JP2000144301A - Tungsten carbide sintered body and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the wear resistance, corrosion resistance and strength of a sintered body by allowing a sintered body to contain at least one or more kinds of VC and Cr3C2 by a specified amt., allowing the balance tungsten carbide to have the specified particle size and composing it of WC single phase having a stoichiometric compsn. SOLUTION: This sintered body contains at least one or more kinds of VC and Cr3C2 by 0.1 to 2 wt.%, and the balance tungsten carbide with <=1 μm average particle size and is composed of WC single phase having a stoichiometric compsn. In the production of the sintered body, a lower punch 3 is set to a molding outer frame 1 made of graphite and is filled with mixed raw material powder 4, and an upper punch 2 is set to assemble a sintering assembly. The mixed raw material powder 4 is formed by mixing WC powder with <=0.8 μm average particle size with one or more kinds of VC and Cr3C2 by 0.1 to 2 wt.%. The sintering assembly is charged to a conductive sintering machine, the atmosphere of the periphery of a graphite mold 5 is controlled to the vacuum or inert gas one, after that, energizing is executed, and the mixed raw material powder 4 is subjected to press sintering.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a tungsten carbide sintered body and a method for producing the same. In particular, it is suitable for the following points. The present invention relates to a high-density, high-strength tungsten carbide sintered body excellent in both wear resistance and corrosion resistance, and used in severe wear and corrosive environments, such as mechanical seals, nozzles, molds, and other wear-resistant materials, sliding. The present invention relates to a tungsten carbide sintered body suitable as a part.

[0002]

2. Description of the Related Art Materials having high hardness and excellent corrosion resistance are required as materials for mechanical seals, nozzles and molds used in severe wear and corrosive environments. In addition, many of these materials are often used by being incorporated as one mechanical part, and are required to have excellent mechanical properties in addition to hardness and corrosion resistance. Until now, in order to develop a material having these necessary properties, development has mainly been carried out in two directions. One is an approach from ceramics that emphasizes corrosion resistance, and the other is mechanical properties,
In particular, there is an approach from cemented carbide which emphasizes strength and hardness. As is well known, ceramic materials have a high hardness and are particularly excellent in corrosion resistance among materials, but are extremely weak to abrasive wear. Hard but brittle, they are not suitable for this type of wear caused by the accumulation of microscopic fractures by hard particles. Further, it is difficult to apply the method to mechanical parts that receive an impact. On the other hand, cemented carbide has excellent mechanical properties and its use is wide. Cemented carbide is a material obtained by sintering tungsten carbide (hereinafter, WC) using an iron group metal as a binder. By controlling the type and amount of the binder, the corrosion resistance can be improved to some extent. However, as the amount of binder for sintering and solidifying WC particles is reduced,
The binder cannot be uniformly dispersed between the particles, and sintering becomes difficult. In particular, when the WC particles were reduced to 1 μm or less and the surface area of the WC particles was significantly increased, a dense sintered body could not be obtained.

[0003] In order to solve such a problem, several approaches have been taken.
Several remedies are disclosed. Japanese Patent Application 3-25043
In No. 7, the amount of Co, the binder in the cemented carbide, was reduced.
Mo or Mo in order to improve the sinterability there_Two 
WC-Mo with C added_Two C-VC-Co cemented carbide
Has been proposed. Also, JP-A-5-230588
Contains 2% or less of Co, Mo or Mo _Two Add C
A hard alloy containing CoxWyCz.
Gold has been proposed. On the other hand, JP-A-6-329427
The publication contains no binder metal, WC-TiC-
Proposal of optical element molding material of TaC binderless cemented carbide
WC particles having an average particle size of 0.7 μm or less
0.1 to 5% by weight of TiC and TaC, and
(Cr_Three C_Two ), Vanadium carbide (VC) 0.1 ~
It is disclosed that 2% by weight is added.

