Vacuum Brazing Furnace Type Characteristics

1) Hot fireplace

Brazing furnace adopts the form of separate vacuum chamber and heater, commonly known as hot fireplace. It is characterized by pumping air out of the container with brazing parts at room temperature, then pushing the container into the furnace. When brazing parts are heated in the furnace, vacuum pumping and heating are carried out simultaneously. After brazing, the container can be cooled out of the furnace, which shortens the production cycle and prevents the grain growth of the base metal. Vacuum brazing in hot wall furnace mostly requires vacuum pump to work continuously in the whole thermal cycle. To remove the gas released from the working load. In terms of size and maximum heating temperature, the hot-wall vacuum furnace is limited by the ability to withstand high pressure at brazing temperature.

The service temperature of this type of brazing furnace can be as high as 1150 C, which is limited to 870 C or lower in most cases. Heating furnace can use ordinary industrial electric furnace. This kind of vacuum vessel has no heating original parts and heat insulation materials. It is not only simple in structure and easy to operate, but also releases less gas during heating, which is conducive to maintaining vacuum. In order to improve productivity, several brazing containers are equipped to braze and cool the furnace gallbladder alternately. Therefore, equipment investment is low and productivity is high. However, the container is easily deformed under the action of external atmospheric pressure under high temperature and vacuum conditions, so it is suitable for small-scale production. The double container structure is often used in large-scale hot fireplace. The shell of the furnace is also designed as a low vacuum vessel, but its application is limited by the complexity of the structure.

2) Cold fireplace

Vacuum chamber is built in heating chamber, that is, heating furnace and vacuum brazing chamber are integrated, commonly known as cold fireplace. With this type of brazing furnace, the brazing parts must be cooled with the furnace after brazing, which limits the productivity. Thermal insulation materials and electric heating elements are included. Vacuum chambers are usually water-cooled. In order to concentrate the radiation heat on the workpiece and prevent the heat from dissipating to the furnace wall. A suitable heat shield (usually made of multi-layer molybdenum, tungsten, silver, graphite or other high temperature materials) is placed between the heating element and the furnace wall. The temperature can be as high as 2200 C and the pressure can be lower than 1.35 *10-4Pa.

Another type of cold-wall vacuum furnace has a double-layer water-cooling structure. Built-in heat reflecting screen, it is composed of thin metal plate with smooth multi-layer surface. According to the different temperature of furnace, molybdenum or stainless steel sheet is selected. Its function is to prevent heat from radiating outward, reduce heating of furnace shell and improve heating efficiency. Heating elements are distributed inside the reflecting screen, and different heaters are selected according to the rated temperature of the furnace.

When the cold hearth furnace is working, the temperature of the shell is not high because of water cooling and shielding by reflecting screen. It can withstand the external atmospheric pressure well, so it is suitable for high temperature brazing of large weldments. Its heating efficiency is also high, but it needs to be vacuum reheated before use. After brazing, the welding pieces can only be cooled with the furnace, and the temperature of the furnace decreases slowly in the low temperature stage, so the productivity is low. However, if two or more chambers continuous cold hearth furnace is adopted, the vacuum degree of heating chamber is not destroyed, and the filling, brazing, cooling and discharging of welding parts can be operated continuously, and the productivity can be improved.

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