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Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

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Improvement of Sealing Structure of Vacuum Furnaces

The vacuum furnace used for cemented carbide vacuum sintering has a design temperature of 1600°C, a volume of 50L, and a power of 50kW. The heating element is assembled with graphite rods, its working voltage is low (phase voltage<20V), and its working current is large (phase current>1000A). In order to ensure the working vacuum of the vacuum furnace and reduce vacuum leakage, the accessories installed on the furnace body, such as the inlet electrode and the thermocouple outlet device, are all designed with a sealing structure, but there are certain problems in the installation and use of the sealing structure.

1.Improvement of the electrode sealing structure

The incoming electrode (also called water-cooled electrode, copper electrode or incoming copper electrode) of the vacuum furnace is a conductive device that introduces electric energy to the electric heating element (graphite rod here) in the furnace. It is water-cooled and made of copper. It is connected with the graphite heating element in the furnace chamber, and it must ensure a good vacuum seal when passing through the furnace shell (the vacuum furnace shell adopts a cylindrical double-layer water-cooled structure); at the same time, the inlet electrode and the furnace shell should have good insulation performance. Therefore, a sealing structure is designed between the feed stage and the furnace shell.

(1)The original advanced electrode sealing structure

The original sealing structure of the inlet electrode of the vacuum sintering furnace (Fig. 1), there are two polytetrafluoroethylene (or bakelite) insulating sleeves between the inlet electrode and the electrode hole of the furnace shell, and the О-shaped rubber ring for sealing is placed on the two sides. between two insulating sleeves. During installation, keep the position between the inlet electrode and the furnace shell relatively fixed, gradually tighten the nuts outside the copper electrode, the О-shaped rubber sealing ring expands under the extrusion of the two insulating sleeves, and finally forms a gap between the inlet electrode and the electrode hole Completely sealed.

This sealing structure seems simple, but once the nut is tightened during installation, the inlet electrode will go out, accompanied by a little rotation, which will drive the graphite transition electrode in the furnace to rotate, and it is difficult to keep its position relatively fixed with the furnace shell , and the excessive displacement of the incoming electrode can easily damage the graphite transition electrode and graphite heating element in the furnace. The gap between the inlet electrode, the insulating sleeve and the electrode hole of the furnace shell in Figure 1 is very small, there is no room for adjustment, and the installation is more difficult. In the normal use process, although there is cooling water in the electrode, there is also carbon felt in the furnace to prevent heat radiation, and a small part of the insulating sleeve extending into the furnace is used under the joint action of conduction heat and remaining radiant heat. It will not take long to deform, the extrusion force on the О-shaped rubber seal ring is reduced, and eventually the seal is damaged, resulting in a vacuum leak.

(2)Improved electrode sealing structure

The improved sealing structure of the inlet electrode is shown in Figure 2. The fastening nuts are replaced by flanges, and the O-rings are moved outside the furnace shell. Flange 1 (welded on the furnace shell) is made of stainless steel (to prevent eddy current effects). From the perspective of insulation, flange 2 should be made of The polytetrafluoroethylene material is connected with a small insulating sleeve extending into the furnace shell into the electrode hole. The end face between flange 1 and flange 2, the gap between inlet electrode, flange 2 and flange 3 are all sealed with О rings. When this structure is fastened, it can ensure that the inlet electrode does not have any displacement. The thinner insulating sleeve increases the gap between the inlet electrode and the hole, and there is a large room for adjustment during installation. It is easy to realize the connection with the graphite transition electrode. Even if the insulating sleeve is deformed, it will not cause damage to the flange seal. After this improvement, it meets the requirements of installation and use, and improves the product quality.

2.Improvement of thermocouple sealing structure

As the temperature sensing element of the temperature measurement and temperature control device, the thermocouple is an important test device for the heating chamber of the vacuum furnace. The working temperature of the cemented carbide vacuum sintering furnace is above 1400°C, and at the same time, the precision of temperature control is required to be high, so the temperature detection in the furnace adopts expensive B-division (platinum-rhodium 30-platinum-rhodium 6) thermocouples. Therefore, it is not only necessary to ensure that the lead-out of the thermocouple wire meets the requirements of vacuum sealing, but also to pay attention to the convenience of use and reduce the cost.

(1)Original thermocouple sealing structure

The sealing structure of the original thermocouple (Figure 3), two small holes are drilled on the vacuum rubber sealing gasket and the polytetrafluoroethylene sleeve, the electrode wire (ψ0.5nn) on the thermocouple passes through the middle, and the outer tightness is tightened. Press the nut, and the rubber gasket is squeezed by the PTFE pad to form a seal. Regardless of whether the protective casing is used in this structure, the thermocouple wire is always exposed in the vacuum atmosphere, and is corroded by the carbon-containing atmosphere at high temperature, and the service life is quickly reduced. Practice shows that after sintering 1~2 furnaces of products, the thermocouple will be broken, which is uneconomical both from the perspective of thermocouple consumption and product production.

(2)Improved thermocouple sealing structure

The improved sealing structure of the thermocouple is shown in Figure 4. The corundum protective sleeve (ψ16nm) extends from the inside of the furnace to the outside of the furnace. The sleeve and the furnace shell are sealed with an О-shaped rubber sealing ring, and the thermocouple is directly inserted into the sleeve. The temperature can be measured inside the tube. At this time, the corundum sleeve isolates the vacuum in the furnace from the atmosphere outside the furnace, and the thermocouple is in the atmosphere, so it will not be damaged after long-term use. The disadvantage of this method is that although the corundum bushing can withstand high temperatures (above 1800°C), long-term thermal expansion and contraction will also cause fine cracks. If the heating element near the bushing is oxidized, it should be replaced in time.

Vacuum furnaces equipment selection: The vacuum heat treatment furnace produced by SIMUWU is a high-quality product for vacuum heat treatment process. Good temperature control accuracy and temperature control uniformity ensure the effective vacuum heat treatment process. The equipment is easy to manage and maintain, greatly reducing maintenance costs. SIMUWU company focuses on the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.