Vacuum brazing 0Cr18Ni9 stainless steel pipe to itself with BNi-2 filler alloy

Focused on the application of special-shaped joints, 0Cr18Ni9 stainless steel pipes were brazed using BNi82CrSiB (BNi-2) as filler alloy to form a bell and spigot joint. Influences of cooling-down velocity, brazing temperature, holding time and fit-up gap on the microstructure and mechanical property of joints were investigated. Results indicated that the fit-up gap had a great effect on the joints due to the special structure of pipe joints, when the width of gap is only 5~10 μm, it is difficult to fill the lap to form a complete joint and the strength of joints is low, but if the gap is too large up to 250~300 μm, large amounts of brittle phases formed in the brazing seam as well as micro-cracks. Cooling velocity almost made no difference on the microstructure, while the joints cooled at 15℃/min present lower strength than that furnace cooled. Increasing brazing temperature or prolonging holding time, the maximum gap increases and erosion of the matrix can be observed, but both parameters have few effects on mechanical property. When the two pipes with the fit-up gap of 20~50 μm were brazed at 1050℃ for 10 min and furnace cooled, no eutectic phases and intermetallics appeared in the seam, and the strength of the joint reached the maximum.