[0004]

The above-mentioned prior art has the following problems. However, the above-mentioned Japanese Patent Application No. Hei 3-250437 and Japanese Patent Application Laid-Open No. Hei 5-230
No. 588, the intended hard alloy can be obtained, but the alloy contains Mo ₂ C or CoxWyCz, which contains a small amount of a metal binder and has poor corrosion resistance, in an environment where corrosion resistance is required. Could not be used. Further, although the hardness of the alloy is improved by the inclusion of Mo ₂ C or CoxWyCz, the toughness of the alloy is significantly impaired, and there is a problem that the alloy cannot be used as a mechanical component requiring high strength. Further, Japanese Patent Application Laid-Open No.
The WC-TiC-TaC binderless cemented carbide disclosed in No. 9427 does not include a metal binder and has a low strength, so that its use has been limited to a press mold for a glass optical element. This type of alloy has a T value to improve oxidation resistance.
Although iC and TaC are added, there is a problem that these additions impair the strength of the cemented carbide. TiC and TaC are added to this type of alloy for the purpose of improving the oxidation resistance required in the use environment (used at a high temperature in the air), and these additives are used for grain growth of WC. It is a material also known as an inhibitor, which contains Cr ₃ C ₂ , V
Since C is further added, there is a problem that the mechanical strength of the obtained alloy is significantly impaired. Since the sintering temperature of alloys that do not include such a metal binder increases, the addition of a grain growth inhibitor may be indispensable,
If the amount is large, even if it can be sintered in fine particles, its strength is deteriorated and it cannot be used as a practical machine part. SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and does not contain any metal binder and contains only a minimum necessary grain growth suppressing material, and has high strength having excellent wear resistance and corrosion resistance. It is an object of the present invention to provide a novel tungsten carbide sintered body and a method for producing the same.

[0005]

In order to solve the above-mentioned problems, the present inventor has conducted intensive studies on sintering of WC fine powder. By using a graphite mold that tends to become an atmosphere and sintering under pressure while simultaneously energizing WC fine powder, tungsten carbide consisting of a stoichiometric WC single phase with the addition of a minimum necessary grain growth inhibitor The inventors have found that a sintered body can be obtained, and have accomplished the present invention. That is, the average particle size is 0.
VC, Cr ₃ C _{2 on} tungsten carbide powder of 8 μm or less
0.1% by weight or more and 2% by weight or more
A step of preparing a mixed raw material powder by mixing as follows, a step of filling the mixed raw material powder into a graphite mold, and loading the graphite mold filled with the mixed raw material powder into an electric sintering machine,
It is intended to provide a method for producing a tungsten carbide sintered body, which comprises sintering the mixed raw material powder under pressure by electric heating, and a tungsten carbide sintered body produced by this method. Here, all or part of the VC and Cr ₃ C ₂ may be added as a composite carbide with tungsten carbide.

[0006] In the sintering of a cemented carbide containing a metal binder, a liquid phase is generated during the heating process, and rearrangement of the WC particles with the liquid phase interposed therebetween and mass transfer by diffusion through the liquid phase occur. Mainly sintering occurs. On the other hand, in sintering in a system that does not include a liquid phase, a densification mechanism mainly comprising a solid phase reaction is mainly used. In the sintering of the WC fine powder according to the present invention, a densification mechanism by a solid phase reaction not involving the liquid phase is considered. In such sintering not involving the liquid phase, the powder properties (particle size, shape, etc.) and mechanical properties of the powder to be sintered, particularly high-temperature strength and the presence of pressure, are important factors that determine whether sintering is possible. A factor. IVb, Vb, V in the periodic table including WC
The transition metal carbide of Ib is generally known as a typical example of a high-strength or high-hardness material, but at a high temperature, its strength rapidly decreases. In many cases, the material softens at a temperature equal to or higher than half the melting point to a position corresponding to the strength of copper at room temperature. At a temperature close to room temperature, the resistance to dislocation of the crystal lattice is extremely large, and it is difficult to deform. However, at a high temperature, it is said that the mobility of metal atoms constituting the crystal sharply increases. Therefore, in solid-phase sintering of this type of material powder, pressure is a factor that plays an important role in accelerating densification, and seems to be essential.

Here, in order to achieve the object of the present invention, in addition to this pressure condition, there are actually some restrictions. These are the minimum required amount of the grain growth suppressing material that does not cause a decrease in strength, the minimum temperature condition under which there is no grain growth and densification can be achieved, and the pressure condition under which the densification can be achieved without exceeding the mold strength of the graphite mold. The present invention provides a range of the best sintering conditions allowed among these constraints and requirements. In the electric sintering, a sample to be sintered, generally a conductive material powder is filled in a molded outer frame having a hole into which a punch is inserted in the center, and conductive punches are set above and below the outer frame.
This is placed in an electric sintering machine, pressurized to a predetermined pressure, and then electrically heated through upper and lower punches. Here, when a conductive molded outer frame is used, the current mainly flows through two paths. One flows through the sample portion via the upper and lower punches, and the other flows through the molded outer frame. Due to the former effect of the current flowing through the sample portion, the electric current sintering can exhibit several unique effects that cannot be obtained by a normal external heat type hot press method. Activation of the particle surface by a micro-discharge phenomenon between grains is one of them. The atmosphere in the electric sintering can be selected from various atmospheres according to the purpose other than the vacuum.

