The vacuum furnace for vacuum sintering of tungsten carbide has a design temperature of 1600 °C, a volume of 50L and a power of 50kW. The heating element is assembled with graphite rods, and its working voltage is low (phase voltage<20V) and the working current is large (phase current: 1000A). In order to ensure the working vacuum degree of the vacuum furnace and reduce the vacuum leakage, the accessories installed on the furnace body, such as the inlet electrode and the thermocouple export device, are designed with a sealing structure, but the sealing structure has certain problems in installation and use.
1. Improvement of the sealing structure of the graphite electrode inlet electrode of the single crystal furnace graphite part of the vacuum furnace.
The inlet electrode of the vacuum furnace (also called the water-cooled electrode copper electrode or the inlet copper electric grade) is a conductive device that introduces electrical energy into the electric heating element (here the graphite rod) in the furnace, which is made of copper in a water-cooled way. It is connected to the graphite heating element in the furnace chamber, and a good vacuum seal should be ensured when passing through the furnace shell (the furnace shell of this vacuum furnace adopts a cylindrical double-layer water cooling structure); At the same time, the inlet electrode and the furnace shell should have good insulation properties. Therefore, a sealing structure is designed between the inlet stage and the furnace shell.
1.Original inlet electrode seal structure.
There are two PTFE (or bakelite) insulating sleeves between the inlet electrode and the furnace shell electrode hole, and the rubber ring for sealing is placed in the middle of the two insulating sleeves. During installation, keep the position between the inlet electrode and the furnace shell relatively fixed, and gradually tighten the nut on the outside of the copper electrode, and the rubber sealing ring expands under the extrusion of the two insulating sleeves, and finally forms a complete seal between the inlet electrode and the electrode hole.
This sealing structure seems simple, but as soon as the nut is tightened during installation, the inlet electrode will go outward, accompanied by a little rotation, thereby driving the graphite transition electrode in the furnace to rotate, and it is difficult to keep the position between it and the furnace shell relatively fixed, and the displacement of the inlet electrode is too large and it is easy to damage the graphite transition electrode and the graphite heating element in the furnace. The gap between the inlet electrode and the electrode hole of the furnace shell is very small, there is no room for adjustment, and the installation is difficult. In the normal use process, although there is cooling water in the inlet electrode, there is also carbon felt in the furnace to prevent heat radiation, a small part of the insulating sleeve extending into the furnace will be deformed in a short time under the combined action of conductive heat and remaining radiant heat, and the extrusion force on the rubber sealing rubber ring is reduced, and the final seal is damaged, resulting in vacuum leakage.
2.Improved inlet electrode seal structure.
The nut for fastening is replaced with a flange, the O-ring is moved to the outside of the furnace shell, and the flange 1 (welded to the furnace shell) is made of stainless steel material (to prevent eddy current effect), and from the perspective of insulation, the flange 2 should be made of polytetrafluoroethylene material, and a small section of insulating sleeve is attached to it and extends into the electrode hole of the furnace shell. The end face between flange 1 and flange 2, the gap between inlet electrode flange 2 and flange 3 are all sealed with rings. This structure ensures that there is no displacement of the inlet electrode when it is tightened. The thinner insulating sleeve increases the gap between the inlet electrode and the hole, and the adjustment room is large during installation, which makes it easy to realize the connection between the graphite transition electrode, and even if the insulating sleeve is deformed, it will not cause the damage of the flange seal. Through this improvement, it meets the requirements of installation and use, and improves product quality.
Second, the improvement of thermocouple sealing structure.
As a temperature sensing element of temperature measurement and temperature control device, thermocouple is an important test device in the heating chamber of vacuum furnace. The working temperature of the cemented carbide vacuum sintering furnace is above 1400, and the accuracy of temperature control is high, so the temperature detection in the furnace adopts the most expensive B index (platinum rhodium 30 - platinum rhodium 6) thermocouple. 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 costs.
1 Original thermocouple seal structure.
Two small holes are drilled on the vacuum rubber gasket and PTFE sleeve, and the electrode wire on the thermocouple (0.).5nn) pass through the middle, tighten the outer compression nut, and the rubber gasket is squeezed by the PTFE gasket to form a seal. Regardless of whether a protective sleeve is used in the furnace, the thermocouple wire is always exposed to a vacuum atmosphere, and is corroded by a carbonaceous atmosphere at high temperatures, and its life is quickly reduced. Practice shows that when sintering 1 2 furnace products, the thermocouple will be broken, which is uneconomical both from the consumption of thermocouples and from the perspective of product production.
2.Improved thermocouple seal structure.
The corundum protective sleeve (16nm) extends from the furnace to the outside of the furnace, and the rubber sealing ring is used between the sleeve and the furnace shell, and the thermocouple is directly placed in the sleeve to measure the temperature. At this time, the corundum sleeve isolates the vacuum in the furnace from the atmosphere outside the furnace, and the thermocouple is in the atmospheric environment, and it will not be damaged for long-term use. The disadvantage of this method is that although the corundum casing can withstand high temperatures (above 1800), long-term thermal expansion and contraction will also cause it to produce fine cracks, and the heating element near the casing should be replaced in time if it is observed to oxidize.