and/or titanium. The dispersed carbon is 10 mass% or less of the composite hard material. The composite hard material is suitable for energization sintering. The dispersed carbon has a particle size of 20 μm or less. The metallic binder phase composed of iron and aluminum is 40 mass% or less of the composite hard material. A new composite hard material and the mold for energization sintering can be provided.

Description

  The present invention relates to a hard mold suitable for energization heating and its material, and more particularly to a mold material suitable for a process of heating by energization. The present invention provides a new composite hard material that can be applied to a process such as electric sintering by imparting heat resistance and oxidation resistance to a hard material used in a mold and uniformly dispersing carbon particles that generate heat during energization. It provides new technologies and new products related to molds.

  In order to produce a bulk sintered body by sintering metal or ceramic powder, heating at a high temperature is required. In particular, a process such as a hot press method in which pressurization and heating are applied simultaneously is applied to a material powder having a high melting point and difficult to sinter. In recent years, an electric current sintering method has attracted attention as a technique for sintering with low energy against such a hardly sinterable material.

  In the electric current sintering method, a short time and high speed heating using Joule heating is realized by energizing the mold and the powder. However, in order to increase the efficiency, it is necessary to use a mold material having a high electric resistance, and generally, a material such as graphite has been used as a mold. However, the strength of graphite is as low as several tens of MPa and cannot be applied to sintering that requires high pressure.

  Therefore, cemented carbide is used as a mold material for sintering under high pressure, but this alloy is a good electrical conductor and cannot be efficiently heated. In addition, it is known that even if an attempt is made to improve the electrical resistivity by combining graphite with cemented carbide, carbon precipitates in the cemented carbide in a flake shape and greatly reduces the strength.

  In addition, since the cemented carbide uses cobalt, which is a pure metal, it also has a drawback of poor heat resistance. That is, as the mold material used for the electric current sintering, the graphite mold having good heat generation efficiency cannot be used at a high pressure, and the reaction with graphite cannot be suppressed for the metal powder. On the other hand, a cemented carbide mold that can be used at high pressure has poor heat resistance and heat generation efficiency.

  However, in current sintering at a low temperature of 500 ° C. or lower, a cemented carbide, which is a hard composite material obtained by baking tungsten carbide with cobalt, can be used, and a highly accurate sintered body can be produced. ing. In order to sinter various material powders with high precision by this electric current sintering technology that excels in energy savings, it is hard like a cemented carbide, heat resistant like graphite and can convert electricity into heat. Development is anxious.

  However, cemented carbides soften at high temperatures and become severely oxidized, so there is a limit to the use temperature as a mold material, and in order to conduct current heating, the efficiency of converting electricity into heat is low even at low temperatures. Furthermore, it is known that even if graphite is added to the cemented carbide, it reacts with cobalt, and flake graphite is precipitated, which deteriorates mechanical properties. Further, graphite has a low mechanical strength, so it is difficult to produce a highly accurate sintered body with a large pressing force, and it is impossible to maintain the same accuracy by repeated molding.

  On the other hand, WC-FeAl is a cemented carbide using a chemically stable intermetallic compound composed of iron and aluminum as a binder phase [Patent Document 1, Non-Patent Documents 1 and 2]. This alloy has been shown to improve heat resistance by using chemically stable Fe-Al as the binder phase [Non-Patent Document 3]. However, the electrical resistivity is comparable to that of conventional cemented carbide, and electrical energy cannot be efficiently converted into heat. Therefore, in this technical field, a hard mold material suitable for current heating is developed. It was an important task.

Japanese Patent No. 2611177

Powder and powder metallurgy, Vol. 49 (2002) 284-289 Powder and powder metallurgy, Vol. 49 (2002) 1089-1093 Powder and powder metallurgy Vol. 48 (2001) 986-989

  Under such circumstances, the present inventors have conducted extensive research with the goal of developing a hard composite material having heat resistance, high strength, oxidation resistance, and high electrical resistance in view of the above-described conventional technology. As a result, high electrical resistance and strength are obtained by compounding spherical graphite particles with a hard composite material of ceramic particles- (Fe-Al) intermetallic compound and suppressing the reaction between the hard composite material and the graphite particles, The inventors have found that a mold material having excellent heat resistance can be produced, and have completed the present invention.

