JPH0798981B2 - Method for producing oxidation-resistant tungsten-based sintered alloy - Google Patents

Description

The present invention relates to a method for producing a tungsten-based sintered alloy having excellent oxidation resistance.

[Conventional technology]

Since the tungsten (W) -based sintered alloy has a small coefficient of thermal expansion and ductility, it is used as a buffer material for joining ceramics and metals.

That is, since the ceramics and the metal have greatly different coefficients of thermal expansion, brazing and joining the two causes a strain to remain in the vicinity of the joint, resulting in the breakage of the brittle ceramic. For the purpose of preventing this destruction, it is known to use a W, Mo, W-based sintered alloy having a thermal expansion coefficient close to that of ceramics as a buffer material (Japanese Patent Laid-Open No. 61-127674).

Further, W-based sintered alloys are known to be used as high-temperature molding members such as die cast dies by utilizing their high hardness and toughness to withstand thermal shock (Japanese Patent Application Laid-Open No. 47-33018).

However, conventional W-based sintered alloys have low oxidation resistance, and
At a temperature of 600 ° C or higher, there was a drawback that the oxidation proceeded rapidly. Therefore, sufficient reliability or durability as a joining cushioning material or a member for high temperature molding cannot be obtained. In particular, regarding the composite of ceramic and metal, it is strongly desired to improve the reliability of the bonding interface strength in a high temperature oxidizing atmosphere, and in order to meet this demand, also to expand the use of the ceramic-metal composite. There is an urgent need to improve the oxidation resistance of composite cushioning materials.

[Problems to be solved by the invention]

In view of the above conventional circumstances, it is an object of the present invention to provide a W-based sintered alloy having both a low coefficient of thermal expansion and sufficient ductility while having excellent oxidation resistance.

[Means for solving problems]

The method for producing an oxidation-resistant W-based sintered alloy of the present invention comprises: 85 to 95% by weight of W powder and the balance of Ni powder and Fe powder having a Ni: Fe weight ratio of 1: 1 to 4: 1. 20 to 60 ℃ from the melting temperature of Ni-Fe phase in a reducing atmosphere
Sintered at a very high temperature, the resulting W-based sintered body has a temperature of 1420 ° C
It is characterized by diffusing chromium at a temperature of 1500 ° C.

Various means can be considered for the diffusion of chromium, but it is sufficient that at least a chromium diffusion layer can be formed on the surface layer of the W-based sintered alloy. As a simple and effective means, for example, the sintered body is embedded in a mixed powder obtained by mixing chromium powder and alumina powder in a ratio of 1: 4 to 9: 1, preferably 4: 1, and the above 1420 ° C to 15
There is a method of heating to a temperature of 00 ° C.

[Action]

When the W content in the mixed powder of raw materials is less than 85% by weight, deformation occurs during sintering and the thermal expansion coefficient of the alloy increases. On the contrary, if the W content exceeds 95% by weight, the Ni-Fe binder layer is reduced and the ductility of the alloy decreases. Regarding the ductility of the alloy, if the weight ratio of Ni-Fe is in the range of 1: 1 to 4: 1, the desired ductility is obtained, and it becomes maximum when this weight ratio is 2: 1. Although the raw material powder contains inevitable impurities, for example, oxygen is 0.05% by weight or less and carbon is 0.
It may be contained in an amount of 005% by weight or less.

The mixed powder is usually pressed at a pressure of 1.0 to 1.5 ton / cm ² to obtain a molded product having a desired shape. The compact is sintered in a reducing atmosphere, preferably a hydrogen atmosphere, and the sintering temperature is Ni.
-Temperature range is 20 ℃ to 60 ℃ higher than the melting temperature of Fe phase. This is because if the sintering temperature exceeds this range, deformation occurs during sintering. Although the sintering time varies depending on the shape of the compact, it is preferable to set the W particle diameter to 20 to 100 μm.

With the above composition and sintering conditions, a sintered body (alloy) having an elongation of 10% or more can be obtained.

Diffusion of chromium into the obtained sintered body is performed at a temperature of 1420 ° C to 1500 ° C. This is because if the treatment temperature is lower than 1420 ° C, diffusion into the sintered body, especially W particles, is slow and sufficient oxidation resistance cannot be obtained, and if it exceeds 1500 ° C, the deformation of the alloy becomes remarkable. The thickness of the chromium diffusion layer can be controlled by the treatment temperature and the treatment time. For example, a treatment at a temperature of 1420 ° C. for 30 minutes forms a chromium diffusion layer having a thickness of 1 mm.

