JPH0112801B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to vacuum sintering of iron-based sintered parts.
The present invention relates to a continuous vacuum sintering furnace which is characterized by excellent productivity and which improves the dimensional accuracy of the sintered product by increasing the accuracy of the sintering temperature. The sintering atmosphere for iron-based sintered parts is generally a modified gas such as butane, ammonia decomposition gas, N ₂ gas or H ₂ gas, but for products containing easily oxidizable elements such as Cr, There was a problem that products were easily oxidized because the reducing power was weak in other atmospheres except H ₂ gas. On the other hand, in the case of H ₂ gas, oxidation of the product can be prevented by keeping the dew point low, but there is a problem in that H ₂ gas is more expensive than other gases. In this respect, vacuum sintering has excellent reducing properties, does not use gas, and is an extremely advantageous method in terms of energy. However, since conventional vacuum furnaces are batch-type, the product is heated after being inserted into the furnace, held at the sintering temperature for a certain period of time, and then cooled down to a temperature at which the heating element does not deteriorate even when exposed to the atmosphere. Since there is a problem that the product cannot be taken out by opening the furnace lid unless the temperature has decreased to 100%, the cycle time is long and usually takes 4 to 5 hours. For this reason, there is a problem that it is difficult to deal with iron-based sintered machine parts that require mass production, and even if it could be solved, the product 1
There was a problem in that the cost was high because the processing time per piece was long. As a means to solve this problem, continuous sintering furnaces that can perform continuous processing have recently been put into practical use, but because the heating element is arranged in a two-sided structure with only the side walls or top and bottom surfaces, the temperature inside the furnace varies by about 20 degrees. ℃, and as a result, there is a problem that the dimensional variation of the products after sintering becomes large. In addition, since three to four cases of boat processing are carried out in the furnace, each time one boat is inserted into the sintering chamber, _N2 gas is introduced to bring it to atmospheric pressure, and then the sintering chamber and preliminary chamber are The structure is such that the intermediate door of the vacuum chamber is opened, and the vacuum atmosphere is interrupted each time, resulting in the problem that continuous atmosphere control is not possible. The continuous vacuum sintering furnace of the present invention was devised to solve these problems. Embodiments of the present invention will be described below based on the drawings. FIG. 1 schematically shows the sintering furnace of the present invention (hereinafter referred to as the present sintering furnace), and 1 is a degassing chamber which is maintained at a temperature of 500 to 700° C. during operation. The reason for providing the degassing chamber 1 here is, firstly, to prevent the heating element 11 in the sintering chamber from being exposed to the atmosphere and deteriorating when an article is inserted into the central sintering chamber 2. The vacuum sintering chamber 2 functions as a preliminary chamber, and the second reason is that the vacuum sintering chamber 2 performs degassing. It is preferable to perform degassing beforehand to avoid contamination and deterioration of pump performance. Thirdly, by making the degassing chamber 1 and the sintering chamber 2 continuous, it is possible to avoid heating during degassing. This preheating can be used effectively during sintering, so the energy required for heating during sintering can be saved accordingly. Table 1 compares the electric energy required for heating in this sintering furnace with the conventional method in which the sintering furnace is a batch type and degassing is performed in a separate furnace.
This is the value when heating kg. This shows that the present sintering furnace is superior to conventional methods in terms of energy savings as well.

