FRACTURE AND FATIGUE OF STAINLESS STEEL WIDE GAP BRAZED WITH NICKEL BASED FILLER METALS

Abstract

Mechanical properties, in terms of fracture and fatigue, of AISI 316 stainless steel wide gap brazed with different nickel based filler metals have been investigated. Tensile tests with extra small strain gauges bonded at the centre of the joints were carried out. Fatigue crack initiation lives and propagation rates were evaluated under displacement control. Crack closure in brazed joints has been determined by means of back face strain on the compact tension specimens used. Crack initiation and propagation toughness of the wide gap brazed with nickel based filler metals have been studied by means of J integral. The scope of the present investigation involves the study of the effects of different filler metals, namely, BNi-1a, BNi-5, BNi-7 and Nicrobraz No 150 containing different melting point depressants, effects of gap filler contents of 0%, 20% and 40%, effects of gap clearances from 0.3 mm to 1.2 mm, and the effects of brazing temperatures of 1100, 1125, 1150, 1175, 12110 and 1225 °C. It was found that joints brazed with nickel based filler metals were often very brittle. The microstructure within the joints consists generally of solid solution, eutectics and discrete metalloid compounds. The composition of the latter was dependent upon the filler metals employed. The volume of solid solution was observed to depend on the gap filler contents and brazing temperature. To improve the mechanical properties of the brazed joints, a diffusion treatment process was carried out at 1040°C for pure Nicrobraz No 150 after brazing. This was found to be able to change the amount and distribution of the phases in the microstructure The nature of the crack paths under quasi-static and fatigue loading was revealed from scanning electron microscope examination of the polished surfaces. Crack branching, meandering and bridging were observed along the fatigue cracks. It is believed that crack closure detected may most likely be caused by roughness of the fracture surfaces and ligament bridging observed in the brazes. Nonlinear fracture behaviour was observed in the fracture toughness tests on the brazed joints. The loading and unloading lines deviated from linearity and could not be extrapolated to the origin. The load versus crack open displacement curves displayed increasing hysteresis behaviour. The J-R curves increased with crack growth. Crack bridging behind the crack tip and small cracks in front of the crack tip were also observed in the fracture toughness testing. Characteristics of the fracture surfaces under monotonic and cyclic loading were examined by means of scanning electron microscope with EDX. Generally, quasi-cleavage was discerned to be the dominant fracture mode in all the fracture surfaces. Among the filler metals investigated, joints brazed with BNi-1a was observed to be the best, while that with BNi-7 was the poorest in terms of strength, fatigue resistance and fracture toughness. The present investigation showed that strength and elongation of the brazed joints increased with brazing temperature. The addition of gap filler was able to improve the load carrying capacity of the brazed joints only when the brazing temperature was high enough. The introduction of gap filler was able to increase the fatigue and fracture resistance of the brazed joints when suitable brazing temperature was used. In addition, experimental results showed that gap filler was able to enhance crack closure caused by roughness and ligament bridging.