Brazing process, microstructure and properties of SUS444/SUS304 joints based on Fe-Cr-Ni based brazing filler metal

A new Fe-Cr-Ni based brazing filler metal was used for vacuum brazing of SUS444/SUS304 dissimilar stainless steels. Diffusion bonding mechanism of lap joints was researched, and effects of brazing temperature and lap gap on microstructure and room temperature shear strength were investigated. Finally, immersion corrosion behavior of T-joints in acid medium for different periods of time was analyzed. The experiment showed that during brazing process, diffusion effect of brazing filler metal on SUS444 base metal side was more obvious, and there existed a wider element diffusion zone. Brazing seam was mainly composed of pre-solidified Fe(Cr,Ni,Si) solid solution in the interface zone on both sides and eutectic microstructure formed by (Fe,Cr,Ni)₃P intermetallic compound and Fe(Cr,Ni,Si) solid solution in the center of brazing seam. Brazing filler metal was sensitive to brazing temperature. If the temperature was too high, it was easy to produce false brazing holes, resulting in a significant decrease of shear strength. Brazing filler metal was less sensitive to lap gap and could meet requirements of brazing with large gap. The maximum shear strength reached 177.3 MPa under conditions of brazing temperature of 1 125 ℃ and lap gap of 35 μm. Corrosion of T- joints preferentially occurred in Fe(Cr,Ni,Si) solid solution of eutectic structure at brazing corner. After immersion for 288 h, corrosion solution eroded to interface area of brazing corner and passed through pre-solidified solid solution to make galvanic corrosion of base metal.