JPH03138301A - Manufacture of high density iron sintered body - Google Patents

Abstract

PURPOSE:To obtain a sintered body having closeness of >=98% relative density by making carbon content in raw material iron powder used to powder metallurgical method the specific value or less. CONSTITUTION:Iron powder having <=0.01wt.% carbon content is compacted, and the obtd. compacted body is sintered. By this method, the iron sintered body having >=98% relative density of the sintered body, is obtd. What the closeness is obstructed with various kinds of gases generated or existing in isolated void in the sintered body, lies in the fact that the gas pressure in the isolated void becomes surface stress in the isolated void (driving force of shrinkage of the void) or higher. As the surface stress of void of 3mu radius is about 1.2MPa at 1400 deg.C, if the gas pressure in the isolated void is <=1.2MPa, this isolated void is shrunk and the closeness is not obstructed. Then, what the CO gas pressure equibrium to the content of carbon and oxygen in the iron becomes 1.2MPa at 1400 deg.C is 0.01wt.% carbon content in the iron. Therefore, in order to develop the closeness, the carbon content in the iron has to be <=0.01wt.%.

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention is characterized in that the relative density of the sintered body is 98 by sintering iron powder.
% or more of high-density iron sintered bodies.

[Prior art and problems to be solved by the invention] Conventionally,
It is common practice to manufacture ferrous material parts by powder metallurgy, and its range of use is expanding due to its cost advantages. However, since sintered parts generally have a bore remaining inside them, their properties are often inferior to those of cast materials. In particular, since elongation, fatigue strength, and impact values are lower than those of cast materials, sintered parts may not be able to be used in areas where repeated stress or impact is applied. That is, in a sintered part in which a bore remains, a crack during fatigue failure occurs from the bore inside the sintered body, and as the crack progresses, it leads to failure. Furthermore, when an impact is applied to the sintered part, the bore inside the sintered body has a notch effect, so it is more likely to break than if there is no bore. Further, pure iron is often used as a soft magnetic material because it has excellent soft magnetic properties, but the soft magnetic properties are degraded by the presence of bores in the sintered body. Therefore, in order to improve these properties and make a molten lumber, it is necessary to eliminate the bore inside the sintered body, that is, to increase the density of the sintered body to the same level as that of molten lumber, but conventional powder metallurgy methods The relative density of sintered parts is about 92% at most, and a recompression process is required to obtain even higher density products. Furthermore, even in the case of metal powder injection molding, which is generally thought to yield high-density sintered crystals, the relative density of the sintered body is 9.
5 to 96%, and there are many bores inside the sintered body. In addition, even if the sintering temperature is increased or the sintering time is lengthened, the relative density of the sintered body is difficult to reach 96% or higher in normal pressure sintering, and properties comparable to those of ingot material cannot be obtained. .

By the way, the reason why the density of the sintered compact cannot be increased in the conventional powder metallurgy method is that the grain size of the raw material powder is large and the bore included in the compact is difficult to shrink. In addition, theoretically, sufficient shrinkage can be achieved by increasing the density of the compact to 92% or more and reducing the size of the bore contained in the compact, but in reality, the relative density of the sintered compact is still over 95%. It is difficult to become. In this way, the reason why the relative density of the sintered body does not reach 95% or more even when the density of the compact is increased or when powder with an average particle size of 20 Ja or less is used as in metal injection molding is that This is because gas continues to be generated even after the voids become isolated and become closed pores. For example, the average particle size 5I, which is commonly used in metal injection molding! m carbonyl iron powder contains about 0.7% by weight of oxygen and about 0.7% by weight of carbon.
This powder is molded and sintered at 1400°C in a hydrogen atmosphere.The sintered body contains approximately 0.3% oxygen by weight.
0.01% by weight, and approximately 0.03% by weight of carbon. On the other hand, the equilibrium between iron oxide, hydrogen and carbon at 1400°C is as follows.

PeO+ H2Fe+HaO ΔG□= RT-In are @ P++zo/a
p@o I pH2F e O+ C-1-→F e
+ CDΔG□= RT ・In ap@e Pco
/arso ” “ccoLm, aFs' also
' ac is the activity of iron, iron oxide (Fen), and carbon, respectively;
'aFeo is 1 and ao follows Ra0ult's law. If calculated, hydrogen 1 atm (P [12= 1 a
tm), the equilibrium gas pressure of H2O generated in the isolated void inside the sintered body is 0.1 MPa, and the equilibrium gas pressure of CO is 2.5 MPa. Furthermore, if all the oxygen and carbon contained in the sintered body become CO gas and are confined in the bore in the sintered body, the pressure will be 25.3M.
Pa, which is higher than the equilibrium pressure of CO gas, so the gas pressure in the isolated void inside the sintered body is 2.5 MPa. on the other hand,
Surface stress (contractile force of voids) σ caused by surface tension of isolated voids of several microns inside the sintered body
is expressed as σ=2r/r. Here, T is the surface energy of iron, and r is the radius of the isolated void. Here, γ is 1.87 N7m, and if r is 3 μ, the value of σ is about 1.2 MPa, and if the gas pressure generated in the isolated void inside the sintered body exceeds this value, the void Shrinkage is inhibited and the density of the sintered body does not increase. The above phenomenon occurs in other reducing atmospheres, inert atmospheres, and vacuums, except for 8.0 gas, and due to the pressure of CO gas generated in isolated voids inside the sintered body, the sintered body density does not increase, and the relative density of the sintered body is 96%.
You won't get anything more than that.

