Problems encountered in vacuum brazing of stainless steel

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The Cr₂O₃ oxide film formed on the surface of stainless steel is quite stable and difficult to remove. To realize vacuum brazing of stainless steel, it is necessary to understand the removal mechanism of Cr₂O₃ oxide film in vacuum during the brazing process.

Stainless steel vacuum film removal mechanism

The vacuum reduces the oxygen partial pressure in the brazing area and helps to decompose oxides. However, the partial pressure of oxygen required for the decomposition of Cr₂O₃ is very low, making it impractical to rely solely on the degree of vacuum to decompose the oxide.

However, under low vacuum conditions, Cr₂O₃ will volatilize significantly when the temperature is higher than 1000°C.

At the same time, the carbon contained in stainless steel is enough to reduce and destroy the Cr₂O₃ oxide film on its surface under low vacuum at temperatures exceeding 900°C.

Therefore, during vacuum brazing of stainless steel, the combined effect of volatilization of oxides and reduction of carbon elements removes oxides on the surface of stainless steel, thereby enabling vacuum brazing.

Stainless steel vacuum brazing problems

It can be seen from the vacuum film removal mechanism of stainless steel that to remove oxides on the surface of stainless steel well, both vacuum degree and brazing temperature are very important, and the brazing temperature is generally required to be above 900°C.

However, in some industrial applications, due to various conditions, the vacuum brazing temperature of stainless steel is limited, lower than 900°C. At this time, the oxides on the surface of the stainless steel cannot be removed well, and the vacuum brazing is difficult to achieve.

Stainless steel vacuum brazing method

To realize medium-temperature vacuum brazing of stainless steel, it is necessary to solve the problem of oxide removal on the surface of stainless steel at medium temperature.

The simplest method is to add flux, which can be very effective in removing surface oxides. However, it is not recommended to add flux during vacuum brazing. The flux forms corrosive gas at high temperatures in the vacuum furnace, which easily corrodes the components in the vacuum furnace and the vacuum pump system, causing damage to the equipment. Therefore, never add flux to the vacuum furnace.

To achieve medium-temperature vacuum brazing of stainless steel, the following two methods can be considered:

Stainless steel surface with easy-brazing metal coating

Common ones are nickel plating or copper plating. After metal plating, there is no Cr2O3 oxide film on the surface, but an oxide film of Ni or Cu instead. Ni or Cu oxides are easily removed under vacuum conditions. When brazing, you can choose Ag-based solder to achieve a firm connection below 900°C.

Use active solder

Common active solder materials such as OKNA AgCuTi have a brazing temperature of about 850°C. During the brazing process, after the solder melts, the Ti element can react with the oxide on the surface of the stainless steel to achieve the connection between the solder and the stainless steel.

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