JPH0128083B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention provides excellent productivity when sintering powder metallurgy products in a reduced pressure gas atmosphere below atmospheric pressure.
Furthermore, the present invention relates to a continuous reduced-pressure atmosphere sintering furnace for the purpose of obtaining a sintered body with good dimensional accuracy. Generally, the sintering atmosphere in powder metallurgy is a metamorphic gas such as butane, ammonia decomposition gas, _N2
Gas, H ₂ gas, or a vacuum atmosphere is used, but there is a problem that materials containing easily oxidizable elements such as Cr and Mn are easily oxidized in the above gas atmosphere other than a vacuum atmosphere. In this regard, sintering in a vacuum atmosphere has excellent reducing properties, but with materials that contain carbon, the reduction progresses as the carbon reacts with oxygen in the product, making it difficult to control the amount of carbon in the product. On the other hand, in the case of materials containing elements with high vapor pressure, there is a problem that they tend to scatter in a vacuum. The idea was devised as a means to solve this problem.
This is a method in which atmospheric gas is introduced while the furnace is evacuated, and sintering is performed in a reduced-pressure gas atmosphere under atmospheric pressure. For products where controlling the amount of C is an issue, it is effective to introduce CO or H ₂ gas under reduced pressure and use these gases to accelerate the reduction reaction, and evaporation of the element is a problem. In such cases, N ₂ ,
It has been confirmed that introducing a gas such as Ar is effective. However, since conventional reduced-pressure atmosphere sintering furnaces are based on batch-type vacuum furnaces, the cycle time of product insertion → temperature rise → sintering → cooling → product removal is long, and usually takes 4 to 5 hours. are doing. For this reason, there was a problem in production capacity that it was difficult to handle iron-based sintered machine parts that required mass production. Moreover, because the conventional batch-type vacuum furnace has a two-sided structure in which the heating element is only on the side wall or the top and bottom, the temperature variation inside the furnace is as large as 20°C, and the dimensional accuracy of the sintered body is poor, requiring high precision. This method has the problem of being difficult to apply to sintered machine parts. The continuous reduced pressure atmosphere sintering furnace of the present invention was devised to solve these problems. Embodiments of the present invention will be described below with reference to the accompanying drawings. Embodiment FIG. 1 schematically shows a sintering furnace of the present invention. 1 in Figure 1 is the degassing chamber, which is heated to a temperature of 500 to 700℃ during operation.
is maintained at a temperature of The reason for providing the degassing chamber here is that firstly, when inserting the product into the central sintering chamber 2, it is a backup to prevent the heating element 11 in the sintering chamber from being exposed to the atmosphere and deteriorating. Second, performing degassing in a vacuum or reduced pressure atmosphere sintering chamber means that some of the lubricant coming out of the compact adheres to the furnace wall and inside the vacuum pump. Thirdly, it is preferable to perform degassing beforehand to avoid contamination of the atmosphere and deterioration of pump performance.
By making the degassing chamber 1 and the sintering chamber 2 continuous, the preheat heated during degassing can be effectively used 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 Kg is heated. This shows that the present sintering furnace is superior to conventional methods in terms of energy savings.

