JPS5852525B2 - Manufacturing method of bronze sintered bearing material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a bronze-based sintered bearing material having higher strength than conventional products.

In the production of bronze-based sintered alloys, there are cases where bronze powder is used as the raw material, and cases where mixed powder of copper and tin powders are used as the raw material, and the technology of the present invention targets the latter case. It was developed as.

First, to describe the general process of powder metallurgy, first, zinc stearate or other powdered lubricant is added to the raw material powder.
The standard method is to add 2 parts by weight, mix well, compression mold it into the desired shape using a mold, then sinter it in a sintering furnace, and then complete it by sizing and oil impregnation treatment. It is a process.

Here, we will touch on the sintering process, which is most closely related to the material properties of sintered alloys. First, there are two types of sintering furnaces: batch-type furnaces, which are usually used for small-scale sintering, and continuous-type furnaces, which are suitable for mass production. There are two main categories.

Continuous furnaces are horizontally long tube-shaped furnaces that use a mechanism to move the charge (formed bodies), such as a mesh belt type furnace using a steel belt made of heat-resistant alloy, or a semi-continuous sintering box containing the formed bodies. There are various types of furnaces, such as pusher type furnaces that are pushed into the furnace in sequence, roller type furnaces, lift beam type furnaces, etc., but all of them basically preheat and preheat the compacts in order from the entrance side of the furnace. The structure includes three zones: a dewaxing chamber, a sintering chamber, and a cooling chamber.

Note that, as the atmosphere in the furnace, a reducing atmosphere such as hydrogen gas or ammonia decomposition gas is generally used.

Next, the sintering process is divided into two stages: an earlier dewaxing process and a later main sintering process.

That is, while the compact passes through the aforementioned furnace, the lubricant in the compact is first evaporated and removed in a preheating/dewaxing chamber at a relatively low temperature, and then in a sintering chamber maintained at a predetermined high temperature. After being sintered in earnest, it is slowly cooled to near room temperature to complete the sintering process.

Here, the conditions are that the dewaxing temperature is approximately 400°C, the temperature increase rate to reach this temperature is 10 to 30°C per minute, and the main sintering temperature is approximately 780°C. be.

The graph in Figure 1 shows the thermal history that the compact undergoes during the above-mentioned sintering process. corresponds to

Now, the bronze-based sintered alloy obtained by removing and sintering in a reducing atmosphere in this way has a strength of 18 Kf/yta in terms of radial crushing strength. '%: Slightly higher.

Therefore, since oil-impregnated bearings are also a type of mechanical element, it is desired to increase their strength as much as possible in order to increase the degree of freedom in design.

However, increasing the strength of bearing materials is not so easy.

This is because in the case of an oil-impregnated bearing, it is essential that it not only has its own wear resistance, but also that it does not wear out the shaft of the mating member, and that it is as porous as possible and capable of retaining a large amount of lubricating oil. For example, in the case of mechanical components such as gears, common strengthening methods in powder metallurgy are used, such as adding reinforcing ingredients to make the material highly alloyed, or densifying the material by compression molding at high pressure. Because you can't.

Therefore, the inventor devoted his efforts to researching the sintering process of the compact, and by improving the preheating and dewaxing process that precedes it, he succeeded in improving the strength of the bearing while maintaining its original characteristics.

These improvements, that is, the features of the present invention, include making the atmosphere in the furnace in the dewaxing process an oxidizing atmosphere, setting the dewaxing 5 temperature at 400 to 750°C, which is higher than the conventional temperature, and increasing the temperature during the dewaxing process. The main idea is to increase the temperature rate to 50° C./min or higher.

Note that although a special oxidizing gas may be used as the oxidizing atmosphere here, the purpose can usually be easily achieved by using the atmosphere.

The present invention will be described below with reference to one embodiment thereof.

Example First, copper powder and tin powder were mixed into a mixture of 91% copper and 9% tin,
Add 0.3% zinc stearate as a lubricant to the total amount and mix thoroughly.The mixed powder is put into a mold and compressed.
icx cylindrical shape, powder density is 6.3311'/c11
Mold a number of samples of i.

Next, regarding the sintering of the samples, a continuous sintering furnace of the type in which the dewaxing chamber and the main sintering chamber are separated and the two are connected by a mesh belt is used, and the samples are divided into the above groups 1 to 5. , respectively, are sintered based on the dewaxing conditions shown in Table 1.

That is, the sample group (■) in the table is related to the present invention. First, the sample was sent to a dewaxing chamber with an oxidizing atmosphere, and the temperature was raised to 700 °C at a rate of 55 °C per minute, held for several minutes, and then lubricated. Evaporate the agent.

The temperature increase rate can be arbitrarily set by adjusting the temperature distribution in the furnace chamber and the feed rate.

The sample after dewaxing is then transferred to the main sintering chamber, but the temperature drop at this time has little effect on the sintering results.

