JPS5839702A - Continuous sintering furnace in reduced pressure atmosphere - Google Patents

Abstract

PURPOSE:To provide a continuous sintering furnace in reduced pressure atmosphere which produces sintered bodies having good diemensional accuracy in high productivity by heating and scattering a lubricant from powder metallurgical moldings in a degassing chamber under reduced pressure, sintering said body in an adjacent sintering chamber under reduced pressure and cooling the bodies in an adjacent cooling chamber under reduced pressure. CONSTITUTION:A boat 21 contg. the above-described moldings contg. a lubricant used for the purpose of lubrication of dies is supplied through an inlet door 4 into a degassing chamber 1, where the moldings are heated to 500-700 deg.C to scatter the lubricant and gases; thereafter the moldings are discharged 8. The boat 21 is carried into the position enclosed by the heat insulating body 10 and heating elements 11 in an adjacent sintering chamber 2 under reduced pressure by evacuation 13 through an intermediate door 5, where the degassed moldings are sintered. The boat 21 contg. the sintered moldings is fed through an intermediate door 6 into a cooling chamber 3 under reduced pressure, where a cooling gas is introduced 20 until the infurnace pressure attains 700-800 Torr, whereby the moldings are cooled and the sintered articles are obtained. The respective chambers are so constituted as to maintain airtightness respectively independently by means of the intermediate doors 5, 6.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides continuous sintering for the purpose of obtaining a sintered body with excellent productivity and good dimensional accuracy when powder metallurgy products are sintered in a reduced pressure gas atmosphere below atmospheric pressure. This relates to a reduced pressure atmosphere sintering furnace. In general, a modified gas such as butane, ammonia decomposition gas, NL gas, H gas, or a vacuum atmosphere is used as the sintering atmosphere in powder metallurgy. There is a problem in that materials containing oxidizing elements 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.

As a means to solve this problem, a method was devised in which atmospheric gas is introduced while the furnace is evacuated, and sintering is performed in a reduced pressure gas atmosphere at atmospheric pressure. For products where controlling the amount of C is a problem, it is effective to introduce CO or H2 gas under reduced pressure and use these gases to accelerate the reduction reaction.In cases where evaporation of elements is a problem, It has been confirmed that it is effective to introduce a gas such as N:L or Ar.

However, since conventional reduced pressure atmosphere sintering furnaces are based on Noguchi type vacuum furnaces, the cycle time of product insertion → temperature rise → sintering → cooling → product removal is long, and the cycle time is usually long.
-It takes 5 hours.

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 vacuum furnace has a two-sided structure with only a side wall heating element, 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. The problem was that it was 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.

〔Example〕

FIG. 1 schematically shows the sintering furnace of the present invention.

1 in Figure 1 is the degassing chamber, and during operation the temperature is jOO~7000C.
is maintained at a temperature of The reason why the degassing chamber was provided here is that firstly, when inserting the product into the central sintering chamber 2, the heating element in the sintering chamber 1 is a preliminary chamber to prevent it from being exposed to the atmosphere and deteriorating. Second, 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, and the atmosphere It is preferable to perform degassing in advance, as contamination of the gas may further cause deterioration of the performance of the pump. Thirdly, by making the degassing chamber 1 and the sintering chamber 2 continuous, the heated Since preheating can be used effectively during sintering, 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. O
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 1: Degassing - Electrical energy required to heat the product to sinteringNext, the reason why the humidity in the degassing chamber was set at 500 to 700°C is that below 5000°C, the lubricant dispersed in the compact will scatter. On the other hand, there is a problem that the lubricant remaining during sintering in a reduced pressure or vacuum atmosphere comes out, which adversely affects the control of the sintering atmosphere.
This is because as soon as the temperature reaches 00@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. As the atmospheric gas, any non-oxidizing gas such as N, ammonia decomposition gas, exosamic gas, H gas, etc. may be used, but since the lubricant coming out of the compact becomes white smoke, it is preferable to use ammonia decomposition gas. From the viewpoint of environmental protection, it is preferable to burn the white smoke using a combustible gas such as gas lH and gas and discharge it outside the furnace.

