Effect of thermal aging on microstructure and carbides of SA508/Alloy 52 dissimilar metal weld

A narrow-gap SA508/Alloy 52 dissimilar metal weld (DMW) mock-up, fully representative of an actual nuclear component, was investigated in this work. The microstructure and carbides formed in the low alloy steel fusion boundary (FB) and heat affected zone (HAZ) can act as brittle fracture initiators and could influence the brittle fracture behavior. However, the amount of information available in the open literature on the microstructural changes and carbide formation in DMW occurring upon post-weld heat treatment and long-term thermal aging is very limited. The microstructure and carbide type, morphology and size in the carbide precipitation zone (CPZ, up to 1.5 ??m from FB), carbon depletion zone (CDZ, up to 40???50 ??m from FB) and HAZ (up to 2 mm from FB) of the plant-relevant DMW in post-weld heat-treated and thermally-aged (400 ???C for 15,000 h, corresponding to 90 years of operation) conditions were analyzed with analytical electron microscopy, wide-angle X-ray scattering and atom probe tomography. Long-term thermal aging increases the microhardness peak close to the FB, triples the width of the CPZ and coarsens the carbide size in the HAZ (up to a magnitude). There is no evidence of a significant phosphorus segregation to grain boundaries due to thermal aging.