JPH062918B2 - Method for producing tungsten sintered alloy - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a tungsten sintered alloy useful for a projectile penetrating a high speed rotating body or a protective object.

[Conventional technology]

The high-speed rotating body must have a high degree of tensile strength and Young's modulus and have sufficient toughness so as not to be broken at high speed rotation. The projectile must also have a high degree of tensile strength, density and hardness, yet be sufficiently ductile and tough so that the projectile will not break before completely penetrating the protective object. .

A method for producing a tungsten alloy having a high specific gravity and a high ductility in order to meet such requirements is disclosed in JP-A-62-185843. This is a tungsten powder 8
A mixed powder of 5 to 97% and the balance of nickel and iron powder was compacted under hydrostatic pressure of 1 to 4 ton / cm ² , and the obtained compact was liquid-phase sintered in a hydrogen stream, The sintered body is subjected to heat treatment in which it is heated in a vacuum and then rapidly cooled.

By performing heat treatment of heating and quenching in a vacuum after the above-mentioned sintering, excess solid-solved hydrogen in the sintered body is removed, and grain boundary precipitation of impurities that cause embrittlement can be prevented, resulting in high ductility. Is said to be obtained.

[Problems to be Solved by the Invention] In a projectile penetrating a high-speed rotating body or a protective object to which the tungsten alloy obtained by the above-mentioned conventional manufacturing method is applied, dimensional accuracy is strictly required, and finally Surface grinding using a centerless machine is an essential process.

According to the results of the intensive studies by the present inventors, the ductility of the tungsten-nickel-iron (W-Ni-Fe) -based sintered alloy is extremely sensitive to the residual stress on the surface as in the case of ceramics. is there. Therefore, it is found that if the surface grinding is applied by a centerless machine after the heat treatment for dehydrogenation and the heat treatment for rapid cooling in the above conventional example, the ductility is remarkably deteriorated, which is a problem in actual use. Was done.

Therefore, an object of the present invention is to solve the above-mentioned conventional problems by providing a method for manufacturing a tungsten alloy capable of removing the adverse effect of residual stress on the surface after machining into a final shape.

[Means for Solving the Problems]

In order to achieve the above object, the present invention provides a tungsten 85
˜97%, the mixed powder consisting of nickel and iron powder as the balance is compression molded, then the compression molded body is densified by liquid phase sintering in hydrogen, and the obtained sintered body is processed into a final shape. Etching the surface will be described in more detail below.

The raw material powder used in the present invention is high-purity tungsten (W), nickel (Ni), iron (Fe) powder.
Tungsten content is 85 in order to maintain a predetermined high density.
% Or more is required. Moreover, in order to secure the amount of the liquid phase which is completely densified in the liquid phase sintering step, it is necessary to be 97% or less. Nickel and iron are added for the purpose of generating a liquid phase during sintering, promoting densification, and increasing the ductility of the material. The addition amount is 3 to 15% of the alloy amount. If it is less than 3%, a sufficient liquid phase is not generated and the effect of increasing the density cannot be exhibited. On the other hand, if it exceeds 15%, the content of tungsten becomes too small, and the high specific gravity of the alloy cannot be obtained. Further, the component ratio of nickel and iron can be adjusted to about 20 to 50% of iron with respect to the entire nickel-iron binding phase, that is, between Ni: Fe = 5: 5 and Ni: Fe = 8: 2. desirable. The reason is that in this composition range,
This is because the liquid phase generation temperature (melting point) in the liquid phase sintering step can be sufficiently lowered as compared with the case of nickel or iron alone, and eventually effective liquid phase sintering can be performed.

Therefore, instead of a single powder of nickel and iron,
Even if both alloy powders having the above composition are used, the same effect can be obtained.

The particle size of the raw material powder is preferably about 1 to 10 μm from the viewpoint of moldability, sinterability and ductility of the material.

The pressure for compression molding the mixture is 1 to 4 ton / cm ²
Hydrostatic pressure. In the case of molding at less than 1 ton / cm ² , 2-3% of pores remain even after liquid phase sintering. Therefore, the density of the compression molded product is too small to be completely densified,
Ductility decreases. Moreover, in the case of molding over 4 ton / cm ² ,
On the contrary, the density becomes too high, and a closed bore occurs in the compression-molded product, which cannot be completely densified.

