There are several types of vacuum brazing furnaces
1) hot fireplace
The brazing furnace adopts the form of vacuum chamber separated from heater, commonly known as hot fireplace. It is characterized by extracting the air from the container containing the brazing parts at room temperature and then pushing the container into the furnace. After brazing, the container can be cooled out of furnace, which shortens the production cycle and prevents the parent material grain from growing up. Most of the vacuum brazing in the hot fireplace requires the vacuum pump to continue working in the whole heat cycle. To remove the gas released by the working load. In terms of size and maximum heating temperature, the hot-wall vacuum furnace is limited by its ability to withstand high pressure at brazing temperature.
The use of this type brazing furnace temperature can be as high as 1150 ℃, in most of them are limited to 870 ℃ or lower. The heating furnace can use the common industrial electric furnace. This kind of vacuum container has no heating elements and heat insulation materials inside, which is not only simple in structure and easy to operate, but also releases less gas during heating, which is conducive to maintaining the vacuum. In order to improve the productivity, several brazing vessels are provided for brazing and cooling by alternately entering and discharging the furnace. Because this equipment investment is little, productivity is high. However, under high temperature and vacuum conditions, the container is subject to external atmospheric pressure and is easy to deform, so it is suitable for small production. Large hot fireplaces often have a double - vessel structure. Even the outer shell of the furnace is designed as a low vacuum vessel, but its application is limited due to the complexity of the structure.
2) cold fireplace
The vacuum chamber is established in the heating chamber, namely the heating furnace and the vacuum brazing chamber, commonly known as the cold fireplace. With this type of brazing furnace, the brazing parts must be cooled with the furnace after brazing, which limits the productivity. It has heat insulation material and electric heating elements. A vacuum chamber is usually water cooled. To concentrate the radiant heat on the workpiece and prevent heat loss to the furnace wall. A suitable heat shield (usually made of multilayer molybdenum, tungsten, silver, graphite or other high temperature materials) is arranged between the heating element and the furnace wall. The temperature can be as high as 2200 ℃, pressure can be lower than 1.35 x 10-4 pa.
The other cold - wall vacuum furnace wall is a double - layer water - cooled structure. Built-in heat reflection screen, it is composed of many layers of surface smooth thin metal plate. Molybdenum sheet or stainless steel sheet is used according to the temperature of furnace. Its function is to prevent the heat from radiating outwards, reduce the heat of furnace shell and improve the heating efficiency. Heating elements are distributed on the inner side of the reflector screen and different heating elements are selected according to the rated temperature of the furnace.
When the cold fireplace is working, the furnace shell is not hot due to the water cooling and the reflection screen shielding. It can withstand the outside atmospheric pressure well, so it is suitable for high temperature brazing of large welded parts. Its heating efficiency is also high, but it needs to vacuum before heating when it is used. After brazing, the welded parts can only be cooled with the furnace, and the furnace temperature drops slowly in the low temperature stage, so the productivity is low. However, if a two-chamber or multi-chamber continuous cold fireplace is used, the vacuum degree of the heating chamber will not be damaged. The welding parts can be continuously operated and the production rate can be improved.