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316L stainless steel vacuum brazing process
316L stainless steel has good corrosion resistance, oxidation resistance and high temperature mechanical properties and is widely used. The corrosion resistance and high temperature performance of nickel-based solder vacuum brazing joints are superior to other solders, especially suitable for vacuum brazing of stainless steel.
Generally adding B, Si, P and other elements to the solder can lower the melting point, and adding Cr and other elements can improve high temperature oxidation resistance.
The mechanical properties are lower due to the presence of more brittle compounds in the brazing joint. In order to reduce joint brittleness and improve strength, ingredients are added to the solder, in order to reduce the number of brittle compounds in the solder joint, improve joint performance, reduce the melting point of nickel-based solder, and promote solder wetting. Adding Cu to the solder can lower the melting point of the nickel-based phase. 316L stainless steel was vacuum brazed with BNi-7 + x% Cu solder, and the effects of Cu addition and vacuum brazing temperature on the microstructure and properties of the joint were studied.
1.316L stainless steel vacuum brazing test
316L stainless steel was used in the test, and the improved BNi-7 solder was made by mixing BNi-7 powder and pure Cu powder. The mass fractions of Cu in the improved solder were 0, 6% and 9%, respectively. A diagram of the connector assembly is shown below. The dimensions of the 316L stainless steel interface and strength analysis specimens are 20 mm×15 mm×3 mm and 40mm×15 mm×3 mm, respectively. Before welding, the surface of vacuum brazing was polished and the test piece was ultrasonically cleaned. The lap lengths of the interface and strength analysis specimens were 12mm and 10mm, respectively. A 100μm thick nickel foil is added to the brazing seam for gap control. The vacuum brazing temperature is 950 C and 980 C, the holding time is 10 min, and the test vacuum degree is lower than 8×10-3 Pa.
2.Stainless steel vacuum brazing test results and discussion
2.1 Joint interface structure
In order to analyze the interface microstructure of vacuum brazed joints, the Cu additions were 0 and 6%, respectively, under the conditions of vacuum brazing temperature of 980C and holding time of 10 min. From the composition and hardness analysis results of each phase, it can be seen that the joints are well bonded and there are no cracks and holes. The joint is mainly composed of irregular band-like reaction phase near the stainless steel side, large pattern-like eutectic structure and fine texture-like structure in the center of the brazing seam. The joint is mainly composed of Ni(Fe,Cr,Cu) solid solution at the stainless steel/brazing filler metal interface and Ni(Fe,Cr,Cu)-CrNiP eutectic structure and Ni,P-Ni (Fe,Cr,Cu) eutectic structure at the center of the brazing joint. crystal structure.
2.2 Effect of Cu addition amount and vacuum brazing temperature on joint interface structure
Similar to the effect of Cu addition, the vacuum brazing temperature increases, the thickness of the Ni-based solid solution at the interface of the brazing seam increases, the Ni(Fe,Cr, Cu)-CrNiP eutectic structure in the center of the brazing seam increases, and the tough Ni(Fe,Cu)-CrNiP eutectic structure in the eutectic structure increases. The number of Cr, Cu) solid solutions increases and the number of brittle compounds decreases.
2.3 Effect of Cu addition amount and vacuum brazing temperature on mechanical properties of joints
The figure below shows the change curve of the shear strength of stainless steel vacuum brazed joints under different Cu additions and vacuum brazing temperatures. It can be seen that with the increase of Cu addition, the shear strength of joints increases. Under the condition that other process parameters remain unchanged, the higher the vacuum brazing temperature, the higher the shear strength of the joint. When the vacuum brazing temperature is 980 C and the holding time is 10 min, the Cu addition is 9%, the shear strength of the joint is the highest, which is 118 MPa. The hardness of each reaction phase of the vacuum brazed joint of the improved nickel-based solder after adding Cu to the solder is lower than that of the vacuum brazed structure without adding Cu. With the increase of vacuum brazing temperature, the amount of Fe in the base metal dissolved into the filler metal increases, resulting in more solid solution phases at the center and interface of the brazing seam, and the amount of brittle compounds decreases, thereby increasing the mechanical properties of the joint.
3.316L stainless steel vacuum brazing conclusion
(1) The vacuum brazed joint consists of Ni( Fe, Cr, Cu) solid solution at the stainless steel/brazing filler metal interface and Ni( Fe, Cr, Cu)-CrNiP eutectic structure in the center of the brazing seam and NijP-Ni( Cr, Fe, Cu) eutectic structure composition.
(2) With the increase of Cu addition and vacuum brazing temperature, the thickness of nickel-based solid solution at the interface of the brazing seam increases, the Ni(Fe,Cr,Cu)-CrNiP eutectic structure in the center of the brazing seam increases, and the ductile phase Ni(Fe , Cr, Cu) content increased.
(3) When the vacuum brazing temperature is 980 C and the Cu addition is 9%, the maximum shear strength of the joint is 118 MPa.
4.Stainless steel vacuum brazing equipment selection
The vacuum brazing furnace produced by SIMUWU not only saves a lot of expensive metal flux, but also does not require complicated flux cleaning procedures, which reduces production costs. Aluminum, aluminum alloy, copper, copper alloy, stainless steel, alloy steel, low carbon steel, titanium, nickel, etc. can all be brazed in a vacuum furnace. Vacuum brazing improves product yield and results in a strong, clean work surface.
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