As part of an overall effort to determine the fundamental mechanisms of neutron irradiation embrittlement of pressure vessel steels, four commercial alloys (two welds and two plates) and eight model alloys have been irradiated to a fluence of ~7 × 10¹⁸ neutrons (n)/cm² (E > 1 MeV) at a flux of ~3 × 10¹² n/cm² · s and at four temperatures from 270°C to 315°C. The alloys had a range of copper and nickel concentrations and microstructures. Post-irradiation microhardness testing showed the following: radiation hardening systematically decreased, albeit modestly, with increasing irradiation temperature; radiation hardening increased nonlinearily with copper concentration, with decreasing incremental effect at high levels; radiation hardening was not influenced by nickel but was sensitive to microstructure. These results were found to be broadly consistent with a physically-based model in which the hardening is primarily due to an irradiation-enhanced copper precipitation with a secondary contribution from radiation-induced defect cluster formation.