  In view of such circumstances, an object of the present invention is to provide a hard composite material having heat resistance, high strength, oxidation resistance, and high electrical resistance in order to provide a hard mold material suitable for electric heating. Is.

The present invention for solving the above-described problems comprises the following technical means.
(1) A composite hard material comprising a sintered body of a hard composite material in which spherical carbon particles are dispersed in a hard composite material comprising conductive ceramic particles and an intermetallic compound of iron and aluminum, the iron A composite hard material having a composite structure in which carbon particles are dispersed in a bonded metal phase of an intermetallic compound composed of aluminum and aluminum.
(2) The composite hard material according to (1), wherein the conductive ceramic particles contain tungsten and / or titanium.
(3) The composite hard material according to (2), wherein the dispersed carbon is 10% by mass or less with respect to the composite material and is suitable for current sintering.
(4) The composite hard material according to (3), wherein the dispersed carbon has a particle size of 20 μm or less.
(5) The composite hard material according to (3), wherein the bonded metal phase composed of iron and aluminum is 40% by mass or less.
(6) A die for electric current sintering, characterized by being composed of the composite hard material according to any one of (1) to (5).

Next, the present invention will be described in more detail.
An object of the present invention is to provide a mold material that has excellent heat generation characteristics and can be used even at high temperatures in view of the problems of the prior art. As a mold material for electric current sintering, there are a graphite mold for high temperature and low pressure and a cemented carbide mold for low temperature and high pressure. However, there is no mold material having these intermediate characteristics in the conventional mold material. Further, the graphite mold is inferior in accuracy to the mold, and there is a problem that graphite and metal react. Accordingly, the present inventors have developed a mold material having characteristics intermediate between the two and excellent in heat resistance.

  In the development, WC-FeAl cemented carbide developed by National Institute of Advanced Industrial Science and Technology was used as seed technology. By compounding carbon with this alloy, we succeeded in producing a mold material having a higher electrical resistivity than the conventional cemented carbide and stronger than the graphite mold.

  The conventional cemented carbide is a composite material (WC-Co) in which WC is bonded with Co. When carbon is compounded with this cemented carbide, the carbon added at the time of fabricating the cemented carbide is separated as free carbon. It is known that it precipitates in the shape of a metal and greatly reduces its strength.

On the other hand, in the WC—FeAl alloy, free carbon is not generated in this way, and significant deterioration in strength can be prevented. Moreover, strength reduction can be further suppressed by reducing the particle size of the added carbon. Furthermore, it is possible to eliminate material strength anisotropy by making the carbon shape spherical. In addition, not only WC—FeAl but also a ceramic particle (TiB ₂ or TiN) having conductivity can be used to replace a part or all of WC.

  The inventors compounded spherical graphite particles with ceramic particles- (Fe-Al) intermetallic compound hard composite material using an intermetallic compound composed of iron and aluminum having excellent oxidation resistance as a binder metal phase, It has been found that a mold material excellent in high electric resistance, strength, heat resistance and the like can be produced by compositing while suppressing the reaction between the hard composite material and the graphite particles. That is, the present invention provides the following new method based on the above findings.

  When graphite is added to a cemented carbide (WC-Co), it reacts at the time of sintering and flake graphite is dispersed. This flake-like graphite is known as a defective structure in cemented carbide as free carbon, and significantly reduces mechanical properties. Therefore, in the present invention, instead of cobalt, an intermetallic compound having a possibility of suppressing the reaction with carbon is used as the binding metal phase. As an intermetallic compound, an intermetallic compound composed of iron and aluminum has been used as a binder phase of cemented carbide, and carbon particles are dispersed based on this alloy.

  As the carbon particles, spherical carbon particles that are unlikely to cause stress concentration are used so as not to deteriorate the mechanical properties of the sintered body. There are various forms of carbon such as amorphous and graphite, but in the present invention, there is no particular restriction due to the form. What is important is the shape of the carbon particles, and the closer to a spherical shape, the better. The size of the carbon particles is easy to disperse in the hard composite material, and considering the heat generation effect during energization, it is preferable that the carbon particles exist as isolated particles. In addition, it is preferable to be fine from the viewpoint of strength.

