Trends in Charpy-V shelf energy degradation and yield strength increase of neutron-embrittled pressure vessel steels

Abstract

Charpy-V shelf energy degradation and yield strength increase have been examined for three neutron-irradiated pressure vessel steel compositions: A302-B, A533 and A543. The effects of radiation exposure at low temperature (<300°F (149°C)) and at elevated temperature (550°F (288°C) to 740°F (393°C)) on overall notch ductility are documented and compared. Summary plots showing the simultaneous degradation in shelf energy and increase of yield strength levels broadly illustrate the progressive change from ductile fracture performance to relatively brittle characteristics.Data for all three steel types suggest a common pattern of properties behavior. Three stages of the shelf level versus yield strength change are indicated with progressive neutron exposure at <300°F (149°C). Data patterns for strong (longitudinal) versus weak (transverse) test orientations of thick section plate imply strong trend similarities. Properties modified by progressive radiation exposure at 550°F (288°C) are interpreted as following the same damage path described by <300°F (149°C) irradiation. The much slower pace of properties change with elevated temperature exposure is readily identified by trend line presentations.Observations show that A533 plate and weld metals (Grades B and C, Classes 1 and 2) have greater toughness retention than the ASTM A302-B reference heat (6-in plate) after 550°F (288°C) radiation. Results for a single reference heat of A543 Class 1, 8-in plate demonstrate that a lower initial brittle/ductile transition temperature in itself may not fully eliminate the general problem of providing sufficient notch ductility to allow for radiation embrittlement in service. Studies of A543 steel also reveal that temper embrittlement introducing during fabrication is not a strong influence on subsequent irradiation-induced changes in Charpy-V notch ductility at exposure temperatures up to 750°F (399°C).