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Stainless steel vacuum brazing process

The cleaning requirements before vacuum brazing of stainless steel are more stringent than that of carbon steel. This is because the oxides on the surface of stainless steel are more difficult to remove with flux or reducing atmosphere during brazing. Cleaning before vacuum brazing of stainless steel should include degreasing to remove any grease and oil film. The surface of the joint to be welded is also cleaned mechanically or with acid.

However, avoid brushing with wire brushes, especially carbon steel wire brushes. After cleaning, prevent dust, grease or finger marks from recontaminating the cleaned surface. The best approach is to vacuum braze the parts as soon as they are cleaned. If this is not possible, the cleaned parts should be transferred to a sealed plastic bag and kept until vacuum brazing. the
Stainless steel can be brazed by a variety of methods, such as soldering iron, flame, induction, furnace brazing and other methods. The furnace used for brazing in the furnace must have a good temperature control system and be able to cool down quickly.

When hydrogen is used as a shielding gas for vacuum brazing, the requirements for the purity of hydrogen depend on the vacuum brazing temperature and the composition of the base metal, that is, the lower the vacuum brazing temperature, the more stabilizers the base metal contains, and the lower the dew point of hydrogen is required . For example, for martensitic stainless steel such as 1Cr13 and Cr17Ni2, the hydrogen dew point is required to be lower than -40°C when vacuum brazing at 1000°C; , the hydrogen dew point is required to be lower than -25°C; but for the titanium stabilizer 1Cr18Ni9Ti, the hydrogen dew point must be lower than -40°C during vacuum brazing at 1150°C.

When using argon protection for vacuum brazing, high-purity argon is required. If the stainless steel surface is plated with copper or nickel, the requirement for the purity of the shielding gas can be reduced. During argon-protected vacuum brazing, in order to ensure the removal of the oxide film on the surface of stainless steel, gas flux can be used. Commonly used is argon-protected vacuum brazing with BF3 gas. When using self-fluxing solder containing lithium or boron, even if the surface of the stainless steel is slightly oxidized, it can ensure the spreading of the solder, thereby improving the quality of vacuum brazing.

When vacuum brazing stainless steel, the degree of vacuum depends on the vacuum brazing temperature.
The main process after vacuum brazing of stainless steel is to clean up residual brazing flux and residual choke agent, and perform heat treatment after vacuum brazing if necessary. Vacuum brazing of non-hardened stainless steel parts in a reducing or inert atmosphere does not require surface cleaning if flux is not used and flow blocking agent removal is not necessary.

Depending on the flux and vacuum brazing method used, residual flux can be removed by water flushing, mechanical cleaning or chemical cleaning. Sand or other non-metallic fine particles should be used if abrasives are used to clean the flux or oxide film in the hot zone near the vacuum brazed joint. Metal fine particles other than stainless steel cannot be used, so as not to cause rust spots or pitting corrosion.

Parts made of martensitic stainless steel and precipitation hardening stainless steel need to be heat treated according to the special requirements of the material after vacuum brazing. Diffusion treatment after vacuum brazing is often an indispensable process when vacuum brazing stainless steel with nickel-chromium-boron and nickel-chromium-silicon filler metals. Diffusion treatment can not only increase the maximum braze gap, but also improve the microstructure of vacuum brazed joints. For example, after vacuum brazing of stainless steel joints with BNi82CrSiBFe filler material and diffusion treatment at 1000 °C, although brittle phases still exist in the brazing joints, only chromium boride phases exist, and other brittle phases have disappeared. Moreover, the chromium boride phase is discontinuously distributed, which is beneficial to improve the plasticity of the joint.

Vacuum brazing furnace equipment selection: A process technology must be matched with excellent equipment to play a role, and the brazing furnace selected in vacuum brazing is very important. The RHVB vacuum high-temperature brazing furnace produced by SIMUWU is an excellent product for handling such processes. With its excellent temperature control accuracy and temperature uniformity, it can realize the brazing of the workpiece evenly and evenly, so as to achieve a high level of brazing level, reduce the scrap rate, increase the efficiency of production and reduce the cost.