  Moreover, although the mixing method of the hard material which makes an intermetallic compound a binder phase is not specified in particular, methods, such as mechanical milling and mortar mixing published so far, can be utilized. Carbon powder is added to this mixed powder and further mixing is performed, and the same process can be used for the mixing method. However, since carbon particles are easy to disintegrate, it is preferable to reduce energy during mixing and mix a buffer material or the like together. As the buffer material, an organic material or a soft metal having a low melting point can be used.

  The obtained mixed powder is preformed by a press or the like and then sintered, whereby a hard composite material having an intermetallic compound in which target carbon particles are dispersed as a binder phase can be produced. The sintering method is not particularly limited, but a sintering technique such as vacuum sintering or pulse current sintering method can be used.

  Further, the preliminary molding is not particularly limited, but press molding or CIP (cold isostatic pressing) known as a general powder metallurgy molding method can be used. As the molding aid, paraffin or stearic acid used in general powder metallurgy can be used, but it is better not to use it in order to maintain the shape of carbon.

  The obtained sintered body can be finished into a desired mold shape by a general cemented carbide processing technique. For example, the sintered body can be subjected to wire cutting, electric discharge machining, grinding using a grindstone, or the like, and further lapping using diamond abrasive grains can be performed.

  When the electrical resistivity of the mold material thus obtained is measured, the electrical resistivity increases with the amount of carbon added, but the bending strength decreases on the contrary. At that time, as the particle size of the carbon is smaller, a decrease in strength due to the addition of carbon can be suppressed, and when the addition amount is 10% by mass or less, particularly more than 0 to 5% by mass, relatively high electrical resistivity and Strength can be realized.

  Further, when the shape is spherical, anisotropy does not occur in the material. When a direct current was passed through such a material and the heat generation characteristics were evaluated, it was found that the larger the amount of carbon, the quicker and higher the heating possible. Moreover, it became clear that the oxidation start temperature by heating in air is higher than that of conventional cemented carbide.

  The strength of the alloy in which carbon is combined with the produced WC- (FeAl) varies depending on the amount of the Fe-Al alloy. That is, the greater the amount of bonded metal phase, the lower the strength of the alloy. When the amount of the binder metal phase exceeds 40% by mass, the hardness of the WC- (Fe-Al) cemented carbide becomes 85HRA or less, and the wear resistance and strength are remarkably reduced.

While low CO ₂ and energy saving are sought in the manufacturing process, the electric current sintering technology is one of the processes that can realize it. However, until now, since only sintering using a graphite mold has been performed, a highly accurate sintered body could not be produced. Further, due to the reaction between the sintered body and the mold, it could not be developed as a continuous automated sintering process. By using the present invention, it is considered that not only a highly accurate sintered body can be produced, but also it can be developed into a continuous automated process.

  Also, in the sintered body, pores inevitably remained and the reliability was lowered, but by realizing sintering with increased pressure, a highly reliable sintered body with reduced generation of defects was produced. It becomes possible to do. Also, a thin sintered body of 1 mm or less can be produced by using the mold according to the present invention. By using the material according to the present invention, it can be applied as a mold material for materials that have been difficult to sinter so far, and it becomes possible to sinter materials having new characteristics.

The present invention has the following effects.
(1) A mold made of a sintered body of a composite material in which spherical carbon is dispersed in a composite material made of conductive ceramic particles and an intermetallic compound of iron and aluminum can be provided.
(2) It is possible to provide the above mold in which the conductive ceramic particles contain tungsten and / or titanium.
(3) Dispersed carbon is 10% by mass or less, and the above-described mold suitable for current sintering can be provided.
(4) The above-mentioned mold having a particle diameter of dispersed carbon of 20 μm or less and suitable for electric current sintering can be provided.
(5) It is possible to provide the above mold in which the bonded metal phase composed of iron and aluminum is 40% by mass or less.
(6) The composite hard material which comprises said metal mold | die can be provided.

The XRD of 98 (WC-30 mass% (Fe-Al)) + 2C is shown. The SEM image of 98 (WC-30 mass% (Fe-Al)) + 2C is shown. The change in electrical resistivity with respect to the carbon addition amount of (100−x) (WC−10 mass% (Fe—Al)) + xC is shown. The bending strength change with respect to the carbon addition amount of (100−x) (WC−10 mass% (Fe—Al)) + xC is shown. The difference in temperature rise characteristics due to energization of (100−x) (WC−30 mass% (Fe—Al)) + xC is shown. The intensity | strength change of (100-x) (WC-30mass% (Fe-Al)) + xC is shown. The oxidation resistance of 96 (WC-10 mass% (Fe-Al)) + 4C is shown. The external appearance photograph under 100MPa of 96 (WC-10mass% (Fe-Al)) + 2C is shown.