This invention was made in view of the above facts, and its purpose is to increase the relative density of the sintered body to 98% or more by sintering iron-based powder, and to manufacture a dense sintered part. It is in.

[Means to solve the problem]

The above purpose is to produce an iron sintered body with a relative density of 98% or more by using iron powder with a carbon content of 0.01% by weight or less and sintering the resulting compact. This is achieved by a method for producing a high-density iron sintered body, which is characterized by obtaining.

The present invention will be explained in detail below.

First, densification is inhibited by various gases generated or present in isolated voids inside the sintered body.
As mentioned above, this is because the gas pressure inside the isolated void exceeds the surface stress of the isolated void (the driving force for contraction of the void), but 14
At 00°C, the surface stress of a void with a radius of 3 J is approximately 1.2
MPa, therefore the gas pressure inside the isolated cavity is 14 MPa.
If it is less than a, this isolated void will shrink and densification will not be inhibited. At 1400°C, the CO gas pressure in equilibrium with carbon and oxygen in iron is 1.2 MPa when the amount of carbon contained in iron is 0.01% by weight, so it is difficult to cause densification. The amount of carbon contained in the iron must be 0.01% by weight or less. Atomized iron powder used in general powder metallurgy and carbonyl iron powder used in metal powder injection molding have a carbon content of 0.0079.
% or more and is not suitable for this condition. Carbon containing io, o
As the powder of i% or less, a pulverized powder obtained by using low carbon iron as a starting material, oxidizing it, pulverizing and reducing it is suitable.

Next, the driving force that causes the isolated void to contract at a certain sintering temperature, that is, the surface stress that acts on the isolated void, is inversely proportional to the radius of the isolated void, as described above. At the sintering temperature, the radius of an isolated void where densification is not inhibited by the gas pressure inside the void is several layers or less, so the density of the compact is changed according to the particle size of the powder, and the internal void size is need to be controlled. For example, for a powder with an average particle size of 10 - or less, the relative density of the compact is 50% or more, and for a powder of 25 #l, it is 8.
It is desirable to set it to 0% or more. The shape of the molded body is not particularly limited, and the molding method can be any conventionally known method.

Further, in order to obtain an iron-based sintered body having a relative density of 98% or more by phase sintering, the sintering temperature is preferably 1250° C. or higher. In particular, it is more preferable to sinter at a temperature of 1400° C. or higher to form the δ phase because the diffusion rate increases. Sintering can be performed in a reducing gas atmosphere that does not contain carbon, in an inert gas atmosphere (at normal pressure to reduced pressure), or under vacuum, but in order to obtain a low-oxygen sintered body, reduction that does not contain carbon is necessary. Preferably, this is carried out in a gas atmosphere.

[For production]

According to the method of this invention, the amount of carbon in the iron-based powder is 0.0
Since it is 1% by weight or less, the contraction of isolated voids inside the sintered body is not hindered by the pressure of CO gas during sintering of the compact, and a sintered body with a relative density of 98% or more can be obtained.

〔Example〕

Hereinafter, the present invention will be specifically explained with reference to Examples.

Example 1 Low carbon iron powder with an average particle size of 25% (oxygen content: 0.4% by weight, carbon content: 0.006% by weight) was used as the raw material powder,
Press-formed at a pressure of 3t/crl, diameter 20 city height 1
A molded article having a diameter of 0 mm and a relative density of 85% was obtained.

For comparison, a molded body of carbonyl iron powder (oxygen content, 0.3% by weight, carbon content, 0.03% by weight) with an average particle size of 5 was also obtained in the same manner.

Next, the molded body was sintered at 1400°C for 1 hour in a vacuum of about 10-' Torr. Table 1 shows the results of the density and structure of the sintered body. With carbonyl iron powder, the relative density of the sintered body did not become higher than 96%, and the presence of voids was observed in the structure of the sintered body, but with the low carbon iron powder, a dense sintered body with almost no voids was obtained.

Table 1 [Effects of the Invention] According to the method of the present invention, the iron powder has a carbon content of 0.
Since low carbon iron powder of 0.1% by weight or less is used, it is possible to reduce the gas pressure in isolated voids inside the sintered body, and a sintered body with a relative density of 98% or more can be produced by one sintering process. Obtainable.

Claims (1)

[Claims] 1. Using iron powder with a carbon content of 0.01% by weight or less as a raw material powder, molding it and sintering the resulting molded body, the relative density of the sintered body is 98. A method for producing a high-density iron sintered body, characterized by obtaining an iron sintered body of % or more. 2. The method for producing a high-density iron sintered body according to claim 1, wherein iron powder having an iron content of 99% by weight or more is used. 3. The method for producing a high-density iron sintered body according to claim 1, wherein the sintering is performed at 1250°C or higher.