# Patent

## Title
Iron-based sintered compact and method for production thereof

## Abstract

Provided is an iron-based sintered body with a rustproof function comprising a layer containing 0.01 to 5 at % of indium on the surface of the iron-based sintered body, or an iron-based sintered body with a rustproof function containing 0.01 to 5 at % of indium throughout the sintered body, and the iron-based sintered body having iron as its principal component is manufactured by performing sintering in a gas atmosphere containing indium vapor or indium. Thereby obtained is an iron-based sintered body, as well as the manufacturing method thereof, capable of easily improving the rustproof effect without having to hardly change the conventional process.

## Background
BACKGROUND OF THE INVENTION
The present invention pertains to mixed powder for powder metallurgy to be employed in the manufacture of sintered components, brushes and so on, and particularly to an iron-based sintered body suitable in manufacturing the likes of iron-based sintered components superior in rustproof performance to be used as a solid lubricant or the like, and the manufacturing method thereof.
Generally, iron powder used in the application of sintered mechanical components, sintered oil retaining bearings, metal graphite brushes and so on rusts easily, and is commonly used upon mixing an organic rust-prevention agent such as benzotriazole therein.
Nevertheless, although such an organic rust-prevention agent possesses a temporary rustproof effect, it decomposes or evaporates at 500° C. or higher, and becomes lost at an ordinarily employed sintering temperature of 700° C. or higher. Therefore, the same condition will occur unless rust prevention is performed after the sintering, and there is a problem in that the sintered object will rust easily.
Meanwhile, in order to obtain the rustproof performance after sintering, a proposal has been made to form a composite powder sintered body by mixing a slight amount of metal powder such as zinc, bismuth, lead or the like With sintering powder having iron as its principal component.
However, this requires an additional step, the manufacturing process will become complex as a result thereof, and there is a problem in that there will be variations in the quality all that much more. Further, even if metal powder of bismuth or lead is mixed in, minute particles are merely dispersed, and it could not be said that it is evenly distributed.
As a conventional additive agent for powder metallurgy, there is an additive agent having organic acid cobalt metallic soap as its component, and technology for manufacturing a sintered body by adding and mixing this additive agent 0.1 to 2.0% by weight, and then molding and sintering this mixed powder has been disclosed (c.f. Japanese Patent Laid-Open Publication No. H10-46201).
Moreover, technology of adding and mixing metal stearate to rare earth-iron-boron permanent magnet coarse powder, which is mainly composed in atomic % of rare earth element R (among rare-earth elements containing Y, one or two or more elements are combined) of 10 to 25%, boron B of 1 to 12%, and the remaining part consisting of iron Fe (a part of Fe is replaced at least with one or more kinds of elements selected from Co, Ni, Al, Nb, Ti, W, Mo, V, Ga, Zn and Si in a range of 0 to 15%, if necessary), and thereafter dry-pulverizing this mixture has also been disclosed (c.f. Japanese Patent Laid-Open Publication No. H6-290919).
Further, a molding improving agent of alloy powder for a permanent magnet consisting of at least one kind selected from polyoxyethylene alkyl ether, polyoxyethylene monofatty acid ester and polyoxyethylene alkylallylether compounded with at least on kind of stearate at 1/20 to 5/1 compounding ratio has also been disclosed (c.f. Japanese Patent Laid-Open Publication No. S61-34101).
SUMMARY OF THE INVENTION
An object of the present invention is to provide an iron-based sintered body, as well as the manufacturing method thereof, capable of easily improving the rust-prevention effect without having to hardly change the conventional process.
As a result of intense study to resolve the foregoing problems, the present inventors discovered that the rust-prevention effect of products after sintering could be significantly improved by making indium exist at least on the surface of the sintered body.
Based on this discovery, the present invention provides:

    1. An iron-based sintered body with a rustproof function, characterized in comprising a layer containing 0.01 to 5at % of indium on the surface of the iron-based sintered body;
    2. An iron-based sintered body with a rustproof function characterized in containing 0.01 to 5at % of indium throughout the sintered body; and
    3. A manufacturing method of an iron-based sintered body having iron as its principal component, characterized in that sintering is performed in a gas atmosphere containing indium vapor or indium.