  EXAMPLES Next, although an Example demonstrates this invention concretely, this invention is not limited at all by these examples. That is, the present invention encompasses all aspects or modifications other than the present embodiment within the scope of the technical idea.

  After weighing tungsten carbide powder, iron powder, aluminum powder, and carbon powder to a composition of 98 (WC-30 mass% (Fe-Al)) + 2C (mass%) and performing mechanical milling in an argon atmosphere A graphite mold having an inner diameter of 30 mm, an outer diameter of 50 mm, and a height of 30 mm was filled to prepare a sintered body of φ30 mm × 4 mm. Sintering was performed in a vacuum, heated under a pressure of 30 MPa, and held at 1150 ° C. for 3 minutes.

  The constituent phase of the obtained sintered body was observed by X-ray diffraction, and the structure was observed by SEM. The results are shown in FIGS. From FIG. 1, the peak of tungsten carbide and the Fe-Al intermetallic compound can be confirmed from the produced hard material. The added carbon does not appear in X-ray diffraction because it has an amorphous crystal structure. FIG. 2 shows that the added carbon is uniformly dispersed in the alloy.

  Tungsten carbide powder, iron powder, aluminum powder, carbon powder are weighed to a composition of (100−x) (WC−10 mass% (Fe—Al)) + xC (mass%), and mechanical milling is performed under an argon atmosphere. After that, a graphite mold having an inner diameter of 30 mm, an outer diameter of 50 mm, and a height of 30 mm was filled to prepare a sintered body of φ30 mm × 4 mm. Sintering was performed in a vacuum, heated under a pressure of 30 MPa, and held at 1150 ° C. for 3 minutes.

  The addition amount (x) of carbon at this time was set to 0-8. The electric resistivity of the obtained sintered body was evaluated by a four-point probe method and the strength was evaluated by three-point bending. 3 and 4 show the electrical resistivity and bending strength of the sintered body produced. From these figures, it can be seen that the electrical resistivity and bending strength increase and decrease as the amount of carbon added increases, respectively.

  Tungsten carbide powder, iron powder, aluminum powder, and carbon powder are weighed so as to have a composition of (100-x) (WC-30 mass% (Fe-Al)) + xC (mass%) and mechanically milled in an argon atmosphere. After that, a graphite mold having an inner diameter of 30 mm, an outer diameter of 50 mm, and a height of 30 mm was filled to prepare a sintered body of φ30 mm × 4 mm. Sintering was performed in a vacuum, heated under a pressure of 30 MPa, and held at 1100 ° C. for 3 minutes.

  The addition amount (x) of carbon at this time was set to 0-4. The produced sintered body was cut out to a size of 3 × 3 × 20 mm, sandwiched between upper and lower electrodes, and a 50 A direct current was applied for 60 seconds, and the temperature change was examined. The result is shown in FIG. It can be seen that the higher the carbon content, the higher the temperature can be increased to a higher temperature.

  Tungsten carbide powder, iron powder, aluminum powder, and carbon powder are weighed so as to have a composition of (100-x) (WC-30 mass% (Fe-Al)) + xC (mass%) and mechanically milled in an argon atmosphere. After that, a graphite mold having an inner diameter of 30 mm, an outer diameter of 50 mm, and a height of 30 mm was filled to prepare a sintered body of φ30 mm × 4 mm. Sintering was performed in a vacuum, heated under a pressure of 30 MPa, and held at 1100 ° C. for 3 minutes. The carbon particle diameter at this time was changed, and the bending strength at that time was evaluated. FIG. 6 shows the result. It can be seen that the finer the added carbon particles are, the more the strength reduction due to the composite can be suppressed.

  After weighing tungsten carbide powder, iron powder, aluminum powder, and carbon powder to a composition of 96 (WC-10 mass% (Fe-Al)) + 4C (mass%), mechanical milling was performed in an argon atmosphere. A graphite mold having an inner diameter of 30 mm, an outer diameter of 50 mm, and a height of 30 mm was filled to prepare a sintered body of φ30 mm × 4 mm. Sintering was performed in a vacuum, heated under a pressure of 30 MPa, and held at 1100 ° C. for 3 minutes.

