JPH03281747A - Sintered alloy excellent in corrosion resistance and machinability and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a sintered alloy having high density ratio and excellent in corrosion resistance and machinability by using the one obtd. by adding specified amounts of Mn and S being free cutting components to Ni ro Co base alloy powder as raw material powder. CONSTITUTION:Alloy powder obtd. by incorporating, by weight, <=2.0% Mn and 0.02 to 0.3% S into an Ni or Co base alloy and having <=15mu average grain size is used, which is mixed with a bond, is compacted an is dewaxed to regulate the C/O molar ratio in the green compact to 0.3 to 3. This green compact is sintered under the reduced pressure of <=1350 deg.C and <=30Torr and is furthermore sintered in a nonoxidizing atmosphere to obtain an objective sintered body having >=92% density ratio. Mn and S in the powder are combined to form MnS an to contribute to the improvement of its machinability, but in the case of >2.0%, the speheroidizing of the powder proceeds, so that the density of the sintered body does not increase and its ductility and toughness are deteriorated. S increases its machinability, but in the case of <0.02%, the desired effect can not be obtd., and in the case of >0.3%, the corrosion resistance, ductility, toughness or the like of the sintered body are reduced to deteriorate the compressibility of the powder.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a sintered alloy with excellent corrosion resistance and machinability produced by a powder metallurgy method, and a method for producing the same.

<Conventional technology> Ni is conventionally used as a corrosion-resistant alloy.

Alloys containing large amounts of Cr, Co, Mo, etc. are processed by forging or rolling, but these alloys are difficult-to-process materials and cannot be cut to a large extent, so the shapes of parts are limited. There are some drawbacks. Furthermore, precision casting methods aimed at enlarging the shape of parts require post-processing such as plating and polishing due to the poor surface condition of the product, which limits its application as a corrosion-resistant material. Another drawback is that the crystal grains of the alloy matrix grow coarsely during solidification and cooling, resulting in deterioration of mechanical properties.

To address the drawback of difficult machining, it is possible to improve machinability by adding free-machining elements to these materials, but in the above-mentioned processing method, free-machining elements such as S and Ca When added to an alloy, there is a problem that it cannot be added because cracks occur during rolling.

As a method for forming these alloys, for example, Japanese Patent Publication No. 56-1
No. 9392 proposes a method for compression molding a mixture of metal oxides by powder metallurgy. According to this method, a Ni-based sintered alloy with a density ratio of 95% or more can be obtained, but it is done by ordinary press forming, and the part shape is thick (restricted) by the shape of the mold. Since shape distortion occurs due to dimensional shrinkage during bonding, it is essential to improve the machinability of the material itself in order to expand its application, and problems remain in this regard.

<Problems to be Solved by the Invention> An object of the present invention is to improve the machinability of Ni-based or Co-based sintered alloys.

Specifically, S, which is a free-cutting component, is mixed in the form of pre-alloy or mixed powder at the powder stage, and finely dispersed as MnS in the sintered body, and the sintered alloy has a density ratio of 92% or more. A manufacturing method thereof is provided.

Means for Solving the Problems> As a result of various studies, the present inventors came to the following conclusion. Since a free-cutting component can be added by adding powder, appropriate amounts of Mn and S, which is a free-cutting component, are added to Ni-based or Co-based alloy powder to make the average particle size 15 μm or less as a raw material powder. After molding and degreasing, the density ratio of the sintered body can be improved by combining vacuum sintering and sintering in a non-oxidizing atmosphere, and MnS is dispersed in the sintered body. It has been found that machinability can be significantly improved while preventing deterioration of corrosion resistance.

That is, according to the present invention, in order to achieve the above object,
A sintered alloy based on Ni-based alloy or Co-based alloy, Mn: 2.0% or less, S: 0.02 to 0.3
% by weight, C: 0.2% by weight or less, C: 0:1.0% by weight or less, and has a density ratio of 92% or more, and has excellent corrosion resistance and machinability.

Furthermore, according to the present invention, Ni-based alloys or Co-
Mn: 2.0% by weight or less, S: 0.02-0.
Using an alloy powder containing 3% by weight and having an average particle size of 15 It m or less, a binder is added and mixed and molded, and then the binder in the molded body is heated under reduced pressure or in a non-oxidizing atmosphere. The C/O molar ratio in the molded body was adjusted to 0.3 to 3, and then sintered at a temperature of 1350°C or less and a reduced pressure of 3°torr or less, and further sintered in a non-oxidizing atmosphere. Provided is a method for producing a sintered alloy with excellent corrosion resistance and machinability.

Here, when adjusting the C/O molar ratio to 0.3 to 3, it is preferable to perform heat treatment under wet hydrogen or in the atmosphere.

The present invention will be explained in more detail below.

