Atom probe tomography of 15Kh2MFA Cr-Mo-V steel surveillance specimens

An atom probe study has been performed on 15Kh2MFA base and 10KhMFT weld metal surveillance specimens from a VVER-440/213C reactor to investigate the mechanisms that produce embrittlement in low copper materials during service. The composition of the base metal was Fe-0.06 at.% Cu, 3.1% Cr, 0.34% V, 0.46% Mn, 0.35% Mo, 0.07% Ni, 0.34% Si, 0.74% C, 0.025% P, and 0.028% S. The base material was characterized after thermal aging for 10 years at 295 degreesC and after neutron irradiation at 270 degreesC for 10 years to a fluence of 1.0 X 10(25) m(-2) (E > 0.5 MeV). The ductile-to-brittle transition temperatures (DBTT) of the base metal were -49, -70 and 141 degreesC, for the unirradiated, thermally aged and neutron irradiated conditions, respectively. The composition of the weld metal was Fe-0.05 at.% Cu, 1.46% Cr, 0.22% V, 1.11% Mn, 0.29% Mo, 1.17% Si, 0.17% C, 0.02% P, and 0.029% S. The weld material was characterized after tempering for 18 h at 690 degreesC plus a simulated stress relief treatment of 43.5 h at 680 degreesC, after thermal aging for 5 years at 295 degreesC, and after neutron irradiation at 275 degreesC for 5 years to a fluence of 5.2 X 10(24) m(2) (E > 0.5 MeV). The DBTTs were 7, 11 and 123 degreesC, respectively, for these three conditions. A high number density of ultrafine manganese- and silicon-enriched regions was observed in both neutron-irradiated materials. Phosphorus segregation was observed at the VC-matrix interface and at grain boundaries. Published by Elsevier Science Ltd.