Principle and application of stainless steel vacuum brazing

Vacuum brazing technology can be used to connect workpieces of different types, special shapes, and complex structures. The joints have better comprehensive performance, high brazing productivity, and easy to realize automated production. Therefore, vacuum brazing technology has developed into a very promising welding technology since the 1940s. It has been rapidly developed and widely used in the world’s industrialized countries. It is one of the most active and most promising professional fields in recent years. Vacuum brazing technology first appeared in the electronics industry to braze copper and stainless steel parts, and later it was applied to the aviation industry. Stainless steel materials are widely used in the petrochemical field due to their good corrosion resistance.

1 Principle of vacuum brazing of stainless steel
Vacuum brazing refers to raising the temperature to the melting temperature of the brazing material under a vacuum protective atmosphere, and using the solidification of the liquid phase brazing material and the diffusion between the brazing material and the base material to achieve the connection between the metals. The remarkable feature of brazing is that the brazing temperature is lower than the melting point of the base material. Compared with other welding methods, it has little effect on the base material and no heat-affected zone will appear.
During brazing, the wetting, spreading and flow process of the brazing material on the base material plays an extremely important role in the brazing quality. If the brazing material does not wet the base material, brazing cannot be achieved; if the brazing material does not spread, flow and fill the gap on the base material, it is impossible to form a good brazing joint. There is always a thin layer of oxide film on the surface of stainless steel, which is the main obstacle for the molten brazing material to wet the base material. Therefore, the removal of the oxide film has an important influence on the brazing quality.

2 Brazing materials for vacuum brazing of stainless steel
The brazing materials used for vacuum brazing usually include Au-based brazing materials, Ag-based brazing materials, Cu-based brazing materials and Ni-based brazing materials. When selecting brazing materials, they are often selected based on their corrosion resistance in special media and their applicability at known operating temperatures.

3 Vacuum brazing equipment for stainless steel
Vacuum brazing equipment mainly consists of a vacuum furnace and a vacuum system. The vacuum furnace consists of a vacuum chamber, a heater, a control system, etc. The vacuum system mainly includes a vacuum unit, a vacuum pipeline, a vacuum valve, etc. The vacuum unit usually consists of a rotary vane mechanical pump and an oil diffusion pump. To obtain a high vacuum, an oil diffusion pump must be used to make the vacuum degree reach 1. 0X10²Pa.

4 Vacuum brazing process of stainless steel
For vacuum brazing of stainless steel, optimization of brazing process parameters is the guarantee for obtaining high-quality brazed joints. The main processes of vacuum brazing of stainless steel include brazing temperature, holding time, brazing gap and post-diffusion treatment.
The brazing temperature is the key to determine product quality. It must ensure that the brazing material can fully liquefy and flow to the greatest extent, and that the risk of deformation of the base material is reduced to a minimum. Depending on the chemical composition of the brazing material, the melting temperature of the brazing material is also different. Generally, it should be higher than the solidus line of the brazing material, but lower than the liquidus line, to ensure that the melting, fluidity and wettability of the brazing material are in the best state at the brazing temperature.
The vacuum degree of the vacuum brazing furnace is also one of the main reasons affecting product quality. It effectively reduces the oxygen partial pressure in the furnace, breaks the oxide film on the surface of the component, and prevents the component from re-oxidizing under high vacuum. Therefore, the vacuum degree of the vacuum furnace is generally required to be above 1X10²Pa.

5 Development Trend of Vacuum Brazing Technology for Stainless Steel
Vacuum brazing technology is widely used in many cutting-edge sectors due to its own advantages. In recent years, with the development of science and technology, it has been more and more widely used in civil industry. With the rapid development of household appliance industry, especially refrigeration industry, including refrigeration equipment, such as refrigerators, air conditioners and their three-way and four-way valves, etc., and automobile industry and electronics industry, vacuum brazing technology is more and more widely used.