JPH03249101A - Iron series powdery raw material for sintering - Google Patents

Abstract

PURPOSE:To prevent excess carburization and to improve dimensional accuracy and mechanical characteristic by blending a material, which generates water with heat decomposition at the temp. of A1 transition point or lower, into iron series powder at the prescribed ratio. CONSTITUTION:The material (e.g. boric acid, etc.) which generates the water with the heat decomposition at the A1 transformation point or lower as the water generating material, is prepared. By blending <=1wt.% of this water generating material into the iron series powder, the iron series powdery raw material for sintering is formed. The above-mentioned powdery material is not affected to the dew point of atmosphere gas and temp. Condition, etc., at the time of sintering and the excess carburization is prevented and abnormal expansion at the time of dewaxing is prevented.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to an iron-based powder raw material for sintering, and in particular to an iron-based powder raw material for sintering that provides a sintered body with excellent dimensional accuracy and mechanical properties. It is related to.

[Prior art] The iron-based powder metallurgy method, in which iron-based powder produced by a reduction method, atomization method, etc., is compression-molded and then sintered to obtain a compact is said to be able to easily form any shape into one piece. Due to its advantages, it is widely used in the production of various mechanical parts including automobile parts, and the demand for it has been increasing rapidly recently.

By the way, in iron-based powder metallurgy, a small amount of lubricant (zinc stearate, lithium stearate, etc.) is used to improve the lubricity of iron-based powder (lubricity between powders and between powder and mold surface) during compression molding. Also, appropriate amounts of copper powder, graphite powder, etc. are blended to improve the physical properties of the sintered product. In addition, after compression molding, the steps of dewaxing, sintering, and cooling are performed sequentially as shown below.
In order to prevent oxidation, decarburization, carburization, etc. of the compression molded body, it is customary to carry out the process in a modified hydrocarbon gas (RX gas) atmosphere.

■A dewaxing process in which the lubricant that is added to improve the compression moldability of iron-based powder is vaporized and removed by heating.

■Sintering process in which the compression molded body is heated and sintered after the lubricant has been removed.

■A cooling process in which the temperature of the sintered product is lowered to a temperature at which it will not undergo atmospheric oxidation.

By the way, in iron-based powder metallurgy, considerable expansion or contraction occurs due to various factors in the film brazing, sintering, and cooling steps (hereinafter sometimes collectively referred to as the sintering step or simply sintering). , the dimensions change before and after sintering. Conventionally, therefore, the dimensions of the mold during compression molding are adjusted in anticipation of dimensional changes during sintering, so that there is no need to adjust the dimensions after sintering. However, even then, sufficient dimensional accuracy is often not obtained, and in such cases, secondary processing such as sizing or machining is performed. In particular, if the dimensional changes during the sintering process are significant, the sintered body dimensions will vary greatly, and not only will secondary processing become essential, but if the dimensional errors are extremely large, it will be extremely difficult to correct the dimensional accuracy by sizing etc. Sometimes it becomes difficult.

Furthermore, if the amount of expansion during the sintering process is large, the density of the compact may decrease, making it impossible to satisfy the mechanical strength required as a mechanical component.

From this point of view, iron-based powder metallurgy materials for machine parts are
Certain standards have been set for the rate of dimensional change that occurs during the sintering process, and the limit of the rate of dimensional change that is currently allowed is approximately 0.4% based on mold dimensions. Regarding iron-based powder metallurgy materials used in the production of various types of molded products,
A very strict range of variation in dimensional change rate of about 0.02% is required.

Various causes can be considered for the expansion/contraction phenomenon, and various solutions have been taken in response to these causes. Typical examples include the method described in Japanese Patent Publication No. 57-9601 and Japanese Patent Publication No. 5B-10963, in which atmospheric gas during sintering is used as an improvement factor, and the method described in Japanese Patent Publication No. 59-1989, in which special additives are blended into iron-based powder raw materials. Examples include the method described in No. 3534, but even these methods cannot sufficiently reduce variations in dimensional change rate and dimensional accuracy.

Under these circumstances, the present inventors developed a method for reducing the variation in dimensional changes by using a non-oxidizing gas that does not contain a carbon source as a dewaxing atmosphere gas. proposed as. However, the normal dewaxing → sintering process is performed continuously in the same furnace,
In addition, since RX gas must be used in the sintering process to prevent decarburization, only the dewaxing process is performed in an atmosphere of non-oxidizing gas (Ar gas, N gas, etc.) that does not contain a carbon source, as in the disclosed invention described above. To do this, the dewaxing and sintering furnaces currently in use would have to be modified, which is not practical.

[Problems to be Solved by the Invention] The present invention has been made with attention to the above-mentioned circumstances, and its purpose is to adjust the type of atmospheric gas, dew point, heating rate, etc. in the dewaxing process. The object of the present invention is to provide an iron-based powder raw material for sintering that can reliably produce a sintered body with excellent dimensional accuracy and mechanical properties while preventing decarburization.

