JPH10121109A - Method for controlling carbon content in metallic sintered body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control a metallic sintered body to a desired carbon content at high accuracy regardless of the history of a degreased body by removing the carbon content and oxygen content in a green compact before sintering as much as possible. SOLUTION: The degreased green compact contains carbon and oxygen mainly as a metallic oxide. The carbon can almost be removed by a hydrogen reduction below 1000 deg.C, but the oxygen remains considerably depending on a kind of the degreased body. A large part of the oxygen is removed in any degreased body by the hydrogen reduction in a prescribed time by slowly raising the temp. from 1000 deg.C to a sintered body producing temp. The sintered body producing temp. is desirable to be in the range of 1150-1250 deg.C and the temp. raising time is sufficient to about 2-4hr regardless of the kind of degreased body. Therefore, it is recommended to execute two steps of the hydrogen reduction process below 1000 deg.C and the hydrogen reduction process raising the temp. to the sintered body producing temp. By this method, since the influence of the history of the degreased body and the material of metallic powder can almost perfectly be rejected, the carbon content in the sintered body can easily be controlled to the desired value in the high accuracy in the same operation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling the amount of carbon in a metal sintered body. More specifically, the present invention relates to a method for controlling the amount of carburization with high accuracy by eliminating the influence of the degreased body history and the material of the metal powder. The present invention relates to a method for controlling the amount of carbon in a metal sintered body that can be easily controlled to the value of

[0002]

2. Description of the Related Art Conventionally, a metal sintered body (also called a sintered alloy)
Is a raw metal powder and a binder (binder, plasticizer,
Lubricant), melt it with high heat and injection mold it.
It is manufactured by evaporating a binder in a degreasing process and then sintering. When producing such a metal sintered body, properly controlling the amount of carbon is an extremely important issue since the amount of carbon affects the mechanical properties of the sintered body.

Conventionally, as a method for controlling the amount of carbon in a metal powder compact, a method of adding a resin having a higher residual carbon amount than the resin of the main binder or a method of reducing the residual carbon amount before sintering to a desired carbon amount has been employed. A method of adding a metal oxide such as iron oxide has been known.

In any of the above conventional methods, the operation of adjusting the amount of carbon is performed before the sintering step, and the operation of selecting experimental conditions for finding an appropriate amount of carbon in the obtained sintered body by trial and error is performed by sintering. It is not enough to just select the conditions in the process,
There is a problem that it becomes extremely complicated because it is necessary to produce many types of molding materials with different amounts of graphite and metal oxide, and to repeat time-consuming degreasing and carbon-containing treatments many times. Not only that, these conventional methods
It was not possible to control the carbon distribution inside the obtained sintered compact.

In order to solve such a problem, one of the present applicants has developed a method of sintering by selecting a mixture ratio of hydrocarbon and hydrogen in a hydrocarbon / hydrogen mixed gas atmosphere. A patent application has been filed (see JP-A-7-118705).

[0006]

The above-mentioned method is extremely excellent in that the entire sintered compact can be easily controlled to a desired carbon content only by sintering while adjusting the mixing ratio of hydrocarbon and hydrogen. It was a way. However, this method should be sufficiently satisfactory in that differences in the history of the production method of the raw metal powder, the type of binder, the degreasing method, etc. make it difficult to control the desired carbon content with high accuracy. It was not a kimono.

The present invention provides a method for controlling the carbon content of a metal sintered body which can control the desired carbon content with high accuracy regardless of the history of the degreased body before sintering. The purpose is to:

[0008]

Means for Solving the Problems To solve the above-mentioned problems, the present inventors have made a series of basic studies and found that the carbon content and the oxygen content in the compact before sintering were reduced to zero or almost constant. The present inventors have found that the desired amount of carbon can be controlled with a high degree of accuracy regardless of the history before sintering by controlling the amount of carbon before sintering.

