JPH079004B2 - Sintering method for iron-based powder compacts - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for sintering an iron-based powder compact, and in particular, a metal salt of stearic acid (eg, zinc stearate or lithium stearate) is blended as a lubricant. The present invention relates to a technique capable of reducing the variation in the dimensional change rate (direction perpendicular to the molding pressure direction) at the time of sintering of an iron-based powder molded body and improving the dimensional accuracy of the final product.

[Prior Art] An iron-based powder compression-molded product obtained by compression-molding and sintering iron-based powder manufactured by a reduction method, an atomizing method, or the like, has the advantage that it can be manufactured in any shape. Because of its excellent dimensional accuracy, it is widely used in the manufacture of various machine parts including automobile parts, and the production amount thereof has been rapidly increasing in recent years.

By the way, in iron-based powder metallurgy, a small amount of stearate (mainly zinc stearate or lithium stearate) is used to improve the lubricity of the iron-based powder during compression molding (lubricity between the powders and between the powder and the mold surface). ) Is added, and an appropriate amount of copper powder, carbon powder, or the like is added to improve the physical properties. After compression molding, the steps of dewaxing, sintering, and cooling are sequentially performed as described below. These steps should prevent oxidation, decarburization, carburization, etc. of the compression molded body. It is customary to do this in an (RX gas) atmosphere.

A dewaxing process in which a lubricant (zinc stearate, etc.) added to improve the compression moldability of the iron-based powder is vaporized and removed by heating.

A sintering step of heating and compressing the compression molded body after the lubricant is removed.

A cooling process in which the temperature of the sintered product is lowered to a temperature at which it will not be oxidized.

As described above, powder metallurgical products are characterized by high dimensional accuracy, but nonetheless, various processes in the above-mentioned dewaxing → sintering → cooling processes (hereinafter sometimes collectively referred to as sintering process or simply sintering) are performed. It causes expansion or contraction depending on the factor, and the dimensions change considerably before and after sintering. Therefore, conventionally, the dimension of the molding die during compression molding is adjusted to a dimension that allows for a dimensional change during sintering, and it is not necessary to adjust the dimension after sintering. However, even then, sufficient dimensional accuracy is often not obtained, and in such a case, secondary processing such as sizing or coining is performed. Especially when the dimensional change in the sintering process is significant, the dimensional variation of the sintered body is large, and not only is secondary processing indispensable, but when the dimensional error is extremely large, the dimensional accuracy correction itself by sizing etc. is extremely difficult. It can be difficult.

Further, if the expansion amount in the sintering step is large, the density of the compression-molded product may decrease, and the mechanical strength required as a mechanical part may not be satisfied.

From such a place, iron-based powder metallurgy materials for machine parts are
A fixed standard is set for the dimensional change rate that occurs in the sintering process, and the limit of the dimensional change rate allowed at this time is about 0.4% based on the dimensional standard of the forming die. For iron-based powder metallurgy materials applied to the production of many types of molded products, the standard deviation of the variation in the dimensional change rate is required to be a very strict value of σ = 0.02%.

There are various possible causes for the expansion / contraction phenomenon in the above-described sintering process, and various solutions have been proposed in response to these causes. Typical of these is Japanese Patent Publication No. Sho 57, which uses the atmospheric gas during sintering as an improvement factor.
-9601 and 58-10963, or the method described in Japanese Examined Patent Publication No. 59-3534 in which an additive is added to the iron-based powder metallurgy material, and each has its own effects. .

Also, "Powder and Powder Metallurgy" Vol. 29, No. 3 (April 1982)
(Page 9 to 13) (Mon.), a method of dewaxing and sintering an iron-based green compact using zinc stearate as a lubricant is also disclosed in a decomposed ammonia gas atmosphere. However, in this method, the surface is severely decarburized during sintering, and the hardness and strength of the sintered body are significantly impaired.

[Problems to be Solved by the Invention] However, even with the improved technique described in the above publication, the dimensional change rate before and after the sintering step cannot be reduced to a satisfactory degree, and particularly, stearic acid metal salt-based When applied to an iron-based powder compression molding containing the above lubricant, the current strict requirement that "the standard deviation of the variation in the rate of change is within the range of σ = 0.02%" cannot be met at all.