DETAILED DESCRIPTION OF THE INVENTION
In order to improve the rustproof effect of the iron-based sintered body according to the present invention, the present inventors focused attention on zinc stearate to be added in a slight amount as a lubricant upon forming powder.
Nevertheless, this zinc stearate has a problem in that it dissipates during sintering, and damages the sintering furnace since it has high corrosiveness, and it has become evident that the rustproof effect is hardly any different from a case when it is additive-free.
As described above, in most cases, this zinc stearate is merely used as a lubricant upon molding, and is insufficient as a material for obtaining a high rustproof effect.
Here, in order to improve the rustproof effect even after sintering, it has become clear that as a result of sintering iron-based material powder in a gas atmosphere containing indium such as indium vapor or indium suboxide (In2O), the rustproof function can be significantly improved by at least forming a layer containing 0.01 to 5 at % of indium on the surface of the iron-based sintered body, or including 0.01 to 5 at % of indium throughout the sintered body.
This rustproof effect is effective even with a slight amount of 0.01 at % or more. Although the rustproof effect can be expected with indium of 5 at % or more, the effect will be saturated, and further addition will be a waste. Thus, it is desirable to contain 0.01 to 5 at % of indium.
As the gas containing indium, compounds such as indium suboxide (In2O) that decomposes with the heat of sintering may be used.
With this type of a sintered body containing 0.01 to 5 at % of indium, in order to increase the function as a molding lubricant, sintering may be performed upon adding metallic soap such as zinc stearate to the powder for powder metallurgy.
Particularly desirable is metallic soap of low-temperature volatile metal, and indium soap, bismuth soap, nickel soap, cobalt soap, copper soap, manganese soap and aluminum soap may be used as such low-temperature volatile metal.
Not only do these metals function as a molding lubricant, they are also capable of increasing the rustproof effect. Needless to say, indium soap is able to obtain extremely superior rustproof effect and lubricant effect.
Moreover, metallic soaps such as metallic soap stearate, metallic soap propionate and metallic soap naphthenate may be used as the soap.
As a result, the rustproof effect of sintered bodies can be improved exponentially without having to significantly change the conventional manufacturing process of a sintered body.
Generally, it is desirable to add 0.1 to 2.0 parts by weight of such metallic soap to 100 parts by weight of metallic powder for powder metallurgy having iron as its principal component.
Nevertheless, this additive amount may be changed in accordance with the type of sintered body, and the additive amount does not necessarily have to be limited to the foregoing additive amount. In other words, the additive amount may be arbitrarily set within a range that is capable of maintaining the characteristics of the target sintered body.
The condition for improving the rustproof effect is that the surface of the iron-based sintered body comprises a layer containing 0.01 to 5 at % of indium.
EXAMPLES AND COMPARATIVE EXAMPLES
Next, the present invention is described based on the Examples. The Examples are for facilitating the understanding of the invention, and the present invention is not in any way limited thereby. In other words, the present invention covers other Examples and modifications based on the technical spirit of the invention.
Example 1
1.0 wt % of graphite powder was mixed with iron powder (Hoganas reduced iron powder). This mixed powder (fill of 1.5 to 2.5 g) was molded into a test piece of approximately 10.06 mmø×2.70 to 4.55 mmH under a molding pressure of 6 t/cm2.
The compact molded into this test piece was sintered in a batch type atmospheric furnace at a sintering temperature of 1150° C., sintering time of 60 min., with indium vapor introduced therein, and under a hydrogen gas atmosphere. 0.05 at % of indium was contained in the outermost surface portion of the sintered body after sintering.
This sintered body was set inside a constant temperature and humidity chamber, and an atmospheric exposure test was conducted for 336 hours at a temperature of 40° C. and humidity of 95% in order to conduct a moisture and oxidation resistance experiment. The results of the moisture and oxidation resistance experiment are shown in Table 1.