The main sintering is carried out in a reducing atmosphere by maintaining a temperature of 780° C. for about 15 minutes, and then a normal cooling process is performed to complete the sintering.

The graph in Figure 2 shows the thermal history that sample ■ undergoes during this period.
The range ←B→ corresponds to the main sintering and cooling process.

For sample I sintered in this way, its radial crushing strength is J
Measurement according to IS z2507 (ISO2739) showed a high value of 26 Kyld.

On the other hand, the 5 steps of dewaxing of sample group
The sintering was carried out under the conditions that the temperature was raised to 400° C. at a temperature increase rate of 400° C., and the subsequent sintering was carried out in the same manner as in the case of sample ①, and the radial crushing strength was 19 edge mem 2.

Next, the groups of samples ■ and ■ are divided into samples ■ and ■, respectively.
This is a comparative example in which a part of the dewaxing conditions 5 was changed in the previous dewaxing method, that is, sample 1 is an example in which only the atmosphere in the dewaxing chamber was changed from reducing to oxidizing in the conventional dewaxing method, and sample 2 is an example in which the atmosphere in the dewaxing chamber was changed from reducing to oxidizing In the dewaxing method of the invention, only the temperature increase rate was lowered from 55° C./min to 30° C., and in both cases, the radial crushing strength is considerably lower than the above two examples.

Next, for the sample group (2), only the dewaxing temperature was increased from 400℃ to 700℃ under the same reducing atmosphere and temperature increase rate as before, and as the temperature was higher, the strength was also slightly higher than before. There is.

Furthermore, when the above five types of samples were sized and oil-impregnated into bearings, and a comparative test was conducted using a bearing performance tester, all of them showed almost the same characteristics, and no particularly significant difference was observed.

Therefore, we conducted further experiments to find the limiting conditions that would cause such a significant difference in radial crushing strength.We found that the influence of the heating rate became clearly significant at temperatures above 50°C per minute, and that the dewaxing temperature was 700°C or higher. Around ℃ is suitable, but 400 to 750
It was found that better results than the conventional method typified by sample ① could be obtained within the temperature range of ℃.

However, if the temperature is increased to 750° C. or higher, the oxide film on the powder surface will peel off, which will reduce the strength.

As is clear from the above results, according to the present invention, a bearing material with higher strength than the conventional product can be obtained, and if the strength is the same as the conventional product, a bearing material with a higher oil content can be obtained. The present invention greatly contributes to expanding the range of application of bronze-based wire-bonded oil-impregnated bearings.

Now, the structure of the present invention and its effects have been described in detail above, and finally, the principle of the operation and effect of the present invention will be considered.

First, if we focus only on the pure sintering phenomenon in a molded body in which no lubricant is present between powder particles, the subsequent sintering progresses better when the surface of the powder is oxidized to some extent.

This is because the oxide film is reduced during the sintering process in a reducing atmosphere and the grain surface becomes active.

Therefore, from this point of view, preheating and dewaxing in an oxidizing atmosphere is advantageous, but on the other hand, this method has the following disadvantages.

That is, the surface and internal pores of the molded body are covered with oxides generated by heating in an oxidizing atmosphere, which inhibits the evaporation and scattering of the lubricant from inside the molded body, and further reduces the lubricant composition. The carbon material therein remains in the form of soot, and as a result, the powders are not sufficiently diffused and sintered.

And this minus is greater than the plus due to activated sintering.

This is the main reason why molded bodies are conventionally preheated and dewaxed in a reducing atmosphere.

However, according to the present invention, it is possible to control the progress of both the reactions of dewaxing of the lubricant and oxidation of the molded body.

That is, when heating is performed in an oxidizing atmosphere under the combination of the dewaxing temperature and temperature increase rate described above, the dewaxing of the lubricant occurs first, and then the oxidation reaction of the molded article proceeds.

Therefore, complete removal of the lubricant and activated sintering are both achieved, and as a result, it is thought that a bearing material with higher strength than in the conventional method can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a one-hour temperature characteristic diagram showing the thermal history in the conventional sintering process, and FIG. 2 is a one-hour temperature characteristic diagram showing the thermal history in the sintering process of the present invention. A: 5 steps of preheating and defiltration, B: Main sintering process.

Claims (1)

[Claims] 1. A method for producing bearing materials by powder metallurgy using copper powder, tin powder, etc. as raw materials, in which copper powder, tin powder, and lubricant are mixed in a predetermined ratio and formed into a desired shape. After that, the lubricant is evaporated and removed by increasing the temperature to 400 to 750 °C in an oxidizing atmosphere at a rate of 50 °C or more per minute and holding it for a certain period of time, and then reducing the lubricant by keeping it at an even higher temperature. A method for manufacturing a bronze-based sintered bearing material, which is characterized by performing sintering for a certain period of time in a neutral atmosphere. 2. The method for manufacturing a bronze-based sintered bearing material according to claim 1, characterized in that the atmosphere is used as the oxidizing atmosphere.