JP2922248B2 - Manufacturing method of sintered alloy with excellent corrosion resistance - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a sintered alloy having excellent corrosion resistance by powder metallurgy.

<Conventional Technology> Conventionally, a molded article of a highly corrosion-resistant alloy containing a large amount of Ni, Cr, Mo, or the like has been manufactured by a precision casting method. However, a molded product (cast product) manufactured by a casting method has poor mechanical properties, and it is difficult to apply it to a corrosion-resistant part that also requires high mechanical properties. Further, since the surface texture of the cast product is poor, it is also difficult to apply it to corrosion-resistant parts used after being subjected to post-processing such as polishing, plating, and coating.

Cast products have the above-mentioned drawbacks, and forging and rolling methods are used depending on the application when manufacturing molded products of alloys containing a large amount of Ni, Cr, Mo, and the like. However, it is impossible to finish to the final part shape by the forging method or the rolling method, so a large amount of cutting work is required, and Ni, Cr,
Since the machinability of an alloy containing a large amount of Mo or the like is inferior, cutting is extremely costly.

Further, in the casting method and the forging method, applicable alloy systems are limited in the production method.

Therefore, a method of manufacturing a molded product containing a large amount of Ni, Cr, Mo, or the like by a sintering method has been proposed.

For example, Japanese Patent Application Laid-Open No. 57-198202 discloses that a granular compound is prepared by mixing a resin and a lubricant as a binder with an alloy powder, and a molding is obtained by injection molding to obtain a molded body, and the resin is removed from the molded body. And then sintering is disclosed.

According to this method, as described in JP-A-57-198202, after removing the binder from the molded article,
Carbon remains. This carbon impairs the properties of the final product.

Therefore, Japanese Patent Laid-Open No. 57-198202 proposes the following means to solve this problem. That is, using a raw metal powder containing less carbon, using a binder that hardly leaves carbon, and performing a heat treatment in a hydrogen atmosphere after sintering the compact.

However, in the case of the above-mentioned solution, the raw metal powder and the binder are restricted. However, since the restriction of the binder particularly affects the injection molding process, it is not advantageous. Heat treatment in a hydrogen atmosphere can be decarburized and deoxidized by controlling the dew point, but it cannot be denied that it is difficult.

<Problems to be Solved by the Invention> As described above, a method for producing a molded article of an alloy containing a large amount of Ni, Cr, Mo, or the like has not yet been established.

The present invention has been made in view of such circumstances, and is a method for producing a molded article of an alloy containing a large amount of Cr and Ni or Co, that is, an alloy having excellent corrosion resistance, comprising a raw metal powder and a binder. It is an object of the present invention to provide a production method by a sintering method without restriction on carbon in the inside.

<Means for Solving the Problems> As a result of various studies, the present inventors have limited the particle size of the raw metal powder and limited the sintering conditions after molding and dewaxing (vacuum sintering and non-oxidation). It has been found that a sintered alloy having excellent corrosion resistance can be obtained by the combined use with a neutral atmosphere sintering), and the present invention has been achieved.

That is, the present invention relates to any one of a nickel-based alloy powder, a cobalt-based alloy powder, and a mixture of these alloy component powders containing 10 to 50% by weight of Cr and having an average particle diameter of 20 μm or less. After adding and mixing the agent and molding to obtain a molded body having a desired shape, the binder in the molded body is removed by heating under reduced pressure and / or in a non-oxidizing atmosphere,
The C / O molar ratio in the compact was adjusted to 0.3-3,
A method for producing a sintered alloy having excellent corrosion resistance, characterized by performing primary sintering under a reduced pressure of 1350 ° C or less and a pressure of 30 Torr or less, and further performing secondary sintering in a non-oxidizing atmosphere. It is.

The C / O molar ratio in the compact is preferably adjusted by heat treatment in wet hydrogen.

The C / O molar ratio in the compact is preferably adjusted by heat treatment in the air.

 Hereinafter, the present invention will be described in detail.

The raw metal powder used in the present invention contains 10 to 50% by weight of Cr.
Contained. The more Cr, the better the corrosion resistance. Therefore, if Cr is less than 10% by weight, the desired corrosion resistance cannot be obtained, while if it exceeds 50% by weight, a large amount of sigma phase is generated and the toughness is remarkably deteriorated.