In the present invention, Ni-based alloy system refers to Ni-Cr-C
The alloy is a Ni-based alloy such as a hastelloy system such as an o-Mo-Fe system, and the Co-based alloy is a Co-based alloy such as a N1-Cr-Mo-Fe system. The reason for using these base alloys is to obtain excellent corrosion resistance. The content of Mn and S in the sintered alloy was specified because these elements are thought to have a significant effect on corrosion resistance, toughness, and ductility in addition to machinability.

Mn combines with S to produce MnS in the sintered body, contributing to improving machinability. However, Mn addition was 2.0% by weight.
If it is too thick, the powder will become spheroidized and the density of the sintered body will not increase.

In addition, the ductility and toughness of the sintered body are significantly reduced.
The content was 2.0% by weight or less.

As a free-cutting component, S has the effect of improving the machinability of the sintered body. However, if it is less than 0.02% by weight, the desired effect will not be obtained, and if it is added excessively, it will deteriorate the corrosion resistance of the sintered body.
0.02 to 0.02 to 0.02 to 0.02, because it causes problems such as inhibiting the compressibility of the powder or reducing the toughness and ductility of the sintered body.
The content was 3% by weight.

In addition to S, Se or Te can be considered as a free-cutting component;
, Te (0,Ol to 0.3% by weight), or a combination of one or more of the above three components may be used.

The amount of C90 should be 0.2% by weight or less, and 1.0% by weight or less, since corrosion resistance deteriorates if it exceeds 0.2% by weight and 1.0% by weight, respectively.
% by weight or less.

Density ratio is a factor that directly affects corrosion resistance. In a sintered body with a density ratio of less than 92%, residual pores are not yet completely occluded, so it is expected that the surface and internal pores are partially connected, and the interior is always exposed to the harsh corrosive environment outside the sintered body. corrosion resistance becomes insufficient. Further, if it is less than 92%, the residual pore size also becomes large, which adversely affects corrosion resistance. Therefore, the lower limit of the density ratio was set to 92%.

Next, a method for producing a sintered alloy with excellent corrosion resistance and machinability according to the present invention will be described.

The alloy powder used in the method of the present invention is a Ni-based alloy or a Co-based alloy with Mn: 2.0% by weight or less and S: 0.
．． Mn and S are added so as to contain 02 to 0.3% by weight, and the average particle size is set to 15 μm or less.

The Ni-based alloy and the Co-based alloy are each a Hastelloy-based N
These are i-based alloys and Co-based alloys such as Ni-Cr-Mo-Fe systems. The reason for using these base alloys is to obtain excellent corrosion resistance.

In the method of the present invention, the M rr and S contents in the raw material alloy powder are specified because they are necessary to obtain the above-mentioned sintered alloy.

The above-mentioned components may be added by pre-alloying at the molten metal stage to form an alloy powder, by a mixed powder of each component, or by a part of the pre-alloy powder.

The average particle size of the raw material powder is a major factor determining the final density of the sintered body. The finer the particle size, the more sintering progresses and the higher the density. However, when particles having an average particle size exceeding 15 μm are used, the density ratio of the sintered body does not increase by 1 and the pores inside the sintered body remain large. Furthermore, the shape of the pores is also irregular, and the mechanical properties of the sintered body are significantly deteriorated.

Therefore, the average particle size of the alloy powder used was specified to be 15 μm or less.

Since the particle size of the alloy powder used is small, it is difficult to mold the powder alone, and even when molded, there are problems such as cracks in the molded product and damage to the mold. Therefore, in the present invention, a binder is added to and mixed with the alloy powder and then molded.

Molding is also possible using wax, resin, or a mixture thereof as the binder. The amount added varies depending on the molding method, and for example, in injection molding, /O to 15% by weight is required, and in mold molding, it is approximately 0.5 to 2% by weight. The molding method may be injection molding, mold molding, or extrusion molding using a press.

After molding, it is heated under reduced pressure or in a non-oxidizing atmosphere to remove the binder. The heating temperature and heating rate are appropriately selected depending on the type of binder.

After removing the binder, the C/O molar ratio is adjusted to a range of 0.3-3. The sintered body contains Cr, which is difficult to reduce. Cr can be reduced by the contained C by sintering under reduced pressure, but in this case, the C/O molar ratio needs to be appropriate in the step before sintering. The amount of C1O in the sintered body is 0.
If it exceeds 2% by weight or 1.0% by weight, the corrosion resistance will deteriorate, but if the C/O molar ratio in the compact is less than 0.3, the corrosion resistance will deteriorate after sintering.
If the amount exceeds 1.0% by weight or exceeds 3.0% by weight, the amount of C after sintering will exceed 0.2% by weight, resulting in poor corrosion resistance. Therefore, the C/O molar ratio is adjusted to 0.3 to 3 before sintering. The method involves heating the shaped degreased body in wet hydrogen or air. An appropriate heating temperature is selected depending on the C/O level of the degreased body, and is usually selected in the range of 300 to 450°C.

