Influence of alloying and holding time on microstructure and corrosion fatigue behaviour of brazed AISI 304L/NiCrSiB joints
For numerous industrial applications, such as turbine or heat exchanger constructions, the corrosion fatigue behaviour of vacuum brazed joints is of great interest for the lifetime determination. In addition to the influence of the joining geometry, this behaviour depends primarily on the microstructure of the brazed seam. This in turn is largely determined by the alloy composition of the brazing filler metal and the brazing process parameters. Knowledge of the alloy-process-structure-property relationships is therefore essential to improve the corrosion fatigue properties. In this study, vacuum brazed AISI 304L/NiCrSiB cylindrical butt joints were investigated in multiple and constant amplitude tests in a self-developed corrosion measuring cell with application-specified sensors. To investigate the influence of the alloy composition with regard to the chromium and additional molybdenum content, two different amorphous-crystalline rapidly quenched filler metal foils were produced. Using two different holding times at 1160°C during the brazing process, the influence on the formation of borides and silicides was also investigated on the basis of cross sections by scanning electron microscopy. In previous investigations on a filler metal with a 20% Cr and 4% Mo content, a clear homogenization of the brazed seam could be observed for longer holding times, whereby a more ductile material behaviour was achieved, which in turn positively influenced the corrosion fatigue properties. The positive influence of chromium on the corrosion resistance could be demonstrated, but the present study shows that improved corrosion properties due to a higher Cr content are not necessarily accompanied by improved fatigue properties. This could be demonstrated with a Mo-free, 7% Cr filler metal by a fourfold increase in the number of cycles to failure. Finally, fractographic investigations were carried out to characterize the microstructure-dependent damage mechanisms.