The reason why the raw material metal powder used in the present invention is a nickel-based alloy powder, a cobalt-based alloy powder, and a mixture of these alloy component powders is to obtain a sintered alloy having excellent corrosion resistance.

In the present invention, nickel-based alloy powder refers to Ni
40 to 90% by weight, 10 to 50% by weight of Cr, the balance being Fe,
Co, Mo, Nb, Ti, Al and other alloy powders that are unavoidable impurities. Cobalt-based alloy powder contains 40 to 90% by weight of Co and 10 to 50% by weight of Cr, with the balance being Fe. , Ni, Mo, N
b, Ti, Al and other alloy powders that are inevitable impurities.
Further, the mixture of these alloy component powders is a mixture of metal powders, and the alloy composition when the mixture is melted to form an alloy is the same as the alloy composition of the nickel-based alloy powder or the cobalt-based alloy powder. A mixture of such metal powders.

The average particle size of the raw metal powder used in the present invention is 20 μm or less, preferably 15 μm or less. This is defined from the relationship between the sintering density ratio, which greatly affects corrosion resistance, and the average particle size of the powder. That is, when the sintered density ratio is low (less than 92%), since the residual pores are not completely closed, the inside of the material is also exposed to a severe corrosive environment, and the corrosion resistance is significantly deteriorated. In order to make the sintering density ratio 92% or more, the average particle size of the raw metal powder must be 20 μm or less.

In the present invention, molding is performed by adding a binder to the above-described raw metal powder. This is because the raw material powder used in the present invention is a fine powder having an average particle size of 20 μm or less, and thus, if the powder alone is used, defects such as lamination and cracks are generated during molding.

Examples of the binder include thermoplastic resins such as polyethylene, polystyrene, acrylic resin, and polyvinyl acetate, and waxes such as paraffin wax and carnaval wax. Any one of them or a mixture of a thermoplastic resin and wax is used.

The mixing ratio between the raw metal powder and the binder is not particularly limited, but is preferably about powder: binder = 100: 7 to 100: 12 (weight ratio).

The mixing and kneading of the raw metal powder and the binder may be performed by a known method using a pressure kneader, a Burberry mixer, a continuous kneader, or the like.

The molding may be performed by any means such as injection molding, compression molding and injection compression molding, but injection molding is preferred.

When a molded article having a desired shape is obtained by molding, heating is performed under reduced pressure and / or a non-oxidizing atmosphere to remove the binder. That is, under reduced pressure of about 50 Torr or less, or Ar
Gas in N ₂ gas, with H ₂ gas or in a non-oxidizing atmosphere of a mixed gas secondary, or to remove the binder by heating to about 200-650 ° C., the first half of the binder removal step The latter half is performed in a non-oxidizing atmosphere under reduced pressure. Alternatively, the removal of the binder under reduced pressure and in a non-oxidizing atmosphere may be alternately repeated.

At this time or after removing the binder, the C / O molar ratio in the molded body is adjusted to 0.3 to 3.0. This step is also important for improving the corrosion resistance of the obtained sintered body.

That is, the reduction of C and O in the sintered body is achieved by the reaction of C + O → CO C + 2O → CO ₂ during sintering. If the amounts of C and O in the sintered body exceed 0.2% by weight and 1% by weight, respectively, good corrosion resistance cannot be obtained. Therefore, if the C / O molar ratio in the molded body before sintering (after removing the binder) is adjusted to 0.3 to 3.0, the C amount in the sintered body is 0.2% by weight or less, and the O amount is 1% by weight. The following is easily accomplished: If the C / O molar ratio in the compact before sintering is less than 0.3, the amount of O in the sintered body exceeds 1% by weight, while if the C / O molar ratio exceeds 3.0, the Since the amount of C in the aggregate exceeds 0.2% by weight, the corrosion resistance deteriorates in any case.

By the way, the C / O molar ratio in the compact after removing the binder is 0.3
There is no particular limitation on the method of keeping the value within the range of ~ 3.0,
There is a method of limiting the amounts of C and O in the raw metal powder to be used, a method of limiting the conditions for removing the binder, a method of performing a heat treatment in wet hydrogen or in the air after the removal of the binder, and the like. What is necessary is just to carry out combining some methods.

Among the methods for adjusting the C / O molar ratio in the above compact, the method of adjusting the C / O molar ratio by performing heat treatment in wet hydrogen or in the air after removing the binder, and adjusting the C / O molar ratio are as follows: This is a particularly preferred method because it can be carried out regardless of the binder removal conditions.

