The effects of thermal aging on microstructure and mechanical property change of delta-ferrite in an austenitic stainless steel weld were investigated. After thermal aging at 400 degrees C for 20,000 h, spinodal decomposition and G phase precipitation were observed, along with similar to 80% increase in yield strength measured by nanopillar compression test. Subsequent reversion heat treatment removed spinodal decomposition while leaving G phase, resulting in partial recovery of hardening in delta-ferrite. It was estimated that similar to 60% of the hardening in the delta-ferrite of the aged specimen was attributed to spinodal decomposition and similar to 40% to G phase, respectively. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.