Fatigue and cyclic deformation behavior of brazed steel joints

To investigate the fatigue assessment of brazed steel joints, stress controlled fatigue tests were conducted with specimens of the steel AISI CA 6-NM (1.4313) and with its brazed joints. Brazing was performed in a shielding gas furnace under H2 atmosphere with Au 18wt% Ni as filler metal. Experiments were performed at a load ratio of R=0.1 with different specimen geometries to compare their fatigue behavior and to investigate the failure mechanism. sults of the experiments—based on a lifetime oriented approach—show the existence of two different regimes depending on the number of cycles to fracture (Nf). For Nf<104 the maximum tolerable loads for all specimens approach the ultimate tensile strength of the substrate material, whereas for Nf>104 the substrate material provides the highest strength, followed by the brazed round specimens and by the brazed T-joint specimens. Investigations on the failure mechanisms revealed that for brazed specimens, fatigue and residual fracture occurred always in the interface of the brazing zone. The crack path is characterized by interfacial jumps, accompanied by ductile deformation features. The analysis of the strain evolution during the cyclic loading experiments shows that the cyclic deformation behavior is significantly influenced by cyclic creep. Furthermore, the experiments show that brazed round specimen exhibit higher strains at similar loading amplitudes, compared to the substrate material. These new findings were also confirmed by FE-calculations, showing an inhomogeneous distribution of local stresses and strains in the proximity of the braze layer. The archived results show the complex interactions of a braze layer on the cyclic deformation behavio—compared to its bulk material—and lead to a better understanding of the fatigue assessment of brazed steel joints.