[i! Means for solving the problem] The above i! The iron-based powder raw material for sintering according to the present invention that can solve the problem is made by blending iron-based powder with 1% or less of a substance that generates water by thermal decomposition at a temperature below the A1 transformation point. However, there is a gist.

[Operations and Examples] The present invention provides a substance that thermally decomposes iron powder for powder metallurgy (including pure iron powder, various alloy steel powders, high speed steel powder, etc.) at a temperature below the A1 transformation point to produce water. It contains less than 1% of (for example, boric acid, etc., sometimes referred to as water-generating substances), which stabilizes the carburization phenomenon during the sintering process.
In addition, dimensional changes during sintering and variations in dimensional accuracy can be significantly reduced. The fact that such excellent effects can be obtained by blending water-generating substances can be confirmed by the experimental results shown below.

First, Figures 1 (A) to (E) show a Fe-2%Cu-0.8%C system compacted at 5t/cm2 in order to investigate the behavior during sintering of iron-based powder raw materials for powder metallurgy. RX the molded object
This figure shows the thermal expansion behavior when sintered in gas.

However, the dewaxing conditions were as follows.

Figure 1 (A); Dew point of atmospheric gas (RX gas) at knee 5°C
, temperature increase rate = 20℃/sin, 600℃ xomin (no lubricant) same (B), RX gas dew point: 10℃, temperature increase rate = 10℃
/win, 600℃ x s o sin same (C);
RX gas dew point: 20°C, heating rate = 30°C/+++i
n, 600°C x 51in.

Same as above (p); Same as above (E) - Same as above Figure 1 (^) To explain, α-γ of Fe at point a
Contraction due to transformation, expansion due to carburization of C (graphite) at point b, expansion due to penetration and diffusion of melted Cu into Fe particles at point 0, contraction during isothermal heating at point d, and γ-α at point e.
Expansion due to metamorphosis is occurring.

Figures 1 (B) and (C) show the above ingredients mixed with 0.75% zinc stearate as a lubricant;
B) lowers the dew point of the atmospheric gas during dewaxing, slows down the temperature rise rate, and lengthens the holding time at 600°C. fSi diagram (＾) showing abnormal contraction at melting point, etc.
It is significantly different from the Also, Figure 1 (C
) is a case in which the dew point of the atmospheric gas during dewaxing is raised and rapid film brazing is performed, and the expansion from point a is relaxed.

Figure 1 (0) is a mixture of the above ingredients with 0.75% zinc stearate and 0.1% boric acid, and the abnormal expansion that occurs from point a is almost no longer observed; （＾）
shows similar behavior.

Figure 1 (E) shows the composition of Figure 1 (D) with the amount of boric acid increased to 1.0%, which suppresses the expansion of graphite due to carburization, while also suppressing the expansion of Cu into Fe particles. The expansion due to penetration and diffusion increases, and a peculiar abnormal expansion occurs at point a.

In addition, in the behavior during sintering as shown in tS1 diagrams (^) to (E), those with a small difference between the volume at the start of temperature rise and the size when the temperature cools down after sintering have excellent dimensional stability. It also shows that the variation in dimensional changes is small, and as is clear from the behavior in Figure 1 (D), by adding a small amount of boric acid to the iron-based raw material for powder metallurgy, abnormal expansion during sintering can be suppressed. It can be seen that the dimensional stability can be significantly improved by suppressing the dimensional stability.

Next, Figure 2 shows Fe-2%Cu-0,8%C-〇,7
Powder consisting of 5% zinc stearate and Fe-2%C
u -0,8% C-0,75% zinc stearate-0,
This figure shows the results of investigating the relationship between the dew point of the atmospheric gas (RX gas) during dewaxing and the dimensional change rate before and after sintering using powder made of 1% boric acid. However, the sintering conditions are
Temperature increase rate in atmospheric gas (RX gas dew point knee 5°C):
20℃/gin.

Heating time: 1130°C x 20 sin.

As is clear from Fig. 2, the rate of dimensional change varies significantly depending on the dew point of the dewaxing atmosphere gas, heating rate and holding time for dewaxing when no boric acid is added, but when an appropriate amount of boric acid is added, Even if the dew point of the wax atmosphere gas, heating rate, or holding time changes, the effect on the dimensional change rate is extremely small.

In addition, Fig. 3 shows Fe-2%Cu-0,8%C-0,7
It is an experimental graph showing the results of investigating the effect on the standard deviation of dimensional change when the basic composition is 5% zinc stearate and the blending ratio of boric acid is changed. However, the experimental conditions were as follows.

Dewaxing process: Temperature increase rate: 20°C/wi in RX gas (dew point: 20°C)
n, 600°C x 5m1n sintering process; RXX gas C8 cony-5), heating rate = 20°C/+i
As is clear from FIG. 3, when a small amount of boric acid is contained in the Fe-based powder for sintering, the standard deviation of the variation in dimensional changes during sintering becomes very small. However, when the amount of boric acid exceeds 0.2 to 0.3%, the standard deviation tends to increase, and when it exceeds 1%, the standard deviation becomes even larger than that of a product without boric acid. For this reason, in the present invention, the amount of boric acid added is set at 1% or less, but in order to reduce the standard deviation, the more preferable amount is in the range of 0.05 to 0.6%.

