Three austenitic stainless steel welds (ASSW), such as E308, ER316L and ER347, with about 10 % of δ-ferrite were thermally aged at $400^\circ C$ up to 20,000 h to accelerate the thermal ageing expected in pressurized water reactor (PWR) environment. Fluctuation of Fe and Cr due to spinodal decomposition is observed with width of 5 - 12 nm within the ferrite phase. For all 3 welds, increase in strength and decrease in tensile elongation is observed after thermal aging at room temperature and $320^\circ C$. The reduction in fracture resistance, measured as J-R curves, is most significant for ER316L weld with decrease by about 61%. However, it is quite small (about 25 % decrease) for ER347 weld even after ageing at $400^\circ C$ for 20,000 h despite the large reduction in tensile elongation. Finally, the double loop-electrochemical potentiodynamic reactivation (DL-EPR) tests show the degree of sensitization (DOS) values are negligible even after the accelerated thermal ageing at $400^\circ C$ for 10,000 h.