More specifically, the compact after removal of the binder may be treated by heating at 400 to 650 ° C in wet hydrogen or at 250 to 450 ° C in the atmosphere. These heat treatments can be performed continuously after removing the binder, or can be performed by reheating after the molded body has been cooled once. The C / O molar ratio in the molded article may be measured and calculated by a usual C and O analysis method.

After the C / O molar ratio in the compact is adjusted, sintering is performed.

The conditions for the primary sintering are a temperature of 1350 ° C. or less and a pressure of 30 Torr or less.

The reason for setting the sintering temperature to 1350 ° C. or lower is to allow a certain amount or more of Cr to remain in the sintered body, particularly on the surface of the sintered body. If sintering is performed at a temperature exceeding this temperature, the amount of Cr evaporation will increase, the amount of Cr on the surface of the sintered body will be insufficient, and the secondary sintering in a non-oxidizing atmosphere will also repair the de-Cr layer. Becomes difficult,
This is because sufficient corrosion resistance is not given to the sintered body.

The pressure during sintering is set to 30 Torr or less in order to easily reduce Cr.

In the present invention, in the raw metal powder, Cr which is hardly reducible
Although it is contained in an amount of 10 to 50% by weight, if sintering is performed under reduced pressure, the effect of carbon contained in the raw metal powder reduces the Cr more easily than in a hydrogen atmosphere. And as a result,
A high density sintered body can be obtained. Atmospheric pressure is 30To
If it exceeds rr, the reduction reaction of Cr is difficult to proceed. Therefore, the upper limit of the atmospheric pressure is set to 30 Torr.

By performing the primary sintering under reduced pressure, decarburization and deoxidation progress, but evaporation of Cr from the surface also progresses. And
If the amount of Cr on the surface of the sintered body is small, good corrosion resistance cannot be obtained. Therefore, in the present invention, after primary sintering under reduced pressure, secondary sintering is further performed in a non-oxidizing atmosphere.

In the secondary sintering, the sintering temperature is not limited, but a temperature of 1000 ° C. or more and 1400 ° C. or less is appropriate. Decarburization below 1000 ℃
If the deoxidation reaction is insufficient, on the other hand, if it exceeds 1400 ° C., a liquid phase appears and the shape may be lost, which is not preferable. The non-oxidizing atmosphere is, as described above, Ar gas, N ₂ gas, H ₂ gas, or a mixed gas thereof.

<Example> Hereinafter, the present invention will be specifically described with reference to examples.

(Example I) Cr: 5.0 to 60.5% by weight, Mo: 1.5 to 15.5% by weight, Fe: 4.5% by weight or less, C: 0.02 to 0.1% by weight, O: 0.5 to 0.7% by weight, the balance being Ni And / or various kinds of metal powder mixtures of Co and impurity elements are prepared, and the average particle size thereof is 10 μm,
It was adjusted to μm or 22 μm.

In addition, a thermoplastic resin (polyethylene) as a binder
And wax (paraffin wax) are added and mixed,
The mixture was kneaded using a pressure kneader. The mixing ratio at this time was metal powder: binder = 9: 1 by weight.

Next, this raw material was molded. The sample size and shape of the molded body were a rectangular parallelepiped having a length of 40 mm, a width of 20 mm and a thickness of 2 mm, and were molded using an injection molding machine.

These compacts were heated in an N ₂ atmosphere or in a + N ₂ atmosphere under reduced pressure to remove the binder. Incidentally, the binder removal conditions, specifically, the former in an N ₂ atmosphere 5-20
The condition is that the temperature is raised to 600 ° C at a temperature rising rate of ° C / h. In the latter case, the temperature is raised to 250 ° C under reduced pressure (0.05 Torr) at a temperature rising rate of 30 ° C / h, and then N ₂ atmosphere The temperature is raised to 600 ° C at a rate of 100 ° C / h.

After removing the binder, the C and O contents were measured by a usual C and O analysis method, and the C / O molar ratio in the molded product was calculated.
~ 1.0.

These compacts were heated and heated under reduced pressure (<1 Torr), kept at 1150 ° C. for 1 hour (primary sintering), and then 1a
It was maintained at 1330 ° C. for 1 hour in tm Ar gas (secondary sintering).

