Mechanical behavior of austenitic stainless-steel welds with variable content of [formula omitted]-ferrite in the heat-affected zone

Abstract

δ-ferrite solidified in the fusion zone (FZ) mitigates the microfissuring susceptibility in multi-pass welded joints performed on austenitic stainless steel (ASS). The herein investigation studied welded joints between two 304L grade ASS with different chemical compositions, which varied δ-ferrite content in the heat-affected zone (HAZ). The pulsed-current gas tungsten arc welding (GTAW-P) process was used along with filler metals ER 308L and ER309L. Additionally, an autogenous joint was made with stainless steel (SS) of high δ-ferrite content (10%). A thermo-mechanical finite element (FE) model was applied to estimate the thermal history, residual stress (RS) and distortion distributions on SS joints. Then, numerical results were correlated with the fracture analysis. The tensile test corroborated the increment in ductility due to δ-ferrite content increasing in the HAZ. The enhancement of mechanical strength observed in the autogenous welded joint was associated with the pinning effect produced by δ-ferrite stringers formed in the HAZ, which inhibited grain growth. Despite the high joint efficiency of SS 304L weldments, the FZ was fractured due to the microfissuring and precipitated particles. The FZ rupture starts in the FZ-HAZ interface of the first weld bead related to the ductility loss with the heat input. Microfissures found in the FZ of all weldments were related to the tensile stress concentration caused by the uneven distortion distribution and the ductility increment (up to 39%) provided by the high δ-ferrite content. Some chromium carbides in the FZ acted as stress raisers.