Figure 4 shows Fe-2%Cu-0,8%C-0,75% zinc stearate powder or Fe-2%Cu-0,8%C
It is an experimental graph showing the results of investigating the relationship between the dew point of the dewaxing and sintering atmosphere gas and the amount of C in the sintered body using -0.75% zinc stearate -0.1% boric acid powder. However, the sintering conditions are: in atmospheric gas (RX gas, dew point knee 5°C);
Temperature increase rate: 20tl:/win, 1130℃×2
It was set to 0 sin.

As is clear from Figure 4, the amount of C in the sintered body varies considerably depending on the dewaxing conditions in the case of the powder containing boric acid, whereas the amount of C in the sintered body varies considerably depending on the dewaxing conditions in the case of the case with the addition of an appropriate amount of boric acid. Almost no fluctuations in the C content were observed, and a sintered body with a stable C content could be obtained.

Further, FIG. 5 shows the results of investigating the effect of varying the amount of boric acid added on the tensile strength of the sintered body, and the experimental conditions were the same as in FIG. 3. As is clear from this figure, if the amount of boric acid added exceeds 1%, it will have an adverse effect on the tensile strength of the sintered body, so from this perspective, the amount of boric acid added should be kept below 1%. be.

As is clear from Figures 1 to 5 above, if an appropriate amount of boric acid is contained in the iron-based powder for sintering, abnormal expansion during dewaxing can be significantly suppressed, and the dewaxing conditions and dew point of the atmospheric gas can be It is possible to obtain a sintered body with a small dimensional change rate and a small variation thereof, without being affected by the above factors, and with little variation in the amount of C due to carburization.

The reason why such an excellent effect is exhibited by the addition of boric acid has not yet been clarified, but the following may be considered.

■For example, zinc stearate, which is blended as a lubricant, is thermally decomposed under dewaxing or sintering conditions to produce metallic Zn, and this Zn acts as a catalyst for the following reaction that occurs in the atmospheric gas (RX gas). it is conceivable that.

2CO→CO, +C CH4→2H2+C The generated C is thought to become a carburizing source and penetrate into the iron powder, causing abnormal expansion.

However, the above-mentioned Zn reacts with water produced by thermal decomposition of boric acid and becomes an oxide. 2HsBOs→B2O3+3H20 Zn+H20→ZnO+H2 Since the above-mentioned catalytic activity is lost, carburization becomes difficult to occur.

■Boric acid thermally decomposes as described above to produce water and turns into boron oxide, but this boron oxide has a low melting point (mp, 450℃
) is a viscous substance that covers the surface of the iron powder to form a carburization prevention layer, which may suppress carburization.

Therefore, in view of the above, it is thought that, in addition to boric acid, various substances capable of releasing water or producing carburization-preventing film-forming substances by thermal decomposition may exhibit similar effects. Therefore, we investigated the effects of adding various substances other than boric acid, and found that any substance that decomposes to produce water at a temperature below the A3 point of the iron-based powder to which it is added exhibits effects similar to boric acid. It has been confirmed that you can get it. Specific examples of such substances include borates such as borax, oxalic acid and its salts, and it has been confirmed that their effects can be effectively exhibited even when added in an amount of 1% or less. However, those whose thermal decomposition temperature exceeds the A1 point have a rather harmful effect, such as inhibiting the carburization of graphite into iron.

[Effects of the Invention] The present invention is configured as described above, and dewaxing can be achieved by simply adding a small amount of a substance that thermally decomposes to produce water below a specific temperature into the iron-based powder for sintering.・Excessive carburization can be prevented almost unaffected by the dew point and temperature conditions of the atmospheric gas during sintering, and as a result, abnormal expansion during dewaxing is prevented and the dimensional change rate is reduced. It has become possible to provide a sintered body that is small and has little variation.

[Brief explanation of drawings]

Figure 1 is a graph showing the change in expansion coefficient during sintering of iron-based powder for powder metallurgy, Figure 2 is a graph showing the relationship between the dew point of the dewaxing atmosphere and the dimensional change rate of the sintered body, and Figure 3 is a graph showing the relationship between the dew point of the dewaxing atmosphere and the dimensional change rate of the sintered body. A graph showing the relationship between the amount of boric acid added and the standard deviation of the variation in the dimensions of the sintered body. Figure 4 is a graph showing the relationship between the dew point of the dewaxing atmosphere and the amount of C in the sintered body. It is a graph showing the relationship between the addition amount and the tensile strength of the sintered body. Temperature f °C) Dew point of dewaxing atmosphere (1) Boric acid content (%)

Claims (1)

[Claims] An iron-based powder raw material for sintering, characterized in that the iron-based powder is blended with 1% by weight or less of a substance that thermally decomposes to produce water at a temperature below the A_1 transformation point.