[Table] Next, the reason why the temperature of the degassing chamber was set to 500 to 700℃ is that below 500℃, the lubricant dispersed in the compact is not sufficiently dispersed, and the lubricant remaining during sintering in a vacuum atmosphere However, once the temperature reaches 700°C, most of the lubricant is gone, and the effect will not change even if the temperature is increased further. It is from. Next, the atmospheric gas for degassing will be described. Reference numeral 18 in FIG. 1 is a degassing atmosphere gas introduction circuit. Any non-oxidizing gas such as N ₂ gas, ammonia decomposition gas, H ₂ gas, etc. may be used as the atmospheric gas, but since the lubricant coming out of the molded product becomes white smoke, it is preferable to use ammonia decomposition gas, H ₂ gas, etc. From an environmental standpoint, it is preferable to use combustible gas such as gas to burn the white smoke and discharge it outside the furnace. The above is the reason why a combustible gas is used as the atmospheric gas in the present invention. However, when using N ₂ gas mixed with ammonia decomposition gas (N ₂ + 3H ₂ ), the volume ratio occupied by H ₂ gas after mixing is 25%.
It is necessary to limit the amount of N ₂ gas mixed so that it is at least %. The reason for this is that when the volume ratio of H ₂ gas becomes 25% or less, the mixed gas becomes nonflammable and the lubricant cannot be burned. Specifically, when mixing N ₂ gas, the mixing amount needs to be 66% or less by volume of the whole. The above description was about a mixed gas of ammonia decomposition gas and _N2 gas, but the mixed gas ultimately becomes _{N2 gas} .
Since it consists of gas and H ₂ gas, it is also possible to use H ₂ gas instead of ammonia decomposition gas, or to use a mixed gas of H ₂ gas and N ₂ gas. In this case, naturally, the amount of H ₂ gas is required to be 25% or more by volume. The boat is inserted by opening the entrance door 4 and using an external loader to set the boat 21 in the degassing chamber 1. When opening the door 4, the upper exhaust port 8 is closed with a lid 9, and the pressure inside the furnace is kept slightly above 1 atm to prevent air from being drawn in from the atmosphere. boat 2
1 is set, the door 4 is closed, the lid 9 of the exhaust port 8 is opened, and the lubricant is discharged from the exhaust port 8 while being burned together with gas. The time for holding in the degassing chamber is determined by the sintering time of the next step, but from the viewpoint of sufficiently scattering the lubricant, it is preferably at least 30 minutes. Next, it is time to move to the sintering chamber, but before moving, first, the gas introduction circuit 19 and the solenoid valve 16 are connected to the sintering chamber.
N ₂ gas or Ar gas is introduced to make the pressure inside the furnace equal to or slightly higher than that of the degassing chamber, and in this state, the intermediate door 5 is opened. Next, the boat is moved into the sintering chamber using a loader set at the bottom of the degassing chamber, and when the setting is completed, the intermediate door 5 is closed, the gas introduction is stopped, the solenoid valve 14 is opened, and the vacuum pump 13 is used to move the boat into the sintering chamber. Make a vacuum inside. It is desirable that the time required during this time be as short as possible, so the gas flow rate from the gas introduction circuit 19 is increased and the exhaust capacity of the vacuum pump is also increased so that the movement can be completed within approximately 5 minutes. It is desirable to do so. Another method for moving the boat 21 from the degassing chamber 1 to the sintering chamber 2 is to add a vacuum pump 1 to the degassing chamber.
3', an exhaust circuit is provided using a solenoid valve 14', and after the degassing is completed, the lid of the exhaust port 8 is closed, and the vacuum pump 1 is closed.
It is also possible to move the boat 21 by opening the intermediate door 5 with the furnace interior evacuated at 3' and the sintering chamber also evacuated. In this case, the gas introduction circuit 19 and the solenoid valve 16 of the sintering chamber become unnecessary. Next, the structure of the heating element in the sintering chamber will be described. In conventional sintering furnaces, the placement of heating elements is limited to the sides or the top and bottom surfaces, so the temperature inside the furnace varies widely, and the temperature difference between the center and the ends of the boat becomes large. The temperature was about 20°C in width, which caused the problem of large dimensional variations in the sintered bodies. Therefore, in order to improve the temperature accuracy of this sintered body, we adopted the structure of the heating element as shown in FIG. 3. In Fig. 3, carbon heating element 31 (front part),
32 (center) and 33 (rear) have a four-sided structure, and are heated separately by a current circuit 35 and a power source 36. Temperature control is performed by detecting the temperature using a thermocouple 34, and by using a microcomputer 38 and a feedback circuit 37.
This is achieved by adjusting the power applied to the heating elements 31, 32, and 33 so that a predetermined temperature or temperature increase rate can be maintained. In other words, temperature variations in the vertical direction can be kept low by the heating elements on the upper and lower surfaces, and variations in the longitudinal direction can be kept low by dividing and controlling the heating elements into three zones.
It has become possible to obtain extremely high temperature accuracy. Table 2 shows the temperature variation inside the furnace at a temperature of 1200°C.

