Influence of fabrication on microstructure and heat affected zone width in weldments of nuclear reactor pressure vessel steel

• Synchrotron X-ray diffraction is utilized to characterize nuclear reactor pressure vessel weldments. • Traditional macroscopic methods underestimate the heat affected zone width in powder metallurgy weldments. • A heat affected zone width model is developed to include porosity in the design of powder metallurgy weldments. • Austenitizing heat treatment eliminates austenite, yielding uniform microstructure. • Hardness in traditional post-weld heat treated welds is driven by dislocation density and hetero-phase interfaces. Advanced manufacturing routes such as electron beam welding and powder metallurgy with hot isostatic pressing are increasingly used across energy and aerospace industries, where the reliable prediction of weld behavior and heat-affected zone (HAZ) evolution is critical. This study examines how fabrication routes and post-weld heat treatments influence phase distribution, crystallite size, microstrain, and dislocation density in nuclear reactor pressure vessel steels using synchrotron X-ray diffraction (SXRD). Retained austenite occurs only in samples that did not undergo austenitization, whereas an austenitizing heat treatment fully eliminates retained austenite and produces a more uniform microstructure across the weldment in terms of phase fraction, dislocation density, and microstrain. The Rosenthal solution underestimates the heat-affected zone width for powder metallurgy samples. A newly proposed modified Rosenthal solution, reducing density by accounting for porosity, matches the SXRD-measured HAZ width with a 0.65% error. Structure–property correlations reveal that dislocation density correlates strongly with nanohardness in homogenous microstructures, while in heterogenous weldments nanohardness is further influenced by the presence of dissimilar phase boundaries. These findings provide new insight into the thermal and microstructural response of powder metallurgy fabricated steels and offer a framework for optimizing welding procedures and heat treatments in advanced manufacturing applications.