After that, sintering is performed. At that time, the density ratio of the sintered body is 92% or more by sintering at a temperature of /O50 to 1350°C under reduced pressure of 30 torr or more, and then sintering at 1250 to 1350°C in a non-oxidizing atmosphere. can be achieved.

The purpose of the first stage of sintering is to reduce Cr, which is difficult to reduce among the added elements.

/O of less than 50° C. is not preferable because the reduction of Cr oxide in the sintered body does not proceed sufficiently and oxygen remains, which inhibits subsequent sintering. Further, at a temperature higher than 1350°C, Cr evaporates from the surface of the sintered body and corrosion resistance deteriorates, so the upper limit temperature was set at 1350°C. Reduced pressure is suitable for reducing Cr oxide, but if the pressure exceeds 30 torr, reduction will be difficult to proceed, so the upper limit of the pressure should be set to 30 torr.
And so.

The purpose of the subsequent stage of sintering is to densify the sintered body and homogenize the alloying elements. Although there is no upper limit to the sintering temperature, a temperature of 1250° C. or higher is required to achieve high density.
However, at temperatures above 1350°C, problems such as excessive evaporation of Cr from the surface of the sintered body and deterioration of the shape of the sintered body occur, so the upper limit temperature was set at 1350°C. In addition, the reason why the atmosphere was made non-oxidizing was to suppress the evaporation of Cr at high temperatures.
, Co, CH, etc. or combustion exhaust gas is used.

As described above, the sintered alloy of the present invention having excellent corrosion resistance and machinability can be manufactured.

<Example> The present invention will be specifically explained below based on Examples.

(Example 1) In order to investigate the effects of Mn and S on machinability and corrosion resistance, Mn = 1.0% by weight and S: 0.02 to 0.3% by weight were added to a Ni-based alloy and a Co-based alloy, respectively. It was produced by atomizing alloy powder containing within the range of . Table 1 shows its composition. In addition, alloy powders with approximately the same composition system but without the addition of S and alloy powders in which S was added in excess of 0.3% by weight were prepared as comparative examples. The average particle size of the powder was measured using a Microtrack and is shown in Table 1, and all were 15 μm or less. These powders are combined with binders (polymer,
A compound was prepared by adding and kneading an appropriate amount of a mixture of wax and paraffin in the range of /O to 15% by weight.
Using this compound, a Charpy test piece and a cylindrical test piece with a diameter of 6 mm and a height of 50 mm were molded by injection molding.

The binder was removed in a nitrogen atmosphere at a heating rate of 6°C/h.
This was done by heating to 00°C.

After binder removal, ensure that the C/O molar ratio is in the range of 0.5-1.0, and 0. 60ml1n sintered at a temperature of 1150°C in a vacuum of 1 torr or less, followed by A
60ml1n was sintered at a temperature of 1350° C. in an r atmosphere to obtain test materials (Inventive Examples 1 to 6, Comparative Examples 1 to 4).

The obtained sintered body was used to evaluate corrosion resistance, machinability, and mechanical properties. Corrosion resistance is determined by holding each Charpy test piece in a 5% NaCβ aqueous solution (pH: 4) at a temperature of 50°C. If no rust is observed in the entire test piece, it is considered good, and even a part of it has chains. It was evaluated as rusting when The machinability test was conducted using a Charpy test piece with a diameter of 1 mm.
A test was conducted on drilling holes using a drill (made of high-speed steel).
Cutting conditions are drill rotation speed 300 rpm, feed rate 15
ff1m/win, the drill hole was drilled to a depth of 5 mm, dry cutting was performed, and the machinability was evaluated by the number of holes until the drill became unusable and broke. In addition, the mechanical properties are Rockwell hardness (HRc) using Charpy test and temperature 80
Evaluation was made by creep rupture stress when maintained at 0°C/00hr.

Table 2 shows the experimental results for each sintered alloy (invention examples 1 to 6,
Comparative Examples 1 to 4). Appropriate amounts of C and O were obtained for each sintered body, and a sintered body density ratio of 92% or more was achieved for each alloy.

Regarding the machinability of the sintered body, alloys with Mn and S added (invention examples 1 to 6) and those without addition (comparative examples), 3)
The number of drilled holes has increased compared to the
, the effect of S is not recognized. Further, the number of holes increases with the amount of S added, but when the amount of S added exceeds 0.3% by weight (Comparative Example 2.4), rusting is observed and corrosion resistance deteriorates.