  The result of investigating the increase in oxidation of the produced sintered body in air is shown in FIG. For comparison, the increase in oxidation of the WC-Co alloy having the same proportion of the binder phase is also shown. From this figure, it can be seen that, when an Fe—Al alloy is used as the binder phase, the weight increase due to oxidation is shifted to a higher temperature than the WC—Co alloy, and the oxidation resistance is improved.

  After weighing tungsten carbide powder, iron powder, aluminum powder, and carbon powder to a composition of 98 (WC-10 mass% (Fe-Al)) + 2C (mass%) and performing mechanical milling in an argon atmosphere. Then, it was filled in a graphite mold having an inner diameter of 10 mm, an outer diameter of 20 mm, and a height of 50 mm to prepare a sintered body of φ10 mm × 15 mm. Sintering was performed in a vacuum, heated under a pressure of 30 MPa, and held at 1200 ° C. for 3 minutes. The upper and lower sides of the produced sintered body were pressurized at 100 MPa. FIG. 8 shows the result of raising the temperature to 800 ° C. in this state. From this figure, it can be seen that deformation does not occur even at a high temperature of 800 ° C., and that it can be used under high temperature and high pressure.

Titanium boride powder, iron powder, aluminum powder, and carbon powder were weighed to a composition of 98 (TiB ₂ -10 mass% (Fe—Al)) + 2C (mass%) and mixed using a mortar. This powder was filled into a graphite mold having an inner diameter of 30 mm, an outer diameter of 45 mm, and a height of 30 mm to produce a sintered body of φ30 mm × 4 mm. Sintering was performed in a vacuum, heated under a pressure of 60 MPa, and held at 1200 ° C. for 4 minutes. The electrical conductivity of the sintered body was measured and found to be 3.5 × 10 ⁻⁵ Ωcm.

Titanium nitride powder, iron powder, aluminum powder, and carbon powder were weighed to a composition of 98 (TiN-10 mass% (Fe-Al)) + 2C (mass%) and mixed using a mortar. This powder was filled into a graphite mold having an inner diameter of 30 mm, an outer diameter of 45 mm, and a height of 30 mm to produce a sintered body of φ30 mm × 4 mm. Sintering was performed in a vacuum, heated under a pressure of 60 MPa, and held at 1250 ° C. for 5 minutes. The electrical conductivity of the sintered body was measured and found to be 14.8 × 10 ⁻⁵ Ωcm.

As described above in detail, the present invention relates to a hard mold suitable for energization heating and its material, and by using the present invention, not only can a highly accurate sintered body be produced, but also continuous automation. Can be deployed in the process. The electric current sintering technology is one of the processes that can achieve this while lowering CO ₂ and saving energy in the manufacturing process. However, since only sintering using a graphite mold has been performed so far, An accurate sintered body could not be produced. Further, due to the reaction between the sintered body and the mold, it could not be developed as a continuous automated sintering process. Moreover, in the sintered body, pores inevitably remained, and reliability was lowered. However, according to the present invention, it is possible to produce a highly reliable sintered body that suppresses the generation of defects by realizing sintering with increased pressure, and a thin sintered body of 1 mm or less is also made of gold according to the present invention. If a mold is used, it can be manufactured. By using the material according to the present invention, it can be applied as a mold material for materials that have been difficult to sinter so far, and it becomes possible to sinter materials having new characteristics.

Claims (6)

  A composite hard material comprising a sintered body of a hard composite material in which spherical carbon particles are dispersed in a hard composite material comprising conductive ceramic particles and an intermetallic compound of iron and aluminum, the iron and aluminum A composite hard material having a composite structure in which carbon particles are dispersed in a bonded metal phase of an intermetallic compound.   The composite hard material according to claim 1, wherein the conductive ceramic particles contain tungsten and / or titanium.   The composite hard material according to claim 2, wherein the dispersed carbon is 10% by mass or less with respect to the composite material, and is suitable for electric current sintering.   The composite hard material according to claim 3, wherein the dispersed carbon has a particle diameter of 20 μm or less.   The composite hard material according to claim 3, wherein the bonded metal phase composed of iron and aluminum is 40% by mass or less.   A die for electric current sintering, comprising the composite hard material according to any one of claims 1 to 5.