AbstractType 308 stainless steel is often used as weld filler metal in light water reactor piping systems. In similar cast CF8 type steels, the ferrite phase has been shown to be responsible for the reported degradation in properties after long term aging at temperatures between 300 and 400°C. Despite the relatively low volume fraction of ferrite in the type 308 materials, long term aging at the maximum operating temperature (343°C) results in a significant loss of impact toughness. Using TEM, G phase precipitation was detected in the ferrite matrix of the aged welds both in the matrix and on dislocations. Composition modulations in the aged ferrite consistent with spinodal decomposition were detected in atom probe composition profiles. A variety of statistical techniques was used to analyse the atom probe composition profiles. The spinodal decomposition of the ferrite into iron rich and chromium enriched regions is the most likely cause of the mechanical property degradation of these materials during aging.MST/1188