Brazing of Nitrogen-Contained Duplex Stainless Steel, UNS32550 using Ni Base Amorphous Filler Metal, MBF-50

摘要

Microstructure and mechanical property of 0.22wt% nitrogen-contained duplex stainless steel UNS32550 brazed in vacuum furnace of 10-4torr have been investigated as a function of bonding temperature and holding time. Amorphous alloy, MBF 50(Ni-19.5Cr-7.3Si-1.5B) was used as a filler metal. In the initial stage of bonding, eutectic phases are not formed but boron nitrides(BN) with spherical morphology and Cr nitrides with rod one were formed in the region of bonded interlayer. Cr nitrides and BN increased with holding time to 0.6ks and decreased with further holding time at 1473K. In case of 1498K, BN had the same tendency of 1473K. However, Cr nitrides continuously decreased with holding time. Isothermal solidification of the liquid filler metal rapidly occurred. It is made clear that the isothermal solidification process of this bonding is controlled by formation of boron nitrides than the diffusion process of depressant element (B and Si) in the base metal. Also, the phase transformation of base metals occurred, forming σ phase in the interface of ferrite/austenite, but decreased with increasing holding time. Furthermore, the volume fraction of ferrite increased with increasing bonding temperature and holding time. In addition, the shear strength of joints was approximately 25% of the tensile strength of base metals.