JPH0717927B2 - Method for producing tungsten sintered alloy - Google Patents

Description

TECHNICAL FIELD The present invention relates to an improvement in a method for producing a high-toughness tungsten sintered alloy suitable for applications such as a core material and quill which are required to have high specific gravity and high toughness. .

[Problems to be Solved by Conventional Techniques and Inventions]

Sintered alloys of W-Ni-Fe system have been used for applications such as core materials and quills that require high specific gravity and high toughness. The demand for improvement of toughness is becoming stronger.

By the way, in the W-Ni-Fe based sintered alloy, the bonding force between W grains in the structure is the weakest. Therefore, in order to improve the toughness of the alloy, it is important to reduce the contact grain boundaries between W powders. In the case of a W-Ni-Fe system sintered alloy, the molding material sent into the liquid phase sintering chamber through the preheating chamber of the continuous sintering furnace is heated at the sintering temperature for a predetermined time. The Ni-Fe-W component, which is a solid solution of W in several percent of the Ni-Fe component, is heated to a temperature 20 to 40 ° C higher than the melting point of about 1450 ° C and liquid-phase-sintered. The liquid phase penetrates into the boundary and promotes densification of the alloy structure. After that, by cooling in a cooling chamber, a structure in which the Ni-Fe-W component solidified from the liquid phase surrounds the W grains that were in the solid phase during sintering becomes a structure, and due to the ductility of the Ni-Fe-W component, sintering bonding occurs. The ductility of gold is improved.

Although it is basically a Ni-Fe-W component, cobalt (Co) may be added as a further component, if necessary. Co is added to improve the strength of the tungsten sintered alloy, and if its content is too large, ductility deteriorates, so 0.5 wt% or less is appropriate, but W-Ni-Fe- The melting point of the Co component is 1470 ° C.

In any case, the cooling rate at the time of cooling from the liquid phase sintering temperature is generally about 3 to 6 ° C./min, but the liquid phase surrounding the W grains is discharged during the cooling process. The W grains contact again and it is difficult to obtain the desired ductility. Therefore, a method is known in which re-contact between W grains is prevented and the ductility is improved by rapidly cooling at a cooling rate from the liquid phase sintering temperature of 8 ° C./min or more. However, rapid cooling from the liquid phase sintering temperature causes shrinkage cavities (vacancy defects) due to rapid solidification shrinkage of the liquid phase, and ductility is greatly deteriorated (G.Petzow et al., “Modern Developmennts in Powder”).
Metallurgy ", Vol. 14 (1981), 189-203.).

According to our detailed study on the cooling rate,
When the cooling rate from the liquidus temperature is increased, deep shrinkage cavities due to solidification shrinkage are particularly likely to occur at the rear end of the material to be sintered that is advancing in the continuous sintering furnace. As a result, there is a problem in that the toughness deteriorates or the yield decreases due to the cutout of the shrinkage cavity.

The present invention has been made by paying attention to such a conventional problem, and prevents the formation of deep shrinkage cavities by supplying heat to the rear end of the material rapidly cooled after liquid phase sintering. It is an object of the present invention to solve the above-mentioned conventional problems by providing a method for manufacturing a high-toughness tungsten sintered alloy.

[Means for Solving the Problems]

The present invention stores a powder compact having a composition of 85 to 98 wt% tungsten, the balance nickel (Ni) and iron (Fe), and cobalt (Co) optionally further contained in a tray,
In a method for producing a tungsten sinter bond, which comprises sintering through steps of preheating, liquid phase sintering, and cooling in a continuous furnace, the tungsten sinter alloy is added to the filler at the rear end of the powder compact in the tray. A metal or alloy having a similar melting point is embedded and sintered, and in the subsequent cooling step, the latent heat of solidification of the metal or alloy melted by heating during sintering is transferred to the sintered compact body. It is a thing.

[Action]

When the material is heated to a temperature above the melting point, the Fe-Ni (-Co) -W component, which is a solid solution of W in the Fe-Ni (-Co) component, becomes a liquid phase,
Penetrate into the contact grain boundary between W grains, which is a solid phase. The W grains thus separated are prevented from re-contacting each other by rapidly cooling at a cooling rate of 8 ° C./min or more, and a structure in which the W grains are surrounded by the Fe-Ni (-Co) -W component Next to
Ductility is improved. In the present invention, the metal or alloy having a melting point similar to that of the tungsten sintered alloy arranged at the rear end of the molding material in the tray melts during the above heating. During cooling, the latent heat of solidification of the molten metal or alloy is applied to the rear end of the molding material. By supplementing the heat from the outside, only the rear end portion of the molding material is gradually solidified without being rapidly cooled. Therefore, deep shrinkage cavities do not occur. Since the product can be processed with a sufficiently good yield if the depth of the sinkhole is shallow, it is possible to produce a high-toughness tungsten sintered alloy with improved toughness by rapid cooling with a good yield.