To insert the boat, open the entrance door 4 and set it in the position of the boat 21 in the degassing chamber using an external rogue. When opening the entrance door 4 of the degassing chamber, the lubricant and gas exhaust port 8 at the top is closed with the lid 9, and the pressure inside the furnace is raised to a level slightly higher than qi to remove air from the atmosphere into the furnace. Avoid getting air in. 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 N or Ar gas to make the furnace pressure equal to that in the degassing chamber 1. In this state, the intermediate door 5 is opened. Next, use the Rogue set at the bottom of the degassing chamber to
1 is moved into the sintering chamber 2, and when the setting is completed, the intermediate door is closed, the gas introduction is stopped, the solenoid valve 14 is opened, and the inside of the furnace is evacuated using the vacuum pump 13. 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 inside the sintering chamber 2 is a heat insulating material.

11 is a heating element.

As another method for moving the bow i from the degassing chamber to the sintering chamber, an exhaust circuit using a vacuum pump is provided in the degassing chamber, and after the degassing is completed, the lid of the exhaust port 8 is closed and the solenoid valve 14' is opened. It is also possible to move the boat by opening the intermediate door 5 with the furnace interior evacuated using the two-row tube 16' and the sintering chamber also evacuated.

When the boat is set in the sintering chamber and reaches a predetermined temperature, the gas doujin circuit 19. Atmospheric gas is introduced through the solenoid valve 16, and the inside of the furnace is maintained at a predetermined reduced pressure by opening and closing the solenoid valve 14 of the exhaust circuit, and sintering is carried out.

Next, the structure of the heating element in the sintering chamber will be described.

In conventional sintering furnaces, the heating elements are limited to two sides, so the temperature inside the furnace varies widely, and the temperature at the top and bottom of the boat tends to be lower than the center of the boat. The normal temperature is about 1-200 Cj, and therefore there is a problem that the dimensional variation of the sintered body becomes large. Therefore, in order to improve temperature accuracy in this sintering furnace, we adopted a heating element structure as shown in Figure 3.

In Fig. 3, 31, 32, and 33 are carbon heating elements each having a T-plane structure, and 35, 33, respectively, are carbon heating elements having a T-plane structure.
It is heated by a current circuit and a power source shown at 6. For temperature control, the temperature is detected by a thermocouple 64, and a microcomputer 68 and a feedback circuit 67 control the temperature control 31 to maintain a predetermined temperature or temperature increase rate.
．． This is achieved by adjusting the power applied to the heating elements of the filters 2 and 33.

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, making it possible to obtain extremely high temperature accuracy. became. Table 2 shows the temperature variation in the furnace at a temperature of /2000C.

Next, the retention time of the article in the sintering chamber will be described. For example, when an article heated to 000C in a degassing chamber is sintered at /2000C, the sintering temperature is reached in about 30 minutes if the temperature increase rate is 20C/min.

If the holding time at the sintering temperature is 30 minutes, the holding time in the 4-stop 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.

Table 2 Temperature variation in the furnace The last sintered product is transferred to the cooling chamber 6. In this case, the inside of the furnace is evacuated by the vacuum pump 16 in both the sintering chamber 2 and the cooling chamber 6, and at that point The intermediate door 6 is opened and the boat 21 inside the sintering chamber is opened using the log provided in the cooling chamber.
Take it out and set it in the cooling room. Immediately after completing the setting, close the intermediate door 6 and close the gas introduction circuit 20. Cooling gas is introduced through the solenoid valve 17 until the furnace internal pressure reaches 700 to 710 Torr.

At this time, if it is desired to increase the cooling speed, after introducing the cooling gas, the cooling fan 12 is rotated to perform forced cooling with the gas fan. After cooling is completed, the pressure inside the furnace is set to /Ki,
When the exit door 7 is opened, the boat 21 inside the furnace is brought out by means of an external log. Through this cycle, degassing, sintering and cooling of the article are carried out continuously. however,
In actual continuous operation, it is necessary to empty the place where the boats are moved, so first the boat in the cooling room is moved outside, then the boat in the sintering room is transferred to the cooling room, and then degassed. It is necessary to move the boats of the chamber into the sintering chamber sequentially, the later boats first.