That is, according to the present invention, a metal powder compact is degreased and hydrogen reduced, and then carburized and sintered in a hydrocarbon / hydrogen mixed gas atmosphere adjusted to a predetermined mixing ratio. In the carbon amount control method of controlling the carbon amount of the molded body to a predetermined value, in the hydrogen reduction step, carbon and oxygen in the molded body are removed to homogenize regardless of the degreased body history and the material of the metal powder. It is characterized by having done.

In short, according to the present invention, the difference in the history of the degreased body can be completely reduced by reducing the amount of carbon and the amount of oxygen before the carburizing and sintering process to zero or a constant value. The gist of the present invention is that it is possible to control the desired amount of carbon with high accuracy by simply selecting the conditions in the carburizing and sintering process so that the desired amount of carbon can be obtained. It is.

In the present invention, to make the carbon amount and the oxygen amount uniform means to reduce the variation between the carbon amount and the oxygen amount before the carburizing and sintering process due to the history of the degreased body. Preferably, the amounts of carbon and oxygen are reduced to zero or to a minimum.

[0012]

Next, an embodiment of the present invention will be described. The metal powder compact may be manufactured by a known method in which a metal fine powder and a binder (a binder, a plasticizer, a lubricant, etc.) are kneaded, heated at an appropriate temperature, and injection molded. In the degreasing step, the binder mixed in the molded body is decomposed and volatilized by a known method.

[0013] The degreased molded body contains carbon and mainly oxygen as a metal oxide. Carbon can be mostly removed by hydrogen reduction at temperatures up to 1000 ° C., but oxygen remains under this condition in considerable amounts depending on the type of degreased body.

Oxygen can be largely removed from any degreased body by gradually reducing the temperature from 1000 ° C. to the sintered body manufacturing temperature (carburizing / sintering temperature) and reducing the hydrogen for a predetermined time. it can. A heating time of about 2 to 4 hours is sufficient, and may be selected so that it can be reduced to zero or minimized regardless of the type of degreased body. Therefore, it is preferable to perform the hydrogen reduction in the first stage up to 1000 ° C. and the hydrogen reduction in the second stage in which the temperature is gradually increased from the temperature of the first stage to the production temperature of the sintered body. good.

[0015] The sintered body production temperature is preferably 1150-150.
It is preferable to carry out at a substantially constant temperature in the range of 1250 ° C.
The sintering holding time varies depending on the desired amount of carbon in the sintered body, the sintering holding temperature, the ratio of the mixed gas of hydrocarbon and hydrogen, and the like, but about 1 to 3 hours is sufficient.

The proportion of the hydrocarbon / hydrogen mixed gas may be selected in accordance with the sintering holding temperature so that the amount of carbon in the sintered body becomes a target amount. The ratio of the hydrocarbon / hydrogen mixed gas may be constant during carburizing and sintering, or may be adjusted so as to change the mixing ratio. As hydrocarbons,
It is preferable to use methane gas, but ethane gas, propane gas, butane gas, etc. may be used. The amount of carbon in the sintered body produced in the present invention can be adjusted in a wide range, but is preferably adjusted to about 0.3 to 0.7% by weight for reasons of the physical properties of the sintered body.

[0017]

Next, the present invention will be further described with reference to examples and reference examples, but the present invention is not limited to these examples. Reference Example The test materials and binders described in the following Table 1 were kneaded, and the test materials 1 to 3 were injection-molded into plate-like tensile test pieces, and the test materials 4 to 5 were injection-molded into Charbie-impact test pieces. . The amount of graphite added to Carbonyl iron powder of Test Material 3 was 0.1% by mass.
4% and 0.8%.

