Structure-property relationships in electron beam welded SA508 nuclear reactor pressure vessel steel

Abstract

The objective of this study is to understand structure-property relationships across electron beam (EB) welds on nuclear reactor pressure vessel (RPV) steel SA508, Grade 3. Modern nuclear reactor designs typically rely on single-forging RPVs in an effort to eliminate arc welds that require costly in-service inspection. By comparison, advanced EB welds are being considered for RPV applications as they can be rapidly produced, quality heat treated, and often exhibit fewer impurities and narrower heat-affected zones (HAZ). But little is known about microstructure evolution during EB welding and its implications on mechanical properties. This work identifies key structure-property relationships in electron beam welds on forged SA-508 and on a powder metallurgy with hot isostatic pressing (PM-HIP) compact produced to match the SA508 Grade 3 chemical composition. In both the forging and PM-HIP compact, the EB weld fusion zone and HAZ exhibit significant hardening due to martensite nucleation during rapid cooling, while the base metal retains a dual-phase ferrite-bainite microstructure. An appropriately designed heat treatment can eliminate hardness gradients by homogeneously recrystallizing a ferrite-bainite microstructure across the weldment. While PM-HIP and forged SA508 exhibit similar EB weld-induced microstructure evolution, the more extensive porosity in the PM-HIP specimen promotes grain growth and stabilizes ferrite and martensite. Hardness is governed by ferrite phase contiguity, wherein dislocation transmission is inhibited at dissimilar phase interfaces. But if the microstructure is dominated by bainite and/or martensite, hardness is instead governed by their phase fractions. This work illustrates the potential for combining EB welding and PM-HIP with an appropriate quality heat treatment to create RPV welds having negligible microstructure gradients and consistent hardening micro-mechanisms.