The details will be described below.

The main composition of the tungsten sintered alloy of the present invention is 85 to 98 wt% of tungsten (W), the balance nickel (Ni) and iron (Fe), and cobalt (Co) optionally contained. The W content needs to be 85% or more in order to maintain a predetermined high density. In addition, in order to secure the amount of the liquid phase that is completely densified in the liquid phase sintering step when manufacturing the tungsten sintered alloy, it is necessary to be 98 wt% or less. Ni and
Fe is added as a binder that generates a liquid phase during sintering, promotes densification, and enhances ductility of the material. On the other hand, Co
Improves the strength of the tungsten sintered alloy, but the content is preferably 0.5 wt% or less so as not to reduce the ductility.

The weight ratio of Ni and Fe is preferably in the range of Ni: Fe = 0.5 to 4 in order to lower the liquid phase formation temperature and perform effective liquid phase sintering.

The high toughness tungsten sintered alloy of the present invention is manufactured by mixing the raw material powders, forming the mixed powder under pressure in a predetermined forming die, and forming the formed material in a liquid phase in a sintering furnace. Sintering process of heating to a liquid-phase sintering temperature above the generation temperature to perform liquid-phase sintering, and then rapidly cooling in a cooling gas flow at a cooling rate of 8 ° C / min or more, and vacuum heat treatment of the material after completion of sintering. The heat treatment is performed in the furnace. However, the inventors of the present invention have diligently studied a method of preventing defects due to deep shrinkage cavities at the rear end of the material that normally occur during cooling after liquid phase sintering in the above-mentioned sintering process, and as a result, the liquid phase In the solidification zone from temperature, it is extremely effective to prevent shrinkage cavities by applying heat from the outside of the material to the rear end of the material. There are various ways of applying that heat, but the latent heat of solidification of the molten metal is used. Was confirmed to be the most practical and sufficient result.

A metal or an alloy having a melting point similar to that of the tungsten sintered alloy is preferable for giving the latent heat of solidification. A melting point similar to this is a temperature of (melting point of tungsten sintered alloy + 25 ° C. or less). If a metal or alloy having a melting point exceeding this range is used, the time of latent heat of solidification and the time of shrinkage cavity formation of the tungsten sintered alloy are deviated,
I cannot achieve my purpose.

For example, when a rod-shaped material is sintered into a rod-shaped material by using a continuous sintering furnace in which a preheating chamber, a liquid-phase sintering chamber and a cooling chamber are continuous, the molding material is the furnace in the longitudinal direction. It is embedded in the filled alumina powder (Al ₂ O ₃ ) in the tray toward the inner traveling direction. At that time, a container such as a crucible containing a metal or an alloy having a melting point similar to that of the tungsten sintered alloy is arranged near the rear end of the material. As the tray moves through the furnace, a liquid phase is generated in the tungsten sintered alloy material that has reached the liquid phase sintering temperature, and a metal or alloy having a similar melting point in the crucible is melted. Then, when the cooling process from the liquid phase sintering temperature is reached, the material is rapidly cooled from the front end in the traveling direction and progresses while solidifying in sequence toward the rear end side. A shrinkage cavity is formed at the rear end of the material that finally solidifies, but the latent heat of solidification is transferred as the metal or alloy with a similar melting point in a nearby crucible cools and solidifies. It Due to the heat supply, only the rear end portion of the material is gradually cooled, and deep shrinkage cavities due to rapid cooling do not occur. On the other hand, good ductility due to rapid cooling can be obtained in portions other than the rear end of the material.

Thus, according to the present invention, in the step of sintering a tungsten sintered alloy, when the material is cooled from the liquid phase sintering temperature at a cooling rate of 8 ° C./min or more, heat is supplied to the rear end of the material. By this, the occurrence of deep shrinkage cavities can be effectively prevented. If the depth of the shrinkage cavities is within 1.5 mm, it is possible to process the molding material after completion of sintering into a product with sufficiently high yield. [Examples] Hereinafter, examples of the present invention will be described with reference to the drawings. It will be described with reference to FIG. FIG. 1 is a vertical cross-sectional view of a tray used for sintering a material in a continuous sintering furnace, and an arrow mark A represents a traveling direction in the furnace.