Under such circumstances, the present invention intends to provide a method capable of suppressing the variation of the dimensional change rate of the sintered part due to the expansion / contraction during sintering to σ = 0.02% or less. is there.

[Means for Solving Problems] The constitution of the sintering method according to the present invention is such that the iron-based powder containing a metal stearate as a lubricant is compression-molded, and then the molded body is dewaxed and sintered. When processing, the non-oxidizing gas containing no carbon source is used as the atmosphere gas in the dewaxing process, and the RX gas is used as the atmosphere gas in the next sintering process. The variation of the dimensional change rate of the molded product before and after the process is standard deviation σ = 0.
The point is to keep it below 02%.

[Operation] The operation of the present invention will be clarified one after another by following the background of the experiment.

The present inventors are most widely used as a representative example of iron-based powder metallurgy materials [iron powder: 96.45%, copper powder: 2%, carbon powder: 0.8%,
Zinc stearate: 0.75%] was selected and the following experiment was conducted. That is, after compression-molding a rectangular parallelepiped molded body by a conventional method using the powder metallurgical material, the most common RX gas in this kind of field is used as an atmospheric gas, and the dewaxing process has a dew point of 20 ° C., sintering and In the cooling step, the dew point was set to −5 ° C., dewaxing → sintering → cooling was performed in the most general heat pattern shown in FIG. 2, and the dimensional change during that time was investigated. The results are as shown in FIG. 3, and in the case of the compression molded product containing the metal stearate as a lubricant, abnormal volume expansion is observed in the dewaxing process. Moreover, when the temperature exceeds the melting point of copper (1083 ° C.) in the sintering process, the molded body causes a shrinkage phenomenon.

Therefore, the dew point of the atmosphere gas used at the time of sintering was first paid attention to, and an experiment was conducted to clarify the effects on the expansion that occurs during the dewaxing process and the contraction that occurs near the melting point of copper. I got the result.

As is clear from FIG. 4, the expansion amount at the time of dewaxing becomes smaller as the dew point of the atmospheric gas becomes higher, and the reason why the contraction amount near the melting point of copper becomes higher as the dew point of the atmospheric gas becomes higher. It is getting smaller. Then, it is considered that the fluctuation of the expansion / contraction amount due to the dew point of the atmospheric gas finally appears as the variation of the dimensional change.

Next, in order to investigate the effect of the type of atmospheric gas, especially the effect of the atmospheric gas in the dewaxing process where the dimensional change is large, the RX gas is used as the atmospheric gas in the sintering process to prevent decarburization. The dimensional change was investigated when only the atmospheric gas in the process was changed. However, Ar, N ₂ and H ₂ which are non-oxidizing gases containing no carbon source were selected as the atmosphere gas. The results are shown in Fig. 5, and when an atmospheric gas containing no carbon source was used in the dewaxing process, the dimensional change during dewaxing could be drastically reduced irrespective of the dew point of the gas. did it. It was also found that the use of these gases can significantly suppress the variation in dimensional change when copper is melted. By the way, Fig. 1 shows that Ar is used as the atmosphere gas during dewaxing and that the conventional atmosphere gas is used during sintering.
This is the expansion curve when using RX, and as is clear from this figure, when Ar is used as the atmosphere gas, the expansion at the time of dewaxing is hardly seen. Although a slight expansion phenomenon was observed near the melting point of copper during the sintering process, this tendency is due to the fact that copper that occurs in the dewaxing / sintering process after compression molding of iron-based powder without the addition of metal stearate is pressed. It is almost equivalent to the amount of expansion near the melting point (Fig. 6 ... Expansion curve of zinc stearate-free additive). The expansion near the copper melting point observed in the expansion curves of FIGS. 1 and 6 causes alloying with iron due to melting of copper, or the molten copper permeates and diffuses into the final grain boundary part of the matrix metallographic structure. It is thought that this occurred because the metal structure was expanded. In any case, the dimensional change during copper melting is almost constant without being affected by the dew point of the atmospheric gas, as is clear from FIG. 5, and the expansion amount during dewaxing is as described above. As a result, the expansion amount generated during dewaxing / sintering can be kept at a substantially constant value regardless of the dew point of the atmospheric gas, and the variation can be suppressed to a very small value. To be Moreover, since the expansion amount can be obtained in advance by preliminary experiments, it is possible to properly design the molding die size during compression molding in anticipation of the expansion amount, and the dimensional accuracy of the sintered compact can be satisfied. Can be increased to
In the above, the iron-based powder metallurgy material containing a small amount of copper and carbon powder was taken as an explanation, but in addition to this, the present invention is Ni, Sn, Mn, S,
The same can be applied to iron-based powder metallurgy materials containing B, P.Sb, Zn, MnS, CaS and the like.