[Table] Next, the holding time of the items in the sintering chamber will be described. 1200 ℃ of goods heated to 600℃ in a degassing chamber
For example, when sintering at ℃, the temperature increase rate is set to 20
If the rate is °C/min, the sintering temperature will be reached in about 30 minutes. If the holding time at the sintering temperature is 30 minutes, the holding time in the sintering chamber will be 60 minutes. The holding time can be changed as appropriate by setting the heating rate, sintering temperature, and holding time at the sintering temperature. Finally, the sintered product is transferred to the cooling chamber. In this case, the cooling chamber is also evacuated by a vacuum pump, and at that point the intermediate door 6 is opened, and the The boat in the sintering chamber is taken out by a loader and set in the cooling chamber. Immediately after the setting is completed, close the intermediate door 6 and supply the cooling gas to the furnace pressure of 700 to 760 Torr using the gas introduction circuit 20 and solenoid valve 17.
Introduce until reaching . If you want to speed up the cooling rate at this time, after introducing the cooling gas,
Turn on the gas fan to perform forced cooling. Alternatively, if it is desired to further increase the cooling rate, the set boat is lowered into the oil tank 22 at the bottom of the cooling chamber using an elevator and oil quenching is performed. Once the oil quenching is complete, it is raised to its original position using an elevator and the oil is removed. When cooling is completed, the pressure inside the furnace is brought to atmospheric pressure, and then the exit door 7 is opened and the boat inside the furnace is taken out by an external loader. The cycles described above sequentially degas, sinter, and cool the article. However, when actually operating continuously,
Because of the need to empty the boats, first the boat in the cooling room was moved outside, then the boat in the sintering room was moved into the cooling room, and then the boat in the degassing room was moved into the sintering room. It is necessary to move the later boats first. Finally, Table 3 shows the comparison results between the conventional batch type furnace and the sintering furnace of the present invention regarding the time required to degas, sinter, and cool a 60 kg product. From Table 3, it is clear that the productivity of the present sintering furnace is much superior to that of the conventional batch-type furnace.

【table】 [Brief explanation of drawings]

Fig. 1 is a schematic explanatory diagram of an embodiment of the continuous vacuum sintering furnace of the present invention, Fig. 2 is a temperature pattern diagram of the electrical energy required to heat the product from degassing to sintering, and A is a diagram of the conventional method; is produced by this sintering furnace,
FIG. 3 shows the structure of a heating element for improving the temperature accuracy of this sintering furnace. 1... Degassing chamber, 2... Sintering chamber, 3... Cooling chamber, 4...
Entrance door, 5, 6... Intermediate door, 7... Exit door, 8... Lubricant and gas exhaust port, 9... Exhaust port cover, 10... Heat insulating material, 11... Heating element, 12... Cooling fan, 13,
13'...Vacuum pump, 14, 14', 15, 16,
17... Solenoid valve, 18, 19, 20... Gas introduction circuit, 21... Boat, 22... Oil tank, 31, 32, 3
3...Carbon heating element, 34...Thermocouple, 35...Power supply circuit for heating the heating element, 36...Heating element power supply, 37...Feedback circuit (temperature control, power control signal circuit), 38...Microcomputer.

Claims (1)

[Scope of Claims] 1. In a sintering furnace in which a compact formed by powder metallurgy is sintered in a vacuum atmosphere, the structure is such that it is added to the raw material powder for the purpose of mold lubrication during molding. A degassing chamber is equipped with a mechanism that heats the mixed lubricant and scatters it from the compact, then burns and discharges the scattered lubricant with combustion gas, and burns the degassed compact in a vacuum atmosphere. It consists of a sintering chamber for sintering and a cooling chamber for cooling the sintered product in a non-oxidizing atmosphere gas.The chambers are successively arranged in the order of degassing chamber → sintering chamber → cooling chamber from the product insertion side. A continuous vacuum sintering furnace characterized by having a structure in which airtightness can be maintained independently of each other by providing an intermediate door between them. 2. A patent claim characterized in that the heating elements in the sintering chamber are arranged on four sides: the upper and lower surfaces and the left and right side walls, and the heating elements are divided into three parts in the direction of movement of the boat, so that the temperature can be controlled separately for each part. The continuous vacuum sintering furnace according to item 1. 3. A gas introduction circuit for introducing non-oxidizing gas for cooling into the cooling chamber, and at least one or more fans in the upper part of the cooling chamber, and an oil tank in the lower part of the cooling chamber, and an elevator to transport the product to the oil tank. 3. A continuous vacuum sintering furnace according to claim 1 or 2, further comprising a device that can take in and out.