The density ratio of the sample thus manufactured was measured. In addition, the sample was treated with a 5% NaCl aqueous solution (pH
(Adjusted to 4) for 24 hours and observed for rust.
Further, a bending test was performed on these samples using a punch having a radius of 15 mm.

 The results are summarized in Table 1.

As is clear from Table 1, only the alloy produced by the method of the present invention showed good properties.

(Example II) Cr: 20% by weight, Mo: 8.5% by weight, Fe: 5% by weight, C: 0.02% by weight, O: 0.5% by weight, the balance being Ni and alloy powder which is an impurity element. A molded product was obtained in the same manner as in Example I, except that the average particle size was adjusted to 10 µm.

These molded bodies, in an N ₂ atmosphere at a heating rate of 20 ° C. / h 60
The temperature was raised to 0 ° C. to remove the binder, and the C / O molar ratio was measured and calculated to be 0.7. The measurement of the C / O molar ratio,
The calculation was performed in the same manner as in Example I.

After removing the binder, sintering was performed under the conditions shown in Table 2,
A sintered body was obtained.

For these, the C and O contents were measured, and a rust test was carried out in the same manner as in Example I. The results are shown in Table 2.

As is clear from Table 2, only the method of the present invention is a method capable of obtaining good corrosion resistance.

(Example III) Cr: 15% by weight, Mo: 15% by weight, Fe: 3% by weight, C: 0.01% by weight, O: 0.15% by weight, the balance being Ni and alloy powder which is an impurity element. A molded product was obtained in the same manner as in Example I, except that the average particle size was adjusted to 10 µm.

These moldings per, at a heating rate of 10 ° C. / h in N ₂ 60
The temperature was raised to 0 ° C. to remove the binder.

After removal of the binder, the C / O
The molar ratio was adjusted, and the C / O molar ratio was measured and calculated in the same manner as in Example I.

These compacts were heated and heated under reduced pressure (<0.5 Torr), kept at 1150 ° C. for 1 hour (primary sintering),
It was kept at 1330 ° C. for 1 hour in Ar gas of 1 atm (secondary sintering).

For these, the C amount and the O amount were measured, and a rusting test was performed in the same manner as in Example I. The results are shown in Table 3.

As is clear from Table 3, adjustment of the C / O molar ratio in the compact before sintering affects the corrosion resistance of the sintered compact.

<Effects of the Invention> According to the present invention, there is provided a method for producing a molded article of an alloy containing a large amount of Cr and Ni or Co, that is, an alloy having excellent corrosion resistance, wherein there is no restriction on the carbon in the raw metal powder or the binder. A manufacturing method is provided by the sintering method.

As described above, the method of the present invention is very advantageous because it is a method that does not limit the composition of the raw metal powder and the type of the binder.

In addition, the method of the present invention is a nickel-based, cobalt-based sintered alloy, which provides a sintered alloy having a high density and a uniform Cr concentration distribution, but nickel-based and cobalt-based alloys are: Since the machinability was poor and it was difficult to manufacture parts with complicated shapes at low cost in the past, according to the present invention, nickel-based or cobalt-based alloy parts having high corrosion resistance were first formed. Regardless, it will be supplied at low cost.

────────────────────────────────────────────────── (5) References JP-A-2-54733 (JP, A) JP-A-57-198202 (JP, A) JP-A-61-253349 (JP, A) (58) Investigation Field (Int.Cl. ⁶ , DB name) B22F 3/00-7/08 C22C 1 / 04,33 / 02

Claims (3)

(57) [Claims] 1. A nickel-based alloy powder, a cobalt-based alloy powder, or a mixture of alloy component powders containing 10 to 50% by weight of Cr and having an average particle size of 20 μm.
m or less, a binder is added, mixed, and molded to obtain a molded body having a desired shape. Then, the binder in the molded body is removed by heating under reduced pressure and / or in a non-oxidizing atmosphere. The C / O molar ratio in the compact was adjusted to 0.3 to 3, followed by primary sintering at a temperature of 1350 ° C. or less and a pressure of 30 Torr or less, and a secondary sintering in a non-oxidizing atmosphere. A method for producing a sintered alloy having excellent corrosion resistance. 2. The method for producing a sintered alloy having excellent corrosion resistance according to claim 1, wherein the C / O molar ratio in the compact is adjusted by heat treatment in wet hydrogen. 3. The method for producing a sintered alloy having excellent corrosion resistance according to claim 1, wherein the C / O molar ratio in the compact is adjusted by heat treatment in the air.