Hydrogen-reduced tungsten powder, carbonyl nickel powder, carbonyl iron powder, and hydrogen-reduced cobalt powder were used as raw material powders and mixed using a V-type mixer. For molding, a cold isostatic press was used to obtain a molded body 1 having a diameter of 25 mm and a length of 180 mm at a pressure of ² ton / cm ² . The composition is 93wt% W-4.9wt
% Ni-2.0wt% Fe-0.1wt% Co. This molded body 1
It is embedded in the alumina powder 3 in the tray 2 made of Mo shown in FIG. An alumina crucible 5 filled with a metal or alloy 4 having a melting point similar to the melting point of 1470 ° C. of the tungsten sintered alloy of the compact 1 was embedded and arranged near the rear end of the compact 1. In this example, various metals or alloys 4 having similar melting points as described above were used and compared with each other as shown in Table 1.

The tray 2 was sent to a pusher continuous sintering furnace (not shown) for liquid phase sintering. In the sintering furnace, a preheating chamber, a liquid phase sintering chamber, and a cooling chamber are continuously connected, and the tray 2 is fed from the inlet of the preheating chamber with the traveling direction of the conveying device in the furnace being the longitudinal direction. After preheating through the preheating chamber, heating was performed in a liquid phase sintering chamber in a H ₂ stream at a liquid phase sintering temperature of 1500 ° C. for 60 minutes. This heating melts the liquid phase forming metal of the compact 1 and also melts the metal or alloy 4 having a similar melting point in the crucible 5. Then, it was cooled from the liquid phase sintering temperature while continuously moving from the liquid phase sintering chamber to the cooling chamber. At this time, the molded body 1
The inside of the first has the lowest temperature at the front end which enters the cooling chamber, and the solidification proceeds rearward in sequence with the progress of the molded body 1. Finally, the liquid phase portion remaining at the rear end of the molded body 1 is cooled,
The solidification latent heat generated by the solidification of the metal or alloy 4 having a similar melting point in the crucible 5 is transferred to this portion, so that the cooling and solidification is relatively gradually performed.

The compact 1 taken out from the sintering furnace was subjected to vacuum heat treatment at a vacuum degree of 10 ^-4 Torr at 1150 ° C for 2 hours, and then cooled with Ar gas at a cooling rate of 20 ° C / min to obtain a test object. It was This test object was cut and the depth of the shrinkage cavity was measured.

The melting point of metal or alloy 4 having a melting point similar to that of Table 1,
The depth of the shrinkage cavity (distance from the rear end) of the test body was shown.
Nos. 1 to 3 are examples of the present invention, whereas Nos. 4 to 4 are
6 is a comparative example.

From Table 1, a clear difference between the tungsten sintered alloy of this example and the comparative example in terms of the depth of the shrinkage cavity was observed.
That is, all of the comparative examples are 20 mm to 10 mm.
However, in the case of the present example, the depth of the shrinkage cavities was 0.5 mm or less, and the generation of the shrinkage cavities that substantially reduce the yield of the product was not generated.

〔The invention's effect〕

As described above, according to the present invention, W85-98 wt%,
Tungsten firing bonding, in which a powder compact having the composition of Co, which contains Ni and Fe as required, and Co, if necessary, is placed in a tray and sintered through preheating, liquid phase sintering, and cooling in a continuous furnace In the method for producing gold, a metal or an alloy having a melting point similar to that of the tungsten sintered alloy is embedded in the filler at the rear end of the powder compact in the tray and sintered, followed by a cooling step. In the above, since the latent heat of solidification of the metal or alloy melted by heating during sintering is transferred to the sintered compacted powder compact, deep shrinkage cavities may occur inside the material due to rapid solidification shrinkage. It is possible to provide a tungsten sintered alloy that can be prevented and that has a remarkable improvement in toughness and that has no shrinkage cavities that substantially reduce the yield.

[Brief description of drawings]

FIG. 1 is an embodiment of the present invention and is a vertical sectional view for explaining the arrangement in the tray in the sintering step. Reference numeral 1 is a compact, 2 is a tray, and 4 is a metal or alloy having a melting point similar to that of a tungsten sintered alloy.

Claims (1)

[Claims] 1. A powder compact having a composition of 85 to 98 wt% tungsten, the balance nickel (Ni) and iron (Fe), and cobalt (Co) optionally contained therein is housed in a tray.
In manufacturing a tungsten sintered alloy through steps of preheating, liquid phase sintering and cooling in a continuous furnace, a melting point similar to that of the tungsten sintered alloy is contained in the filler at the rear end of the powder compact in the tray. Characterized in that the metal or alloy having the above is embedded and sintered, and in the subsequent cooling step, the latent heat of solidification of the metal or alloy melted by heating during sintering is transferred to the sintered compact body. And a method for producing a tungsten sintered alloy.