The reason why the hydrostatic pressure is used instead of the usual uniaxial compression is to increase the homogeneity of the alloy and thus the ductility by uniformly pressing from all sides.

Liquid phase sintering needs to be performed in hydrogen at 1430 ° C. or higher, which is the temperature at which the nickel / iron component forms a liquid phase.

The sintering time is a time required for complete densification, that is, 20 minutes or more, and 60 minutes or less is desirable in order to prevent coarse porosity from being generated during sintering.

Next, the sintered body is processed into a final shape. The processing method may be cutting, grinding, plastic working such as swaging, or a combination thereof.

In the present invention, the surface is etched after the final shape is processed. The reason is that the adverse effect of the residual surface stress cannot be removed by performing mechanical processing such as surface grinding after etching.

As a method for etching the surface, various known methods can be used, but it is preferable to etch the tungsten portion using NaOH, KOH, hydrofluoric acid or the like. When the etching depth is 1 μm or more, the effect appears, but it is preferably 10
It is at least μm.

Although it is possible to remove stress by etching nickel and iron, it is preferable to etch the tungsten portion as described above. The reason is that the residual stress is mainly present in the tungsten grains and, in the case of nickel and iron, the notch effect occurs, and on the contrary, there is a risk of degrading the toughness.

It is well known that the residual stress on the surface generally affects the material properties such as fatigue strength. In particular, as mentioned above, brittle materials such as ceramics and tungsten are sensitive to surface residual stress. No matter how carefully the material is processed, it is impossible to generate residual stress. The present inventors have found that this residual stress on the surface has an extremely large effect on the ductility of the tungsten sintered alloy. FIG. 1 shows the relationship between surface residual stress and elongation in a tungsten alloy. The surface residual stress is measured by the half-value width midpoint method using X-ray diffraction. From FIG. 1, it can be seen that the elongation decreases in inverse proportion to the increase in the surface residual stress. In other words, removal of surface residual stress is very important for improving ductility.

According to the present invention, after compression-molding a predetermined metal powder is liquid-phase sintered and then processed into a final shape, the surface layer having residual stress due to the processing is removed by etching to remove the residual stress, thereby improving ductility. Could be effectively improved.

[Examples] Examples of the present invention will be described below.

Tungsten powder 95 wt% -nickel powder 3.5 wt% -iron powder 1.5 wt% were mixed and mixed using a V-type mixer. The obtained mixed powder was compression-molded under a hydrostatic pressure of ² ton / cm ² , and the molded body was liquid-phase sintered in hydrogen at 1530 ° C. for 40 minutes. Next, the sintered body was heated at 1200 ° C. for 2 hours,
After heat treatment in vacuum, tensile specimens were machined.

This machining causes surface residual stress in the test piece.
The tensile test piece was electrolytically etched with a 5N-KOH solution to remove the surface of 1 to 50 μm. The results of the tensile test conducted on the thus obtained tensile test pieces are shown in Table 1.

In addition, as a comparative example, the results of those in which the tensile test piece was not machined and then subjected to no etching treatment are also shown.

From Table 1, it can be seen that the method according to the present example exhibits higher elongation than that not subjected to the etching treatment.

[Effects of the Invention] As described above, according to the present invention, the tungsten 8
A mixed powder of 5 to 97%, the balance being nickel and iron powder, is compression molded, and then the compression molded body is densified by liquid phase sintering, and the obtained sintered body is processed into a final shape. Was etched to remove the portion where the surface stress due to processing remained. Therefore, the ductility of the tungsten sintered alloy in the final product shape can be remarkably enhanced, and a substantially excellent projectile and high-speed rotating body can be obtained.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between surface compressive residual stress and elongation in the tungsten alloy of the present invention.

Claims (1)

[Claims] 1. A sintered body obtained by compression-molding a mixed powder comprising 85-97% tungsten and the balance being powder of nickel and iron, and then densifying the compression-molded body by liquid phase sintering in hydrogen. A method for producing a tungsten sintered alloy, which comprises etching the surface after processing into a final shape.