Microstructure Evolution and Precipitates Transformation in a Duplex Stainless Steel Bond Brazed with a Ni-B-Si Foil

摘要

MBF-35 (Ni-7.3Si-2.2B (wt.%)) was selected as a filler alloy for brazing of a duplex stainless steel conducted at the temperature range of 1060-1090 ℃ with various times from 60 s to 3600 s. The experimental results revealed that there was no significant grain growth of ferrite and austenite in the base metal. The precipitation of σ phase was not found in any of the brazed bonds. The results also indicated, prior to completion of isothermal solidification, BN and Ni3Si appeared in the brazing-affected zone and the bonded interlayer, respectively. The noted enrichment of Ni and Fe was detected in the brazing-affected zone. At 1060 ℃ and 1090 ℃ after 2700 s and 900 s, respectively, the isothermal solidification of the melt was completed at the center of the joints. The brazing was finished within 1200 s and 3600 s, respectively. The SEM and EPMA results also showed the transformation of secondary-phase precipitates from BN to BN and (N, Mo) boride and further to BN at the interface with increasing holding time. This transformation appeared to be largely due to the change in N composition with increase of holding time at the interface. BN precipitate was not removed in terms of the increased holding time at the bonding temperature. The formation of Ni3Si and BN was responsible for the appearance of hardness peak values in the joint area.