Fatigue performance and failure mechanisms of vacuum brazed AISI 304L joints with experimental nickel-based filler metals in air and corrosive environments

Brazing with nickel-based filler metals under vacuum is frequently applied for joining metals in applications that require high resistance to mechanical loads, corrosive environments and elevated temperatures. However, melting-point depressant elements in the filler can lead to brittle phases in the brazing seam, affecting crack initiation and propagation. This study investigated cylindrical butt joints of metastable austenitic AISI 304L, brazed with NiCrSiB–based fillers containing iron and molybdenum. Tensile, fatigue and corrosion tests were applied to identify the most promising filler for subsequent corrosion fatigue experiments in a self-developed test chamber. Results in air showed that fatigue performance close to AISI 304L can be achieved, when problematic Kirkendall voids and athermal brittle phases are avoided. Corrosion fatigue tests in aqueous environments with varying sodium chloride concentrations and elevated temperatures revealed that cracked chromium-rich borides in the diffusion-affected zone act as microgalvanic elements, accelerating corrosion fatigue via hydrogen embrittlement. At 80 °C, these mechanisms became widespread, leading to multiple crack initiations across the steel surface and not just in the brazing seam. Hence, this study highlights the complex interplay of microstructure, cyclic loading, and environmental effects, offering key insights into the failure mechanisms of vacuum-brazed joints under combined mechanical and corrosive stresses.