Brazing Stainless Steel Using a New MBF-Series of Ni-Cr-B-Si Amorphous Brazing Foils

摘要

A group of new high chromium containing amorphous brazing filler metals in foil form has been developed for applications in highly corrosive and/or high temperature environments. These new alloys contain 10-16 wt-% chromium, 1.2-1.6 wt-% boron, i.e., minimal amount needed for amorphability, 7.0-7.5 wt-% silicon, 0-5 wt-% molybdenum, and nickel as the balance. Their T_S is within 975-1030℃ (1787-1886℉) and T_L is within 1090-1140℃ (1994-2084℉) ranges. The alloys exhibit low erosion of base metal and show no significant detrimental effects on base metal strength because boron concentration is kept to a minimum. It was found that optimal combination of strength, ductility and fatigue resistance of joints is obtained when brazing is combined with high temperature annealing in one extended cycle. The joints after such treatment are practically free of the brittle central eutectic line and the base metals adjoining braze area have only limited and localized segregation of chromium borides. The joint itself is uniform and has a strong and ductile microstructure with only one nickel-chromium-based solid solution phase. The fracture of brazements occurs predominantly in the base metal with the joint ultimate strength higher than the yield strength of the virgin 316L base metal. The brazements have a high corrosion resistance in sea water and water solutions of ammonia and phosphoric acid. This new series of brazing filler metals is currently produced on a regular basis by RS technology as a ductile brazing foil having up to 200 mm width and 50-60 μm thickness. The foil has already been used successfully in brazing of hundreds of large corrosion resistant 316L heat exchangers.