Brazing of UNS S32101 and UNS S32304 lean duplex stainless steels and UNS S32750 super duplex with AWS A5.3 BNi-7 (Ni–Cr–P) filler metal

Abstract

Due to their corrosion resistance, duplex stainless steels are widely applied in various industries in which corrosion resistance is a required property. Unlike ferritic and austenitic stainless steels, duplex stainless steels have superior mechanical strength and can be used to produce lighter equipment. When thermally cycled, the properties of these steels can change, especially when using joining processes such as brazing, and when the austenite/ferrite volume fraction becomes imbalanced. This study aims to describe the brazing of lean duplex and superduplex stainless steels, verifying the effect of the joint gap and brazing time on the formation of continuous intermetallics inside the brazed joint. In this study, samples of duplex stainless steel UNS S32101, UNS S32304 and UNS S32750 were brazed in a continuous hydrogen atmosphere furnace, using BNi-7 (Ni–Cr–P) filler metal for brazing at a temperature of 1100°C, for 12 and 32 min, using joint gaps of 0.0 and 0.3 mm, followed by in-furnace cooling. The samples were characterized using optical microscopy, scanning electron microscopy with energy dispersion and X-ray diffraction. The results showed that the best condition for the in-furnace brazing used was a joint gap of 0.0 mm. Both of the joint gaps had different microstructures, yet the joint gap of 0.0 mm showed a continuous centred face cubic phase rich in nickel, which is frequently called γ-Ni. This region is composed of intermetallic phases rich in phosphorus. Sigma phase precipitation in the base metal on the duplex stainless steel UNS S35750 (32750?) was observed in the brazing conditions studied.