In the method according to the present invention, sintering of WC fine particles with the necessary minimum amount of the grain growth suppressing material is possible. The sintering mechanism is considered as follows. It is considered that the surface of each particle of the WC fine powder is covered with a thin film of an oxide such as WO ₃ . The amount of oxygen at the time of purchase of a commercially available 0.5 μm WC fine powder is 0.2 to 0.3%, but the amount of oxygen immediately before sintering after subsequent handling has reached nearly 0.5%. When this is converted to WO ₃ becomes about 2.4%. The oxide film on the surface of the particles is formed by carbon (C) added for the purpose of the reduction or CO (2CO →
CO2 + C is supplied and carbon is supplied), reduced and carbonized to return to WC and sintered. Here, when the average particle diameter of the WC fine powder is reduced to 0.8 μm or less, the ratio occupied by the particle surface becomes extremely large, and the reduction and carbonization reaction of this oxide film is important for the sintering mechanism of this system, and , Will play an effective role. The direct carbonization reaction of WO ₃ with carbon (C) is described as WO ₃ + 4C → WC + 3CO in the chemical formula, but it is reported that when this is decomposed into elementary reactions, the following process is performed. WO ₃ → WO _2.9 → WO _2.7 → WO ₂ → W → W ₂ C → W
C The crystal system from the left to WO _{2 of} this reaction is monoclinic, W is cubic, and W ₂ C and WC all have a hexagonal structure. The reduction reaction of WO _2.7 → WO ₂ is a solid-phase reaction involving nucleation. A point to be noted in the above series of reaction processes is a solid-phase reaction (WO _2.7 → WO ₂ ) in which the nucleation of the product is accompanied by a large change in particle shape during the reduction reaction.
And that a series of reactions involves changes in crystal structure with rearrangement of atoms. The former reaction indicates that the diffusion and movement of the constituent atoms occur in a relatively large order accompanying this reaction, and the latter shows that the order of displacement of the atoms is small, but the atoms alone or as clusters. It shows that it is displaced. Therefore, in the sintering of the WC fine powder according to the present invention which does not contain a liquid phase, reduction and carbonization of a thin oxide film covering the surface of the raw material provide an important means of mass transfer necessary for sintering. it is conceivable that. Since the mass transfer involved in this reaction is a solid phase reaction as described above, it requires temperature and time, but is considered to have the same action as the liquid phase in the system containing the metal binder.

WC fine powder alone and similar fine powder to 0.5
% Cr ₃ C ₂ was added, and sintering was performed under the same conditions. When the grain growth in the obtained sintered body was observed, no grain growth was observed in the case where Cr ₃ C _{2 was} added. This result is
This was similar to the phenomenon in sintering involving a liquid phase such as -Co system. The mechanism of WC grain growth in the WC-Co system is due to the dissolution and precipitation of WC in the liquid phase, and the grain growth inhibitor reduces the solubility of WC in the liquid phase here.
As a result, it is considered that grain growth is suppressed. It is unclear whether grain growth is suppressed by the same mechanism in sintering not involving a liquid phase according to the present invention, but at least the addition of a small amount of a substance such as Cr ₃ C ₂ causes It was found that this had a negative effect on the mobility of W or WC in the medium, and as a result, grain growth could be suppressed.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below based on embodiments with reference to the drawings. FIG. 1 schematically shows an electric current sintering method for explaining one embodiment. In the method for manufacturing a tungsten carbide sintered body according to the present invention, the molded outer frame 1, the upper punch 2, and the lower punch 3 are made of graphite. This is for adjusting the atmosphere during the heating and sintering of the portion filled with the mixed raw material powder 4 to a CO atmosphere (reducing and carbonizing atmosphere).
Reduction and carbonization reactions of the oxides in the fine powder occur, which promotes densification. There is also a method of adding carbon powder in another form for this purpose, but it is difficult to adjust the optimal amount of carbon in a composition system that does not contain a metal binder, and there is no solid solution of carbon when it becomes excess , Resulting in residual carbon and lowering the strength. On the other hand, a carbon deficiency is not preferable because a harmful phase such as W ₂ C is formed. In addition, since the molding outer frame and the upper and lower punches are made of graphite, uniform heating of the raw material powder and sintering for a short time can be performed, and mold strength at a high temperature can be secured. By simultaneously applying electricity to the molding outer frame 2 and the mixed raw material powder 4 and utilizing self-heating in the mixed raw material powder portion, rapid heating and short-time sintering become possible, thereby suppressing WC grain growth. Can be. In the manufacturing method according to the method of the present invention, it is desirable that the heating time and the holding time of sintering be as short as possible.