[Example] An iron-based powder metallurgy material consisting of iron powder: 96.45%, copper powder: 2%, carbon powder: 0.8%, and zinc stearate: 0.75% was used at a molding pressure of 5 t / cm ² and 5 x 5 x. A 30 mm compression molded body was produced. Using the obtained compression-molded article, according to the heat pattern shown in FIG.
The dewaxing / sintering experiments were conducted one by one to examine the variation in dimensional change of the sintered compact. The increase / decrease in carbon content in each sintered molded product was also examined. The results are collectively shown in Table 1.

As can be seen from Table 1, in the conventional example (No. 1 and 2) in which RX gas was used as the atmosphere gas during dewaxing, the expansion during dewaxing was significant and the variation was large, so the sintered compact The variation (σ) in dimensional change exceeds 0.02%, and a carburization phenomenon occurs in the sintering process, and an increase in carbon content is observed.
On the other hand, when a gas that does not contain a carbon source is used as the atmosphere gas during dewaxing (No. 3 to 8), expansion during dewaxing is avoided and firing is almost independent of the dew point of the gas. The variation in the dimensional change of the binder was as small as 0.013% or less, and the carbon content did not fluctuate at all. Experiment No. 9 is an example of using an iron-based powder compression molding without a lubricant (zinc stearate). In this case, even when RX gas was used as the dewaxing atmosphere gas, the dimensional changes No large variations are seen.
On the other hand, in the examples of the present invention, it can be seen that even when an appropriate amount of metal stearate is used in combination, a high level of dimensional accuracy surpassing the case where no metal stearate is added can be ensured.

In the above experiment, Ar, N ₂ and H were used as the atmosphere gas during dewaxing.
_Although the examples using ₂ respectively were shown, the same effect can be obtained even when these gases are mixed and used, and particularly when Ar or N ₂ is used, oxidation during dewaxing is prevented. In order to prevent it, more preferable results can be obtained when H ₂ gas of about 5 to 70% is used together.

[Effects of the Invention] The present invention is configured as described above, but the point is to reduce the volume change occurring in the dewaxing step when a metal stearate salt is mixed as a lubricant in an iron-based powder metallurgy material, in a dewaxing atmosphere. By specifying the gas as much as possible, the variation in dimensional change during dewaxing / sintering can be suppressed to a very small level, and the dimensional accuracy of the sintered compact can be dramatically improved. The atmosphere gas of the sintering process is RX
By using gas, it is possible to prevent a decrease in hardness and strength due to decarburization, and it is possible to obtain a sintered compact having excellent dimensional accuracy and physical properties.

[Brief description of drawings]

FIG. 1 is a diagram showing an expansion curve when the method of the present invention is adopted, FIG. 2 is a diagram showing a heat pattern adopted in an experiment, FIG. 3 is a diagram showing an expansion curve in the conventional method, and FIG. Fig. 5 is a graph showing the dew point of the atmospheric gas and the dimensional changes during dewaxing and copper melting when the method is adopted, and Fig. 5 shows the dew point of the atmospheric gas and the time during dewaxing and copper melting when the method of the present invention is adopted. FIG. 6 is a graph showing a dimensional change, and is a diagram showing an expansion curve when an iron-based powder metallurgy material containing no metal stearate added is used.

Claims (1)

[Claims] 1. An iron-based powder containing a metal salt of stearic acid as a lubricant is compression-molded, and then a dewaxing / sintering process is performed on the green body. Using non-oxidizing gas and RX gas as the atmosphere gas in the next sintering process, the standard deviation σ = 0.02% or less of the variation in the dimensional change rate of the molded product before and after the dewaxing / sintering process. A method for sintering an iron-based powder compact, characterized in that