MICROSTRUCTURAL AND MICROCHEMICAL CHARACTERIZATION OF NUCLEAR PRESSURE VESSEL WELDS (MICROANALYSIS, ELECTRON MICROSCOPY)

Quenched and tempered ASTM A533 Grade B steel is being widely used in the current construction of nuclear reactor pressure vessels. Studies on submerged arc A533 steel weldments show that the weld is more susceptible to irradiation embrittlement than either the base plate or the heat affected zone.;The A533 base plate material was characterized to discern those microstructural features that occur during welding. Microstructures and microchemistries in a high and low copper content welds were compared in order to identify those microstructural features that may be responsible for the enhanced embrittlement seen in the high copper content weldment during irradiation. Characterization studies were performed on welds made with proprietary fluxes--Linde Grade 80 and Linde Grade 0091 to account for differences in mechanical properties in these welds. The effects of multipass welding and long term post weld heat treatments on the microstructure and mechanical properties of the welds were also studied.;Results show that the A533 base plate steel resists softening even after 400 hours of tempering at 620(DEGREES)C (1150(DEGREES)F). This has been attributed to precipitation of spherical copper particles and additional precipitation of acicular and spherical molybdeum carbides during tempering. In the higher copper content weldment, precipitation of copper particles and Mo(,2)C cause the weld to reach a peak hardness at 260 hours of post weld heat treatment (PWHT) at 620(DEGREES)C (1150(DEGREES)F). The improvement in toughness properties of the weldment by the use of Linde Grade 0091 flux instead of Linde Grade 80 flux was attributed to the low inclusion content and fine interlocking acicular ferrite microstructure that was seen in welds made with Grade 0091 flux.;Because of the complexity of the weld microstructure, few attempts have been made to correlate the microstructure with the mechanical properties on a sub-micron scale. In this investigation modern analytical electron microscopy (AEM) techniques have been used to correlate the mechanical properties data of various welds with microstructural and microchemical studies.