[Table] Next, the reason why the temperature of the degassing chamber was set at 500 to 700℃.
At temperatures below 500°C, the lubricant dispersed in the compact is not sufficiently dispersed, and the remaining lubricant comes out during sintering in a reduced pressure or vacuum atmosphere.
This is because there is a problem that it has an adverse effect on the control of the sintering atmosphere, and on the other hand, when the temperature reaches 700°C, most of the lubricant is gone, and the effect will not change even if the temperature is raised further. 18 in FIG. 1 is a degassing atmosphere gas introduction circuit, and 15 is a solenoid valve. Atmospheric gas is N ₂ ,
Ammonia decomposition gas, exosamic gas, _H2
Any non-oxidizing gas such as gas may be used, but since the lubricant coming out of the molded product will turn into white smoke, if possible, use a combustible gas such as ammonia decomposition gas or _H2 gas to burn off the white smoke. It is preferable from an environmental point of view to discharge the waste outside the furnace. To insert the boat, open the entrance door 4 and use an external loader to insert the boat 2 into the degassing chamber.
Set to position 1. When opening the inlet door 4 of the degassing chamber, close the upper lubricant and gas exhaust port 8 with the lid 9, and keep the pressure inside the furnace slightly above 1 atm to remove air from the inside of the furnace. Avoid letting air into the After the boat 21 is set, the door 4 is closed, the exhaust port cover 9 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 1 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 or more. Next, we move to the sintering chamber 2, but before moving, we first open the solenoid valve 16 from the gas introduction circuit 19 into the sintering chamber to introduce _N2 or Ar gas to bring the pressure inside the furnace to the degassing chamber 1. Make it the same or slightly higher, and open the intermediate door 5 in this state. Next, the boat 21 is moved to the sintering chamber 2 using a loader set at the bottom of the degassing chamber.
When the setting is completed, close the intermediate door, stop the gas introduction, open the solenoid valve 14, and turn on the vacuum pump 1.
Vacuum the furnace in Step 3. Since it is desirable that the time required during this time be as short as possible, the introduced gas flow rate adjustment range should be wide, and the structure should be designed so that the flow rate can be changed from a small amount when introducing atmospheric gas to a large amount when moving the boat. It is desirable that the evacuation capacity of the vacuum pump 13 is also increased so that the movement can be completed within approximately 3 minutes. Note that 10 shown in the sintering chamber 2 is a heat insulating material, and 11 is a heating element. Another method for moving the boat from the degassing chamber to the sintering chamber is to install an exhaust circuit using a vacuum pump in the degassing chamber, and after degassing, close the lid of the exhaust port 8, open the solenoid valve 14', and release the vacuum. It is also possible to move the boat by opening the intermediate door 5 with the furnace interior evacuated by the pump 13' and the sintering chamber also evacuated. When the boat is set in the sintering chamber and reaches a predetermined temperature, atmospheric gas is introduced through the gas admission circuit 19 and solenoid valve 16, and the interior of the furnace is adjusted to a predetermined temperature by opening and closing the solenoid valve 14 in the exhaust circuit. Sintering is carried out under reduced pressure. 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 side walls or two surfaces, the top and bottom, resulting in large temperature variations within the furnace.There is a large temperature difference between the center and end of the boat, and the temperature range is The temperature was around 20°C, which caused the problem of large dimensional variations in the sintered bodies.
Therefore, in order to improve the temperature accuracy in this sintering furnace, we adopted the structure of the heating element as shown in Figure 3. In FIG. 3, reference numerals 31, 32, and 33 are carbon heating elements each having a four-sided structure, and are energized by current circuits and power sources shown separately at 35 and 36, respectively. Temperature control is thermocouple 34
This is achieved by detecting the temperature using the microcomputer 38 and the feedback circuit 37, and adjusting the power applied to the heating elements 31, 32, and 33 so as to maintain a predetermined temperature or temperature increase rate. In other words, temperature variations in the vertical direction can be kept low by the heating elements on the top and bottom surfaces, and variations in the front and back directions can be kept low by dividing and controlling the heating elements into three zones, resulting in extremely high temperature accuracy. It became possible. Table 2 shows the temperature variation inside the furnace at a temperature of 1200°C. Next, the retention time of the article 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, the sintering temperature, and the holding time at the sintering temperature.