In the method of diffusion using the chromium powder and alumina powder described above, the chromium powder is preferably 100 mesh or less in order to ensure uniform diffusion. or,
Since the alumina powder is used to prevent the seizure of the W-based sintered alloy and the alumina powder, other stable ceramic powder can be used instead.

〔Example〕

191 kg of W powder, 6 kg of Ni powder and 3 kg of Fe powder were mixed in an attritor for 5 hours using alcohol as a solvent. The particle size of the mixed powder after vacuum removal of alcohol was 2.2 μm and the carbon content was 0.003% by weight.
And the oxygen content was 0.02% by weight.

To this mixed powder, 0.2% by weight of Kanha dissolved in methylene chloride was mixed, and then 1 ton / cm ² was applied using a mold.
A pressure was applied to obtain a molded body. Next, this molded body was subjected to intermediate firing at 500 ° C. in an N ₂ atmosphere to remove the Kanha, and then sintered at 1460 ° C. for 3 hours in a hydrogen sintering furnace. The obtained sintered body has a diameter of 10 mm and a length of 50 mm, and the average particle size of W particles is 60 μm.
The tensile strength was 60 kg / mm ² and the elongation was 25%. The metallographic structure of this sintered body is shown in FIG. 1 as a 100 × micrograph.

Next, the sintered body was cut into a disk shape with a diameter of 9 mm and a thickness of 5 mm, and embedded in a mixed powder of 20 g of chrome powder of -325 mesh and 5 g of alumina powder of -325 mesh, in a hydrogen sintering furnace. Heat treatment was performed at 1460 ° C. for 30 minutes. Cr of the obtained sintered alloy
The metallographic structure of the diffusion layer is shown in FIG. 2 as a 100 × micrograph. The thickness of the Cr diffusion layer is about 1 mm, and its metallographic structure has changed significantly from the state shown in Fig. 1, and it can be seen that Cr is uniformly diffused in the Ni-Fe binder phase and the W particle phase. .

The W-based sintered alloy thus obtained was heated in the air at different temperatures, and the amount of increased oxidation was measured. As a comparative example, the amount of oxidation increase was measured in the same manner for a sample that did not undergo Cr diffusion treatment up to the sintered body. The results are summarized in the table below. Cr diffusion treatment Present invention example Comparative example 600 ° C × 2h 09 × 10 ^-6 g / mm ² 700 ° C × 2h 0 82 × 10 ^-6 g / mm ² 800 ° C × 2h 0 210 × 10 ^-6 g / mm ² 900 ° C. × 2 h 20 × 10 ^-6 g / mm ² 650 × 10 ^-6 g / mm ^{2 From the} above results, the Cr-diffused W-based sintered alloy of the present invention has extremely excellent oxidation resistance even at high temperatures. You know that you have.

〔The invention's effect〕

According to the present invention, since the Cr diffusion layer is formed on the surface of the W-based sintered alloy, while maintaining a low thermal expansion coefficient and sufficient ductility,
It is possible to provide a W-based sintered alloy that also has excellent oxidation resistance.

Therefore, when the W-based sintered alloy according to the present invention is used as a buffer material for bonding ceramics and metals, the bonding is performed by the internal alloy having ductility, while the other exposed portions are Cr diffusion excellent in oxidation resistance. Since it becomes a layer, the reliability of the bonding interface strength in a high temperature oxidizing atmosphere can be improved. Also, when used as a high-temperature molding member in a portion that is directly contacted with a high-temperature molten metal, it is possible to create a mold or the like that has bending resistance, heat resistance, high-temperature oxidation resistance, and sufficient reliability or durability. .

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a 100 × photomicrograph showing the metallographic structure of a sintered body after completion of sintering in the method of the present invention, and FIG. 2 is a W-based firing bond produced by the method of the present invention. It is a 100 times micrograph showing the metal structure of the Cr diffusion layer of gold.

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Claims (1)

[Claims] 1. A compacted body of a mixed powder comprising 85 to 95% by weight of W powder and the balance Ni powder and Ni powder having a weight ratio of Ni: Fe of 1: 1 to 4: 1 in a reducing atmosphere. Sintered at a temperature 20 to 60 ℃ higher than the melting temperature of the Ni-Fe phase, and diffused chromium into the resulting sintered body at a temperature of 1420 ℃ to 1500 ℃.
A method for producing an oxidation resistant tungsten-based sintered alloy, which comprises diffusing chromium in an e-bonded phase and tungsten particles.