"Microstructural Evolution, Residual Stress Development, and Irradiation Effects in Electron Beam Welded PM-HIP and Forged Nuclear Reactor Pressure Vessel Steels" 

Jasmyne Emerson, MSE PhD Candidate 

Advisor: Professor Maria Okuniewski

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ABSTRACT

Reactor pressure vessel (RPV) steels serve as non-replaceable safety-critical components in commercial light water reactors. The structural integrity of these vessels depends not only on base metal properties, but also on the microstructural and residual stress gradients introduced by welding and post-weld processing across the fusion zone (FZ), heat-affected zone (HAZ), and base metal. Despite extensive research on arc-welded RPV steels, far less is understood about RPVs fabricated via advanced manufacturing methods such as powder metallurgy with hot isostatic pressing (PM-HIP) or welded by electron beam (EB) welding. Additionally, although the HAZ is often treated as mechanically intermediate between the FZ and base metal, it represents a microstructurally distinct region whose unique phase, strain, and stress gradients may evolve differently under irradiation, making independent characterization essential for accurate reactor life assessment.

This dissertation presents a systematic, multi-scale investigation of SA508-3 RPV steel fabricated via conventional forging and PM-HIP, joined by single-pass autogenous EB welding, and subjected to either a traditional post-weld heat treatment (PWHT) or a novel solution anneal, quench, normalization, and temper (SQNT) treatment. Characterization spans optical and scanning electron microscopy alongside synchrotron-based X-ray diffraction (SXRD). SXRD is employed for phase identification and phase fraction quantification, crystallite size and microstrain estimation, dislocation density determination, and residual strain and stress analysis performed across the weldment in both unirradiated and proton-irradiated conditions. This framework enables direct, spatially resolved comparison of microstructural and stress states across fabrication routes, heat treatment conditions, and irradiation exposures within the same weldment geometry.

First, this work establishes that PM-HIP and forged SA508 exhibit different microstructural and residual stress gradients across EB weldments. Second, this work demonstrates that the SQNT treatment homogenizes phase and strain distribution across the weldment, which is a marked improvement over the PWHT. Third, this work introduces a modified Rosenthal solution incorporating porosity-corrected density, which accurately predicts the wider HAZ experimentally observed in PM-HIP materials. Fourth, proton irradiation reveals that the forged PWHT weldment exhibits irradiation-induced precipitation and localized microstrain relaxation while the PM-HIP SQNT weldment shows minimal irradiation-induced change, consistent with its more homogeneous and lower-strain starting microstructure. Together, these findings demonstrate that fabrication route, welding, and post-weld heat treatment exert influence on both the overall microstructure and residual stress profile of EB weldments, and on their subsequent response to irradiation, with direct implications for the qualification of PM-HIP and EB welding technologies for next-generation RPV fabrication.