In the electric current sintering, first, a molded outer frame 1 made of graphite is used.
, A lower punch 3 is set therein, the mixed raw material powder 4 is filled therein, and an upper punch 2 is set thereon, thereby assembling a sintered assembly. The sintering assembly is charged into an electric sintering machine, the atmosphere around the graphite mold 5 is adjusted to a vacuum or an inert gas atmosphere, and then the energization is started, and the mixed raw material powder 4 is sintered under pressure. The energization here may be a direct current, an alternating current, a direct current pulse, or a superimposed energization form thereof. In the figure, reference numeral 6 denotes a power supply. The pressing force here is
At least 100 kg to achieve the purpose of the present invention
/ Cm ² or more is required. In the tungsten carbide sintered body according to the present invention, the average particle size of WC needs to be 1 μm or less, and the average size of WC particles in the mixed raw material powder before sintering is 0.8 μm or less. It was necessary. If the average particle diameter of the raw WC particles is larger than this, the WC particles after sintering will be too large, and the strength and hardness of the sintered body will be reduced. In order to achieve high density and high hardness of the sintered body, the average particle diameter of the WC particles of the mixed raw material powder is desirably 0.2 to 0.5 μm. Furthermore, it is necessary that the tungsten carbide sintered body be composed of a stoichiometric WC single phase. In a tungsten carbide sintered body without a metal binder, W ₂
C is easily generated. Due to the formation of W ₂ C, both the hardness and the strength are reduced, the corrosion resistance is poor, and the object of the present invention cannot be achieved.

Further, the tungsten carbide sintered body according to the present invention is characterized in that at least one of VC and Cr ₃ C ₂ is used in 0.1%.
It contains 1% by weight or more and 2% by weight or less. IVb of the periodic table in 0.5 μm WC fine powder containing no metal binder,
Vb, VIb transition metal carbides, nitrides and carbonitrides were added, and their grain growth inhibiting effects were investigated.
A remarkable effect was observed with VC, Cr ₃ C ₂ and their simultaneous addition. Further, based on the evaluation results of the strength and corrosion resistance of the obtained sintered body, the amount was set to 0.1% or more and 2% or less. If it is 0.1% or less, a sufficient effect of suppressing grain growth cannot be obtained, and both hardness and strength decrease. Further, when the content is 2% or more, the corrosion resistance is reduced along with the strength. Where VC, C
a composite carbide of r ₃ C ₂ or a part thereof with WC,
For example, when mixed as a composite carbide such as (W, Cr) C and (W, V) C, the effect of suppressing grain growth is remarkable, and particularly, the effect can be exerted in a very small amount. Another object of the present invention is to provide a high-density, high-strength tungsten carbide sintered body having both wear resistance and corrosion resistance. For this purpose, a WC sintered body according to the present invention is provided. Has a relative density of 99% or more and a micro Vickers hardness of 2200 kg
/ Mm ² or more is required. At a relative density of 99% or less, residual pores are present, and corrosion and abrasion starting from the pores are likely to occur, deteriorating abrasion resistance and corrosion resistance, and reducing the strength of the sintered body. If the hardness is 2200 kg / mm ² or less, practical wear resistance cannot be obtained.

[0013]

EXAMPLES Further, as a result of an experiment under the following conditions,
Very good results were obtained. Example 1 A mixed raw material powder having the composition shown in Table 1 was prepared by mixing for 48 hours using a carbide ball mill. Sample Nos. 1 to 5 in Table 1 are Examples of the present invention, and Sample Nos.
Are comparative examples outside the scope of the present invention. The average particle size of the raw material powder used here is WC 0.5 μm, VC 0.7 μm
m, Cr ₃ C ₂ 1 μm. These mixed raw material powders were individually filled into the positions of the mixed raw material powders 4 of the graphite mold 5 shown in FIG. The graphite mold is a cylindrical outer frame 1 having an outer diameter of 70 mm, an inner diameter of 30 mm, and a height of 60 mm. The upper and lower punches 2 and 3 have a diameter of 30 m.
m, a columnar shape having a height of 30 mm was used.