[Table] The last step is to move the sintered product to the cooling chamber 3. In this case, the inside of the furnace is evacuated by the vacuum pump 13 in both the sintering chamber 2 and the cooling chamber 3, and at that point the intermediate door is closed. 6 is opened, and the boat 21 in the sintering chamber is taken out by a loader provided in the cooling chamber and set in the cooling chamber. Immediately after the setting is completed, the intermediate door 6 is closed, and cooling gas is introduced from the gas introduction circuit 20 and the solenoid valve 17 until the furnace internal pressure reaches 700 to 760 Torr. At this time, if it is desired to increase the cooling rate, after introducing cooling gas, the cooling fan 12 is turned to perform forced cooling with a gas fan. When the cooling is finished, the pressure inside the furnace is set to 1 atmosphere, and then the exit door 7 is opened and the boat 21 inside the furnace is taken out by an external loader. Through such cycles, the article is continuously degassed, sintered, and cooled. However, in actual continuous operation, the boats in the cooling room must first be moved outside, and then the boats in the sintering room must be moved into the cooling room. It is necessary to move the boats of the degassing chamber into the sintering chamber sequentially, the latter boats first. Finally, Table 3 shows the comparison results between a batch type furnace and a full-scale sintering furnace regarding the time required to degas, sinter, and cool a 60 kg product. From Table 3, it is clear that the productivity of this sintering furnace is much superior to that of the batch type.

[Table] * In the case of batch type, the carbon heating element
It takes time for the material to reach a low temperature that does not deteriorate even when exposed to the atmosphere.

[Brief explanation of drawings]

Fig. 1 is a schematic explanatory diagram of the continuous reduced pressure atmosphere sintering furnace of the present invention, Fig. 2 is a diagram of the degassing → sintering pattern of the electrical energy required to heat the product from degassing → sintering, and In the case of the method, B shows the case of the sintering furnace of the present invention, and FIG. 3 is an explanatory diagram of the structure of a heating element for improving the temperature accuracy of the sintering furnace of the present invention. 1...Degassing chamber, 2...Sintering chamber, 3...Cooling chamber, 4...Inlet door, 5, 6...Intermediate door, 7...Exit door, 8...Lubricant and gas exhaust port , 9...Exhaust port cover, 10...Insulating material, 11...Heating element, 12
...cooling fan, 13,13'...vacuum pump,
14, 14', 15, 16, 17...Solenoid valve, 1
8, 19, 20...Gas introduction circuit, 21...Boat, 31, 32, 33...Carbon heating elements at the front, center, and rear, respectively, 34...Thermocouple, 3
5... Current circuit for heating the heating element, 36... Power supply for the heating element, 37... Feedback circuit, 38... Microcomputer.

Claims (1)

[Claims] 1. A sintering furnace in which a compact formed by powder metallurgy is sintered in a reduced pressure gas atmosphere below atmospheric pressure, the structure of which is designed to lubricate the mold during molding. A degassing chamber is equipped with a mechanism that scatters the lubricant mixed into the raw material powder from the molded body and burns the lubricant using combustion gas, and the degassed molded body is sintered under reduced pressure. It consists of a sintering chamber for sintering, a cooling chamber for cooling the sintered article in a non-oxidizing atmosphere gas, and a degassing chamber from the article insertion side.
A continuous reduced-pressure atmosphere sintering furnace characterized in that a sintering chamber and a cooling chamber are successively arranged in that order, and each chamber can be kept airtight independently by an intermediate door provided between each chamber. 2 The heating elements in the sintering chamber are arranged not only on the left and right sides, but also on the top and bottom, making it a four-sided structure.The heating elements are divided into three parts in the direction of boat movement, and the temperature is controlled independently for each part. A continuous reduced-pressure atmosphere sintering furnace according to claim 1. 3 The sintering chamber is equipped with an exhaust circuit that uses a vacuum pump to exhaust the gas in the furnace, and a gas introduction circuit that introduces non-oxidizing atmospheric gas, so that the gas is introduced while drawing a vacuum inside the furnace. Claim 1, characterized in that the inside of the furnace is maintained at a predetermined pressure below atmospheric pressure by a solenoid valve provided in the middle of the exhaust circuit, and the article is sintered in the reduced pressure gas atmosphere. The continuous reduced pressure atmosphere sintering furnace according to item 1 or 2. 4 Claim 1, characterized in that a gas introduction circuit for introducing non-oxidizing gas into the cooling chamber is provided, and one or more fans are installed in the room.
The continuous reduced pressure atmosphere sintering furnace according to item 1, 2 or 3.