Duplex stainless steels, due to their superior corrosion resistance, have wide application in various industries where corrosion resistance is an important issue. Equipment made of theses stainless steels are lighter than equipment produced with conventional ferritic and austenitic stainless steels, due to superior mechanical properties of the duplex stainless steels. These steels can have their properties changed when thermally cycled, especially employing joining processes, such as brazing, and when the austenite/ferrite balanced microstructure is changed. The objective of this work is to study the brazing of lean duplex and superduplex, analyzing the effect of the joint gap and the brazing time on the formation of continuous intermetallic phases in the brazed joint. In this work, samples were brazed in a continuous hydrogen atmosphere furnace, using lean duplex stainless steel UNS S32101, UNS S32304 and UNS S32750 with the brazing filler metal BNi-7 (Ni-Cr-P). The experiments were conducted at 1100ºC , during 12 min and 32 min, using joint gaps of 0.0 and 0.3 mm, followed by a cooling thermal cycles in the oven. The samples were characterized using optical microscope, scanning electron microscope with EDS and X-rays diffraction. The results showed that the best condition for furnace brazing of theses stainless steels was the joint gap of 0.0 mm and 32 min. brazing time. The 0.0 mm joint gap showed the presence of a continuous Ni-rich FCC face-centered cubic) phase, which is often referred as γ-Ni. For joint gap of 0.3 mm, a continuous eutectic region in the center of the joint was produced, indicating the joint gap is larger than the ideal gap. A continuous eutectic phase in the joint middle can affect the mechanical properties of the joint. This region consists of intermetallic phases rich in phosphorus. It was observed the precipitation of sigma phase in the base metal UNS S35750, for all brazing conditions studied.
Brazing; lean duplex stainless steels; Ni-Cr-P brazing filler metal