TABLE 1









Indium




content in
Oxidation Resistance












surface
After 96
After 168
After 336



layer
Hours
Hours
Hours















Example 1
 0.05 at %
No change in
Slight change
Slight change




color
in color
in color


Example 2
 0.1 at %
No change in
Slight change
Slight change




color
in color
in color


Example 3
 0.5 at %
No change in
Slight change
Slight change




color
in color
in color


Example 4
   1 at %
No change in
Slight change
Slight change




color
in color
in color


Example 5
   3 at %
No change in
Slight change
Slight change




color
in color
in color


Example 6
   5 at %
No change in
Slight change
Slight change




color
in color
in color


Example 7
 0.05 at %
No change in
Slight change
Slight change




color
in color
in color


Comparative
   0 at %
Severe
Severe change
Severe change


Example 1

change in
in color
in color




color



Comparative
   0 at %
Severe
Severe change
Severe change


Example 2

change in
in color
in color




color



Comparative
0.005 at %
Some change
Severe change
Severe change


Example 3

in color
in color
in color









Examples 2 to 6
Under the same conditions, sintered bodies were prepared in which the indium content contained in the surface portion was changed to 0.1 at %, 0.5 at %, 1 at %, 3 at % and 5 at %, these sintered bodies were set inside a constant temperature and humidity chamber, and an atmospheric exposure test was conducted for 336 hours at a temperature of 40° C. and humidity of 95% in order to conduct a moisture and oxidation resistance experiment. The results of the moisture and oxidation resistance experiment are shown in Table 1.
Comparative Example 1
Zinc stearate SZ-2000 (manufactured by Sakai Chemical Industry Co., Ltd.) was used, and 0.8 wt % of this zinc stearate and 1.0 wt % of graphite powder were mixed with the iron powder. This mixed powder (fill of 1.5 to 2.5 g) was molded into a test piece of approximately 10.04 mmø×2.73 to 4.58 mmH under a molding pressure of 6 t/cm2.
The compact molded into this test piece was sintered in a batch type atmospheric furnace at a sintering temperature of 1150° C., sintering time of 60 min., and under a hydrogen gas atmosphere.
This sintered body was set inside a constant temperature and humidity chamber, and an atmospheric exposure test was conducted for 336 hours at a temperature of 40° C. and humidity of 95% in order to conduct a moisture and oxidation resistance experiment. The results of the moisture and oxidation resistance experiment are shown in Table 1.
Comparative Example 2
1.0 wt % of graphite powder was mixed with iron powder (Hoganas reduced iron powder). This mixed powder (fill of 1.5 to 2.5 g) was molded into a test piece of approximately 10.06 mmø×2.70 to 4.55 mmH under a molding pressure of 6 t/cm2.
The compact molded into this test piece was sintered in a batch type atmospheric furnace at a sintering temperature of 1150° C., sintering time of 60 min., without indium vapor introduced therein, and under a hydrogen gas atmosphere. An indium layer did not exist in surface of the sintered body.
This sintered body was set inside a constant temperature and humidity chamber, and an atmospheric exposure test was conducted for 336 hours at a temperature of 40° C. and humidity of 95% in order to conduct a moisture and oxidation resistance experiment. The results of the moisture and oxidation resistance experiment are shown in Table 1.
Comparative Example 3
Similarly, 1.0 wt % of graphite powder was mixed with iron powder (Hoganas reduced iron powder). This mixed powder (fill of 1.5 to 2.5 g) was molded into a test piece of approximately 10.06 mmø×2.70 to 4.55 mmH under a molding pressure of 6 t/cm2.
The compact molded into this test piece was sintered in a batch type atmospheric furnace at a sintering temperature of 1150° C., sintering time of 60 min., with a slight amount of indium vapor introduced therein, and under a hydrogen gas atmosphere. 0.005 at % of indium was contained in the outermost surface portion of the sintered body after sintering.
This sintered body was set inside a constant temperature and humidity chamber, and an atmospheric exposure test was conducted for 336 hours at a temperature of 40° C. and humidity of 95% in order to conduct a moisture and oxidation resistance experiment. The results of the moisture and oxidation resistance experiment are shown in Table 1.
Example 7
Synthesized indium stearate (In content of 12.0 wt %) was pulverized, and this was put through a sieve to obtain fine powder of 250 meshes or less.
0.8 wt % of this indium stearate (abbreviated as “In” in Table 1 above) and 1.0 wt % wt of graphite powder were mixed with the iron powder (Hoganas reduced iron powder). This mixed powder (fill of 1.5 to 2.5 g) was molded into a test piece of approximately 10.06 mmø×2.70 to 4.55 mmH under a molding pressure of 6 t/cm2.
The compact molded into this test piece was sintered in a batch type atmospheric furnace at a sintering temperature of 1150° C., sintering time of 60 min., with indium vapor introduced therein, and under a hydrogen gas atmosphere. 0.05 at % of indium was contained in the outermost surface portion of the sintered body after sintering.
This sintered body was set inside a constant temperature and humidity chamber, and an atmospheric exposure test was conducted for 336 hours at a temperature of 40° C. and humidity of 95% in order to conduct a moisture and oxidation resistance experiment. The results of the moisture and oxidation resistance experiment are shown in Table 1.
The moldability of the sintered body according to the present Example was extremely favorable since it is using indium stearate.
Likewise, results similar as Example 7 could be obtained in cases where soaps such as bismuth stearate (Bi content of 12.0 wt %), nickel stearate (Ni content of 12.0 wt %), cobalt stearate (Co content of 12.0 wt %), copper stearate (Cu content of 12.0 wt %), manganese stearate (Mn content of 12.0 wt %) or the compounds thereof were added to the iron powder (Hoganas reduced iron powder).
Next, as evident from Table 1, regarding Comparative Examples 1 to 3 in which a lubricant was not added to the iron powder, in the moisture resistance and oxidation resistance experiment after sintering, change in color (corrosion) occurred after 96 hours (4 days), and, together with the lapse in time, the degree of change in color increased gradually. The change in color was severe after 336 hours.
Meanwhile, with Examples 1 to 7 according to the present invention, it is clear that each of the Examples only has a slight change in color from the foregoing moisture resistance and oxidation resistance experiment after the lapse of 336 hours, and each of such Examples has moisture resistance and oxidation resistance properties.
Accordingly, it has been confirmed that the sintered body of the present invention containing a prescribed amount of indium in the surface portion thereof has favorable moisture resistance and oxidation resistance properties. Further, the moldability was favorable when using mixed powder for powder metallurgy to which metallic soap was added thereto, and the obtained result was that the corrosion resistance improved even more.
EFFECT OF THE INVENTION
As described above, the present invention provides an iron-based sintered body, as well as the manufacturing method thereof, capable of easily and exponentially improving the rustproof effect without having to significantly change the conventional manufacturing process of a sintered body, and the rustproof effect of sintered bodies such as sintered mechanical components, sintered oil retaining bearings, metal graphite brushes and so on can thereby be improved remarkably.