[0018]

[Table 1]

The specimen was heated at a rate of 12 ° C./hour in the air, kept at 300 ° C. for 4 hours and degreased. The degreased body was placed in a silicon knit tube furnace, and the temperature was raised as shown in FIG. Then, carburizing and sintering were performed. The amount of carbon was measured for the obtained degreased body, reduced body and sintered body, and the amount of oxygen was also measured for the reduced body. The results are shown in FIG. The reduction treatment body is a sample that was interrupted immediately before the carburizing and sintering step in the step of FIG. 1, was evacuated, cooled, and then taken out.
To measure the amount of carbon, use a Horiba Seisakusho EMIA-720 type carbon / sulfur analyzer, and to measure the amount of oxygen, use Horiba EM
A GA-650 oxygen / nitrogen analyzer was used.

The symbols and abbreviations in FIG. 2 represent the following test materials. ○ SCM. W: SCM40 water atomized powder ● SCM. G: SCM40 gas atomized powder △ CO. 4%: Graphite Carbonyl iron powder added with 4% ▲ CO. 8%: Graphite 8% added carbonyl iron powder □ 4600L: 4600 steel low oxygen powder ■ 4600H: 4600 steel high oxygen powder

As shown in FIG. 2, the defatted body generally has a large amount of carbon since a large amount of carbon and added graphite remain in the binder. Among the test materials, the carbon content of SCM440 was the smallest, the carbon content of 4600 steel powder was the largest, the carbon iron powder was in the middle, and the difference in the amount of graphite added to the carbon iron powder was approximately It is the difference in carbon content.

In the reduced material, the carbon content of all the test materials is almost zero due to the strong decarburizing action of hydrogen. In this way, all the test materials are carburized from the same amount of carbon, so the carburized amount should be the same,
In practice, as shown in FIG.
The percentage varies greatly.

The oxygen content of the reduced product is as follows. Carbonyl iron powder is the smallest, 4600 steel powder and SCM440 gas atomized powder are intermediate, and SCM440 water atomized powder is the largest. Are in the reverse order of the oxygen amount as shown in FIG. For this reason, oxygen content is an important factor influencing carburization,
It has become clear that not only the amount of carbon but also the amount of oxygen needs to be precisely controlled in order to enhance the control accuracy of the amount of carbon.

Example From the above reference example, in order to improve the control accuracy of the amount of carbon in the sintered body, the amount of carbon and the amount of oxygen at the start of carburizing are fixed (for example, almost zero), and It is inferred that if started, carburization can be performed substantially uniformly regardless of the history of the degreased body and the material of the metal powder. The following experiment was performed to confirm such inference.

A degreased body was produced from the test materials shown in Table 1 in the same manner as in the above reference example, and the temperature was raised as shown in FIG. 3 to carry out carburizing and sintering. That is, as shown in FIG. 3, from room temperature to 1000 ° C. is the same as the reference example,
The temperature was gradually raised from 1000 ° C to 1200 ° C over 3 hours. After reaching 1200 ° C., in a step of maintaining the temperature at this temperature for 2 hours, a methane / hydrogen mixed gas of low-concentration methane (CH ₄ 0.2%) was flowed to carry out carburizing and sintering. The temperature lowering and cooling steps are the same as in the reference example of FIG.

The carbon content of the sintered body was measured in the same manner as in the reference example, and the results are shown in FIG. The symbols and abbreviations in FIG. 4 have the same meaning as in FIG. From the results shown in FIG. 4, the sintered bodies from all the test materials have a carbon content of 0.09%.
Is focused on As is clear from these results, according to the method of the present invention, the effects of the degreased body history and the material of the metal powder are eliminated, and carburization can be performed almost uniformly.

With respect to the 1000 ° C. reduction treated body (taken out immediately before raising the temperature) and the 1200 ° C. reduction treated body (taken out at the time of reaching 1200 ° C.) in the above embodiment, the oxygen amount and The carbon content was measured. Regarding the amount of carbon, all the samples were almost decarburized by the 1000 ° C. reduction treatment, and completely decarbonized by the 1200 ° C. reduction treatment. The amount of oxygen is 1000 ° C.
Although it varied from 0.05 to 0.82% by weight, 120
In the 0 ° C. reduction treated body, it was homogenized to 0.02 to 0.3% by weight.