Finally, Table 3 shows the comparison results between the batch type furnace and the main sintering furnace regarding the time required to degas, sinter, and cool the Otsu Okg product.

From Table 3, it is clear that the productivity of this sintering furnace is much superior to that of the batch type.

[Brief explanation of the drawing]

FIG. 1 is a schematic explanatory diagram of the continuous reduced pressure ambient air sintering furnace of the present invention,
Figure 2 is a diagram of the degassing → sintering pattern of the electrical energy required to heat the product from degassing → sintering, (A) is for the conventional method, (→ is for the sintering of the present invention. ... Degassing chamber, 2... Sintering chamber, 6.1. Cooling chamber,
4... Entrance door, 5, 6... Intermediate door, 7... Exit door 98... Lubricant and gas exhaust port, 9... Exhaust port lid, 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...Baud), 3
1, 32, 33... Carbon heating elements at the front, center, and rear, respectively, 34... Thermocouple, 35... Current circuit for heating the heating element, 36... Heating electricity agent Patent attorney Kiyoshi Urata - Figure 1 on the left, Figure 2 on the tree

Claims (1)

[Claims] 1. A sintering furnace for sintering compacts formed by powder metallurgy in a reduced-pressure gas atmosphere below atmospheric pressure. A degassing chamber is provided to heat the lubricant mixed in the powder at a temperature range of 500°C to 700°C and scatter it from the compact, and the degassed compact is sintered under reduced pressure. sintering chamber,
and a cooling chamber for cooling the sintered product in a non-oxidizing atmospheric gas.The degassing chamber, the sintering chamber, and the cooling chamber are successively arranged from the product insertion side in this order, with a cooling chamber provided between each chamber. By using intermediate doors, airtightness can be maintained independently, and by limiting the amount of processing in each chamber to boats/cases, temperature and atmosphere control during sintering can be improved.
A continuous reduced pressure atmosphere sintering furnace characterized by obtaining sintered bodies with good dimensional accuracy. 2. The processing time of the product in each of the degassing chamber, sintering chamber, and cooling chamber, that is, the residence time of the boat filled with the processed material, is the same, and every time this processing time ends, the intermediate door is opened and the boat is closed. Continuous reduced pressure atmosphere according to claim 7, characterized in that the cycle of boat insertion → degassing → sintering → cooling → removal is performed continuously by sequentially moving to the next chamber using a ROGOO. Sintering furnace. 6. The heating element in the sintering chamber is arranged not only on the left and right sides, but also on the top and bottom, making it a four-sided structure.The heating element is divided into three parts in the direction of boat movement, and each part can be adjusted independently to adjust the temperature. A continuous reduced-pressure atmosphere sintering furnace according to claim 1 or 2, characterized in that temperature variation within the volume range occupied by the boat in the furnace is within 000 by controlling the furnace. . 4. 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, and the gas is introduced while drawing a vacuum inside the furnace. Claims 1 and 2 are 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 2 or 3. 5. The gas introduced into the sintering chamber is Co, N, , H,...
and Ar, and any one of them can be selected depending on the material of the object to be treated or the sintering conditions. Atmosphere sintering furnace. 6. In addition to installing a gas introduction circuit to introduce non-oxidizing gas into the cooling room, install seven or more fans in the room.
Claims 1, 2, 3, 4, or 5, characterized in that cooling with gas or forced cooling with a gas fan can be performed by rotating a fan after introducing gas. Continuous reduced pressure atmosphere sintering furnace. Z The non-oxidizing gas introduced into the cooling chamber is N! , H Yugas, and any of them can be selected depending on the cooling conditions. Claims 1, 2, and 3,
The continuous reduced pressure atmosphere sintering furnace according to item 1, 5 or 3.