(Example 2) As shown in Table 3, two types of Nf-based alloys and Co-based alloys, those with average particle diameters of 15 μm or less and those around 20 μm, were manufactured by spraying with water atomization (Example of the present invention) 7.8, Comparative Example 5.6). Test pieces were prepared using these powders under the same conditions as in Example 1, and the corrosion resistance, machinability, and mechanical properties were measured and evaluated in the same manner as in Example 1. The results are shown in Table 4. Those with an average particle size of 15 μm or less (Example 7.8 of the present invention) achieve a sintered body density ratio of 92% or more and have good properties, but the average particle size of both Ni and Co groups exceeds 15 μm. (Comparative Examples 5 and 6), the sintered body density did not increase, and as a result, the corrosion resistance and mechanical properties were significantly deteriorated. Moreover, machinability is also reduced.

(Example 3) Using the alloy powder shown in Table 1 (Inventive Example 2.5), a test piece was molded and the binder was removed under the same conditions as in Example 1. Sintering is performed at ■1150℃Xlh (</
O-'torr)-+1350℃X1h(Ar), 01
350"Cx1, h (0,1torr), 01350
℃X1h (/O0torr), ■1350℃Xlh (
The test was carried out under four conditions: hydrogen (dew point -40°C). The experimental results are shown in Table 5 (Table 1 Invention Example 9 corresponding to Invention Example 2, Comparative Examples 7 to 9 and Table 1 Invention Example 1 O1 Comparative Example/O to 12 corresponding to Invention Example 5). Only the product sintered under the above conditions (invention example 9./O) showed good properties, and under other conditions (comparative examples 7 to 12), the C, O
The amount does not reach an appropriate value, and corrosion resistance deteriorates.

(Example 4) Using the alloy powder shown in Table 6, test pieces were molded under the same conditions as in Example 1. Thereafter, the binder was removed by heating to 600° C. at a temperature increase rate of 10° C./h in a nitrogen atmosphere. The C/O molar ratio at this stage is 3.2. Next, for some of the test pieces,
/Om1n held at 0℃, lomin at 350℃ in air
C/O adjustment was performed under two retention conditions. Thereafter, it was sintered under the same conditions as in Example 1, and its properties were evaluated.

The results are shown in Table 7 (Comparative Example 13 with Ni group, Inventive Example 11.12 and Comparative Example 14 with Co group, Inventive Example 13.14). C/O adjusted (invention example 11~
14), an appropriate C/O molar ratio is obtained, and as a result, the amount of C/O in the sintered body is also appropriate, and the corrosion resistance is good.
Those without O adjustment (Comparative Example 13.14) had residual C
It can be seen that the corrosion resistance has deteriorated.

(Example 5) Alloy powders having the compositions shown in Table 8 were sprayed and manufactured by water atomization (Invention Examples 15 to 21; Comparative Examples 15 to 21). Each powder is a suitable fine powder with an average particle size of 15 μm or less. Using these alloy powders, molding, binder removal, and sintering were performed under the same conditions as in Example 1, and properties were evaluated in the same manner as in Example 1 (Inventive Examples 15 to 21, Comparative Example 15). ~21). The experimental results are shown in Table 9. It was confirmed that adding S to each sintered body significantly improved machinability (Inventive Examples 15 to 21), and there was almost no deterioration in corrosion resistance and mechanical properties. There wasn't.

<Effects of the Invention> Since the present invention is configured as described above, a sintered alloy with excellent corrosion resistance and machinability can be obtained.

Further, by adding appropriate amounts of Mn and S to a Ni-based or Co-based alloy using the manufacturing method of the present invention, a sintered alloy with excellent corrosion resistance and machinability can be easily obtained.

Claims (4)

[Claims] (1) Ni-based alloy or Co-based sintered alloy, Mn: 2.0% by weight or less, S: 0.02~
A sintered alloy containing 0.3% by weight or less, C: 0.2% by weight or less, and O: 1.0% by weight or less, and having a density ratio of 92% or more and excellent in corrosion resistance and machinability. (2) Mn:2 in Ni-based alloy or Co-based alloy
．． 0% by weight or less, S: 0.02 to 0.3% by weight,
Using an alloy powder with an average particle size of 15 μm or less, a binder is added and mixed into the powder, and then the binder in the compact is removed by heating under reduced pressure or in a non-oxidizing atmosphere. The C/O molar ratio of was adjusted to 0.3-3, and then 1350
2. The method for producing a sintered alloy with excellent corrosion resistance and machinability according to claim 1, wherein the sintering is performed under reduced pressure of 30 torr or less at a temperature of 30 torr or less, and further sintering in a non-oxidizing atmosphere. (3) The method for producing a sintered alloy with excellent corrosion resistance and machinability according to claim 2, wherein heat treatment is performed under wet hydrogen when adjusting the C/O molar ratio to 0.3 to 3. (4) The method for producing a sintered alloy with excellent corrosion resistance and machinability according to claim 2, wherein heat treatment is performed in the atmosphere when adjusting the C/O molar ratio to 0.3 to 3.