  Powder metallurgy with hot isostatic pressing (PM-HIP) is an advanced
manufacturing process that is envisioned to replace forging for heavy nuclear
components, including the reactor pressure vessel (RPV). But PM-HIP products
must at least demonstrate comparable irradiation tolerance than forgings in
order to be qualified for nuclear applications. The objective of this study is
to directly compare PM-HIP to forged SA508 Grade 3 Class 1 low-alloy RPV steel
at two neutron irradiation conditions: ~0.5-1.0 displacements per atom (dpa) at
~270C and ~370C. PM-HIP SA508 experiences greater irradiation hardening and
embrittlement (total elongation) than forged SA508. However, uniform elongation
and approximate toughness are comparable across all irradiated materials,
suggesting irradiated PM-HIP SA508 exhibits superior ductility at maximum
load-bearing capacity. The irradiation hardening mechanism is linked to
composition rather than fabrication method. Since PM-HIP SA508 has higher Mn
and Ni concentration, it is more susceptible to irradiation-induced nucleation
of Mn-Ni-Si-P (MNSP) nanoprecipitates and dislocation loops, which both
contribute to hardening. Conversely, the forged material nucleates fewer MNSPs,
causing dislocation loops to control irradiation hardening. These results show
promise for the irradiation performance of PM-HIP SA508 and can motivate future
nuclear code qualification of PM-HIP fabrication for RPVs.
