Nanopillar compression tests were applied on delta-ferrites in austenitic stainless steel welds to measure the thermal ageing activation energy caused by spinodal decomposition. Welds of an austenitic stainless steel were thermally aged at 343, 375, and 400 degrees C for up to 20,000 h. To avoid the interference of surrounding austenite matrix on embrittlement measurement, austenite matrix was selectively dissolved before the nanopillar fabrication. Using the yield stress changes measured by nanopillar compression tests, the thermal ageing activation energy of delta-ferrite was estimated as 154 KJ/mol, which was compared with literature results and the reasons of discrepancy were discussed. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.