The current pressure sintering was performed in the following procedure. First,
The graphite mold 5 filled with the mixed raw material powder 4 was set in an electric sintering machine, evacuated, and then 2.1 tons (about 300 kg / m
The pressure was increased to c ² ), and energization was started. Temperature measurement is φ70m
m on the outer peripheral surface of the molding outer frame, and the temperature measurement there is 1750
C was taken as the sintering temperature. This temperature was maintained for 3 minutes, and the energization was stopped. After furnace cooling as it was to 500 ° C., it was taken out into the air and cooled to room temperature. Thereafter, a punch was pressed to collect the tungsten carbide sintered body therein. The collected sintered body was measured for density, hardness, and cohesive strength, further observed for structure, and analyzed for constituent phases. Table 1 shows some of the results.
From the observation results of X-ray diffraction and etching structure on the polished surface of the sintered body, the constituent phases of each sintered body obtained here
C was confirmed to consist of a single phase.

[0015]

[Table 1]

Example 6 W powder having an average particle diameter of 0.8 μm, Cr ₃ C ₂ powder having an average particle diameter of 1 μm, and carbon powder having an average particle diameter of 0.1 μm were used as raw material powders, with W: Cr ₃ C ₂ : C = 93. .12: 0.8
0: 6.08, and the mixed powder was heat-treated at 1550 ° C. for 30 minutes in a hydrogen atmosphere to obtain (W, C
r) A composite carbide of C was obtained. The composite carbide was pulverized by a carbide ball mill for 40 hours to obtain a mixed raw material powder having an average particle size of 0.5 μm. In the X-ray diffraction analysis of this mixed raw material powder, no single diffraction line of Cr ₃ C ₂ was observed. This mixed raw material powder was subjected to current pressure sintering in the same manner as in Example 1 to obtain a tungsten carbide sintered body. The obtained sintered body has W ₂ C shifted from the stoichiometric ratio.
Was not recognized, and the relative density was 100%. The hardness and bending strength of this sintered body were 2550 k, respectively.
g / mm ² and 150 kg / mm ² . Cr ₃ C ₂
Has been confirmed to be present in the form of a composite carbide with WC also in the obtained sintered body.

[0017]

Since the present invention is configured as described above, the following effects can be obtained. According to the method of the present invention, it is possible to produce a high-strength tungsten carbide sintered body having both wear resistance and corrosion resistance higher than conventional cemented carbide, and this sintered body has abrasion resistance, especially sliding resistance. It can be used for a wide range of applications as various mechanical parts such as water jets, various nozzles and dies, which require abrasion resistance, abrasive wear resistance, and erosion wear resistance.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating an electric current sintering method used in a production method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Forming outer frame 2 Upper punch 3 Lower punch 4 Mixed raw material powder 5 Graphite type 6 Power supply

Claims (5)

[Claims] 1. A tungsten carbide containing at least one of VC and Cr ₃ C _{2 in} an amount of 0.1% by weight or more and 2% by weight or less, the balance being tungsten carbide having an average particle size of 1 μm or less, wherein the tungsten carbide is chemically A tungsten carbide sintered body comprising a stoichiometric WC single phase. _2. The tungsten carbide sintered body according to claim 1, wherein all or a part of VC and Cr ₃ C ₂ form a composite carbide with tungsten carbide. 3. The relative density of the tungsten carbide sintered body is 9
9% or more, micro Vickers hardness is 2200kg / m
^{2. The} tungsten carbide sintered body according to claim 1, which has a m ² or more. 4. A mixed raw material obtained by mixing at least one of VC and Cr ₃ C _{2 with} a tungsten carbide powder having an average particle size of 0.8 μm or less so as to be 0.1% by weight or more and 2% by weight or less. A step of producing powder; a step of filling the mixed raw material powder into a graphite mold; a step of charging the graphite mold filled with the mixed raw material powder into an electric sintering machine; and pressing the mixed raw material powder by electric heating. A method for producing a tungsten carbide sintered body, comprising sintering. 5. The method according to claim 4, wherein all or part of VC and Cr ₃ C ₂ are mixed as a composite carbide with tungsten carbide.
A method for producing the tungsten carbide sintered body according to the above.