Effects of Thermal Aging Induced Microstructure Evolution on Mechanical and Corrosion Properties of Delta-Ferrite in Austenitic Stainless Steel Weld

347 austenitic stainless steel weld (ASSW) was thermally aged at 343, 400 and 450 °C up to 20,000 h in this study. Effects of thermal aging induced microstructure evolution on the mechanical properties and corrosion resistance of delta-ferrite in the 347 ASSW were qualitatively and quantitatively assessed by high resolution transmission electron microscopy (0.07 nm) with energy-dispersive spectroscope (EDS) and Fast Fourier Transform (FFT). After thermal aging at 343 °C for 20,000 h and 400 °C over 5,000 h, fluctuation of major alloying elements such as Fe, Cr, and Ni was observed by spinodal decomposition in the delta-ferrite. Meanwhile, a Ni+Si-rich G phase was developed as thermal aging progressed at 400 °C for 20,000 h and 450 °C for 5,000 h in the delta-ferrite. Such microstructure evolutions tended to be accelerated with increasing aging temperature and exposure period, while the G phase was formed at a higher exposure temperature and/ or period compared to spinidal decomposition. These effects increased the tensile strength and decreased the elongation of 347 ASSW at room temperature, compared to the as-welded condition. Moreover, when spinodal decomposition and G phase were observed, the degree of sensitization values of the 347 ASSW in the double loop-electrochemical potentiodynamic reactivation tests were significantly increased, due to localized Cr depletion in the delta-ferrite.