Fatigue Studies of Welded Stainless Steel 304L: Effect of Manufacturability and Weld Type, Choice of Fatigue Model, and Comparison to Pristine Material

Abstract

This study investigates fatigue life and fracture in welded stainless steel 304L (SS 304L) resulting from load-controlled fatigue. This was done for specimens from two welding manufacturers. Mechanical testing was carried out using an MTS test frame in uniaxial tension. Specimens fabricated in three weld configurations (butt, lap, and butt-with-backer) were cycled at stress ratio R = 0.1, frequency f = 5 Hz, and experienced varying mean tensile stress. A comparison of the fatigue life of welded and unwelded (or pristine) specimens determined that the fatigue life of welded specimens is considerably shorter. The microstructure in the parent material (PM), heat-affected (HAZ), and fusion zones (FZ) was analyzed using SEM, and results suggest that, for a limited number of specimens (backer welds), fracture in the (FZ) was primarily the result of Zener–Stroh cracks. Most welded specimens fractured at the weld-HAZ interface (weld toe), and we postulate that stress concentrations primarily due to abrupt geometrical change at the weld interface led to crack initiation there. Additional stress concentrations at welding-induced defects (resulting from high temperature) such as second-phase particles, fatigue-induced defects, and preexisting impurities are responsible for crack initiation, crack growth, and eventual fracture. Comparison of unwelded and welded SN curve data using numerical curve-fitting methods reveals that the fatigue life of pristine SS 304L follows the expected sigmoid shape found in literature (specifically the Chandran fatigue model), while that of welded SS 304L deviates considerably in low cycle fatigue (LCF) and follows the Basquin power law equation.