Fatigue behavior and microstructural characteristics of a duplex stainless steel weld metal under vibration-assisted welding

The fatigue behavior and dislocation structures of SAF 2507 alloy were investigated. Specifically, as-received superduplex stainless steel SAF 2507 (AS) alloy and its weld metals from direct current welding without and with electrodynamic vibration (VIB) were studied. Fatigue experiments were conducted at total strain amplitudes of 1.2% and 2.4% (R = −1), respectively; the VIB samples exhibited higher strength but shorter fatigue lives. In the fatigued AS sample, ferrite exhibited characteristic dislocation structures, namely veins, walls, and cells, and austenite had persistent Lüders bands (PLBs) and mechanical twins. In the fatigued weld metals, the dislocation structures in austenite exhibited only PLBs; however, in ferrite, two types of walls formed due to entanglement of dislocations and rearrangement of Cr2N dislocation bundles, indicating that the fatigue strains are accommodated mostly by ferrite. The shorter fatigue lives of the VIB samples are attributed to the increased amount of intragranular austenite, and increased boundaries between intragranular austenite and δ-ferrite result in faster dislocation entanglement. Consequently, the initial response stress was the highest and work hardening rate was higher, which eventually induced cracks in δ-ferrite.