The difference between the above embodiment and the method described in the reference example is that firstly, the temperature range for reduction by hydrogen is extended to the sintering temperature, and secondly, a mixed gas of methane and hydrogen is introduced. That is, the carburizing step is overlapped with the sintering step. By extending the temperature range for reduction by hydrogen to the sintering temperature, the amount of carbon and oxygen in the degreased body can be reduced to zero or to a minimum regardless of the history of the degreased body and the material of the metal powder.

Since the carburizing step of introducing the methane / hydrogen mixed gas is repeated in the sintering step, the methane / hydrogen mixed gas is introduced in a state where the carbon amount and the oxygen amount are reduced to zero or to a minimum. Therefore, the carbon content in the sintered body can be easily controlled to a desired value irrespective of the history of the degreased body and the material of the metal powder only by controlling the mixing ratio of the methane / hydrogen mixed gas.

[0030]

According to the present invention, the amount of carbon and the amount of oxygen before the introduction of the methane / hydrogen mixed gas (the carburizing / sintering step) can be reduced to zero or minimum regardless of the history of the degreased body and the material of the metal powder. Since it can be reduced and made uniform, the influence of the degreased body history and the material of the metal powder can be almost completely eliminated. Therefore, regardless of the degreased body history and the material of the metal powder, the amount of carbon in the metal sintered body can be easily and precisely controlled to a desired value by the same operation.

[0031]

[Brief description of the drawings]

FIG. 1 is a diagram showing a manufacturing process of a sintered body from a degreased body according to a reference example.

FIG. 2 is a diagram showing the amounts of carbon and oxygen in a degreased body, a reduced body, and a carburized / sintered body according to a reference example.

FIG. 3 is a diagram showing a manufacturing process of a sintered body from a degreased body according to an example.

FIG. 4 is a diagram showing the amounts of carbon and oxygen in a degreased body, a reduced body, and a carburized / sintered body according to an example.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Takashi Amano 5th-9th, Yonbancho, Chiyoda-ku, Tokyo Topy Industries Co., Ltd.

Claims (7)

[Claims] 1. A metal powder compact is degreased and hydrogen reduced, then carburized and sintered in a hydrocarbon / hydrogen mixed gas atmosphere adjusted to a predetermined mixing ratio to reduce the amount of carbon in the metal sintered body. In the carbon amount control method of controlling to a predetermined value, in the hydrogen reduction step, carbon and oxygen in the molded body are removed, and the molded body is made uniform regardless of the history of the degreased body and the material of the metal powder. A method for controlling the amount of carbon in a metal sintered body. 2. The method according to claim 1, wherein the amount of carbon and the amount of oxygen in the compact are reduced to zero or to a minimum. 3. The method according to claim 1, wherein the hydrogen reduction step is performed in two stages: a hydrogen reduction step for mainly removing carbon and a hydrogen reduction step for mainly removing oxygen. 4. The hydrogen reduction step of removing carbon mainly in the first step is performed at a temperature of up to 1000 ° C., and the hydrogen reduction step of removing mainly oxygen in the second step is carried out from the temperature of the first step. 4. The method according to claim 3, wherein the temperature is raised to a carburizing / sintering temperature. 5. The method according to claim 4, wherein in the reduction step of the second step, after raising the temperature to the carburizing / sintering temperature, the hydrocarbon / hydrogen mixed gas is introduced to simultaneously perform carburizing and sintering. The method for controlling the amount of carbon described above. 6. The carburizing and sintering step is performed at 1150 to 125
The method according to claim 5, wherein the method is performed at a substantially constant temperature of 0 ° C. 7. The amount of carbon in the sintered metal is 0.3 to 0.3.
The method for controlling carbon content according to any one of claims 1 to 6, wherein the carbon content is controlled to 0.7% by weight.