## Claims

1. A manufacturing method of an iron-based sintered body having iron as its principal component, characterized in that sintering is performed in a gas atmosphere containing indium vapor or indium, and during said sintering, the iron-based sintered body is provided with a rustproof outer surface containing 0.01 to 5at % of indium.

2. A method according to claim 1, wherein said sintering is performed in a furnace under a hydrogen gas atmosphere.

3. A method according to claim 2, wherein the furnace is a batch type atmospheric furnace and said sintering is performed to the iron-based body at a sintering temperature of 1150° C. for 60 minutes.

4. A method according to claim 2, wherein, during said sintering, indium vapor is introduced into the furnace.

5. A method according to claim 2, wherein, during said sintering, a compound that contains indium and that decomposes with the heat of sintering is introduced into the furnace.

6. A method according to claim 5, wherein the compound is indium suboxide (In2O).

7. A manufacturing method of an iron-based sintered body having iron as its principal component, characterized in that sintering is performed in a gas atmosphere containing indium vapor or indium, and further comprising the steps of mixing graphite powder with iron powder to form a powder mixture consisting essentially of iron and graphite and molding the mixture into said iron-based body, and then thereafter, performing said sintering step to said iron-based body.

8. A manufacturing method of an iron-based sintered body having iron as its principal component, characterized in that sintering is performed in a gas atmosphere containing indium vapor or indium, and further comprising the steps of mixing graphite powder with iron powder and a metallic soap to form a powder mixture consisting essentially of iron, graphite and metallic soap, and molding the mixture into said iron-based body, and then thereafter, performing said sintering step to said iron-based body.